Limit DHE groups to 4096-bit.

dh.c had a 10k-bit limit but it wasn't quite correctly enforced. However,
that's still 1.12s of jank on the IO thread, which is too long. Since the SSL
code consumes DHE groups from the network, it should be responsible for
enforcing what sanity it needs on them.

Costs of various bit lengths on 2013 Macbook Air:
1024 - 1.4ms
2048 - 14ms
3072 - 24ms
4096 - 55ms
5000 - 160ms
10000 - 1.12s

UMA says that DHE groups are 0.2% 4096-bit and otherwise are 5.5% 2048-bit and
94% 1024-bit and some noise. Set the limit to 4096-bit to be conservative,
although that's already quite a lot of jank.


Change-Id: I8e167748a67e4e1adfb62d73dfff094abfa7d215
Reviewed-by: Adam Langley <>
5 files changed
tree: d2d84607437b31221c5e0ed6141ae93a7fd41088
  1. crypto/
  2. decrepit/
  3. fuzz/
  4. include/
  5. ssl/
  6. tool/
  7. util/
  8. .clang-format
  9. .gitignore
  11. CMakeLists.txt
  12. codereview.settings


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:

  • how to port OpenSSL-using code to BoringSSL.
  • how to build BoringSSL
  • rules and guidelines for coding style.
  • include/openssl: public headers with API documentation in comments. Also available online.
  • information about fuzzing BoringSSL.