GHSA-4hff-hh47-7788: Duplicate Advisory: curve25519-dalek has timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub`

The vulnerability, identified as GHSA-4hff-hh47-7788, is a timing side-channel issue in the curve25519-dalek crate. The root cause is a compiler optimization in LLVM that introduces a data-dependent branch in the scalar subtraction functions, specifically Scalar52::sub and, as mentioned in the advisory, Scalar29::sub. This timing variability could allow an attacker to leak secret information. The analysis of the provided patch 8f38163e5cd6ddb048b0bd5a3737927b79e6d80f confirms the vulnerability in Scalar52::sub located in curve25519-dalek/src/backend/serial/u64/scalar.rs. The patch mitigates this by introducing a black_box function using a volatile read, which acts as an optimization barrier and prevents the compiler from introducing the vulnerable branch. While the advisory also mentions Scalar29::sub, the provided commit only contains the fix for Scalar52::sub. A security engineer should ensure that all instances of this pattern are patched in their environment, as other scalar sizes might be affected as well.