mime: implement encoders.

curl_mime_encoder() is operational and documented.
curl tool -F option is extended with ";encoder=".
curl tool --libcurl option generates calls to curl_mime_encoder().
New encoder tests 648 & 649.
Test 1404 extended with an encoder specification.

12 files changed, 809 insertions, 45 deletions

diff --git a/docs/cmdline-opts/form.d b/docs/cmdline-opts/form.d
index 14261d3ad..232d30c07 100644
--- a/docs/cmdline-opts/form.d
+++ b/docs/cmdline-opts/form.d
@@ -101,6 +101,20 @@ text file:
 .br
       -F '=)' -F '=@textfile.txt' ...  smtp://example.com
 
+Data can be encoded for transfer using encoder=. Available encodings are
+\fIbinary\fP and \fI8bit\fP that do nothing else than adding the corresponding
+Content-Transfer-Encoding header, \fI7bit\fP that only rejects 8-bit characters
+with a transfer error, \fIquoted-printable\fP and \fIbase64\fP that encodes
+data according to the corresponding schemes, limiting lines length to
+76 characters.
+
+Example: send multipart mail with a quoted-printable text message and a
+base64 attached file:
+
+ curl -F '=text message;encoder=quoted-printable' \\
+.br
+      -F '=@localfile;encoder=base64' ... smtp://example.com
+
 See further examples and details in the MANUAL.
 
 This option can be used multiple times.
diff --git a/docs/libcurl/Makefile.inc b/docs/libcurl/Makefile.inc
index 2414ecc12..86a002680 100644
--- a/docs/libcurl/Makefile.inc
+++ b/docs/libcurl/Makefile.inc
@@ -21,4 +21,4 @@ man_MANS = curl_easy_cleanup.3 curl_easy_getinfo.3 curl_easy_init.3      \
   curl_mime_init.3 curl_mime_free.3 curl_mime_addpart.3 curl_mime_name.3 \
   curl_mime_data.3 curl_mime_data_cb.3 curl_mime_filedata.3              \
   curl_mime_filename.3 curl_mime_subparts.3                              \
-  curl_mime_type.3 curl_mime_headers.3
+  curl_mime_type.3 curl_mime_headers.3 curl_mime_encoder.3
diff --git a/docs/libcurl/curl_mime_addpart.3 b/docs/libcurl/curl_mime_addpart.3
index cbb861852..5ea8171de 100644
--- a/docs/libcurl/curl_mime_addpart.3
+++ b/docs/libcurl/curl_mime_addpart.3
@@ -62,4 +62,5 @@ A mime part structure handle, or NULL upon failure.
 .BR curl_mime_filename "(3),"
 .BR curl_mime_subparts "(3),"
 .BR curl_mime_type "(3),"
-.BR curl_mime_headers "(3)"
+.BR curl_mime_headers "(3),"
+.BR curl_mime_encoder "(3)"
diff --git a/docs/libcurl/curl_mime_encoder.3 b/docs/libcurl/curl_mime_encoder.3
new file mode 100644
index 000000000..c17cf25b3
--- /dev/null
+++ b/docs/libcurl/curl_mime_encoder.3
@@ -0,0 +1,97 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.TH curl_mime_encoder 3 "22 August 2017" "libcurl 7.56.0" "libcurl Manual"
+.SH NAME
+curl_mime_encoder - set a mime part's encoder and content transfer encoding
+.SH SYNOPSIS
+.B #include <curl/curl.h>
+.sp
+.BI "CURLcode curl_mime_encoder(curl_mimepart * " part ,
+.BI "const char * " encoding ");"
+.ad
+.SH DESCRIPTION
+curl_mime_encoder() requests a mime part's content to be encoded before being
+transmitted.
+
+\fIpart\fP is the part's handle to assign an encoder.
+\fIencoding\fP is a pointer to a zero-terminated encoding scheme. It may be
+set to NULL to disable an encoder previously attached to the part. The encoding
+scheme storage may safely be reused after this function returns.
+
+Setting a part's encoder twice is valid: only the value set by the last call is
+retained.
+
+Upon multipart rendering, the part's content is encoded according to the
+pertaining scheme and a corresponding \fIContent-Transfer-Encoding"\fP header
+is added to the part.
+
+Supported encoding schemes are:
+.br
+"\fIbinary\fP": the data is left unchanged, the header is added.
+.br
+"\fI8bit\fP": header added, no data change.
+.br
+"\fI7bit\fP": the data is unchanged, but is each byte is checked
+to be a 7-bit value; if not, a read error occurs.
+.br
+"\fIbase64\fP": Data is converted to base64 encoding, then split in
+CRLF-terminated lines of at most 76 characters.
+.br
+"\fIquoted-printable\fP": data is encoded in quoted printable lines of
+at most 76 characters. Since the resulting size of the final data cannot be
+determined prior to reading the original data, it is left as unknown, causing
+chunked transfer in HTTP. For the same reason, this encoder may not be used
+with IMAP. This encoder targets text data that is mostly ASCII and should
+not be used with other types of data.
+
+If the original data is already encoded in such a scheme, a custom
+\fIContent-Transfer-Encoding\fP header should be added with
+\FIcurl_mime_headers\fP() instead of setting a part encoder.
+
+Encoding should not be applied to multiparts, thus the use of this
+function on a part with content set with \fIcurl_mime_subparts\fP() is
+strongly discouraged.
+.SH AVAILABILITY
+As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
+.SH RETURN VALUE
+CURLE_OK or a CURL error code upon failure.
+.SH EXAMPLE
+.nf
+ curl_mime *mime;
+ curl_mimepart *part;
+
+ /* create a mime handle */
+ mime = curl_mime_init(easy);
+
+ /* add a part */
+ part = curl_mime_addpart(mime);
+
+ /* send a file */
+ curl_mime_filedata(part, "image.png");
+
+ /* encode file data in base64 for transfer */
+ curl_mime_encoder(part, "base64");
+.fi
+.SH "SEE ALSO"
+.BR curl_mime_addpart "(3),"
+.BR curl_mime_headers "(3),"
+.BR curl_mime_subparts "(3)"
diff --git a/lib/mime.c b/lib/mime.c
index f6a626316..38a20ed2d 100644
--- a/lib/mime.c
+++ b/lib/mime.c
@@ -55,6 +55,73 @@
 
 #define READ_ERROR                      ((size_t) -1)
 
+/* Encoders. */
+static size_t encoder_nop_read(char *buffer, size_t size, bool ateof,
+                                struct Curl_mimepart *part);
+static curl_off_t encoder_nop_size(struct Curl_mimepart *part);
+static size_t encoder_7bit_read(char *buffer, size_t size, bool ateof,
+                                struct Curl_mimepart *part);
+static size_t encoder_base64_read(char *buffer, size_t size, bool ateof,
+                                struct Curl_mimepart *part);
+static curl_off_t encoder_base64_size(struct Curl_mimepart *part);
+static size_t encoder_qp_read(char *buffer, size_t size, bool ateof,
+                              struct Curl_mimepart *part);
+static curl_off_t encoder_qp_size(struct Curl_mimepart *part);
+
+static const mime_encoder encoders[] = {
+  {"binary", encoder_nop_read, encoder_nop_size},
+  {"8bit", encoder_nop_read, encoder_nop_size},
+  {"7bit", encoder_7bit_read, encoder_nop_size},
+  {"base64", encoder_base64_read, encoder_base64_size},
+  {"quoted-printable", encoder_qp_read, encoder_qp_size},
+  {ZERO_NULL, ZERO_NULL, ZERO_NULL}
+};
+
+/* Base64 encoding table */
+static const char base64[] =
+  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+/* Quoted-printable character class table.
+ *
+ * We cannot rely on ctype functions since quoted-printable input data
+ * is assumed to be ascii-compatible, even on non-ascii platforms. */
+#define QP_OK           1       /* Can be represented by itself. */
+#define QP_SP           2       /* Space or tab. */
+#define QP_CR           3       /* Carriage return. */
+#define QP_LF           4       /* Line-feed. */
+static const unsigned char qp_class[] = {
+ 0,     0,     0,     0,     0,     0,     0,     0,            /* 00 - 07 */
+ 0,     QP_SP, QP_LF, 0,     0,     QP_CR, 0,     0,            /* 08 - 0F */
+ 0,     0,     0,     0,     0,     0,     0,     0,            /* 10 - 17 */
+ 0,     0,     0,     0,     0,     0,     0,     0,            /* 18 - 1F */
+ QP_SP, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 20 - 27 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 28 - 2F */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 30 - 37 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, 0    , QP_OK, QP_OK,        /* 38 - 3F */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 40 - 47 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 48 - 4F */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 50 - 57 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 58 - 5F */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 60 - 67 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 68 - 6F */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK,        /* 70 - 77 */
+ QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, QP_OK, 0,            /* 78 - 7F */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* 80 - 8F */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* 90 - 9F */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* A0 - AF */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* B0 - BF */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* C0 - CF */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* D0 - DF */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,                /* E0 - EF */
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0                 /* F0 - FF */
+};
+
+
+/* Binary --> hexadecimal ASCII table. */
+static const char aschex[] =
+  "\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x41\x42\x43\x44\x45\x46";
+
+
 
 #ifndef __VMS
 #define filesize(name, stat_data) (stat_data.st_size)
@@ -277,6 +344,279 @@ static char *strippath(const char *fullfile)
   return base; /* returns an allocated string or NULL ! */
 }
 
+/* Initialize data encoder state. */
+static void cleanup_encoder_state(mime_encoder_state *p)
+{
+  p->pos = 0;
+  p->bufbeg = 0;
+  p->bufend = 0;
+}
+
+
+/* Dummy encoder. This is used for 8bit and binary content encodings. */
+static size_t encoder_nop_read(char *buffer, size_t size, bool ateof,
+                               struct Curl_mimepart *part)
+{
+  mime_encoder_state *st = &part->encstate;
+  size_t insize = st->bufend - st->bufbeg;
+
+  (void) ateof;
+
+  if(size > insize)
+    size = insize;
+  if(size)
+    memcpy(buffer, st->buf, size);
+  st->bufbeg += size;
+  return size;
+}
+
+static curl_off_t encoder_nop_size(struct Curl_mimepart *part)
+{
+  return part->datasize;
+}
+
+
+/* 7bit encoder: the encoder is just a data validity check. */
+static size_t encoder_7bit_read(char *buffer, size_t size, bool ateof,
+                                struct Curl_mimepart *part)
+{
+  size_t i;
+  mime_encoder_state *st = &part->encstate;
+  size_t cursize = st->bufend - st->bufbeg;
+
+  (void) ateof;
+
+  if(size > cursize)
+    size = cursize;
+
+  for(cursize = 0; cursize < size; cursize++) {
+    *buffer = st->buf[st->bufbeg];
+    if(*buffer++ & 0x80)
+      return cursize? cursize: READ_ERROR;
+    st->bufbeg++;
+  }
+
+  return cursize;
+}
+
+
+/* Base64 content encoder. */
+static size_t encoder_base64_read(char *buffer, size_t size, bool ateof,
+                                struct Curl_mimepart *part)
+{
+  mime_encoder_state *st = &part->encstate;
+  size_t cursize = 0;
+  int i;
+  char *ptr = buffer;
+
+  while(st->bufbeg < st->bufend) {
+    /* Line full ? */
+    if(st->pos >= MAX_ENCODED_LINE_LENGTH - 4) {
+      /* Yes, we need 2 characters for CRLF. */
+      if(size < 2)
+        break;
+      *ptr++ = '\r';
+      *ptr++ = '\n';
+      st->pos = 0;
+      cursize += 2;
+      size -= 2;
+    }
+
+    /* Be sure there is enough space and input data for a base64 group. */
+    if(size < 4 || st->bufend - st->bufbeg < 3)
+      break;
+
+    /* Encode three bytes a four characters. */
+    i = st->buf[st->bufbeg++] & 0xFF;
+    i = (i << 8) | (st->buf[st->bufbeg++] & 0xFF);
+    i = (i << 8) | (st->buf[st->bufbeg++] & 0xFF);
+    *ptr++ = base64[(i >> 18) & 0x3F];
+    *ptr++ = base64[(i >> 12) & 0x3F];
+    *ptr++ = base64[(i >> 6) & 0x3F];
+    *ptr++ = base64[i & 0x3F];
+    cursize += 4;
+    st->pos += 4;
+    size -= 4;
+  }
+
+  /* If at eof, we have to flush the buffered data. */
+  if(ateof && size >= 4) {
+    /* Buffered data size can only be 0, 1 or 2. */
+    ptr[2] = ptr[3] = '=';
+    i = 0;
+    switch(st->bufend - st->bufbeg) {
+    case 2:
+      i = (st->buf[st->bufbeg + 1] & 0xFF) << 8;
+      /* FALLTHROUGH */
+    case 1:
+      i |= (st->buf[st->bufbeg] & 0xFF) << 16;
+      ptr[0] = base64[(i >> 18) & 0x3F];
+      ptr[1] = base64[(i >> 12) & 0x3F];
+      if(++st->bufbeg != st->bufend) {
+        ptr[2] = base64[(i >> 6) & 0x3F];
+        st->bufbeg++;
+      }
+      cursize += 4;
+      st->pos += 4;
+      break;
+    }
+  }
+
+#ifdef CURL_DOES_CONVERSIONS
+  /* This is now textual data, Convert character codes. */
+  if(part->easy && cursize) {
+    CURLcode result = Curl_convert_to_network(part->easy, buffer, cursize);
+    if(result)
+      return READ_ERROR;
+  }
+#endif
+
+  return cursize;
+}
+
+static curl_off_t encoder_base64_size(struct Curl_mimepart *part)
+{
+  curl_off_t size = part->datasize;
+
+  if(size <= 0)
+    return size;    /* Unknown size or no data. */
+
+  /* Compute base64 character count. */
+  size = 4 * (1 + (size - 1) / 3);
+
+  /* Effective character count must include CRLFs. */
+  return size + 2 * ((size - 1) / MAX_ENCODED_LINE_LENGTH);
+}
+
+
+/* Quoted-printable lookahead.
+ *
+ * Check if a CRLF or end of data is in input buffer at current position + n.
+ * Return -1 if more data needed, 1 if CRLF or end of data, else 0.
+ */
+static int qp_lookahead_eol(mime_encoder_state *st, int ateof, size_t n)
+{
+  n += st->bufbeg;
+  if(n >= st->bufend && ateof)
+    return 1;
+  if(n + 2 > st->bufend)
+    return ateof? 0: -1;
+  if(qp_class[st->buf[n] & 0xFF] == QP_CR &&
+     qp_class[st->buf[n + 1] & 0xFF] == QP_LF)
+    return 1;
+  return 0;
+}
+
+/* Quoted-printable encoder. */
+static size_t encoder_qp_read(char *buffer, size_t size, bool ateof,
+                              struct Curl_mimepart *part)
+{
+  mime_encoder_state *st = &part->encstate;
+  char *ptr = buffer;
+  size_t cursize = 0;
+  int i;
+  size_t len;
+  size_t consumed;
+  int softlinebreak;
+  char buf[4];
+
+  /* On all platforms, input is supposed to be ASCII compatible: for this
+     reason, we use hexadecimal ASCII codes in this function rather than
+     character constants that can be interpreted as non-ascii on some
+     platforms. Preserve ASCII encoding on output too. */
+  while(st->bufbeg < st->bufend) {
+    len = 1;
+    consumed = 1;
+    i = st->buf[st->bufbeg];
+    buf[0] = (char) i;
+    buf[1] = aschex[(i >> 4) & 0xF];
+    buf[2] = aschex[i & 0xF];
+
+    switch(qp_class[st->buf[st->bufbeg] & 0xFF]) {
+    case QP_OK:          /* Not a special character. */
+      break;
+    case QP_SP:          /* Space or tab. */
+      /* Spacing must be escaped if followed by CRLF. */
+      switch(qp_lookahead_eol(st, ateof, 1)) {
+      case -1:          /* More input data needed. */
+        return cursize;
+      case 0:           /* No encoding needed. */
+        break;
+      default:          /* CRLF after space or tab. */
+        buf[0] = '\x3D';    /* '=' */
+        len = 3;
+        break;
+      }
+      break;
+    case QP_CR:         /* Carriage return. */
+      /* If followed by a line-feed, output the CRLF pair.
+         Else escape it. */
+      switch(qp_lookahead_eol(st, ateof, 0)) {
+      case -1:          /* Need more data. */
+        return cursize;
+      case 1:           /* CRLF found. */
+        buf[len++] = '\x0A';    /* Append '\n'. */
+        consumed = 2;
+        break;
+      default:          /* Not followed by LF: escape. */
+        buf[0] = '\x3D';    /* '=' */
+        len = 3;
+        break;
+      }
+      break;
+    default:            /* Character must be escaped. */
+      buf[0] = '\x3D';    /* '=' */
+      len = 3;
+      break;
+    }
+
+    /* Be sure the encoded character fits within maximum line length. */
+    if(buf[len - 1] != '\x0A') {    /* '\n' */
+      softlinebreak = st->pos + len > MAX_ENCODED_LINE_LENGTH;
+      if(!softlinebreak && st->pos + len == MAX_ENCODED_LINE_LENGTH) {
+        /* We may use the current line only if end of data or followed by
+           a CRLF. */
+        switch(qp_lookahead_eol(st, ateof, consumed)) {
+        case -1:        /* Need more data. */
+          return cursize;
+          break;
+        case 0:         /* Not followed by a CRLF. */
+          softlinebreak = 1;
+          break;
+        }
+      }
+      if(softlinebreak) {
+        strcpy(buf, "\x3D\x0D\x0A");    /* "=\r\n" */
+        len = 3;
+        consumed = 0;
+      }
+    }
+
+    /* If the output buffer would overflow, do not store. */
+    if(len > size)
+      break;
+
+    /* Append to output buffer. */
+    memcpy(ptr, buf, len);
+    cursize += len;
+    ptr += len;
+    size -= len;
+    st->pos += len;
+    if(buf[len - 1] == '\x0A')    /* '\n' */
+      st->pos = 0;
+    st->bufbeg += consumed;
+  }
+
+  return cursize;
+}
+
+static curl_off_t encoder_qp_size(struct Curl_mimepart *part)
+{
+  /* Determining the size can only be done by reading the data: unless the
+     data size is 0, we return it as unknown (-1). */
+  return part->datasize? -1: 0;
+}
+
 
 /* In-memory data callbacks. */
 /* Argument is a pointer to the mime part. */
@@ -435,6 +775,77 @@ static size_t readback_bytes(struct mime_state *state,
   return sz;
 }
 
+/* Read a non-encoded part content. */
+static size_t read_part_content(struct Curl_mimepart *part,
+                                char *buffer, size_t bufsize)
+{
+  size_t sz = 0;
+
+  if(part->readfunc)
+    sz = part->readfunc(buffer, 1, bufsize, part->arg);
+  return sz;
+}
+
+/* Read and encode part content. */
+static size_t read_encoded_part_content(struct Curl_mimepart *part,
+                                        char *buffer, size_t bufsize)
+{
+  mime_encoder_state *st = &part->encstate;
+  size_t cursize = 0;
+  size_t sz;
+  bool ateof = FALSE;
+
+  while(bufsize) {
+    if(st->bufbeg < st->bufend || ateof) {
+      /* Encode buffered data. */
+      sz = part->encoder->encodefunc(buffer, bufsize, ateof, part);
+      switch(sz) {
+      case 0:
+        if(ateof)
+          return cursize;
+        break;
+      case CURL_READFUNC_ABORT:
+      case CURL_READFUNC_PAUSE:
+      case READ_ERROR:
+        return cursize? cursize: sz;
+      default:
+        cursize += sz;
+        buffer += sz;
+        bufsize -= sz;
+        continue;
+      }
+    }
+
+    /* We need more data in input buffer. */
+    if(st->bufbeg) {
+      size_t len = st->bufend - st->bufbeg;
+
+      if(len)
+        memmove(st->buf, st->buf + st->bufbeg, len);
+      st->bufbeg = 0;
+      st->bufend = len;
+    }
+    if(st->bufend >= sizeof st->buf)
+      return cursize? cursize: READ_ERROR;    /* Buffer full. */
+    sz = read_part_content(part, st->buf + st->bufend,
+                           sizeof st->buf - st->bufend);
+    switch(sz) {
+    case 0:
+      ateof = TRUE;
+      break;
+    case CURL_READFUNC_ABORT:
+    case CURL_READFUNC_PAUSE:
+    case READ_ERROR:
+      return cursize? cursize: sz;
+    default:
+      st->bufend += sz;
+      break;
+    }
+  }
+
+  return cursize;
+}
+
 /* Readback a mime part. */
 static size_t readback_part(curl_mimepart *part,
                             char *buffer, size_t bufsize)
@@ -491,11 +902,14 @@ static size_t readback_part(curl_mimepart *part,
         convbuf = buffer;
       }
 #endif
+      cleanup_encoder_state(&part->encstate);
       mimesetstate(&part->state, MIMESTATE_CONTENT, NULL);
       break;
     case MIMESTATE_CONTENT:
-      if(part->readfunc)
-        sz = part->readfunc(buffer, 1, bufsize, part->arg);
+      if(part->encoder)
+        sz = read_encoded_part_content(part, buffer, bufsize);
+      else
+        sz = read_part_content(part, buffer, bufsize);
       switch(sz) {
       case 0:
         mimesetstate(&part->state, MIMESTATE_END, NULL);
@@ -634,6 +1048,7 @@ static int mime_part_rewind(curl_mimepart *part)
 
   if(part->flags & MIME_BODY_ONLY)
     targetstate = MIMESTATE_BODY;
+  cleanup_encoder_state(&part->encstate);
   if(part->state.state > targetstate) {
     res = CURL_SEEKFUNC_CANTSEEK;
     if(part->seekfunc) {
@@ -643,6 +1058,9 @@ static int mime_part_rewind(curl_mimepart *part)
       case CURL_SEEKFUNC_FAIL:
       case CURL_SEEKFUNC_CANTSEEK:
         break;
+      case -1:    /* For fseek() error. */
+        res = CURL_SEEKFUNC_CANTSEEK;
+        break;
       default:
         res = CURL_SEEKFUNC_FAIL;
         break;
@@ -702,6 +1120,8 @@ static void cleanup_part_content(curl_mimepart *part)
   part->namedfp = NULL;
   part->origin = 0;
   part->datasize = (curl_off_t) 0;    /* No size yet. */
+  part->encoder = NULL;
+  cleanup_encoder_state(&part->encstate);
   part->kind = MIMEKIND_NONE;
 }
 
@@ -976,15 +1396,22 @@ CURLcode curl_mime_type(curl_mimepart *part, const char *mimetype)
 /* Set mime data transfer encoder. */
 CURLcode curl_mime_encoder(curl_mimepart *part, const char *encoding)
 {
-  CURLcode result = CURLE_OK;
-
-  /* Encoding feature not yet implemented. */
+  CURLcode result = CURLE_BAD_FUNCTION_ARGUMENT;
+  const mime_encoder *mep;
 
   if(!part)
-    return CURLE_BAD_FUNCTION_ARGUMENT;
+    return result;
 
-  if(encoding)
-    return CURLE_BAD_FUNCTION_ARGUMENT;
+  part->encoder = NULL;
+
+  if(!encoding)
+    return CURLE_OK;    /* Removing current encoder. */
+
+  for(mep = encoders; mep->name; mep++)
+    if(strcasecompare(encoding, mep->name)) {
+      part->encoder = mep;
+      result = CURLE_OK;
+    }
 
   return result;
 }
@@ -1130,6 +1557,10 @@ curl_off_t Curl_mime_size(curl_mimepart *part)
     part->datasize = multipart_size(part->arg);
 
   size = part->datasize;
+
+  if(part->encoder)
+    size = part->encoder->sizefunc(part);
+
   if(size >= 0 && !(part->flags & MIME_BODY_ONLY)) {
     /* Compute total part size. */
     size += slist_size(part->curlheaders, 2, NULL);
@@ -1219,6 +1650,7 @@ CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
   curl_mime *mime = NULL;
   const char *boundary = NULL;
   char *s;
+  const char *cte = NULL;
   CURLcode ret = CURLE_OK;
 
   /* Get rid of previously prepared headers. */
@@ -1315,13 +1747,18 @@ CURLcode Curl_mime_prepare_headers(curl_mimepart *part,
   }
 
   /* Content-Transfer-Encoding header. */
-  if(contenttype && strategy == MIMESTRATEGY_MAIL &&
-     part->kind != MIMEKIND_MULTIPART &&
-     !search_header(part->userheaders, "Content-Transfer-Encoding")) {
-    ret = Curl_mime_add_header(&part->curlheaders,
-                               "Content-Transfer-Encoding: 8bit");
-    if(ret)
-      return ret;
+  if(!search_header(part->userheaders, "Content-Transfer-Encoding")) {
+    if(part->encoder)
+      cte = part->encoder->name;
+    else if(contenttype && strategy == MIMESTRATEGY_MAIL &&
+     part->kind != MIMEKIND_MULTIPART)
+      cte = "8bit";
+    if(cte) {
+      ret = Curl_mime_add_header(&part->curlheaders,
+                                 "Content-Transfer-Encoding: %s", cte);
+      if(ret)
+        return ret;
+    }
   }
 
   /* If we were reading curl-generated headers, restart with new ones (this
diff --git a/lib/mime.h b/lib/mime.h
index 369f9774d..5d2e9477b 100644
--- a/lib/mime.h
+++ b/lib/mime.h
@@ -23,6 +23,8 @@
  ***************************************************************************/
 
 #define MIME_RAND_BOUNDARY_CHARS        16  /* Nb. of random boundary chars. */
+#define MAX_ENCODED_LINE_LENGTH         76  /* Maximum encoded line length. */
+#define ENCODING_BUFFER_SIZE            256 /* Encoding temp buffers size. */
 
 /* Part flags. */
 #define MIME_USERHEADERS_OWNER  (1 << 0)
@@ -60,6 +62,22 @@ enum mimestrategy {
   MIMESTRATEGY_LAST
 };
 
+/* Content transfer encoder. */
+typedef struct {
+  const char *   name;          /* Encoding name. */
+  size_t         (*encodefunc)(char *buffer, size_t size, bool ateof,
+                             curl_mimepart *part);  /* Encoded read. */
+  curl_off_t     (*sizefunc)(curl_mimepart *part);  /* Encoded size. */
+}  mime_encoder;
+
+/* Content transfer encoder state. */
+typedef struct {
+  size_t         pos;           /* Position on output line. */
+  size_t         bufbeg;        /* Next data index in input buffer. */
+  size_t         bufend;        /* First unused byte index in input buffer. */
+  char           buf[ENCODING_BUFFER_SIZE]; /* Input buffer. */
+}  mime_encoder_state;
+
 /* Mime readback state. */
 struct mime_state {
   enum mimestate state;       /* Current state token. */
@@ -67,7 +85,7 @@ struct mime_state {
   size_t offset;              /* State-dependent offset. */
 };
 
-/* A mime context. */
+/* A mime multipart. */
 struct curl_mime_s {
   struct Curl_easy *easy;          /* The associated easy handle. */
   curl_mimepart *parent;           /* Parent part. */
@@ -99,6 +117,8 @@ struct curl_mimepart_s {
   curl_off_t datasize;             /* Expected data size. */
   unsigned int flags;              /* Flags. */
   struct mime_state state;         /* Current readback state. */
+  const mime_encoder *encoder;     /* Content data encoder. */
+  mime_encoder_state encstate;     /* Data encoder state. */
 };
 
 
diff --git a/src/tool_formparse.c b/src/tool_formparse.c
index de83d6e05..95e0980d1 100644
--- a/src/tool_formparse.c
+++ b/src/tool_formparse.c
@@ -172,11 +172,12 @@ static int read_field_headers(struct OperationConfig *config,
 
 static int get_param_part(struct OperationConfig *config, char **str,
                           char **pdata, char **ptype, char **pfilename,
-                          struct curl_slist **pheaders)
+                          char **pencoder, struct curl_slist **pheaders)
 {
   char *p = *str;
   char *type = NULL;
   char *filename = NULL;
+  char *encoder = NULL;
   char *endpos;
   char *tp;
   char sep;
@@ -191,6 +192,8 @@ static int get_param_part(struct OperationConfig *config, char **str,
     *pfilename = NULL;
   if(pheaders)
     *pheaders = NULL;
+  if(pencoder)
+    *pencoder = NULL;
   while(ISSPACE(*p))
     p++;
   tp = p;
@@ -300,6 +303,22 @@ static int get_param_part(struct OperationConfig *config, char **str,
         }
       }
     }
+    else if(checkprefix("encoder=", p)) {
+      if(endct) {
+        *endct = '\0';
+        endct = NULL;
+      }
+      for(p += 8; ISSPACE(*p); p++)
+        ;
+      tp = p;
+      encoder = get_param_word(&p, &endpos);
+      /* If not quoted, strip trailing spaces. */
+      if(encoder == tp)
+        while(endpos > encoder && ISSPACE(endpos[-1]))
+          endpos--;
+      sep = *p;
+      *endpos = '\0';
+    }
     else {
       /* unknown prefix, skip to next block */
       char *unknown = get_param_word(&p, &endpos);
@@ -335,6 +354,12 @@ static int get_param_part(struct OperationConfig *config, char **str,
     warnf(config->global,
           "Field file name not allowed here: %s\n", filename);
 
+  if(pencoder)
+    *pencoder = encoder;
+  else if(encoder)
+    warnf(config->global,
+          "Field encoder not allowed here: %s\n", encoder);
+
   if(pheaders)
     *pheaders = headers;
   else if(headers) {
@@ -421,6 +446,7 @@ int formparse(struct OperationConfig *config,
   char *data;
   char *type = NULL;
   char *filename = NULL;
+  char *encoder = NULL;
   struct curl_slist *headers = NULL;
   curl_mimepart *part = NULL;
   CURLcode res;
@@ -454,7 +480,7 @@ int formparse(struct OperationConfig *config,
       curl_mime *subparts;
 
       /* Starting a multipart. */
-      sep = get_param_part(config, &contp, &data, &type, NULL, &headers);
+      sep = get_param_part(config, &contp, &data, &type, NULL, NULL, &headers);
       if(sep < 0) {
         Curl_safefree(contents);
         return 3;
@@ -513,8 +539,8 @@ int formparse(struct OperationConfig *config,
         /* since this was a file, it may have a content-type specifier
            at the end too, or a filename. Or both. */
         ++contp;
-        sep = get_param_part(config,
-                             &contp, &data, &type, &filename, &headers);
+        sep = get_param_part(config, &contp,
+                             &data, &type, &filename, &encoder, &headers);
         if(sep < 0) {
           if(subparts != *mimecurrent)
             curl_mime_free(subparts);
@@ -577,13 +603,20 @@ int formparse(struct OperationConfig *config,
           Curl_safefree(contents);
           return 15;
         }
-        if(type && curl_mime_type(part, type)) {
+        if(curl_mime_type(part, type)) {
           warnf(config->global, "curl_mime_type failed!\n");
           if(subparts != *mimecurrent)
             curl_mime_free(subparts);
           Curl_safefree(contents);
           return 16;
         }
+        if(curl_mime_encoder(part, encoder)) {
+          warnf(config->global, "curl_mime_encoder failed!\n");
+          if(subparts != *mimecurrent)
+            curl_mime_free(subparts);
+          Curl_safefree(contents);
+          return 17;
+        }
 
         /* *contp could be '\0', so we just check with the delimiter */
       } while(sep); /* loop if there's another file name */
@@ -595,13 +628,13 @@ int formparse(struct OperationConfig *config,
           warnf(config->global, "curl_mime_addpart failed!\n");
           curl_mime_free(subparts);
           Curl_safefree(contents);
-          return 17;
+          return 18;
         }
         if(curl_mime_subparts(part, subparts)) {
           warnf(config->global, "curl_mime_subparts failed!\n");
           curl_mime_free(subparts);
           Curl_safefree(contents);
-          return 18;
+          return 19;
         }
       }
     }
@@ -611,16 +644,16 @@ int formparse(struct OperationConfig *config,
         if(!part) {
           warnf(config->global, "curl_mime_addpart failed!\n");
           Curl_safefree(contents);
-          return 19;
+          return 20;
         }
 
       if(*contp == '<' && !literal_value) {
         ++contp;
-        sep = get_param_part(config,
-                             &contp, &data, &type, &filename, &headers);
+        sep = get_param_part(config, &contp,
+                             &data, &type, &filename, &encoder, &headers);
         if(sep < 0) {
           Curl_safefree(contents);
-          return 20;
+          return 21;
         }
 
         /* Set part headers. */
@@ -628,7 +661,7 @@ int formparse(struct OperationConfig *config,
           warnf(config->global, "curl_mime_headers failed!\n");
           curl_slist_free_all(headers);
           Curl_safefree(contents);
-          return 21;
+          return 22;
         }
 
         /* Setup file in part. */
@@ -637,7 +670,7 @@ int formparse(struct OperationConfig *config,
           warnf(config->global, "setting file %s failed!\n", data);
           if(res != CURLE_READ_ERROR) {
             Curl_safefree(contents);
-            return 22;
+            return 23;
           }
         }
       }
@@ -645,11 +678,11 @@ int formparse(struct OperationConfig *config,
         if(literal_value)
           data = contp;
         else {
-          sep = get_param_part(config,
-                               &contp, &data, &type, &filename, &headers);
+          sep = get_param_part(config, &contp,
+                               &data, &type, &filename, &encoder, &headers);
           if(sep < 0) {
             Curl_safefree(contents);
-            return 23;
+            return 24;
           }
         }
 
@@ -658,33 +691,38 @@ int formparse(struct OperationConfig *config,
           warnf(config->global, "curl_mime_headers failed!\n");
           curl_slist_free_all(headers);
           Curl_safefree(contents);
-          return 24;
+          return 25;
         }
 
 #ifdef CURL_DOES_CONVERSIONS
         if(convert_to_network(data, strlen(data))) {
           warnf(config->global, "curl_formadd failed!\n");
           Curl_safefree(contents);
-          return 25;
+          return 26;
         }
 #endif
 
         if(curl_mime_data(part, data, CURL_ZERO_TERMINATED)) {
           warnf(config->global, "curl_mime_data failed!\n");
           Curl_safefree(contents);
-          return 26;
+          return 27;
         }
       }
 
       if(curl_mime_filename(part, filename)) {
         warnf(config->global, "curl_mime_filename failed!\n");
         Curl_safefree(contents);
-        return 27;
+        return 28;
       }
       if(curl_mime_type(part, type)) {
         warnf(config->global, "curl_mime_type failed!\n");
         Curl_safefree(contents);
-        return 28;
+        return 29;
+      }
+      if(curl_mime_encoder(part, encoder)) {
+        warnf(config->global, "curl_mime_encoder failed!\n");
+        Curl_safefree(contents);
+        return 30;
       }
 
       if(sep) {
@@ -698,13 +736,13 @@ int formparse(struct OperationConfig *config,
     if(name && curl_mime_name(part, name, CURL_ZERO_TERMINATED)) {
       warnf(config->global, "curl_mime_name failed!\n");
       Curl_safefree(contents);
-      return 29;
+      return 31;
     }
   }
   else {
     warnf(config->global, "Illegally formatted input field!\n");
     Curl_safefree(contents);
-    return 30;
+    return 32;
   }
   Curl_safefree(contents);
   return 0;
diff --git a/src/tool_setopt.c b/src/tool_setopt.c
index b0f319814..635304a8f 100644
--- a/src/tool_setopt.c
+++ b/src/tool_setopt.c
@@ -506,6 +506,14 @@ static CURLcode libcurl_generate_mime(curl_mime *mime, int *mimeno)
         break;
       }
 
+      if(part->encoder) {
+        Curl_safefree(escaped);
+        escaped = c_escape(part->encoder->name, CURL_ZERO_TERMINATED);
+        if(!escaped)
+          return CURLE_OUT_OF_MEMORY;
+        CODE2("curl_mime_encoder(part%d, \"%s\");", *mimeno, escaped);
+      }
+
       if(filename) {
         Curl_safefree(escaped);
         escaped = c_escape(filename, CURL_ZERO_TERMINATED);
diff --git a/tests/data/Makefile.inc b/tests/data/Makefile.inc
index 464faeeaf..d7ea5c951 100644
--- a/tests/data/Makefile.inc
+++ b/tests/data/Makefile.inc
@@ -78,7 +78,7 @@ test608 test609 test610 test611 test612 test613 test614 test615 test616 \
 test617 test618 test619 test620 test621 test622 test623 test624 test625 \
 test626 test627 test628 test629 test630 test631 test632 test633 test634 \
 test635 test636 test637 test638 test639 test640 test641 test642 \
-test643 test644 test645 test646 test647 \
+test643 test644 test645 test646 test647 test648 test649 \
 \
 test700 test701 test702 test703 test704 test705 test706 test707 test708 \
 test709 test710 test711 test712 test713 test714 test715 \
diff --git a/tests/data/test1404 b/tests/data/test1404
index a3ec66ef7..20dc01333 100644
--- a/tests/data/test1404
+++ b/tests/data/test1404
@@ -27,13 +27,13 @@ Connection: close
 http
 </server>
  <name>
---libcurl for HTTP RFC1867-type formposting - -F with three files, one with explicit type
+--libcurl for HTTP RFC1867-type formposting - -F with 3 files, one with explicit type & encoder
  </name>
 <setenv>
 SSL_CERT_FILE=
 </setenv>
  <command>
-http://%HOSTIP:%HTTPPORT/we/want/1404 -F name=value -F 'file=@log/test1404.txt,log/test1404.txt;type=magic/content,log/test1404.txt;headers=X-testheader-1: header 1;headers=X-testheader-2: header 2' --libcurl log/test1404.c
+http://%HOSTIP:%HTTPPORT/we/want/1404 -F name=value -F 'file=@log/test1404.txt,log/test1404.txt;type=magic/content;encoder=8bit,log/test1404.txt;headers=X-testheader-1: header 1;headers=X-testheader-2: header 2' --libcurl log/test1404.c
 </command>
 # We create this file before the command is invoked!
 <file name="log/test1404.txt">
@@ -51,7 +51,7 @@ POST /we/want/1404 HTTP/1.1
 User-Agent: curl/7.18.2 (i686-pc-linux-gnu) libcurl/7.18.2 OpenSSL/0.9.7a ipv6 zlib/1.1.4

 Host: %HOSTIP:%HTTPPORT

 Accept: */*

-Content-Length: 849

+Content-Length: 882

 Content-Type: multipart/form-data; boundary=----------------------------9ef8d6205763

 

 ------------------------------9ef8d6205763

@@ -70,6 +70,7 @@ dummy data
 ------------------------------9ef8d6205763

 Content-Disposition: attachment; filename="test1404.txt"

 Content-Type: magic/content

+Content-Transfer-Encoding: 8bit

 

 dummy data
 

@@ -131,6 +132,7 @@ int main(int argc, char *argv[])
   curl_mime_filedata(part2, "log/test1404.txt");
   part2 = curl_mime_addpart(mime2);
   curl_mime_filedata(part2, "log/test1404.txt");
+  curl_mime_encoder(part2, "8bit");
   curl_mime_type(part2, "magic/content");
   part2 = curl_mime_addpart(mime2);
   curl_mime_filedata(part2, "log/test1404.txt");
diff --git a/tests/data/test648 b/tests/data/test648
new file mode 100644
index 000000000..cd8f02085
--- /dev/null
+++ b/tests/data/test648
@@ -0,0 +1,75 @@
+<testcase>
+<info>
+<keywords>
+SMTP
+MULTIPART
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+smtp
+</server>
+ <name>
+SMTP multipart with transfer content encoders
+ </name>
+<stdin>
+From: different

+To: another

+

+body

+</stdin>
+ <command>
+smtp://%HOSTIP:%SMTPPORT/648 --mail-rcpt recipient@example.com --mail-from sender@example.com -F '=This is the e-mail inline text with a very long line containing the special character = and that should be split by encoder.;headers=Content-disposition: "inline";encoder=quoted-printable' -F "=@log/test648.txt;encoder=base64" -H "From: different" -H "To: another"
+</command>
+<file name="log/test648.txt">
+This is an attached file.
+
+It may contain any type of data and will be encoded in base64 for transfer.
+</file>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strippart>
+s/^--------------------------[a-z0-9]*/------------------------------/
+s/boundary=------------------------[a-z0-9]*/boundary=----------------------------/
+</strippart>
+<protocol>
+EHLO 648

+MAIL FROM:<sender@example.com>

+RCPT TO:<recipient@example.com>

+DATA

+QUIT

+</protocol>
+<upload>
+Content-Type: multipart/mixed; boundary=----------------------------

+Mime-Version: 1.0

+From: different

+To: another

+

+------------------------------

+Content-Transfer-Encoding: quoted-printable

+Content-disposition: "inline"

+

+This is the e-mail inline text with a very long line containing the special=

+ character =3D and that should be split by encoder.

+------------------------------

+Content-Disposition: attachment; filename="test648.txt"

+Content-Transfer-Encoding: base64

+

+VGhpcyBpcyBhbiBhdHRhY2hlZCBmaWxlLgoKSXQgbWF5IGNvbnRhaW4gYW55IHR5cGUgb2Yg

+ZGF0YSBhbmQgd2lsbCBiZSBlbmNvZGVkIGluIGJhc2U2NCBmb3IgdHJhbnNmZXIuCg==

+--------------------------------

+.

+</upload>
+</verify>
+</testcase>
diff --git a/tests/data/test649 b/tests/data/test649
new file mode 100644
index 000000000..46c01cd00
--- /dev/null
+++ b/tests/data/test649
@@ -0,0 +1,72 @@
+<testcase>
+<info>
+<keywords>
+SMTP
+MULTIPART
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+smtp
+</server>
+ <name>
+SMTP multipart with 7bit encoder error
+ </name>
+<stdin>
+From: different

+To: another

+

+body

+</stdin>
+ <command>
+smtp://%HOSTIP:%SMTPPORT/649 --mail-rcpt recipient@example.com --mail-from sender@example.com -F '=This is valid;encoder=7bit' -F "=@log/test649.txt;encoder=7bit" -H "From: different" -H "To: another"
+</command>
+<file name="log/test649.txt">
+This is an attached file (in french: pièce jointe).
+
+It contains at least an 8-bit byte value.
+</file>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strippart>
+s/^--------------------------[a-z0-9]*/------------------------------/
+s/boundary=------------------------[a-z0-9]*/boundary=----------------------------/
+</strippart>
+<protocol>
+EHLO 649

+MAIL FROM:<sender@example.com>

+RCPT TO:<recipient@example.com>

+DATA

+</protocol>
+<upload nonewline="yes">
+Content-Type: multipart/mixed; boundary=----------------------------

+Mime-Version: 1.0

+From: different

+To: another

+

+------------------------------

+Content-Transfer-Encoding: 7bit

+

+This is valid

+------------------------------

+Content-Disposition: attachment; filename="test649.txt"

+Content-Transfer-Encoding: 7bit

+

+This is an attached file (in french: pi
+</upload>
+<errorcode>
+26
+</errorcode>
+</verify>
+</testcase>