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 | `3.2.0` |
| URL | [https://github.com/dalek-cryptography/curve25519-dalek/pull/659](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](https://rustsec.org/advisories/RUSTSEC-2024-0344.html) for additional details.

Review the repository's dependency declarations and lockfile for curve25519-dalek, starting with the current 3.2.0 requirement. Compare the dependency update with the upstream fix linked in the issue and target patched version 4.1.3 or later. Run the Rust test suite and dependency or security checks; done means the vulnerable version is no longer selected.

Written by the indexing model from the issue text.