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Diffstat (limited to 'crypto/dsa/dsa_ossl.c')
-rw-r--r--crypto/dsa/dsa_ossl.c42
1 files changed, 27 insertions, 15 deletions
diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c
index 58013a4a13b5..aa10dd12f6f8 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,