The following program in C$++$ is able to encrypt a file using the Vigen$$re cypher, and writes the file to encrypted.txt file, but it is not able to decrypt the file again. Please modify the program so that it can decrypt the file back to the original as well as encrypt. This is due by tomorrow. Thanks!

Code:

#include

#include

#include

#include

using namespace std;

char encryptChar$($char ch$,$ char key$)\{$

if $($isalpha$($ch$))\{$

bool isUpper $=$ isupper$($ch$)$;

char base $=$ isUpper $?\text{'}$A$\text{'}$ : $\text{'}$a$\text{'}$;

return base $+($ch $+$ key $-2*$ base$)\%26$;

$\}$

else $\{$

return ch;

$\}$

$\}$

char decryptChar$($char ch$,$ char key$)\{$

if $($isalpha$($ch$))\{$

bool isUpper $=$ isupper$($ch$)$;

char base $=$ isUpper $?\text{'}$A$\text{'}$ : $\text{'}$a$\text{'}$;

return base $+($ch $-$ key $+26-$ base$)\%26$;

$\}$

else $\{$

return ch;

$\}$

$\}$

void processFile$($string filename, string key, bool encrypt$)\{$

ifstream inFile$($filename$)$;

ofstream outFile$($encrypt $?$ "encrypted.txt$"$ : "decrypted.txt$")$;

if $(!$inFile.is$\_$open$()||!$outFile.is$\_$open$())\{$

cerr $<<$ "Error opening file." $<<$ endl;

return;

$\}$

char ch;

int keyIndex $=0$;

while $($inFile$.$get$($ch$))\{$

char encryptedChar $=$ encrypt $?$ encryptChar$($ch$,$ key$[$keyIndex$])$ : decryptChar$($ch$,$ key$[$keyIndex$])$;

outFile $<<$ encryptedChar;

cout $<<$ encryptedChar;

keyIndex $=($keyIndex $+1)\%$ key.length$()$;

$\}$

inFile.close$()$;

outFile.close$()$;

$\}$

int main$()\{$

string choice, key, filename;

while $($true$)\{$

cout $<<$ "Encrypt or decrypt $($e$/$d$)?($q to quit$)$: $"$;

cin $>>$ choice;

if $($choice $=="$e$")\{$

cout $<<$ "Enter key: $"$;

cin $>>$ key;

cout $<<$ "Enter plaintext filename: $"$;

cin $>>$ filename;

processFile$($filename$,$ key, true$)$;

$\}$

else if $($choice $=="$d$")\{$

cout $<<$ "Enter key: $"$;

cin $>>$ key;

cout $<<$ "Enter encrypted filename: $"$;

cin $>>$ filename;

processFile$($filename$,$ key, false$)$;

$\}$

else if $($choice $=="$q$")\{$

break;

$\}$

else $\{$

cout $<<$ "Invalid choice." $<<$ endl;

$\}$

$\}$

return $0$;

$\}$