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

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Details
Package	curve25519-dalek
Version	4.1.2
URL	dalek-cryptography/curve25519-dalek#659
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

[bnjbvr]

Fixed by #3574.