src/crypto/cipher/tls_cbc.c

/* ====================================================================
 * Copyright (c) 2012 The OpenSSL Project.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. 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.
 *
 * 3. All advertising materials mentioning features or use of this
 *    software must display the following acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please contact
 *    openssl-core@openssl.org.
 *
 * 5. Products derived from this software may not be called "OpenSSL"
 *    nor may "OpenSSL" appear in their names without prior written
 *    permission of the OpenSSL Project.
 *
 * 6. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
 *
 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
 * EXPRESSED 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 OpenSSL PROJECT OR
 * ITS 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.
 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * (eay@cryptsoft.com).  This product includes software written by Tim
 * Hudson (tjh@cryptsoft.com). */

#include <assert.h>
#include <string.h>

#include <openssl/digest.h>
#include <openssl/nid.h>
#include <openssl/sha.h>

#include "../internal.h"
#include "internal.h"


/* TODO(davidben): unsigned should be size_t. The various constant_time
 * functions need to be switched to size_t. */

/* MAX_HASH_BIT_COUNT_BYTES is the maximum number of bytes in the hash's length
 * field. (SHA-384/512 have 128-bit length.) */
#define MAX_HASH_BIT_COUNT_BYTES 16

/* MAX_HASH_BLOCK_SIZE is the maximum hash block size that we'll support.
 * Currently SHA-384/512 has a 128-byte block size and that's the largest
 * supported by TLS.) */
#define MAX_HASH_BLOCK_SIZE 128

int EVP_tls_cbc_remove_padding(unsigned *out_padding_ok, unsigned *out_len,
                               const uint8_t *in, unsigned in_len,
                               unsigned block_size, unsigned mac_size) {
  unsigned padding_length, good, to_check, i;
  const unsigned overhead = 1 /* padding length byte */ + mac_size;

  /* These lengths are all public so we can test them in non-constant time. */
  if (overhead > in_len) {
    return 0;
  }

  padding_length = in[in_len - 1];

  good = constant_time_ge(in_len, overhead + padding_length);
  /* The padding consists of a length byte at the end of the record and
   * then that many bytes of padding, all with the same value as the
   * length byte. Thus, with the length byte included, there are i+1
   * bytes of padding.
   *
   * We can't check just |padding_length+1| bytes because that leaks
   * decrypted information. Therefore we always have to check the maximum
   * amount of padding possible. (Again, the length of the record is
   * public information so we can use it.) */
  to_check = 256; /* maximum amount of padding, inc length byte. */
  if (to_check > in_len) {
    to_check = in_len;
  }

  for (i = 0; i < to_check; i++) {
    uint8_t mask = constant_time_ge_8(padding_length, i);
    uint8_t b = in[in_len - 1 - i];
    /* The final |padding_length+1| bytes should all have the value
     * |padding_length|. Therefore the XOR should be zero. */
    good &= ~(mask & (padding_length ^ b));
  }

  /* If any of the final |padding_length+1| bytes had the wrong value,
   * one or more of the lower eight bits of |good| will be cleared. */
  good = constant_time_eq(0xff, good & 0xff);

  /* Always treat |padding_length| as zero on error. If, assuming block size of
   * 16, a padding of [<15 arbitrary bytes> 15] treated |padding_length| as 16
   * and returned -1, distinguishing good MAC and bad padding from bad MAC and
   * bad padding would give POODLE's padding oracle. */
  padding_length = good & (padding_length + 1);
  *out_len = in_len - padding_length;
  *out_padding_ok = good;
  return 1;
}

/* If CBC_MAC_ROTATE_IN_PLACE is defined then EVP_tls_cbc_copy_mac is performed
 * with variable accesses in a 64-byte-aligned buffer. Assuming that this fits
 * into a single or pair of cache-lines, then the variable memory accesses don't
 * actually affect the timing. CPUs with smaller cache-lines [if any] are not
 * multi-core and are not considered vulnerable to cache-timing attacks. */
#define CBC_MAC_ROTATE_IN_PLACE

void EVP_tls_cbc_copy_mac(uint8_t *out, unsigned md_size,
                          const uint8_t *in, unsigned in_len,
                          unsigned orig_len) {
#if defined(CBC_MAC_ROTATE_IN_PLACE)
  uint8_t rotated_mac_buf[64 + EVP_MAX_MD_SIZE];
  uint8_t *rotated_mac;
#else
  uint8_t rotated_mac[EVP_MAX_MD_SIZE];
#endif

  /* mac_end is the index of |in| just after the end of the MAC. */
  unsigned mac_end = in_len;
  unsigned mac_start = mac_end - md_size;
  /* scan_start contains the number of bytes that we can ignore because
   * the MAC's position can only vary by 255 bytes. */
  unsigned scan_start = 0;
  unsigned i, j;
  unsigned rotate_offset;

  assert(orig_len >= in_len);
  assert(in_len >= md_size);
  assert(md_size <= EVP_MAX_MD_SIZE);

#if defined(CBC_MAC_ROTATE_IN_PLACE)
  rotated_mac = rotated_mac_buf + ((0 - (size_t)rotated_mac_buf) & 63);
#endif

  /* This information is public so it's safe to branch based on it. */
  if (orig_len > md_size + 255 + 1) {
    scan_start = orig_len - (md_size + 255 + 1);
  }

  /* Ideally the next statement would be:
   *
   *   rotate_offset = (mac_start - scan_start) % md_size;
   *
   * However, division is not a constant-time operation (at least on Intel
   * chips). Thus we enumerate the possible values of md_size and handle each
   * separately. The value of |md_size| is public information (it's determined
   * by the cipher suite in the ServerHello) so our timing can vary based on
   * its value. */

  rotate_offset = mac_start - scan_start;
  /* rotate_offset can be, at most, 255 (bytes of padding) + 1 (padding length)
   * + md_size = 256 + 48 (since SHA-384 is the largest hash) = 304. */
  assert(rotate_offset <= 304);

  /* Below is an SMT-LIB2 verification that the Barrett reductions below are
   * correct within this range:
   *
   * (define-fun barrett (
   *     (x (_ BitVec 32))
   *     (mul (_ BitVec 32))
   *     (shift (_ BitVec 32))
   *     (divisor (_ BitVec 32)) ) (_ BitVec 32)
   *   (let ((q (bvsub x (bvmul divisor (bvlshr (bvmul x mul) shift))) ))
   *     (ite (bvuge q divisor)
   *       (bvsub q divisor)
   *       q)))
   *
   * (declare-fun x () (_ BitVec 32))
   *
   * (assert (or
   *   (let (
   *     (divisor (_ bv20 32))
   *     (mul (_ bv25 32))
   *     (shift (_ bv9 32))
   *     (limit (_ bv853 32)))
   *
   *     (and (bvule x limit) (not (= (bvurem x divisor)
   *                                  (barrett x mul shift divisor)))))
   *
   *   (let (
   *     (divisor (_ bv48 32))
   *     (mul (_ bv10 32))
   *     (shift (_ bv9 32))
   *     (limit (_ bv768 32)))
   *
   *     (and (bvule x limit) (not (= (bvurem x divisor)
   *                                  (barrett x mul shift divisor)))))
   * ))
   *
   * (check-sat)
   * (get-model)
   */

  if (md_size == 16) {
    rotate_offset &= 15;
  } else if (md_size == 20) {
    /* 1/20 is approximated as 25/512 and then Barrett reduction is used.
     * Analytically, this is correct for 0 <= rotate_offset <= 853. */
    unsigned q = (rotate_offset * 25) >> 9;
    rotate_offset -= q * 20;
    rotate_offset -=
        constant_time_select(constant_time_ge(rotate_offset, 20), 20, 0);
  } else if (md_size == 32) {
    rotate_offset &= 31;
  } else if (md_size == 48) {
    /* 1/48 is approximated as 10/512 and then Barrett reduction is used.
     * Analytically, this is correct for 0 <= rotate_offset <= 768. */
    unsigned q = (rotate_offset * 10) >> 9;
    rotate_offset -= q * 48;
    rotate_offset -=
        constant_time_select(constant_time_ge(rotate_offset, 48), 48, 0);
  } else {
    /* This should be impossible therefore this path doesn't run in constant
     * time. */
    assert(0);
    rotate_offset = rotate_offset % md_size;
  }

  memset(rotated_mac, 0, md_size);
  for (i = scan_start, j = 0; i < orig_len; i++) {
    uint8_t mac_started = constant_time_ge_8(i, mac_start);
    uint8_t mac_ended = constant_time_ge_8(i, mac_end);
    uint8_t b = in[i];
    rotated_mac[j++] |= b & mac_started & ~mac_ended;
    j &= constant_time_lt(j, md_size);
  }

/* Now rotate the MAC */
#if defined(CBC_MAC_ROTATE_IN_PLACE)
  j = 0;
  for (i = 0; i < md_size; i++) {
    /* in case cache-line is 32 bytes, touch second line */
    ((volatile uint8_t *)rotated_mac)[rotate_offset ^ 32];
    out[j++] = rotated_mac[rotate_offset++];
    rotate_offset &= constant_time_lt(rotate_offset, md_size);
  }
#else
  memset(out, 0, md_size);
  rotate_offset = md_size - rotate_offset;
  rotate_offset &= constant_time_lt(rotate_offset, md_size);
  for (i = 0; i < md_size; i++) {
    for (j = 0; j < md_size; j++) {
      out[j] |= rotated_mac[i] & constant_time_eq_8(j, rotate_offset);
    }
    rotate_offset++;
    rotate_offset &= constant_time_lt(rotate_offset, md_size);
  }
#endif
}

/* u32toBE serialises an unsigned, 32-bit number (n) as four bytes at (p) in
 * big-endian order. The value of p is advanced by four. */
#define u32toBE(n, p) \
  (*((p)++)=(uint8_t)(n>>24), \
   *((p)++)=(uint8_t)(n>>16), \
   *((p)++)=(uint8_t)(n>>8), \
   *((p)++)=(uint8_t)(n))

/* u64toBE serialises an unsigned, 64-bit number (n) as eight bytes at (p) in
 * big-endian order. The value of p is advanced by eight. */
#define u64toBE(n, p) \
  (*((p)++)=(uint8_t)(n>>56), \
   *((p)++)=(uint8_t)(n>>48), \
   *((p)++)=(uint8_t)(n>>40), \
   *((p)++)=(uint8_t)(n>>32), \
   *((p)++)=(uint8_t)(n>>24), \
   *((p)++)=(uint8_t)(n>>16), \
   *((p)++)=(uint8_t)(n>>8), \
   *((p)++)=(uint8_t)(n))

/* These functions serialize the state of a hash and thus perform the standard
 * "final" operation without adding the padding and length that such a function
 * typically does. */
static void tls1_sha1_final_raw(void *ctx, uint8_t *md_out) {
  SHA_CTX *sha1 = ctx;
  u32toBE(sha1->h[0], md_out);
  u32toBE(sha1->h[1], md_out);
  u32toBE(sha1->h[2], md_out);
  u32toBE(sha1->h[3], md_out);
  u32toBE(sha1->h[4], md_out);
}
#define LARGEST_DIGEST_CTX SHA_CTX

static void tls1_sha256_final_raw(void *ctx, uint8_t *md_out) {
  SHA256_CTX *sha256 = ctx;
  unsigned i;

  for (i = 0; i < 8; i++) {
    u32toBE(sha256->h[i], md_out);
  }
}
#undef  LARGEST_DIGEST_CTX
#define LARGEST_DIGEST_CTX SHA256_CTX

static void tls1_sha512_final_raw(void *ctx, uint8_t *md_out) {
  SHA512_CTX *sha512 = ctx;
  unsigned i;

  for (i = 0; i < 8; i++) {
    u64toBE(sha512->h[i], md_out);
  }
}
#undef  LARGEST_DIGEST_CTX
#define LARGEST_DIGEST_CTX SHA512_CTX

int EVP_tls_cbc_record_digest_supported(const EVP_MD *md) {
  switch (EVP_MD_type(md)) {
    case NID_sha1:
    case NID_sha256:
    case NID_sha384:
      return 1;

    default:
      return 0;
  }
}

int EVP_tls_cbc_digest_record(const EVP_MD *md, uint8_t *md_out,
                              size_t *md_out_size, const uint8_t header[13],
                              const uint8_t *data, size_t data_plus_mac_size,
                              size_t data_plus_mac_plus_padding_size,
                              const uint8_t *mac_secret,
                              unsigned mac_secret_length) {
  union {
    double align;
    uint8_t c[sizeof(LARGEST_DIGEST_CTX)];
  } md_state;
  void (*md_final_raw)(void *ctx, uint8_t *md_out);
  void (*md_transform)(void *ctx, const uint8_t *block);
  unsigned md_size, md_block_size = 64;
  unsigned len, max_mac_bytes, num_blocks, num_starting_blocks, k,
           mac_end_offset, c, index_a, index_b;
  unsigned int bits; /* at most 18 bits */
  uint8_t length_bytes[MAX_HASH_BIT_COUNT_BYTES];
  /* hmac_pad is the masked HMAC key. */
  uint8_t hmac_pad[MAX_HASH_BLOCK_SIZE];
  uint8_t first_block[MAX_HASH_BLOCK_SIZE];
  uint8_t mac_out[EVP_MAX_MD_SIZE];
  unsigned i, j, md_out_size_u;
  EVP_MD_CTX md_ctx;
  /* mdLengthSize is the number of bytes in the length field that terminates
  * the hash. */
  unsigned md_length_size = 8;

  /* This is a, hopefully redundant, check that allows us to forget about
   * many possible overflows later in this function. */
  assert(data_plus_mac_plus_padding_size < 1024 * 1024);

  switch (EVP_MD_type(md)) {
    case NID_sha1:
      SHA1_Init((SHA_CTX *)md_state.c);
      md_final_raw = tls1_sha1_final_raw;
      md_transform =
          (void (*)(void *ctx, const uint8_t *block))SHA1_Transform;
      md_size = 20;
      break;

    case NID_sha256:
      SHA256_Init((SHA256_CTX *)md_state.c);
      md_final_raw = tls1_sha256_final_raw;
      md_transform =
          (void (*)(void *ctx, const uint8_t *block))SHA256_Transform;
      md_size = 32;
      break;

    case NID_sha384:
      SHA384_Init((SHA512_CTX *)md_state.c);
      md_final_raw = tls1_sha512_final_raw;
      md_transform =
          (void (*)(void *ctx, const uint8_t *block))SHA512_Transform;
      md_size = 384 / 8;
      md_block_size = 128;
      md_length_size = 16;
      break;

    default:
      /* EVP_tls_cbc_record_digest_supported should have been called first to
       * check that the hash function is supported. */
      assert(0);
      *md_out_size = 0;
      return 0;
  }

  assert(md_length_size <= MAX_HASH_BIT_COUNT_BYTES);
  assert(md_block_size <= MAX_HASH_BLOCK_SIZE);
  assert(md_size <= EVP_MAX_MD_SIZE);

  static const unsigned kHeaderLength = 13;

  /* kVarianceBlocks is the number of blocks of the hash that we have to
   * calculate in constant time because they could be altered by the
   * padding value.
   *
   * TLSv1 has MACs up to 48 bytes long (SHA-384) and the padding is not
   * required to be minimal. Therefore we say that the final six blocks
   * can vary based on the padding. */
  static const unsigned kVarianceBlocks = 6;

  /* From now on we're dealing with the MAC, which conceptually has 13
   * bytes of `header' before the start of the data. */
  len = data_plus_mac_plus_padding_size + kHeaderLength;
  /* max_mac_bytes contains the maximum bytes of bytes in the MAC, including
  * |header|, assuming that there's no padding. */
  max_mac_bytes = len - md_size - 1;
  /* num_blocks is the maximum number of hash blocks. */
  num_blocks =
      (max_mac_bytes + 1 + md_length_size + md_block_size - 1) / md_block_size;
  /* In order to calculate the MAC in constant time we have to handle
   * the final blocks specially because the padding value could cause the
   * end to appear somewhere in the final |kVarianceBlocks| blocks and we
   * can't leak where. However, |num_starting_blocks| worth of data can
   * be hashed right away because no padding value can affect whether
   * they are plaintext. */
  num_starting_blocks = 0;
  /* k is the starting byte offset into the conceptual header||data where
   * we start processing. */
  k = 0;
  /* mac_end_offset is the index just past the end of the data to be
   * MACed. */
  mac_end_offset = data_plus_mac_size + kHeaderLength - md_size;
  /* c is the index of the 0x80 byte in the final hash block that
   * contains application data. */
  c = mac_end_offset % md_block_size;
  /* index_a is the hash block number that contains the 0x80 terminating
   * value. */
  index_a = mac_end_offset / md_block_size;
  /* index_b is the hash block number that contains the 64-bit hash
   * length, in bits. */
  index_b = (mac_end_offset + md_length_size) / md_block_size;
  /* bits is the hash-length in bits. It includes the additional hash
   * block for the masked HMAC key. */

  if (num_blocks > kVarianceBlocks) {
    num_starting_blocks = num_blocks - kVarianceBlocks;
    k = md_block_size * num_starting_blocks;
  }

  bits = 8 * mac_end_offset;

  /* Compute the initial HMAC block. */
  bits += 8 * md_block_size;
  memset(hmac_pad, 0, md_block_size);
  assert(mac_secret_length <= sizeof(hmac_pad));
  memcpy(hmac_pad, mac_secret, mac_secret_length);
  for (i = 0; i < md_block_size; i++) {
    hmac_pad[i] ^= 0x36;
  }

  md_transform(md_state.c, hmac_pad);

  memset(length_bytes, 0, md_length_size - 4);
  length_bytes[md_length_size - 4] = (uint8_t)(bits >> 24);
  length_bytes[md_length_size - 3] = (uint8_t)(bits >> 16);
  length_bytes[md_length_size - 2] = (uint8_t)(bits >> 8);
  length_bytes[md_length_size - 1] = (uint8_t)bits;

  if (k > 0) {
    /* k is a multiple of md_block_size. */
    memcpy(first_block, header, 13);
    memcpy(first_block + 13, data, md_block_size - 13);
    md_transform(md_state.c, first_block);
    for (i = 1; i < k / md_block_size; i++) {
      md_transform(md_state.c, data + md_block_size * i - 13);
    }
  }

  memset(mac_out, 0, sizeof(mac_out));

  /* We now process the final hash blocks. For each block, we construct
   * it in constant time. If the |i==index_a| then we'll include the 0x80
   * bytes and zero pad etc. For each block we selectively copy it, in
   * constant time, to |mac_out|. */
  for (i = num_starting_blocks; i <= num_starting_blocks + kVarianceBlocks;
       i++) {
    uint8_t block[MAX_HASH_BLOCK_SIZE];
    uint8_t is_block_a = constant_time_eq_8(i, index_a);
    uint8_t is_block_b = constant_time_eq_8(i, index_b);
    for (j = 0; j < md_block_size; j++) {
      uint8_t b = 0, is_past_c, is_past_cp1;
      if (k < kHeaderLength) {
        b = header[k];
      } else if (k < data_plus_mac_plus_padding_size + kHeaderLength) {
        b = data[k - kHeaderLength];
      }
      k++;

      is_past_c = is_block_a & constant_time_ge_8(j, c);
      is_past_cp1 = is_block_a & constant_time_ge_8(j, c + 1);
      /* If this is the block containing the end of the
       * application data, and we are at the offset for the
       * 0x80 value, then overwrite b with 0x80. */
      b = constant_time_select_8(is_past_c, 0x80, b);
      /* If this the the block containing the end of the
       * application data and we're past the 0x80 value then
       * just write zero. */
      b = b & ~is_past_cp1;
      /* If this is index_b (the final block), but not
       * index_a (the end of the data), then the 64-bit
       * length didn't fit into index_a and we're having to
       * add an extra block of zeros. */
      b &= ~is_block_b | is_block_a;

      /* The final bytes of one of the blocks contains the
       * length. */
      if (j >= md_block_size - md_length_size) {
        /* If this is index_b, write a length byte. */
        b = constant_time_select_8(
            is_block_b, length_bytes[j - (md_block_size - md_length_size)], b);
      }
      block[j] = b;
    }

    md_transform(md_state.c, block);
    md_final_raw(md_state.c, block);
    /* If this is index_b, copy the hash value to |mac_out|. */
    for (j = 0; j < md_size; j++) {
      mac_out[j] |= block[j] & is_block_b;
    }
  }

  EVP_MD_CTX_init(&md_ctx);
  if (!EVP_DigestInit_ex(&md_ctx, md, NULL /* engine */)) {
    EVP_MD_CTX_cleanup(&md_ctx);
    return 0;
  }

  /* Complete the HMAC in the standard manner. */
  for (i = 0; i < md_block_size; i++) {
    hmac_pad[i] ^= 0x6a;
  }

  EVP_DigestUpdate(&md_ctx, hmac_pad, md_block_size);
  EVP_DigestUpdate(&md_ctx, mac_out, md_size);
  EVP_DigestFinal(&md_ctx, md_out, &md_out_size_u);
  *md_out_size = md_out_size_u;
  EVP_MD_CTX_cleanup(&md_ctx);

  return 1;
}