Question

*Below is a project I have been assigned (in which I am struggling to understand) in my Computer Science 150 class*

It must be done in:

- C++

- void functions

Here is the project: Write a Program the Decodes a Secret Message

The secret message will be encoded using an affine cipher. The cipher works by replacing the letter represented by the numeric value p (‘a’ = 0, ‘b’ = 1,…) with the letter represented by the numeric value (a*p + k) % 26, where a and k are constants (note, gcd(a, 26) = 1). The encoded message can then be decoded by replacing the encoded letters q with the expression (A*q + A*K) % 26, where A is the multiplicative inverse of a modulo 26 and K is the additive inverse of k modulo 26; i.e. (a*A) % 26 = 1 (k + K) % 26 = 0.

When a = 1 this type of cipher is known as a Caeser cipher which is simply a shift of the alphabet. Input will consist of a file with a and k on the first line (space separated) and an affine encoded message (only lowercase letters and spaces) on the second line with an added ‘0’ to denote the end of the message (note, the included input file is just one example, a and k are not fixed). Do not use features of C++ (like arrays & classes) covered after this program’s assignment date.

Write a C++ program that includes functions for each of the following tasks (use reference and value parameters appropriately based on the conventions of our class):

- Prompts user for file name

-reads in for the file name

-associates the input file (with validation) to an ifstream variable

-Reads in a and k from file line 1

-Computes and prints A and K Using A and K from (3.)

-decodes and prints the message from file line 2 (character by character)

-Example: Input File (input.txt): 3 1 erojd urzogd zq vrja uartabl buunokd b dnhang lnddbtn gr gwn zoujg qzin0

-Program Output: What is the name of the file? input 9 225 bonus points if your program appends a secret message to the input file.

*I am honestly very confused and don't know where to start with this program, than you so much for your help*

Answer 1

PROGRAM

#include<bits/stdc++.h>

using namespace std;

const int a = 17;

const int b = 20;

  

string encryptMessage(string msg)

{

string cipher = "";

for (int i = 0; i < msg.length(); i++)

{

if(msg[i]!=' ')

cipher = cipher +

(char) ((((a * (msg[i]-'A') ) + b) % 26) + 'A');

else

cipher += msg[i];

}

return cipher;

}

  

string decryptCipher(string cipher)

{

string msg = "";

int a_inv = 0;

int flag = 0;

(int i = 0; i < 26; i++)

{

flag = (a * i) % 26;

  

if (flag == 1)

{

a_inv = i;

}

}

for (int i = 0; i < cipher.length(); i++)

{

if(cipher[i]!=' ')

msg = msg +

(char) (((a_inv * ((cipher[i]+'A' - b)) % 26)) + 'A');

else

msg += cipher[i];

}

  

return msg;

}

int main(void)

{

string msg = "AFFINE CIPHER";

  

string cipherText = encryptMessage(msg);

cout << "Encrypted Message is : " << cipherText<<endl;

cout << "Decrypted Message is: " << decryptCipher(cipherText);

  

return 0;

}

OUTPUT:

Encode Message is : UBBAHK CAPJKX

Decode Message is : AFFINE CIPHER