Make primality testing mostly constant-time.
The extra details in Enhanced Rabin-Miller are only used in
RSA_check_key_fips, on the public RSA modulus, which the static linker
will drop in most of our consumers anyway. Implement normal Rabin-Miller
for RSA keygen and use Montgomery reduction so it runs in constant-time.
Note that we only need to avoid leaking information about the input if
it's a large prime. If the number ends up composite, or we find it in
our table of small primes, we can return immediately.
The leaks not addressed by this CL are:
- The difficulty of selecting |b| leaks information about |w|.
- The distribution of whether step 4.4 runs leaks information about w.
- We leak |a| (the largest power of two which divides w) everywhere.
- BN_mod_word in the trial division is not constant-time.
These will be resolved in follow-up changes.
Median of 29 RSA keygens: 0m0.521 -> 0m0.621s
(Accuracy beyond 0.1s is questionable.)
Bug: 238
Change-Id: I0cf0ff22079732a0a3ababfe352bb4327e95b879
Reviewed-on: https://boringssl-review.googlesource.com/25886
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index 0a844ed..eeb25a3 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -706,7 +706,7 @@
// than the number-field sieve security level of |w| is used. It returns one on
// success and zero on failure. If |cb| is not NULL, then it is called during
// each iteration of the primality test.
-int BN_enhanced_miller_rabin_primality_test(
+OPENSSL_EXPORT int BN_enhanced_miller_rabin_primality_test(
enum bn_primality_result_t *out_result, const BIGNUM *w, int iterations,
BN_CTX *ctx, BN_GENCB *cb);
