Implementation of Affine Cipher
``````
// CSci 4509 Affine Cipher implementation
// Elena Machkasova, 1/19/05

public class AffineCipher {
public static void main(String[] args) {
// testing mod
/*
System.out.println(mod(7, 3));
System.out.println(mod(-7, 3));
System.out.println(mod(-101, 7));
*/
System.out.println(mod(11 * 19, 26));

if (args.length < 4) {
System.out.println("Usage: java ShiftCipher mode a b message");
System.out.println("mode: e for encrypt. d for decrypt");
System.out.println("a: an integer <= 25, b: an integer <= 25");
System.out.println("message: the message to encrypt or decrypt");
System.exit(0);
}

int a = (new Integer(args[1])).intValue();
int b = (new Integer(args[2])).intValue();
if (a < 1 || a > 25 || a < 0 || b > 25) {
System.out.println("Illegal value of a or b " + a + " " + b);
System.exit(0);
}

if (args[0].equals("e") || args[0].equals("E")) {
// encrypting
System.out.println(encrypt(args[3], a, b));
} else if (args[0].equals("d") || args[0].equals("D")) {
// decrypting
System.out.println(decrypt(args[3], a, b));
} else {
System.out.println("the first argument must be e or d");
System.exit(0);
}

}

// computes n mod m defined as the smallest
// non-negative integer r such that n = k * m + r,
// where k is an integer
// Assume that m > 0
public static int mod(int n, int m) {
int r = n % m;
if (r < 0) r = r + m;
return r;
}

// assume that the key is valid
public static String encrypt(String plaintext, int a, int b) {
int length = plaintext.length();
StringBuffer ciphertext = new StringBuffer();
for (int i = 0; i < length; ++i) {
char letter = plaintext.charAt(i);
if (!Character.isLetter(letter)) {
System.out.println("Illegal character " + letter);
System.exit(0);
}
int position = -1;
if (Character.isUpperCase(letter)) {
position = letter - 'A';
}
else if (Character.isLowerCase(letter)) {
position = letter - 'a';
}
int cipherposition = mod(a * position + b, 26);
//System.out.println(cipherposition);
ciphertext.append((char) ('A' + cipherposition));
}
return ciphertext.toString();
}

// assume that the key is valid
public static String decrypt(String ciphertext, int a, int b) {
int aInverse = inverse(a, 26);
int length = ciphertext.length();
StringBuffer plaintext = new StringBuffer();
for (int i = 0; i < length; ++i) {
char letter = ciphertext.charAt(i);
if (!Character.isLetter(letter)) {
System.out.println("Illegal character " + letter);
System.exit(0);
}
int position = -1;
if (Character.isUpperCase(letter)) {
position = letter - 'A';
}
else if (Character.isLowerCase(letter)) {
position = letter - 'a';
}
int plainposition = mod(aInverse * (position - b), 26);
//System.out.println(plainposition);
plaintext.append((char) ('A' + plainposition));
}
return plaintext.toString();
}

// find a multiplicative inverse of an integer num modulo an integer base
// If num is not relatively prime to base, throw a RuntimeException
// The method implements Extended Euclidean algorithm
public static int inverse(int num, int base) {
int b = base;
int a = num;
int k1 = 1;
int k2 = 0;
int k3 = 0;
int k4 = 1;
while (b != 0) {
int rem = mod(a, b);
int quot = a / b;
a = b;
b = rem;
int new1 = k1 - quot * k3;
int new2 = k2 - quot * k4;
k1 = k3;
k2 = k4;
k3 = new1;
k4 = new2;
}

//System.out.println("k1 = " + k1 + " k2 = " + k2 + " k3 = " + k3 + " k4 = " + k4);
//System.out.println(mod(k1, 26));

if (mod (k1 * num, base) != 1) {
throw new RuntimeException(a + " is not relatively prime with " + base);
}
return mod(k1, base);
}
}

``````
Try decrypting this:
``````
DMLQIFVMHQGLRXDFFY
``````

The course main page.

The views and opinions expressed in this page are strictly those of the page author. The contents of this page have not been reviewed or approved by the University of Minnesota.