HILL CIPHER

  HILL CIPHER

AIM

To write a program to encrypt and decrypt using  Hill cipher substitution technique

ALGORITHM DESCRIPTION

· The Hill cipher is a substitution cipher invented by Lester S. Hill in 1929.

· Each letter is represented by a number modulo 26. To encrypt a message, each block of n letters is multiplied by an invertible n × n matrix, again modulus 26.

· To decrypt the message, each block is multiplied by the inverse of the matrix used for encryption. The matrix used for encryption is the cipher key, and it should be chosen randomly from the set of invertible n × n matrices (modulo 26).

· The cipher can, be adapted to an alphabet with any number of letters.

· All arithmetic just needs to be done modulo the number of letters instead of modulo 26.

PROGRAM

import java.util.*;

class hill

{

/* 3x3 key matrix for 3 characters at once */

public static int[][] keymat = new int[][]

{ { 1, 2, 1 }, { 2, 3, 2 }, { 2, 2, 1 } };

/* key inverse matrix */

public static int[][] invkeymat = new int[][]

{ { -1, 0, 1 }, { 2, -1, 0 }, { -2, 2, -1 } };

public static String key = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

private static String encode(char a, char b, char c)

{

String ret = "";

int x,y, z;

int posa = (int)a - 65;

int posb = (int)b - 65;

int posc = (int)c - 65;

x = posa * keymat[0][0] + posb * keymat[1][0] + posc * keymat[2][0];

y = posa * keymat[0][1] + posb * keymat[1][1] + posc * keymat[2][1];

z = posa * keymat[0][2] + posb * keymat[1][2] + posc * keymat[2][2];

a = key.charAt(x%26);

b = key.charAt(y%26);

c = key.charAt(z%26);

ret = "" + a + b + c;

return ret;

}

private static String decode(char a, char b, char c)

{

String ret = "";

int x,y,z;

int posa = (int)a - 65;

int posb = (int)b - 65;

int posc = (int)c - 65;

x = posa * invkeymat[0][0]+ posb * invkeymat[1][0] + posc * invkeymat[2][0];

y = posa * invkeymat[0][1]+ posb * invkeymat[1][1] + posc * invkeymat[2][1];

z = posa * invkeymat[0][2]+ posb * invkeymat[1][2] + posc * invkeymat[2][2];

a = key.charAt((x%26<0) ? (26+x%26) : (x%26));

b = key.charAt((y%26<0) ? (26+y%26) : (y%26));

c = key.charAt((z%26<0) ? (26+z%26) : (z%26));

ret = "" + a + b + c;

return ret;

}

public static void main (String[] args) throws java.lang.Exception

{

String msg;

String enc = "";

String dec = "";

int n;

msg = ("SecurityLaboratory");

System.out.println("simulation of Hill Cipher");

System.out.println("input message : " + msg);

msg = msg.toUpperCase();

msg = msg.replaceAll("\\s", ""); /* remove spaces */

n = msg.length() % 3;

/* append padding text X */

if (n != 0)

{

for(int i = 1; i<= (3-n);i++)

{

msg+= 'X';

}

}

System.out.println("padded message : " + msg);

char[] pdchars = msg.toCharArray();

for (int i=0; i < msg.length(); i+=3)

{

enc += encode(pdchars[i], pdchars[i+1], pdchars[i+2]);

}

System.out.println("encoded message : " + enc);

char[] dechars = enc.toCharArray();

for (int i=0; i< enc.length(); i+=3)

{

dec += decode(dechars[i], dechars[i+1], dechars[i+2]);

}

System.out.println("decoded message : " + dec);

}

}

OUTPUT

simulation of hill cipher

input message: SecurityLaboratory

padded message:SECURITYLABORATORY

encoded message:EACSDKLCAEFQDUKSXU

decoded message:SECURITYLABORATORY