GHSA-x4gp-pqpj-f43q

Suggest an improvement
Source
https://github.com/advisories/GHSA-x4gp-pqpj-f43q
Import Source
https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2024/06/GHSA-x4gp-pqpj-f43q/GHSA-x4gp-pqpj-f43q.json
JSON Data
https://api.osv.dev/v1/vulns/GHSA-x4gp-pqpj-f43q
Published
2024-06-18T21:56:24Z
Modified
2024-06-18T21:56:24Z
Summary
curve25519-dalek has timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub`
Details

Timing variability of any kind is problematic when working with potentially secret values such as elliptic curve scalars, and such issues can potentially leak private keys and other secrets. Such a problem was recently discovered in curve25519-dalek.

The Scalar29::sub (32-bit) and Scalar52::sub (64-bit) functions contained usage of a mask value inside a loop where LLVM saw an opportunity to insert a branch instruction (jns on x86) to conditionally bypass this code section when the mask value is set to zero as can be seen in godbolt:

  • 32-bit (see L106): https://godbolt.org/z/zvaWxzvqv
  • 64-bit (see L48): https://godbolt.org/z/PczYj7Pda

A similar problem was recently discovered in the Kyber reference implementation:

https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/hqbtIGFKIpU/m/cnE3pbueBgAJ

As discussed on that thread, one portable solution, which is also used in this PR, is to introduce a volatile read as an optimization barrier, which prevents the compiler from optimizing it away.

The fix can be validated in godbolt here:

  • 32-bit: https://godbolt.org/z/jc9j7eb8E
  • 64-bit: https://godbolt.org/z/x8d46Yfah

The problem was discovered and the solution independently verified by Alexander Wagner alexander.wagner@aisec.fraunhofer.de and Lea Themint lea.thiemt@tum.de using their DATA tool:

https://github.com/Fraunhofer-AISEC/DATA

References

Affected packages

crates.io / curve25519-dalek

Package

Name
curve25519-dalek
View open source insights on deps.dev
Purl
pkg:cargo/curve25519-dalek

Affected ranges

Type
SEMVER
Events
Introduced
0Unknown introduced version / All previous versions are affected
Fixed
4.1.3