Constant-time utilities

[openssl.git] / 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,
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 * 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 (int)((good & 1) | (~good & -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 (int)((good & 1) | (~good & -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 = (b&~is_past_c) | (0x80&is_past_c);
                        /* 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 = (b&~is_block_b) | (is_block_b&length_bytes[j-(md_block_size-md_length_size)]);
                                }
                        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);
        }