Implement HKDF with CryptoKit on Apple

[vcsjones]

CryptoKit has a native HKDF implementation. For platforms where CryptoKit HKDF is available, this implements the HKDF algorithm using CryptoKit. On platforms where CryptoKit HKDF is not available (< iOS 14) the managed fallback is used.

There was also a notable lack any tests for MD5, SHA-384, and SHA-512. The entire HKDF test suite would pass even if I did not implement those algorithms at all. So lets create some tests while we are here.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

This PR implements native HKDF (HMAC-based Key Derivation Function) support on Apple platforms using CryptoKit. The implementation provides better performance by leveraging Apple's native cryptographic libraries when available (iOS 14+, tvOS 14+, macOS, Mac Catalyst), while falling back to the existing managed implementation on older platforms.

Key changes:

• Added native CryptoKit HKDF implementation with platform detection
• Enhanced test coverage for previously untested hash algorithms (MD5, SHA-384, SHA-512)
• Refactored shared code by extracting hash algorithm mapping utility

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 16 comments.

Show a summary per file

File	Description
pal_swiftbindings.swift	Implements HKDF extract, expand, and derive operations using CryptoKit
pal_swiftbindings.h	Declares native function entry points for HKDF operations
entrypoints.c	Registers HKDF native functions for P/Invoke
HKDFTests.cs	Adds comprehensive test cases for MD5, SHA-384, and SHA-512 algorithms
HKDF.cs	Fixes documentation comment parameter reference
HKDF.Apple.cs	Provides Apple-specific HKDF implementation with platform detection and fallback
System.Security.Cryptography.csproj	Updates project file to include Apple HKDF implementation
Interop.RSA.cs	Removes duplicate hash algorithm mapping function
Interop.PAL_HashAlgorithm.cs	Centralizes hash algorithm mapping utility function
Interop.HKDF.cs	Provides managed wrapper for native HKDF P/Invoke calls

[vcsjones]

Hum. CoreCLR passed, Mono failed.

[vcsjones]

Hum. CoreCLR passed, Mono failed.

Of course I cannot reproduce it...

[vcsjones]

Draft / NO-MERGE because this has debugging logs for CI since issue cannot be reproduced locally.