[nbari]

Allow applications to use Argon2 0.6 as the optional OPAQUE key stretching function. The existing implementation already supports its API, so the production change is one dependency line; features, KSF parameters and the Rust 1.87 MSRV stay the same.

Add a compatibility test with outputs generated by Argon2 0.5.3 for 32-byte and 64-byte inputs, plus a changelog entry. This is deliberately limited to Argon2; the elliptic-curve/voprf migration is separate.

Validation:

• All-feature tests pass on stable and Rust 1.87: 143 unit tests, the v3 migration test and 19 doctests (2 existing ignored doctests; all 7 SoftHSM integration tests also pass).
• Tests with --no-default-features --features argon2 pass.
• No-std builds pass for wasm32-unknown-unknown and thumbv6m-none-eabi with argon2,ristretto255.
• Documentation builds with warnings denied; the full tree passes nightly rustfmt, Taplo and strict Clippy.
• An out-of-tree deterministic registration/login check matches the legacy Argon2 setup's serialized record, states, messages, proof and session key byte for byte.

Full-tree nightly formatting, Taplo, strict Clippy and documentation checks now pass. The CI compatibility changes preserve the existing protocol vectors and transcript bytes. Clippy permits the unavoidable syn 2/3 procedural macro dependencies while keeping the Cargo lint group enabled.

The dependency audit remains blocked by the existing crypto-bigint 0.5.5 advisory (RUSTSEC-2026-0387) and cryptoki 0.9 advisory (RUSTSEC-2026-0286). Both are addressed separately in #428; this PR should be rebased after that security migration lands. No advisory exceptions were added.

[meta-cla]

Hi @nbari!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[kevinlewi]

Merged via #428: your commit is included on main with your authorship (c3a559d), including the argon2 0.5.3 output regression test. Thanks @nbari!