Commit a3e9d5aa authored by Matt Caswell's avatar Matt Caswell
Browse files

Add blinding to an ECDSA signature 



Keegan Ryan (NCC Group) has demonstrated a side channel attack on an
ECDSA signature operation. During signing the signer calculates:

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

The addition operation above provides a sufficient signal for a
flush+reload attack to derive the private key given sufficient signature
operations.

As a mitigation (based on a suggestion from Keegan) we add blinding to
the operation so that:

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

Since this attack is a localhost side channel only no CVE is assigned.

Reviewed-by: default avatarRich Salz <rsalz@openssl.org>
parent d3273ef6

CHANGES

+4 −0
Original line number Diff line number Diff line
@@ -8,6 +8,10 @@ 
 release branch.















 Changes between 1.1.0h and 1.1.1 [xx XXX xxxx]













  *) Add blinding to an ECDSA signature to protect against side channel attacks













     discovered by Keegan Ryan (NCC Group).













     [Matt Caswell]



























  *) Enforce checking in the pkeyutl command line app to ensure that the input















     length does not exceed the maximum supported digest length when performing















     a sign, verify or verifyrecover operation.

































crypto/ec/ecdsa_ossl.c



+63
−7






























Original line number
Diff line number
Diff line








@@ -196,7 +196,8 @@ 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;













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













    BIGNUM *blindm = NULL;















    const BIGNUM *order, *ckinv;















    BN_CTX *ctx = NULL;















    const EC_GROUP *group;










@@ -229,8 +230,18 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,













    }















    s = ret->s;





























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













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













    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) {















        ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);















        goto err;















    }










@@ -270,18 +281,64 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,













            }















        }





























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













        /*













         * 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 := k^-1 * blind^-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)) {















            ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_BN_LIB);















            goto err;















        }













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



























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













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















            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;













        }



























        /* 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;















        }





























        if (BN_is_zero(s)) {















            /*















             * if kinv and r have been supplied by the caller, don't









@@ -303,9 +360,8 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,













        ECDSA_SIG_free(ret);















        ret = NULL;















    }













    BN_CTX_end(ctx);















    BN_CTX_free(ctx);













    BN_clear_free(m);













    BN_clear_free(tmp);















    BN_clear_free(kinv);















    return ret;















}