Remove cacheline striping in copy_from_prebuf.

The standard computation model for constant-time code is that memory
access patterns must be independent of secret data.
BN_mod_exp_mont_consttime was previously written to a slightly weaker
model: only cacheline access patterns must be independent of secret
data. It assumed accesses within a cacheline were indistinguishable.

The CacheBleed attack ( showed this
assumption was false. Cache lines may be divided into cache banks, and
the researchers were able to measure cache bank contention pre-Haswell.
For Haswell, the researchers note "But, as Haswell does show timing
variations that depend on low address bits [19], it may be vulnerable to
similar attacks."

OpenSSL's fix to CacheBleed was not to adopt the standard constant-time
computation model. Rather, it now assumes accesses within a 16-byte
cache bank are indistinguishable, at least in the C copy_from_prebuf
path. These weaker models failed before with CacheBleed, so avoiding
such assumptions seems prudent. (The [19] citation above notes a false
dependence between memory addresses with a distance of 4k, which may be
what the paper was referring to.) Moreover, the C path is largely unused
on x86_64 (which uses mont5 asm), so it is especially questionable for
the generic C code to make assumptions based on x86_64.

Just walk the entire table in the C implementation. Doing so as-is comes
with a performance hit, but the striped memory layout is, at that point,
useless. We regain the performance loss (and then some) by using a more
natural layout. Benchmarks below.

This CL does not touch the mont5 assembly; I haven't figured out what
it's doing yet.

Pixel 3, aarch64:
Did 3146 RSA 2048 signing operations in 10009070us (314.3 ops/sec)
Did 447 RSA 4096 signing operations in 10026666us (44.6 ops/sec)
Did 3210 RSA 2048 signing operations in 10010712us (320.7 ops/sec)
Did 456 RSA 4096 signing operations in 10063543us (45.3 ops/sec)

Pixel 3, armv7:
Did 2688 RSA 2048 signing operations in 10002266us (268.7 ops/sec)
Did 459 RSA 4096 signing operations in 10004785us (45.9 ops/sec)
Did 2709 RSA 2048 signing operations in 10001299us (270.9 ops/sec)
Did 459 RSA 4096 signing operations in 10063737us (45.6 ops/sec)

x86_64 Broadwell, mont5 assembly disabled:
(This configuration is not actually shipped anywhere, but seemed a
useful data point.)
Did 14274 RSA 2048 signing operations in 10009130us (1426.1 ops/sec)
Did 2448 RSA 4096 signing operations in 10046921us (243.7 ops/sec)
Did 14706 RSA 2048 signing operations in 10037908us (1465.0 ops/sec)
Did 2538 RSA 4096 signing operations in 10059986us (252.3 ops/sec)

Change-Id: If41da911d4281433856a86c6c8eadf99cd33e2d8
Reviewed-by: Adam Langley <>
Commit-Queue: David Benjamin <>
1 file changed
tree: e63222410c1b11a4248ab8500090480fd0ee64a6
  1. .github/
  2. crypto/
  3. decrepit/
  4. fipstools/
  5. fuzz/
  6. include/
  7. infra/
  8. ssl/
  9. third_party/
  10. tool/
  11. util/
  12. .clang-format
  13. .gitignore
  17. CMakeLists.txt
  18. codereview.settings
  21. go.mod
  26. sources.cmake


BoringSSL is a fork of OpenSSL that is designed to meet Google's needs.

Although BoringSSL is an open source project, it is not intended for general use, as OpenSSL is. We don't recommend that third parties depend upon it. Doing so is likely to be frustrating because there are no guarantees of API or ABI stability.

Programs ship their own copies of BoringSSL when they use it and we update everything as needed when deciding to make API changes. This allows us to mostly avoid compromises in the name of compatibility. It works for us, but it may not work for you.

BoringSSL arose because Google used OpenSSL for many years in various ways and, over time, built up a large number of patches that were maintained while tracking upstream OpenSSL. As Google's product portfolio became more complex, more copies of OpenSSL sprung up and the effort involved in maintaining all these patches in multiple places was growing steadily.

Currently BoringSSL is the SSL library in Chrome/Chromium, Android (but it's not part of the NDK) and a number of other apps/programs.

There are other files in this directory which might be helpful: