Make bn_mul_recursive constant-time.

I left the input length as int because the calling convention passes
these messy deltas around. This micro-optimization is almost certainly
pointless, but bn_sub_part_words is written in assembly, so I've left it
alone for now. The documented preconditions were also all completely
wrong, so I've fixed them. We actually only call them for even tighter
bounds (one of dna or dnb is 0 and the other is 0 or -1), at least
outside bn_mul_part_recursive which I still need to read through.

This leaves bn_mul_part_recursive, which is reachable for RSA keys which
are not a power of two in bit width.

The first iteration of this had an uncaught bug, so I added a few more
aggressive tests generated with:

  A = 0x...
  B = 0x...

  # Chop off 0, 1 and > 1 word for both 32 and 64-bit.
  for i in (0, 1, 2, 4):
    for j in (0, 1, 2, 4):
      a = A >> (32*i)
      b = B >> (32*j)
      p = a * b
      print "Product = %x" % p
      print "A = %x" % a
      print "B = %x" % b

Bug: 234
Change-Id: I72848d992637c0390cdd3c4f81cb919393b59eb8
Commit-Queue: David Benjamin <>
CQ-Verified: CQ bot account: <>
Reviewed-by: Adam Langley <>
2 files changed
tree: 0622790a2e6ed2b67552a920a838531c4a9067e5
  1. .clang-format
  2. .github/
  3. .gitignore
  6. CMakeLists.txt
  14. codereview.settings
  15. crypto/
  16. decrepit/
  17. fipstools/
  18. fuzz/
  19. include/
  20. infra/
  21. sources.cmake
  22. ssl/
  23. third_party/
  24. tool/
  25. util/


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.

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