RUSTSEC-2024-0344: Timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub`

Timing variability in curve25519-dalek's Scalar29::sub/Scalar52::sub

Details
Package	`curve25519-dalek`
Version	`2.1.3`
URL	https://github.com/dalek-cryptography/curve25519-dalek/pull/659
Date	2024-06-18
Patched versions	`>=4.1.3`

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

See advisory page for additional details.

KasarLabs / barknet

RUSTSEC-2024-0344: Timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub` #47