Commit 949ff366 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 3984ef0b

CHANGES

+4 −0
Original line number Diff line number Diff line
@@ -9,6 +9,10 @@ 


 Changes between 1.0.2o and 1.0.2p [xx XXX xxxx]



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

     discovered by Keegan Ryan (NCC Group).

     [Matt Caswell]



  *) When unlocking a pass phrase protected PEM file or PKCS#8 container, we

     now allow empty (zero character) pass phrases.

     [Richard Levitte]

crypto/ecdsa/ecdsatest.c

+8 −1
Original line number Diff line number Diff line
@@ -137,7 +137,7 @@ int restore_rand(void) 
        return 1;

}



static int fbytes_counter = 0;

static int fbytes_counter = 0, use_fake = 0;

static const char *numbers[8] = {

    "651056770906015076056810763456358567190100156695615665659",

    "6140507067065001063065065565667405560006161556565665656654",
@@ -158,6 +158,11 @@ int fbytes(unsigned char *buf, int num) 
    int ret;

    BIGNUM *tmp = NULL;



    if (use_fake == 0)

        return old_rand->bytes(buf, num);



    use_fake = 0;



    if (fbytes_counter >= 8)

        return 0;

    tmp = BN_new();
@@ -199,11 +204,13 @@ int x9_62_test_internal(BIO *out, int nid, const char *r_in, const char *s_in) 
    /* create the key */

    if ((key = EC_KEY_new_by_curve_name(nid)) == NULL)

        goto x962_int_err;

    use_fake = 1;

    if (!EC_KEY_generate_key(key))

        goto x962_int_err;

    BIO_printf(out, ".");

    (void)BIO_flush(out);

    /* create the signature */

    use_fake = 1;

    signature = ECDSA_do_sign(digest, 20, key);

    if (signature == NULL)

        goto x962_int_err;

crypto/ecdsa/ecs_ossl.c

+67 −15
Original line number Diff line number Diff line
@@ -252,6 +252,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 
{

    int ok = 0, i;

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

    BIGNUM *blind = NULL, *blindm = NULL;

    const BIGNUM *ckinv;

    BN_CTX *ctx = NULL;

    const EC_GROUP *group;
@@ -269,14 +270,25 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 
    }



    ret = ECDSA_SIG_new();

    if (!ret) {

    if (ret == NULL) {

        ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);

        return NULL;

    }

    s = ret->s;



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

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

    ctx = BN_CTX_new();

    if (ctx == NULL) {

        ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);

        goto err;

    }



    BN_CTX_start(ctx);

    order = BN_CTX_get(ctx);

    tmp = BN_CTX_get(ctx);

    m = BN_CTX_get(ctx);

    blind = BN_CTX_get(ctx);

    blindm = BN_CTX_get(ctx);

    if (blindm == NULL) {

        ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_MALLOC_FAILURE);

        goto err;

    }
@@ -315,26 +327,70 @@ static ECDSA_SIG *ecdsa_do_sign(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_rand(blind, BN_num_bits(order) - 1, -1, 0))

                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)) {

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }



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

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, 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)) {

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }



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

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }



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

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }



        if (BN_is_zero(s)) {

            /*

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

             * generate new kinv and r values

             */

            if (in_kinv != NULL && in_r != NULL) {

                ECDSAerr(ECDSA_F_ECDSA_DO_SIGN,

                         ECDSA_R_NEED_NEW_SETUP_VALUES);

                ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ECDSA_R_NEED_NEW_SETUP_VALUES);

                goto err;

            }

        } else
@@ -349,15 +405,11 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 
        ECDSA_SIG_free(ret);

        ret = NULL;

    }

    if (ctx)

    if (ctx != NULL) {

        BN_CTX_end(ctx);

        BN_CTX_free(ctx);

    if (m)

        BN_clear_free(m);

    if (tmp)

        BN_clear_free(tmp);

    if (order)

        BN_free(order);

    if (kinv)

    }

    if (kinv != NULL)

        BN_clear_free(kinv);

    return ret;

}