Commit 3fc7a9b9 authored by Andy Polyakov's avatar Andy Polyakov
Browse files

ec/ecdsa_ossl.c: revert blinding in ECDSA signature. 



Originally suggested solution for "Return Of the Hidden Number Problem"
is arguably too expensive. While it has marginal impact on slower
curves, none to ~6%, optimized implementations suffer real penalties.
Most notably sign with P-256 went more than 2 times[!] slower. Instead,
just implement constant-time BN_mod_add_quick.

Reviewed-by: default avatarRich Salz <rsalz@openssl.org>
Reviewed-by: default avatarDavid Benjamin <davidben@google.com>
(Merged from https://github.com/openssl/openssl/pull/6664)
parent 83e03437

crypto/bn/bn_mod.c

+60 −6
Original line number Diff line number Diff line
@@ -35,18 +35,72 @@ int BN_mod_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m, 


/*

 * BN_mod_add variant that may be used if both a and b are non-negative and

 * less than m

 * less than m. The original algorithm was

 *

 *    if (!BN_uadd(r, a, b))

 *       return 0;

 *    if (BN_ucmp(r, m) >= 0)

 *       return BN_usub(r, r, m);

 *

 * which is replaced with addition, subtracting modulus, and conditional

 * move depending on whether or not subtraction borrowed.

 */

int BN_mod_add_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,

int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,

                         const BIGNUM *m)

{

    if (!BN_uadd(r, a, b))

    size_t i, ai, bi, mtop = m->top;

    BN_ULONG storage[1024 / BN_BITS2];

    BN_ULONG carry, temp, mask, *rp, *tp = storage;

    const BN_ULONG *ap, *bp;



    if (bn_wexpand(r, mtop) == NULL)

        return 0;



    if (mtop > sizeof(storage) / sizeof(storage[0])

        && (tp = OPENSSL_malloc(mtop * sizeof(BN_ULONG))) == NULL)

	return 0;

    if (BN_ucmp(r, m) >= 0)

        return BN_usub(r, r, m);



    ap = a->d != NULL ? a->d : tp;

    bp = b->d != NULL ? b->d : tp;



    for (i = 0, ai = 0, bi = 0, carry = 0; i < mtop;) {

        mask = (BN_ULONG)0 - ((i - a->top) >> (8 * sizeof(i) - 1));

        temp = ((ap[ai] & mask) + carry) & BN_MASK2;

        carry = (temp < carry);



        mask = (BN_ULONG)0 - ((i - b->top) >> (8 * sizeof(i) - 1));

        tp[i] = ((bp[bi] & mask) + temp) & BN_MASK2;

        carry += (tp[i] < temp);



        i++;

        ai += (i - a->dmax) >> (8 * sizeof(i) - 1);

        bi += (i - b->dmax) >> (8 * sizeof(i) - 1);

    }

    rp = r->d;

    carry -= bn_sub_words(rp, tp, m->d, mtop);

    for (i = 0; i < mtop; i++) {

        rp[i] = (carry & tp[i]) | (~carry & rp[i]);

        ((volatile BN_ULONG *)tp)[i] = 0;

    }

    r->top = mtop;



    if (tp != storage)

        OPENSSL_free(tp);



    return 1;

}



int BN_mod_add_quick(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,

                     const BIGNUM *m)

{

    int ret = bn_mod_add_fixed_top(r, a, b, m);



    if (ret)

        bn_correct_top(r);



    return ret;

}



int BN_mod_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, const BIGNUM *m,

               BN_CTX *ctx)

{

crypto/ec/ecdsa_ossl.c

+7 −64
Original line number Diff line number Diff line
@@ -172,8 +172,7 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len, 
                               EC_KEY *eckey)

{

    int ok = 0, i;

    BIGNUM *kinv = NULL, *s, *m = NULL, *tmp = NULL, *blind = NULL;

    BIGNUM *blindm = NULL;

    BIGNUM *kinv = NULL, *s, *m = NULL, *tmp = NULL;

    const BIGNUM *order, *ckinv;

    BN_CTX *ctx = NULL;

    const EC_GROUP *group;
@@ -206,18 +205,8 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len, 
    }

    s = ret->s;



    ctx = BN_CTX_secure_new();

    if (ctx == NULL) {

        ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);

        goto err;

    }



    BN_CTX_start(ctx);

    tmp = BN_CTX_get(ctx);

    m = BN_CTX_get(ctx);

    blind = BN_CTX_get(ctx);

    blindm = BN_CTX_get(ctx);

    if (blindm == NULL) {

    if ((ctx = BN_CTX_new()) == NULL ||

        (tmp = BN_new()) == NULL || (m = BN_new()) == NULL) {

        ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);

        goto err;

    }
@@ -257,64 +246,18 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len, 
            }

        }



        /*

         * The normal signature calculation is:

         *

         *   s := k^-1 * (m + r * priv_key) mod order

         *

         * We will blind this to protect against side channel attacks

         *

         *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod order

         */



        /* Generate a blinding value */

        do {

            if (!BN_priv_rand(blind, BN_num_bits(order) - 1,

                              BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))

                goto err;

        } while (BN_is_zero(blind));

        BN_set_flags(blind, BN_FLG_CONSTTIME);

        BN_set_flags(blindm, BN_FLG_CONSTTIME);

        BN_set_flags(tmp, BN_FLG_CONSTTIME);



        /* tmp := blind * priv_key * r mod order */

        if (!BN_mod_mul(tmp, blind, priv_key, order, ctx)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }

        if (!BN_mod_mul(tmp, tmp, ret->r, order, ctx)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }



        /* blindm := blind * m mod order */

        if (!BN_mod_mul(blindm, blind, m, order, ctx)) {

        if (!BN_mod_mul(tmp, priv_key, ret->r, order, ctx)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }



        /* s : = (blind * priv_key * r) + (blind * m) mod order */

        if (!BN_mod_add_quick(s, tmp, blindm, order)) {

        if (!BN_mod_add_quick(s, tmp, m, order)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }



        /* s := s * k^-1 mod order */

        if (!BN_mod_mul(s, s, ckinv, order, ctx)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }



        /* s:= s * blind^-1 mod order */

        if (BN_mod_inverse(blind, blind, order, ctx) == NULL) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }

        if (!BN_mod_mul(s, s, blind, order, ctx)) {

            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);

            goto err;

        }



        if (BN_is_zero(s)) {

            /*

             * if kinv and r have been supplied by the caller, don't
@@ -336,9 +279,9 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len, 
        ECDSA_SIG_free(ret);

        ret = NULL;

    }

    if (ctx != NULL)

        BN_CTX_end(ctx);

    BN_CTX_free(ctx);

    BN_clear_free(m);

    BN_clear_free(tmp);

    BN_clear_free(kinv);

    return ret;

}

crypto/include/internal/bn_int.h

+2 −0
Original line number Diff line number Diff line
@@ -71,5 +71,7 @@ int bn_mul_mont_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, 
                          BN_MONT_CTX *mont, BN_CTX *ctx);

int bn_to_mont_fixed_top(BIGNUM *r, const BIGNUM *a, BN_MONT_CTX *mont,

                         BN_CTX *ctx);

int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,

                         const BIGNUM *m);



#endif