Ecosyste.ms: Advisories

Security Advisories: GSA_kwCzR0hTQS14NGdwLXBxcGotZjQzcc4AA9KH

curve25519-dalek has timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub`

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 [email protected] and Lea Themint [email protected] using
their DATA tool:

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

Permalink: https://github.com/advisories/GHSA-x4gp-pqpj-f43q
JSON: https://advisories.ecosyste.ms/api/v1/advisories/GSA_kwCzR0hTQS14NGdwLXBxcGotZjQzcc4AA9KH
Source: GitHub Advisory Database
Origin: Unspecified
Severity: Moderate
Classification: General
Published: 4 months ago
Updated: 4 months ago

Identifiers: GHSA-x4gp-pqpj-f43q
References:

• https://github.com/dalek-cryptography/curve25519-dalek/pull/659
• https://github.com/dalek-cryptography/curve25519-dalek/commit/415892acf1cdf9161bd6a4c99bc2f4cb8fae5e6a
• https://rustsec.org/advisories/RUSTSEC-2024-0344.html
• https://github.com/advisories/GHSA-x4gp-pqpj-f43q

Repository: https://github.com/dalek-cryptography/curve25519-dalek
Blast Radius: 0.0

Affected Packages