test(core,nativeshims): export sanity + interop KATs + Hkdf relocate

[DennisDyallo]

Summary

Yubico.NativeShims had no direct test coverage — CI verified the library built but never executed an exported function or validated per-platform export lists against the implementation. This PR adds that coverage and bundles related Core test-tree cleanups that surfaced while integrating with the parent webauthn previewSign branch (#468).

Stacking: merge before #468. The previewSign branch depends on this for the new export, the consumed-NuGet bump, and the relocated HkdfUtilities tests.

What's in the PR

• Test infrastructure — Yubico.NativeShims/tests/check_exports.{sh,ps1} + expected_symbols.txt validate the export table on every per-platform build via build-nativeshims.yml. 30 xUnit P/Invoke KAT tests (BigNum / EcPoint / GcmEvp / Cmac) using NIST SP 800-38D, RFC 4493, SEC 1/2 vectors. build-macOS-local.sh for fast vcpkg-bypassing arm64 dev iteration.
• New native export — Native_EC_POINT_is_on_curve (C function + per-platform export-list entries + managed wrapper + 2 on-curve tests). Consumed by ARKG-P256 on feat(fido2): WebAuthn previewSign extension (ARKG-P256) #468.
• Consumed-NuGet bump — Yubico.NativeShims 1.16.1-prerelease.20260428.1 so Core sees the new symbol at runtime.
• Test-tree cleanup — moves HkdfUtilities from Yubico.YubiKey to Yubico.Core and relocates its tests to the Core test project to match the SUT's assembly. Adds spec-citation file headers (URLs verified resolvable + content-matching) on the new interop suites.

Verification

```
dotnet test Yubico.Core/tests/Yubico.Core.UnitTests.csproj \
--filter "FullyQualifiedNamePlatformInterop|FullyQualifiedNameHkdf"
```
→ 58 / 58 PASS on macOS arm64. Export-table check verifies all 36 expected symbols against built binaries; removing any symbol from the canonical list correctly produces exit 1.

CI note

submit-nuget will fail until Yubico.NativeShims 1.16.1-prerelease.20260428.1 is reachable from the public NuGet feed (currently published only to Yubico's internal feed). Build + test CI checks across Windows/Linux/macOS all green.

[github-actions]

Test Results: Windows

    2 files      2 suites   41s ⏱️
4 117 tests 4 095 ✅ 22 💤 0 ❌
4 119 runs  4 097 ✅ 22 💤 0 ❌

Results for commit fc51473.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results: Ubuntu

    2 files      2 suites   1m 10s ⏱️
4 109 tests 4 087 ✅ 22 💤 0 ❌
4 111 runs  4 089 ✅ 22 💤 0 ❌

Results for commit fc51473.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results: MacOS

    4 files      4 suites   54s ⏱️
4 091 tests 4 088 ✅ 3 💤 0 ❌
4 093 runs  4 090 ✅ 3 💤 0 ❌

Results for commit fc51473.

♻️ This comment has been updated with latest results.

[DennisDyallo]

I'm aware. Will be updated later

[Copilot]

Pull request overview

This PR strengthens the SDK’s native/managed cryptography integration by adding export-table verification for Yubico.NativeShims, introducing P/Invoke known-answer tests (KATs) for key OpenSSL wrappers, adding a new EC-point on-curve native export + managed wrapper, and relocating HKDF utilities/tests into Yubico.Core to match usage.

Changes:

• Add canonical export-symbol validation scripts and wire them into the NativeShims build workflow across Windows/Linux/macOS.
• Add new Native_EC_POINT_is_on_curve export plus managed P/Invoke wrapper and interop tests.
• Move HkdfUtilities into Yubico.Core (and adjust tests/usages), and bump the consumed Yubico.NativeShims NuGet version.

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Yubico.YubiKey/src/Yubico/YubiKey/Fido2/AuthenticatorInfo.cs	Switch HKDF usage to Core cryptography namespace after HKDF relocation.
Yubico.NativeShims/tests/expected_symbols.txt	Define canonical exported-symbol list for NativeShims.
Yubico.NativeShims/tests/check_exports.sh	POSIX export-table verifier using nm + canonical list.
Yubico.NativeShims/tests/check_exports.ps1	Windows export-table verifier using dumpbin + canonical list.
Yubico.NativeShims/ssl.ecpoint.c	Add Native_EC_POINT_is_on_curve export implementation.
Yubico.NativeShims/exports.msvc	Add new export to Windows .def list.
Yubico.NativeShims/exports.llvm	Add new export to macOS export list.
Yubico.NativeShims/exports.gnu	Add new export to GNU/Linux version script.
Yubico.NativeShims/build-macOS-local.sh	Add local macOS arm64 build helper that also runs export checks.
Yubico.Core/tests/unit/Yubico/PlatformInterop/Cryptography/GcmEvpInteropTests.cs	Add AES-256-GCM EVP wrapper interop KATs + tamper checks.
Yubico.Core/tests/unit/Yubico/PlatformInterop/Cryptography/EcPointInteropTests.cs	Add EC group/point lifecycle + mul + on-curve interop coverage.
Yubico.Core/tests/unit/Yubico/PlatformInterop/Cryptography/CmacInteropTests.cs	Add RFC 4493 CMAC interop KATs + chunking equivalence tests.
Yubico.Core/tests/unit/Yubico/PlatformInterop/Cryptography/BigNumInteropTests.cs	Add basic BN marshaling interop round-trip tests.
Yubico.Core/tests/unit/Yubico/Core/Cryptography/HkdfUtilitiesTests.cs	Relocate HKDF tests to Core test project/namespace.
Yubico.Core/src/Yubico/PlatformInterop/Desktop/Cryptography/EcPoint.Interop.cs	Add managed P/Invoke wrapper for Native_EC_POINT_is_on_curve.
Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs	Relocate HKDF utility to Core and adjust implementation to use BCL HMACSHA256.
Yubico.Core/src/Yubico.Core.csproj	Bump consumed Yubico.NativeShims package version.
.github/workflows/build-nativeshims.yml	Run export-table checks as part of NativeShims CI builds.

Comments suppressed due to low confidence (6)

Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs:87

• In HkdfExpand, pseudoRandomKey.ToArray() creates an additional copy of the PRK that is not cleared, even though pseudoRandomKey itself is zeroed by the caller. Consider reusing/owning a single PRK byte[] for the HMAC key and zeroing it (and any intermediate blocks) when done.
  Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs:20
• The repository’s C# code in this folder consistently uses block-scoped namespaces (e.g., CmacPrimitives.cs). This file uses a file-scoped namespace, which makes the style inconsistent within Yubico.Core.Cryptography. Consider switching to the block-scoped namespace style for consistency.
  Yubico.Core/tests/unit/Yubico/Core/Cryptography/HkdfUtilitiesTests.cs:22
• This test file is the only one in Yubico.Core tests using a file-scoped namespace (and it also has a redundant using for the same namespace). For consistency with the rest of the test suite (block-scoped namespaces), consider converting it to the same namespace style used elsewhere.
  Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs:89
• In HkdfExpand, the loop counter is a byte. When length == 8160, numberOfBlocks becomes 255 and the for-loop overflows from 255 back to 0, causing an infinite loop. Use an int loop variable and only cast to byte when appending the counter to the HMAC input.
  Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs:22
• HkdfUtilities is now public, but the type itself has no XML doc comment. With GenerateDocumentationFile enabled and warnings treated as errors, this will typically raise CS1591 for a public type. Add a
  for the class (and keep it consistent with other public crypto primitives).
  Yubico.Core/src/Yubico/Core/Cryptography/HkdfUtilities.cs:73
• HkdfExtract allocates copies of potentially sensitive material (salt.ToArray() and inputKeyMaterial.ToArray()) but never clears them. If this utility is intended for key-derivation with secret inputs, consider zeroing the temporary byte[] buffers after ComputeHash (similar to how pseudoRandomKey is cleared).

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Yubico.Core	53%	48%	1584	➖
Yubico.YubiKey	50%	45%	7180	➖
Summary	51% (12922 / 25583)	46% (3162 / 6912)	8764	➖

Minimum allowed line rate is 40%