feat: Implement DefaultCryptoHmac for compliance-aware HMAC operations

- Added DefaultCryptoHmac class implementing ICryptoHmac interface.
- Introduced purpose-based HMAC computation methods.
- Implemented verification methods for HMACs with constant-time comparison.
- Created HmacAlgorithms and HmacPurpose classes for well-known identifiers.
- Added compliance profile support for HMAC algorithms.
- Included asynchronous methods for HMAC computation from streams.

src/Scanner/StellaOps.Scanner.Worker/Processing/Surface/HmacDsseEnvelopeSigner.cs

@@ -8,6 +8,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
+using StellaOps.Cryptography;
 using StellaOps.Scanner.Surface.Env;
 using StellaOps.Scanner.Surface.Secrets;
 using StellaOps.Scanner.Worker.Options;
@@ -18,20 +19,23 @@ namespace StellaOps.Scanner.Worker.Processing.Surface;
 /// DSSE envelope signer that prefers an HMAC key (deterministic) and falls back to
 /// the deterministic hash-only signer when no key is configured.
 /// </summary>
-internal sealed class HmacDsseEnvelopeSigner : IDsseEnvelopeSigner, IDisposable
+internal sealed class HmacDsseEnvelopeSigner : IDsseEnvelopeSigner
 {
     private readonly ILogger<HmacDsseEnvelopeSigner> _logger;
     private readonly ScannerWorkerOptions _options;
+    private readonly ICryptoHmac _cryptoHmac;
     private readonly DeterministicDsseEnvelopeSigner _deterministic = new();
-    private readonly HMACSHA256? _hmac;
+    private readonly byte[]? _secretBytes;
     private readonly string _keyId;
 
     public HmacDsseEnvelopeSigner(
         IOptions<ScannerWorkerOptions> options,
+        ICryptoHmac cryptoHmac,
         ILogger<HmacDsseEnvelopeSigner> logger,
         IServiceProvider serviceProvider)
     {
         ArgumentNullException.ThrowIfNull(options);
+        _cryptoHmac = cryptoHmac ?? throw new ArgumentNullException(nameof(cryptoHmac));
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
         _options = options.Value ?? throw new ArgumentNullException(nameof(options));
 
@@ -52,8 +56,8 @@ internal sealed class HmacDsseEnvelopeSigner : IDsseEnvelopeSigner, IDisposable
 
         if (secretBytes is not null && secretBytes.Length > 0)
         {
-            _hmac = new HMACSHA256(secretBytes);
-            _logger.LogInformation("DSSE signing enabled using HMAC-SHA256 with key id {KeyId}", _keyId);
+            _secretBytes = secretBytes;
+            _logger.LogInformation("DSSE signing enabled using HMAC with key id {KeyId}", _keyId);
         }
         else if (!signing.AllowDeterministicFallback)
         {
@@ -67,13 +71,13 @@ internal sealed class HmacDsseEnvelopeSigner : IDsseEnvelopeSigner, IDisposable
 
     public Task<DsseEnvelope> SignAsync(string payloadType, ReadOnlyMemory<byte> content, string suggestedKind, string merkleRoot, string? view, CancellationToken cancellationToken)
     {
-        if (_hmac is null)
+        if (_secretBytes is null)
         {
             return _deterministic.SignAsync(payloadType, content, suggestedKind, merkleRoot, view, cancellationToken);
         }
 
         var pae = BuildPae(payloadType, content.Span);
-        var signatureBytes = _hmac.ComputeHash(pae);
+        var signatureBytes = _cryptoHmac.ComputeHmacForPurpose(_secretBytes, pae, HmacPurpose.Signing);
         var envelope = new
         {
             payloadType,
@@ -96,11 +100,6 @@ internal sealed class HmacDsseEnvelopeSigner : IDsseEnvelopeSigner, IDisposable
         return Task.FromResult(new DsseEnvelope("application/vnd.dsse+json", uri, digest, bytes));
     }
 
-    public void Dispose()
-    {
-        _hmac?.Dispose();
-    }
-
     private static byte[]? LoadSecret(ScannerWorkerOptions.SigningOptions signing)
     {
         if (!string.IsNullOrWhiteSpace(signing.SharedSecretFile) && File.Exists(signing.SharedSecretFile))