Merge pull request #6549 from libgit2/ethomson/sha256_experimental

sha256: less hardcoded SHA1 types and lengths

58 files changed, 1087 insertions, 527 deletions

diff --git a/src/libgit2/annotated_commit.c b/src/libgit2/annotated_commit.c
index 7bd8b6077..c5c8ace78 100644
--- a/src/libgit2/annotated_commit.c
+++ b/src/libgit2/annotated_commit.c
@@ -39,8 +39,8 @@ static int annotated_commit_init(
 	if ((error = git_commit_dup(&annotated_commit->commit, commit)) < 0)
 		goto done;
 
-	git_oid_fmt(annotated_commit->id_str, git_commit_id(commit));
-	annotated_commit->id_str[GIT_OID_SHA1_HEXSIZE] = '\0';
+	git_oid_tostr(annotated_commit->id_str, GIT_OID_MAX_HEXSIZE + 1,
+		git_commit_id(commit));
 
 	if (!description)
 		description = annotated_commit->id_str;
diff --git a/src/libgit2/annotated_commit.h b/src/libgit2/annotated_commit.h
index c87eaa805..1f805fe9b 100644
--- a/src/libgit2/annotated_commit.h
+++ b/src/libgit2/annotated_commit.h
@@ -41,7 +41,7 @@ struct git_annotated_commit {
 	const char *ref_name;
 	const char *remote_url;
 
-	char id_str[GIT_OID_SHA1_HEXSIZE+1];
+	char id_str[GIT_OID_MAX_HEXSIZE + 1];
 };
 
 extern int git_annotated_commit_from_head(git_annotated_commit **out,
diff --git a/src/libgit2/apply.c b/src/libgit2/apply.c
index 18304da4d..6b55b812f 100644
--- a/src/libgit2/apply.c
+++ b/src/libgit2/apply.c
@@ -19,6 +19,7 @@
 #include "zstream.h"
 #include "reader.h"
 #include "index.h"
+#include "repository.h"
 #include "apply.h"
 
 typedef struct {
@@ -644,7 +645,7 @@ int git_apply_to_tree(
 	 * put the current tree into the postimage as-is - the diff will
 	 * replace any entries contained therein
 	 */
-	if ((error = git_index_new(&postimage)) < 0 ||
+	if ((error = git_index__new(&postimage, repo->oid_type)) < 0 ||
 		(error = git_index_read_tree(postimage, preimage)) < 0 ||
 		(error = git_reader_for_index(&post_reader, repo, postimage)) < 0)
 		goto done;
@@ -851,8 +852,8 @@ int git_apply(
 	 * having the full repo index, so we will limit our checkout
 	 * to only write these files that were affected by the diff.
 	 */
-	if ((error = git_index_new(&preimage)) < 0 ||
-	    (error = git_index_new(&postimage)) < 0 ||
+	if ((error = git_index__new(&preimage, repo->oid_type)) < 0 ||
+	    (error = git_index__new(&postimage, repo->oid_type)) < 0 ||
 	    (error = git_reader_for_index(&post_reader, repo, postimage)) < 0)
 		goto done;
 
diff --git a/src/libgit2/blame.c b/src/libgit2/blame.c
index b70cd615e..d93dd5e76 100644
--- a/src/libgit2/blame.c
+++ b/src/libgit2/blame.c
@@ -60,10 +60,11 @@ static bool hunk_starts_at_or_after_line(git_blame_hunk *hunk, size_t line)
 }
 
 static git_blame_hunk *new_hunk(
-		size_t start,
-		size_t lines,
-		size_t orig_start,
-		const char *path)
+	size_t start,
+	size_t lines,
+	size_t orig_start,
+	const char *path,
+	git_blame *blame)
 {
 	git_blame_hunk *hunk = git__calloc(1, sizeof(git_blame_hunk));
 	if (!hunk) return NULL;
@@ -72,8 +73,8 @@ static git_blame_hunk *new_hunk(
 	hunk->final_start_line_number = start;
 	hunk->orig_start_line_number = orig_start;
 	hunk->orig_path = path ? git__strdup(path) : NULL;
-	git_oid_clear(&hunk->orig_commit_id, GIT_OID_SHA1);
-	git_oid_clear(&hunk->final_commit_id, GIT_OID_SHA1);
+	git_oid_clear(&hunk->orig_commit_id, blame->repository->oid_type);
+	git_oid_clear(&hunk->final_commit_id, blame->repository->oid_type);
 
 	return hunk;
 }
@@ -86,13 +87,14 @@ static void free_hunk(git_blame_hunk *hunk)
 	git__free(hunk);
 }
 
-static git_blame_hunk *dup_hunk(git_blame_hunk *hunk)
+static git_blame_hunk *dup_hunk(git_blame_hunk *hunk, git_blame *blame)
 {
 	git_blame_hunk *newhunk = new_hunk(
 			hunk->final_start_line_number,
 			hunk->lines_in_hunk,
 			hunk->orig_start_line_number,
-			hunk->orig_path);
+			hunk->orig_path,
+			blame);
 
 	if (!newhunk)
 		return NULL;
@@ -237,7 +239,8 @@ static git_blame_hunk *split_hunk_in_vector(
 		git_vector *vec,
 		git_blame_hunk *hunk,
 		size_t rel_line,
-		bool return_new)
+		bool return_new,
+		git_blame *blame)
 {
 	size_t new_line_count;
 	git_blame_hunk *nh;
@@ -250,8 +253,9 @@ static git_blame_hunk *split_hunk_in_vector(
 	}
 
 	new_line_count = hunk->lines_in_hunk - rel_line;
-	nh = new_hunk(hunk->final_start_line_number + rel_line, new_line_count,
-			hunk->orig_start_line_number + rel_line, hunk->orig_path);
+	nh = new_hunk(hunk->final_start_line_number + rel_line,
+		new_line_count, hunk->orig_start_line_number + rel_line,
+		hunk->orig_path, blame);
 
 	if (!nh)
 		return NULL;
@@ -304,7 +308,8 @@ static int index_blob_lines(git_blame *blame)
 static git_blame_hunk *hunk_from_entry(git_blame__entry *e, git_blame *blame)
 {
 	git_blame_hunk *h = new_hunk(
-			e->lno+1, e->num_lines, e->s_lno+1, e->suspect->path);
+		e->lno+1, e->num_lines, e->s_lno+1, e->suspect->path,
+		blame);
 
 	if (!h)
 		return NULL;
@@ -445,14 +450,16 @@ static int buffer_hunk_cb(
 	blame->current_hunk = (git_blame_hunk*)git_blame_get_hunk_byline(blame, wedge_line);
 	if (!blame->current_hunk) {
 		/* Line added at the end of the file */
-		blame->current_hunk = new_hunk(wedge_line, 0, wedge_line, blame->path);
+		blame->current_hunk = new_hunk(wedge_line, 0, wedge_line,
+			blame->path, blame);
 		GIT_ERROR_CHECK_ALLOC(blame->current_hunk);
 
 		git_vector_insert(&blame->hunks, blame->current_hunk);
 	} else if (!hunk_starts_at_or_after_line(blame->current_hunk, wedge_line)){
 		/* If this hunk doesn't start between existing hunks, split a hunk up so it does */
 		blame->current_hunk = split_hunk_in_vector(&blame->hunks, blame->current_hunk,
-				wedge_line - blame->current_hunk->orig_start_line_number, true);
+				wedge_line - blame->current_hunk->orig_start_line_number, true,
+				blame);
 		GIT_ERROR_CHECK_ALLOC(blame->current_hunk);
 	}
 
@@ -481,7 +488,7 @@ static int buffer_line_cb(
 		} else {
 			/* Create a new buffer-blame hunk with this line */
 			shift_hunks_by(&blame->hunks, blame->current_diff_line, 1);
-			blame->current_hunk = new_hunk(blame->current_diff_line, 1, 0, blame->path);
+			blame->current_hunk = new_hunk(blame->current_diff_line, 1, 0, blame->path, blame);
 			GIT_ERROR_CHECK_ALLOC(blame->current_hunk);
 
 			git_vector_insert_sorted(&blame->hunks, blame->current_hunk, NULL);
@@ -529,7 +536,7 @@ int git_blame_buffer(
 
 	/* Duplicate all of the hunk structures in the reference blame */
 	git_vector_foreach(&reference->hunks, i, hunk) {
-		git_blame_hunk *h = dup_hunk(hunk);
+		git_blame_hunk *h = dup_hunk(hunk, blame);
 		GIT_ERROR_CHECK_ALLOC(h);
 
 		git_vector_insert(&blame->hunks, h);
diff --git a/src/libgit2/branch.c b/src/libgit2/branch.c
index 4cbd1e26f..9a31c9c6f 100644
--- a/src/libgit2/branch.c
+++ b/src/libgit2/branch.c
@@ -134,9 +134,9 @@ int git_branch_create(
 	const git_commit *commit,
 	int force)
 {
-	char commit_id[GIT_OID_SHA1_HEXSIZE + 1];
+	char commit_id[GIT_OID_MAX_HEXSIZE + 1];
 
-	git_oid_tostr(commit_id, GIT_OID_SHA1_HEXSIZE + 1, git_commit_id(commit));
+	git_oid_tostr(commit_id, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit));
 	return create_branch(ref_out, repository, branch_name, commit, commit_id, force);
 }
 
diff --git a/src/libgit2/cherrypick.c b/src/libgit2/cherrypick.c
index 04812b1c6..3ef42d5e7 100644
--- a/src/libgit2/cherrypick.c
+++ b/src/libgit2/cherrypick.c
@@ -106,10 +106,10 @@ static int cherrypick_state_cleanup(git_repository *repo)
 
 static int cherrypick_seterr(git_commit *commit, const char *fmt)
 {
-	char commit_oidstr[GIT_OID_SHA1_HEXSIZE + 1];
+	char commit_oidstr[GIT_OID_MAX_HEXSIZE + 1];
 
 	git_error_set(GIT_ERROR_CHERRYPICK, fmt,
-		git_oid_tostr(commit_oidstr, GIT_OID_SHA1_HEXSIZE + 1, git_commit_id(commit)));
+		git_oid_tostr(commit_oidstr, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit)));
 
 	return -1;
 }
@@ -173,7 +173,7 @@ int git_cherrypick(
 	git_cherrypick_options opts;
 	git_reference *our_ref = NULL;
 	git_commit *our_commit = NULL;
-	char commit_oidstr[GIT_OID_SHA1_HEXSIZE + 1];
+	char commit_oidstr[GIT_OID_MAX_HEXSIZE + 1];
 	const char *commit_msg, *commit_summary;
 	git_str their_label = GIT_STR_INIT;
 	git_index *index = NULL;
diff --git a/src/libgit2/commit_graph.c b/src/libgit2/commit_graph.c
index bf557f7ad..4edd71106 100644
--- a/src/libgit2/commit_graph.c
+++ b/src/libgit2/commit_graph.c
@@ -138,19 +138,22 @@ static int commit_graph_parse_oid_lookup(
 		struct git_commit_graph_chunk *chunk_oid_lookup)
 {
 	uint32_t i;
-	unsigned char *oid, *prev_oid, zero_oid[GIT_OID_SHA1_SIZE] = {0};
+	unsigned char *oid, *prev_oid, zero_oid[GIT_OID_MAX_SIZE] = {0};
+	size_t oid_size;
+
+	oid_size = git_oid_size(file->oid_type);
 
 	if (chunk_oid_lookup->offset == 0)
 		return commit_graph_error("missing OID Lookup chunk");
 	if (chunk_oid_lookup->length == 0)
 		return commit_graph_error("empty OID Lookup chunk");
-	if (chunk_oid_lookup->length != file->num_commits * GIT_OID_SHA1_SIZE)
+	if (chunk_oid_lookup->length != file->num_commits * oid_size)
 		return commit_graph_error("OID Lookup chunk has wrong length");
 
 	file->oid_lookup = oid = (unsigned char *)(data + chunk_oid_lookup->offset);
 	prev_oid = zero_oid;
-	for (i = 0; i < file->num_commits; ++i, oid += GIT_OID_SHA1_SIZE) {
-		if (git_oid_raw_cmp(prev_oid, oid, GIT_OID_SHA1_SIZE) >= 0)
+	for (i = 0; i < file->num_commits; ++i, oid += oid_size) {
+		if (git_oid_raw_cmp(prev_oid, oid, oid_size) >= 0)
 			return commit_graph_error("OID Lookup index is non-monotonic");
 		prev_oid = oid;
 	}
@@ -163,11 +166,13 @@ static int commit_graph_parse_commit_data(
 		const unsigned char *data,
 		struct git_commit_graph_chunk *chunk_commit_data)
 {
+	size_t oid_size = git_oid_size(file->oid_type);
+
 	if (chunk_commit_data->offset == 0)
 		return commit_graph_error("missing Commit Data chunk");
 	if (chunk_commit_data->length == 0)
 		return commit_graph_error("empty Commit Data chunk");
-	if (chunk_commit_data->length != file->num_commits * (GIT_OID_SHA1_SIZE + 16))
+	if (chunk_commit_data->length != file->num_commits * (oid_size + 16))
 		return commit_graph_error("Commit Data chunk has wrong length");
 
 	file->commit_data = data + chunk_commit_data->offset;
@@ -209,7 +214,9 @@ int git_commit_graph_file_parse(
 
 	GIT_ASSERT_ARG(file);
 
-	if (size < sizeof(struct git_commit_graph_header) + GIT_OID_SHA1_SIZE)
+	checksum_size = git_oid_size(file->oid_type);
+
+	if (size < sizeof(struct git_commit_graph_header) + checksum_size)
 		return commit_graph_error("commit-graph is too short");
 
 	hdr = ((struct git_commit_graph_header *)data);
@@ -226,8 +233,7 @@ int git_commit_graph_file_parse(
 	 * headers, and a special zero chunk.
 	 */
 	last_chunk_offset = sizeof(struct git_commit_graph_header) + (1 + hdr->chunks) * 12;
-	trailer_offset = size - GIT_OID_SHA1_SIZE;
-	checksum_size = GIT_HASH_SHA1_SIZE;
+	trailer_offset = size - checksum_size;
 
 	if (trailer_offset < last_chunk_offset)
 		return commit_graph_error("wrong commit-graph size");
@@ -295,25 +301,35 @@ int git_commit_graph_file_parse(
 	return 0;
 }
 
-int git_commit_graph_new(git_commit_graph **cgraph_out, const char *objects_dir, bool open_file)
+int git_commit_graph_new(
+	git_commit_graph **cgraph_out,
+	const char *objects_dir,
+	bool open_file,
+	git_oid_t oid_type)
 {
 	git_commit_graph *cgraph = NULL;
 	int error = 0;
 
 	GIT_ASSERT_ARG(cgraph_out);
 	GIT_ASSERT_ARG(objects_dir);
+	GIT_ASSERT_ARG(oid_type);
 
 	cgraph = git__calloc(1, sizeof(git_commit_graph));
 	GIT_ERROR_CHECK_ALLOC(cgraph);
 
+	cgraph->oid_type = oid_type;
+
 	error = git_str_joinpath(&cgraph->filename, objects_dir, "info/commit-graph");
 	if (error < 0)
 		goto error;
 
 	if (open_file) {
-		error = git_commit_graph_file_open(&cgraph->file, git_str_cstr(&cgraph->filename));
+		error = git_commit_graph_file_open(&cgraph->file,
+				git_str_cstr(&cgraph->filename), oid_type);
+
 		if (error < 0)
 			goto error;
+
 		cgraph->checked = 1;
 	}
 
@@ -326,14 +342,18 @@ error:
 }
 
 int git_commit_graph_validate(git_commit_graph *cgraph) {
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
-	size_t trailer_offset = cgraph->file->graph_map.len - checksum_size;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	git_hash_algorithm_t checksum_type;
+	size_t checksum_size, trailer_offset;
+
+	checksum_type = git_oid_algorithm(cgraph->oid_type);
+	checksum_size = git_hash_size(checksum_type);
+	trailer_offset = cgraph->file->graph_map.len - checksum_size;
 
 	if (cgraph->file->graph_map.len < checksum_size)
 		return commit_graph_error("map length too small");
 
-	if (git_hash_buf(checksum, cgraph->file->graph_map.data, trailer_offset, GIT_HASH_ALGORITHM_SHA1) < 0)
+	if (git_hash_buf(checksum, cgraph->file->graph_map.data, trailer_offset, checksum_type) < 0)
 		return commit_graph_error("could not calculate signature");
 	if (memcmp(checksum, cgraph->file->checksum, checksum_size) != 0)
 		return commit_graph_error("index signature mismatch");
@@ -341,16 +361,32 @@ int git_commit_graph_validate(git_commit_graph *cgraph) {
 	return 0;
 }
 
-int git_commit_graph_open(git_commit_graph **cgraph_out, const char *objects_dir)
+int git_commit_graph_open(
+	git_commit_graph **cgraph_out,
+	const char *objects_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+	, git_oid_t oid_type
+#endif
+	)
 {
-	int error = git_commit_graph_new(cgraph_out, objects_dir, true);
-	if (!error) {
+#ifndef GIT_EXPERIMENTAL_SHA256
+	git_oid_t oid_type = GIT_OID_SHA1;
+#endif
+	int error;
+
+	error = git_commit_graph_new(cgraph_out, objects_dir, true,
+			oid_type);
+
+	if (!error)
 		return git_commit_graph_validate(*cgraph_out);
-	}
+
 	return error;
 }
 
-int git_commit_graph_file_open(git_commit_graph_file **file_out, const char *path)
+int git_commit_graph_file_open(
+	git_commit_graph_file **file_out,
+	const char *path,
+	git_oid_t oid_type)
 {
 	git_commit_graph_file *file;
 	git_file fd = -1;
@@ -379,6 +415,8 @@ int git_commit_graph_file_open(git_commit_graph_file **file_out, const char *pat
 	file = git__calloc(1, sizeof(git_commit_graph_file));
 	GIT_ERROR_CHECK_ALLOC(file);
 
+	file->oid_type = oid_type;
+
 	error = git_futils_mmap_ro(&file->graph_map, fd, 0, cgraph_size);
 	p_close(fd);
 	if (error < 0) {
@@ -395,7 +433,9 @@ int git_commit_graph_file_open(git_commit_graph_file **file_out, const char *pat
 	return 0;
 }
 
-int git_commit_graph_get_file(git_commit_graph_file **file_out, git_commit_graph *cgraph)
+int git_commit_graph_get_file(
+	git_commit_graph_file **file_out,
+	git_commit_graph *cgraph)
 {
 	if (!cgraph->checked) {
 		int error = 0;
@@ -405,7 +445,8 @@ int git_commit_graph_get_file(git_commit_graph_file **file_out, git_commit_graph
 		cgraph->checked = 1;
 
 		/* Best effort */
-		error = git_commit_graph_file_open(&result, git_str_cstr(&cgraph->filename));
+		error = git_commit_graph_file_open(&result,
+			git_str_cstr(&cgraph->filename), cgraph->oid_type);
 
 		if (error < 0)
 			return error;
@@ -441,6 +482,7 @@ static int git_commit_graph_entry_get_byindex(
 		size_t pos)
 {
 	const unsigned char *commit_data;
+	size_t oid_size = git_oid_size(file->oid_type);
 
 	GIT_ASSERT_ARG(e);
 	GIT_ASSERT_ARG(file);
@@ -450,15 +492,15 @@ static int git_commit_graph_entry_get_byindex(
 		return GIT_ENOTFOUND;
 	}
 
-	commit_data = file->commit_data + pos * (GIT_OID_SHA1_SIZE + 4 * sizeof(uint32_t));
-	git_oid__fromraw(&e->tree_oid, commit_data, GIT_OID_SHA1);
-	e->parent_indices[0] = ntohl(*((uint32_t *)(commit_data + GIT_OID_SHA1_SIZE)));
+	commit_data = file->commit_data + pos * (oid_size + 4 * sizeof(uint32_t));
+	git_oid__fromraw(&e->tree_oid, commit_data, file->oid_type);
+	e->parent_indices[0] = ntohl(*((uint32_t *)(commit_data + oid_size)));
 	e->parent_indices[1] = ntohl(
-			*((uint32_t *)(commit_data + GIT_OID_SHA1_SIZE + sizeof(uint32_t))));
+			*((uint32_t *)(commit_data + oid_size + sizeof(uint32_t))));
 	e->parent_count = (e->parent_indices[0] != GIT_COMMIT_GRAPH_MISSING_PARENT)
 			+ (e->parent_indices[1] != GIT_COMMIT_GRAPH_MISSING_PARENT);
-	e->generation = ntohl(*((uint32_t *)(commit_data + GIT_OID_SHA1_SIZE + 2 * sizeof(uint32_t))));
-	e->commit_time = ntohl(*((uint32_t *)(commit_data + GIT_OID_SHA1_SIZE + 3 * sizeof(uint32_t))));
+	e->generation = ntohl(*((uint32_t *)(commit_data + oid_size + 2 * sizeof(uint32_t))));
+	e->commit_time = ntohl(*((uint32_t *)(commit_data + oid_size + 3 * sizeof(uint32_t))));
 
 	e->commit_time |= (e->generation & UINT64_C(0x3)) << UINT64_C(32);
 	e->generation >>= 2u;
@@ -485,7 +527,7 @@ static int git_commit_graph_entry_get_byindex(
 		}
 	}
 
-	git_oid__fromraw(&e->sha1, &file->oid_lookup[pos * GIT_OID_SHA1_SIZE], GIT_OID_SHA1);
+	git_oid__fromraw(&e->sha1, &file->oid_lookup[pos * oid_size], file->oid_type);
 	return 0;
 }
 
@@ -494,8 +536,8 @@ bool git_commit_graph_file_needs_refresh(const git_commit_graph_file *file, cons
 	git_file fd = -1;
 	struct stat st;
 	ssize_t bytes_read;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	size_t checksum_size = git_oid_size(file->oid_type);
 
 	/* TODO: properly open the file without access time using O_NOATIME */
 	fd = git_futils_open_ro(path);
@@ -530,35 +572,40 @@ int git_commit_graph_entry_find(
 	int pos, found = 0;
 	uint32_t hi, lo;
 	const unsigned char *current = NULL;
+	size_t oid_size, oid_hexsize;
 
 	GIT_ASSERT_ARG(e);
 	GIT_ASSERT_ARG(file);
 	GIT_ASSERT_ARG(short_oid);
 
+	oid_size = git_oid_size(file->oid_type);
+	oid_hexsize = git_oid_hexsize(file->oid_type);
+
 	hi = ntohl(file->oid_fanout[(int)short_oid->id[0]]);
 	lo = ((short_oid->id[0] == 0x0) ? 0 : ntohl(file->oid_fanout[(int)short_oid->id[0] - 1]));
 
-	pos = git_pack__lookup_id(file->oid_lookup, GIT_OID_SHA1_SIZE, lo, hi, short_oid->id, GIT_OID_SHA1);
+	pos = git_pack__lookup_id(file->oid_lookup, oid_size, lo, hi,
+		short_oid->id, file->oid_type);
 
 	if (pos >= 0) {
 		/* An object matching exactly the oid was found */
 		found = 1;
-		current = file->oid_lookup + (pos * GIT_OID_SHA1_SIZE);
+		current = file->oid_lookup + (pos * oid_size);
 	} else {
 		/* No object was found */
 		/* pos refers to the object with the "closest" oid to short_oid */
 		pos = -1 - pos;
 		if (pos < (int)file->num_commits) {
-			current = file->oid_lookup + (pos * GIT_OID_SHA1_SIZE);
+			current = file->oid_lookup + (pos * oid_size);
 
 			if (!git_oid_raw_ncmp(short_oid->id, current, len))
 				found = 1;
 		}
 	}
 
-	if (found && len != GIT_OID_SHA1_HEXSIZE && pos + 1 < (int)file->num_commits) {
+	if (found && len != oid_hexsize && pos + 1 < (int)file->num_commits) {
 		/* Check for ambiguousity */
-		const unsigned char *next = current + GIT_OID_SHA1_SIZE;
+		const unsigned char *next = current + oid_size;
 
 		if (!git_oid_raw_ncmp(short_oid->id, next, len))
 			found = 2;
@@ -637,11 +684,27 @@ static int packed_commit__cmp(const void *a_, const void *b_)
 	return git_oid_cmp(&a->sha1, &b->sha1);
 }
 
-int git_commit_graph_writer_new(git_commit_graph_writer **out, const char *objects_info_dir)
+int git_commit_graph_writer_new(
+	git_commit_graph_writer **out,
+	const char *objects_info_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+	, git_oid_t oid_type
+#endif
+	)
 {
-	git_commit_graph_writer *w = git__calloc(1, sizeof(git_commit_graph_writer));
+	git_commit_graph_writer *w;
+
+#ifndef GIT_EXPERIMENTAL_SHA256
+	git_oid_t oid_type = GIT_OID_SHA1;
+#endif
+
+	GIT_ASSERT_ARG(out && objects_info_dir && oid_type);
+
+	w = git__calloc(1, sizeof(git_commit_graph_writer));
 	GIT_ERROR_CHECK_ALLOC(w);
 
+	w->oid_type = oid_type;
+
 	if (git_str_sets(&w->objects_info_dir, objects_info_dir) < 0) {
 		git__free(w);
 		return -1;
@@ -993,8 +1056,9 @@ static int commit_graph_write(
 	off64_t offset;
 	git_str oid_lookup = GIT_STR_INIT, commit_data = GIT_STR_INIT,
 		extra_edge_list = GIT_STR_INIT;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	git_hash_algorithm_t checksum_type;
+	size_t checksum_size, oid_size;
 	git_hash_ctx ctx;
 	struct commit_graph_write_hash_context hash_cb_data = {0};
 
@@ -1007,8 +1071,11 @@ static int commit_graph_write(
 	hash_cb_data.cb_data = cb_data;
 	hash_cb_data.ctx = &ctx;
 
-	checksum_size = GIT_HASH_SHA1_SIZE;
-	error = git_hash_ctx_init(&ctx, GIT_HASH_ALGORITHM_SHA1);
+	oid_size = git_oid_size(w->oid_type);
+	checksum_type = git_oid_algorithm(w->oid_type);
+	checksum_size = git_hash_size(checksum_type);
+
+	error = git_hash_ctx_init(&ctx, checksum_type);
 	if (error < 0)
 		return error;
 	cb_data = &hash_cb_data;
@@ -1035,7 +1102,7 @@ static int commit_graph_write(
 	git_vector_foreach (&w->commits, i, packed_commit) {
 		error = git_str_put(&oid_lookup,
 			(const char *)&packed_commit->sha1.id,
-			GIT_OID_SHA1_SIZE);
+			oid_size);
 
 		if (error < 0)
 			goto cleanup;
@@ -1052,7 +1119,7 @@ static int commit_graph_write(
 
 		error = git_str_put(&commit_data,
 			(const char *)&packed_commit->tree_oid.id,
-			GIT_OID_SHA1_SIZE);
+			oid_size);
 
 		if (error < 0)
 			goto cleanup;
diff --git a/src/libgit2/commit_graph.h b/src/libgit2/commit_graph.h
index 517abb239..ecf4379bd 100644
--- a/src/libgit2/commit_graph.h
+++ b/src/libgit2/commit_graph.h
@@ -30,6 +30,9 @@
 typedef struct git_commit_graph_file {
 	git_map graph_map;
 
+	/* The type of object IDs in the commit graph file. */
+	git_oid_t oid_type;
+
 	/* The OID Fanout table. */
 	const uint32_t *oid_fanout;
 	/* The total number of commits in the graph. */
@@ -84,10 +87,10 @@ typedef struct git_commit_graph_entry {
 	/* The index within the Extra Edge List of any parent after the first two. */
 	size_t extra_parents_index;
 
-	/* The SHA-1 hash of the root tree of the commit. */
+	/* The object ID of the root tree of the commit. */
 	git_oid tree_oid;
 
-	/* The SHA-1 hash of the requested commit. */
+	/* The object ID hash of the requested commit. */
 	git_oid sha1;
 } git_commit_graph_entry;
 
@@ -99,18 +102,28 @@ struct git_commit_graph {
 	/* The underlying commit-graph file. */
 	git_commit_graph_file *file;
 
+	/* The object ID types in the commit graph. */
+	git_oid_t oid_type;
+
 	/* Whether the commit-graph file was already checked for validity. */
 	bool checked;
 };
 
 /** Create a new commit-graph, optionally opening the underlying file. */
-int git_commit_graph_new(git_commit_graph **cgraph_out, const char *objects_dir, bool open_file);
+int git_commit_graph_new(
+	git_commit_graph **cgraph_out,
+	const char *objects_dir,
+	bool open_file,
+	git_oid_t oid_type);
 
 /** Validate the checksum of a commit graph */
 int git_commit_graph_validate(git_commit_graph *cgraph);
 
 /** Open and validate a commit-graph file. */
-int git_commit_graph_file_open(git_commit_graph_file **file_out, const char *path);
+int git_commit_graph_file_open(
+	git_commit_graph_file **file_out,
+	const char *path,
+	git_oid_t oid_type);
 
 /*
  * Attempt to get the git_commit_graph's commit-graph file. This object is
@@ -134,6 +147,9 @@ struct git_commit_graph_writer {
 	 */
 	git_str objects_info_dir;
 
+	/* The object ID type of the commit graph. */
+	git_oid_t oid_type;
+
 	/* The list of packed commits. */
 	git_vector commits;
 };
diff --git a/src/libgit2/commit_list.c b/src/libgit2/commit_list.c
index 12b329b25..485871db3 100644
--- a/src/libgit2/commit_list.c
+++ b/src/libgit2/commit_list.c
@@ -125,7 +125,7 @@ static int commit_quick_parse(
 	git_oid *parent_oid;
 	git_commit *commit;
 	git_commit__parse_options parse_opts = {
-		GIT_OID_SHA1,
+		walk->repo->oid_type,
 		GIT_COMMIT_PARSE_QUICK
 	};
 	size_t i;
@@ -176,7 +176,9 @@ int git_commit_list_parse(git_revwalk *walk, git_commit_list_node *commit)
 	if (cgraph_file) {
 		git_commit_graph_entry e;
 
-		error = git_commit_graph_entry_find(&e, cgraph_file, &commit->oid, GIT_OID_SHA1_SIZE);
+		error = git_commit_graph_entry_find(&e, cgraph_file,
+			&commit->oid, git_oid_size(walk->repo->oid_type));
+
 		if (error == 0 && git__is_uint16(e.parent_count)) {
 			size_t i;
 			commit->generation = (uint32_t)e.generation;
diff --git a/src/libgit2/config_file.c b/src/libgit2/config_file.c
index 932ca7601..716924de6 100644
--- a/src/libgit2/config_file.c
+++ b/src/libgit2/config_file.c
@@ -26,7 +26,7 @@
 
 typedef struct config_file {
 	git_futils_filestamp stamp;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	unsigned char checksum[GIT_HASH_SHA256_SIZE];
 	char *path;
 	git_array_t(struct config_file) includes;
 } config_file;
@@ -133,7 +133,7 @@ static int config_file_is_modified(int *modified, config_file *file)
 {
 	config_file *include;
 	git_str buf = GIT_STR_INIT;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	unsigned char checksum[GIT_HASH_SHA256_SIZE];
 	uint32_t i;
 	int error = 0;
 
@@ -145,10 +145,10 @@ static int config_file_is_modified(int *modified, config_file *file)
 	if ((error = git_futils_readbuffer(&buf, file->path)) < 0)
 		goto out;
 
-	if ((error = git_hash_buf(checksum, buf.ptr, buf.size, GIT_HASH_ALGORITHM_SHA1)) < 0)
+	if ((error = git_hash_buf(checksum, buf.ptr, buf.size, GIT_HASH_ALGORITHM_SHA256)) < 0)
 		goto out;
 
-	if (memcmp(checksum, file->checksum, GIT_HASH_SHA1_SIZE) != 0) {
+	if (memcmp(checksum, file->checksum, GIT_HASH_SHA256_SIZE) != 0) {
 		*modified = 1;
 		goto out;
 	}
@@ -881,7 +881,7 @@ static int config_file_read(
 		goto out;
 
 	git_futils_filestamp_set_from_stat(&file->stamp, &st);
-	if ((error = git_hash_buf(file->checksum, contents.ptr, contents.size, GIT_HASH_ALGORITHM_SHA1)) < 0)
+	if ((error = git_hash_buf(file->checksum, contents.ptr, contents.size, GIT_HASH_ALGORITHM_SHA256)) < 0)
 		goto out;
 
 	if ((error = config_file_read_buffer(entries, repo, file, level, depth,
@@ -1116,7 +1116,12 @@ static int write_on_eof(
 /*
  * This is pretty much the parsing, except we write out anything we don't have
  */
-static int config_file_write(config_file_backend *cfg, const char *orig_key, const char *key, const git_regexp *preg, const char *value)
+static int config_file_write(
+	config_file_backend *cfg,
+	const char *orig_key,
+	const char *key,
+	const git_regexp *preg,
+	const char *value)
 
 {
 	char *orig_section = NULL, *section = NULL, *orig_name, *name, *ldot;
@@ -1131,8 +1136,9 @@ static int config_file_write(config_file_backend *cfg, const char *orig_key, con
 	if (cfg->locked) {
 		error = git_str_puts(&contents, git_str_cstr(&cfg->locked_content) == NULL ? "" : git_str_cstr(&cfg->locked_content));
 	} else {
-		if ((error = git_filebuf_open(&file, cfg->file.path, GIT_FILEBUF_HASH_CONTENTS,
-					      GIT_CONFIG_FILE_MODE)) < 0)
+		if ((error = git_filebuf_open(&file, cfg->file.path,
+				GIT_FILEBUF_HASH_SHA256,
+				GIT_CONFIG_FILE_MODE)) < 0)
 			goto done;
 
 		/* We need to read in our own config file */
diff --git a/src/libgit2/describe.c b/src/libgit2/describe.c
index 3f73d87d6..044534723 100644
--- a/src/libgit2/describe.c
+++ b/src/libgit2/describe.c
@@ -363,12 +363,15 @@ static int find_unique_abbrev_size(
 	size_t size = abbreviated_size;
 	git_odb *odb;
 	git_oid dummy;
+	size_t hexsize;
 	int error;
 
 	if ((error = git_repository_odb__weakptr(&odb, repo)) < 0)
 		return error;
 
-	while (size < GIT_OID_SHA1_HEXSIZE) {
+	hexsize = git_oid_hexsize(repo->oid_type);
+
+	while (size < hexsize) {
 		if ((error = git_odb_exists_prefix(&dummy, odb, oid_in, size)) == 0) {
 			*out = (int) size;
 			return 0;
@@ -383,7 +386,7 @@ static int find_unique_abbrev_size(
 	}
 
 	/* If we didn't find any shorter prefix, we have to do the whole thing */
-	*out = GIT_OID_SHA1_HEXSIZE;
+	*out = (int)hexsize;
 
 	return 0;
 }
@@ -397,7 +400,7 @@ static int show_suffix(
 {
 	int error, size = 0;
 
-	char hex_oid[GIT_OID_SHA1_HEXSIZE];
+	char hex_oid[GIT_OID_MAX_HEXSIZE];
 
 	if ((error = find_unique_abbrev_size(&size, repo, id, abbrev_size)) < 0)
 		return error;
@@ -414,7 +417,7 @@ static int show_suffix(
 #define MAX_CANDIDATES_TAGS FLAG_BITS - 1
 
 static int describe_not_found(const git_oid *oid, const char *message_format) {
-	char oid_str[GIT_OID_SHA1_HEXSIZE + 1];
+	char oid_str[GIT_OID_MAX_HEXSIZE + 1];
 	git_oid_tostr(oid_str, sizeof(oid_str), oid);
 
 	git_error_set(GIT_ERROR_DESCRIBE, message_format, oid_str);
@@ -525,7 +528,7 @@ static int describe(
 		if (annotated_cnt && (git_pqueue_size(&list) == 0)) {
 			/*
 			if (debug) {
-				char oid_str[GIT_OID_SHA1_HEXSIZE + 1];
+				char oid_str[GIT_OID_MAX_HEXSIZE + 1];
 				git_oid_tostr(oid_str, sizeof(oid_str), &c->oid);
 
 				fprintf(stderr, "finished search at %s\n", oid_str);
@@ -592,7 +595,7 @@ static int describe(
 			"head", "lightweight", "annotated",
 		};
 
-		char oid_str[GIT_OID_SHA1_HEXSIZE + 1];
+		char oid_str[GIT_OID_MAX_HEXSIZE + 1];
 
 		if (debug) {
 			for (cur_match = 0; cur_match < match_cnt; cur_match++) {
@@ -816,7 +819,7 @@ static int git_describe__format(
 
 	/* If we didn't find *any* tags, we fall back to the commit's id */
 	if (result->fallback_to_id) {
-		char hex_oid[GIT_OID_SHA1_HEXSIZE + 1] = {0};
+		char hex_oid[GIT_OID_MAX_HEXSIZE + 1] = {0};
 		int size = 0;
 
 		if ((error = find_unique_abbrev_size(
diff --git a/src/libgit2/diff.c b/src/libgit2/diff.c
index 20a18c4b9..db12ccd68 100644
--- a/src/libgit2/diff.c
+++ b/src/libgit2/diff.c
@@ -19,8 +19,10 @@
 #include "git2/email.h"
 
 struct patch_id_args {
+	git_diff *diff;
 	git_hash_ctx ctx;
 	git_oid result;
+	git_oid_t oid_type;
 	int first_file;
 };
 
@@ -280,17 +282,19 @@ int git_diff_find_options_init(
 	return 0;
 }
 
-static int flush_hunk(git_oid *result, git_hash_ctx *ctx)
+static int flush_hunk(git_oid *result, struct patch_id_args *args)
 {
+	git_hash_ctx *ctx = &args->ctx;
 	git_oid hash;
 	unsigned short carry = 0;
-	int error, i;
+	size_t i;
+	int error;
 
 	if ((error = git_hash_final(hash.id, ctx)) < 0 ||
 	    (error = git_hash_init(ctx)) < 0)
 		return error;
 
-	for (i = 0; i < GIT_OID_SHA1_SIZE; i++) {
+	for (i = 0; i < git_oid_size(args->oid_type); i++) {
 		carry += result->id[i] + hash.id[i];
 		result->id[i] = (unsigned char)carry;
 		carry >>= 8;
@@ -338,7 +342,7 @@ static int diff_patchid_print_callback_to_buf(
 
 	if (line->origin == GIT_DIFF_LINE_FILE_HDR &&
 	    !args->first_file &&
-	    (error = flush_hunk(&args->result, &args->ctx) < 0))
+	    (error = flush_hunk(&args->result, args) < 0))
 			goto out;
 
 	if ((error = git_hash_update(&args->ctx, buf.ptr, buf.size)) < 0)
@@ -362,14 +366,19 @@ int git_diff_patchid_options_init(git_diff_patchid_options *opts, unsigned int v
 int git_diff_patchid(git_oid *out, git_diff *diff, git_diff_patchid_options *opts)
 {
 	struct patch_id_args args;
+	git_hash_algorithm_t algorithm;
 	int error;
 
 	GIT_ERROR_CHECK_VERSION(
 		opts, GIT_DIFF_PATCHID_OPTIONS_VERSION, "git_diff_patchid_options");
 
+	algorithm = git_oid_algorithm(diff->opts.oid_type);
+
 	memset(&args, 0, sizeof(args));
+	args.diff = diff;
 	args.first_file = 1;
-	if ((error = git_hash_ctx_init(&args.ctx, GIT_HASH_ALGORITHM_SHA1)) < 0)
+	args.oid_type = diff->opts.oid_type;
+	if ((error = git_hash_ctx_init(&args.ctx, algorithm)) < 0)
 		goto out;
 
 	if ((error = git_diff_print(diff,
@@ -378,11 +387,11 @@ int git_diff_patchid(git_oid *out, git_diff *diff, git_diff_patchid_options *opt
 				    &args)) < 0)
 		goto out;
 
-	if ((error = (flush_hunk(&args.result, &args.ctx))) < 0)
+	if ((error = (flush_hunk(&args.result, &args))) < 0)
 		goto out;
 
 #ifdef GIT_EXPERIMENTAL_SHA256
-	args.result.type = GIT_OID_SHA1;
+	args.result.type = diff->opts.oid_type;
 #endif
 
 	git_oid_cpy(out, &args.result);
diff --git a/src/libgit2/diff.h b/src/libgit2/diff.h
index 2cc35e65b..f21b27645 100644
--- a/src/libgit2/diff.h
+++ b/src/libgit2/diff.h
@@ -30,15 +30,15 @@ typedef enum {
 } git_diff_origin_t;
 
 struct git_diff {
-	git_refcount     rc;
+	git_refcount      rc;
 	git_repository   *repo;
-	git_attr_session attrsession;
+	git_attr_session  attrsession;
 	git_diff_origin_t type;
-	git_diff_options opts;
-	git_vector       deltas;    /* vector of git_diff_delta */
+	git_diff_options  opts;
+	git_vector        deltas;    /* vector of git_diff_delta */
 	git_pool pool;
-	git_iterator_t old_src;
-	git_iterator_t new_src;
+	git_iterator_t    old_src;
+	git_iterator_t    new_src;
 	git_diff_perfdata perf;
 
 	int (*strcomp)(const char *, const char *);
diff --git a/src/libgit2/diff_file.c b/src/libgit2/diff_file.c
index c2d08675a..6b7f9590c 100644
--- a/src/libgit2/diff_file.c
+++ b/src/libgit2/diff_file.c
@@ -144,7 +144,7 @@ int git_diff_file_content__init_from_src(
 
 	if (!src->blob && !src->buf) {
 		fc->flags |= GIT_DIFF_FLAG__NO_DATA;
-		git_oid_clear(&fc->file->id, GIT_OID_SHA1);
+		git_oid_clear(&fc->file->id, opts->oid_type);
 	} else {
 		fc->flags |= GIT_DIFF_FLAG__LOADED;
 		fc->file->flags |= GIT_DIFF_FLAG_VALID_ID;
@@ -154,7 +154,7 @@ int git_diff_file_content__init_from_src(
 			git_blob_dup((git_blob **)&fc->blob, (git_blob *) src->blob);
 			fc->file->size = git_blob_rawsize(src->blob);
 			git_oid_cpy(&fc->file->id, git_blob_id(src->blob));
-			fc->file->id_abbrev = GIT_OID_SHA1_HEXSIZE;
+			fc->file->id_abbrev = (uint16_t)git_oid_hexsize(repo->oid_type);
 
 			fc->map.len  = (size_t)fc->file->size;
 			fc->map.data = (char *)git_blob_rawcontent(src->blob);
@@ -162,10 +162,10 @@ int git_diff_file_content__init_from_src(
 			fc->flags |= GIT_DIFF_FLAG__FREE_BLOB;
 		} else {
 			int error;
-			if ((error = git_odb__hash(&fc->file->id, src->buf, src->buflen, GIT_OBJECT_BLOB, GIT_OID_SHA1)) < 0)
+			if ((error = git_odb__hash(&fc->file->id, src->buf, src->buflen, GIT_OBJECT_BLOB, opts->oid_type)) < 0)
 				return error;
 			fc->file->size = src->buflen;
-			fc->file->id_abbrev = GIT_OID_SHA1_HEXSIZE;
+			fc->file->id_abbrev = (uint16_t)git_oid_hexsize(opts->oid_type);
 
 			fc->map.len  = src->buflen;
 			fc->map.data = (char *)src->buf;
@@ -178,7 +178,7 @@ int git_diff_file_content__init_from_src(
 static int diff_file_content_commit_to_str(
 	git_diff_file_content *fc, bool check_status)
 {
-	char oid[GIT_OID_SHA1_HEXSIZE+1];
+	char oid[GIT_OID_MAX_HEXSIZE+1];
 	git_str content = GIT_STR_INIT;
 	const char *status = "";
 
@@ -420,7 +420,7 @@ static int diff_file_content_load_workdir(
 	if (!error && (fc->file->flags & GIT_DIFF_FLAG_VALID_ID) == 0) {
 		error = git_odb__hash(
 			&fc->file->id, fc->map.data, fc->map.len,
-			GIT_OBJECT_BLOB, GIT_OID_SHA1);
+			GIT_OBJECT_BLOB, diff_opts->oid_type);
 		fc->file->flags |= GIT_DIFF_FLAG_VALID_ID;
 	}
 
diff --git a/src/libgit2/diff_generate.c b/src/libgit2/diff_generate.c
index a88ce8c32..78fe510e7 100644
--- a/src/libgit2/diff_generate.c
+++ b/src/libgit2/diff_generate.c
@@ -61,8 +61,8 @@ static git_diff_delta *diff_delta__alloc(
 	}
 	delta->status = status;
 
-	git_oid_clear(&delta->old_file.id, GIT_OID_SHA1);
-	git_oid_clear(&delta->new_file.id, GIT_OID_SHA1);
+	git_oid_clear(&delta->old_file.id, diff->base.opts.oid_type);
+	git_oid_clear(&delta->new_file.id, diff->base.opts.oid_type);
 
 	return delta;
 }
@@ -149,10 +149,13 @@ static int diff_delta__from_one(
 	const git_index_entry *entry = nitem;
 	bool has_old = false;
 	git_diff_delta *delta;
+	git_oid_t oid_type;
 	const char *matched_pathspec;
 
 	GIT_ASSERT_ARG((oitem != NULL) ^ (nitem != NULL));
 
+	oid_type = diff->base.opts.oid_type;
+
 	if (oitem) {
 		entry = oitem;
 		has_old = true;
@@ -186,20 +189,23 @@ static int diff_delta__from_one(
 	GIT_ASSERT(status != GIT_DELTA_MODIFIED);
 	delta->nfiles = 1;
 
+	git_oid_clear(&delta->old_file.id, diff->base.opts.oid_type);
+	git_oid_clear(&delta->new_file.id, diff->base.opts.oid_type);
+
 	if (has_old) {
 		delta->old_file.mode = entry->mode;
 		delta->old_file.size = entry->file_size;
 		delta->old_file.flags |= GIT_DIFF_FLAG_EXISTS;
 		git_oid_cpy(&delta->old_file.id, &entry->id);
-		git_oid_clear(&delta->new_file.id, GIT_OID_SHA1);
-		delta->old_file.id_abbrev = GIT_OID_SHA1_HEXSIZE;
+		git_oid_clear(&delta->new_file.id, oid_type);
+		delta->old_file.id_abbrev = (uint16_t)git_oid_hexsize(oid_type);
 	} else /* ADDED, IGNORED, UNTRACKED */ {
 		delta->new_file.mode = entry->mode;
 		delta->new_file.size = entry->file_size;
 		delta->new_file.flags |= GIT_DIFF_FLAG_EXISTS;
-		git_oid_clear(&delta->old_file.id, GIT_OID_SHA1);
+		git_oid_clear(&delta->old_file.id, oid_type);
 		git_oid_cpy(&delta->new_file.id, &entry->id);
-		delta->new_file.id_abbrev = GIT_OID_SHA1_HEXSIZE;
+		delta->new_file.id_abbrev = (uint16_t)git_oid_hexsize(oid_type);
 	}
 
 	delta->old_file.flags |= GIT_DIFF_FLAG_VALID_ID;
@@ -225,6 +231,9 @@ static int diff_delta__from_two(
 	const git_oid *old_id = &old_entry->id;
 	git_diff_delta *delta;
 	const char *canonical_path = old_entry->path;
+	git_oid_t oid_type;
+
+	oid_type = diff->base.opts.oid_type;
 
 	if (status == GIT_DELTA_UNMODIFIED &&
 		DIFF_FLAG_ISNT_SET(diff, GIT_DIFF_INCLUDE_UNMODIFIED))
@@ -254,14 +263,14 @@ static int diff_delta__from_two(
 		delta->old_file.size = old_entry->file_size;
 		delta->old_file.mode = old_mode;
 		git_oid_cpy(&delta->old_file.id, old_id);
-		delta->old_file.id_abbrev = GIT_OID_SHA1_HEXSIZE;
+		delta->old_file.id_abbrev = (uint16_t)git_oid_hexsize(oid_type);
 		delta->old_file.flags |= GIT_DIFF_FLAG_VALID_ID |
 			GIT_DIFF_FLAG_EXISTS;
 	}
 
 	if (!git_index_entry_is_conflict(new_entry)) {
 		git_oid_cpy(&delta->new_file.id, new_id);
-		delta->new_file.id_abbrev = GIT_OID_SHA1_HEXSIZE;
+		delta->new_file.id_abbrev = (uint16_t)git_oid_hexsize(oid_type);
 		delta->new_file.size = new_entry->file_size;
 		delta->new_file.mode = new_mode;
 		delta->old_file.flags |= GIT_DIFF_FLAG_EXISTS;
@@ -490,6 +499,14 @@ static int diff_generated_apply_options(
 			return -1;
 	}
 
+	if (!diff->base.opts.oid_type) {
+		diff->base.opts.oid_type = repo->oid_type;
+	} else if (diff->base.opts.oid_type != repo->oid_type) {
+		git_error_set(GIT_ERROR_INVALID,
+			"specified object ID type does not match repository object ID type");
+		return -1;
+	}
+
 	/* flag INCLUDE_TYPECHANGE_TREES implies INCLUDE_TYPECHANGE */
 	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_INCLUDE_TYPECHANGE_TREES))
 		diff->base.opts.flags |= GIT_DIFF_INCLUDE_TYPECHANGE;
@@ -603,7 +620,7 @@ int git_diff__oid_for_file(
 	entry.mode = mode;
 	entry.file_size = (uint32_t)size;
 	entry.path = (char *)path;
-	git_oid_clear(&entry.id, GIT_OID_SHA1);
+	git_oid_clear(&entry.id, diff->opts.oid_type);
 
 	return git_diff__oid_for_entry(out, diff, &entry, mode, NULL);
 }
@@ -624,7 +641,7 @@ int git_diff__oid_for_entry(
 	GIT_ASSERT(d->type == GIT_DIFF_TYPE_GENERATED);
 	diff = (git_diff_generated *)d;
 
-	git_oid_clear(out, GIT_OID_SHA1);
+	git_oid_clear(out, diff->base.opts.oid_type);
 
 	if (git_repository_workdir_path(&full_path, diff->base.repo, entry.path) < 0)
 		return -1;
@@ -660,7 +677,8 @@ int git_diff__oid_for_entry(
 			git_error_clear();
 		}
 	} else if (S_ISLNK(mode)) {
-		error = git_odb__hashlink(out, full_path.ptr, GIT_OID_SHA1);
+		error = git_odb__hashlink(out, full_path.ptr,
+			diff->base.opts.oid_type);
 		diff->base.perf.oid_calculations++;
 	} else if (!git__is_sizet(entry.file_size)) {
 		git_error_set(GIT_ERROR_NOMEMORY, "file size overflow (for 32-bits) on '%s'",
@@ -676,7 +694,8 @@ int git_diff__oid_for_entry(
 		else {
 			error = git_odb__hashfd_filtered(
 				out, fd, (size_t)entry.file_size,
-				GIT_OBJECT_BLOB, GIT_OID_SHA1, fl);
+				GIT_OBJECT_BLOB, diff->base.opts.oid_type,
+				fl);
 			p_close(fd);
 			diff->base.perf.oid_calculations++;
 		}
@@ -785,7 +804,7 @@ static int maybe_modified(
 	git_diff_generated *diff,
 	diff_in_progress *info)
 {
-	git_oid noid = GIT_OID_SHA1_ZERO;
+	git_oid noid;
 	git_delta_t status = GIT_DELTA_MODIFIED;
 	const git_index_entry *oitem = info->oitem;
 	const git_index_entry *nitem = info->nitem;
@@ -796,6 +815,8 @@ static int maybe_modified(
 	const char *matched_pathspec;
 	int error = 0;
 
+	git_oid_clear(&noid, diff->base.opts.oid_type);
+
 	if (!diff_pathspec_match(&matched_pathspec, diff, oitem))
 		return 0;
 
@@ -1700,11 +1721,11 @@ int git_diff__commit(
 	*out = NULL;
 
 	if ((parents = git_commit_parentcount(commit)) > 1) {
-		char commit_oidstr[GIT_OID_SHA1_HEXSIZE + 1];
+		char commit_oidstr[GIT_OID_MAX_HEXSIZE + 1];
 
 		error = -1;
 		git_error_set(GIT_ERROR_INVALID, "commit %s is a merge commit",
-			git_oid_tostr(commit_oidstr, GIT_OID_SHA1_HEXSIZE + 1, git_commit_id(commit)));
+			git_oid_tostr(commit_oidstr, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit)));
 		goto on_error;
 	}
 
diff --git a/src/libgit2/diff_parse.c b/src/libgit2/diff_parse.c
index 75e41a544..04603969e 100644
--- a/src/libgit2/diff_parse.c
+++ b/src/libgit2/diff_parse.c
@@ -29,7 +29,7 @@ static void diff_parsed_free(git_diff *d)
 	git__free(diff);
 }
 
-static git_diff_parsed *diff_parsed_alloc(void)
+static git_diff_parsed *diff_parsed_alloc(git_oid_t oid_type)
 {
 	git_diff_parsed *diff;
 
@@ -51,6 +51,7 @@ static git_diff_parsed *diff_parsed_alloc(void)
 	}
 
 	diff->base.opts.flags &= ~GIT_DIFF_IGNORE_CASE;
+	diff->base.opts.oid_type = oid_type;
 
 	if (git_pool_init(&diff->base.pool, 1) < 0 ||
 	    git_vector_init(&diff->patches, 0, NULL) < 0 ||
@@ -67,19 +68,34 @@ static git_diff_parsed *diff_parsed_alloc(void)
 int git_diff_from_buffer(
 	git_diff **out,
 	const char *content,
-	size_t content_len)
+	size_t content_len
+#ifdef GIT_EXPERIMENTAL_SHA256
+	, git_diff_parse_options *opts
+#endif
+	)
 {
 	git_diff_parsed *diff;
 	git_patch *patch;
 	git_patch_parse_ctx *ctx = NULL;
+	git_patch_options patch_opts = GIT_PATCH_OPTIONS_INIT;
+	git_oid_t oid_type;
 	int error = 0;
 
 	*out = NULL;
 
-	diff = diff_parsed_alloc();
+#ifdef GIT_EXPERIMENTAL_SHA256
+	oid_type = (opts && opts->oid_type) ? opts->oid_type :
+		GIT_OID_DEFAULT;
+#else
+	oid_type = GIT_OID_DEFAULT;
+#endif
+
+	patch_opts.oid_type = oid_type;
+
+	diff = diff_parsed_alloc(oid_type);
 	GIT_ERROR_CHECK_ALLOC(diff);
 
-	ctx = git_patch_parse_ctx_init(content, content_len, NULL);
+	ctx = git_patch_parse_ctx_init(content, content_len, &patch_opts);
 	GIT_ERROR_CHECK_ALLOC(ctx);
 
 	while (ctx->parse_ctx.remain_len) {
diff --git a/src/libgit2/diff_print.c b/src/libgit2/diff_print.c
index 3077e11e1..32c936826 100644
--- a/src/libgit2/diff_print.c
+++ b/src/libgit2/diff_print.c
@@ -29,6 +29,7 @@ typedef struct {
 	const char *new_prefix;
 	uint32_t flags;
 	int id_strlen;
+	git_oid_t oid_type;
 
 	int (*strcomp)(const char *, const char *);
 } diff_print_info;
@@ -46,6 +47,8 @@ static int diff_print_info_init__common(
 	pi->payload = payload;
 	pi->buf = out;
 
+	GIT_ASSERT(pi->oid_type);
+
 	if (!pi->id_strlen) {
 		if (!repo)
 			pi->id_strlen = GIT_ABBREV_DEFAULT;
@@ -53,8 +56,9 @@ static int diff_print_info_init__common(
 			return -1;
 	}
 
-	if (pi->id_strlen > GIT_OID_SHA1_HEXSIZE)
-		pi->id_strlen = GIT_OID_SHA1_HEXSIZE;
+	if (pi->id_strlen > 0 &&
+	    (size_t)pi->id_strlen > git_oid_hexsize(pi->oid_type))
+		pi->id_strlen = (int)git_oid_hexsize(pi->oid_type);
 
 	memset(&pi->line, 0, sizeof(pi->line));
 	pi->line.old_lineno = -1;
@@ -78,6 +82,7 @@ static int diff_print_info_init_fromdiff(
 
 	if (diff) {
 		pi->flags = diff->opts.flags;
+		pi->oid_type = diff->opts.oid_type;
 		pi->id_strlen = diff->opts.id_abbrev;
 		pi->old_prefix = diff->opts.old_prefix;
 		pi->new_prefix = diff->opts.new_prefix;
@@ -101,6 +106,7 @@ static int diff_print_info_init_frompatch(
 	memset(pi, 0, sizeof(diff_print_info));
 
 	pi->flags = patch->diff_opts.flags;
+	pi->oid_type = patch->diff_opts.oid_type;
 	pi->id_strlen = patch->diff_opts.id_abbrev;
 	pi->old_prefix = patch->diff_opts.old_prefix;
 	pi->new_prefix = patch->diff_opts.new_prefix;
@@ -212,7 +218,10 @@ static int diff_print_one_raw(
 	git_str *out = pi->buf;
 	int id_abbrev;
 	char code = git_diff_status_char(delta->status);
-	char start_oid[GIT_OID_SHA1_HEXSIZE+1], end_oid[GIT_OID_SHA1_HEXSIZE+1];
+	char start_oid[GIT_OID_MAX_HEXSIZE + 1],
+	     end_oid[GIT_OID_MAX_HEXSIZE + 1];
+	size_t oid_hexsize;
+	bool id_is_abbrev;
 
 	GIT_UNUSED(progress);
 
@@ -231,12 +240,21 @@ static int diff_print_one_raw(
 		return -1;
 	}
 
+#ifdef GIT_EXPERIMENTAL_SHA256
+	GIT_ASSERT(delta->old_file.id.type == delta->new_file.id.type);
+	oid_hexsize = git_oid_hexsize(delta->old_file.id.type);
+#else
+	oid_hexsize = GIT_OID_SHA1_HEXSIZE;
+#endif
+
+	id_is_abbrev = (pi->id_strlen > 0 &&
+	                (size_t)pi->id_strlen <= oid_hexsize);
+
 	git_oid_tostr(start_oid, pi->id_strlen + 1, &delta->old_file.id);
 	git_oid_tostr(end_oid, pi->id_strlen + 1, &delta->new_file.id);
 
-	git_str_printf(
-		out, (pi->id_strlen <= GIT_OID_SHA1_HEXSIZE) ?
-			":%06o %06o %s... %s... %c" : ":%06o %06o %s %s %c",
+	git_str_printf(out,
+		id_is_abbrev ? ":%06o %06o %s... %s... %c" : ":%06o %06o %s %s %c",
 		delta->old_file.mode, delta->new_file.mode, start_oid, end_oid, code);
 
 	if (delta->similarity > 0)
@@ -273,7 +291,8 @@ static int diff_print_oid_range(
 	git_str *out, const git_diff_delta *delta, int id_strlen,
 	bool print_index)
 {
-	char start_oid[GIT_OID_SHA1_HEXSIZE+1], end_oid[GIT_OID_SHA1_HEXSIZE+1];
+	char start_oid[GIT_OID_MAX_HEXSIZE + 1],
+	     end_oid[GIT_OID_MAX_HEXSIZE + 1];
 
 	if (delta->old_file.mode &&
 			id_strlen > delta->old_file.id_abbrev) {
diff --git a/src/libgit2/diff_tform.c b/src/libgit2/diff_tform.c
index 8c0c1b7fc..4a156c7a3 100644
--- a/src/libgit2/diff_tform.c
+++ b/src/libgit2/diff_tform.c
@@ -364,7 +364,7 @@ static int insert_delete_side_of_split(
 	memset(&deleted->new_file, 0, sizeof(deleted->new_file));
 	deleted->new_file.path = deleted->old_file.path;
 	deleted->new_file.flags |= GIT_DIFF_FLAG_VALID_ID;
-	git_oid_clear(&deleted->new_file.id, GIT_OID_SHA1);
+	git_oid_clear(&deleted->new_file.id, diff->opts.oid_type);
 
 	return git_vector_insert(onto, deleted);
 }
@@ -398,7 +398,7 @@ static int apply_splits_and_deletes(
 			memset(&delta->old_file, 0, sizeof(delta->old_file));
 			delta->old_file.path = delta->new_file.path;
 			delta->old_file.flags |= GIT_DIFF_FLAG_VALID_ID;
-			git_oid_clear(&delta->old_file.id, GIT_OID_SHA1);
+			git_oid_clear(&delta->old_file.id, diff->opts.oid_type);
 		}
 
 		/* clean up delta before inserting into new list */
@@ -997,7 +997,7 @@ find_best_matches:
 				memset(&src->new_file, 0, sizeof(src->new_file));
 				src->new_file.path = src->old_file.path;
 				src->new_file.flags |= GIT_DIFF_FLAG_VALID_ID;
-				git_oid_clear(&src->new_file.id, GIT_OID_SHA1);
+				git_oid_clear(&src->new_file.id, diff->opts.oid_type);
 
 				num_updates++;
 
@@ -1023,7 +1023,7 @@ find_best_matches:
 				memset(&src->old_file, 0, sizeof(src->old_file));
 				src->old_file.path = src->new_file.path;
 				src->old_file.flags |= GIT_DIFF_FLAG_VALID_ID;
-				git_oid_clear(&src->old_file.id, GIT_OID_SHA1);
+				git_oid_clear(&src->old_file.id, diff->opts.oid_type);
 
 				src->flags &= ~GIT_DIFF_FLAG__TO_SPLIT;
 				num_rewrites--;
diff --git a/src/libgit2/email.c b/src/libgit2/email.c
index 0a75021c8..8a10a12b7 100644
--- a/src/libgit2/email.c
+++ b/src/libgit2/email.c
@@ -130,11 +130,12 @@ static int append_header(
 	const git_signature *author,
 	git_email_create_options *opts)
 {
-	char id[GIT_OID_SHA1_HEXSIZE];
+	char id[GIT_OID_MAX_HEXSIZE + 1];
 	int error;
 
-	if ((error = git_oid_fmt(id, commit_id)) < 0 ||
-	    (error = git_str_printf(out, "From %.*s %s\n", GIT_OID_SHA1_HEXSIZE, id, EMAIL_TIMESTAMP)) < 0 ||
+	git_oid_tostr(id, GIT_OID_MAX_HEXSIZE + 1, commit_id);
+
+	if ((error = git_str_printf(out, "From %s %s\n", id, EMAIL_TIMESTAMP)) < 0 ||
 	    (error = git_str_printf(out, "From: %s <%s>\n", author->name, author->email)) < 0 ||
 	    (error = append_date(out, &author->when)) < 0 ||
 	    (error = append_subject(out, patch_idx, patch_count, summary, opts)) < 0)
diff --git a/src/libgit2/fetch.c b/src/libgit2/fetch.c
index 003b5198a..ff10bde02 100644
--- a/src/libgit2/fetch.c
+++ b/src/libgit2/fetch.c
@@ -76,7 +76,7 @@ static int maybe_want_oid(git_remote *remote, git_refspec *spec)
 	oid_head = git__calloc(1, sizeof(git_remote_head));
 	GIT_ERROR_CHECK_ALLOC(oid_head);
 
-	git_oid__fromstr(&oid_head->oid, spec->src, GIT_OID_SHA1);
+	git_oid__fromstr(&oid_head->oid, spec->src, remote->repo->oid_type);
 
 	if (spec->dst) {
 		oid_head->name = git__strdup(spec->dst);
@@ -137,7 +137,7 @@ static int filter_wants(git_remote *remote, const git_fetch_options *opts)
 
 	/* Handle explicitly specified OID specs */
 	git_vector_foreach(&remote->active_refspecs, i, spec) {
-		if (!git_oid__is_hexstr(spec->src, GIT_OID_SHA1))
+		if (!git_oid__is_hexstr(spec->src, remote->repo->oid_type))
 			continue;
 
 		if (!(remote_caps & oid_mask)) {
diff --git a/src/libgit2/fetchhead.c b/src/libgit2/fetchhead.c
index 0ebfe5c43..2f276e526 100644
--- a/src/libgit2/fetchhead.c
+++ b/src/libgit2/fetchhead.c
@@ -105,15 +105,14 @@ static int fetchhead_ref_write(
 	git_filebuf *file,
 	git_fetchhead_ref *fetchhead_ref)
 {
-	char oid[GIT_OID_SHA1_HEXSIZE + 1];
+	char oid[GIT_OID_MAX_HEXSIZE + 1];
 	const char *type, *name;
 	int head = 0;
 
 	GIT_ASSERT_ARG(file);
 	GIT_ASSERT_ARG(fetchhead_ref);
 
-	git_oid_fmt(oid, &fetchhead_ref->oid);
-	oid[GIT_OID_SHA1_HEXSIZE] = '\0';
+	git_oid_tostr(oid, GIT_OID_MAX_HEXSIZE + 1, &fetchhead_ref->oid);
 
 	if (git__prefixcmp(fetchhead_ref->ref_name, GIT_REFS_HEADS_DIR) == 0) {
 		type = "branch ";
@@ -174,7 +173,8 @@ static int fetchhead_ref_parse(
 	git_str *ref_name,
 	const char **remote_url,
 	char *line,
-	size_t line_num)
+	size_t line_num,
+	git_oid_t oid_type)
 {
 	char *oid_str, *is_merge_str, *desc, *name = NULL;
 	const char *type = NULL;
@@ -196,13 +196,13 @@ static int fetchhead_ref_parse(
 		*is_merge = 1;
 	}
 
-	if (strlen(oid_str) != GIT_OID_SHA1_HEXSIZE) {
+	if (strlen(oid_str) != git_oid_hexsize(oid_type)) {
 		git_error_set(GIT_ERROR_FETCHHEAD,
 			"invalid object ID in FETCH_HEAD line %"PRIuZ, line_num);
 		return -1;
 	}
 
-	if (git_oid__fromstr(oid, oid_str, GIT_OID_SHA1) < 0) {
+	if (git_oid__fromstr(oid, oid_str, oid_type) < 0) {
 		const git_error *oid_err = git_error_last();
 		const char *err_msg = oid_err ? oid_err->message : "invalid object ID";
 
@@ -269,7 +269,8 @@ static int fetchhead_ref_parse(
 	return error;
 }
 
-int git_repository_fetchhead_foreach(git_repository *repo,
+int git_repository_fetchhead_foreach(
+	git_repository *repo,
 	git_repository_fetchhead_foreach_cb cb,
 	void *payload)
 {
@@ -296,8 +297,9 @@ int git_repository_fetchhead_foreach(git_repository *repo,
 	while ((line = git__strsep(&buffer, "\n")) != NULL) {
 		++line_num;
 
-		if ((error = fetchhead_ref_parse(
-				&oid, &is_merge, &name, &remote_url, line, line_num)) < 0)
+		if ((error = fetchhead_ref_parse(&oid, &is_merge, &name,
+				&remote_url, line, line_num,
+				repo->oid_type)) < 0)
 			goto done;
 
 		if (git_str_len(&name) > 0)
diff --git a/src/libgit2/ident.c b/src/libgit2/ident.c
index bf9a4998e..97110c664 100644
--- a/src/libgit2/ident.c
+++ b/src/libgit2/ident.c
@@ -42,7 +42,7 @@ static int ident_find_id(
 static int ident_insert_id(
 	git_str *to, const git_str *from, const git_filter_source *src)
 {
-	char oid[GIT_OID_SHA1_HEXSIZE+1];
+	char oid[GIT_OID_MAX_HEXSIZE + 1];
 	const char *id_start, *id_end, *from_end = from->ptr + from->size;
 	size_t need_size;
 
@@ -57,7 +57,7 @@ static int ident_insert_id(
 		return GIT_PASSTHROUGH;
 
 	need_size = (size_t)(id_start - from->ptr) +
-		5 /* "$Id: " */ + GIT_OID_SHA1_HEXSIZE + 2 /* " $" */ +
+		5 /* "$Id: " */ + GIT_OID_MAX_HEXSIZE + 2 /* " $" */ +
 		(size_t)(from_end - id_end);
 
 	if (git_str_grow(to, need_size) < 0)
@@ -65,7 +65,7 @@ static int ident_insert_id(
 
 	git_str_set(to, from->ptr, (size_t)(id_start - from->ptr));
 	git_str_put(to, "$Id: ", 5);
-	git_str_put(to, oid, GIT_OID_SHA1_HEXSIZE);
+	git_str_puts(to, oid);
 	git_str_put(to, " $", 2);
 	git_str_put(to, id_end, (size_t)(from_end - id_end));
 
diff --git a/src/libgit2/index.c b/src/libgit2/index.c
index d4532c005..9d919093b 100644
--- a/src/libgit2/index.c
+++ b/src/libgit2/index.c
@@ -32,8 +32,6 @@ static int index_apply_to_wd_diff(git_index *index, int action, const git_strarr
 				  unsigned int flags,
 				  git_index_matched_path_cb cb, void *payload);
 
-#define minimal_entry_size (offsetof(struct entry_short, path))
-
 static const size_t INDEX_HEADER_SIZE = 12;
 
 static const unsigned int INDEX_VERSION_NUMBER_DEFAULT = 2;
@@ -65,7 +63,7 @@ struct entry_time {
 	uint32_t nanoseconds;
 };
 
-struct entry_short {
+struct entry_common {
 	struct entry_time ctime;
 	struct entry_time mtime;
 	uint32_t dev;
@@ -74,25 +72,35 @@ struct entry_short {
 	uint32_t uid;
 	uint32_t gid;
 	uint32_t file_size;
-	unsigned char oid[GIT_OID_SHA1_SIZE];
-	uint16_t flags;
-	char path[1]; /* arbitrary length */
 };
 
-struct entry_long {
-	struct entry_time ctime;
-	struct entry_time mtime;
-	uint32_t dev;
-	uint32_t ino;
-	uint32_t mode;
-	uint32_t uid;
-	uint32_t gid;
-	uint32_t file_size;
-	unsigned char oid[GIT_OID_SHA1_SIZE];
-	uint16_t flags;
-	uint16_t flags_extended;
-	char path[1]; /* arbitrary length */
-};
+#define entry_short(oid_size)                        \
+	struct {                                     \
+		struct entry_common common;          \
+		unsigned char oid[oid_size];         \
+		uint16_t flags;                      \
+		char path[1]; /* arbitrary length */ \
+	}
+
+#define entry_long(oid_size)                         \
+	struct {                                     \
+		struct entry_common common;          \
+		unsigned char oid[oid_size];         \
+		uint16_t flags;                      \
+		uint16_t flags_extended;             \
+		char path[1]; /* arbitrary length */ \
+	}
+
+typedef entry_short(GIT_OID_SHA1_SIZE) index_entry_short_sha1;
+typedef entry_long(GIT_OID_SHA1_SIZE) index_entry_long_sha1;
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+typedef entry_short(GIT_OID_SHA256_SIZE) index_entry_short_sha256;
+typedef entry_long(GIT_OID_SHA256_SIZE) index_entry_long_sha256;
+#endif
+
+#undef entry_short
+#undef entry_long
 
 struct entry_srch_key {
 	const char *path;
@@ -115,12 +123,12 @@ struct reuc_entry_internal {
 bool git_index__enforce_unsaved_safety = false;
 
 /* local declarations */
-static int read_extension(size_t *read_len, git_index *index, const char *buffer, size_t buffer_size);
+static int read_extension(size_t *read_len, git_index *index, size_t checksum_size, const char *buffer, size_t buffer_size);
 static int read_header(struct index_header *dest, const void *buffer);
 
 static int parse_index(git_index *index, const char *buffer, size_t buffer_size);
 static bool is_index_extended(git_index *index);
-static int write_index(unsigned char checksum[GIT_HASH_SHA1_SIZE], size_t *checksum_size, git_index *index, git_filebuf *file);
+static int write_index(unsigned char checksum[GIT_HASH_MAX_SIZE], size_t *checksum_size, git_index *index, git_filebuf *file);
 
 static void index_entry_free(git_index_entry *entry);
 static void index_entry_reuc_free(git_index_reuc_entry *reuc);
@@ -401,7 +409,10 @@ void git_index__set_ignore_case(git_index *index, bool ignore_case)
 	git_vector_sort(&index->reuc);
 }
 
-int git_index_open(git_index **index_out, const char *index_path)
+int git_index__open(
+	git_index **index_out,
+	const char *index_path,
+	git_oid_t oid_type)
 {
 	git_index *index;
 	int error = -1;
@@ -411,6 +422,8 @@ int git_index_open(git_index **index_out, const char *index_path)
 	index = git__calloc(1, sizeof(git_index));
 	GIT_ERROR_CHECK_ALLOC(index);
 
+	index->oid_type = oid_type;
+
 	if (git_pool_init(&index->tree_pool, 1) < 0)
 		goto fail;
 
@@ -451,10 +464,34 @@ fail:
 	return error;
 }
 
+#ifdef GIT_EXPERIMENTAL_SHA256
+int git_index_open(git_index **index_out, const char *index_path, git_oid_t oid_type)
+{
+	return git_index__open(index_out, index_path, oid_type);
+}
+#else
+int git_index_open(git_index **index_out, const char *index_path)
+{
+	return git_index__open(index_out, index_path, GIT_OID_SHA1);
+}
+#endif
+
+int git_index__new(git_index **out, git_oid_t oid_type)
+{
+	return git_index__open(out, NULL, oid_type);
+}
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+int git_index_new(git_index **out, git_oid_t oid_type)
+{
+	return git_index__new(out, oid_type);
+}
+#else
 int git_index_new(git_index **out)
 {
-	return git_index_open(out, NULL);
+	return git_index__new(out, GIT_OID_SHA1);
 }
+#endif
 
 static void index_free(git_index *index)
 {
@@ -620,8 +657,8 @@ static int compare_checksum(git_index *index)
 {
 	int fd;
 	ssize_t bytes_read;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	size_t checksum_size = git_oid_size(index->oid_type);
 
 	if ((fd = p_open(index->index_file_path, O_RDONLY)) < 0)
 		return fd;
@@ -2306,6 +2343,7 @@ static int index_error_invalid(const char *message)
 static int read_reuc(git_index *index, const char *buffer, size_t size)
 {
 	const char *endptr;
+	size_t oid_size = git_oid_size(index->oid_type);
 	size_t len;
 	int i;
 
@@ -2354,16 +2392,16 @@ static int read_reuc(git_index *index, const char *buffer, size_t size)
 		for (i = 0; i < 3; i++) {
 			if (!lost->mode[i])
 				continue;
-			if (size < GIT_OID_SHA1_SIZE) {
+			if (size < oid_size) {
 				index_entry_reuc_free(lost);
 				return index_error_invalid("reading reuc entry oid");
 			}
 
-			if (git_oid__fromraw(&lost->oid[i], (const unsigned char *) buffer, GIT_OID_SHA1) < 0)
+			if (git_oid__fromraw(&lost->oid[i], (const unsigned char *) buffer, index->oid_type) < 0)
 				return -1;
 
-			size -= GIT_OID_SHA1_SIZE;
-			buffer += GIT_OID_SHA1_SIZE;
+			size -= oid_size;
+			buffer += oid_size;
 		}
 
 		/* entry was read successfully - insert into reuc vector */
@@ -2433,73 +2471,157 @@ out_err:
 	return 0;
 }
 
-static size_t index_entry_size(size_t path_len, size_t varint_len, uint32_t flags)
+GIT_INLINE(size_t) index_entry_path_offset(
+	git_oid_t oid_type,
+	uint32_t flags)
+{
+	if (oid_type == GIT_OID_SHA1)
+		return (flags & GIT_INDEX_ENTRY_EXTENDED) ?
+			offsetof(index_entry_long_sha1, path) :
+			offsetof(index_entry_short_sha1, path);
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+	else if (oid_type == GIT_OID_SHA256)
+		return (flags & GIT_INDEX_ENTRY_EXTENDED) ?
+			offsetof(index_entry_long_sha256, path) :
+			offsetof(index_entry_short_sha256, path);
+#endif
+
+	git_error_set(GIT_ERROR_INTERNAL, "invalid oid type");
+	return 0;
+}
+
+GIT_INLINE(size_t) index_entry_flags_offset(git_oid_t oid_type)
 {
+	if (oid_type == GIT_OID_SHA1)
+		return offsetof(index_entry_long_sha1, flags_extended);
+
+#ifdef GIT_EXPERIMENTAL_SHA256
+	else if (oid_type == GIT_OID_SHA256)
+		return offsetof(index_entry_long_sha256, flags_extended);
+#endif
+
+	git_error_set(GIT_ERROR_INTERNAL, "invalid oid type");
+	return 0;
+}
+
+static size_t index_entry_size(
+	size_t path_len,
+	size_t varint_len,
+	git_oid_t oid_type,
+	uint32_t flags)
+{
+	size_t offset, size;
+
+	if (!(offset = index_entry_path_offset(oid_type, flags)))
+		return 0;
+
 	if (varint_len) {
-		if (flags & GIT_INDEX_ENTRY_EXTENDED)
-			return offsetof(struct entry_long, path) + path_len + 1 + varint_len;
-		else
-			return offsetof(struct entry_short, path) + path_len + 1 + varint_len;
+		if (GIT_ADD_SIZET_OVERFLOW(&size, offset, path_len) ||
+		    GIT_ADD_SIZET_OVERFLOW(&size, size, 1) ||
+		    GIT_ADD_SIZET_OVERFLOW(&size, size, varint_len))
+			return 0;
 	} else {
-#define entry_size(type,len) ((offsetof(type, path) + (len) + 8) & ~7)
-		if (flags & GIT_INDEX_ENTRY_EXTENDED)
-			return entry_size(struct entry_long, path_len);
-		else
-			return entry_size(struct entry_short, path_len);
-#undef entry_size
+		if (GIT_ADD_SIZET_OVERFLOW(&size, offset, path_len) ||
+		    GIT_ADD_SIZET_OVERFLOW(&size, size, 8))
+			return 0;
+
+		size &= ~7;
 	}
+
+	return size;
 }
 
 static int read_entry(
 	git_index_entry **out,
 	size_t *out_size,
 	git_index *index,
+	size_t checksum_size,
 	const void *buffer,
 	size_t buffer_size,
 	const char *last)
 {
-	size_t path_length, entry_size;
+	size_t path_length, path_offset, entry_size;
 	const char *path_ptr;
-	struct entry_short source;
+	struct entry_common *source_common;
+	index_entry_short_sha1 source_sha1;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	index_entry_short_sha256 source_sha256;
+#endif
 	git_index_entry entry = {{0}};
 	bool compressed = index->version >= INDEX_VERSION_NUMBER_COMP;
 	char *tmp_path = NULL;
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
+
+	size_t minimal_entry_size = index_entry_path_offset(index->oid_type, 0);
 
 	if (checksum_size + minimal_entry_size > buffer_size)
 		return -1;
 
 	/* buffer is not guaranteed to be aligned */
-	memcpy(&source, buffer, sizeof(struct entry_short));
-
-	entry.ctime.seconds = (git_time_t)ntohl(source.ctime.seconds);
-	entry.ctime.nanoseconds = ntohl(source.ctime.nanoseconds);
-	entry.mtime.seconds = (git_time_t)ntohl(source.mtime.seconds);
-	entry.mtime.nanoseconds = ntohl(source.mtime.nanoseconds);
-	entry.dev = ntohl(source.dev);
-	entry.ino = ntohl(source.ino);
-	entry.mode = ntohl(source.mode);
-	entry.uid = ntohl(source.uid);
-	entry.gid = ntohl(source.gid);
-	entry.file_size = ntohl(source.file_size);
-	entry.flags = ntohs(source.flags);
-
-	if (git_oid__fromraw(&entry.id, source.oid, GIT_OID_SHA1) < 0)
+	switch (index->oid_type) {
+	case GIT_OID_SHA1:
+		source_common = &source_sha1.common;
+		memcpy(&source_sha1, buffer, sizeof(source_sha1));
+		break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	case GIT_OID_SHA256:
+		source_common = &source_sha256.common;
+		memcpy(&source_sha256, buffer, sizeof(source_sha256));
+		break;
+#endif
+	default:
+		GIT_ASSERT(!"invalid oid type");
+	}
+
+	entry.ctime.seconds = (git_time_t)ntohl(source_common->ctime.seconds);
+	entry.ctime.nanoseconds = ntohl(source_common->ctime.nanoseconds);
+	entry.mtime.seconds = (git_time_t)ntohl(source_common->mtime.seconds);
+	entry.mtime.nanoseconds = ntohl(source_common->mtime.nanoseconds);
+	entry.dev = ntohl(source_common->dev);
+	entry.ino = ntohl(source_common->ino);
+	entry.mode = ntohl(source_common->mode);
+	entry.uid = ntohl(source_common->uid);
+	entry.gid = ntohl(source_common->gid);
+	entry.file_size = ntohl(source_common->file_size);
+
+	switch (index->oid_type) {
+	case GIT_OID_SHA1:
+		if (git_oid__fromraw(&entry.id, source_sha1.oid,
+		                     GIT_OID_SHA1) < 0)
+			return -1;
+		entry.flags = ntohs(source_sha1.flags);
+		break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	case GIT_OID_SHA256:
+		if (git_oid__fromraw(&entry.id, source_sha256.oid,
+		                     GIT_OID_SHA256) < 0)
+			return -1;
+		entry.flags = ntohs(source_sha256.flags);
+		break;
+#endif
+	default:
+		GIT_ASSERT(!"invalid oid type");
+	}
+
+	if (!(path_offset = index_entry_path_offset(index->oid_type, entry.flags)))
 		return -1;
 
+
 	if (entry.flags & GIT_INDEX_ENTRY_EXTENDED) {
 		uint16_t flags_raw;
 		size_t flags_offset;
 
-		flags_offset = offsetof(struct entry_long, flags_extended);
-		memcpy(&flags_raw, (const char *) buffer + flags_offset,
-			sizeof(flags_raw));
+		if (!(flags_offset = index_entry_flags_offset(index->oid_type)))
+			return -1;
+
+		memcpy(&flags_raw, (const char *)buffer + flags_offset, sizeof(flags_raw));
 		flags_raw = ntohs(flags_raw);
 
 		memcpy(&entry.flags_extended, &flags_raw, sizeof(flags_raw));
-		path_ptr = (const char *) buffer + offsetof(struct entry_long, path);
-	} else
-		path_ptr = (const char *) buffer + offsetof(struct entry_short, path);
+		path_ptr = (const char *)buffer + path_offset;
+	} else {
+		path_ptr = (const char *)buffer + path_offset;
+	}
 
 	if (!compressed) {
 		path_length = entry.flags & GIT_INDEX_ENTRY_NAMEMASK;
@@ -2511,12 +2633,12 @@ static int read_entry(
 
 			path_end = memchr(path_ptr, '\0', buffer_size);
 			if (path_end == NULL)
-				return -1;
+				return index_error_invalid("invalid path name");
 
 			path_length = path_end - path_ptr;
 		}
 
-		entry_size = index_entry_size(path_length, 0, entry.flags);
+		entry_size = index_entry_size(path_length, 0, index->oid_type, entry.flags);
 		entry.path = (char *)path_ptr;
 	} else {
 		size_t varint_len, last_len, prefix_len, suffix_len, path_len;
@@ -2542,15 +2664,18 @@ static int read_entry(
 
 		memcpy(tmp_path, last, prefix_len);
 		memcpy(tmp_path + prefix_len, path_ptr + varint_len, suffix_len + 1);
-		entry_size = index_entry_size(suffix_len, varint_len, entry.flags);
+
+		entry_size = index_entry_size(suffix_len, varint_len, index->oid_type, entry.flags);
 		entry.path = tmp_path;
 	}
 
 	if (entry_size == 0)
 		return -1;
 
-	if (checksum_size + entry_size > buffer_size)
+	if (checksum_size + entry_size > buffer_size) {
+		git_error_set(GIT_ERROR_INTERNAL, "invalid index checksum");
 		return -1;
+	}
 
 	if (index_entry_dup(out, index, &entry) < 0) {
 		git__free(tmp_path);
@@ -2579,11 +2704,10 @@ static int read_header(struct index_header *dest, const void *buffer)
 	return 0;
 }
 
-static int read_extension(size_t *read_len, git_index *index, const char *buffer, size_t buffer_size)
+static int read_extension(size_t *read_len, git_index *index, size_t checksum_size, const char *buffer, size_t buffer_size)
 {
 	struct index_extension dest;
 	size_t total_size;
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
 
 	/* buffer is not guaranteed to be aligned */
 	memcpy(&dest, buffer, sizeof(struct index_extension));
@@ -2602,7 +2726,7 @@ static int read_extension(size_t *read_len, git_index *index, const char *buffer
 	if (dest.signature[0] >= 'A' && dest.signature[0] <= 'Z') {
 		/* tree cache */
 		if (memcmp(dest.signature, INDEX_EXT_TREECACHE_SIG, 4) == 0) {
-			if (git_tree_cache_read(&index->tree, buffer + 8, dest.extension_size, &index->tree_pool) < 0)
+			if (git_tree_cache_read(&index->tree, buffer + 8, dest.extension_size, index->oid_type, &index->tree_pool) < 0)
 				return -1;
 		} else if (memcmp(dest.signature, INDEX_EXT_UNMERGED_SIG, 4) == 0) {
 			if (read_reuc(index, buffer + 8, dest.extension_size) < 0)
@@ -2630,8 +2754,8 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 	int error = 0;
 	unsigned int i;
 	struct index_header header = { 0 };
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size = GIT_HASH_SHA1_SIZE;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	size_t checksum_size = git_hash_size(git_oid_algorithm(index->oid_type));
 	const char *last = NULL;
 	const char *empty = "";
 
@@ -2646,9 +2770,12 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 	if (buffer_size < INDEX_HEADER_SIZE + checksum_size)
 		return index_error_invalid("insufficient buffer space");
 
-	/* Precalculate the SHA1 of the files's contents -- we'll match it to
-	 * the provided SHA1 in the footer */
-	git_hash_buf(checksum, buffer, buffer_size - checksum_size, GIT_HASH_ALGORITHM_SHA1);
+	/*
+	 * Precalculate the hash of the files's contents -- we'll match
+	 * it to the provided checksum in the footer.
+	 */
+	git_hash_buf(checksum, buffer, buffer_size - checksum_size,
+		git_oid_algorithm(index->oid_type));
 
 	/* Parse header */
 	if ((error = read_header(&header, buffer)) < 0)
@@ -2670,7 +2797,7 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 		git_index_entry *entry = NULL;
 		size_t entry_size;
 
-		if ((error = read_entry(&entry, &entry_size, index, buffer, buffer_size, last)) < 0) {
+		if ((error = read_entry(&entry, &entry_size, index, checksum_size, buffer, buffer_size, last)) < 0) {
 			error = index_error_invalid("invalid entry");
 			goto done;
 		}
@@ -2701,7 +2828,7 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 	while (buffer_size > checksum_size) {
 		size_t extension_size;
 
-		if ((error = read_extension(&extension_size, index, buffer, buffer_size)) < 0) {
+		if ((error = read_extension(&extension_size, index, checksum_size, buffer, buffer_size)) < 0) {
 			goto done;
 		}
 
@@ -2714,7 +2841,10 @@ static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
 		goto done;
 	}
 
-	/* 160-bit SHA-1 over the content of the index file before this checksum. */
+	/*
+	 * SHA-1 or SHA-256 (depending on the repository's object format)
+	 * over the content of the index file before this checksum.
+	 */
 	if (memcmp(checksum, buffer, checksum_size) != 0) {
 		error = index_error_invalid(
 			"calculated checksum does not match expected");
@@ -2754,16 +2884,40 @@ static bool is_index_extended(git_index *index)
 	return (extended > 0);
 }
 
-static int write_disk_entry(git_filebuf *file, git_index_entry *entry, const char *last)
+static int write_disk_entry(
+	git_index *index,
+	git_filebuf *file,
+	git_index_entry *entry,
+	const char *last)
 {
 	void *mem = NULL;
-	struct entry_short ondisk;
-	size_t path_len, disk_size;
+	struct entry_common *ondisk_common;
+	size_t path_len, path_offset, disk_size;
 	int varint_len = 0;
 	char *path;
 	const char *path_start = entry->path;
 	size_t same_len = 0;
 
+	index_entry_short_sha1 ondisk_sha1;
+	index_entry_long_sha1 ondisk_ext_sha1;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	index_entry_short_sha256 ondisk_sha256;
+	index_entry_long_sha256 ondisk_ext_sha256;
+#endif
+
+	switch (index->oid_type) {
+	case GIT_OID_SHA1:
+		ondisk_common = &ondisk_sha1.common;
+		break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	case GIT_OID_SHA256:
+		ondisk_common = &ondisk_sha256.common;
+		break;
+#endif
+	default:
+		GIT_ASSERT(!"invalid oid type");
+	}
+
 	path_len = ((struct entry_internal *)entry)->pathlen;
 
 	if (last) {
@@ -2780,9 +2934,9 @@ static int write_disk_entry(git_filebuf *file, git_index_entry *entry, const cha
 		varint_len = git_encode_varint(NULL, 0, strlen(last) - same_len);
 	}
 
-	disk_size = index_entry_size(path_len, varint_len, entry->flags);
+	disk_size = index_entry_size(path_len, varint_len, index->oid_type, entry->flags);
 
-	if (git_filebuf_reserve(file, &mem, disk_size) < 0)
+	if (!disk_size || git_filebuf_reserve(file, &mem, disk_size) < 0)
 		return -1;
 
 	memset(mem, 0x0, disk_size);
@@ -2797,35 +2951,77 @@ static int write_disk_entry(git_filebuf *file, git_index_entry *entry, const cha
 	 *
 	 * In 2038 I will be either too dead or too rich to care about this
 	 */
-	ondisk.ctime.seconds = htonl((uint32_t)entry->ctime.seconds);
-	ondisk.mtime.seconds = htonl((uint32_t)entry->mtime.seconds);
-	ondisk.ctime.nanoseconds = htonl(entry->ctime.nanoseconds);
-	ondisk.mtime.nanoseconds = htonl(entry->mtime.nanoseconds);
-	ondisk.dev = htonl(entry->dev);
-	ondisk.ino = htonl(entry->ino);
-	ondisk.mode = htonl(entry->mode);
-	ondisk.uid = htonl(entry->uid);
-	ondisk.gid = htonl(entry->gid);
-	ondisk.file_size = htonl((uint32_t)entry->file_size);
-	git_oid_raw_cpy(ondisk.oid, entry->id.id, GIT_OID_SHA1_SIZE);
-	ondisk.flags = htons(entry->flags);
+	ondisk_common->ctime.seconds = htonl((uint32_t)entry->ctime.seconds);
+	ondisk_common->mtime.seconds = htonl((uint32_t)entry->mtime.seconds);
+	ondisk_common->ctime.nanoseconds = htonl(entry->ctime.nanoseconds);
+	ondisk_common->mtime.nanoseconds = htonl(entry->mtime.nanoseconds);
+	ondisk_common->dev = htonl(entry->dev);
+	ondisk_common->ino = htonl(entry->ino);
+	ondisk_common->mode = htonl(entry->mode);
+	ondisk_common->uid = htonl(entry->uid);
+	ondisk_common->gid = htonl(entry->gid);
+	ondisk_common->file_size = htonl((uint32_t)entry->file_size);
+
+	switch (index->oid_type) {
+	case GIT_OID_SHA1:
+		git_oid_raw_cpy(ondisk_sha1.oid, entry->id.id, GIT_OID_SHA1_SIZE);
+		ondisk_sha1.flags = htons(entry->flags);
+		break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+	case GIT_OID_SHA256:
+		git_oid_raw_cpy(ondisk_sha256.oid, entry->id.id, GIT_OID_SHA256_SIZE);
+		ondisk_sha256.flags = htons(entry->flags);
+		break;
+#endif
+	default:
+		GIT_ASSERT(!"invalid oid type");
+	}
+
+	path_offset = index_entry_path_offset(index->oid_type, entry->flags);
 
 	if (entry->flags & GIT_INDEX_ENTRY_EXTENDED) {
-		const size_t path_offset = offsetof(struct entry_long, path);
-		struct entry_long ondisk_ext;
-		memcpy(&ondisk_ext, &ondisk, sizeof(struct entry_short));
-		ondisk_ext.flags_extended = htons(entry->flags_extended &
+		struct entry_common *ondisk_ext;
+		uint16_t flags_extended = htons(entry->flags_extended &
 			GIT_INDEX_ENTRY_EXTENDED_FLAGS);
-		memcpy(mem, &ondisk_ext, path_offset);
-		path = (char *)mem + path_offset;
-		disk_size -= path_offset;
+
+		switch (index->oid_type) {
+		case GIT_OID_SHA1:
+			memcpy(&ondisk_ext_sha1, &ondisk_sha1,
+				sizeof(index_entry_short_sha1));
+			ondisk_ext_sha1.flags_extended = flags_extended;
+			ondisk_ext = &ondisk_ext_sha1.common;
+			break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+		case GIT_OID_SHA256:
+			memcpy(&ondisk_ext_sha256, &ondisk_sha256,
+				sizeof(index_entry_short_sha256));
+			ondisk_ext_sha256.flags_extended = flags_extended;
+			ondisk_ext = &ondisk_ext_sha256.common;
+			break;
+#endif
+		default:
+			GIT_ASSERT(!"invalid oid type");
+		}
+
+		memcpy(mem, ondisk_ext, path_offset);
 	} else {
-		const size_t path_offset = offsetof(struct entry_short, path);
-		memcpy(mem, &ondisk, path_offset);
-		path = (char *)mem + path_offset;
-		disk_size -= path_offset;
+		switch (index->oid_type) {
+		case GIT_OID_SHA1:
+			memcpy(mem, &ondisk_sha1, path_offset);
+			break;
+#ifdef GIT_EXPERIMENTAL_SHA256
+		case GIT_OID_SHA256:
+			memcpy(mem, &ondisk_sha256, path_offset);
+			break;
+#endif
+		default:
+			GIT_ASSERT(!"invalid oid type");
+		}
 	}
 
+	path = (char *)mem + path_offset;
+	disk_size -= path_offset;
+
 	if (last) {
 		varint_len = git_encode_varint((unsigned char *) path,
 					  disk_size, strlen(last) - same_len);
@@ -2877,7 +3073,7 @@ static int write_entries(git_index *index, git_filebuf *file)
 		last = "";
 
 	git_vector_foreach(entries, i, entry) {
-		if ((error = write_disk_entry(file, entry, last)) < 0)
+		if ((error = write_disk_entry(index, file, entry, last)) < 0)
 			break;
 		if (index->version >= INDEX_VERSION_NUMBER_COMP)
 			last = entry->path;
@@ -2955,8 +3151,9 @@ done:
 	return error;
 }
 
-static int create_reuc_extension_data(git_str *reuc_buf, git_index_reuc_entry *reuc)
+static int create_reuc_extension_data(git_str *reuc_buf, git_index *index, git_index_reuc_entry *reuc)
 {
+	size_t oid_size = git_oid_size(index->oid_type);
 	int i;
 	int error = 0;
 
@@ -2970,7 +3167,7 @@ static int create_reuc_extension_data(git_str *reuc_buf, git_index_reuc_entry *r
 	}
 
 	for (i = 0; i < 3; i++) {
-		if (reuc->mode[i] && (error = git_str_put(reuc_buf, (char *)&reuc->oid[i].id, GIT_OID_SHA1_SIZE)) < 0)
+		if (reuc->mode[i] && (error = git_str_put(reuc_buf, (char *)&reuc->oid[i].id, oid_size)) < 0)
 			return error;
 	}
 
@@ -2987,7 +3184,7 @@ static int write_reuc_extension(git_index *index, git_filebuf *file)
 	int error = 0;
 
 	git_vector_foreach(out, i, reuc) {
-		if ((error = create_reuc_extension_data(&reuc_buf, reuc)) < 0)
+		if ((error = create_reuc_extension_data(&reuc_buf, index, reuc)) < 0)
 			goto done;
 	}
 
@@ -3036,7 +3233,7 @@ static void clear_uptodate(git_index *index)
 }
 
 static int write_index(
-	unsigned char checksum[GIT_HASH_SHA1_SIZE],
+	unsigned char checksum[GIT_HASH_MAX_SIZE],
 	size_t *checksum_size,
 	git_index *index,
 	git_filebuf *file)
@@ -3048,7 +3245,9 @@ static int write_index(
 	GIT_ASSERT_ARG(index);
 	GIT_ASSERT_ARG(file);
 
-	*checksum_size = GIT_HASH_SHA1_SIZE;
+	GIT_ASSERT(index->oid_type);
+
+	*checksum_size = git_hash_size(git_oid_algorithm(index->oid_type));
 
 	if (index->version <= INDEX_VERSION_NUMBER_EXT)  {
 		is_extended = is_index_extended(index);
@@ -3209,7 +3408,7 @@ cleanup:
 	if (error < 0)
 		return error;
 
-	error = git_tree_cache_read_tree(&index->tree, tree, &index->tree_pool);
+	error = git_tree_cache_read_tree(&index->tree, tree, index->oid_type, &index->tree_pool);
 
 	return error;
 }
@@ -3668,19 +3867,23 @@ int git_indexwriter_init(
 	git_indexwriter *writer,
 	git_index *index)
 {
-	int error;
+	int filebuf_hash, error;
 
 	GIT_REFCOUNT_INC(index);
 
 	writer->index = index;
 
+	filebuf_hash = git_filebuf_hash_flags(git_oid_algorithm(index->oid_type));
+	GIT_ASSERT(filebuf_hash);
+
 	if (!index->index_file_path)
 		return create_index_error(-1,
 			"failed to write index: The index is in-memory only");
 
-	if ((error = git_filebuf_open(
-		&writer->file, index->index_file_path, GIT_FILEBUF_HASH_CONTENTS, GIT_INDEX_FILE_MODE)) < 0) {
-
+	if ((error = git_filebuf_open(&writer->file,
+			index->index_file_path,
+			git_filebuf_hash_flags(filebuf_hash),
+			GIT_INDEX_FILE_MODE)) < 0) {
 		if (error == GIT_ELOCKED)
 			git_error_set(GIT_ERROR_INDEX, "the index is locked; this might be due to a concurrent or crashed process");
 
@@ -3712,7 +3915,7 @@ int git_indexwriter_init_for_operation(
 
 int git_indexwriter_commit(git_indexwriter *writer)
 {
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
 	size_t checksum_size;
 	int error;
 
diff --git a/src/libgit2/index.h b/src/libgit2/index.h
index 71bb096f7..53c29977d 100644
--- a/src/libgit2/index.h
+++ b/src/libgit2/index.h
@@ -27,7 +27,7 @@ struct git_index {
 
 	char *index_file_path;
 	git_futils_filestamp stamp;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
 
 	git_vector entries;
 	git_idxmap *entries_map;
@@ -35,6 +35,8 @@ struct git_index {
 	git_vector deleted; /* deleted entries if readers > 0 */
 	git_atomic32 readers; /* number of active iterators */
 
+	git_oid_t oid_type;
+
 	unsigned int on_disk:1;
 	unsigned int ignore_case:1;
 	unsigned int distrust_filemode:1;
@@ -141,6 +143,17 @@ GIT_INLINE(unsigned char *) git_index__checksum(git_index *index)
 	return index->checksum;
 }
 
+/* SHA256-aware internal functions */
+
+extern int git_index__new(
+	git_index **index_out,
+	git_oid_t oid_type);
+
+extern int git_index__open(
+	git_index **index_out,
+	const char *index_path,
+	git_oid_t oid_type);
+
 /* Copy the current entries vector *and* increment the index refcount.
  * Call `git_index__release_snapshot` when done.
  */
diff --git a/src/libgit2/indexer.c b/src/libgit2/indexer.c
index fa55fb5ea..7357a4aa5 100644
--- a/src/libgit2/indexer.c
+++ b/src/libgit2/indexer.c
@@ -1232,6 +1232,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 	git_filebuf index_file = {0};
 	void *packfile_trailer;
 	size_t checksum_size;
+	int filebuf_hash;
 	bool mismatch;
 
 	if (!idx->parsed_header) {
@@ -1240,6 +1241,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 	}
 
 	checksum_size = git_hash_size(indexer_hash_algorithm(idx));
+	filebuf_hash = git_filebuf_hash_flags(indexer_hash_algorithm(idx));
 	GIT_ASSERT(checksum_size);
 
 	/* Test for this before resolve_deltas(), as it plays with idx->off */
@@ -1314,8 +1316,7 @@ int git_indexer_commit(git_indexer *idx, git_indexer_progress *stats)
 		return -1;
 
 	if (git_filebuf_open(&index_file, filename.ptr,
-		GIT_FILEBUF_HASH_CONTENTS |
-		(idx->do_fsync ? GIT_FILEBUF_FSYNC : 0),
+		filebuf_hash | (idx->do_fsync ? GIT_FILEBUF_FSYNC : 0),
 		idx->mode) < 0)
 		goto on_error;
 
diff --git a/src/libgit2/iterator.c b/src/libgit2/iterator.c
index 1ee8e25f5..95ded1046 100644
--- a/src/libgit2/iterator.c
+++ b/src/libgit2/iterator.c
@@ -1036,6 +1036,8 @@ typedef struct {
 	git_index *index;
 	git_vector index_snapshot;
 
+	git_oid_t oid_type;
+
 	git_array_t(filesystem_iterator_frame) frames;
 	git_ignores ignores;
 
@@ -1271,7 +1273,7 @@ static int filesystem_iterator_entry_hash(
 	int error;
 
 	if (S_ISDIR(entry->st.st_mode)) {
-		memset(&entry->id, 0, GIT_OID_SHA1_SIZE);
+		memset(&entry->id, 0, git_oid_size(iter->oid_type));
 		return 0;
 	}
 
@@ -1281,7 +1283,7 @@ static int filesystem_iterator_entry_hash(
 
 	if (!(error = git_str_joinpath(&fullpath, iter->root, entry->path)) &&
 	    !(error = git_path_validate_str_length(iter->base.repo, &fullpath)))
-		error = git_odb__hashfile(&entry->id, fullpath.ptr, GIT_OBJECT_BLOB, GIT_OID_SHA1);
+		error = git_odb__hashfile(&entry->id, fullpath.ptr, GIT_OBJECT_BLOB, iter->oid_type);
 
 	git_str_dispose(&fullpath);
 	return error;
@@ -1530,7 +1532,7 @@ static void filesystem_iterator_set_current(
 	if (iter->base.flags & GIT_ITERATOR_INCLUDE_HASH)
 		git_oid_cpy(&iter->entry.id, &entry->id);
 	else
-		git_oid_clear(&iter->entry.id, GIT_OID_SHA1);
+		git_oid_clear(&iter->entry.id, iter->oid_type);
 
 	iter->entry.path = entry->path;
 
@@ -1975,6 +1977,8 @@ static int iterator_for_filesystem(
 		(iterator__flag(&iter->base, PRECOMPOSE_UNICODE) ?
 			GIT_FS_PATH_DIR_PRECOMPOSE_UNICODE : 0);
 
+	iter->oid_type = options->oid_type;
+
 	if ((error = filesystem_iterator_init(iter)) < 0)
 		goto on_error;
 
@@ -1989,10 +1993,15 @@ on_error:
 int git_iterator_for_filesystem(
 	git_iterator **out,
 	const char *root,
-	git_iterator_options *options)
+	git_iterator_options *given_opts)
 {
+	git_iterator_options options = GIT_ITERATOR_OPTIONS_INIT;
+
+	if (given_opts)
+		memcpy(&options, given_opts, sizeof(git_iterator_options));
+
 	return iterator_for_filesystem(out,
-		NULL, root, NULL, NULL, GIT_ITERATOR_FS, options);
+		NULL, root, NULL, NULL, GIT_ITERATOR_FS, &options);
 }
 
 int git_iterator_for_workdir_ext(
@@ -2019,6 +2028,12 @@ int git_iterator_for_workdir_ext(
 	options.flags |= GIT_ITERATOR_HONOR_IGNORES |
 		GIT_ITERATOR_IGNORE_DOT_GIT;
 
+	if (!options.oid_type)
+		options.oid_type = repo->oid_type;
+	else if (options.oid_type != repo->oid_type)
+		git_error_set(GIT_ERROR_INVALID,
+			"specified object ID type does not match repository object ID type");
+
 	return iterator_for_filesystem(out,
 		repo, repo_workdir, index, tree, GIT_ITERATOR_WORKDIR, &options);
 }
diff --git a/src/libgit2/iterator.h b/src/libgit2/iterator.h
index 6bb8489d0..7963ce7e2 100644
--- a/src/libgit2/iterator.h
+++ b/src/libgit2/iterator.h
@@ -63,6 +63,9 @@ typedef struct {
 
 	/* flags, from above */
 	unsigned int flags;
+
+	/* oid type - necessary for non-workdir filesystem iterators */
+	git_oid_t oid_type;
 } git_iterator_options;
 
 #define GIT_ITERATOR_OPTIONS_INIT {0}
diff --git a/src/libgit2/merge.c b/src/libgit2/merge.c
index df2cefb29..0114e4b75 100644
--- a/src/libgit2/merge.c
+++ b/src/libgit2/merge.c
@@ -611,13 +611,13 @@ int git_repository_mergehead_foreach(
 	buffer = merge_head_file.ptr;
 
 	while ((line = git__strsep(&buffer, "\n")) != NULL) {
-		if (strlen(line) != GIT_OID_SHA1_HEXSIZE) {
+		if (strlen(line) != git_oid_hexsize(repo->oid_type)) {
 			git_error_set(GIT_ERROR_INVALID, "unable to parse OID - invalid length");
 			error = -1;
 			goto cleanup;
 		}
 
-		if ((error = git_oid__fromstr(&oid, line, GIT_OID_SHA1)) < 0)
+		if ((error = git_oid__fromstr(&oid, line, repo->oid_type)) < 0)
 			goto cleanup;
 
 		if ((error = cb(&oid, payload)) != 0) {
@@ -1061,7 +1061,7 @@ static int index_entry_similarity_calc(
 	const git_merge_options *opts)
 {
 	git_blob *blob;
-	git_diff_file diff_file = { GIT_OID_SHA1_ZERO };
+	git_diff_file diff_file;
 	git_object_size_t blobsize;
 	int error;
 
@@ -1070,6 +1070,8 @@ static int index_entry_similarity_calc(
 
 	*out = NULL;
 
+	git_oid_clear(&diff_file.id, repo->oid_type);
+
 	if ((error = git_blob_lookup(&blob, repo, &entry->id)) < 0)
 		return error;
 
@@ -1997,8 +1999,11 @@ static int index_update_reuc(git_index *index, git_merge_diff_list *diff_list)
 	return 0;
 }
 
-static int index_from_diff_list(git_index **out,
-	git_merge_diff_list *diff_list, bool skip_reuc)
+static int index_from_diff_list(
+	git_index **out,
+	git_merge_diff_list *diff_list,
+	git_oid_t oid_type,
+	bool skip_reuc)
 {
 	git_index *index;
 	size_t i;
@@ -2007,7 +2012,7 @@ static int index_from_diff_list(git_index **out,
 
 	*out = NULL;
 
-	if ((error = git_index_new(&index)) < 0)
+	if ((error = git_index__new(&index, oid_type)) < 0)
 		return error;
 
 	if ((error = git_index__fill(index, &diff_list->staged)) < 0)
@@ -2157,7 +2162,7 @@ int git_merge__iterators(
 		}
 	}
 
-	error = index_from_diff_list(out, diff_list,
+	error = index_from_diff_list(out, diff_list, repo->oid_type,
 		(opts.flags & GIT_MERGE_SKIP_REUC));
 
 done:
@@ -2200,8 +2205,8 @@ int git_merge_trees(
 			result = our_tree;
 
 		if (result) {
-			if ((error = git_index_new(out)) == 0)
-    			error = git_index_read_tree(*out, result);
+			if ((error = git_index__new(out, repo->oid_type)) == 0)
+				error = git_index_read_tree(*out, result);
 
 			return error;
 		}
diff --git a/src/libgit2/midx.c b/src/libgit2/midx.c
index b09055237..f6071f0bb 100644
--- a/src/libgit2/midx.c
+++ b/src/libgit2/midx.c
@@ -115,19 +115,20 @@ static int midx_parse_oid_lookup(
 		struct git_midx_chunk *chunk_oid_lookup)
 {
 	uint32_t i;
-	unsigned char *oid, *prev_oid, zero_oid[GIT_OID_SHA1_SIZE] = {0};
+	unsigned char *oid, *prev_oid, zero_oid[GIT_OID_MAX_SIZE] = {0};
+	size_t oid_size = git_oid_size(idx->oid_type);
 
 	if (chunk_oid_lookup->offset == 0)
 		return midx_error("missing OID Lookup chunk");
 	if (chunk_oid_lookup->length == 0)
 		return midx_error("empty OID Lookup chunk");
-	if (chunk_oid_lookup->length != idx->num_objects * GIT_OID_SHA1_SIZE)
+	if (chunk_oid_lookup->length != idx->num_objects * oid_size)
 		return midx_error("OID Lookup chunk has wrong length");
 
 	idx->oid_lookup = oid = (unsigned char *)(data + chunk_oid_lookup->offset);
 	prev_oid = zero_oid;
-	for (i = 0; i < idx->num_objects; ++i, oid += GIT_OID_SHA1_SIZE) {
-		if (git_oid_raw_cmp(prev_oid, oid, GIT_OID_SHA1_SIZE) >= 0)
+	for (i = 0; i < idx->num_objects; ++i, oid += oid_size) {
+		if (git_oid_raw_cmp(prev_oid, oid, oid_size) >= 0)
 			return midx_error("OID Lookup index is non-monotonic");
 		prev_oid = oid;
 	}
@@ -178,7 +179,7 @@ int git_midx_parse(
 	struct git_midx_chunk *last_chunk;
 	uint32_t i;
 	off64_t last_chunk_offset, chunk_offset, trailer_offset;
-	size_t checksum_size;
+	size_t checksum_size, oid_size;
 	int error;
 	struct git_midx_chunk chunk_packfile_names = {0},
 					 chunk_oid_fanout = {0},
@@ -188,7 +189,9 @@ int git_midx_parse(
 
 	GIT_ASSERT_ARG(idx);
 
-	if (size < sizeof(struct git_midx_header) + GIT_OID_SHA1_SIZE)
+	oid_size = git_oid_size(idx->oid_type);
+
+	if (size < sizeof(struct git_midx_header) + oid_size)
 		return midx_error("multi-pack index is too short");
 
 	hdr = ((struct git_midx_header *)data);
@@ -209,7 +212,7 @@ int git_midx_parse(
 			sizeof(struct git_midx_header) +
 			(1 + hdr->chunks) * 12;
 
-	checksum_size = GIT_HASH_SHA1_SIZE;
+	checksum_size = oid_size;
 	trailer_offset = size - checksum_size;
 
 	if (trailer_offset < last_chunk_offset)
@@ -287,8 +290,9 @@ int git_midx_parse(
 }
 
 int git_midx_open(
-		git_midx_file **idx_out,
-		const char *path)
+	git_midx_file **idx_out,
+	const char *path,
+	git_oid_t oid_type)
 {
 	git_midx_file *idx;
 	git_file fd = -1;
@@ -296,6 +300,8 @@ int git_midx_open(
 	struct stat st;
 	int error;
 
+	GIT_ASSERT_ARG(idx_out && path && oid_type);
+
 	/* TODO: properly open the file without access time using O_NOATIME */
 	fd = git_futils_open_ro(path);
 	if (fd < 0)
@@ -317,6 +323,8 @@ int git_midx_open(
 	idx = git__calloc(1, sizeof(git_midx_file));
 	GIT_ERROR_CHECK_ALLOC(idx);
 
+	idx->oid_type = oid_type;
+
 	error = git_str_sets(&idx->filename, path);
 	if (error < 0)
 		return error;
@@ -344,7 +352,7 @@ bool git_midx_needs_refresh(
 	git_file fd = -1;
 	struct stat st;
 	ssize_t bytes_read;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
 	size_t checksum_size;
 
 	/* TODO: properly open the file without access time using O_NOATIME */
@@ -364,8 +372,8 @@ bool git_midx_needs_refresh(
 		return true;
 	}
 
-	checksum_size = GIT_HASH_SHA1_SIZE;
-	bytes_read = p_pread(fd, checksum, checksum_size, st.st_size - GIT_OID_SHA1_SIZE);
+	checksum_size = git_oid_size(idx->oid_type);
+	bytes_read = p_pread(fd, checksum, checksum_size, st.st_size - checksum_size);
 	p_close(fd);
 
 	if (bytes_read != (ssize_t)checksum_size)
@@ -381,7 +389,7 @@ int git_midx_entry_find(
 		size_t len)
 {
 	int pos, found = 0;
-	size_t pack_index;
+	size_t pack_index, oid_size, oid_hexsize;
 	uint32_t hi, lo;
 	unsigned char *current = NULL;
 	const unsigned char *object_offset;
@@ -389,30 +397,33 @@ int git_midx_entry_find(
 
 	GIT_ASSERT_ARG(idx);
 
+	oid_size = git_oid_size(idx->oid_type);
+	oid_hexsize = git_oid_hexsize(idx->oid_type);
+
 	hi = ntohl(idx->oid_fanout[(int)short_oid->id[0]]);
 	lo = ((short_oid->id[0] == 0x0) ? 0 : ntohl(idx->oid_fanout[(int)short_oid->id[0] - 1]));
 
-	pos = git_pack__lookup_id(idx->oid_lookup, GIT_OID_SHA1_SIZE, lo, hi, short_oid->id, GIT_OID_SHA1);
+	pos = git_pack__lookup_id(idx->oid_lookup, oid_size, lo, hi, short_oid->id, idx->oid_type);
 
 	if (pos >= 0) {
 		/* An object matching exactly the oid was found */
 		found = 1;
-		current = idx->oid_lookup + (pos * GIT_OID_SHA1_SIZE);
+		current = idx->oid_lookup + (pos * oid_size);
 	} else {
 		/* No object was found */
 		/* pos refers to the object with the "closest" oid to short_oid */
 		pos = -1 - pos;
 		if (pos < (int)idx->num_objects) {
-			current = idx->oid_lookup + (pos * GIT_OID_SHA1_SIZE);
+			current = idx->oid_lookup + (pos * oid_size);
 
 			if (!git_oid_raw_ncmp(short_oid->id, current, len))
 				found = 1;
 		}
 	}
 
-	if (found && len != GIT_OID_SHA1_HEXSIZE && pos + 1 < (int)idx->num_objects) {
+	if (found && len != oid_hexsize && pos + 1 < (int)idx->num_objects) {
 		/* Check for ambiguousity */
-		const unsigned char *next = current + GIT_OID_SHA1_SIZE;
+		const unsigned char *next = current + oid_size;
 
 		if (!git_oid_raw_ncmp(short_oid->id, next, len))
 			found = 2;
@@ -443,7 +454,7 @@ int git_midx_entry_find(
 		return midx_error("invalid index into the packfile names table");
 	e->pack_index = pack_index;
 	e->offset = offset;
-	git_oid__fromraw(&e->sha1, current, GIT_OID_SHA1);
+	git_oid__fromraw(&e->sha1, current, idx->oid_type);
 	return 0;
 }
 
@@ -453,13 +464,15 @@ int git_midx_foreach_entry(
 		void *data)
 {
 	git_oid oid;
-	size_t i;
+	size_t oid_size, i;
 	int error;
 
 	GIT_ASSERT_ARG(idx);
 
+	oid_size = git_oid_size(idx->oid_type);
+
 	for (i = 0; i < idx->num_objects; ++i) {
-		if ((error = git_oid__fromraw(&oid, &idx->oid_lookup[i * GIT_OID_SHA1_SIZE], GIT_OID_SHA1)) < 0)
+		if ((error = git_oid__fromraw(&oid, &idx->oid_lookup[i * oid_size], idx->oid_type)) < 0)
 			return error;
 
 		if ((error = cb(&oid, data)) != 0)
@@ -501,9 +514,21 @@ static int packfile__cmp(const void *a_, const void *b_)
 
 int git_midx_writer_new(
 		git_midx_writer **out,
-		const char *pack_dir)
+		const char *pack_dir
+#ifdef GIT_EXPERIMENTAL_SHA256
+		, git_oid_t oid_type
+#endif
+		)
 {
-	git_midx_writer *w = git__calloc(1, sizeof(git_midx_writer));
+	git_midx_writer *w;
+
+#ifndef GIT_EXPERIMENTAL_SHA256
+	git_oid_t oid_type = GIT_OID_SHA1;
+#endif
+
+	GIT_ASSERT_ARG(out && pack_dir && oid_type);
+
+	w = git__calloc(1, sizeof(git_midx_writer));
 	GIT_ERROR_CHECK_ALLOC(w);
 
 	if (git_str_sets(&w->pack_dir, pack_dir) < 0) {
@@ -518,6 +543,8 @@ int git_midx_writer_new(
 		return -1;
 	}
 
+	w->oid_type = oid_type;
+
 	*out = w;
 	return 0;
 }
@@ -662,12 +689,13 @@ static int midx_write(
 		oid_lookup = GIT_STR_INIT,
 		object_offsets = GIT_STR_INIT,
 		object_large_offsets = GIT_STR_INIT;
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
-	size_t checksum_size;
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+	size_t checksum_size, oid_size;
 	git_midx_entry *entry;
 	object_entry_array_t object_entries_array = GIT_ARRAY_INIT;
 	git_vector object_entries = GIT_VECTOR_INIT;
 	git_hash_ctx ctx;
+	git_hash_algorithm_t checksum_type;
 	struct midx_write_hash_context hash_cb_data = {0};
 
 	hdr.signature = htonl(MIDX_SIGNATURE);
@@ -679,10 +707,14 @@ static int midx_write(
 	hash_cb_data.cb_data = cb_data;
 	hash_cb_data.ctx = &ctx;
 
-	checksum_size = GIT_HASH_SHA1_SIZE;
-	error = git_hash_ctx_init(&ctx, GIT_HASH_ALGORITHM_SHA1);
-	if (error < 0)
+	oid_size = git_oid_size(w->oid_type);
+
+	GIT_ASSERT((checksum_type = git_oid_algorithm(w->oid_type)));
+	checksum_size = git_hash_size(checksum_type);
+
+	if ((error = git_hash_ctx_init(&ctx, checksum_type)) < 0)
 		return error;
+
 	cb_data = &hash_cb_data;
 	write_cb = midx_write_hash;
 
@@ -749,7 +781,9 @@ static int midx_write(
 
 	/* Fill the OID Lookup table. */
 	git_vector_foreach (&object_entries, i, entry) {
-		error = git_str_put(&oid_lookup, (char *)&entry->sha1.id, GIT_OID_SHA1_SIZE);
+		error = git_str_put(&oid_lookup,
+			(char *)&entry->sha1.id, oid_size);
+
 		if (error < 0)
 			goto cleanup;
 	}
diff --git a/src/libgit2/midx.h b/src/libgit2/midx.h
index bcb0d9a0a..5107f69cb 100644
--- a/src/libgit2/midx.h
+++ b/src/libgit2/midx.h
@@ -51,8 +51,14 @@ typedef struct git_midx_file {
 	/* The number of entries in the Object Large Offsets table. Each entry has an 8-byte with an offset */
 	size_t num_object_large_offsets;
 
-	/* The trailer of the file. Contains the SHA1-checksum of the whole file. */
-	unsigned char checksum[GIT_HASH_SHA1_SIZE];
+	/*
+	 * The trailer of the file. Contains the checksum of the whole
+	 * file, in the repository's object format hash.
+	 */
+	unsigned char checksum[GIT_HASH_MAX_SIZE];
+
+	/* The type of object IDs in the midx. */
+	git_oid_t oid_type;
 
 	/* something like ".git/objects/pack/multi-pack-index". */
 	git_str filename;
@@ -82,11 +88,15 @@ struct git_midx_writer {
 
 	/* The list of `git_pack_file`s. */
 	git_vector packs;
+
+	/* The object ID type of the writer. */
+	git_oid_t oid_type;
 };
 
 int git_midx_open(
 		git_midx_file **idx_out,
-		const char *path);
+		const char *path,
+		git_oid_t oid_type);
 bool git_midx_needs_refresh(
 		const git_midx_file *idx,
 		const char *path);
diff --git a/src/libgit2/notes.c b/src/libgit2/notes.c
index 1b1935330..13ca3824b 100644
--- a/src/libgit2/notes.c
+++ b/src/libgit2/notes.c
@@ -460,7 +460,7 @@ int git_note_commit_read(
 {
 	int error;
 	git_tree *tree = NULL;
-	char target[GIT_OID_SHA1_HEXSIZE + 1];
+	char target[GIT_OID_MAX_HEXSIZE + 1];
 
 	git_oid_tostr(target, sizeof(target), oid);
 
@@ -507,7 +507,7 @@ int git_note_commit_create(
 {
 	int error;
 	git_tree *tree = NULL;
-	char target[GIT_OID_SHA1_HEXSIZE + 1];
+	char target[GIT_OID_MAX_HEXSIZE + 1];
 
 	git_oid_tostr(target, sizeof(target), oid);
 
@@ -578,7 +578,7 @@ int git_note_commit_remove(
 {
 	int error;
 	git_tree *tree = NULL;
-	char target[GIT_OID_SHA1_HEXSIZE + 1];
+	char target[GIT_OID_MAX_HEXSIZE + 1];
 
 	git_oid_tostr(target, sizeof(target), oid);
 
@@ -665,8 +665,9 @@ void git_note_free(git_note *note)
 }
 
 static int process_entry_path(
-	const char *entry_path,
-	git_oid *annotated_object_id)
+	git_oid *annotated_object_id,
+	git_note_iterator *it,
+	const char *entry_path)
 {
 	int error = 0;
 	size_t i = 0, j = 0, len;
@@ -698,12 +699,12 @@ static int process_entry_path(
 	buf.ptr[j] = '\0';
 	buf.size = j;
 
-	if (j != GIT_OID_SHA1_HEXSIZE) {
+	if (j != git_oid_hexsize(it->repo->oid_type)) {
 		/* This is not a note entry */
 		goto cleanup;
 	}
 
-	error = git_oid__fromstr(annotated_object_id, buf.ptr, GIT_OID_SHA1);
+	error = git_oid__fromstr(annotated_object_id, buf.ptr, it->repo->oid_type);
 
 cleanup:
 	git_str_dispose(&buf);
@@ -799,7 +800,7 @@ int git_note_next(
 
 	git_oid_cpy(note_id, &item->id);
 
-	if ((error = process_entry_path(item->path, annotated_id)) < 0)
+	if ((error = process_entry_path(annotated_id, it, item->path)) < 0)
 		return error;
 
 	if ((error = git_iterator_advance(NULL, it)) < 0 && error != GIT_ITEROVER)
diff --git a/src/libgit2/object.c b/src/libgit2/object.c
index d87d40cf1..b7796f057 100644
--- a/src/libgit2/object.c
+++ b/src/libgit2/object.c
@@ -181,6 +181,7 @@ int git_object_lookup_prefix(
 	git_object *object = NULL;
 	git_odb *odb = NULL;
 	git_odb_object *odb_obj = NULL;
+	size_t oid_hexsize;
 	int error = 0;
 
 	GIT_ASSERT_ARG(repo);
@@ -196,10 +197,12 @@ int git_object_lookup_prefix(
 	if (error < 0)
 		return error;
 
-	if (len > GIT_OID_SHA1_HEXSIZE)
-		len = GIT_OID_SHA1_HEXSIZE;
+	oid_hexsize = git_oid_hexsize(repo->oid_type);
 
-	if (len == GIT_OID_SHA1_HEXSIZE) {
+	if (len > oid_hexsize)
+		len = oid_hexsize;
+
+	if (len == oid_hexsize) {
 		git_cached_obj *cached = NULL;
 
 		/* We want to match the full id : we can first look up in the cache,
@@ -233,8 +236,9 @@ int git_object_lookup_prefix(
 			error = git_odb_read(&odb_obj, odb, id);
 		}
 	} else {
-		git_oid short_oid = GIT_OID_SHA1_ZERO;
+		git_oid short_oid;
 
+		git_oid_clear(&short_oid, repo->oid_type);
 		git_oid__cpy_prefix(&short_oid, id, len);
 
 		/* If len < GIT_OID_SHA1_HEXSIZE (a strict short oid was given), we have
@@ -262,7 +266,8 @@ int git_object_lookup_prefix(
 }
 
 int git_object_lookup(git_object **object_out, git_repository *repo, const git_oid *id, git_object_t type) {
-	return git_object_lookup_prefix(object_out, repo, id, GIT_OID_SHA1_HEXSIZE, type);
+	return git_object_lookup_prefix(object_out,
+		repo, id, git_oid_hexsize(repo->oid_type), type);
 }
 
 void git_object_free(git_object *object)
@@ -503,31 +508,36 @@ cleanup:
 static int git_object__short_id(git_str *out, const git_object *obj)
 {
 	git_repository *repo;
-	int len = GIT_ABBREV_DEFAULT, error;
-	git_oid id = GIT_OID_SHA1_ZERO;
+	git_oid id;
 	git_odb *odb;
+	size_t oid_hexsize;
+	int len = GIT_ABBREV_DEFAULT, error;
 
 	GIT_ASSERT_ARG(out);
 	GIT_ASSERT_ARG(obj);
 
 	repo = git_object_owner(obj);
 
+	git_oid_clear(&id, repo->oid_type);
+	oid_hexsize = git_oid_hexsize(repo->oid_type);
+
 	if ((error = git_repository__configmap_lookup(&len, repo, GIT_CONFIGMAP_ABBREV)) < 0)
 		return error;
 
+	if (len < 0 || (size_t)len > oid_hexsize) {
+		git_error_set(GIT_ERROR_CONFIG, "invalid oid abbreviation setting: '%d'", len);
+		return -1;
+	}
+
 	if ((error = git_repository_odb(&odb, repo)) < 0)
 		return error;
 
-	while (len < GIT_OID_SHA1_HEXSIZE) {
+	while ((size_t)len < oid_hexsize) {
 		/* set up short oid */
 		memcpy(&id.id, &obj->cached.oid.id, (len + 1) / 2);
 		if (len & 1)
 			id.id[len / 2] &= 0xf0;
 
-#ifdef GIT_EXPERIMENTAL_SHA256
-		id.type = GIT_OID_SHA1;
-#endif
-
 		error = git_odb_exists_prefix(NULL, odb, &id, len);
 		if (error != GIT_EAMBIGUOUS)
 			break;
diff --git a/src/libgit2/odb.c b/src/libgit2/odb.c
index 0fc48035a..68872e1a1 100644
--- a/src/libgit2/odb.c
+++ b/src/libgit2/odb.c
@@ -748,7 +748,8 @@ int git_odb__add_default_backends(
 		git_error_set(GIT_ERROR_ODB, "failed to acquire the odb lock");
 		return -1;
 	}
-	if (!db->cgraph && git_commit_graph_new(&db->cgraph, objects_dir, false) < 0) {
+	if (!db->cgraph &&
+	    git_commit_graph_new(&db->cgraph, objects_dir, false, db->options.oid_type) < 0) {
 		git_mutex_unlock(&db->lock);
 		return -1;
 	}
diff --git a/src/libgit2/odb_pack.c b/src/libgit2/odb_pack.c
index 1b1d122b0..fc533ae83 100644
--- a/src/libgit2/odb_pack.c
+++ b/src/libgit2/odb_pack.c
@@ -309,11 +309,17 @@ static int pack_entry_find_prefix(
 {
 	int error;
 	size_t i;
-	git_oid found_full_oid = GIT_OID_SHA1_ZERO;
+	git_oid found_full_oid;
 	bool found = false;
 	struct git_pack_file *last_found = backend->last_found, *p;
 	git_midx_entry midx_entry;
 
+#ifdef GIT_EXPERIMENTAL_SHA256
+	git_oid_clear(&found_full_oid, short_oid->type);
+#else
+	git_oid_clear(&found_full_oid, GIT_OID_SHA1);
+#endif
+
 	if (backend->midx) {
 		error = git_midx_entry_find(&midx_entry, backend->midx, short_oid, len);
 		if (error == GIT_EAMBIGUOUS)
@@ -473,7 +479,9 @@ static int refresh_multi_pack_index(struct pack_backend *backend)
 		}
 	}
 
-	error = git_midx_open(&backend->midx, git_str_cstr(&midx_path));
+	error = git_midx_open(&backend->midx, git_str_cstr(&midx_path),
+		backend->opts.oid_type);
+
 	git_str_dispose(&midx_path);
 	if (error < 0)
 		return error;
@@ -792,7 +800,12 @@ static int pack_backend__writemidx(git_odb_backend *_backend)
 
 	backend = (struct pack_backend *)_backend;
 
-	error = git_midx_writer_new(&w, backend->pack_folder);
+	error = git_midx_writer_new(&w, backend->pack_folder
+#ifdef GIT_EXPERIMENTAL_SHA256
+		, backend->opts.oid_type
+#endif
+		);
+
 	if (error < 0)
 		return error;
 
diff --git a/src/libgit2/pack-objects.c b/src/libgit2/pack-objects.c
index 20a5dfcbd..d6fd60326 100644
--- a/src/libgit2/pack-objects.c
+++ b/src/libgit2/pack-objects.c
@@ -127,6 +127,7 @@ out:
 
 int git_packbuilder_new(git_packbuilder **out, git_repository *repo)
 {
+	git_hash_algorithm_t hash_algorithm;
 	git_packbuilder *pb;
 
 	*out = NULL;
@@ -134,6 +135,11 @@ int git_packbuilder_new(git_packbuilder **out, git_repository *repo)
 	pb = git__calloc(1, sizeof(*pb));
 	GIT_ERROR_CHECK_ALLOC(pb);
 
+	pb->oid_type = repo->oid_type;
+
+	hash_algorithm = git_oid_algorithm(pb->oid_type);
+	GIT_ASSERT(hash_algorithm);
+
 	if (git_oidmap_new(&pb->object_ix) < 0 ||
 	    git_oidmap_new(&pb->walk_objects) < 0 ||
 	    git_pool_init(&pb->object_pool, sizeof(struct walk_object)) < 0)
@@ -142,7 +148,7 @@ int git_packbuilder_new(git_packbuilder **out, git_repository *repo)
 	pb->repo = repo;
 	pb->nr_threads = 1; /* do not spawn any thread by default */
 
-	if (git_hash_ctx_init(&pb->ctx, GIT_HASH_ALGORITHM_SHA1) < 0 ||
+	if (git_hash_ctx_init(&pb->ctx, hash_algorithm) < 0 ||
 		git_zstream_init(&pb->zstream, GIT_ZSTREAM_DEFLATE) < 0 ||
 		git_repository_odb(&pb->odb, repo) < 0 ||
 		packbuilder_config(pb) < 0)
@@ -315,9 +321,11 @@ static int write_object(
 	git_object_t type;
 	unsigned char hdr[10], *zbuf = NULL;
 	void *data = NULL;
-	size_t hdr_len, zbuf_len = COMPRESS_BUFLEN, data_len;
+	size_t hdr_len, zbuf_len = COMPRESS_BUFLEN, data_len, oid_size;
 	int error;
 
+	oid_size = git_oid_size(pb->oid_type);
+
 	/*
 	 * If we have a delta base, let's use the delta to save space.
 	 * Otherwise load the whole object. 'data' ends up pointing to
@@ -347,8 +355,8 @@ static int write_object(
 		goto done;
 
 	if (type == GIT_OBJECT_REF_DELTA) {
-		if ((error = write_cb(po->delta->id.id, GIT_OID_SHA1_SIZE, cb_data)) < 0 ||
-			(error = git_hash_update(&pb->ctx, po->delta->id.id, GIT_OID_SHA1_SIZE)) < 0)
+		if ((error = write_cb(po->delta->id.id, oid_size, cb_data)) < 0 ||
+		    (error = git_hash_update(&pb->ctx, po->delta->id.id, oid_size)) < 0)
 			goto done;
 	}
 
@@ -668,7 +676,7 @@ static int write_pack(git_packbuilder *pb,
 	if ((error = git_hash_final(entry_oid.id, &pb->ctx)) < 0)
 		goto done;
 
-	error = write_cb(entry_oid.id, GIT_OID_SHA1_SIZE, cb_data);
+	error = write_cb(entry_oid.id, git_oid_size(pb->oid_type), cb_data);
 
 done:
 	/* if callback cancelled writing, we must still free delta_data */
diff --git a/src/libgit2/pack-objects.h b/src/libgit2/pack-objects.h
index 2faa3ec7f..c6bc52fdc 100644
--- a/src/libgit2/pack-objects.h
+++ b/src/libgit2/pack-objects.h
@@ -56,6 +56,8 @@ struct git_packbuilder {
 	git_repository *repo; /* associated repository */
 	git_odb *odb; /* associated object database */
 
+	git_oid_t oid_type;
+
 	git_hash_ctx ctx;
 	git_zstream zstream;
 
diff --git a/src/libgit2/parse.c b/src/libgit2/parse.c
index 55d3cb10e..9eb86a3f5 100644
--- a/src/libgit2/parse.c
+++ b/src/libgit2/parse.c
@@ -102,13 +102,16 @@ int git_parse_advance_digit(int64_t *out, git_parse_ctx *ctx, int base)
 	return 0;
 }
 
-int git_parse_advance_oid(git_oid *out, git_parse_ctx *ctx)
+int git_parse_advance_oid(git_oid *out, git_parse_ctx *ctx, git_oid_t oid_type)
 {
-	if (ctx->line_len < GIT_OID_SHA1_HEXSIZE)
+	size_t oid_hexsize = git_oid_hexsize(oid_type);
+	GIT_ASSERT(oid_hexsize);
+
+	if (ctx->line_len < oid_hexsize)
 		return -1;
-	if ((git_oid__fromstrn(out, ctx->line, GIT_OID_SHA1_HEXSIZE, GIT_OID_SHA1)) < 0)
+	if ((git_oid__fromstrn(out, ctx->line, oid_hexsize, oid_type)) < 0)
 		return -1;
-	git_parse_advance_chars(ctx, GIT_OID_SHA1_HEXSIZE);
+	git_parse_advance_chars(ctx, oid_hexsize);
 	return 0;
 }
 
diff --git a/src/libgit2/parse.h b/src/libgit2/parse.h
index 0ecb7c103..beef1de12 100644
--- a/src/libgit2/parse.h
+++ b/src/libgit2/parse.h
@@ -50,7 +50,7 @@ int git_parse_advance_expected(
 int git_parse_advance_ws(git_parse_ctx *ctx);
 int git_parse_advance_nl(git_parse_ctx *ctx);
 int git_parse_advance_digit(int64_t *out, git_parse_ctx *ctx, int base);
-int git_parse_advance_oid(git_oid *out, git_parse_ctx *ctx);
+int git_parse_advance_oid(git_oid *out, git_parse_ctx *ctx, git_oid_t oid_type);
 
 enum GIT_PARSE_PEEK_FLAGS {
 	GIT_PARSE_PEEK_SKIP_WHITESPACE = (1 << 0)
diff --git a/src/libgit2/patch.h b/src/libgit2/patch.h
index 1e1471ed6..86328e886 100644
--- a/src/libgit2/patch.h
+++ b/src/libgit2/patch.h
@@ -59,9 +59,15 @@ typedef struct {
 	 * This prefix will be removed when looking for files.  The default is 1.
 	 */
 	uint32_t prefix_len;
+
+	/**
+	 * The type of object IDs in the patch file. The default is
+	 * `GIT_OID_DEFAULT`.
+	 */
+	git_oid_t oid_type;
 } git_patch_options;
 
-#define GIT_PATCH_OPTIONS_INIT { 1 }
+#define GIT_PATCH_OPTIONS_INIT { 1, GIT_OID_DEFAULT }
 
 extern int git_patch__to_buf(git_str *out, git_patch *patch);
 extern void git_patch_free(git_patch *patch);
diff --git a/src/libgit2/patch_generate.c b/src/libgit2/patch_generate.c
index bc598fea8..079bc53ae 100644
--- a/src/libgit2/patch_generate.c
+++ b/src/libgit2/patch_generate.c
@@ -81,7 +81,8 @@ static void patch_generated_init_common(git_patch_generated *patch)
 
 static int patch_generated_normalize_options(
 	git_diff_options *out,
-	const git_diff_options *opts)
+	const git_diff_options *opts,
+	git_repository *repo)
 {
 	if (opts) {
 		GIT_ERROR_CHECK_VERSION(opts, GIT_DIFF_OPTIONS_VERSION, "git_diff_options");
@@ -91,6 +92,23 @@ static int patch_generated_normalize_options(
 		memcpy(out, &default_opts, sizeof(git_diff_options));
 	}
 
+	if (repo && opts && opts->oid_type && repo->oid_type != opts->oid_type) {
+		/*
+		 * This limitation feels unnecessary - we should consider
+		 * allowing users to generate diffs with a different object
+		 * ID format than the repository.
+		 */
+		git_error_set(GIT_ERROR_INVALID,
+			"specified object ID type does not match repository object ID type");
+		return -1;
+	} else if (repo) {
+		out->oid_type = repo->oid_type;
+	} else if (opts && opts->oid_type) {
+		out->oid_type = opts->oid_type;
+	} else {
+		out->oid_type = GIT_OID_DEFAULT;
+	}
+
 	out->old_prefix = opts && opts->old_prefix ?
 		git__strdup(opts->old_prefix) :
 		git__strdup(DIFF_OLD_PREFIX_DEFAULT);
@@ -118,7 +136,7 @@ static int patch_generated_init(
 	patch->delta_index = delta_index;
 
 	if ((error = patch_generated_normalize_options(
-			&patch->base.diff_opts, &diff->opts)) < 0 ||
+			&patch->base.diff_opts, &diff->opts, diff->repo)) < 0 ||
 		(error = git_diff_file_content__init_from_diff(
 			&patch->ofile, diff, patch->base.delta, true)) < 0 ||
 		(error = git_diff_file_content__init_from_diff(
@@ -449,7 +467,7 @@ static int patch_generated_from_sources(
 	git_xdiff_output *xo,
 	git_diff_file_content_src *oldsrc,
 	git_diff_file_content_src *newsrc,
-	const git_diff_options *opts)
+	const git_diff_options *given_opts)
 {
 	int error = 0;
 	git_repository *repo =
@@ -457,11 +475,12 @@ static int patch_generated_from_sources(
 		newsrc->blob ? git_blob_owner(newsrc->blob) : NULL;
 	git_diff_file *lfile = &pd->delta.old_file, *rfile = &pd->delta.new_file;
 	git_diff_file_content *ldata = &pd->patch.ofile, *rdata = &pd->patch.nfile;
+	git_diff_options *opts = &pd->patch.base.diff_opts;
 
-	if ((error = patch_generated_normalize_options(&pd->patch.base.diff_opts, opts)) < 0)
+	if ((error = patch_generated_normalize_options(opts, given_opts, repo)) < 0)
 		return error;
 
-	if (opts && (opts->flags & GIT_DIFF_REVERSE) != 0) {
+	if ((opts->flags & GIT_DIFF_REVERSE) != 0) {
 		void *tmp = lfile; lfile = rfile; rfile = tmp;
 		tmp = ldata; ldata = rdata; rdata = tmp;
 	}
diff --git a/src/libgit2/patch_parse.c b/src/libgit2/patch_parse.c
index ffdb99231..c06915537 100644
--- a/src/libgit2/patch_parse.c
+++ b/src/libgit2/patch_parse.c
@@ -166,15 +166,19 @@ static int parse_header_oid(
 	uint16_t *oid_len,
 	git_patch_parse_ctx *ctx)
 {
-	size_t len;
+	size_t hexsize, len;
+
+	hexsize = git_oid_hexsize(ctx->opts.oid_type);
 
-	for (len = 0; len < ctx->parse_ctx.line_len && len < GIT_OID_SHA1_HEXSIZE; len++) {
+	for (len = 0;
+	     len < ctx->parse_ctx.line_len && len < hexsize;
+	     len++) {
 		if (!git__isxdigit(ctx->parse_ctx.line[len]))
 			break;
 	}
 
-	if (len < GIT_OID_MINPREFIXLEN || len > GIT_OID_SHA1_HEXSIZE ||
-		git_oid__fromstrn(oid, ctx->parse_ctx.line, len, GIT_OID_SHA1) < 0)
+	if (len < GIT_OID_MINPREFIXLEN || len > hexsize ||
+		git_oid__fromstrn(oid, ctx->parse_ctx.line, len, ctx->opts.oid_type) < 0)
 		return git_parse_err("invalid hex formatted object id at line %"PRIuZ,
 			ctx->parse_ctx.line_num);
 
@@ -1065,12 +1069,14 @@ static int check_patch(git_patch_parsed *patch)
 		return git_parse_err("patch with no hunks");
 
 	if (delta->status == GIT_DELTA_ADDED) {
-		git_oid_clear(&delta->old_file.id, GIT_OID_SHA1);
+		git_oid_clear(&delta->old_file.id,
+			patch->base.diff_opts.oid_type);
 		delta->old_file.id_abbrev = 0;
 	}
 
 	if (delta->status == GIT_DELTA_DELETED) {
-		git_oid_clear(&delta->new_file.id, GIT_OID_SHA1);
+		git_oid_clear(&delta->new_file.id,
+			patch->base.diff_opts.oid_type);
 		delta->new_file.id_abbrev = 0;
 	}
 
@@ -1187,11 +1193,13 @@ int git_patch_parse(
 	patch->base.delta->status = GIT_DELTA_MODIFIED;
 	patch->base.delta->nfiles = 2;
 
+	patch->base.diff_opts.oid_type = ctx->opts.oid_type;
+
 	start = ctx->parse_ctx.remain_len;
 
 	if ((error = parse_patch_header(patch, ctx)) < 0 ||
-		(error = parse_patch_body(patch, ctx)) < 0 ||
-		(error = check_patch(patch)) < 0)
+	    (error = parse_patch_body(patch, ctx)) < 0 ||
+	    (error = check_patch(patch)) < 0)
 		goto done;
 
 	used = start - ctx->parse_ctx.remain_len;
diff --git a/src/libgit2/push.c b/src/libgit2/push.c
index e25681870..8b47abc24 100644
--- a/src/libgit2/push.c
+++ b/src/libgit2/push.c
@@ -118,8 +118,8 @@ static int parse_refspec(git_push *push, push_spec **spec, const char *str)
 	s = git__calloc(1, sizeof(*s));
 	GIT_ERROR_CHECK_ALLOC(s);
 
-	git_oid_clear(&s->loid, GIT_OID_SHA1);
-	git_oid_clear(&s->roid, GIT_OID_SHA1);
+	git_oid_clear(&s->loid, push->repo->oid_type);
+	git_oid_clear(&s->roid, push->repo->oid_type);
 
 	if (git_refspec__parse(&s->refspec, str, false) < 0) {
 		git_error_set(GIT_ERROR_INVALID, "invalid refspec %s", str);
diff --git a/src/libgit2/reader.c b/src/libgit2/reader.c
index be29bb41c..df2b2807f 100644
--- a/src/libgit2/reader.c
+++ b/src/libgit2/reader.c
@@ -125,7 +125,7 @@ static int workdir_reader_read(
 		goto done;
 
 	if (out_id || reader->index) {
-		if ((error = git_odb__hash(&id, out->ptr, out->size, GIT_OBJECT_BLOB, GIT_OID_SHA1)) < 0)
+		if ((error = git_odb__hash(&id, out->ptr, out->size, GIT_OBJECT_BLOB, reader->repo->oid_type)) < 0)
 			goto done;
 	}
 
diff --git a/src/libgit2/rebase.c b/src/libgit2/rebase.c
index 1970d5ddc..77e442e98 100644
--- a/src/libgit2/rebase.c
+++ b/src/libgit2/rebase.c
@@ -65,6 +65,9 @@ struct git_rebase {
 	git_rebase_t type;
 	char *state_path;
 
+	/* Temporary buffer for paths within the state path. */
+	git_str state_filename;
+
 	unsigned int head_detached:1,
 	             inmemory:1,
 	             quiet:1,
@@ -134,33 +137,42 @@ done:
 
 GIT_INLINE(int) rebase_readfile(
 	git_str *out,
-	git_str *state_path,
+	git_rebase *rebase,
 	const char *filename)
 {
-	size_t state_path_len = state_path->size;
+	/*
+	 * `rebase->state_filename` is a temporary buffer to avoid
+	 * unnecessary allocations and copies of `rebase->state_path`.
+	 * At the start and end of this function it always contains the
+	 * contents of `rebase->state_path` itself.
+	 */
+	size_t state_path_len = rebase->state_filename.size;
 	int error;
 
 	git_str_clear(out);
 
-	if ((error = git_str_joinpath(state_path, state_path->ptr, filename)) < 0 ||
-		(error = git_futils_readbuffer(out, state_path->ptr)) < 0)
+	if ((error = git_str_joinpath(&rebase->state_filename, rebase->state_filename.ptr, filename)) < 0 ||
+	    (error = git_futils_readbuffer(out, rebase->state_filename.ptr)) < 0)
 		goto done;
 
 	git_str_rtrim(out);
 
 done:
-	git_str_truncate(state_path, state_path_len);
+	git_str_truncate(&rebase->state_filename, state_path_len);
 	return error;
 }
 
 GIT_INLINE(int) rebase_readint(
-	size_t *out, git_str *asc_out, git_str *state_path, const char *filename)
+	size_t *out,
+	git_str *asc_out,
+	git_rebase *rebase,
+	const char *filename)
 {
 	int32_t num;
 	const char *eol;
 	int error = 0;
 
-	if ((error = rebase_readfile(asc_out, state_path, filename)) < 0)
+	if ((error = rebase_readfile(asc_out, rebase, filename)) < 0)
 		return error;
 
 	if (git__strntol32(&num, asc_out->ptr, asc_out->size, &eol, 10) < 0 || num < 0 || *eol) {
@@ -174,15 +186,18 @@ GIT_INLINE(int) rebase_readint(
 }
 
 GIT_INLINE(int) rebase_readoid(
-	git_oid *out, git_str *str_out, git_str *state_path, const char *filename)
+	git_oid *out,
+	git_str *str_out,
+	git_rebase *rebase,
+	const char *filename)
 {
 	int error;
 
-	if ((error = rebase_readfile(str_out, state_path, filename)) < 0)
+	if ((error = rebase_readfile(str_out, rebase, filename)) < 0)
 		return error;
 
-	if (str_out->size != GIT_OID_SHA1_HEXSIZE ||
-	    git_oid__fromstr(out, str_out->ptr, GIT_OID_SHA1) < 0) {
+	if (str_out->size != git_oid_hexsize(rebase->repo->oid_type) ||
+	    git_oid__fromstr(out, str_out->ptr, rebase->repo->oid_type) < 0) {
 		git_error_set(GIT_ERROR_REBASE, "the file '%s' contains an invalid object ID", filename);
 		return -1;
 	}
@@ -213,17 +228,14 @@ static git_rebase_operation *rebase_operation_alloc(
 
 static int rebase_open_merge(git_rebase *rebase)
 {
-	git_str state_path = GIT_STR_INIT, buf = GIT_STR_INIT, cmt = GIT_STR_INIT;
+	git_str buf = GIT_STR_INIT, cmt = GIT_STR_INIT;
 	git_oid id;
 	git_rebase_operation *operation;
 	size_t i, msgnum = 0, end;
 	int error;
 
-	if ((error = git_str_puts(&state_path, rebase->state_path)) < 0)
-		goto done;
-
 	/* Read 'msgnum' if it exists (otherwise, let msgnum = 0) */
-	if ((error = rebase_readint(&msgnum, &buf, &state_path, MSGNUM_FILE)) < 0 &&
+	if ((error = rebase_readint(&msgnum, &buf, rebase, MSGNUM_FILE)) < 0 &&
 		error != GIT_ENOTFOUND)
 		goto done;
 
@@ -233,11 +245,11 @@ static int rebase_open_merge(git_rebase *rebase)
 	}
 
 	/* Read 'end' */
-	if ((error = rebase_readint(&end, &buf, &state_path, END_FILE)) < 0)
+	if ((error = rebase_readint(&end, &buf, rebase, END_FILE)) < 0)
 		goto done;
 
 	/* Read 'current' if it exists */
-	if ((error = rebase_readoid(&id, &buf, &state_path, CURRENT_FILE)) < 0 &&
+	if ((error = rebase_readoid(&id, &buf, rebase, CURRENT_FILE)) < 0 &&
 		error != GIT_ENOTFOUND)
 		goto done;
 
@@ -249,7 +261,7 @@ static int rebase_open_merge(git_rebase *rebase)
 		git_str_clear(&cmt);
 
 		if ((error = git_str_printf(&cmt, "cmt.%" PRIuZ, (i+1))) < 0 ||
-			(error = rebase_readoid(&id, &buf, &state_path, cmt.ptr)) < 0)
+			(error = rebase_readoid(&id, &buf, rebase, cmt.ptr)) < 0)
 			goto done;
 
 		operation = rebase_operation_alloc(rebase, GIT_REBASE_OPERATION_PICK, &id, NULL);
@@ -257,14 +269,13 @@ static int rebase_open_merge(git_rebase *rebase)
 	}
 
 	/* Read 'onto_name' */
-	if ((error = rebase_readfile(&buf, &state_path, ONTO_NAME_FILE)) < 0)
+	if ((error = rebase_readfile(&buf, rebase, ONTO_NAME_FILE)) < 0)
 		goto done;
 
 	rebase->onto_name = git_str_detach(&buf);
 
 done:
 	git_str_dispose(&cmt);
-	git_str_dispose(&state_path);
 	git_str_dispose(&buf);
 
 	return error;
@@ -308,9 +319,9 @@ int git_rebase_open(
 	const git_rebase_options *given_opts)
 {
 	git_rebase *rebase;
-	git_str path = GIT_STR_INIT, orig_head_name = GIT_STR_INIT,
-		orig_head_id = GIT_STR_INIT, onto_id = GIT_STR_INIT;
-	size_t state_path_len;
+	git_str orig_head_name = GIT_STR_INIT,
+		orig_head_id = GIT_STR_INIT,
+		onto_id = GIT_STR_INIT;
 	int error;
 
 	GIT_ASSERT_ARG(repo);
@@ -332,13 +343,10 @@ int git_rebase_open(
 		goto done;
 	}
 
-	if ((error = git_str_puts(&path, rebase->state_path)) < 0)
+	if ((error = git_str_puts(&rebase->state_filename, rebase->state_path)) < 0)
 		goto done;
 
-	state_path_len = git_str_len(&path);
-
-	if ((error = git_str_joinpath(&path, path.ptr, HEAD_NAME_FILE)) < 0 ||
-		(error = git_futils_readbuffer(&orig_head_name, path.ptr)) < 0)
+	if ((error = rebase_readfile(&orig_head_name, rebase, HEAD_NAME_FILE)) < 0)
 		goto done;
 
 	git_str_rtrim(&orig_head_name);
@@ -346,36 +354,16 @@ int git_rebase_open(
 	if (strcmp(ORIG_DETACHED_HEAD, orig_head_name.ptr) == 0)
 		rebase->head_detached = 1;
 
-	git_str_truncate(&path, state_path_len);
-
-	if ((error = git_str_joinpath(&path, path.ptr, ORIG_HEAD_FILE)) < 0)
-		goto done;
-
-	if (!git_fs_path_isfile(path.ptr)) {
+	if ((error = rebase_readoid(&rebase->orig_head_id, &orig_head_id, rebase, ORIG_HEAD_FILE)) < 0) {
 		/* Previous versions of git.git used 'head' here; support that. */
-		git_str_truncate(&path, state_path_len);
+		if (error == GIT_ENOTFOUND)
+			error = rebase_readoid(&rebase->orig_head_id, &orig_head_id, rebase, HEAD_FILE);
 
-		if ((error = git_str_joinpath(&path, path.ptr, HEAD_FILE)) < 0)
+		if (error < 0)
 			goto done;
 	}
 
-	if ((error = git_futils_readbuffer(&orig_head_id, path.ptr)) < 0)
-		goto done;
-
-	git_str_rtrim(&orig_head_id);
-
-	if ((error = git_oid__fromstr(&rebase->orig_head_id, orig_head_id.ptr, GIT_OID_SHA1)) < 0)
-		goto done;
-
-	git_str_truncate(&path, state_path_len);
-
-	if ((error = git_str_joinpath(&path, path.ptr, ONTO_FILE)) < 0 ||
-		(error = git_futils_readbuffer(&onto_id, path.ptr)) < 0)
-		goto done;
-
-	git_str_rtrim(&onto_id);
-
-	if ((error = git_oid__fromstr(&rebase->onto_id, onto_id.ptr, GIT_OID_SHA1)) < 0)
+	if ((error = rebase_readoid(&rebase->onto_id, &onto_id, rebase, ONTO_FILE)) < 0)
 		goto done;
 
 	if (!rebase->head_detached)
@@ -403,7 +391,6 @@ done:
 	else
 		git_rebase_free(rebase);
 
-	git_str_dispose(&path);
 	git_str_dispose(&orig_head_name);
 	git_str_dispose(&orig_head_id);
 	git_str_dispose(&onto_id);
@@ -453,13 +440,13 @@ static const char *rebase_onto_name(const git_annotated_commit *onto)
 static int rebase_setupfiles_merge(git_rebase *rebase)
 {
 	git_str commit_filename = GIT_STR_INIT;
-	char id_str[GIT_OID_SHA1_HEXSIZE];
+	char id_str[GIT_OID_MAX_HEXSIZE + 1];
 	git_rebase_operation *operation;
 	size_t i;
 	int error = 0;
 
 	if ((error = rebase_setupfile(rebase, END_FILE, 0, "%" PRIuZ "\n", git_array_size(rebase->operations))) < 0 ||
-		(error = rebase_setupfile(rebase, ONTO_NAME_FILE, 0, "%s\n", rebase->onto_name)) < 0)
+	    (error = rebase_setupfile(rebase, ONTO_NAME_FILE, 0, "%s\n", rebase->onto_name)) < 0)
 		goto done;
 
 	for (i = 0; i < git_array_size(rebase->operations); i++) {
@@ -468,10 +455,9 @@ static int rebase_setupfiles_merge(git_rebase *rebase)
 		git_str_clear(&commit_filename);
 		git_str_printf(&commit_filename, CMT_FILE_FMT, i+1);
 
-		git_oid_fmt(id_str, &operation->id);
+		git_oid_tostr(id_str, GIT_OID_MAX_HEXSIZE + 1, &operation->id);
 
-		if ((error = rebase_setupfile(rebase, commit_filename.ptr, 0,
-				"%.*s\n", GIT_OID_SHA1_HEXSIZE, id_str)) < 0)
+		if ((error = rebase_setupfile(rebase, commit_filename.ptr, 0, "%s\n", id_str)) < 0)
 			goto done;
 	}
 
@@ -482,11 +468,11 @@ done:
 
 static int rebase_setupfiles(git_rebase *rebase)
 {
-	char onto[GIT_OID_SHA1_HEXSIZE], orig_head[GIT_OID_SHA1_HEXSIZE];
+	char onto[GIT_OID_MAX_HEXSIZE + 1], orig_head[GIT_OID_MAX_HEXSIZE + 1];
 	const char *orig_head_name;
 
-	git_oid_fmt(onto, &rebase->onto_id);
-	git_oid_fmt(orig_head, &rebase->orig_head_id);
+	git_oid_tostr(onto, GIT_OID_MAX_HEXSIZE + 1, &rebase->onto_id);
+	git_oid_tostr(orig_head, GIT_OID_MAX_HEXSIZE + 1, &rebase->orig_head_id);
 
 	if (p_mkdir(rebase->state_path, REBASE_DIR_MODE) < 0) {
 		git_error_set(GIT_ERROR_OS, "failed to create rebase directory '%s'", rebase->state_path);
@@ -498,8 +484,8 @@ static int rebase_setupfiles(git_rebase *rebase)
 
 	if (git_repository__set_orig_head(rebase->repo, &rebase->orig_head_id) < 0 ||
 		rebase_setupfile(rebase, HEAD_NAME_FILE, 0, "%s\n", orig_head_name) < 0 ||
-		rebase_setupfile(rebase, ONTO_FILE, 0, "%.*s\n", GIT_OID_SHA1_HEXSIZE, onto) < 0 ||
-		rebase_setupfile(rebase, ORIG_HEAD_FILE, 0, "%.*s\n", GIT_OID_SHA1_HEXSIZE, orig_head) < 0 ||
+		rebase_setupfile(rebase, ONTO_FILE, 0, "%s\n", onto) < 0 ||
+		rebase_setupfile(rebase, ORIG_HEAD_FILE, 0, "%s\n", orig_head) < 0 ||
 		rebase_setupfile(rebase, QUIET_FILE, 0, rebase->quiet ? "t\n" : "\n") < 0)
 		return -1;
 
@@ -644,7 +630,8 @@ static int rebase_init_merge(
 
 	GIT_UNUSED(upstream);
 
-	if ((error = git_str_joinpath(&state_path, repo->gitdir, REBASE_MERGE_DIR)) < 0)
+	if ((error = git_str_joinpath(&state_path, repo->gitdir, REBASE_MERGE_DIR)) < 0 ||
+	    (error = git_str_put(&rebase->state_filename, state_path.ptr, state_path.size)) < 0)
 		goto done;
 
 	rebase->state_path = git_str_detach(&state_path);
@@ -814,7 +801,7 @@ static int rebase_next_merge(
 	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
 	git_rebase_operation *operation;
 	git_checkout_options checkout_opts;
-	char current_idstr[GIT_OID_SHA1_HEXSIZE];
+	char current_idstr[GIT_OID_MAX_HEXSIZE + 1];
 	unsigned int parent_count;
 	int error;
 
@@ -837,13 +824,13 @@ static int rebase_next_merge(
 			goto done;
 	}
 
-	git_oid_fmt(current_idstr, &operation->id);
+	git_oid_tostr(current_idstr, GIT_OID_MAX_HEXSIZE + 1, &operation->id);
 
 	normalize_checkout_options_for_apply(&checkout_opts, rebase, current_commit);
 
 	if ((error = git_indexwriter_init_for_operation(&indexwriter, rebase->repo, &checkout_opts.checkout_strategy)) < 0 ||
 		(error = rebase_setupfile(rebase, MSGNUM_FILE, 0, "%" PRIuZ "\n", rebase->current+1)) < 0 ||
-		(error = rebase_setupfile(rebase, CURRENT_FILE, 0, "%.*s\n", GIT_OID_SHA1_HEXSIZE, current_idstr)) < 0 ||
+		(error = rebase_setupfile(rebase, CURRENT_FILE, 0, "%s\n", current_idstr)) < 0 ||
 		(error = git_merge_trees(&index, rebase->repo, parent_tree, head_tree, current_tree, &rebase->options.merge_options)) < 0 ||
 		(error = git_merge__check_result(rebase->repo, index)) < 0 ||
 		(error = git_checkout_index(rebase->repo, index, &checkout_opts)) < 0 ||
@@ -1103,7 +1090,7 @@ static int rebase_commit_merge(
 	git_reference *head = NULL;
 	git_commit *head_commit = NULL, *commit = NULL;
 	git_index *index = NULL;
-	char old_idstr[GIT_OID_SHA1_HEXSIZE], new_idstr[GIT_OID_SHA1_HEXSIZE];
+	char old_idstr[GIT_OID_MAX_HEXSIZE + 1], new_idstr[GIT_OID_MAX_HEXSIZE + 1];
 	int error;
 
 	operation = git_array_get(rebase->operations, rebase->current);
@@ -1119,11 +1106,11 @@ static int rebase_commit_merge(
 			rebase->repo, NULL, "HEAD", git_commit_id(commit), "rebase")) < 0)
 		goto done;
 
-	git_oid_fmt(old_idstr, &operation->id);
-	git_oid_fmt(new_idstr, git_commit_id(commit));
+	git_oid_tostr(old_idstr, GIT_OID_MAX_HEXSIZE + 1, &operation->id);
+	git_oid_tostr(new_idstr, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit));
 
 	if ((error = rebase_setupfile(rebase, REWRITTEN_FILE, O_CREAT|O_WRONLY|O_APPEND,
-		"%.*s %.*s\n", GIT_OID_SHA1_HEXSIZE, old_idstr, GIT_OID_SHA1_HEXSIZE, new_idstr)) < 0)
+			"%s %s\n", old_idstr, new_idstr)) < 0)
 		goto done;
 
 	git_oid_cpy(commit_id, git_commit_id(commit));
@@ -1306,7 +1293,9 @@ static int rebase_copy_notes(
 	git_rebase *rebase,
 	const git_signature *committer)
 {
-	git_str path = GIT_STR_INIT, rewritten = GIT_STR_INIT, notes_ref = GIT_STR_INIT;
+	git_str path = GIT_STR_INIT,
+	        rewritten = GIT_STR_INIT,
+	        notes_ref = GIT_STR_INIT;
 	char *pair_list, *fromstr, *tostr, *end;
 	git_oid from, to;
 	unsigned int linenum = 1;
@@ -1342,10 +1331,10 @@ static int rebase_copy_notes(
 		tostr = end+1;
 		*end = '\0';
 
-		if (strlen(fromstr) != GIT_OID_SHA1_HEXSIZE ||
-			strlen(tostr) != GIT_OID_SHA1_HEXSIZE ||
-			git_oid__fromstr(&from, fromstr, GIT_OID_SHA1) < 0 ||
-			git_oid__fromstr(&to, tostr, GIT_OID_SHA1) < 0)
+		if (strlen(fromstr) != git_oid_hexsize(rebase->repo->oid_type) ||
+		    strlen(tostr) != git_oid_hexsize(rebase->repo->oid_type) ||
+		    git_oid__fromstr(&from, fromstr, rebase->repo->oid_type) < 0 ||
+		    git_oid__fromstr(&to, tostr, rebase->repo->oid_type) < 0)
 			goto on_error;
 
 		if ((error = rebase_copy_note(rebase, notes_ref.ptr, &from, &to, committer)) < 0)
@@ -1373,17 +1362,15 @@ static int return_to_orig_head(git_rebase *rebase)
 	git_reference *terminal_ref = NULL, *branch_ref = NULL, *head_ref = NULL;
 	git_commit *terminal_commit = NULL;
 	git_str branch_msg = GIT_STR_INIT, head_msg = GIT_STR_INIT;
-	char onto[GIT_OID_SHA1_HEXSIZE];
+	char onto[GIT_OID_MAX_HEXSIZE + 1];
 	int error = 0;
 
-	git_oid_fmt(onto, &rebase->onto_id);
+	git_oid_tostr(onto, GIT_OID_MAX_HEXSIZE + 1, &rebase->onto_id);
 
 	if ((error = git_str_printf(&branch_msg,
-			"rebase finished: %s onto %.*s",
-			rebase->orig_head_name, GIT_OID_SHA1_HEXSIZE, onto)) == 0 &&
+			"rebase finished: %s onto %s", rebase->orig_head_name, onto)) == 0 &&
 		(error = git_str_printf(&head_msg,
-			"rebase finished: returning to %s",
-			rebase->orig_head_name)) == 0 &&
+			"rebase finished: returning to %s", rebase->orig_head_name)) == 0 &&
 		(error = git_repository_head(&terminal_ref, rebase->repo)) == 0 &&
 		(error = git_reference_peel((git_object **)&terminal_commit,
 			terminal_ref, GIT_OBJECT_COMMIT)) == 0 &&
@@ -1475,6 +1462,7 @@ void git_rebase_free(git_rebase *rebase)
 	git__free(rebase->onto_name);
 	git__free(rebase->orig_head_name);
 	git__free(rebase->state_path);
+	git_str_dispose(&rebase->state_filename);
 	git_array_clear(rebase->operations);
 	git__free((char *)rebase->options.rewrite_notes_ref);
 	git__free(rebase);
diff --git a/src/libgit2/refdb_fs.c b/src/libgit2/refdb_fs.c
index 9ce1a9608..c5b292ae3 100644
--- a/src/libgit2/refdb_fs.c
+++ b/src/libgit2/refdb_fs.c
@@ -805,7 +805,9 @@ static void refdb_fs_backend__iterator_free(git_reference_iterator *_iter)
 	git__free(iter);
 }
 
-static int iter_load_loose_paths(refdb_fs_backend *backend, refdb_fs_iter *iter)
+static int iter_load_loose_paths(
+	refdb_fs_backend *backend,
+	refdb_fs_iter *iter)
 {
 	int error = 0;
 	git_str path = GIT_STR_INIT;
@@ -819,6 +821,7 @@ static int iter_load_loose_paths(refdb_fs_backend *backend, refdb_fs_iter *iter)
 		return 0;
 
 	fsit_opts.flags = backend->iterator_flags;
+	fsit_opts.oid_type = backend->oid_type;
 
 	if (iter->glob) {
 		const char *last_sep = NULL;
@@ -1949,9 +1952,9 @@ static int reflog_parse(git_reflog *log, const char *buf, size_t buf_size)
 		entry->committer = git__calloc(1, sizeof(*entry->committer));
 		GIT_ERROR_CHECK_ALLOC(entry->committer);
 
-		if (git_parse_advance_oid(&entry->oid_old, &parser) < 0 ||
+		if (git_parse_advance_oid(&entry->oid_old, &parser, log->oid_type) < 0 ||
 		    git_parse_advance_expected(&parser, " ", 1) < 0 ||
-		    git_parse_advance_oid(&entry->oid_cur, &parser) < 0)
+		    git_parse_advance_oid(&entry->oid_cur, &parser, log->oid_type) < 0)
 			goto next;
 
 		sig = parser.line;
diff --git a/src/libgit2/refs.c b/src/libgit2/refs.c
index 8e4abaccc..5e2fe97e7 100644
--- a/src/libgit2/refs.c
+++ b/src/libgit2/refs.c
@@ -72,6 +72,7 @@ git_reference *git_reference__alloc(
 	const git_oid *oid,
 	const git_oid *peel)
 {
+	git_oid_t oid_type;
 	git_reference *ref;
 
 	GIT_ASSERT_ARG_WITH_RETVAL(name, NULL);
@@ -84,10 +85,16 @@ git_reference *git_reference__alloc(
 	ref->type = GIT_REFERENCE_DIRECT;
 	git_oid_cpy(&ref->target.oid, oid);
 
+#ifdef GIT_EXPERIMENTAL_SHA256
+	oid_type = oid->type;
+#else
+	oid_type = GIT_OID_SHA1;
+#endif
+
 	if (peel != NULL)
 		git_oid_cpy(&ref->peel, peel);
 	else
-		git_oid_clear(&ref->peel, GIT_OID_SHA1);
+		git_oid_clear(&ref->peel, oid_type);
 
 	return ref;
 }
diff --git a/src/libgit2/remote.c b/src/libgit2/remote.c
index c3e2a324d..c1dccbe32 100644
--- a/src/libgit2/remote.c
+++ b/src/libgit2/remote.c
@@ -1631,7 +1631,10 @@ int git_remote_prune(git_remote *remote, const git_remote_callbacks *callbacks)
 	const git_refspec *spec;
 	const char *refname;
 	int error;
-	git_oid zero_id = GIT_OID_SHA1_ZERO;
+	git_oid zero_id;
+
+	GIT_ASSERT(remote && remote->repo);
+	git_oid_clear(&zero_id, remote->repo->oid_type);
 
 	if (callbacks)
 		GIT_ERROR_CHECK_VERSION(callbacks, GIT_REMOTE_CALLBACKS_VERSION, "git_remote_callbacks");
@@ -1733,9 +1736,12 @@ static int update_ref(
 	const git_remote_callbacks *callbacks)
 {
 	git_reference *ref;
-	git_oid old_id = GIT_OID_SHA1_ZERO;
+	git_oid old_id;
 	int error;
 
+	GIT_ASSERT(remote && remote->repo);
+	git_oid_clear(&old_id, remote->repo->oid_type);
+
 	error = git_reference_name_to_id(&old_id, remote->repo, ref_name);
 
 	if (error < 0 && error != GIT_ENOTFOUND)
@@ -1779,6 +1785,8 @@ static int update_one_tip(
 	int valid;
 	int error;
 
+	GIT_ASSERT(remote && remote->repo);
+
 	if ((error = git_repository_odb__weakptr(&odb, remote->repo)) < 0)
 		goto done;
 
@@ -1839,7 +1847,7 @@ static int update_one_tip(
 	}
 
 	if (error == GIT_ENOTFOUND) {
-		git_oid_clear(&old, GIT_OID_SHA1);
+		git_oid_clear(&old, remote->repo->oid_type);
 		error = 0;
 
 		if (autotag && (error = git_vector_insert(update_heads, head)) < 0)
@@ -1885,7 +1893,7 @@ static int update_tips_for_spec(
 	int error = 0;
 	size_t i;
 
-	GIT_ASSERT_ARG(remote);
+	GIT_ASSERT_ARG(remote && remote->repo);
 
 	if (git_refspec__parse(&tagspec, GIT_REFSPEC_TAGS, true) < 0)
 		return -1;
@@ -1901,10 +1909,10 @@ static int update_tips_for_spec(
 	}
 
 	/* Handle specified oid sources */
-	if (git_oid__is_hexstr(spec->src, GIT_OID_SHA1)) {
+	if (git_oid__is_hexstr(spec->src, remote->repo->oid_type)) {
 		git_oid id;
 
-		if ((error = git_oid__fromstr(&id, spec->src, GIT_OID_SHA1)) < 0)
+		if ((error = git_oid__fromstr(&id, spec->src, remote->repo->oid_type)) < 0)
 			goto on_error;
 
 		if (spec->dst &&
diff --git a/src/libgit2/repository.c b/src/libgit2/repository.c
index 3f57fb23c..804e436ab 100644
--- a/src/libgit2/repository.c
+++ b/src/libgit2/repository.c
@@ -934,7 +934,7 @@ static int obtain_config_and_set_oid_type(
 		if ((error = load_objectformat(repo, config)) < 0)
 			goto out;
 	} else {
-		repo->oid_type = GIT_OID_SHA1;
+		repo->oid_type = GIT_OID_DEFAULT;
 	}
 
 out:
@@ -1547,7 +1547,8 @@ int git_repository_index__weakptr(git_index **out, git_repository *repo)
 		if ((error = repository_index_path(&index_path, repo)) < 0)
 			return error;
 
-		error = git_index_open(&index, index_path.ptr);
+		error = git_index__open(&index, index_path.ptr, repo->oid_type);
+
 		if (!error) {
 			GIT_REFCOUNT_OWN(index, repo);
 
@@ -1803,7 +1804,7 @@ static int load_objectformat(git_repository *repo, git_config *config)
 
 	if ((error = git_config_get_entry(&entry, config, "extensions.objectformat")) < 0) {
 		if (error == GIT_ENOTFOUND) {
-			repo->oid_type = GIT_OID_SHA1;
+			repo->oid_type = GIT_OID_DEFAULT;
 			git_error_clear();
 			error = 0;
 		}
@@ -2239,7 +2240,7 @@ static int repo_init_config(
 		SET_REPO_CONFIG(bool, "receive.denyNonFastforwards", true);
 	}
 
-	if (oid_type != GIT_OID_SHA1) {
+	if (oid_type != GIT_OID_DEFAULT) {
 		SET_REPO_CONFIG(int32, "core.repositoryformatversion", 1);
 		SET_REPO_CONFIG(string, "extensions.objectformat", git_oid_type_name(oid_type));
 	}
diff --git a/src/libgit2/reset.c b/src/libgit2/reset.c
index 9574819cb..605c4afd5 100644
--- a/src/libgit2/reset.c
+++ b/src/libgit2/reset.c
@@ -188,9 +188,9 @@ int git_reset(
 	git_reset_t reset_type,
 	const git_checkout_options *checkout_opts)
 {
-	char to[GIT_OID_SHA1_HEXSIZE + 1];
+	char to[GIT_OID_MAX_HEXSIZE + 1];
 
-	git_oid_tostr(to, GIT_OID_SHA1_HEXSIZE + 1, git_object_id(target));
+	git_oid_tostr(to, GIT_OID_MAX_HEXSIZE + 1, git_object_id(target));
 	return reset(repo, target, to, reset_type, checkout_opts);
 }
 
diff --git a/src/libgit2/revert.c b/src/libgit2/revert.c
index 1106dfe2f..4a31ad40a 100644
--- a/src/libgit2/revert.c
+++ b/src/libgit2/revert.c
@@ -107,12 +107,10 @@ static int revert_state_cleanup(git_repository *repo)
 
 static int revert_seterr(git_commit *commit, const char *fmt)
 {
-	char commit_oidstr[GIT_OID_SHA1_HEXSIZE + 1];
+	char commit_id[GIT_OID_MAX_HEXSIZE + 1];
 
-	git_oid_fmt(commit_oidstr, git_commit_id(commit));
-	commit_oidstr[GIT_OID_SHA1_HEXSIZE] = '\0';
-
-	git_error_set(GIT_ERROR_REVERT, fmt, commit_oidstr);
+	git_oid_tostr(commit_id, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit));
+	git_error_set(GIT_ERROR_REVERT, fmt, commit_id);
 
 	return -1;
 }
@@ -176,7 +174,7 @@ int git_revert(
 	git_revert_options opts;
 	git_reference *our_ref = NULL;
 	git_commit *our_commit = NULL;
-	char commit_oidstr[GIT_OID_SHA1_HEXSIZE + 1];
+	char commit_id[GIT_OID_MAX_HEXSIZE + 1];
 	const char *commit_msg;
 	git_str their_label = GIT_STR_INIT;
 	git_index *index = NULL;
@@ -191,19 +189,18 @@ int git_revert(
 	if ((error = git_repository__ensure_not_bare(repo, "revert")) < 0)
 		return error;
 
-	git_oid_fmt(commit_oidstr, git_commit_id(commit));
-	commit_oidstr[GIT_OID_SHA1_HEXSIZE] = '\0';
+	git_oid_tostr(commit_id, GIT_OID_MAX_HEXSIZE + 1, git_commit_id(commit));
 
 	if ((commit_msg = git_commit_summary(commit)) == NULL) {
 		error = -1;
 		goto on_error;
 	}
 
-	if ((error = git_str_printf(&their_label, "parent of %.7s... %s", commit_oidstr, commit_msg)) < 0 ||
+	if ((error = git_str_printf(&their_label, "parent of %.7s... %s", commit_id, commit_msg)) < 0 ||
 		(error = revert_normalize_opts(repo, &opts, given_opts, git_str_cstr(&their_label))) < 0 ||
 		(error = git_indexwriter_init_for_operation(&indexwriter, repo, &opts.checkout_opts.checkout_strategy)) < 0 ||
-		(error = write_revert_head(repo, commit_oidstr)) < 0 ||
-		(error = write_merge_msg(repo, commit_oidstr, commit_msg)) < 0 ||
+		(error = write_revert_head(repo, commit_id)) < 0 ||
+		(error = write_merge_msg(repo, commit_id, commit_msg)) < 0 ||
 		(error = git_repository_head(&our_ref, repo)) < 0 ||
 		(error = git_reference_peel((git_object **)&our_commit, our_ref, GIT_OBJECT_COMMIT)) < 0 ||
 		(error = git_revert_commit(&index, repo, commit, our_commit, opts.mainline, &opts.merge_opts)) < 0 ||
diff --git a/src/libgit2/stash.c b/src/libgit2/stash.c
index 319ae3a3f..b49e95cdb 100644
--- a/src/libgit2/stash.c
+++ b/src/libgit2/stash.c
@@ -284,7 +284,7 @@ static int build_untracked_tree(
 	struct stash_update_rules data = {0};
 	int error;
 
-	if ((error = git_index_new(&i_index)) < 0)
+	if ((error = git_index__new(&i_index, repo->oid_type)) < 0)
 		goto cleanup;
 
 	if (flags & GIT_STASH_INCLUDE_UNTRACKED) {
@@ -487,7 +487,7 @@ static int commit_worktree(
 	int error = 0, ignorecase;
 
 	if ((error = git_repository_index(&r_index, repo) < 0) ||
-	    (error = git_index_new(&i_index)) < 0 ||
+	    (error = git_index__new(&i_index, repo->oid_type)) < 0 ||
 	    (error = git_index__fill(i_index, &r_index->entries) < 0) ||
 	    (error = git_repository__configmap_lookup(&ignorecase, repo, GIT_CONFIGMAP_IGNORECASE)) < 0)
 		goto cleanup;
@@ -732,7 +732,7 @@ int git_stash_save_with_opts(
 					     i_commit, b_commit, u_commit)) < 0)
 			goto cleanup;
 	} else {
-		if ((error = git_index_new(&paths_index)) < 0 ||
+		if ((error = git_index__new(&paths_index, repo->oid_type)) < 0 ||
 		    (error = retrieve_head(&head, repo)) < 0 ||
 		    (error = git_reference_peel((git_object**)&tree, head, GIT_OBJECT_TREE)) < 0 ||
 		    (error = git_index_read_tree(paths_index, tree)) < 0 ||
@@ -1003,6 +1003,7 @@ static int stage_new_file(const git_index_entry **entries, void *data)
 
 static int stage_new_files(
 	git_index **out,
+	git_repository *repo,
 	git_tree *parent_tree,
 	git_tree *tree)
 {
@@ -1011,7 +1012,7 @@ static int stage_new_files(
 	git_index *index = NULL;
 	int error;
 
-	if ((error = git_index_new(&index)) < 0 ||
+	if ((error = git_index__new(&index, repo->oid_type)) < 0 ||
 		(error = git_iterator_for_tree(
 			&iterators[0], parent_tree, &iterator_options)) < 0 ||
 		(error = git_iterator_for_tree(
@@ -1095,10 +1096,10 @@ int git_stash_apply(
 	 * previously unstaged contents are staged, not the previously staged.)
 	 */
 	} else if ((opts.flags & GIT_STASH_APPLY_REINSTATE_INDEX) == 0) {
-		if ((error = stage_new_files(
-				&stash_adds, stash_parent_tree, stash_tree)) < 0 ||
-			(error = merge_indexes(
-				&unstashed_index, repo, stash_parent_tree, repo_index, stash_adds)) < 0)
+		if ((error = stage_new_files(&stash_adds, repo,
+				stash_parent_tree, stash_tree)) < 0 ||
+		    (error = merge_indexes(&unstashed_index, repo,
+				stash_parent_tree, repo_index, stash_adds)) < 0)
 			goto cleanup;
 	}
 
diff --git a/src/libgit2/submodule.h b/src/libgit2/submodule.h
index 7fa982486..40b7b70f7 100644
--- a/src/libgit2/submodule.h
+++ b/src/libgit2/submodule.h
@@ -69,9 +69,9 @@
  * - `repo` is the parent repository that contains this submodule.
  * - `flags` after for internal use, tracking where this submodule has been
  *   found (head, index, config, workdir) and known status info, etc.
- * - `head_oid` is the SHA1 for the submodule path in the repo HEAD.
- * - `index_oid` is the SHA1 for the submodule recorded in the index.
- * - `wd_oid` is the SHA1 for the HEAD of the checked out submodule.
+ * - `head_oid` is the oid for the submodule path in the repo HEAD.
+ * - `index_oid` is the oid for the submodule recorded in the index.
+ * - `wd_oid` is the oid for the HEAD of the checked out submodule.
  *
  * If the submodule has been added to .gitmodules but not yet git added,
  * then the `index_oid` will be zero but still marked valid.  If the
diff --git a/src/libgit2/threadstate.h b/src/libgit2/threadstate.h
index f9e7ba7bf..65edec717 100644
--- a/src/libgit2/threadstate.h
+++ b/src/libgit2/threadstate.h
@@ -13,7 +13,7 @@ typedef struct {
 	git_error *last_error;
 	git_error error_t;
 	git_str error_buf;
-	char oid_fmt[GIT_OID_SHA1_HEXSIZE+1];
+	char oid_fmt[GIT_OID_MAX_HEXSIZE+1];
 } git_threadstate;
 
 extern int git_threadstate_global_init(void);
diff --git a/src/libgit2/tree-cache.c b/src/libgit2/tree-cache.c
index 19fad85ae..95d879860 100644
--- a/src/libgit2/tree-cache.c
+++ b/src/libgit2/tree-cache.c
@@ -71,12 +71,16 @@ const git_tree_cache *git_tree_cache_get(const git_tree_cache *tree, const char
 	}
 }
 
-static int read_tree_internal(git_tree_cache **out,
-			      const char **buffer_in, const char *buffer_end,
-			      git_pool *pool)
+static int read_tree_internal(
+	git_tree_cache **out,
+	const char **buffer_in,
+	const char *buffer_end,
+	git_oid_t oid_type,
+	git_pool *pool)
 {
 	git_tree_cache *tree = NULL;
 	const char *name_start, *buffer;
+	size_t oid_size = git_oid_size(oid_type);
 	int count;
 
 	buffer = name_start = *buffer_in;
@@ -87,7 +91,7 @@ static int read_tree_internal(git_tree_cache **out,
 	if (++buffer >= buffer_end)
 		goto corrupted;
 
-	if (git_tree_cache_new(&tree, name_start, pool) < 0)
+	if (git_tree_cache_new(&tree, name_start, oid_type, pool) < 0)
 		return -1;
 
 	/* Blank-terminated ASCII decimal number of entries in this tree */
@@ -108,14 +112,14 @@ static int read_tree_internal(git_tree_cache **out,
 	if (*buffer != '\n' || ++buffer > buffer_end)
 		goto corrupted;
 
-	/* The SHA1 is only there if it's not invalidated */
+	/* The OID is only there if it's not invalidated */
 	if (tree->entry_count >= 0) {
 		/* 160-bit SHA-1 for this tree and it's children */
-		if (buffer + GIT_OID_SHA1_SIZE > buffer_end)
+		if (buffer + oid_size > buffer_end)
 			goto corrupted;
 
-		git_oid__fromraw(&tree->oid, (const unsigned char *)buffer, GIT_OID_SHA1);
-		buffer += GIT_OID_SHA1_SIZE;
+		git_oid__fromraw(&tree->oid, (const unsigned char *)buffer, oid_type);
+		buffer += oid_size;
 	}
 
 	/* Parse children: */
@@ -130,7 +134,7 @@ static int read_tree_internal(git_tree_cache **out,
 		memset(tree->children, 0x0, bufsize);
 
 		for (i = 0; i < tree->children_count; ++i) {
-			if (read_tree_internal(&tree->children[i], &buffer, buffer_end, pool) < 0)
+			if (read_tree_internal(&tree->children[i], &buffer, buffer_end, oid_type, pool) < 0)
 				goto corrupted;
 		}
 	}
@@ -144,11 +148,16 @@ static int read_tree_internal(git_tree_cache **out,
 	return -1;
 }
 
-int git_tree_cache_read(git_tree_cache **tree, const char *buffer, size_t buffer_size, git_pool *pool)
+int git_tree_cache_read(
+	git_tree_cache **tree,
+	const char *buffer,
+	size_t buffer_size,
+	git_oid_t oid_type,
+	git_pool *pool)
 {
 	const char *buffer_end = buffer + buffer_size;
 
-	if (read_tree_internal(tree, &buffer, buffer_end, pool) < 0)
+	if (read_tree_internal(tree, &buffer, buffer_end, oid_type, pool) < 0)
 		return -1;
 
 	if (buffer < buffer_end) {
@@ -201,7 +210,7 @@ static int read_tree_recursive(git_tree_cache *cache, const git_tree *tree, git_
 			continue;
 		}
 
-		if ((error = git_tree_cache_new(&cache->children[j], git_tree_entry_name(entry), pool)) < 0)
+		if ((error = git_tree_cache_new(&cache->children[j], git_tree_entry_name(entry), cache->oid_type, pool)) < 0)
 			return error;
 
 		if ((error = git_tree_lookup(&subtree, repo, git_tree_entry_id(entry))) < 0)
@@ -219,12 +228,12 @@ static int read_tree_recursive(git_tree_cache *cache, const git_tree *tree, git_
 	return 0;
 }
 
-int git_tree_cache_read_tree(git_tree_cache **out, const git_tree *tree, git_pool *pool)
+int git_tree_cache_read_tree(git_tree_cache **out, const git_tree *tree, git_oid_t oid_type, git_pool *pool)
 {
 	int error;
 	git_tree_cache *cache;
 
-	if ((error = git_tree_cache_new(&cache, "", pool)) < 0)
+	if ((error = git_tree_cache_new(&cache, "", oid_type, pool)) < 0)
 		return error;
 
 	if ((error = read_tree_recursive(cache, tree, pool)) < 0)
@@ -234,7 +243,7 @@ int git_tree_cache_read_tree(git_tree_cache **out, const git_tree *tree, git_poo
 	return 0;
 }
 
-int git_tree_cache_new(git_tree_cache **out, const char *name, git_pool *pool)
+int git_tree_cache_new(git_tree_cache **out, const char *name, git_oid_t oid_type, git_pool *pool)
 {
 	size_t name_len, alloc_size;
 	git_tree_cache *tree;
@@ -248,6 +257,7 @@ int git_tree_cache_new(git_tree_cache **out, const char *name, git_pool *pool)
 
 	memset(tree, 0x0, sizeof(git_tree_cache));
 	/* NUL-terminated tree name */
+	tree->oid_type = oid_type;
 	tree->namelen = name_len;
 	memcpy(tree->name, name, name_len);
 	tree->name[name_len] = '\0';
@@ -263,7 +273,7 @@ static void write_tree(git_str *out, git_tree_cache *tree)
 	git_str_printf(out, "%s%c%"PRIdZ" %"PRIuZ"\n", tree->name, 0, tree->entry_count, tree->children_count);
 
 	if (tree->entry_count != -1)
-		git_str_put(out, (char *)&tree->oid.id, GIT_OID_SHA1_SIZE);
+		git_str_put(out, (char *)&tree->oid.id, git_oid_size(tree->oid_type));
 
 	for (i = 0; i < tree->children_count; i++)
 		write_tree(out, tree->children[i]);
diff --git a/src/libgit2/tree-cache.h b/src/libgit2/tree-cache.h
index a27e30466..e4a73f277 100644
--- a/src/libgit2/tree-cache.h
+++ b/src/libgit2/tree-cache.h
@@ -18,6 +18,8 @@ typedef struct git_tree_cache {
 	struct git_tree_cache **children;
 	size_t children_count;
 
+	git_oid_t oid_type;
+
 	ssize_t entry_count;
 	git_oid oid;
 	size_t namelen;
@@ -25,14 +27,14 @@ typedef struct git_tree_cache {
 } git_tree_cache;
 
 int git_tree_cache_write(git_str *out, git_tree_cache *tree);
-int git_tree_cache_read(git_tree_cache **tree, const char *buffer, size_t buffer_size, git_pool *pool);
+int git_tree_cache_read(git_tree_cache **tree, const char *buffer, size_t buffer_size, git_oid_t oid_type, git_pool *pool);
 void git_tree_cache_invalidate_path(git_tree_cache *tree, const char *path);
 const git_tree_cache *git_tree_cache_get(const git_tree_cache *tree, const char *path);
-int git_tree_cache_new(git_tree_cache **out, const char *name, git_pool *pool);
+int git_tree_cache_new(git_tree_cache **out, const char *name, git_oid_t oid_type, git_pool *pool);
 /**
  * Read a tree as the root of the tree cache (like for `git read-tree`)
  */
-int git_tree_cache_read_tree(git_tree_cache **out, const git_tree *tree, git_pool *pool);
+int git_tree_cache_read_tree(git_tree_cache **out, const git_tree *tree, git_oid_t oid_type, git_pool *pool);
 void git_tree_cache_free(git_tree_cache *tree);
 
 #endif
diff --git a/src/libgit2/tree.c b/src/libgit2/tree.c
index 9293d9422..236a87f7e 100644
--- a/src/libgit2/tree.c
+++ b/src/libgit2/tree.c
@@ -731,7 +731,7 @@ int git_tree__write_index(
 		return ret;
 
 	/* Read the tree cache into the index */
-	ret = git_tree_cache_read_tree(&index->tree, tree, &index->tree_pool);
+	ret = git_tree_cache_read_tree(&index->tree, tree, index->oid_type, &index->tree_pool);
 	git_tree_free(tree);
 
 	return ret;
diff --git a/src/util/filebuf.c b/src/util/filebuf.c
index e014d43b2..7afb76b88 100644
--- a/src/util/filebuf.c
+++ b/src/util/filebuf.c
@@ -302,11 +302,16 @@ int git_filebuf_open_withsize(git_filebuf *file, const char *path, int flags, mo
 	}
 
 	/* If we are hashing on-write, allocate a new hash context */
-	if (flags & GIT_FILEBUF_HASH_CONTENTS) {
+	if (flags & GIT_FILEBUF_HASH_SHA1) {
 		file->compute_digest = 1;
 
 		if (git_hash_ctx_init(&file->digest, GIT_HASH_ALGORITHM_SHA1) < 0)
 			goto cleanup;
+	} else if (flags & GIT_FILEBUF_HASH_SHA256) {
+		file->compute_digest = 1;
+
+		if (git_hash_ctx_init(&file->digest, GIT_HASH_ALGORITHM_SHA256) < 0)
+			goto cleanup;
 	}
 
 	compression = flags >> GIT_FILEBUF_DEFLATE_SHIFT;
diff --git a/src/util/filebuf.h b/src/util/filebuf.h
index 4a61ae4e3..e23b9ed2a 100644
--- a/src/util/filebuf.h
+++ b/src/util/filebuf.h
@@ -17,13 +17,14 @@
 #	define GIT_FILEBUF_THREADS
 #endif
 
-#define GIT_FILEBUF_HASH_CONTENTS		(1 << 0)
-#define GIT_FILEBUF_APPEND				(1 << 2)
+#define GIT_FILEBUF_HASH_SHA1           (1 << 0)
+#define GIT_FILEBUF_HASH_SHA256         (1 << 1)
+#define GIT_FILEBUF_APPEND              (1 << 2)
 #define GIT_FILEBUF_CREATE_LEADING_DIRS	(1 << 3)
-#define GIT_FILEBUF_TEMPORARY			(1 << 4)
-#define GIT_FILEBUF_DO_NOT_BUFFER		(1 << 5)
-#define GIT_FILEBUF_FSYNC				(1 << 6)
-#define GIT_FILEBUF_DEFLATE_SHIFT		(7)
+#define GIT_FILEBUF_TEMPORARY           (1 << 4)
+#define GIT_FILEBUF_DO_NOT_BUFFER       (1 << 5)
+#define GIT_FILEBUF_FSYNC               (1 << 6)
+#define GIT_FILEBUF_DEFLATE_SHIFT       (7)
 
 #define GIT_FILELOCK_EXTENSION ".lock\0"
 #define GIT_FILELOCK_EXTLENGTH 6
@@ -91,4 +92,16 @@ int git_filebuf_hash(unsigned char *out, git_filebuf *file);
 int git_filebuf_flush(git_filebuf *file);
 int git_filebuf_stats(time_t *mtime, size_t *size, git_filebuf *file);
 
+GIT_INLINE(int) git_filebuf_hash_flags(git_hash_algorithm_t algorithm)
+{
+	switch (algorithm) {
+	case GIT_HASH_ALGORITHM_SHA1:
+		return GIT_FILEBUF_HASH_SHA1;
+	case GIT_HASH_ALGORITHM_SHA256:
+		return GIT_FILEBUF_HASH_SHA256;
+	default:
+		return 0;
+	}
+}
+
 #endif