tree 198f0a8f877f338f7be4b58fda6b41dd507f7fe0
parent 02d2715bcc95d0f03ae394310d32739f4fcfa1ac
author David Benjamin <davidben@google.com> 1686513905 -0400
committer Boringssl LUCI CQ <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com> 1686696433 +0000

Remove p > q normalization in RSA keys

RSA CRT is tiny bit messier when p < q.
https://boringssl-review.googlesource.com/25263 solved this by
normalizing to p > q. The cost was we sometimes had to compute a new
iqmp.

Modular inversion is expensive. We did it only once per key, but it's
still a performance cliff in per-key costs. When later work moves
freeze_private_key into RSA private key parsing, it will be a
performance cliff in the private key parser.

Instead, just handle p < q in the CRT function. The only difference is
needing one extra reduction before the modular subtraction. Even using
the fully general mod_montgomery function (as opposed to checking p < q,
or using bn_reduce_once when num_bits(p) == num_bits(q)) was not
measurable.

In doing so, I noticed we didn't actually have tests that exercise the
reduction step. I added one to evp_tests.txt, but it is only meaningful
when blinding is disabled. (Another cost of blinding.) When blinding is
enabled, the answers mod p and q are randomized and we hit this case
with about 1.8% probability. See comment in evp_test.txt.

I kept the optimization where we store iqmp in Montgomery form, not
because the optimization matters, but because we need to store a
corrected, fixed-width version of the value anyway, so we may as well
store it in a more convenient form.

M1 Max
Before:
Did 9048 RSA 2048 signing operations in 5033403us (1797.6 ops/sec)
Did 1500 RSA 4096 signing operations in 5009288us (299.4 ops/sec)
After:
Did 9116 RSA 2048 signing operations in 5053802us (1803.8 ops/sec) [+0.3%]
Did 1500 RSA 4096 signing operations in 5008283us (299.5 ops/sec) [+0.0%]

Intel(R) Xeon(R) Gold 6154 CPU @ 3.00GHz
Before:
Did 9282 RSA 2048 signing operations in 5019395us (1849.2 ops/sec)
Did 1302 RSA 4096 signing operations in 5055011us (257.6 ops/sec)
After:
Did 9240 RSA 2048 signing operations in 5024845us (1838.9 ops/sec) [-0.6%]
Did 1302 RSA 4096 signing operations in 5046157us (258.0 ops/sec) [+0.2%]

Bug: 316
Change-Id: Icb90c7d5f5188f9b69a6d7bcc63db13d92ec26d5
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/60705
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>