base64.c

/* base64.c -- Encode binary data using printable characters.
   Copyright (C) 1999-2001, 2004-2006, 2009-2024 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* Written by Simon Josefsson.  Partially adapted from GNU MailUtils
 * (mailbox/filter_trans.c, as of 2004-11-28).  Improved by review
 * from Paul Eggert, Bruno Haible, and Stepan Kasal.
 *
 * See also RFC 4648 <https://www.ietf.org/rfc/rfc4648.txt>.
 *
 * Be careful with error checking.  Here is how you would typically
 * use these functions:
 *
 * bool ok = base64_decode_alloc (in, inlen, &out, &outlen);
 * if (!ok)
 *   FAIL: input was not valid base64
 * if (out == NULL)
 *   FAIL: memory allocation error
 * OK: data in OUT/OUTLEN
 *
 * idx_t outlen = base64_encode_alloc (in, inlen, &out);
 * if (out == NULL && outlen == 0 && inlen != 0)
 *   FAIL: input too long
 * if (out == NULL)
 *   FAIL: memory allocation error
 * OK: data in OUT/OUTLEN.
 *
 */

#include <config.h>

/* Get prototype. */
#define BASE64_INLINE _GL_EXTERN_INLINE
#include "base64.h"

/* Get imalloc. */
#include <ialloc.h>

#include <stdckdint.h>

#include <string.h>

/* Convert 'char' to 'unsigned char' without casting.  */
static unsigned char
to_uchar (char ch)
{
  return ch;
}

static const char b64c[64] =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/* Base64 encode IN array of size INLEN into OUT array. OUT needs
   to be of length >= BASE64_LENGTH(INLEN), and INLEN needs to be
   a multiple of 3.  */
static void
base64_encode_fast (const char *restrict in, idx_t inlen, char *restrict out)
{
  while (inlen)
    {
      *out++ = b64c[(to_uchar (in[0]) >> 2) & 0x3f];
      *out++ = b64c[((to_uchar (in[0]) << 4) + (to_uchar (in[1]) >> 4)) & 0x3f];
      *out++ = b64c[((to_uchar (in[1]) << 2) + (to_uchar (in[2]) >> 6)) & 0x3f];
      *out++ = b64c[to_uchar (in[2]) & 0x3f];

      inlen -= 3;
      in += 3;
    }
}

/* Base64 encode IN array of size INLEN into OUT array of size OUTLEN.
   If OUTLEN is less than BASE64_LENGTH(INLEN), write as many bytes as
   possible.  If OUTLEN is larger than BASE64_LENGTH(INLEN), also zero
   terminate the output buffer. */
void
base64_encode (const char *restrict in, idx_t inlen,
               char *restrict out, idx_t outlen)
{
  /* Note this outlen constraint can be enforced at compile time.
     I.E. that the output buffer is exactly large enough to hold
     the encoded inlen bytes.  The inlen constraints (of corresponding
     to outlen, and being a multiple of 3) can change at runtime
     at the end of input.  However the common case when reading
     large inputs is to have both constraints satisfied, so we depend
     on both in base_encode_fast().  */
  if (outlen % 4 == 0 && inlen == (outlen >> 2) * 3)
    {
      base64_encode_fast (in, inlen, out);
      return;
    }

  while (inlen && outlen)
    {
      *out++ = b64c[(to_uchar (in[0]) >> 2) & 0x3f];
      if (!--outlen)
        break;
      *out++ = b64c[((to_uchar (in[0]) << 4)
                       + (--inlen ? to_uchar (in[1]) >> 4 : 0))
                      & 0x3f];
      if (!--outlen)
        break;
      *out++ =
        (inlen
         ? b64c[((to_uchar (in[1]) << 2)
                   + (--inlen ? to_uchar (in[2]) >> 6 : 0))
                  & 0x3f]
         : '=');
      if (!--outlen)
        break;
      *out++ = inlen ? b64c[to_uchar (in[2]) & 0x3f] : '=';
      if (!--outlen)
        break;
      if (inlen)
        inlen--;
      if (inlen)
        in += 3;
    }

  if (outlen)
    *out = '\0';
}

/* Allocate a buffer and store zero terminated base64 encoded data
   from array IN of size INLEN, returning BASE64_LENGTH(INLEN), i.e.,
   the length of the encoded data, excluding the terminating zero.  On
   return, the OUT variable will hold a pointer to newly allocated
   memory that must be deallocated by the caller.  If output string
   length would overflow, 0 is returned and OUT is set to NULL.  If
   memory allocation failed, OUT is set to NULL, and the return value
   indicates length of the requested memory block, i.e.,
   BASE64_LENGTH(inlen) + 1. */
idx_t
base64_encode_alloc (const char *in, idx_t inlen, char **out)
{
  /* Check for overflow in outlen computation.
     Treat negative INLEN as overflow, for better compatibility with
     pre-2021-08-27 API, which used size_t.  */
  idx_t in_over_3 = inlen / 3 + (inlen % 3 != 0), outlen;
  if (ckd_mul (&outlen, in_over_3, 4) || inlen < 0)
    {
      *out = NULL;
      return 0;
    }
  outlen++;

  *out = imalloc (outlen);
  if (!*out)
    return outlen;

  base64_encode (in, inlen, *out, outlen);

  return outlen - 1;
}

/* With this approach this file works independent of the charset used
   (think EBCDIC).  However, it does assume that the characters in the
   Base64 alphabet (A-Za-z0-9+/) are encoded in 0..255.  POSIX
   1003.1-2001 require that char and unsigned char are 8-bit
   quantities, though, taking care of that problem.  But this may be a
   potential problem on non-POSIX C99 platforms.

   IBM C V6 for AIX mishandles "#define B64(x) ...'x'...", so use "_"
   as the formal parameter rather than "x".  */
#define B64(_)                                  \
  ((_) == 'A' ? 0                               \
   : (_) == 'B' ? 1                             \
   : (_) == 'C' ? 2                             \
   : (_) == 'D' ? 3                             \
   : (_) == 'E' ? 4                             \
   : (_) == 'F' ? 5                             \
   : (_) == 'G' ? 6                             \
   : (_) == 'H' ? 7                             \
   : (_) == 'I' ? 8                             \
   : (_) == 'J' ? 9                             \
   : (_) == 'K' ? 10                            \
   : (_) == 'L' ? 11                            \
   : (_) == 'M' ? 12                            \
   : (_) == 'N' ? 13                            \
   : (_) == 'O' ? 14                            \
   : (_) == 'P' ? 15                            \
   : (_) == 'Q' ? 16                            \
   : (_) == 'R' ? 17                            \
   : (_) == 'S' ? 18                            \
   : (_) == 'T' ? 19                            \
   : (_) == 'U' ? 20                            \
   : (_) == 'V' ? 21                            \
   : (_) == 'W' ? 22                            \
   : (_) == 'X' ? 23                            \
   : (_) == 'Y' ? 24                            \
   : (_) == 'Z' ? 25                            \
   : (_) == 'a' ? 26                            \
   : (_) == 'b' ? 27                            \
   : (_) == 'c' ? 28                            \
   : (_) == 'd' ? 29                            \
   : (_) == 'e' ? 30                            \
   : (_) == 'f' ? 31                            \
   : (_) == 'g' ? 32                            \
   : (_) == 'h' ? 33                            \
   : (_) == 'i' ? 34                            \
   : (_) == 'j' ? 35                            \
   : (_) == 'k' ? 36                            \
   : (_) == 'l' ? 37                            \
   : (_) == 'm' ? 38                            \
   : (_) == 'n' ? 39                            \
   : (_) == 'o' ? 40                            \
   : (_) == 'p' ? 41                            \
   : (_) == 'q' ? 42                            \
   : (_) == 'r' ? 43                            \
   : (_) == 's' ? 44                            \
   : (_) == 't' ? 45                            \
   : (_) == 'u' ? 46                            \
   : (_) == 'v' ? 47                            \
   : (_) == 'w' ? 48                            \
   : (_) == 'x' ? 49                            \
   : (_) == 'y' ? 50                            \
   : (_) == 'z' ? 51                            \
   : (_) == '0' ? 52                            \
   : (_) == '1' ? 53                            \
   : (_) == '2' ? 54                            \
   : (_) == '3' ? 55                            \
   : (_) == '4' ? 56                            \
   : (_) == '5' ? 57                            \
   : (_) == '6' ? 58                            \
   : (_) == '7' ? 59                            \
   : (_) == '8' ? 60                            \
   : (_) == '9' ? 61                            \
   : (_) == '+' ? 62                            \
   : (_) == '/' ? 63                            \
   : -1)

signed char const base64_to_int[256] = {
  B64 (0), B64 (1), B64 (2), B64 (3),
  B64 (4), B64 (5), B64 (6), B64 (7),
  B64 (8), B64 (9), B64 (10), B64 (11),
  B64 (12), B64 (13), B64 (14), B64 (15),
  B64 (16), B64 (17), B64 (18), B64 (19),
  B64 (20), B64 (21), B64 (22), B64 (23),
  B64 (24), B64 (25), B64 (26), B64 (27),
  B64 (28), B64 (29), B64 (30), B64 (31),
  B64 (32), B64 (33), B64 (34), B64 (35),
  B64 (36), B64 (37), B64 (38), B64 (39),
  B64 (40), B64 (41), B64 (42), B64 (43),
  B64 (44), B64 (45), B64 (46), B64 (47),
  B64 (48), B64 (49), B64 (50), B64 (51),
  B64 (52), B64 (53), B64 (54), B64 (55),
  B64 (56), B64 (57), B64 (58), B64 (59),
  B64 (60), B64 (61), B64 (62), B64 (63),
  B64 (64), B64 (65), B64 (66), B64 (67),
  B64 (68), B64 (69), B64 (70), B64 (71),
  B64 (72), B64 (73), B64 (74), B64 (75),
  B64 (76), B64 (77), B64 (78), B64 (79),
  B64 (80), B64 (81), B64 (82), B64 (83),
  B64 (84), B64 (85), B64 (86), B64 (87),
  B64 (88), B64 (89), B64 (90), B64 (91),
  B64 (92), B64 (93), B64 (94), B64 (95),
  B64 (96), B64 (97), B64 (98), B64 (99),
  B64 (100), B64 (101), B64 (102), B64 (103),
  B64 (104), B64 (105), B64 (106), B64 (107),
  B64 (108), B64 (109), B64 (110), B64 (111),
  B64 (112), B64 (113), B64 (114), B64 (115),
  B64 (116), B64 (117), B64 (118), B64 (119),
  B64 (120), B64 (121), B64 (122), B64 (123),
  B64 (124), B64 (125), B64 (126), B64 (127),
  B64 (128), B64 (129), B64 (130), B64 (131),
  B64 (132), B64 (133), B64 (134), B64 (135),
  B64 (136), B64 (137), B64 (138), B64 (139),
  B64 (140), B64 (141), B64 (142), B64 (143),
  B64 (144), B64 (145), B64 (146), B64 (147),
  B64 (148), B64 (149), B64 (150), B64 (151),
  B64 (152), B64 (153), B64 (154), B64 (155),
  B64 (156), B64 (157), B64 (158), B64 (159),
  B64 (160), B64 (161), B64 (162), B64 (163),
  B64 (164), B64 (165), B64 (166), B64 (167),
  B64 (168), B64 (169), B64 (170), B64 (171),
  B64 (172), B64 (173), B64 (174), B64 (175),
  B64 (176), B64 (177), B64 (178), B64 (179),
  B64 (180), B64 (181), B64 (182), B64 (183),
  B64 (184), B64 (185), B64 (186), B64 (187),
  B64 (188), B64 (189), B64 (190), B64 (191),
  B64 (192), B64 (193), B64 (194), B64 (195),
  B64 (196), B64 (197), B64 (198), B64 (199),
  B64 (200), B64 (201), B64 (202), B64 (203),
  B64 (204), B64 (205), B64 (206), B64 (207),
  B64 (208), B64 (209), B64 (210), B64 (211),
  B64 (212), B64 (213), B64 (214), B64 (215),
  B64 (216), B64 (217), B64 (218), B64 (219),
  B64 (220), B64 (221), B64 (222), B64 (223),
  B64 (224), B64 (225), B64 (226), B64 (227),
  B64 (228), B64 (229), B64 (230), B64 (231),
  B64 (232), B64 (233), B64 (234), B64 (235),
  B64 (236), B64 (237), B64 (238), B64 (239),
  B64 (240), B64 (241), B64 (242), B64 (243),
  B64 (244), B64 (245), B64 (246), B64 (247),
  B64 (248), B64 (249), B64 (250), B64 (251),
  B64 (252), B64 (253), B64 (254), B64 (255)
};

/* If CTX->i is 0 or 4, there are four or more bytes in [*IN..IN_END), and
   none of those four is a newline, then return *IN.  Otherwise, copy up to
   4 - CTX->i non-newline bytes from that range into CTX->buf, starting at
   index CTX->i and setting CTX->i to reflect the number of bytes copied,
   and return CTX->buf.  In either case, advance *IN to point to the byte
   after the last one processed, and set *N_NON_NEWLINE to the number of
   verified non-newline bytes accessible through the returned pointer.  */
static char *
get_4 (struct base64_decode_context *ctx,
       char const *restrict *in, char const *restrict in_end,
       idx_t *n_non_newline)
{
  if (ctx->i == 4)
    ctx->i = 0;

  if (ctx->i == 0)
    {
      char const *t = *in;
      if (4 <= in_end - *in && memchr (t, '\n', 4) == NULL)
        {
          /* This is the common case: no newline.  */
          *in += 4;
          *n_non_newline = 4;
          return (char *) t;
        }
    }

  {
    /* Copy non-newline bytes into BUF.  */
    char const *p = *in;
    while (p < in_end)
      {
        char c = *p++;
        if (c != '\n')
          {
            ctx->buf[ctx->i++] = c;
            if (ctx->i == 4)
              break;
          }
      }

    *in = p;
    *n_non_newline = ctx->i;
    return ctx->buf;
  }
}

#define return_false                            \
  do                                            \
    {                                           \
      *outp = out;                              \
      return false;                             \
    }                                           \
  while (false)

/* Decode up to four bytes of base64-encoded data, IN, of length INLEN
   into the output buffer, *OUT, of size *OUTLEN bytes.  Return true if
   decoding is successful, false otherwise.  If *OUTLEN is too small,
   as many bytes as possible are written to *OUT.  On return, advance
   *OUT to point to the byte after the last one written, and decrement
   *OUTLEN to reflect the number of bytes remaining in *OUT.  */
static bool
decode_4 (char const *restrict in, idx_t inlen,
          char *restrict *outp, idx_t *outleft)
{
  char *out = *outp;
  if (inlen < 2)
    return false;

  if (!isbase64 (in[0]) || !isbase64 (in[1]))
    return false;

  if (*outleft)
    {
      *out++ = ((base64_to_int[to_uchar (in[0])] << 2)
                | (base64_to_int[to_uchar (in[1])] >> 4));
      --*outleft;
    }

  if (inlen == 2)
    return_false;

  if (in[2] == '=')
    {
      if (inlen != 4)
        return_false;

      if (in[3] != '=')
        return_false;

      /* Reject non-canonical encodings.  */
      if (base64_to_int[to_uchar (in[1])] & 0x0f)
        return_false;
    }
  else
    {
      if (!isbase64 (in[2]))
        return_false;

      if (*outleft)
        {
          *out++ = (((base64_to_int[to_uchar (in[1])] << 4) & 0xf0)
                    | (base64_to_int[to_uchar (in[2])] >> 2));
          --*outleft;
        }

      if (inlen == 3)
        return_false;

      if (in[3] == '=')
        {
          if (inlen != 4)
            return_false;

          /* Reject non-canonical encodings.  */
          if (base64_to_int[to_uchar (in[2])] & 0x03)
            return_false;
        }
      else
        {
          if (!isbase64 (in[3]))
            return_false;

          if (*outleft)
            {
              *out++ = (((base64_to_int[to_uchar (in[2])] << 6) & 0xc0)
                        | base64_to_int[to_uchar (in[3])]);
              --*outleft;
            }
        }
    }

  *outp = out;
  return true;
}

/* Decode base64-encoded input array IN of length INLEN to output array
   OUT that can hold *OUTLEN bytes.  The input data may be interspersed
   with newlines.  Return true if decoding was successful, i.e. if the
   input was valid base64 data, false otherwise.  If *OUTLEN is too
   small, as many bytes as possible will be written to OUT.  On return,
   *OUTLEN holds the length of decoded bytes in OUT.  Note that as soon
   as any non-alphabet, non-newline character is encountered, decoding
   is stopped and false is returned.  If INLEN is zero, then process
   only whatever data is stored in CTX.

   Initially, CTX must have been initialized via base64_decode_ctx_init.
   Subsequent calls to this function must reuse whatever state is recorded
   in that buffer.  It is necessary for when a quadruple of base64 input
   bytes spans two input buffers.

   If CTX is NULL then newlines are treated as garbage and the input
   buffer is processed as a unit.  */

bool
base64_decode_ctx (struct base64_decode_context *ctx,
                   const char *restrict in, idx_t inlen,
                   char *restrict out, idx_t *outlen)
{
  idx_t outleft = *outlen;
  bool ignore_newlines = ctx != NULL;
  bool flush_ctx = false;
  unsigned int ctx_i = 0;

  if (ignore_newlines)
    {
      ctx_i = ctx->i;
      flush_ctx = inlen == 0;
    }


  while (true)
    {
      idx_t outleft_save = outleft;
      if (ctx_i == 0 && !flush_ctx)
        {
          while (true)
            {
              /* Save a copy of outleft, in case we need to re-parse this
                 block of four bytes.  */
              outleft_save = outleft;
              if (!decode_4 (in, inlen, &out, &outleft))
                break;

              in += 4;
              inlen -= 4;
            }
        }

      if (inlen == 0 && !flush_ctx)
        break;

      /* Handle the common case of 72-byte wrapped lines.
         This also handles any other multiple-of-4-byte wrapping.  */
      if (inlen && *in == '\n' && ignore_newlines)
        {
          ++in;
          --inlen;
          continue;
        }

      /* Restore OUT and OUTLEFT.  */
      out -= outleft_save - outleft;
      outleft = outleft_save;

      {
        char const *in_end = in + inlen;
        char const *non_nl;

        if (ignore_newlines)
          non_nl = get_4 (ctx, &in, in_end, &inlen);
        else
          non_nl = in;  /* Might have nl in this case. */

        /* If the input is empty or consists solely of newlines (0 non-newlines),
           then we're done.  Likewise if there are fewer than 4 bytes when not
           flushing context and not treating newlines as garbage.  */
        if (inlen == 0 || (inlen < 4 && !flush_ctx && ignore_newlines))
          {
            inlen = 0;
            break;
          }
        if (!decode_4 (non_nl, inlen, &out, &outleft))
          break;

        inlen = in_end - in;
      }
    }

  *outlen -= outleft;

  return inlen == 0;
}

/* Allocate an output buffer in *OUT, and decode the base64 encoded
   data stored in IN of size INLEN to the *OUT buffer.  On return, the
   size of the decoded data is stored in *OUTLEN.  OUTLEN may be NULL,
   if the caller is not interested in the decoded length.  *OUT may be
   NULL to indicate an out of memory error, in which case *OUTLEN
   contains the size of the memory block needed.  The function returns
   true on successful decoding and memory allocation errors.  (Use the
   *OUT and *OUTLEN parameters to differentiate between successful
   decoding and memory error.)  The function returns false if the
   input was invalid, in which case *OUT is NULL and *OUTLEN is
   undefined. */
bool
base64_decode_alloc_ctx (struct base64_decode_context *ctx,
                         const char *in, idx_t inlen, char **out,
                         idx_t *outlen)
{
  /* This may allocate a few bytes too many, depending on input,
     but it's not worth the extra CPU time to compute the exact size.
     The exact size is 3 * (inlen + (ctx ? ctx->i : 0)) / 4, minus 1 if the
     input ends with "=" and minus another 1 if the input ends with "==".
     Shifting before multiplying avoids the possibility of overflow.  */
  idx_t needlen = 3 * ((inlen >> 2) + 1);

  *out = imalloc (needlen);
  if (!*out)
    return true;

  if (!base64_decode_ctx (ctx, in, inlen, *out, &needlen))
    {
      free (*out);
      *out = NULL;
      return false;
    }

  if (outlen)
    *outlen = needlen;

  return true;
}