Make RSA_check_key more than 2x as fast.

The bulk of RSA_check_key is spent in bn_div_consttime, which is a naive
but constant-time long-division algorithm for the few places that divide
by a secret even divisor: RSA keygen and RSA import. RSA import is
somewhat performance-sensitive, so pick some low-hanging fruit:

The main observation is that, in all but one call site, the bit width of
the divisor is public. That means, for an N-bit divisor, we can skip the
first N-1 iterations of long division because an N-1-bit remainder
cannot exceed the N-bit divisor.

One minor nuisance is bn_lcm_consttime, used in RSA keygen has a case
that does *not* have a public bit width. Apply the optimization there
would leak information. I've implemented this as an optional public
lower bound on num_bits(divisor), which all but that call fills in.

Did 5060 RSA 2048 private key parse operations in 1058526us (4780.2 ops/sec)
Did 1551 RSA 4096 private key parse operations in 1082343us (1433.0 ops/sec)

Did 11532 RSA 2048 private key parse operations in 1084145us (10637.0 ops/sec) [+122.5%]
Did 3542 RSA 4096 private key parse operations in 1036374us (3417.7 ops/sec) [+138.5%]

Bug: b/192484677
Change-Id: I893ebb8886aeb8200a1a365673b56c49774221a2
Reviewed-by: Adam Langley <>
6 files changed
tree: c1d0227382fd96ce24e6bd5768207f7d6c5e1263
  1. .clang-format
  2. .github/
  3. .gitignore
  7. CMakeLists.txt
  16. codereview.settings
  17. crypto/
  18. decrepit/
  19. fuzz/
  20. go.mod
  21. go.sum
  22. include/
  23. sources.cmake
  24. ssl/
  25. third_party/
  26. tool/
  27. 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.

Project links:

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