openssl/ssl/s3_cbc.c

/* ssl/s3_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 "../crypto/constant_time_locl.h"
#include "ssl_locl.h"

#include <openssl/md5.h>
#include <openssl/sha.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 (s->version == DTLS1_VERSION || s->version == DTLS1_BAD_VER) {
        /*
         * 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];

    /*
     * NB: if compression is in operation the first packet may not be of even
     * length so the padding bug check cannot be performed. This bug
     * workaround has been around since SSLeay so hopefully it is either
     * fixed now or no buggy implementation supports compression [steve]
     */
    if ((s->options & SSL_OP_TLS_BLOCK_PADDING_BUG) && !s->expand) {
        /* First packet is even in size, so check */
        if ((memcmp(s->s3->read_sequence, "\0\0\0\0\0\0\0\0", 8) == 0) &&
            !(padding_length & 1)) {
            s->s3->flags |= TLS1_FLAGS_TLS_PADDING_BUG;
        }
        if ((s->s3->flags & TLS1_FLAGS_TLS_PADDING_BUG) && padding_length > 0) {
            padding_length--;
        }
    }

    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 = 255;             /* maximum amount of padding. */
    if (to_check > rec->length - 1)
        to_check = rec->length - 1;

    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;

    OPENSSL_assert(orig_len >= md_size);
    OPENSSL_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_md5_final_raw(void *ctx, unsigned char *md_out)
{
    MD5_CTX *md5 = ctx;
    u32toLE(md5->A, md_out);
    u32toLE(md5->B, md_out);
    u32toLE(md5->C, md_out);
    u32toLE(md5->D, md_out);
}

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

#ifndef OPENSSL_NO_SHA256
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
#endif

#ifndef OPENSSL_NO_SHA512
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
#endif

/*
 * 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 (ctx->digest->type) {
    case NID_md5:
    case NID_sha1:
#ifndef OPENSSL_NO_SHA256
    case NID_sha224:
    case NID_sha256:
#endif
#ifndef OPENSSL_NO_SHA512
    case NID_sha384:
    case NID_sha512:
#endif
        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: if non-NULL, 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. )
 */
void 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;
    char length_is_big_endian = 1;

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

    switch (ctx->digest->type) {
    case NID_md5:
        MD5_Init((MD5_CTX *)md_state.c);
        md_final_raw = tls1_md5_final_raw;
        md_transform =
            (void (*)(void *ctx, const unsigned char *block))MD5_Transform;
        md_size = 16;
        sslv3_pad_length = 48;
        length_is_big_endian = 0;
        break;
    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;
#ifndef OPENSSL_NO_SHA256
    case NID_sha224:
        SHA224_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 = 224 / 8;
        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;
#endif
#ifndef OPENSSL_NO_SHA512
    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;
    case NID_sha512:
        SHA512_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 = 64;
        md_block_size = 128;
        md_length_size = 16;
        break;
#endif
    default:
        /*
         * ssl3_cbc_record_digest_supported should have been called first to
         * check that the hash function is supported.
         */
        OPENSSL_assert(0);
        if (md_out_size)
            *md_out_size = -1;
        return;
    }

    OPENSSL_assert(md_length_size <= MAX_HASH_BIT_COUNT_BYTES);
    OPENSSL_assert(md_block_size <= MAX_HASH_BLOCK_SIZE);
    OPENSSL_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);
        OPENSSL_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);
    }

    if (length_is_big_endian) {
        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;
    } else {
        memset(length_bytes, 0, md_length_size);
        length_bytes[md_length_size - 5] = (unsigned char)(bits >> 24);
        length_bytes[md_length_size - 6] = (unsigned char)(bits >> 16);
        length_bytes[md_length_size - 7] = (unsigned char)(bits >> 8);
        length_bytes[md_length_size - 8] = (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: either 7 bytes (SHA1) or 11 bytes (MD5).
             */
            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);
    EVP_DigestInit_ex(&md_ctx, ctx->digest, NULL /* engine */ );
    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);
    if (md_out_size)
        *md_out_size = md_out_size_u;
    EVP_MD_CTX_cleanup(&md_ctx);
}