Retire legacy implementations and flatten managed layout

src/VoiceCat.Crypto/MediaDecryptor.cs

@@ -0,0 +1,60 @@
+using VoiceCat.Protocol;
+
+namespace VoiceCat.Crypto;
+
+public sealed class MediaDecryptor : IDisposable
+{
+    private readonly MediaCipher cipher;
+    private ulong highestSequence;
+    private ulong replayWindow;
+    private bool initialized;
+    private bool disposed;
+
+    public MediaDecryptor(ReadOnlySpan<byte> key) : this(key, false) { }
+
+    internal MediaDecryptor(ReadOnlySpan<byte> key, bool useManaged) => cipher = new(key, useManaged);
+
+    public bool TryDecrypt(ReadOnlySpan<byte> packet, Span<byte> plaintext, out VoiceFrameHeader header, out int bytesWritten)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        header = default;
+        bytesWritten = 0;
+        if (packet.Length < VoiceFrameHeader.Size + MediaEncryptor.TagSize) return false;
+        int length = packet.Length - VoiceFrameHeader.Size - MediaEncryptor.TagSize;
+        ArgumentOutOfRangeException.ThrowIfLessThan(plaintext.Length, length);
+        if (packet.Overlaps(plaintext)) throw new ArgumentException("Input and output must not overlap.", nameof(plaintext));
+        VoiceFrameHeader.TryRead(packet, out var candidate);
+        ulong sequence = candidate.Sequence;
+        if (initialized && sequence <= highestSequence)
+        {
+            ulong offset = highestSequence - sequence;
+            if (offset >= 64 || (replayWindow & (1UL << (int)offset)) != 0) return false;
+        }
+        if (!cipher.TryDecrypt(sequence, packet[VoiceFrameHeader.Size..], packet[..VoiceFrameHeader.Size], plaintext[..length])) return false;
+
+        // Only authenticated counters may move the replay window.
+        if (!initialized)
+        {
+            highestSequence = sequence;
+            replayWindow = 1;
+            initialized = true;
+        }
+        else if (sequence > highestSequence)
+        {
+            ulong shift = sequence - highestSequence;
+            replayWindow = (shift >= 64 ? 0 : replayWindow << (int)shift) | 1;
+            highestSequence = sequence;
+        }
+        else replayWindow |= 1UL << (int)(highestSequence - sequence);
+        header = candidate;
+        bytesWritten = length;
+        return true;
+    }
+
+    public void Dispose()
+    {
+        if (disposed) return;
+        disposed = true;
+        cipher.Dispose();
+    }
+}