import java.io$.$File;

import java.io$.$FileNotFoundException;

import java.util.Scanner;

public class pa$01\{$

$//$ Define the maximum size for the plaintext and ciphertext arrays

private static final int MAX$\_$SIZE $=10000$;

$//$ Function to read the key file and store the key matrix

public static int read$\_$key$\_$file$($String key$\_$filename, int$[][]$ key$\_$matrix$)$ throws FileNotFoundException $\{$

File key$\_$file $=$ new File$($key$\_$filename$)$;

Scanner keyScanner $=$ new Scanner$($key$\_$file$)$;

$//$ Read the size of the key matrix from the first line of the file

if $(!$keyScanner.hasNextInt$())\{$

System.err.println$("$Error: Invalid key file format."$)$;

keyScanner.close$()$;

System.exit$(1)$;

$\}$

int n $=$ keyScanner.nextInt$()$;

$//$ Validate the key size

if $($n $<2||$ n $>9)\{$

System.err.println$("$Error: Key size n must be between $2$ and $9.")$;

keyScanner.close$()$;

System.exit$(1)$;

$\}$

$//$ Read the key matrix from the remaining lines of the file

for $($int i $=0$; i $<$ n; i$++)\{$

for $($int j $=0$; j $<$ n; j$++)\{$

if $($keyScanner$.$hasNextInt$())\{$

key$\_$matrix$[$i$][$j$]=$ keyScanner.nextInt$()$;

$\}$ else $\{$

System.err.println$("$Error: Invalid key matrix format."$)$;

keyScanner.close$()$;

System.exit$(1)$;

$\}$

$\}$

$\}$

keyScanner.close$()$;

return n;

$\}$

$//$ Function to read the plaintext file and store the processed plaintext

public static String read$\_$plaintext$\_$file$($String plaintext$\_$filename, int n$)$ throws FileNotFoundException $\{$

File plaintext$\_$file $=$ new File$($plaintext$\_$filename$)$;

Scanner plaintextScanner $=$ new Scanner$($plaintext$\_$file$)$;

StringBuilder plaintextBuilder $=$ new StringBuilder$()$;

$//$ Read each character from the file and store only the alphabetic characters in lowercase

while $($plaintextScanner$.$hasNextLine$())\{$

String line $=$ plaintextScanner.nextLine$()$;

for $($char c : line.toCharArray$())\{$

if $($Character$.$isLetter$($c$))\{$

if $($plaintextBuilder$.$length$()<9991)\{$

plaintextBuilder.append$($Character$.$toLowerCase$($c$))$;

$\}$ else $\{$

break;

$\}$

$\}$

$\}$

$\}$

plaintextScanner.close$()$;

$//$ Pad the plaintext with $\text{'}$x$\text{'}$ characters if necessary to match the block size of the key

while $($plaintextBuilder$.$length$()\%$ n $!=0$ && plaintextBuilder.length$()<$ MAX$\_$SIZE $-1)\{$

plaintextBuilder.append$(\text{'}$x$\text{'})$;

$\}$

return plaintextBuilder.toString$()$;

$\}$

$//$ Function to print text in lines of $80$ characters

public static void print$\_$in$\_80\_$char$\_$lines$($String text, String title$)\{$

System.out.println$("$

$"+$ title $+"$:$")$;

int len $=$ text.length$()$;

for $($int i $=0$; i $<$ len; i$++)\{$

System.out.print$($text$.$charAt$($i$))$;

if $(($i $+1)\%80==0||$ i $==$ len $-1)\{$

System.out.println$()$;

$\}$

$\}$

$\}$

$//$ Function to encrypt the plaintext using the key matrix and store the ciphertext

public static String encrypt$($String plaintext, int n$,$ int$[][]$ key$\_$matrix$)\{$

int len $=$ plaintext.length$()$;

char$[]$ ciphertext $=$ new char$[$len$]$;

$//$ Process the plaintext in blocks of size n and encrypt each block using the key matrix

for $($int i $=0$; i $<$ len; i $+=$ n$)\{$

for $($int j $=0$; j $<$ n; j$++)\{$

int c $=0$;

for $($int k $=0$; k $<$ n; k$++)\{$

c $+=$ key$\_$matrix$[$j$][$k$]*($plaintext$.$charAt$($i $+$ k$)-\text{'}$a$\text{'})$;

$\}$

$//$ Store the encrypted character in the ciphertext array

ciphertext$[$i $+$ j$]=($char$)((($c $\%26+26)\%26)+\text{'}$a$\text{'})$;

$\}$

$\}$

return new String$($ciphertext$)$;

$\}$

public static void main$($String$[]$ args$)\{$

$/*=============================================================================$

$|$ I $[$Rosa Siprien$]([5525879])$ affirm that this program is

$|$ entirely my own work and that I have neither developed my code together with

$|$ any another person, nor copied any code from any other person, nor permitted

$|$ my code to be copied or otherwise used by any other person, nor have I

$|$ copied, modified, or otherwise used programs created by others. I acknowledge

$|$ that any violation of the above terms will be treated as academic dishonesty.

$+=============================================================================*/$

$//$ Check that the program is called with the correct number of arguments

if $($args$.$length $!=2)\{$

System.err.println$("$Usage: java pa$01$ key$\_$filename.txt plaintext$\_$filename.txt$")$;

System.exit$(1)$;

$\}$