/// <summary>Determines whether a stream uses X3:TC file encryption.</summary>
/// <param name="s">stream.</param>
/// <returns></returns>
bool EncryptedX3Stream::IsEncrypted(StreamPtr s)
{
// Verify length
if (s->GetLength() < 3)
return false;
// Prepare
DWORD origin = s->GetPosition();
WORD header, key;
// Generate key: XOR first byte with 0xC8
s->Seek(0, SeekOrigin::Begin);
s->Read(reinterpret_cast<byte*>(&key), 1);
key ^= DECRYPT_SEED;
// Generate WORD from first byte
reinterpret_cast<byte*>(&key)[1] = reinterpret_cast<byte*>(&key)[0];
// Read encrypted 2-byte header. reset position
s->Read(reinterpret_cast<BYTE*>(&header), 2);
s->Seek(origin, SeekOrigin::Begin);
// Check for encrypted GZip header
return (header ^ key) == 0x8b1f;
}
