Affected: dalek-cryptography curve25519-dalek

The curve25519-dalek crate before 4.1.3 for Rust has a constant-time operation on elliptic curve scalars that is removed by LLVM.

Affected: crates.io: curve25519-dalek

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