rsa/rsa_ssl.c: make RSA_padding_check_SSLv23 constant-time.

#include <openssl/bn.h>

#include <openssl/rsa.h>

#include <openssl/rand.h>

#include "internal/constant_time_locl.h"

int RSA_padding_add_SSLv23(unsigned char *to, int tlen,

                           const unsigned char *from, int flen)

    return 1;

}

/*

 * Copy of RSA_padding_check_PKCS1_type_2 with a twist that rejects padding

 * if nul delimiter is preceded by 8 consecutive 0x03 bytes. It also

 * preserves error code reporting for backward compatibility.

 */

int RSA_padding_check_SSLv23(unsigned char *to, int tlen,

                             const unsigned char *from, int flen, int num)

{

    int i, j, k;

    const unsigned char *p;

    int i;

    /* |em| is the encoded message, zero-padded to exactly |num| bytes */

    unsigned char *em = NULL;

    unsigned int good, found_zero_byte, mask, threes_in_row;

    int zero_index = 0, msg_index, mlen = -1, err;

    p = from;

    if (flen < 10) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_DATA_TOO_SMALL);

        return -1;

    }

    /* Accept even zero-padded input */

    if (flen == num) {

        if (*(p++) != 0) {

            RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_BLOCK_TYPE_IS_NOT_02);

    em = OPENSSL_malloc(num);

    if (em == NULL) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, ERR_R_MALLOC_FAILURE);

        return -1;

    }

        flen--;

    }

    if ((num != (flen + 1)) || (*(p++) != 02)) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_BLOCK_TYPE_IS_NOT_02);

        return -1;

    /*

     * Caller is encouraged to pass zero-padded message created with

     * BN_bn2binpad. Trouble is that since we can't read out of |from|'s

     * bounds, it's impossible to have an invariant memory access pattern

     * in case |from| was not zero-padded in advance.

     */

    for (from += flen, em += num, i = 0; i < num; i++) {

        mask = ~constant_time_is_zero(flen);

        flen -= 1 & mask;

        from -= 1 & mask;

        *--em = *from & mask;

    }

    from = em;

    good = constant_time_is_zero(from[0]);

    good &= constant_time_eq(from[1], 2);

    err = constant_time_select_int(good, 0, RSA_R_BLOCK_TYPE_IS_NOT_02);

    mask = ~good;

    /* scan over padding data */

    j = flen - 1;               /* one for type */

    for (i = 0; i < j; i++)

        if (*(p++) == 0)

            break;

    found_zero_byte = 0;

    threes_in_row = 0;

    for (i = 2; i < num; i++) {

        unsigned int equals0 = constant_time_is_zero(from[i]);

    if ((i == j) || (i < 8)) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23,

               RSA_R_NULL_BEFORE_BLOCK_MISSING);

        return -1;

    }

    for (k = -9; k < -1; k++) {

        if (p[k] != 0x03)

            break;

    }

    if (k == -1) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_SSLV3_ROLLBACK_ATTACK);

        return -1;

        zero_index = constant_time_select_int(~found_zero_byte & equals0,

                                              i, zero_index);

        found_zero_byte |= equals0;

        threes_in_row += 1 & ~found_zero_byte;

        threes_in_row &= found_zero_byte | constant_time_eq(from[i], 3);

    }

    i++;                        /* Skip over the '\0' */

    j -= i;

    if (j > tlen) {

        RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, RSA_R_DATA_TOO_LARGE);

        return -1;

    /*

     * PS must be at least 8 bytes long, and it starts two bytes into |from|.

     * If we never found a 0-byte, then |zero_index| is 0 and the check

     * also fails.

     */

    good &= constant_time_ge(zero_index, 2 + 8);

    err = constant_time_select_int(mask | good, err,

                                   RSA_R_NULL_BEFORE_BLOCK_MISSING);

    mask = ~good;

    good &= constant_time_lt(threes_in_row, 8);

    err = constant_time_select_int(mask | good, err,

                                   RSA_R_SSLV3_ROLLBACK_ATTACK);

    mask = ~good;

    /*

     * Skip the zero byte. This is incorrect if we never found a zero-byte

     * but in this case we also do not copy the message out.

     */

    msg_index = zero_index + 1;

    mlen = num - msg_index;

    /*

     * For good measure, do this check in constant time as well.

     */

    good &= constant_time_ge(tlen, mlen);

    err = constant_time_select_int(mask | good, err, RSA_R_DATA_TOO_LARGE);

    /*

     * Even though we can't fake result's length, we can pretend copying

     * |tlen| bytes where |mlen| bytes would be real. Last |tlen| of |num|

     * bytes are viewed as circular buffer with start at |tlen|-|mlen'|,

     * where |mlen'| is "saturated" |mlen| value. Deducing information

     * about failure or |mlen| would take attacker's ability to observe

     * memory access pattern with byte granularity *as it occurs*. It

     * should be noted that failure is indistinguishable from normal

     * operation if |tlen| is fixed by protocol.

     */

    tlen = constant_time_select_int(constant_time_lt(num, tlen), num, tlen);

    msg_index = constant_time_select_int(good, msg_index, num - tlen);

    mlen = num - msg_index;

    for (from += msg_index, mask = good, i = 0; i < tlen; i++) {

        unsigned int equals = constant_time_eq(i, mlen);

        from -= tlen & equals;  /* if (i == mlen) rewind   */

        mask &= mask ^ equals;  /* if (i == mlen) mask = 0 */

        to[i] = constant_time_select_8(mask, from[i], to[i]);

    }

    memcpy(to, p, (unsigned int)j);

    return j;

    OPENSSL_clear_free(em, num);

    RSAerr(RSA_F_RSA_PADDING_CHECK_SSLV23, err);

    err_clear_last_constant_time(1 & good);

    return constant_time_select_int(good, mlen, -1);

}