sha256-crypt.c

/* SHA256-based Unix crypt implementation.
   Released into the Public Domain by Ulrich Drepper <drepper@redhat.com>.  */

/*
   Slightly modified by Daniel Vérité, 2018:
   - no declaration of variables in for loops for pre-C99 compilers
   - replaced mempcpy with equivalent memcpy calls.
   - moved some definitions into sha256.h
   - removed inclusion of endian.h.

   See https://www.akkadia.org/drepper/SHA-crypt.txt
   for the original source code.
*/
#include <errno.h>
#include <limits.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/param.h>
#include <sys/types.h>

#include "sha256.h"


#if __BYTE_ORDER == __LITTLE_ENDIAN
# define SWAP(n) \
    (((n) << 24) | (((n) & 0xff00) << 8) | (((n) >> 8) & 0xff00) | ((n) >> 24))
#else
# define SWAP(n) (n)
#endif


/* This array contains the bytes used to pad the buffer to the next
   64-byte boundary.  (FIPS 180-2:5.1.1)  */
static const unsigned char fillbuf[64] = { 0x80, 0 /* , 0, 0, ...  */ };


/* Constants for SHA256 from FIPS 180-2:4.2.2.  */
static const uint32_t K[64] =
  {
    0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5,
    0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
    0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
    0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
    0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc,
    0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
    0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7,
    0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
    0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
    0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
    0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3,
    0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
    0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5,
    0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
    0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
    0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2
  };


/* Process LEN bytes of BUFFER, accumulating context into CTX.
   It is assumed that LEN % 64 == 0.  */
static void
sha256_process_block (const void *buffer, size_t len, struct sha256_ctx *ctx)
{
  const uint32_t *words = buffer;
  size_t nwords = len / sizeof (uint32_t);
  uint32_t a = ctx->H[0];
  uint32_t b = ctx->H[1];
  uint32_t c = ctx->H[2];
  uint32_t d = ctx->H[3];
  uint32_t e = ctx->H[4];
  uint32_t f = ctx->H[5];
  uint32_t g = ctx->H[6];
  uint32_t h = ctx->H[7];

  /* First increment the byte count.  FIPS 180-2 specifies the possible
     length of the file up to 2^64 bits.  Here we only compute the
     number of bytes.  Do a double word increment.  */
  ctx->total[0] += len;
  if (ctx->total[0] < len)
    ++ctx->total[1];

  /* Process all bytes in the buffer with 64 bytes in each round of
     the loop.  */
  while (nwords > 0)
    {
      unsigned int t;
      uint32_t W[64];
      uint32_t a_save = a;
      uint32_t b_save = b;
      uint32_t c_save = c;
      uint32_t d_save = d;
      uint32_t e_save = e;
      uint32_t f_save = f;
      uint32_t g_save = g;
      uint32_t h_save = h;

      /* Operators defined in FIPS 180-2:4.1.2.  */
#define Ch(x, y, z) ((x & y) ^ (~x & z))
#define Maj(x, y, z) ((x & y) ^ (x & z) ^ (y & z))
#define S0(x) (CYCLIC (x, 2) ^ CYCLIC (x, 13) ^ CYCLIC (x, 22))
#define S1(x) (CYCLIC (x, 6) ^ CYCLIC (x, 11) ^ CYCLIC (x, 25))
#define R0(x) (CYCLIC (x, 7) ^ CYCLIC (x, 18) ^ (x >> 3))
#define R1(x) (CYCLIC (x, 17) ^ CYCLIC (x, 19) ^ (x >> 10))

      /* It is unfortunate that C does not provide an operator for
	 cyclic rotation.  Hope the C compiler is smart enough.  */
#define CYCLIC(w, s) ((w >> s) | (w << (32 - s)))

      /* Compute the message schedule according to FIPS 180-2:6.2.2 step 2.  */
      for (t = 0; t < 16; ++t)
	{
	  W[t] = SWAP (*words);
	  ++words;
	}
      for (t = 16; t < 64; ++t)
	W[t] = R1 (W[t - 2]) + W[t - 7] + R0 (W[t - 15]) + W[t - 16];

      /* The actual computation according to FIPS 180-2:6.2.2 step 3.  */
      for (t = 0; t < 64; ++t)
	{
	  uint32_t T1 = h + S1 (e) + Ch (e, f, g) + K[t] + W[t];
	  uint32_t T2 = S0 (a) + Maj (a, b, c);
	  h = g;
	  g = f;
	  f = e;
	  e = d + T1;
	  d = c;
	  c = b;
	  b = a;
	  a = T1 + T2;
	}

      /* Add the starting values of the context according to FIPS 180-2:6.2.2
	 step 4.  */
      a += a_save;
      b += b_save;
      c += c_save;
      d += d_save;
      e += e_save;
      f += f_save;
      g += g_save;
      h += h_save;

      /* Prepare for the next round.  */
      nwords -= 16;
    }

  /* Put checksum in context given as argument.  */
  ctx->H[0] = a;
  ctx->H[1] = b;
  ctx->H[2] = c;
  ctx->H[3] = d;
  ctx->H[4] = e;
  ctx->H[5] = f;
  ctx->H[6] = g;
  ctx->H[7] = h;
}


/* Initialize structure containing state of computation.
   (FIPS 180-2:5.3.2)  */
static void
sha256_init_ctx (struct sha256_ctx *ctx)
{
  ctx->H[0] = 0x6a09e667;
  ctx->H[1] = 0xbb67ae85;
  ctx->H[2] = 0x3c6ef372;
  ctx->H[3] = 0xa54ff53a;
  ctx->H[4] = 0x510e527f;
  ctx->H[5] = 0x9b05688c;
  ctx->H[6] = 0x1f83d9ab;
  ctx->H[7] = 0x5be0cd19;

  ctx->total[0] = ctx->total[1] = 0;
  ctx->buflen = 0;
}


/* Process the remaining bytes in the internal buffer and the usual
   prolog according to the standard and write the result to RESBUF.

   IMPORTANT: On some systems it is required that RESBUF is correctly
   aligned for a 32 bits value.  */
static void *
sha256_finish_ctx (struct sha256_ctx *ctx, void *resbuf)
{
  /* Take yet unprocessed bytes into account.  */
  uint32_t bytes = ctx->buflen;
  size_t pad;
  int i;

  /* Now count remaining bytes.  */
  ctx->total[0] += bytes;
  if (ctx->total[0] < bytes)
    ++ctx->total[1];

  pad = bytes >= 56 ? 64 + 56 - bytes : 56 - bytes;
  memcpy (&ctx->buffer[bytes], fillbuf, pad);

  /* Put the 64-bit file length in *bits* at the end of the buffer.  */
  *(uint32_t *) &ctx->buffer[bytes + pad + 4] = SWAP (ctx->total[0] << 3);
  *(uint32_t *) &ctx->buffer[bytes + pad] = SWAP ((ctx->total[1] << 3) |
						  (ctx->total[0] >> 29));

  /* Process last bytes.  */
  sha256_process_block (ctx->buffer, bytes + pad + 8, ctx);

  /* Put result from CTX in first 32 bytes following RESBUF.  */
  for (i = 0; i < 8; ++i)
    ((uint32_t *) resbuf)[i] = SWAP (ctx->H[i]);

  return resbuf;
}


static void
sha256_process_bytes (const void *buffer, size_t len, struct sha256_ctx *ctx)
{
  /* When we already have some bits in our internal buffer concatenate
     both inputs first.  */
  if (ctx->buflen != 0)
    {
      size_t left_over = ctx->buflen;
      size_t add = 128 - left_over > len ? len : 128 - left_over;

      memcpy (&ctx->buffer[left_over], buffer, add);
      ctx->buflen += add;

      if (ctx->buflen > 64)
	{
	  sha256_process_block (ctx->buffer, ctx->buflen & ~63, ctx);

	  ctx->buflen &= 63;
	  /* The regions in the following copy operation cannot overlap.  */
	  memcpy (ctx->buffer, &ctx->buffer[(left_over + add) & ~63],
		  ctx->buflen);
	}

      buffer = (const char *) buffer + add;
      len -= add;
    }

  /* Process available complete blocks.  */
  if (len >= 64)
    {
/* To check alignment gcc has an appropriate operator.  Other
   compilers don't.  */
#if __GNUC__ >= 2
# define UNALIGNED_P(p) (((uintptr_t) p) % __alignof__ (uint32_t) != 0)
#else
# define UNALIGNED_P(p) (((uintptr_t) p) % sizeof (uint32_t) != 0)
#endif
      if (UNALIGNED_P (buffer))
	while (len > 64)
	  {
	    sha256_process_block (memcpy (ctx->buffer, buffer, 64), 64, ctx);
	    buffer = (const char *) buffer + 64;
	    len -= 64;
	  }
      else
	{
	  sha256_process_block (buffer, len & ~63, ctx);
	  buffer = (const char *) buffer + (len & ~63);
	  len &= 63;
	}
    }

  /* Move remaining bytes into internal buffer.  */
  if (len > 0)
    {
      size_t left_over = ctx->buflen;

      memcpy (&ctx->buffer[left_over], buffer, len);
      left_over += len;
      if (left_over >= 64)
	{
	  sha256_process_block (ctx->buffer, 64, ctx);
	  left_over -= 64;
	  memcpy (ctx->buffer, &ctx->buffer[64], left_over);
	}
      ctx->buflen = left_over;
    }
}


/* Define our magic string to mark salt for SHA256 "encryption"
   replacement.  */
static const char sha256_salt_prefix[] = SHA256_SALT_PREFIX;

/* Prefix for optional rounds specification.  */
static const char sha256_rounds_prefix[] = SHA256_ROUNDS_PREFIX;

/* Maximum salt string length.  */
#define SALT_LEN_MAX 16
/* Default number of rounds if not explicitly specified.  */
#define ROUNDS_DEFAULT 5000
/* Minimum number of rounds.  */
#define ROUNDS_MIN 1000
/* Maximum number of rounds.  */
#define ROUNDS_MAX 999999999

/* Table with characters for base64 transformation.  */
static const char b64t[64] =
"./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";


char *
sha256_crypt_r (const char *key, const char *salt, char *buffer, int buflen)
{
  unsigned char alt_result[32]
    __attribute__ ((__aligned__ (__alignof__ (uint32_t))));
  unsigned char temp_result[32]
    __attribute__ ((__aligned__ (__alignof__ (uint32_t))));
  struct sha256_ctx ctx;
  struct sha256_ctx alt_ctx;
  size_t salt_len;
  size_t key_len;
  size_t cnt;
  char *cp;
  char *copied_key = NULL;
  char *copied_salt = NULL;
  char *p_bytes;
  char *s_bytes;
  /* Default number of rounds.  */
  size_t rounds = ROUNDS_DEFAULT;
  bool rounds_custom = false;

  /* Find beginning of salt string.  The prefix should normally always
     be present.  Just in case it is not.  */
  if (strncmp (sha256_salt_prefix, salt, sizeof (sha256_salt_prefix) - 1) == 0)
    /* Skip salt prefix.  */
    salt += sizeof (sha256_salt_prefix) - 1;

  if (strncmp (salt, sha256_rounds_prefix, sizeof (sha256_rounds_prefix) - 1)
      == 0)
    {
      const char *num = salt + sizeof (sha256_rounds_prefix) - 1;
      char *endp;
      unsigned long int srounds = strtoul (num, &endp, 10);
      if (*endp == '$')
	{
	  salt = endp + 1;
	  rounds = MAX (ROUNDS_MIN, MIN (srounds, ROUNDS_MAX));
	  rounds_custom = true;
	}
    }

  salt_len = MIN (strcspn (salt, "$"), SALT_LEN_MAX);
  key_len = strlen (key);

  if ((key - (char *) 0) % __alignof__ (uint32_t) != 0)
    {
      char *tmp = (char *) alloca (key_len + __alignof__ (uint32_t));
      key = copied_key =
	memcpy (tmp + __alignof__ (uint32_t)
		- (tmp - (char *) 0) % __alignof__ (uint32_t),
		key, key_len);
    }

  if ((salt - (char *) 0) % __alignof__ (uint32_t) != 0)
    {
      char *tmp = (char *) alloca (salt_len + __alignof__ (uint32_t));
      salt = copied_salt =
	memcpy (tmp + __alignof__ (uint32_t)
		- (tmp - (char *) 0) % __alignof__ (uint32_t),
		salt, salt_len);
    }

  /* Prepare for the real work.  */
  sha256_init_ctx (&ctx);

  /* Add the key string.  */
  sha256_process_bytes (key, key_len, &ctx);

  /* The last part is the salt string.  This must be at most 16
     characters and it ends at the first `$' character (for
     compatibility with existing implementations).  */
  sha256_process_bytes (salt, salt_len, &ctx);


  /* Compute alternate SHA256 sum with input KEY, SALT, and KEY.  The
     final result will be added to the first context.  */
  sha256_init_ctx (&alt_ctx);

  /* Add key.  */
  sha256_process_bytes (key, key_len, &alt_ctx);

  /* Add salt.  */
  sha256_process_bytes (salt, salt_len, &alt_ctx);

  /* Add key again.  */
  sha256_process_bytes (key, key_len, &alt_ctx);

  /* Now get result of this (32 bytes) and add it to the other
     context.  */
  sha256_finish_ctx (&alt_ctx, alt_result);

  /* Add for any character in the key one byte of the alternate sum.  */
  for (cnt = key_len; cnt > 32; cnt -= 32)
    sha256_process_bytes (alt_result, 32, &ctx);
  sha256_process_bytes (alt_result, cnt, &ctx);

  /* Take the binary representation of the length of the key and for every
     1 add the alternate sum, for every 0 the key.  */
  for (cnt = key_len; cnt > 0; cnt >>= 1)
    if ((cnt & 1) != 0)
      sha256_process_bytes (alt_result, 32, &ctx);
    else
      sha256_process_bytes (key, key_len, &ctx);

  /* Create intermediate result.  */
  sha256_finish_ctx (&ctx, alt_result);

  /* Start computation of P byte sequence.  */
  sha256_init_ctx (&alt_ctx);

  /* For every character in the password add the entire password.  */
  for (cnt = 0; cnt < key_len; ++cnt)
    sha256_process_bytes (key, key_len, &alt_ctx);

  /* Finish the digest.  */
  sha256_finish_ctx (&alt_ctx, temp_result);

  /* Create byte sequence P.  */
  cp = p_bytes = alloca (key_len);
  for (cnt = key_len; cnt >= 32; cnt -= 32)
    cp = (char*)memcpy (cp, temp_result, 32) + 32;
  memcpy (cp, temp_result, cnt);

  /* Start computation of S byte sequence.  */
  sha256_init_ctx (&alt_ctx);

  /* For every character in the password add the entire password.  */
  for (cnt = 0; cnt < 16 + alt_result[0]; ++cnt)
    sha256_process_bytes (salt, salt_len, &alt_ctx);

  /* Finish the digest.  */
  sha256_finish_ctx (&alt_ctx, temp_result);

  /* Create byte sequence S.  */
  cp = s_bytes = alloca (salt_len);
  for (cnt = salt_len; cnt >= 32; cnt -= 32)
    cp = (char*)memcpy (cp, temp_result, 32) + 32;
  memcpy (cp, temp_result, cnt);

  /* Repeatedly run the collected hash value through SHA256 to burn
     CPU cycles.  */
  for (cnt = 0; cnt < rounds; ++cnt)
    {
      /* New context.  */
      sha256_init_ctx (&ctx);

      /* Add key or last result.  */
      if ((cnt & 1) != 0)
	sha256_process_bytes (p_bytes, key_len, &ctx);
      else
	sha256_process_bytes (alt_result, 32, &ctx);

      /* Add salt for numbers not divisible by 3.  */
      if (cnt % 3 != 0)
	sha256_process_bytes (s_bytes, salt_len, &ctx);

      /* Add key for numbers not divisible by 7.  */
      if (cnt % 7 != 0)
	sha256_process_bytes (p_bytes, key_len, &ctx);

      /* Add key or last result.  */
      if ((cnt & 1) != 0)
	sha256_process_bytes (alt_result, 32, &ctx);
      else
	sha256_process_bytes (p_bytes, key_len, &ctx);

      /* Create intermediate result.  */
      sha256_finish_ctx (&ctx, alt_result);
    }

  /* Now we can construct the result string.  It consists of three
     parts.  */
  cp = stpncpy (buffer, sha256_salt_prefix, MAX (0, buflen));
  buflen -= sizeof (sha256_salt_prefix) - 1;

  if (rounds_custom)
    {
      int n = snprintf (cp, MAX (0, buflen), "%s%zu$",
			sha256_rounds_prefix, rounds);
      cp += n;
      buflen -= n;
    }

  cp = stpncpy (cp, salt, MIN ((size_t) MAX (0, buflen), salt_len));
  buflen -= MIN ((size_t) MAX (0, buflen), salt_len);

  if (buflen > 0)
    {
      *cp++ = '$';
      --buflen;
    }

#define b64_from_24bit(B2, B1, B0, N)					      \
  do {									      \
    unsigned int w = ((B2) << 16) | ((B1) << 8) | (B0);			      \
    int n = (N);							      \
    while (n-- > 0 && buflen > 0)					      \
      {									      \
	*cp++ = b64t[w & 0x3f];						      \
	--buflen;							      \
	w >>= 6;							      \
      }									      \
  } while (0)

  b64_from_24bit (alt_result[0], alt_result[10], alt_result[20], 4);
  b64_from_24bit (alt_result[21], alt_result[1], alt_result[11], 4);
  b64_from_24bit (alt_result[12], alt_result[22], alt_result[2], 4);
  b64_from_24bit (alt_result[3], alt_result[13], alt_result[23], 4);
  b64_from_24bit (alt_result[24], alt_result[4], alt_result[14], 4);
  b64_from_24bit (alt_result[15], alt_result[25], alt_result[5], 4);
  b64_from_24bit (alt_result[6], alt_result[16], alt_result[26], 4);
  b64_from_24bit (alt_result[27], alt_result[7], alt_result[17], 4);
  b64_from_24bit (alt_result[18], alt_result[28], alt_result[8], 4);
  b64_from_24bit (alt_result[9], alt_result[19], alt_result[29], 4);
  b64_from_24bit (0, alt_result[31], alt_result[30], 3);
  if (buflen <= 0)
    {
      errno = ERANGE;
      buffer = NULL;
    }
  else
    *cp = '\0';		/* Terminate the string.  */

  /* Clear the buffer for the intermediate result so that people
     attaching to processes or reading core dumps cannot get any
     information.  We do it in this way to clear correct_words[]
     inside the SHA256 implementation as well.  */
  sha256_init_ctx (&ctx);
  sha256_finish_ctx (&ctx, alt_result);
  memset (temp_result, '\0', sizeof (temp_result));
  memset (p_bytes, '\0', key_len);
  memset (s_bytes, '\0', salt_len);
  memset (&ctx, '\0', sizeof (ctx));
  memset (&alt_ctx, '\0', sizeof (alt_ctx));
  if (copied_key != NULL)
    memset (copied_key, '\0', key_len);
  if (copied_salt != NULL)
    memset (copied_salt, '\0', salt_len);

  return buffer;
}


/* This entry point is equivalent to the `crypt' function in Unix
   libcs.  */
char *
sha256_crypt (const char *key, const char *salt)
{
  /* We don't want to have an arbitrary limit in the size of the
     password.  We can compute an upper bound for the size of the
     result in advance and so we can prepare the buffer we pass to
     `sha256_crypt_r'.  */
  static char *buffer;
  static int buflen;
  int needed = (sizeof (sha256_salt_prefix) - 1
		+ sizeof (sha256_rounds_prefix) + 9 + 1
		+ strlen (salt) + 1 + 43 + 1);

  if (buflen < needed)
    {
      char *new_buffer = (char *) realloc (buffer, needed);
      if (new_buffer == NULL)
	return NULL;

      buffer = new_buffer;
      buflen = needed;
    }

  return sha256_crypt_r (key, salt, buffer, buflen);
}