Commit 02a36fda authored by Matt Caswell's avatar Matt Caswell
Browse files

Move more SSL3_RECORD oriented functions into ssl3_record.c

parent fe589e61
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+703 −0
Original line number Diff line number Diff line
@@ -110,6 +110,26 @@
 */

#include "../ssl_locl.h"
#include "../../crypto/constant_time_locl.h"
#include <openssl/rand.h>

static const unsigned char ssl3_pad_1[48] = {
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36,
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36,
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36,
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36,
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36,
    0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36, 0x36
};

static const unsigned char ssl3_pad_2[48] = {
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c,
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c,
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c,
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c,
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c,
    0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c, 0x5c
};

void SSL3_RECORD_clear(SSL3_RECORD *r)
{
@@ -485,6 +505,689 @@ int ssl3_do_compress(SSL *ssl)
    return (1);
}

/*-
 * ssl3_enc encrypts/decrypts the record in |s->wrec| / |s->rrec|, respectively.
 *
 * Returns:
 *   0: (in non-constant time) if the record is publically invalid (i.e. too
 *       short etc).
 *   1: if the record's padding is valid / the encryption was successful.
 *   -1: if the record's padding is invalid or, if sending, an internal error
 *       occurred.
 */
int ssl3_enc(SSL *s, int send)
{
    SSL3_RECORD *rec;
    EVP_CIPHER_CTX *ds;
    unsigned long l;
    int bs, i, mac_size = 0;
    const EVP_CIPHER *enc;

    if (send) {
        ds = s->enc_write_ctx;
        rec = RECORD_LAYER_get_wrec(&s->rlayer);
        if (s->enc_write_ctx == NULL)
            enc = NULL;
        else
            enc = EVP_CIPHER_CTX_cipher(s->enc_write_ctx);
    } else {
        ds = s->enc_read_ctx;
        rec = RECORD_LAYER_get_rrec(&s->rlayer);
        if (s->enc_read_ctx == NULL)
            enc = NULL;
        else
            enc = EVP_CIPHER_CTX_cipher(s->enc_read_ctx);
    }

    if ((s->session == NULL) || (ds == NULL) || (enc == NULL)) {
        memmove(rec->data, rec->input, rec->length);
        rec->input = rec->data;
    } else {
        l = rec->length;
        bs = EVP_CIPHER_block_size(ds->cipher);

        /* COMPRESS */

        if ((bs != 1) && send) {
            i = bs - ((int)l % bs);

            /* we need to add 'i-1' padding bytes */
            l += i;
            /*
             * the last of these zero bytes will be overwritten with the
             * padding length.
             */
            memset(&rec->input[rec->length], 0, i);
            rec->length += i;
            rec->input[l - 1] = (i - 1);
        }

        if (!send) {
            if (l == 0 || l % bs != 0)
                return 0;
            /* otherwise, rec->length >= bs */
        }

        if (EVP_Cipher(ds, rec->data, rec->input, l) < 1)
            return -1;

        if (EVP_MD_CTX_md(s->read_hash) != NULL)
            mac_size = EVP_MD_CTX_size(s->read_hash);
        if ((bs != 1) && !send)
            return ssl3_cbc_remove_padding(s, rec, bs, mac_size);
    }
    return (1);
}

/*-
 * tls1_enc encrypts/decrypts the record in |s->wrec| / |s->rrec|, respectively.
 *
 * Returns:
 *   0: (in non-constant time) if the record is publically invalid (i.e. too
 *       short etc).
 *   1: if the record's padding is valid / the encryption was successful.
 *   -1: if the record's padding/AEAD-authenticator is invalid or, if sending,
 *       an internal error occurred.
 */
int tls1_enc(SSL *s, int send)
{
    SSL3_RECORD *rec;
    EVP_CIPHER_CTX *ds;
    unsigned long l;
    int bs, i, j, k, pad = 0, ret, mac_size = 0;
    const EVP_CIPHER *enc;

    if (send) {
        if (EVP_MD_CTX_md(s->write_hash)) {
            int n = EVP_MD_CTX_size(s->write_hash);
            OPENSSL_assert(n >= 0);
        }
        ds = s->enc_write_ctx;
        rec = RECORD_LAYER_get_wrec(&s->rlayer);
        if (s->enc_write_ctx == NULL)
            enc = NULL;
        else {
            int ivlen;
            enc = EVP_CIPHER_CTX_cipher(s->enc_write_ctx);
            /* For TLSv1.1 and later explicit IV */
            if (SSL_USE_EXPLICIT_IV(s)
                && EVP_CIPHER_mode(enc) == EVP_CIPH_CBC_MODE)
                ivlen = EVP_CIPHER_iv_length(enc);
            else
                ivlen = 0;
            if (ivlen > 1) {
                if (rec->data != rec->input)
                    /*
                     * we can't write into the input stream: Can this ever
                     * happen?? (steve)
                     */
                    fprintf(stderr,
                            "%s:%d: rec->data != rec->input\n",
                            __FILE__, __LINE__);
                else if (RAND_bytes(rec->input, ivlen) <= 0)
                    return -1;
            }
        }
    } else {
        if (EVP_MD_CTX_md(s->read_hash)) {
            int n = EVP_MD_CTX_size(s->read_hash);
            OPENSSL_assert(n >= 0);
        }
        ds = s->enc_read_ctx;
        rec = RECORD_LAYER_get_rrec(&s->rlayer);
        if (s->enc_read_ctx == NULL)
            enc = NULL;
        else
            enc = EVP_CIPHER_CTX_cipher(s->enc_read_ctx);
    }

#ifdef KSSL_DEBUG
    fprintf(stderr, "tls1_enc(%d)\n", send);
#endif                          /* KSSL_DEBUG */

    if ((s->session == NULL) || (ds == NULL) || (enc == NULL)) {
        memmove(rec->data, rec->input, rec->length);
        rec->input = rec->data;
        ret = 1;
    } else {
        l = rec->length;
        bs = EVP_CIPHER_block_size(ds->cipher);

        if (EVP_CIPHER_flags(ds->cipher) & EVP_CIPH_FLAG_AEAD_CIPHER) {
            unsigned char buf[13], *seq;

            seq = send ? s->s3->write_sequence : s->s3->read_sequence;

            if (SSL_IS_DTLS(s)) {
                unsigned char dtlsseq[9], *p = dtlsseq;

                s2n(send ? s->d1->w_epoch : s->d1->r_epoch, p);
                memcpy(p, &seq[2], 6);
                memcpy(buf, dtlsseq, 8);
            } else {
                memcpy(buf, seq, 8);
                for (i = 7; i >= 0; i--) { /* increment */
                    ++seq[i];
                    if (seq[i] != 0)
                        break;
                }
            }

            buf[8] = rec->type;
            buf[9] = (unsigned char)(s->version >> 8);
            buf[10] = (unsigned char)(s->version);
            buf[11] = rec->length >> 8;
            buf[12] = rec->length & 0xff;
            pad = EVP_CIPHER_CTX_ctrl(ds, EVP_CTRL_AEAD_TLS1_AAD, 13, buf);
            if (send) {
                l += pad;
                rec->length += pad;
            }
        } else if ((bs != 1) && send) {
            i = bs - ((int)l % bs);

            /* Add weird padding of upto 256 bytes */

            /* we need to add 'i' padding bytes of value j */
            j = i - 1;
            if (s->options & SSL_OP_TLS_BLOCK_PADDING_BUG) {
                if (s->s3->flags & TLS1_FLAGS_TLS_PADDING_BUG)
                    j++;
            }
            for (k = (int)l; k < (int)(l + i); k++)
                rec->input[k] = j;
            l += i;
            rec->length += i;
        }
#ifdef KSSL_DEBUG
        {
            unsigned long ui;
            fprintf(stderr,
                    "EVP_Cipher(ds=%p,rec->data=%p,rec->input=%p,l=%ld) ==>\n",
                    ds, rec->data, rec->input, l);
            fprintf(stderr,
                    "\tEVP_CIPHER_CTX: %d buf_len, %d key_len [%lu %lu], %d iv_len\n",
                    ds->buf_len, ds->cipher->key_len, DES_KEY_SZ,
                    DES_SCHEDULE_SZ, ds->cipher->iv_len);
            fprintf(stderr, "\t\tIV: ");
            for (i = 0; i < ds->cipher->iv_len; i++)
                fprintf(stderr, "%02X", ds->iv[i]);
            fprintf(stderr, "\n");
            fprintf(stderr, "\trec->input=");
            for (ui = 0; ui < l; ui++)
                fprintf(stderr, " %02x", rec->input[ui]);
            fprintf(stderr, "\n");
        }
#endif                          /* KSSL_DEBUG */

        if (!send) {
            if (l == 0 || l % bs != 0)
                return 0;
        }

        i = EVP_Cipher(ds, rec->data, rec->input, l);
        if ((EVP_CIPHER_flags(ds->cipher) & EVP_CIPH_FLAG_CUSTOM_CIPHER)
            ? (i < 0)
            : (i == 0))
            return -1;          /* AEAD can fail to verify MAC */
        if (EVP_CIPHER_mode(enc) == EVP_CIPH_GCM_MODE && !send) {
            rec->data += EVP_GCM_TLS_EXPLICIT_IV_LEN;
            rec->input += EVP_GCM_TLS_EXPLICIT_IV_LEN;
            rec->length -= EVP_GCM_TLS_EXPLICIT_IV_LEN;
        }
#ifdef KSSL_DEBUG
        {
            unsigned long i;
            fprintf(stderr, "\trec->data=");
            for (i = 0; i < l; i++)
                fprintf(stderr, " %02x", rec->data[i]);
            fprintf(stderr, "\n");
        }
#endif                          /* KSSL_DEBUG */

        ret = 1;
        if (!SSL_USE_ETM(s) && EVP_MD_CTX_md(s->read_hash) != NULL)
            mac_size = EVP_MD_CTX_size(s->read_hash);
        if ((bs != 1) && !send)
            ret = tls1_cbc_remove_padding(s, rec, bs, mac_size);
        if (pad && !send)
            rec->length -= pad;
    }
    return ret;
}

int n_ssl3_mac(SSL *ssl, unsigned char *md, int send)
{
    SSL3_RECORD *rec;
    unsigned char *mac_sec, *seq;
    EVP_MD_CTX md_ctx;
    const EVP_MD_CTX *hash;
    unsigned char *p, rec_char;
    size_t md_size;
    int npad;
    int t;

    if (send) {
        rec = RECORD_LAYER_get_wrec(&ssl->rlayer);
        mac_sec = &(ssl->s3->write_mac_secret[0]);
        seq = &(ssl->s3->write_sequence[0]);
        hash = ssl->write_hash;
    } else {
        rec = RECORD_LAYER_get_rrec(&ssl->rlayer);
        mac_sec = &(ssl->s3->read_mac_secret[0]);
        seq = &(ssl->s3->read_sequence[0]);
        hash = ssl->read_hash;
    }

    t = EVP_MD_CTX_size(hash);
    if (t < 0)
        return -1;
    md_size = t;
    npad = (48 / md_size) * md_size;

    if (!send &&
        EVP_CIPHER_CTX_mode(ssl->enc_read_ctx) == EVP_CIPH_CBC_MODE &&
        ssl3_cbc_record_digest_supported(hash)) {
        /*
         * This is a CBC-encrypted record. We must avoid leaking any
         * timing-side channel information about how many blocks of data we
         * are hashing because that gives an attacker a timing-oracle.
         */

        /*-
         * npad is, at most, 48 bytes and that's with MD5:
         *   16 + 48 + 8 (sequence bytes) + 1 + 2 = 75.
         *
         * With SHA-1 (the largest hash speced for SSLv3) the hash size
         * goes up 4, but npad goes down by 8, resulting in a smaller
         * total size.
         */
        unsigned char header[75];
        unsigned j = 0;
        memcpy(header + j, mac_sec, md_size);
        j += md_size;
        memcpy(header + j, ssl3_pad_1, npad);
        j += npad;
        memcpy(header + j, seq, 8);
        j += 8;
        header[j++] = rec->type;
        header[j++] = rec->length >> 8;
        header[j++] = rec->length & 0xff;

        /* Final param == is SSLv3 */
        ssl3_cbc_digest_record(hash,
                               md, &md_size,
                               header, rec->input,
                               rec->length + md_size, rec->orig_len,
                               mac_sec, md_size, 1);
    } else {
        unsigned int md_size_u;
        /* Chop the digest off the end :-) */
        EVP_MD_CTX_init(&md_ctx);

        EVP_MD_CTX_copy_ex(&md_ctx, hash);
        EVP_DigestUpdate(&md_ctx, mac_sec, md_size);
        EVP_DigestUpdate(&md_ctx, ssl3_pad_1, npad);
        EVP_DigestUpdate(&md_ctx, seq, 8);
        rec_char = rec->type;
        EVP_DigestUpdate(&md_ctx, &rec_char, 1);
        p = md;
        s2n(rec->length, p);
        EVP_DigestUpdate(&md_ctx, md, 2);
        EVP_DigestUpdate(&md_ctx, rec->input, rec->length);
        EVP_DigestFinal_ex(&md_ctx, md, NULL);

        EVP_MD_CTX_copy_ex(&md_ctx, hash);
        EVP_DigestUpdate(&md_ctx, mac_sec, md_size);
        EVP_DigestUpdate(&md_ctx, ssl3_pad_2, npad);
        EVP_DigestUpdate(&md_ctx, md, md_size);
        EVP_DigestFinal_ex(&md_ctx, md, &md_size_u);
        md_size = md_size_u;

        EVP_MD_CTX_cleanup(&md_ctx);
    }

    ssl3_record_sequence_update(seq);
    return (md_size);
}

int tls1_mac(SSL *ssl, unsigned char *md, int send)
{
    SSL3_RECORD *rec;
    unsigned char *seq;
    EVP_MD_CTX *hash;
    size_t md_size;
    int i;
    EVP_MD_CTX hmac, *mac_ctx;
    unsigned char header[13];
    int stream_mac = (send ? (ssl->mac_flags & SSL_MAC_FLAG_WRITE_MAC_STREAM)
                      : (ssl->mac_flags & SSL_MAC_FLAG_READ_MAC_STREAM));
    int t;

    if (send) {
        rec = RECORD_LAYER_get_wrec(&ssl->rlayer);
        seq = &(ssl->s3->write_sequence[0]);
        hash = ssl->write_hash;
    } else {
        rec = RECORD_LAYER_get_rrec(&ssl->rlayer);
        seq = &(ssl->s3->read_sequence[0]);
        hash = ssl->read_hash;
    }

    t = EVP_MD_CTX_size(hash);
    OPENSSL_assert(t >= 0);
    md_size = t;

    /* I should fix this up TLS TLS TLS TLS TLS XXXXXXXX */
    if (stream_mac) {
        mac_ctx = hash;
    } else {
        if (!EVP_MD_CTX_copy(&hmac, hash))
            return -1;
        mac_ctx = &hmac;
    }

    if (SSL_IS_DTLS(ssl)) {
        unsigned char dtlsseq[8], *p = dtlsseq;

        s2n(send ? ssl->d1->w_epoch : ssl->d1->r_epoch, p);
        memcpy(p, &seq[2], 6);

        memcpy(header, dtlsseq, 8);
    } else
        memcpy(header, seq, 8);

    header[8] = rec->type;
    header[9] = (unsigned char)(ssl->version >> 8);
    header[10] = (unsigned char)(ssl->version);
    header[11] = (rec->length) >> 8;
    header[12] = (rec->length) & 0xff;

    if (!send && !SSL_USE_ETM(ssl) &&
        EVP_CIPHER_CTX_mode(ssl->enc_read_ctx) == EVP_CIPH_CBC_MODE &&
        ssl3_cbc_record_digest_supported(mac_ctx)) {
        /*
         * This is a CBC-encrypted record. We must avoid leaking any
         * timing-side channel information about how many blocks of data we
         * are hashing because that gives an attacker a timing-oracle.
         */
        /* Final param == not SSLv3 */
        ssl3_cbc_digest_record(mac_ctx,
                               md, &md_size,
                               header, rec->input,
                               rec->length + md_size, rec->orig_len,
                               ssl->s3->read_mac_secret,
                               ssl->s3->read_mac_secret_size, 0);
    } else {
        EVP_DigestSignUpdate(mac_ctx, header, sizeof(header));
        EVP_DigestSignUpdate(mac_ctx, rec->input, rec->length);
        t = EVP_DigestSignFinal(mac_ctx, md, &md_size);
        OPENSSL_assert(t > 0);
        if (!send && !SSL_USE_ETM(ssl) && FIPS_mode())
            tls_fips_digest_extra(ssl->enc_read_ctx,
                                  mac_ctx, rec->input,
                                  rec->length, rec->orig_len);
    }

    if (!stream_mac)
        EVP_MD_CTX_cleanup(&hmac);
#ifdef TLS_DEBUG
    fprintf(stderr, "seq=");
    {
        int z;
        for (z = 0; z < 8; z++)
            fprintf(stderr, "%02X ", seq[z]);
        fprintf(stderr, "\n");
    }
    fprintf(stderr, "rec=");
    {
        unsigned int z;
        for (z = 0; z < rec->length; z++)
            fprintf(stderr, "%02X ", rec->data[z]);
        fprintf(stderr, "\n");
    }
#endif

    if (!SSL_IS_DTLS(ssl)) {
        for (i = 7; i >= 0; i--) {
            ++seq[i];
            if (seq[i] != 0)
                break;
        }
    }
#ifdef TLS_DEBUG
    {
        unsigned int z;
        for (z = 0; z < md_size; z++)
            fprintf(stderr, "%02X ", md[z]);
        fprintf(stderr, "\n");
    }
#endif
    return (md_size);
}

/*-
 * 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);
    rec->length -= good & (padding_length + 1);
    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;
        rec->orig_len -= 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--;
        }
    }

    if (EVP_CIPHER_flags(s->enc_read_ctx->cipher) & EVP_CIPH_FLAG_AEAD_CIPHER) {
        /* padding is already verified */
        rec->length -= padding_length + 1;
        return 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 = 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);
    rec->length -= good & (padding_length + 1);

    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)
{
#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(rec->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 (rec->orig_len > md_size + 255 + 1)
        scan_start = rec->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 < rec->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
}

int dtls1_process_record(SSL *s)
{
    int i, al;
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