public static BitString16 forwardPermutation(BitString16 bitString)
{
String input = bitString.BitString;
String output = "";
output += input[1 - 1]; //1st bit
output += input[5 - 1]; //2nd bit
output += input[9 - 1];
output += input[13 - 1];
output += input[2 - 1];
output += input[6 - 1];
output += input[10 - 1];
output += input[14 - 1];
output += input[3 - 1];
output += input[7 - 1];
output += input[11 - 1];
output += input[15 - 1];
output += input[4 - 1];
output += input[8 - 1];
output += input[12 - 1];
output += input[16 - 1];
return(new BitString16(output));
}
//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);
}
//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);
}
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);
}
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);
}
static BitString16 partial_decrypt(BitString16 ciphertext, BitString16 key)
{
SBox sbox = new SBox();
//Pre
BitString16 plaintext = Util.xor(ciphertext, key);
//Round 1
plaintext = sbox.backwardsTransform(plaintext);
return plaintext;
}
public BitString16 forwardTransform(BitString16 bitString)
{
String input = bitString.BitString;
String output = "";
for (int i = 0; i < 4; ++i)
{
String subBlock = input.Substring(i * 4, 4);
output += sBoxMapping[subBlock];
}
return(new BitString16(output));
}
static BitString16 partial_decrypt(BitString16 ciphertext, BitString16 key)
{
SBox sbox = new SBox();
//Pre
BitString16 plaintext = Util.xor(ciphertext, key);
//Round 1
plaintext = sbox.backwardsTransform(plaintext);
return(plaintext);
}
public BitString16 forwardTransform(BitString16 bitString)
{
String input = bitString.BitString;
String output = "";
for (int i = 0; i < 4; ++i)
{
String subBlock = input.Substring(i * 4, 4);
output += sBoxMapping[subBlock];
}
return new BitString16(output);
}
public BitString16 backwardsTransform(BitString16 bitString)
{
String input = bitString.BitString;
String output = "";
for (int i = 0; i < 4; ++i)
{
String subBlock = input.Substring(i * 4, 4);
output += sBoxMapping.First(x => x.Value.Equals(subBlock)).Key;
}
return(new BitString16(output));
}
public BitString16 backwardsTransform(BitString16 bitString)
{
String input = bitString.BitString;
String output = "";
for (int i = 0; i < 4; ++i)
{
String subBlock = input.Substring(i * 4, 4);
output += sBoxMapping.First(x => x.Value.Equals(subBlock)).Key;
}
return new BitString16(output);
}
static bool testPartialKey(BitString16 plaintext, BitString16 partialPlaintext)
{
String p = plaintext.BitString;
String u = partialPlaintext.BitString;
//Implementation of formula 5: U[4,6] XOR U[4,8] XOR U[4,14] XOR U[4,16] XOR P[5] XOR P[7] XOR P[8]
char result = Util.bit_xor(u[5], Util.bit_xor(u[7], Util.bit_xor(u[13], Util.bit_xor(u[15], Util.bit_xor(p[4], Util.bit_xor(p[6], p[7]))))));
if (result == '0')
{
return(true);
}
return(false);
}
public static BitString16 xor(BitString16 bs1, BitString16 bs2)
{
String output = "";
String s1 = bs1.BitString;
String s2 = bs2.BitString;
for (int i = 0; i < s1.Length; ++i)
{
if (s1[i] != s2[i])
output += '1';
else
output += '0';
}
return new BitString16(output);
}
public static BitString16 PerformLinearCryptanalysis(Dictionary <String, String> plain_cipher_pairs)
{
//Get the list of 256 possible subkeys
List <BitString16> partialSubKeys = generateAllPartialSubkeys();
double highestBias = 0; //Keep track of the highest bias
String subkey = ""; //Keep track of the subkey with the highest bias
//Iterate through each possible subkey and check which has the highest bias, this is the actual subkey.
foreach (BitString16 partialSubKey in partialSubKeys)
{
int count = 0;
foreach (var x in plain_cipher_pairs)
{
BitString16 plaintext = new BitString16(x.Key);
BitString16 ciphertext = new BitString16(x.Value);
BitString16 partial_decrypted = partial_decrypt(ciphertext, partialSubKey);
//Check if the partially decrypted ciphertext satisfies equation 5
if (testPartialKey(plaintext, partial_decrypted))
{
count++;
}
}
double bias = Math.Abs(count - 5000.0) / 10000.0; //Get this key's bias
String _subkey = partialSubKey.BitString; //Get this subkey
System.Console.WriteLine("KEY: {1}, BIAS: {0}", bias, _subkey);
//If this key has the highest bias so far, store it
if (bias > highestBias)
{
highestBias = bias;
subkey = _subkey;
}
}
//Return the best subkey we found
return(new BitString16(subkey));
}
public static BitString16 xor(BitString16 bs1, BitString16 bs2)
{
String output = "";
String s1 = bs1.BitString;
String s2 = bs2.BitString;
for (int i = 0; i < s1.Length; ++i)
{
if (s1[i] != s2[i])
{
output += '1';
}
else
{
output += '0';
}
}
return(new BitString16(output));
}
public static BitString16 PerformLinearCryptanalysis(Dictionary<String, String> plain_cipher_pairs)
{
//Get the list of 256 possible subkeys
List<BitString16> partialSubKeys = generateAllPartialSubkeys();
double highestBias = 0; //Keep track of the highest bias
String subkey = ""; //Keep track of the subkey with the highest bias
//Iterate through each possible subkey and check which has the highest bias, this is the actual subkey.
foreach (BitString16 partialSubKey in partialSubKeys)
{
int count = 0;
foreach (var x in plain_cipher_pairs)
{
BitString16 plaintext = new BitString16(x.Key);
BitString16 ciphertext = new BitString16(x.Value);
BitString16 partial_decrypted = partial_decrypt(ciphertext, partialSubKey);
//Check if the partially decrypted ciphertext satisfies equation 5
if (testPartialKey(plaintext, partial_decrypted))
count++;
}
double bias = Math.Abs(count - 5000.0) / 10000.0; //Get this key's bias
String _subkey = partialSubKey.BitString; //Get this subkey
System.Console.WriteLine("KEY: {1}, BIAS: {0}", bias, _subkey);
//If this key has the highest bias so far, store it
if (bias > highestBias)
{
highestBias = bias;
subkey = _subkey;
}
}
//Return the best subkey we found
return new BitString16(subkey);
}
//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;
}
static bool testPartialKey(BitString16 plaintext, BitString16 partialPlaintext)
{
String p = plaintext.BitString;
String u = partialPlaintext.BitString;
//Implementation of formula 5: U[4,6] XOR U[4,8] XOR U[4,14] XOR U[4,16] XOR P[5] XOR P[7] XOR P[8]
char result = Util.bit_xor(u[5], Util.bit_xor(u[7], Util.bit_xor(u[13], Util.bit_xor(u[15], Util.bit_xor(p[4], Util.bit_xor(p[6], p[7]))))));
if (result == '0')
return true;
return false;
}