IRandomNumberGenerator csprng = new RandomNumberGenerator();

byte[] key = new byte[32];
csprng.Read(key, 0, key.Length);

ISymmetricAlgorithm algorithm = new Serpent(key);

IStream input = new FileStream("plaintext.bin");
IStream output = new FileStream("ciphertext.bin");
IStream transformer = algorithm.CreateEncryptor();

transformer.Input = input;
transformer.Output = output;
transformer.Transform();