Question

Computer Science C++ Help, here's the question that needs to be answered (TASK D):

Task D. Decryption

Implement two decryption functions corresponding to the above ciphers. When decrypting ciphertext, ensure that the produced decrypted string is equal to the original plaintext:

decryptCaesar(ciphertext, rshift) == plaintext

decryptVigenere(ciphertext, keyword) == plaintext

Write a program decryption.cpp that uses the above functions to demonstrate encryption and decryption for both ciphers.

It should first ask the user to input plaintext, then ask for a right shift for the Caesar cipher and report the ciphertext and its subsequent decryption. After that, it should do the same for the Vigenere cipher.

Example:

$ ./decryption

Enter plaintext: Hello, World!

= Caesar =
Enter shift    : 10
Ciphertext     : Rovvy, Gybvn!
Decrypted      : Hello, World!

= Vigenere =
Enter keyword  : cake
Ciphertext     : Jevpq, Wyvnd!
Decrypted      : Hello, World!

(When reporting decrypted strings, they should be the result of applying decryption functions to the ciphertext, not the original plaintext variable.)

This question piggybacks off of two previous tasks (Tasks B & C). The following shows the questions for both Task B and C (NOTE: These are only shown to provide background info and continue the code using the variables/functions that were used in this code), I'm also including the code I've written for both these tasks, which is correct, only task D needs to be answered.

Question & Code for TASK B:

Task B. Implementing Caesar cipher encryption Write a program caesar.cpp with functions implementing Caesar cipher encryption: // A helper function to shift one character by rshift char shiftChar(char c, int rshift); // Caesar cipher encryption string encryptCaesar(string plaintext, int rshift); The argument rshift is the magnitude of the right shift. For the sake of the lab, you may assume it to be in the range 0 ≤ rshift ≤ 25 (although, implementing it correctly for an arbitrary integer shift is also possible, of course). Your functions should preserve case, and any non-alphabetic characters should be left unchanged. For example, encryptCaesar("Way to Go!", 5) == "Bfd yt Lt!" Feel free to write more additional helper functions when you need them. The main function should ask the user to input a plaintext sentence, then enter the right shift, and report the ciphertext computed using your encryption function.

Code for Task B:

#include <iostream> #include <string> using namespace std; char shiftChar(char character, int rShift); string encryptCaesar(string textPlain, int rShift); int main() {    string userInput; cout << "Enter plaintext: ";    getline(cin, userInput);       int encryptionKey;    cout << "Enter shift : "; cin >> encryptionKey;    cout << "Ciphertext : " << encryptCaesar(userInput, encryptionKey) << endl;        /*            This piece of int main will ask for user input, store the user input, create            an ecryption key and encrypt it using Caesar.        */ } char shiftChar(char character, int rShift) { if(character >= 'A' && character <= 'Z') { character = 'A' + (character - 'A' + rShift + 26) %26; } if(character >= 'a' && character <= 'z') { character = 'a' + (character-'a' + rShift + 26) %26; } return character; /*        This piece of code will implement the Caesar shift, it will push characters within        a certain boundary to the next character in line. */ } string encryptCaesar(string userInput, int rShift) { string endResult = ""; char character; for(int i = 0; i < userInput.length(); ++i) { endResult += shiftChar(userInput[i], rShift); } return endResult; }

Question for Task C:

Write a program vigenere.cpp. It should contain a function encryptVigenere implementing this cipher: string encryptVigenere(string plaintext, string keyword); You may assume that the keyword contains only lowercase alphabetic characters a - z. The main should implement a testing interface similar to the one in Task B, the user enters the plaintext and the keyword, and the program reports the ciphertext. Example: $ ./vigenere Enter plaintext: Hello, World! Enter keyword : cake Ciphertext : Jevpq, Wyvnd!

Code for Task C:

#include <iostream> #include <string> using namespace std; string encryptVigenere(string textPlain, string encryptionKey); int main() {    string textPlain; cout << "Enter plaintext: ";    getline(cin, textPlain);    string encryptionKey;    cout << "Enter keyword: "; cin >> encryptionKey;    cout << "Ciphertext: " << encryptVigenere(textPlain, encryptionKey) << endl;          /*        This part of int main function will get user input, it will store our values and        create an ecryption key to later decipher.    */ } string encryptVigenere(string textPlain, string keyword) { string endResult = ""; char character; int ind = 0, encryptionKey; //Starting our nested if loops inside a for loop. for(int k = 0; k < textPlain.length(); ++k) { character = textPlain[k];       if(character >= 'A' && character <= 'Z')    { encryptionKey = keyword[ind] - 'a'; ind = (ind + 1) % keyword.length(); character = 'A' + (character - 'A' + encryptionKey + 26) %26; } if(character >= 'a' && character <= 'z')    { encryptionKey = keyword[ind] - 'a'; ind = (ind + 1) % keyword.length(); character = 'a' + (character - 'a' + encryptionKey + 26) %26; } endResult += character; } return endResult; /*        This wall of text is the code that actually implements the encryption, it takes        characters within a certain boundary and then pushes them however many ASCII        characters necessary. */ }

Once again, please only answer Task D, and please use the strings/variables/functions that were used in Tasks B & C so that the code lines up. Including comments so that I can follow along with would be greatly appreciated. Thank you for the help!

Answer 1

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Solution :-

C++ code :- I have tried to use most of your existing code and modify it accordingly.

#include <iostream>
#include <string>

using namespace std;

// shift key function used for encryption
char shiftChar(char character, int rShift)
{
if(character >= 'A' && character <= 'Z')
{
character = 'A' + (character - 'A' + rShift + 26) %26;
}
if(character >= 'a' && character <= 'z')
{
character = 'a' + (character-'a' + rShift + 26) %26;
}

return character;

}

// shift key function used for decryption
// instead of adding rshift subtract it for the decryption
char shiftCharDecrypt(char character, int rShift)
{
if(character >= 'A' && character <= 'Z')
{
character = 'A' + (character - 'A' - rShift + 26) %26;
}
if(character >= 'a' && character <= 'z')
{
character = 'a' + (character-'a' - rShift + 26) %26;
}
return character;
}

string encryptCaesar(string userInput, int rShift)
{
string endResult = "";
char character;
for(int i = 0; i < userInput.length(); ++i)
{
endResult += shiftChar(userInput[i], rShift);
}
return endResult;
}

// decryption part is almost same with little change in shift key function
string decryptCaesar(string cipherText, int rShift)
{
string endResult = "";
char character;
for(int i = 0; i < cipherText.length(); ++i)
{
endResult += shiftCharDecrypt(cipherText[i], rShift);
}
return endResult;
}

// Vigenere encryption
string encryptVigenere(string textPlain, string keyword)
{
string endResult = "";

char character;
int ind = 0, encryptionKey;

//Starting our nested if loops inside a for loop.
for(int k = 0; k < textPlain.length(); ++k)
{
character = textPlain[k];
  
if(character >= 'A' && character <= 'Z')
{
encryptionKey = keyword[ind] - 'a';
ind = (ind + 1) % keyword.length();
character = 'A' + (character - 'A' + encryptionKey + 26) %26;
}
if(character >= 'a' && character <= 'z')
{
encryptionKey = keyword[ind] - 'a';
ind = (ind + 1) % keyword.length();
character = 'a' + (character - 'a' + encryptionKey + 26) %26;
}
endResult += character;
}
return endResult;

}

// Vigenere decryption, instead of adding keyword subtract it
// almost same as encryption
string decryptVigenere(string cipherText, string keyword)
{
string endResult = "";

char character;
int ind = 0, decryptionKey;

for(int k = 0; k < cipherText.length(); ++k)
{
character = cipherText[k];
  
if(character >= 'A' && character <= 'Z')
{
decryptionKey = keyword[ind] - 'a';
ind = (ind + 1) % keyword.length();
character = 'A' + (character - 'A' - decryptionKey + 26) %26;
}
if(character >= 'a' && character <= 'z')
{
decryptionKey = keyword[ind] - 'a';
ind = (ind + 1) % keyword.length();
character = 'a' + (character - 'a' - decryptionKey + 26) %26;
}
endResult += character;
}
return endResult;
  
}
int main()
{
string plainText;
cout << "Enter plaintext: ";
  
getline(cin, plainText);

string choice;
cout<<"choose from below to proceed :-\n";
cout<<"caesar : to apply caesar cipher encryption-decryption\n";
cout<<"vigenere : to apply vigenere algo encryption-decryption\n";

cin>>choice;

if(choice == "caesar")
{
int encryptionKey;
  
cout << "Enter shift : ";
cin >> encryptionKey;
  
cout << "Ciphertext : " << encryptCaesar(plainText, encryptionKey) << endl;
cout<<"Decrypted :- "<<decryptCaesar(encryptCaesar(plainText,encryptionKey), encryptionKey) << endl;
}
else
{
string encryptionKey;
  
cout << "Enter keyword: ";
cin >> encryptionKey;
  
cout << "Ciphertext: " << encryptVigenere(plainText, encryptionKey) << endl;   

cout<<"Decrypted :- "<<decryptVigenere(encryptVigenere(plainText,encryptionKey), encryptionKey) << endl;
}
  
  
}