Commit 37258dad authored by Matt Caswell's avatar Matt Caswell
Browse files

Fix BN_mod_word bug 



On systems where we do not have BN_ULLONG (e.g. typically 64 bit systems)
then BN_mod_word() can return incorrect results if the supplied modulus is
too big.

RT#4501

Reviewed-by: default avatarAndy Polyakov <appro@openssl.org>
parent 4692340e

crypto/bn/bn_word.c

+22 −0
Original line number Diff line number Diff line
@@ -22,10 +22,32 @@ BN_ULONG BN_mod_word(const BIGNUM *a, BN_ULONG w) 
    if (w == 0)

        return (BN_ULONG)-1;



#ifndef BN_LLONG

    /*

     * If |w| is too long and we don't have BN_ULLONG then we need to fall

     * back to using BN_div_word

     */

    if (w > ((BN_ULONG)1 << BN_BITS4)) {

        BIGNUM *tmp = BN_dup(a);

        if (tmp == NULL)

            return (BN_ULONG)-1;



        ret = BN_div_word(tmp, w);

        BN_free(tmp);



        return ret;

    }

#endif



    bn_check_top(a);

    w &= BN_MASK2;

    for (i = a->top - 1; i >= 0; i--) {

#ifndef BN_LLONG

        /*

         * We can assume here that | w <= ((BN_ULONG)1 << BN_BITS4) | and so

         * | ret < ((BN_ULONG)1 << BN_BITS4) | and therefore the shifts here are

         * safe and will not overflow

         */

        ret = ((ret << BN_BITS4) | ((a->d[i] >> BN_BITS4) & BN_MASK2l)) % w;

        ret = ((ret << BN_BITS4) | (a->d[i] & BN_MASK2l)) % w;

#else