/* ====================================================================
 * 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 <openssl/obj.h>
#include <openssl/sha.h>

#include "../crypto/internal.h"
#include "ssl_locl.h"


/* 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

/* ssl3_cbc_remove_padding removes padding from the decrypted, SSLv3, CBC
 * record in |rec| by updating |rec->length| in constant time.
 *
 * block_size: the block size of the cipher used to encrypt the record.
 * returns:
 *   0: (in non-constant time) if the record is publicly invalid.
 *   1: if the padding was valid
 *  -1: otherwise. */
int ssl3_cbc_remove_padding(const SSL *s, SSL3_RECORD *rec, unsigned block_size,
                            unsigned mac_size) {
  unsigned padding_length, good;
  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 > rec->length) {
    return 0;
  }

  padding_length = rec->data[rec->length - 1];
  good = constant_time_ge(rec->length, padding_length + overhead);
  /* SSLv3 requires that the padding is minimal. */
  good &= constant_time_ge(block_size, padding_length + 1);
  padding_length = good & (padding_length + 1);
  rec->length -= padding_length;
  rec->type |= padding_length << 8; /* kludge: pass padding length */
  return constant_time_select_int(good, 1, -1);
}

/* tls1_cbc_remove_padding removes the CBC padding from the decrypted, TLS, CBC
 * record in |rec| in constant time and returns 1 if the padding is valid and
 * -1 otherwise. It also removes any explicit IV from the start of the record
 * without leaking any timing about whether there was enough space after the
 * padding was removed.
 *
 * block_size: the block size of the cipher used to encrypt the record.
 * returns:
 *   0: (in non-constant time) if the record is publicly invalid.
 *   1: if the padding was valid
 *  -1: otherwise. */
int tls1_cbc_remove_padding(const SSL *s, SSL3_RECORD *rec, unsigned block_size,
                            unsigned mac_size) {
  unsigned padding_length, good, to_check, i;
  const unsigned overhead = 1 /* padding length byte */ + mac_size;

  /* Check if version requires explicit IV */
  if (SSL_USE_EXPLICIT_IV(s)) {
    /* These lengths are all public so we can test them in
     * non-constant time. */
    if (overhead + block_size > rec->length) {
      return 0;
    }
    /* We can now safely skip explicit IV */
    rec->data += block_size;
    rec->input += block_size;
    rec->length -= block_size;
  } else if (overhead > rec->length) {
    return 0;
  }

  padding_length = rec->data[rec->length - 1];

  good = constant_time_ge(rec->length, 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 > rec->length) {
    to_check = rec->length;
  }

  for (i = 0; i < to_check; i++) {
    unsigned char mask = constant_time_ge_8(padding_length, i);
    unsigned char b = rec->data[rec->length - 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);

  padding_length = good & (padding_length + 1);
  rec->length -= padding_length;
  rec->type |= padding_length << 8; /* kludge: pass padding length */

  return constant_time_select_int(good, 1, -1);
}

/* ssl3_cbc_copy_mac copies |md_size| bytes from the end of |rec| to |out| in
 * constant time (independent of the concrete value of rec->length, which may
 * vary within a 256-byte window).
 *
 * ssl3_cbc_remove_padding or tls1_cbc_remove_padding must be called prior to
 * this function.
 *
 * On entry:
 *   rec->orig_len >= md_size
 *   md_size <= EVP_MAX_MD_SIZE
 *
 * If CBC_MAC_ROTATE_IN_PLACE is defined then the rotation 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 ssl3_cbc_copy_mac(unsigned char *out, const SSL3_RECORD *rec,
                       unsigned md_size, unsigned orig_len) {
#if defined(CBC_MAC_ROTATE_IN_PLACE)
  unsigned char rotated_mac_buf[64 + EVP_MAX_MD_SIZE];
  unsigned char *rotated_mac;
#else
  unsigned char rotated_mac[EVP_MAX_MD_SIZE];
#endif

  /* mac_end is the index of |rec->data| just after the end of the MAC. */
  unsigned mac_end = rec->length;
  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 div_spoiler;
  unsigned rotate_offset;

  assert(orig_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);
  }
  /* div_spoiler contains a multiple of md_size that is used to cause the
   * modulo operation to be constant time. Without this, the time varies
   * based on the amount of padding when running on Intel chips at least.
   *
   * The aim of right-shifting md_size is so that the compiler doesn't
   * figure out that it can remove div_spoiler as that would require it
   * to prove that md_size is always even, which I hope is beyond it. */
  div_spoiler = md_size >> 1;
  div_spoiler <<= (sizeof(div_spoiler) - 1) * 8;
  rotate_offset = (div_spoiler + mac_start - scan_start) % md_size;

  memset(rotated_mac, 0, md_size);
  for (i = scan_start, j = 0; i < orig_len; i++) {
    unsigned char mac_started = constant_time_ge_8(i, mac_start);
    unsigned char mac_ended = constant_time_ge_8(i, mac_end);
    unsigned char b = rec->data[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 unsigned char *)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
}

/* u32toLE serialises an unsigned, 32-bit number (n) as four bytes at (p) in
 * little-endian order. The value of p is advanced by four. */
#define u32toLE(n, p) \
  (*((p)++)=(unsigned char)(n), \
   *((p)++)=(unsigned char)(n>>8), \
   *((p)++)=(unsigned char)(n>>16), \
   *((p)++)=(unsigned char)(n>>24))

/* 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, unsigned char *md_out) {
  SHA_CTX *sha1 = ctx;
  l2n(sha1->h0, md_out);
  l2n(sha1->h1, md_out);
  l2n(sha1->h2, md_out);
  l2n(sha1->h3, md_out);
  l2n(sha1->h4, md_out);
}
#define LARGEST_DIGEST_CTX SHA_CTX

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

  for (i = 0; i < 8; i++) {
    l2n(sha256->h[i], md_out);
  }
}
#undef  LARGEST_DIGEST_CTX
#define LARGEST_DIGEST_CTX SHA256_CTX

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

  for (i = 0; i < 8; i++) {
    l2n8(sha512->h[i], md_out);
  }
}
#undef  LARGEST_DIGEST_CTX
#define LARGEST_DIGEST_CTX SHA512_CTX

/* ssl3_cbc_record_digest_supported returns 1 iff |ctx| uses a hash function
 * which ssl3_cbc_digest_record supports. */
char ssl3_cbc_record_digest_supported(const EVP_MD_CTX *ctx) {
  switch (EVP_MD_CTX_type(ctx)) {
    case NID_sha1:
    case NID_sha256:
    case NID_sha384:
      return 1;

    default:
      return 0;
  }
}

/* ssl3_cbc_digest_record computes the MAC of a decrypted, padded SSLv3/TLS
 * record.
 *
 *   ctx: the EVP_MD_CTX from which we take the hash function.
 *     ssl3_cbc_record_digest_supported must return true for this EVP_MD_CTX.
 *   md_out: the digest output. At most EVP_MAX_MD_SIZE bytes will be written.
 *   md_out_size: the number of output bytes is written here.
 *   header: the 13-byte, TLS record header.
 *   data: the record data itself, less any preceeding explicit IV.
 *   data_plus_mac_size: the secret, reported length of the data and MAC
 *     once the padding has been removed.
 *   data_plus_mac_plus_padding_size: the public length of the whole
 *     record, including padding.
 *   is_sslv3: non-zero if we are to use SSLv3. Otherwise, TLS.
 *
 * On entry: by virtue of having been through one of the remove_padding
 * functions, above, we know that data_plus_mac_size is large enough to contain
 * a padding byte and MAC. (If the padding was invalid, it might contain the
 * padding too. ) */
int ssl3_cbc_digest_record(const EVP_MD_CTX *ctx, unsigned char *md_out,
                           size_t *md_out_size, const unsigned char header[13],
                           const unsigned char *data, size_t data_plus_mac_size,
                           size_t data_plus_mac_plus_padding_size,
                           const unsigned char *mac_secret,
                           unsigned mac_secret_length, char is_sslv3) {
  union {
    double align;
    unsigned char c[sizeof(LARGEST_DIGEST_CTX)];
  } md_state;
  void (*md_final_raw)(void *ctx, unsigned char *md_out);
  void (*md_transform)(void *ctx, const unsigned char *block);
  unsigned md_size, md_block_size = 64;
  unsigned sslv3_pad_length = 40, header_length, variance_blocks, 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 */
  unsigned char length_bytes[MAX_HASH_BIT_COUNT_BYTES];
  /* hmac_pad is the masked HMAC key. */
  unsigned char hmac_pad[MAX_HASH_BLOCK_SIZE];
  unsigned char first_block[MAX_HASH_BLOCK_SIZE];
  unsigned char 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_CTX_type(ctx)) {
    case NID_sha1:
      SHA1_Init((SHA_CTX *)md_state.c);
      md_final_raw = tls1_sha1_final_raw;
      md_transform =
          (void (*)(void *ctx, const unsigned char *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 unsigned char *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 unsigned char *block))SHA512_Transform;
      md_size = 384 / 8;
      md_block_size = 128;
      md_length_size = 16;
      break;

    default:
      /* ssl3_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);

  header_length = 13;
  if (is_sslv3) {
    header_length = mac_secret_length + sslv3_pad_length +
                    8 /* sequence number */ + 1 /* record type */ +
                    2 /* record length */;
  }

  /* variance_blocks 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.
   *
   * In SSLv3, the padding must be minimal so the end of the plaintext
   * varies by, at most, 15+20 = 35 bytes. (We conservatively assume that
   * the MAC size varies from 0..20 bytes.) In case the 9 bytes of hash
   * termination (0x80 + 64-bit length) don't fit in the final block, we
   * say that the final two blocks can vary based on the padding.
   *
   * 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.
   *
   * Later in the function, if the message is short and there obviously
   * cannot be this many blocks then variance_blocks can be reduced. */
  variance_blocks = is_sslv3 ? 2 : 6;
  /* From now on we're dealing with the MAC, which conceptually has 13
   * bytes of `header' before the start of the data (TLS) or 71/75 bytes
   * (SSLv3) */
  len = data_plus_mac_plus_padding_size + header_length;
  /* 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 |variance_blocks| 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 + header_length - 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, or whole of |header| in the case of
   * SSLv3. */

  /* For SSLv3, if we're going to have any starting blocks then we need
   * at least two because the header is larger than a single block. */
  if (num_blocks > variance_blocks + (is_sslv3 ? 1 : 0)) {
    num_starting_blocks = num_blocks - variance_blocks;
    k = md_block_size * num_starting_blocks;
  }

  bits = 8 * mac_end_offset;
  if (!is_sslv3) {
    /* Compute the initial HMAC block. For SSLv3, the padding and
     * secret bytes are included in |header| because they take more
     * than a single 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] = (unsigned char)(bits >> 24);
  length_bytes[md_length_size - 3] = (unsigned char)(bits >> 16);
  length_bytes[md_length_size - 2] = (unsigned char)(bits >> 8);
  length_bytes[md_length_size - 1] = (unsigned char)bits;

  if (k > 0) {
    if (is_sslv3) {
      /* The SSLv3 header is larger than a single block.
       * overhang is the number of bytes beyond a single
       * block that the header consumes: 7 bytes (SHA1). */
      unsigned overhang = header_length - md_block_size;
      md_transform(md_state.c, header);
      memcpy(first_block, header + md_block_size, overhang);
      memcpy(first_block + overhang, data, md_block_size - overhang);
      md_transform(md_state.c, first_block);
      for (i = 1; i < k / md_block_size - 1; i++) {
        md_transform(md_state.c, data + md_block_size * i - overhang);
      }
    } else {
      /* 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 + variance_blocks;
       i++) {
    unsigned char block[MAX_HASH_BLOCK_SIZE];
    unsigned char is_block_a = constant_time_eq_8(i, index_a);
    unsigned char is_block_b = constant_time_eq_8(i, index_b);
    for (j = 0; j < md_block_size; j++) {
      unsigned char b = 0, is_past_c, is_past_cp1;
      if (k < header_length) {
        b = header[k];
      } else if (k < data_plus_mac_plus_padding_size + header_length) {
        b = data[k - header_length];
      }
      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, ctx->digest, NULL /* engine */)) {
    EVP_MD_CTX_cleanup(&md_ctx);
    return 0;
  }

  if (is_sslv3) {
    /* We repurpose |hmac_pad| to contain the SSLv3 pad2 block. */
    memset(hmac_pad, 0x5c, sslv3_pad_length);

    EVP_DigestUpdate(&md_ctx, mac_secret, mac_secret_length);
    EVP_DigestUpdate(&md_ctx, hmac_pad, sslv3_pad_length);
    EVP_DigestUpdate(&md_ctx, mac_out, md_size);
  } else {
    /* 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;
}