public byte[] HandleFinalFullPayloadBlockDecryption(BitString payload, IBlockCipherEngine engine, int numberOfBlocks, int originalPayloadBitLength)
{
// Decrypt the last full payload block (when there is more than one block)
if (numberOfBlocks > 1)
{
var originalPayloadPaddedToBlockSize = payload.PadToModulus(engine.BlockSizeBits).ToBytes();
// Decrypt the last full payload block (in this case the second to last block)
var secondToLastBlock = new byte[engine.BlockSizeBytes];
var secondToLastBlockStartIndex = (numberOfBlocks - 2) * engine.BlockSizeBytes;
Array.Copy(originalPayloadPaddedToBlockSize, secondToLastBlockStartIndex, secondToLastBlock, 0, engine.BlockSizeBytes);
var decryptedSecondToLastBlockBuffer = new byte[engine.BlockSizeBytes];
engine.ProcessSingleBlock(secondToLastBlock, decryptedSecondToLastBlockBuffer, 0);
var decryptedBlock = new BitString(decryptedSecondToLastBlockBuffer);
// Pad the payload to the nearest multiple of the block size using the last B−M bits of block cipher decryption of the second-to-last ciphertext block.
var amountToPad = (engine.BlockSizeBits - payload.BitLength % engine.BlockSizeBits);
if (amountToPad > 0)
{
payload = payload.ConcatenateBits(BitString.Substring(decryptedBlock, 0, amountToPad));
}
var payloadBytes = payload.ToBytes();
TransformText(payloadBytes, engine, numberOfBlocks, originalPayloadBitLength);
payload = new BitString(payloadBytes);
return(payload.ToBytes());
}
return(payload.ToBytes());
}
public byte[] HandleFinalFullPayloadBlockDecryption(BitString payload, IBlockCipherEngine engine, int numberOfBlocks, int originalPayloadBitLength)
{
// When payload is not a multiple of the block size
if (numberOfBlocks > 1 && payload.BitLength % engine.BlockSizeBits != 0)
{
var numberOfBitsToAdd = engine.BlockSizeBits - payload.BitLength % engine.BlockSizeBits;
// Decrypt the last full payload block (in this case the last block)
var decryptedLastBlockBuffer = new byte[engine.BlockSizeBytes];
var lastBlock = payload.GetLeastSignificantBits(engine.BlockSizeBits).ToBytes();
engine.ProcessSingleBlock(lastBlock, decryptedLastBlockBuffer, 0);
var paddedPayload = payload
// The original payload minus the final full block
.GetMostSignificantBits(payload.BitLength - engine.BlockSizeBits)
// Add the least significant bits of the decrypted last block to pad to a multiple of the block size
.ConcatenateBits(new BitString(decryptedLastBlockBuffer).GetLeastSignificantBits(numberOfBitsToAdd))
// Add the last block back onto the payload
.ConcatenateBits(payload.GetLeastSignificantBits(engine.BlockSizeBits));
return(paddedPayload.ToBytes());
}
return(payload.ToBytes());
}
