Implementing Hill Cipher Substitution In Java:

import java.io.*;
public class hillcipher
{
int keymatrix[][];
int linematrix[];
int resultmatrix[];
public void divide(String temp,int s)
{
while(temp.length()>s)
{
String sub=temp.substring(0,s);
temp=temp.substring(s,temp.length());
perform(sub);
}
if(temp.length()==s)
perform(temp);
else if(temp.length()<s)
{
for(int i=temp.length();i<s;i++)
temp=temp+'x';
perform(temp);
}
}
public void perform(String line)
{
linetomatrix(line);
linemultiplykey(line.length());
result(line.length());
}
public void keytomatrix(String key,int len)
{
keymatrix=new int[len][len];
int c=0;
for(int i=0;i<len;i++)
{
for(int j=0;j<len;j++)
{
keymatrix[i][j]=((int)key.charAt(c))-97;
c++;
}
}
}
public void linetomatrix(String line)
{
linematrix=new int[line.length()];
for(int i=0;i<line.length();i++)
{
linematrix[i]=((int)line.charAt(i))-97;
}
}
public void linemultiplykey(int len)
{
resultmatrix=new int[len];
for(int i=0;i<len;i++)
{
for(int j=0;j<len;j++)
{
resultmatrix[i]+=keymatrix[i][j]*linematrix[j];
}
resultmatrix[i]%=26;
}
}
public void result(int len)
{
String result="";
for(int i=0;i<len;i++)
{
result+=(char)(resultmatrix[i]+97);
}
System.out.print(result);
}
public boolean check(String key,int len)
{
keytomatrix(key,len);
int d=determinant(keymatrix,len);
d=d%26;
if(d==0)
{
System.out.println("Invalid key!!! Key is not invertible because determinant=0...");
return false;
}
else if(d%2==0||d%13==0)
{
System.out.println("Invalid key!!! Key is not invertible because determinant has common factor with 26...");
return false;
}
else
{
return true;
}
}
public int determinant(int A[][], int N){
int det=0;
int res;
if(N == 1)
res = A[0][0];
else if (N == 2){
res = A[0][0]*A[1][1] - A[1][0]*A[0][1];
}
else{
res=0;
for(int j1=0;j1<N;j1++){
int m[][]=new int[N-1][N-1];
for(int i=1;i<N;i++){
int j2=0;
for(int j=0;j<N;j++){
if(j==j1)
continue;
m[i-1][j2]=A[i][j];
j2++;
}
}
res+=Math.pow(-1.0,1.0+j1+1.0)* A[0][j1]*determinant(m,N-1);
}
}
return res;
}
public void cofact(int num[][],int f)
{
int b[][],fac[][];
b=new int[f][f];
fac=new int[f][f];
int p,q,m,n,i,j;
for(q=0;q<f;q++)
{
for(p=0;p<f;p++)
{
m=0;
n=0;
for(i=0;i<f;i++)
{
for(j=0;j<f;j++)
{
b[i][j]=0;
if(i!=q&&j!=p)
{
b[m][n]=num[i][j];
if(n<(f-2))
n++;
else
{
n=0;
m++;
}
}
}
}
fac[q][p]=(int)Math.pow(-1,q+p)*determinant(b,f-1);
}
}
trans(fac,f);
}
void trans(int fac[][],int r)
{
int i,j;
int b[][],inv[][];
b=new int[r][r];
inv=new int[r][r];
int d=determinant(keymatrix,r);
int mi=mi(d%26);
mi%=26;
if(mi<0)
mi+=26;
for(i=0;i<r;i++)
{
for(j=0;j<r;j++)
{
b[i][j]=fac[j][i];
}
}
for(i=0;i<r;i++)
{
for(j=0;j<r;j++)
{
inv[i][j]=b[i][j]%26;
if(inv[i][j]<0)
inv[i][j]+=26;
inv[i][j]*=mi;
inv[i][j]%=26;
}
}
System.out.println("\nInverse key:");
matrixtoinvkey(inv,r);
}
public int mi(int d)
{
int q,r1,r2,r,t1,t2,t;
r1=26;
r2=d;
t1=0;
t2=1;
while(r1!=1&&r2!=0)
{
q=r1/r2;
r=r1%r2;
t=t1-(t2*q);
r1=r2;
r2=r;
t1=t2;
t2=t;
}
return (t1+t2);
}
public void matrixtoinvkey(int inv[][],int n)
{
String invkey="";
for(int i=0;i<n;i++)
{
for(int j=0;j<n;j++)
{
invkey+=(char)(inv[i][j]+97);
}
}
System.out.print(invkey);
}
public static void main(String args[])throws IOException
{
hillcipher obj=new hillcipher();
BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
int choice;
System.out.println("Menu:\n1: Encryption\n2: Decryption");
choice=Integer.parseInt(in.readLine());
System.out.println("Enter the line: ");
String line=in.readLine();
System.out.println("Enter the key: ");
String key=in.readLine();
double sq=java.lang.Math.sqrt(key.length());
if(sq!=(long)sq)
System.out.println("Invalid key length!!! Does not form a square matrix...");
else
{
int s=(int)sq;
if(obj.check(key,s))
{
System.out.println("Result:");
obj.divide(line,s);
obj.cofact(obj.keymatrix,s);
}
}
}
}