tree 3416fffe8c4caa5c7355f753e3bc60b8879dac83
parent 00e434d67e15c9b6d535a29e9ef05a7ab48ad122
author David Benjamin <davidben@google.com> 1617826670 -0400
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1617916136 +0000

Simplify the Lucky13 mitigation.

Rather than computing kVarianceBlocks, which is hard to reason about,
use a sha1_final_with_secret_suffix abstraction. This lets us separate
reasoning in bytes about the minimum and maximum values of |data_size|
and the interaction with HMAC, separately from the core constant-time
SHA-1 update.

It's also faster. I'm guessing it's the more accurate block counts.

Before:
Did 866000 AES-128-CBC-SHA1 (16 bytes) open operations in 2000697us (6.9 MB/sec)
Did 616000 AES-128-CBC-SHA1 (256 bytes) open operations in 2001403us (78.8 MB/sec)
Did 432000 AES-128-CBC-SHA1 (1350 bytes) open operations in 2003898us (291.0 MB/sec)
Did 148000 AES-128-CBC-SHA1 (8192 bytes) open operations in 2006042us (604.4 MB/sec)
Did 83000 AES-128-CBC-SHA1 (16384 bytes) open operations in 2010885us (676.3 MB/sec)

After:
Did 2089000 AES-128-CBC-SHA1 (16 bytes) open operations in 2000049us (16.7 MB/sec) [+141.3%]
Did 851000 AES-128-CBC-SHA1 (256 bytes) open operations in 2000034us (108.9 MB/sec) [+38.2%]
Did 553000 AES-128-CBC-SHA1 (1350 bytes) open operations in 2002169us (372.9 MB/sec) [+28.1%]
Did 178000 AES-128-CBC-SHA1 (8192 bytes) open operations in 2008596us (726.0 MB/sec) [+20.1%]
Did 98000 AES-128-CBC-SHA1 (16384 bytes) open operations in 2001509us (802.2 MB/sec) [+18.6%]

Confirmed with valgrind tooling that this is still constant-time. In
doing so, I ran into a new nuisance with GCC. In loops where we run
constant_time_lt with a counter value, GCC sometimes offsets the loop
counter by the secret. It cancels it out before dereferencing memory,
etc., but valgrind does not know that x + uninit - uninit = x and gets
upset. I've worked around this with a barrier for now.

Change-Id: Ieff8d2cad1b56c07999002e67ce4e6d6aa59e0d3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46686
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>