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