Этот код должен работать в качестве примера:
byte[] key = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
byte[] iv = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
byte[] data = { 1, 2, 3, 4, 5 };
using (var symmetrickey = Rijndael.Create().CreateEncryptor(key, iv))
{
using (Stream f = File.Create("anencryptedfile.bin"))
{
using (Stream c = new CryptoStream(f, symmetrickey, CryptoStreamMode.Write))
{
c.Write(data, 0, data.Length);
}
}
}