Matrices

import java.util.Arrays;
import java.io.*;

public class matrix {

    public static final int N = 16;

    public static int add(int a, int b){
        return (a+b)%251;
    }

    public static int multiply(int a, int b){
        return (a*b)%251;
    }

    public static int[][] random(int m, int n) {
        int[][] C = new int[m][n];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                C[i][j] = (int) (Math.random()*254+1);
        return C;
    }

    public static int dot(int[] x, int[] y) {
        if (x.length != y.length) throw new RuntimeException("Illegal vector dimensions.");
        int sum = 0;
        for (int i = 0; i < x.length; i++)
            sum = add(sum, multiply(x[i], y[i]));
        return sum;
    }

    public static int[][] multiply(int[][] A, int[][] B) {
        int mA = A.length;
        int nA = A[0].length;
        int mB = B.length;
        int nB = B[0].length;
        if (nA != mB) throw new RuntimeException("Illegal matrix dimensions.");
        int[][] C = new int[mA][nB];
        for (int i = 0; i < mA; i++)
            for (int j = 0; j < nB; j++)
                for (int k = 0; k < nA; k++)
                    C[i][j] = add(C[i][j], multiply(A[i][k], B[k][j]));
        return C;
    }

    public static int[] multiply(int[][] A, int[] x) {
        int m = A.length;
        int n = A[0].length;
        if (x.length != n) throw new RuntimeException("Illegal matrix dimensions.");
        int[] y = new int[m];
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                y[i] = add(y[i], multiply(A[i][j], x[j]));
        return y;
    }

    public static int[] multiply(int[] x, int[][] A) {
        int m = A.length;
        int n = A[0].length;
        if (x.length != m) throw new RuntimeException("Illegal matrix dimensions.");
        int[] y = new int[n];
        for (int j = 0; j < n; j++)
            for (int i = 0; i < m; i++)
                y[j] = add(y[j], multiply(A[i][j], x[i]));
        return y;
    }

    public static void main(String[] args) {
        int [][] key = random(N,N);
        printArr(key);

        String message = args[0];

        int[] messagebuf = new int[N];

        String result = "";
        byte[] resultarr = new byte[(message.length()/N+1)*N];


        for (int i=0;i<message.length();i+=N){
            for (int j=0;j<N;j++){
                if (i+j<message.length()){
                    messagebuf[j]=messagebuf[j]^(int) message.charAt(i+j);
                }else{
                    messagebuf[j]=0;
                }
            }
            messagebuf = multiply(key,messagebuf);
            for (int j=0;j<N;j++){
                resultarr[i+j] = (byte) messagebuf[j];
                result += (char) messagebuf[j];
            }
        }

        try (FileOutputStream fos = new FileOutputStream("output1")) {
            fos.write(resultarr);
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }

    public static void printArr(int[][] arr){
        for (int i=0;i<arr.length;i++){
            System.out.println(Arrays.toString(arr[i]));
        }
    }
}