Use CryptoKit for recovering public key from private EC keys on macOS

[vcsjones]

In #134682 we added support for iOS and tvOS to import private-key only values (ECParameters that contain only D).

This worked on macOS previously by round-tripping through PKCS#8. This is significantly more expensive, both in CPU and memory. Let's use the CryptoKit approach so that we have a single implementation and use the better one on macOS.

This is around a 100x improvement in time.

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
ImportP256	After	127.2 μs	0.32 μs	0.30 μs	0.002	256 B	0.06
ImportP256	Before	75,153.2 μs	1,433.16 μs	2,862.18 μs	1.000	4320 B	1.00
ImportP384	After	341.7 μs	3.57 μs	3.34 μs	0.003	288 B	0.06
ImportP384	Before	114,843.9 μs	2,236.42 μs	3,547.20 μs	1.000	4469 B	1.00
ImportP521	After	987.9 μs	9.93 μs	9.29 μs	0.006	337 B	0.07
ImportP521	Before	158,300.2 μs	3,068.62 μs	3,151.25 μs	1.000	4664 B	1.00

Benchmark code:

[MemoryDiagnoser]
public class EccDImportBenchmark
{
    private static readonly ECParameters s_p256 = CreateParameters(ECCurve.NamedCurves.nistP256);
    private static readonly ECParameters s_p384 = CreateParameters(ECCurve.NamedCurves.nistP384);
    private static readonly ECParameters s_p521 = CreateParameters(ECCurve.NamedCurves.nistP521);

    private ECDiffieHellman _p256 = null!;
    private ECDiffieHellman _p384 = null!;
    private ECDiffieHellman _p521 = null!;

    [GlobalSetup]
    public void Setup()
    {
        _p256 = ECDiffieHellman.Create();
        _p384 = ECDiffieHellman.Create();
        _p521 = ECDiffieHellman.Create();
    }

    [GlobalCleanup]
    public void Cleanup()
    {
        _p256.Dispose();
        _p384.Dispose();
        _p521.Dispose();
    }

    [Benchmark]
    public void ImportP256()
    {
        _p256.ImportParameters(s_p256);
    }

    [Benchmark]
    public void ImportP384()
    {
        _p384.ImportParameters(s_p384);
    }

    [Benchmark]
    public void ImportP521()
    {
        _p521.ImportParameters(s_p521);
    }

    private static ECParameters CreateParameters(ECCurve curve)
    {
        using (ECDiffieHellman ecdh = ECDiffieHellman.Create(curve))
        {
            ECParameters privateParameters = ecdh.ExportParameters(includePrivateParameters: true);

            return new ECParameters
            {
                Curve = curve,
                D = privateParameters.D,
            };
        }
    }
}

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved issues were identified that would block approval.

Review effort: Lite
Findings: None

What changed in this PR

This PR centralizes Apple EC public-key derivation from private scalar-only parameters using CryptoKit, enabling the faster path on macOS as well as mobile Apple platforms.

Changes:

• Moves the CryptoKit-based derivation helper to shared Apple code.
• Removes the macOS PKCS#8 round-trip implementation.
• Removes obsolete platform-specific usings and duplicate code.

Validation: No build or tests were run in this review.

File	Description
src/​libraries/​Common/​src/​System/​Security/​Cryptography/​EccAppleCrypto.macOS.cs	Updated as part of this pull request.
src/​libraries/​Common/​src/​System/​Security/​Cryptography/​EccAppleCrypto.iOS.cs	Updated as part of this pull request.
src/​libraries/​Common/​src/​System/​Security/​Cryptography/​EccAppleCrypto.cs	Updated as part of this pull request.