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C++ Program to Implement Affine Cipher

This is a C++ Program to implement Affine Cipher. The affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which. As such, it has the weaknesses of all substitution ciphers. Each letter is enciphered with the function (ax+b)mod(26), where b is the magnitude of the shift.

Here is source code of the C++ Program to Implement Affine Cipher. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include<iostream>
```
```
#include<string.h>
```
```
#include<stdlib.h>
```
```
using namespace std;
```
```
string encryptionMessage(string Msg)
```
```
{
```
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    string CTxt = "";
```
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    int a = 3;
```
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    int b = 6;
```

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

```
    {
```

        CTxt = CTxt + (char) ((((a * Msg[i]) + b) % 26) + 65);

```
    }
```
```
    return CTxt;
```
```
}
```
```
 
```
```
string decryptionMessage(string CTxt)
```
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{
```
```
    string Msg = "";
```
```
    int a = 3;
```
```
    int b = 6;
```
```
    int a_inv = 0;
```
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    int flag = 0;
```
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    for (int i = 0; i < 26; i++)
```
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    {
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        flag = (a * i) % 26;
```
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        if (flag == 1)
```
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        {
```
```
            a_inv = i;
```
```
        }
```
```
    }
```

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

```
    {
```

        Msg = Msg + (char) (((a_inv * ((CTxt[i] - b)) % 26)) + 65);

```
    }
```
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    return Msg;
```
```
}
```
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int main(int argc, char **argv)
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{
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    cout << "Enter the message: ";
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    string message;
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    cin >> message;
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    cout << "Message is :" << message;
```

    cout << "\nEncrypted Message is : " << encryptionMessage(message);

```
 
```

    cout << "\nDecrypted Message is: " << decryptionMessage(

            encryptionMessage(message));

```
}
```

Output:

$ g++ AffineCipher.cpp
$ a.out
 
Enter the message: SANFOUNDRY
Message is :SANFOUNDRY
Encrypted Message is : VTGIJBGCSN
Decrypted Message is: SANFOUNDRY
------------------
(program exited with code: 0)
Press return to continue