Add a simple DER support header

1 files changed, 149 insertions, 0 deletions

diff --git a/src/include/k5-der.h b/src/include/k5-der.h
new file mode 100644
index 000000000..b8371d9b4
--- /dev/null
+++ b/src/include/k5-der.h
@@ -0,0 +1,149 @@
+/* -*- mode: c; c-basic-offset: 4; indent-tabs-mode: nil -*- */
+/* include/k5-der.h - Distinguished Encoding Rules (DER) declarations */
+/*
+ * Copyright (C) 2023 by the Massachusetts Institute of Technology.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in
+ *   the documentation and/or other materials provided with the
+ *   distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/*
+ * Most ASN.1 encoding and decoding is done using the table-driven framework in
+ * libkrb5.  When that is not an option, these helpers can be used to encode
+ * and decode simple types.
+ */
+
+#ifndef K5_DER_H
+#define K5_DER_H
+
+#include <stdint.h>
+#include <stdbool.h>
+#include "k5-buf.h"
+#include "k5-input.h"
+
+/* Return the number of bytes needed to encode len as a DER encoding length. */
+static inline size_t
+k5_der_len_len(size_t len)
+{
+    size_t llen;
+
+    if (len < 128)
+        return 1;
+    llen = 1;
+    while (len > 0) {
+        len >>= 8;
+        llen++;
+    }
+    return llen;
+}
+
+/* Return the number of bytes needed to encode a DER value (with identifier
+ * byte and length) for a given contents length. */
+static inline size_t
+k5_der_value_len(size_t contents_len)
+{
+    return 1 + k5_der_len_len(contents_len) + contents_len;
+}
+
+/* Add a DER identifier byte (composed by the caller, including the ASN.1
+ * class, tag, and constructed bit) and length. */
+static inline void
+k5_der_add_taglen(struct k5buf *buf, uint8_t idbyte, size_t len)
+{
+    uint8_t *p;
+    size_t llen = k5_der_len_len(len);
+
+    p = k5_buf_get_space(buf, 1 + llen);
+    if (p == NULL)
+        return;
+    *p++ = idbyte;
+    if (len < 128) {
+        *p = len;
+    } else {
+        *p = 0x80 | (llen - 1);
+        /* Encode the length bytes backwards so the most significant byte is
+         * first. */
+        p += llen;
+        while (len > 0) {
+            *--p = len & 0xFF;
+            len >>= 8;
+        }
+    }
+}
+
+/* Add a DER value (identifier byte, length, and contents). */
+static inline void
+k5_der_add_value(struct k5buf *buf, uint8_t idbyte, const void *contents,
+                 size_t len)
+{
+    k5_der_add_taglen(buf, idbyte, len);
+    k5_buf_add_len(buf, contents, len);
+}
+
+/*
+ * If the next byte in in matches idbyte and the subsequent DER length is
+ * valid, advance in past the value, set *contents_out to the value contents,
+ * and return true.  Otherwise return false.  Only set an error on in if the
+ * next bytes matches idbyte but the ensuing length is invalid.  contents_out
+ * may be aliased to in; it will only be written to on successful decoding of a
+ * value.
+ */
+static inline bool
+k5_der_get_value(struct k5input *in, uint8_t idbyte,
+                 struct k5input *contents_out)
+{
+    uint8_t lenbyte, i;
+    size_t len;
+    const void *bytes;
+
+    /* Do nothing if in is empty or the next byte doesn't match idbyte. */
+    if (in->status || in->len == 0 || *in->ptr != idbyte)
+        return false;
+
+    /* Advance past the identifier byte and decode the length. */
+    (void)k5_input_get_byte(in);
+    lenbyte = k5_input_get_byte(in);
+    if (lenbyte < 128) {
+        len = lenbyte;
+    } else {
+        len = 0;
+        for (i = 0; i < (lenbyte & 0x7F); i++) {
+            if (len > (SIZE_MAX >> 8)) {
+                k5_input_set_status(in, EOVERFLOW);
+                return false;
+            }
+            len = (len << 8) | k5_input_get_byte(in);
+        }
+    }
+
+    bytes = k5_input_get_bytes(in, len);
+    if (bytes == NULL)
+        return false;
+    k5_input_init(contents_out, bytes, len);
+    return true;
+}
+
+#endif /* K5_DER_H */