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Remove DSA k+q kludge.

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With fixed-width BIGNUMs, this is no longer a concern. With this CL, I
believe we now no longer call BN_num_bits on BIGNUMs with secret
magnitude.

Of course, DSA then turns around and calls the variable-time BN_mod
immediately afterwards anyway. But the DSA is deprecated and doomed to
be removed someday anyway.

Change-Id: Iac1dab22aa51c0e7f5ca0f7f44a026a242a4eaa2
Reviewed-on: https://boringssl-review.googlesource.com/25284
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
This commit is contained in:
David Benjamin 2018-01-25 11:39:22 -05:00 committed by CQ bot account: commit-bot@chromium.org
parent 08805fe279
commit eaa80b7069
1 changed files with 13 additions and 47 deletions
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60
crypto/dsa/dsa.c
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@ -835,7 +835,7 @@ int DSA_size(const DSA *dsa) {
static int dsa_sign_setup(const DSA *dsa, BN_CTX *ctx_in, BIGNUM **out_kinv,
BIGNUM **out_r) {
BN_CTX *ctx;
BIGNUM k, kq, *kinv = NULL, *r = NULL;
BIGNUM k, *kinv = NULL, *r = NULL;
int ret = 0;
if (!dsa->p || !dsa->q || !dsa->g) {
@ -844,7 +844,6 @@ static int dsa_sign_setup(const DSA *dsa, BN_CTX *ctx_in, BIGNUM **out_kinv,
}
BN_init(&k);
BN_init(&kq);
ctx = ctx_in;
if (ctx == NULL) {
@ -855,54 +854,22 @@ static int dsa_sign_setup(const DSA *dsa, BN_CTX *ctx_in, BIGNUM **out_kinv,
}
r = BN_new();
if (r == NULL) {
goto err;
}
// Get random k
if (!BN_rand_range_ex(&k, 1, dsa->q)) {
goto err;
}
if (!BN_MONT_CTX_set_locked((BN_MONT_CTX **)&dsa->method_mont_p,
kinv = BN_new();
if (r == NULL || kinv == NULL ||
// Get random k
!BN_rand_range_ex(&k, 1, dsa->q) ||
!BN_MONT_CTX_set_locked((BN_MONT_CTX **)&dsa->method_mont_p,
(CRYPTO_MUTEX *)&dsa->method_mont_lock, dsa->p,
ctx) ||
!BN_MONT_CTX_set_locked((BN_MONT_CTX **)&dsa->method_mont_q,
(CRYPTO_MUTEX *)&dsa->method_mont_lock, dsa->q,
ctx)) {
goto err;
}
// Compute r = (g^k mod p) mod q
if (!BN_copy(&kq, &k)) {
goto err;
}
// 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.)
if (!BN_add(&kq, &kq, dsa->q)) {
goto err;
}
if (BN_num_bits(&kq) <= BN_num_bits(dsa->q) && !BN_add(&kq, &kq, dsa->q)) {
goto err;
}
if (!BN_mod_exp_mont_consttime(r, dsa->g, &kq, dsa->p, ctx,
dsa->method_mont_p)) {
goto err;
}
if (!BN_mod(r, r, dsa->q, ctx)) {
goto err;
}
// Compute part of 's = inv(k) (m + xr) mod q' using Fermat's Little
// Theorem.
kinv = BN_new();
if (kinv == NULL ||
ctx) ||
// Compute r = (g^k mod p) mod q
!BN_mod_exp_mont_consttime(r, dsa->g, &k, dsa->p, ctx,
dsa->method_mont_p) ||
!BN_mod(r, r, dsa->q, ctx) ||
// Compute part of 's = inv(k) (m + xr) mod q' using Fermat's Little
// Theorem.
!bn_mod_inverse_prime(kinv, &k, dsa->q, ctx, dsa->method_mont_q)) {
goto err;
}
@ -926,7 +893,6 @@ err:
BN_CTX_free(ctx);
}
BN_clear_free(&k);
BN_clear_free(&kq);
BN_clear_free(kinv);
return ret;
}