static void Main(string[] args)
{
Random rand = new Random(); //Random number generator which will be used to randomize the key
/*
* Generate the round keys. For this SPN the keys bits are random and independent.
* However, we fix K5 such that [K(5,5)...K(5,8) , K(5,13)...K(5,16)] = [3,1]
*/
List<BitString16> keys = new List<BitString16>(); //Temporary storage container for the keys
keys.Add(new BitString16(rand)); //Round 1 key (random)
keys.Add(new BitString16(rand)); //Round 2 key (random)
keys.Add(new BitString16(rand)); //Round 3 key (random)
keys.Add(new BitString16(rand)); //Round 4 key (random)
keys.Add(new BitString16("0000001100000001")); //Round 5 key (fixed)
Key key = new Key(keys);
//Create an instance of SBox which will be used by the SPN
SBox sbox = new SBox();
Dictionary<String, String> plain_cipher_pairs = new Dictionary<string, string>(); //Plaintext/Ciphertext pairs
StreamWriter writer = new StreamWriter("pairs.txt"); //Text writer
//Generate 10,000 Plaintext/Ciphertext pairs
while (plain_cipher_pairs.Keys.Count < 10000)
{
BitString16 message = new BitString16(rand); //Generate a random message to encrypt
//Check if we have already encrypted this plaintext. If so, skip this iteration of the loop
//Without this check we may end up with duplicate plain/ciphertext pairs
if (plain_cipher_pairs.ContainsKey(message.BitString))
continue;
//Encrypt the message via the four round SPN
BitString16 encrypt_message = FourRoundSPN.Encrypt(message, key);
//Extract the plaintext/ciphertext strings from the objects
String plaintext = message.BitString;
String ciphertext = encrypt_message.BitString;
//Write the pair to the textfile and store in the dictionary for further processing
writer.WriteLine("{0},{1}", plaintext, ciphertext);
plain_cipher_pairs.Add(plaintext, ciphertext);
}
writer.Close();
//Using only the plaintext/ciphertext pairs, perform linear cryptanalysis
BitString16 r5BitString = LinearCryptanalysis.PerformLinearCryptanalysis(plain_cipher_pairs);
String r5Key = r5BitString.BitString;
String subkey1 = r5Key.Substring(4, 4);
String subkey2 = r5Key.Substring(12, 4);
System.Console.WriteLine("RESULT OF LINEAR CRYPTANALYSIS:\nK[5,5]-K[5,8] = {0}\nK[5,13]-K[5,16] = {1}", subkey1, subkey2);
}
//Encrypt a message under a key using the 4 round SPN
public static BitString16 Encrypt(BitString16 message, Key key)
{
SBox sbox = new SBox();
//Rounds 1-3
for (int round = 1; round < 4; ++round)
{
BitString16 subkey = key.getEncryptionSubKey(round);
message = Util.xor(message, subkey);
message = sbox.forwardTransform(message);
message = Permutation.forwardPermutation(message);
}
//Round 4
BitString16 subkey_4 = key.getEncryptionSubKey(4);
message = Util.xor(message, subkey_4);
message = sbox.forwardTransform(message);
//Post
BitString16 last_subkey = key.getEncryptionSubKey(5);
message = Util.xor(message, last_subkey);
return message;
}
//Decrypt a ciphertext using a key and the 4 round SPN
public static BitString16 Decrypt(BitString16 ciphertext, Key key)
{
SBox sbox = new SBox();
//Pre
BitString16 first_subkey = key.getDecryptionSubKey(1);
BitString16 plaintext = Util.xor(ciphertext, first_subkey);
//Round 1
BitString16 second_subkey = key.getDecryptionSubKey(2);
plaintext = sbox.backwardsTransform(plaintext);
plaintext = Util.xor(plaintext, second_subkey);
//Rounds 2-4
for (int round = 2; round < 5; ++round)
{
BitString16 subkey = key.getDecryptionSubKey(round + 1);
plaintext = Permutation.backwardsPermutation(plaintext);
plaintext = sbox.backwardsTransform(plaintext);
plaintext = Util.xor(plaintext, subkey);
}
return plaintext;
}