Commit f1b12b87 authored by Pauli's avatar Pauli
Browse files

DSA mod inverse fix 



There is a side channel attack against the division used to calculate one of
the modulo inverses in the DSA algorithm.  This change takes advantage of the
primality of the modulo and Fermat's little theorem to calculate the inverse
without leaking information.

Thanks to Samuel Weiser for finding and reporting this.

Reviewed-by: default avatarMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: default avatarBernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7487)

(cherry picked from commit 415c33563528667868c3c653a612e6fc8736fd79)
parent d2953e5e

crypto/dsa/dsa_ossl.c

+31 −1
Original line number Diff line number Diff line
@@ -23,6 +23,8 @@ static int dsa_do_verify(const unsigned char *dgst, int dgst_len, 
                         DSA_SIG *sig, DSA *dsa);

static int dsa_init(DSA *dsa);

static int dsa_finish(DSA *dsa);

static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,

                                      BN_CTX *ctx);



static DSA_METHOD openssl_dsa_meth = {

    "OpenSSL DSA method",
@@ -259,7 +261,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 
        goto err;



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

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

    if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)

        goto err;



    BN_clear_free(*kinvp);
@@ -393,3 +395,31 @@ static int dsa_finish(DSA *dsa) 
    BN_MONT_CTX_free(dsa->method_mont_p);

    return 1;

}



/*

 * Compute the inverse of k modulo q.

 * Since q is prime, Fermat's Little Theorem applies, which reduces this to

 * mod-exp operation.  Both the exponent and modulus are public information

 * so a mod-exp that doesn't leak the base is sufficient.  A newly allocated

 * BIGNUM is returned which the caller must free.

 */

static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,

                                      BN_CTX *ctx)

{

    BIGNUM *res = NULL;

    BIGNUM *r, *e;



    if ((r = BN_new()) == NULL)

        return NULL;



    BN_CTX_start(ctx);

    if ((e = BN_CTX_get(ctx)) != NULL

            && BN_set_word(r, 2)

            && BN_sub(e, q, r)

            && BN_mod_exp_mont(r, k, e, q, ctx, NULL))

        res = r;

    else

        BN_free(r);

    BN_CTX_end(ctx);

    return res;

}