Commit 41d23d43 authored by Matt Caswell's avatar Matt Caswell
Browse files

Add blinding to a DSA signature 



This extends the recently added ECDSA signature blinding to blind DSA too.

This is based on side channel attacks demonstrated by Keegan Ryan (NCC
Group) for ECDSA which are likely to be able to be applied to DSA.

Normally, as in ECDSA, during signing the signer calculates:

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

In ECDSA, 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.

This commit also tweaks the previous ECDSA blinding so that blinding is
only removed at the last possible step.

Reviewed-by: default avatarRich Salz <rsalz@openssl.org>
Reviewed-by: default avatarRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6524)
parent 8fbbbdd5

CHANGES

+2 −2
Original line number Diff line number Diff line
@@ -9,8 +9,8 @@ 


 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).

  *) Add blinding to ECDSA and DSA signatures 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

crypto/dsa/dsa_ossl.c

+52 −21
Original line number Diff line number Diff line
@@ -133,17 +133,13 @@ const DSA_METHOD *DSA_OpenSSL(void) 
static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)

{

    BIGNUM *kinv = NULL, *r = NULL, *s = NULL;

    BIGNUM m;

    BIGNUM xr;

    BIGNUM *m, *blind, *blindm, *tmp;

    BN_CTX *ctx = NULL;

    int reason = ERR_R_BN_LIB;

    DSA_SIG *ret = NULL;

    int noredo = 0;



    BN_init(&m);

    BN_init(&xr);



    if (!dsa->p || !dsa->q || !dsa->g) {

    if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) {

        reason = DSA_R_MISSING_PARAMETERS;

        goto err;

    }
@@ -154,6 +150,13 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) 
    ctx = BN_CTX_new();

    if (ctx == NULL)

        goto err;

    m = BN_CTX_get(ctx);

    blind = BN_CTX_get(ctx);

    blindm = BN_CTX_get(ctx);

    tmp = BN_CTX_get(ctx);

    if (tmp == NULL)

        goto err;



 redo:

    if ((dsa->kinv == NULL) || (dsa->r == NULL)) {

        if (!DSA_sign_setup(dsa, ctx, &kinv, &r))
@@ -173,20 +176,52 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) 
         * 4.2

         */

        dlen = BN_num_bytes(dsa->q);

    if (BN_bin2bn(dgst, dlen, &m) == NULL)

    if (BN_bin2bn(dgst, dlen, m) == NULL)

        goto err;



    /*

     * The normal signature calculation is:

     *

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

     *

     * We will blind this to protect against side channel attacks

     *

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

     */



    /* Generate a blinding value */

    do {

        if (!BN_rand(blind, BN_num_bits(dsa->q) - 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);



    /* Compute  s = inv(k) (m + xr) mod q */

    if (!BN_mod_mul(&xr, dsa->priv_key, r, dsa->q, ctx))

        goto err;               /* s = xr */

    if (!BN_add(s, &xr, &m))

        goto err;               /* s = m + xr */

    if (BN_cmp(s, dsa->q) > 0)

        if (!BN_sub(s, s, dsa->q))

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

    if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))

        goto err;

    if (!BN_mod_mul(tmp, tmp, r, dsa->q, ctx))

        goto err;



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

    if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))

        goto err;



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

    if (!BN_mod_add_quick(s, tmp, blindm, dsa->q))

        goto err;



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

    if (!BN_mod_mul(s, s, kinv, dsa->q, ctx))

        goto err;



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

    if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)

        goto err;

    if (!BN_mod_mul(s, s, blind, dsa->q, ctx))

        goto err;



    /*

     * Redo if r or s is zero as required by FIPS 186-3: this is very

     * unlikely.
@@ -210,13 +245,9 @@ static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa) 
        BN_free(r);

        BN_free(s);

    }

    if (ctx != NULL)

    BN_CTX_free(ctx);

    BN_clear_free(&m);

    BN_clear_free(&xr);

    if (kinv != NULL)           /* dsa->kinv is NULL now if we used it */

    BN_clear_free(kinv);

    return (ret);

    return ret;

}



static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,

crypto/ecdsa/ecs_ossl.c

+7 −7
Original line number Diff line number Diff line
@@ -334,7 +334,7 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 
         *

         * We will blind this to protect against side channel attacks

         *

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

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

         */



        /* Generate a blinding value */
@@ -368,18 +368,18 @@ static ECDSA_SIG *ecdsa_do_sign(const unsigned char *dgst, int dgst_len, 
            goto err;

        }



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

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

        /* 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_mod_mul(s, s, blind, order, ctx)) {



        /* 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;

        }



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

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

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

            ECDSAerr(ECDSA_F_ECDSA_DO_SIGN, ERR_R_BN_LIB);

            goto err;

        }