RUSTSEC-2024-0344

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 <alexander.wagner@aisec.fraunhofer.de> and Lea Themint <lea.thiemt@tum.de> using their DATA tool: <https://github.com/Fraunhofer-AISEC/DATA>