private static BitString GetLengthAsByte(BitString content)
{
if (content.BitLength < 1024) // 128 * 8 bits
{
return(BitString.To8BitString((byte)(content.BitLength / 8))); // To add to below, byte is naturally unsigned, 0 to 255.
}
else if (content.BitLength < 524288) // 65536 * 8 bits
{
// Length is a multi-part field, the first bit is set to 1. The next 7 bits communicate the length (bytes) of the upcoming length property
// Then the actual length of the content is present after.
var bytesInContent = content.BitLength / 8;
var length = BitString.PadToNextByteBoundry(((BigInteger)bytesInContent).ExactBitString());
var bytesNeededForLength = length.BitLength / 8;
var preamble = BitString.PadToNextByteBoundry(((BigInteger)bytesNeededForLength).ExactBitString());
preamble.Set(0, true);
return(preamble.ConcatenateBits(length));
}
else // We shouldn't be generating DER encoded strings this long
{
throw new Exception("Content too large to encode in DER encoder");
}
}
public override SymmetricCipherResult ProcessPayload(IModeBlockCipherParameters param)
{
CheckPayloadRequirements(param.Payload);
var key = param.Key.ToBytes();
var actualBitsToProcess = param.Payload.BitLength;
param.Payload = BitString.PadToNextByteBoundry(param.Payload);
// CFB always utilizes engine in encrypt mode
var engineParam = new EngineInitParameters(BlockCipherDirections.Encrypt, key, param.UseInverseCipherMode);
_engine.Init(engineParam);
var numberOfSegments = actualBitsToProcess / _shiftRegisterStrategy.ShiftSize;
var outBuffer = GetOutputBuffer(param.Payload.BitLength);
if (param.Direction == BlockCipherDirections.Encrypt)
{
Encrypt(param, numberOfSegments, outBuffer);
}
else
{
Decrypt(param, numberOfSegments, outBuffer);
}
return(new SymmetricCipherResult(
new BitString(outBuffer).GetMostSignificantBits(actualBitsToProcess)
));
}
