diff options
Diffstat (limited to 'crypto/dsa/dsa_ossl.c')
| -rw-r--r-- | crypto/dsa/dsa_ossl.c | 38 |
1 files changed, 34 insertions, 4 deletions
diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index 2dcfedeeee7e..c887c3c34e37 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -73,6 +73,8 @@ 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", @@ -279,7 +281,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, goto err; /* Preallocate space */ - q_bits = BN_num_bits(dsa->q); + q_bits = BN_num_bits(dsa->q) + sizeof(dsa->q->d[0]) * 16; if (!BN_set_bit(&k, q_bits) || !BN_set_bit(&l, q_bits) || !BN_set_bit(&m, q_bits)) @@ -293,9 +295,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { BN_set_flags(&k, BN_FLG_CONSTTIME); + BN_set_flags(&l, BN_FLG_CONSTTIME); } - if (dsa->flags & DSA_FLAG_CACHE_MONT_P) { if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p, CRYPTO_LOCK_DSA, dsa->p, ctx)) @@ -333,8 +335,8 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, if (!BN_mod(r, r, dsa->q, ctx)) goto err; - /* Compute part of 's = inv(k) (m + xr) mod q' */ - if ((kinv = BN_mod_inverse(NULL, &k, dsa->q, ctx)) == NULL) + /* Compute part of 's = inv(k) (m + xr) mod q' */ + if ((kinv = dsa_mod_inverse_fermat(&k, dsa->q, ctx)) == NULL) goto err; if (*kinvp != NULL) @@ -468,3 +470,31 @@ 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_init(&e); + + if (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_free(&e); + return res; +} |