Don't loop forever in BN_mod_sqrt on invalid inputs.

BN_mod_sqrt implements the Tonelli–Shanks algorithm, which requires a
prime modulus. It was written such that, given a composite modulus, it
would sometimes loop forever. This change fixes the algorithm to always
terminate. However, callers must still pass a prime modulus for the
function to have a defined output.

In OpenSSL, this loop resulted in a DoS vulnerability, CVE-2022-0778.
BoringSSL is mostly unaffected by this. In particular, this case is not
reachable in BoringSSL from certificate and other ASN.1 elliptic curve
parsing code. Any impact in BoringSSL is limited to:

- Callers of EC_GROUP_new_curve_GFp that take untrusted curve parameters
- Callers of BN_mod_sqrt that take untrusted moduli

This CL updates documentation of those functions to clarify that callers
should not pass attacker-controlled values. Even with the infinite loop
fixed, doing so breaks preconditions and will give undefined output.

Change-Id: I64dc1220aaaaafedba02d2ac0e4232a3a0648160
Reviewed-by: Adam Langley <>
Reviewed-by: Martin Kreichgauer <>
Commit-Queue: Adam Langley <>
4 files changed
tree: bd99202e416aa75c96ff38e8b6f0c36a020ba44b
  1. .github/
  2. crypto/
  3. decrepit/
  4. fuzz/
  5. include/
  6. rust/
  7. ssl/
  8. third_party/
  9. tool/
  10. util/
  11. .clang-format
  12. .gitignore
  16. CMakeLists.txt
  17. codereview.settings
  20. go.mod
  21. go.sum
  27. 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.

Project links:

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