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RUSTSEC-2024-0344: Timing variability in curve25519-dalek's Scalar29::sub/Scalar52::sub #842

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github-actions bot opened this issue Jun 19, 2024 · 0 comments

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

Details
Package curve25519-dalek
Version 3.2.0
URL dalek-cryptography/curve25519-dalek#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:

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:

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>

See advisory page for additional details.

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