feat: Implement BLAKE2X XOF (Blake2Xb and Blake2Xs)

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This PR introduces an implementation of Blake2X – inspired from, but greatly extends on #677. This contributes to #1.

Category	Lines
Test Vector Files	6148
Unit Tests	762
Dependency Management	124
Implementation	631

This is a complete implementation of the Blake2X extensible-output function (XOF) for both its 64-bit (Blake2Xb) and 32-bit (Blake2Xs) variants. This new functionality is gated behind the blake2x cargo feature flag.

1. Implementation and Design

The implementation follows the design principles of the existing crate, utilizing a macro-driven approach to provide generic logic for both Blake2Xb and Blake2Xs, which minimizes code duplication.

• Core Logic (macros.rs): A new macro, blake2x_impl!, has been introduced.

• Algorithm (blake2x.rs): The implementation follows the two-phase process specified by the Blake2X algorithm:

  1. Root Hash Generation: It first computes a root hash (H₀) using the underlying Blake2bVarCore or Blake2sVarCore. Crucially, it incorporates the total desired output length (xof_len) into the parameter block during this phase. This ensures that outputs of different lengths are cryptographically distinct, a key security feature of Blake2X.
  2. Output Expansion: The finalize_xof() method returns a Reader struct. This reader generates the final hash output incrementally. For each block of output requested, it computes a new hash by feeding the root hash H₀ into the base BLAKE2 function, but with a unique parameter block for each "expansion node" (differentiated by an incrementing node_offset). This logic is encapsulated in the expand_node helper function.

• Public API: New public-facing structs Blake2xb, Blake2xs, and their corresponding Reader types are exposed.

2. Testing and Validation

The implementation is supported by a comprehensive test suite in tests/blake2x.rs that validates correctness through two primary resources: official test vectors and a reference implementation.

• Test Vectors:

  • The tests/data/ directory now includes blake2xb-kat.json and blake2xs-kat.json. These are the test vectors sourced directly from the BLAKE2 RFC repository.
  • The tests parse these JSON files and validate the implementation against a wide range of input and output sizes for both keyed and unkeyed hashing.

• Reference Implementation (b2rs):

  • To further ensure correctness, the test suite now uses the b2rs crate as a reference implementation. This crate is maintained on GitHub by Jean-Philippe Aumasson (@veorq), one of the original authors of the BLAKE2 algorithm.
  • The tests use b2rs in two ways:
    1. Fuzz-like comparison: Random inputs and output lengths are generated, and the output of our implementation is compared against the output of b2rs.
    2. Internal State Validation: The tests go a step further by using b2rs to verify intermediate values of the Blake2X computation, such as the value of the root hash (H₀) and the output of the first expansion node. This confirms that our internal parameter block construction and hashing logic are correct, not just the final result.

All tests, including functional checks for progressive reads and constructor behavior, are passing.

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Edit: the last 2 commits of the PR respectively fix a typo-detection CI false positive, and unrelated clippy warnings.

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@newpavlov Happy to add a CHANGELOG line as part of this PR, or remove the b2rs unit tests, or anything else you might think needs adapting.