HMAC Verify

[vcsjones]

This adds HMAC "Verify" APIs for validating an HMAC signature.

KMAC will be a followup PR.

Contributes to #116028

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[Copilot]

Pull Request Overview

This PR adds HMAC "Verify" APIs for validating HMAC signatures across the cryptographic library. The changes introduce new verification methods that perform fixed-time comparisons to prevent timing attacks.

Key changes include:

• Addition of new Verify and VerifyAsync methods to all HMAC classes (HMACSHA1, HMACSHA256, HMACSHA384, HMACSHA512, HMACSHA3_256, HMACSHA3_384, HMACSHA3_512, HMACMD5)
• New verification APIs in CryptographicOperations class for general HMAC verification
• Addition of VerifyCurrentHash and VerifyHashAndReset methods to IncrementalHash class
• Creation of shared implementation infrastructure through HMACShared class

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
IncrementalHashTests.cs	Adds comprehensive test coverage for new verify methods in IncrementalHash
HmacTests.cs	Adds base test infrastructure and validation tests for HMAC verification APIs
HmacSha*.cs (test files)	Implements abstract verify methods for each HMAC algorithm test class
IncrementalHash.cs	Implements VerifyCurrentHash and VerifyHashAndReset methods
HMACShared.cs	New shared implementation for HMAC verification logic across all algorithms
HMAC*.cs (implementation files)	Adds Verify and VerifyAsync static methods to each HMAC class
CryptographicOperations.cs	Adds general-purpose HMAC verification APIs
Project and reference files	Updates to include new HMACShared.cs file and API surface

[bartonjs]

Unlike CryptographicOperations, I think IncrementalHash supports asking the provider (at least, CNG) for an algorithm we don't know about. So the defensive up-sizing makes sense here.

Wish we could test it, though 😄

[vcsjones]

I'm open to ideas. We could do something like

#if DEBUG
const int MaxStackAlloc = 32;
#else
const int MaxStackAlloc = 64;
#endif

So that a debug run always puts SHA-X-512 through the allocating path. But I really also dislike different behaviors between debug vs. release. Up to you though.