tree edb09a6582fa116c0611ec5070af6209c6c3db61
parent 7e2a8a34baf87ca12a92cca87b51ee2ee1bfe997
author David Benjamin <davidben@google.com> 1523393193 -0400
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1524257415 +0000

Restore the BN_mod codepath for public Montgomery moduli.

https://boringssl-review.googlesource.com/10520 and then later
https://boringssl-review.googlesource.com/25285 made BN_MONT_CTX_set
constant-time, which is necessary for RSA's mont_p and mont_q. However,
due to a typo in the benchmark, they did not correctly measure.

Split BN_MONT_CTX creation into a constant-time and variable-time one.
The constant-time one uses our current algorithm and the latter restores
the original BN_mod codepath.

Should we wish to avoid BN_mod, I have an alternate version lying
around:

First, BN_set_bit + bn_mod_lshift1_consttime as now to count up to 2*R.
Next, observe that 2*R = BN_to_montgomery(2) and R*R =
BN_to_montgomery(R) = BN_to_montgomery(2^r_bits) Also observe that
BN_mod_mul_montgomery only needs n0, not RR. Split the core of
BN_mod_exp_mont into its own function so the caller handles conversion.
Raise 2*R to the r_bits power to get 2^r_bits*R = R*R.

The advantage of that algorithm is that it is still constant-time, so we
only need one BN_MONT_CTX_new. Additionally, it avoids BN_mod which is
otherwise (almost, but the remaining links should be easy to cut) out of
the critical path for correctness. One less operation to worry about.

The disadvantage is that it is gives a 25% (RSA-2048) or 32% (RSA-4096)
slower RSA verification speed. I went with the BN_mod one for the time
being.

Before:
Did 9204 RSA 2048 signing operations in 10052053us (915.6 ops/sec)
Did 326000 RSA 2048 verify (same key) operations in 10028823us (32506.3 ops/sec)
Did 50830 RSA 2048 verify (fresh key) operations in 10033794us (5065.9 ops/sec)
Did 1269 RSA 4096 signing operations in 10019204us (126.7 ops/sec)
Did 88435 RSA 4096 verify (same key) operations in 10031129us (8816.1 ops/sec)
Did 14552 RSA 4096 verify (fresh key) operations in 10053411us (1447.5 ops/sec)

After:
Did 9150 RSA 2048 signing operations in 10022831us (912.9 ops/sec)
Did 322000 RSA 2048 verify (same key) operations in 10028604us (32108.2 ops/sec)
Did 289000 RSA 2048 verify (fresh key) operations in 10017205us (28850.4 ops/sec)
Did 1270 RSA 4096 signing operations in 10072950us (126.1 ops/sec)
Did 87480 RSA 4096 verify (same key) operations in 10036328us (8716.3 ops/sec)
Did 80730 RSA 4096 verify (fresh key) operations in 10073614us (8014.0 ops/sec)

Change-Id: Ie8916d1634ccf8513ceda458fa302f09f3e93c07
Reviewed-on: https://boringssl-review.googlesource.com/27287
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>