Add managed TLS interoperability and persisted credentials

dotnet/src/VoiceCat.Crypto/ServerIdentity.cs

@@ -0,0 +1,58 @@
+using System.Security.Cryptography;
+using Org.BouncyCastle.Crypto.Parameters;
+
+namespace VoiceCat.Crypto;
+
+public sealed class ServerIdentity : IDisposable
+{
+    private readonly byte[] seed;
+    private readonly byte[] publicKey;
+    private bool disposed;
+
+    private ServerIdentity(byte[] seed)
+    {
+        this.seed = seed;
+        publicKey = new Ed25519PrivateKeyParameters(seed, 0).GeneratePublicKey().GetEncoded();
+    }
+
+    public byte[] PublicKey => (byte[])publicKey.Clone();
+    public string Fingerprint => Convert.ToHexString(SHA256.HashData(publicKey));
+
+    public static ServerIdentity Generate() => new(RandomNumberGenerator.GetBytes(32));
+
+    public static ServerIdentity Load(string path)
+    {
+        byte[] data = File.ReadAllBytes(path);
+        try
+        {
+            if (data.Length != 96) throw new InvalidDataException("Server identity must contain 96 bytes.");
+            var identity = new ServerIdentity(data.AsSpan(32, 32).ToArray());
+            if (!CryptographicOperations.FixedTimeEquals(identity.publicKey, data.AsSpan(0, 32)) ||
+                !CryptographicOperations.FixedTimeEquals(identity.publicKey, data.AsSpan(64, 32)))
+            {
+                identity.Dispose();
+                throw new InvalidDataException("Server identity public key does not match its seed.");
+            }
+            return identity;
+        }
+        finally { CryptographicOperations.ZeroMemory(data); }
+    }
+
+    public void Save(string path)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        byte[] data = new byte[96];
+        publicKey.CopyTo(data, 0);
+        seed.CopyTo(data, 32);
+        publicKey.CopyTo(data, 64);
+        try { PrivateFiles.Write(path, data); }
+        finally { CryptographicOperations.ZeroMemory(data); }
+    }
+
+    public void Dispose()
+    {
+        if (disposed) return;
+        disposed = true;
+        CryptographicOperations.ZeroMemory(seed);
+    }
+}