Don't use BN_mod_inverse for inverses mod p in RSA keygen.
Instead, use BN_mod_exp_mont_consttime of p - 2. This removes two more
call sites sensitive to BN_FLG_CONSTTIME. We're down to just that last
BN_mod_inverse modulo φ(n). (Sort of. It's actually not sensitive
because even mod inverses always hit the other codepath. Perhaps we
should just leave it alone.)
Note this comes with a slight behavior change. The BN_MONT_CTXs are
initialized a little earlier. If a caller calls RSA_generate_* and then
reaches into the struct to scrap all the fields on it, they'll get
confused. Before, they had to perform an operation on it to get
confused. This is a completely ridiculous thing to do.
Since we do this a lot, this introduces some convenience functions for
doing the Fermat's Little Theorem mod inverse and fixes a leak in the
DSA code should computing kinv hit a malloc error.
BUG=125
Change-Id: Iafcae2fc6fd379d161f015c90ff7050e2282e905
Reviewed-on: https://boringssl-review.googlesource.com/12925
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/crypto/bn/internal.h b/crypto/bn/internal.h
index f2141d7..1ee29fd 100644
--- a/crypto/bn/internal.h
+++ b/crypto/bn/internal.h
@@ -238,6 +238,18 @@
#error "Either BN_ULLONG or BN_UMULT_LOHI must be defined on every platform."
#endif
+/* bn_mod_inverse_prime sets |out| to the modular inverse of |a| modulo |p|,
+ * computed with Fermat's Little Theorem. It returns one on success and zero on
+ * error. If |mont_p| is NULL, one will be computed temporarily. */
+int bn_mod_inverse_prime(BIGNUM *out, const BIGNUM *a, const BIGNUM *p,
+ BN_CTX *ctx, const BN_MONT_CTX *mont_p);
+
+/* bn_mod_inverse_secret_prime behaves like |bn_mod_inverse_prime| but uses
+ * |BN_mod_exp_mont_consttime| instead of |BN_mod_exp_mont| in hopes of
+ * protecting the exponent. */
+int bn_mod_inverse_secret_prime(BIGNUM *out, const BIGNUM *a, const BIGNUM *p,
+ BN_CTX *ctx, const BN_MONT_CTX *mont_p);
+
#if defined(__cplusplus)
} /* extern C */
