diff options
| -rw-r--r-- | crypto/dsa/dsa_ossl.c | 42 |
1 files changed, 27 insertions, 15 deletions
diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index 58013a4a13..aa10dd12f6 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -224,7 +224,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, { BN_CTX *ctx; BIGNUM k, kq, *K, *kinv = NULL, *r = NULL; + BIGNUM l, m; int ret = 0; + int q_bits; if (!dsa->p || !dsa->q || !dsa->g) { DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS); @@ -233,6 +235,8 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BN_init(&k); BN_init(&kq); + BN_init(&l); + BN_init(&m); if (ctx_in == NULL) { if ((ctx = BN_CTX_new()) == NULL) @@ -243,6 +247,13 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, if ((r = BN_new()) == NULL) goto err; + /* Preallocate space */ + q_bits = BN_num_bits(dsa->q); + if (!BN_set_bit(&k, q_bits) + || !BN_set_bit(&l, q_bits) + || !BN_set_bit(&m, q_bits)) + goto err; + /* Get random k */ do if (!BN_rand_range(&k, dsa->q)) @@ -263,24 +274,23 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, /* Compute r = (g^k mod p) mod q */ if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { - if (!BN_copy(&kq, &k)) - goto err; - - BN_set_flags(&kq, BN_FLG_CONSTTIME); - /* * We do not want timing information to leak the length of k, so we - * compute g^k using an equivalent exponent of fixed length. (This - * is a kludge that we need because the BN_mod_exp_mont() does not - * let us specify the desired timing behaviour.) + * compute G^k using an equivalent scalar of fixed bit-length. + * + * We unconditionally perform both of these additions to prevent a + * small timing information leakage. We then choose the sum that is + * one bit longer than the modulus. + * + * TODO: revisit the BN_copy aiming for a memory access agnostic + * conditional copy. */ - - if (!BN_add(&kq, &kq, dsa->q)) + if (!BN_add(&l, &k, dsa->q) + || !BN_add(&m, &l, dsa->q) + || !BN_copy(&kq, BN_num_bits(&l) > q_bits ? &l : &m)) goto err; - if (BN_num_bits(&kq) <= BN_num_bits(dsa->q)) { - if (!BN_add(&kq, &kq, dsa->q)) - goto err; - } + + BN_set_flags(&kq, BN_FLG_CONSTTIME); K = &kq; } else { @@ -314,7 +324,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BN_CTX_free(ctx); BN_clear_free(&k); BN_clear_free(&kq); - return (ret); + BN_clear_free(&l); + BN_clear_free(&m); + return ret; } static int dsa_do_verify(const unsigned char *dgst, int dgst_len, |