private Stream GetDataStream(FirmwareHeader header)
{
// offset into the file past the header
var substream = new SubStream(_stream, header.Offset + FirmwareHeader.HeaderLength, header.Length);
var hasEncryptionKey = !header.KeyBlob.All(b => b == 0);
if (hasEncryptionKey)
{
// Pull AES key from header, init AES decryptor, return stream
var rawAesKey = _cipher.PublicDecrypt(header.KeyBlob);
var aesKey = AESKey.Parse(rawAesKey);
var aesAlg = new AesCryptoServiceProvider
{
Key = aesKey.Key,
IV = aesKey.IV,
Mode = CipherMode.CBC,
Padding = PaddingMode.None // No reference to padding seen in binary, using default (PKCS7) always fails.
};
var decryptor = aesAlg.CreateDecryptor();
var cryptoStream = new CryptoStream(substream, decryptor, CryptoStreamMode.Read, leaveOpen: true);
return(cryptoStream);
}
else
{
return(new XorStream(substream, Constants.DataXOR));
}
}
private FirmwareHeader ReadHeader(long offset)
{
Span <byte> buf = stackalloc byte[FirmwareHeader.HeaderLength];
_stream.Seek(offset, SeekOrigin.Begin);
_stream.Read(buf);
var header = FirmwareHeader.Parse(buf);
if (header.Magic != Constants.FirmwareMagic)
{
throw new InvalidDataException($"read invalid header at {offset:X}, bad magic");
}
header.Offset = offset;
return(header);
}