C++ Program to Implement the Vigenere Cypher

Vigenere Cipher is a kind of polyalphabetic substitution method of encrypting alphabetic text.
Vigenere Cipher Table is used in which alphabets from A to Z are written in 26 rows, for encryption and decryption in this method.
Key: WELCOME
Message: Thisistutorialspoint
Here we have to obtain a key by repeating the given key till its length becomes equal to original message length.
For encryption take first letter of message and key i.e. T and W. Take the alphabet in Vigenere Cipher Table where T row and W column coincides i.e. P.
Repeat the same process for all remaining alphabets in message text.
Finally, the encrypted message text is −
Encrypted Message: PLTUWEXQXZTWMPOTZKBF.
The cipher text can be generated by below equation.
Ei = (Pi + Ki) mod 26
Here P is plain text and K is key.
Key: WELCOME
Encrypted Message: PLTUWEXQXZTWMPOTZKBF
Take generated key and first alphabet of encrypted message i.e. P and W. Analyze Vigenere Cipher Table, look for alphabet P in column W, the corresponding row will be the first alphabet of original message i.e. T.
Repeat this process for all the alphabets in encrypted message.
Original Message: Thisistutorialspoint
This can be represented in algebraic form by following equation.
Pi = (Ei – Ki + 26) mod 26
Here is a C++ program to implement Vigenere cipher.
Begin
   Function encryption(string t)
   for i = 0, j = 0 to t.length() - 1
      char c = t[i]
      if (c >= 'a' and c <= 'z')
         c = c + 'A' - 'a'
      else if (c < 'A' or c > 'Z')
         continue
      output = output + (c + k[j] ) % 26 + 'A'
      j = (j + 1) % k.length()
   return output
End
Begin
   Function decryption(string t)
   for i = 0, j = 0 to t.length() - 1
      char c = t[i]
      if (c >= 'a' and c <= 'z')
         c = c + 'A' - 'a'
      else if (c < 'A' or c > 'Z')
         continue
      output =output + (c - k[j] + 26) % 26 + 'A'
      j = (j + 1) % k.length()
   return output
End
#include <iostream>
#include <string>
using namespace std;
class Vig {
   public:
      string k;
   Vig(string k) {
      for (int i = 0; i < k.size(); ++i) {
         if (k[i] >= 'A' && k[i] <= 'Z')
            this->k += k[i];
         else if (k[i] >= 'a' && k[i] <= 'z')
            this->k += k[i] + 'A' - 'a';
      }
   }
   string encryption(string t) {
      string output;
      for (int i = 0, j = 0; i < t.length(); ++i) {
         char c = t[i];
         if (c >= 'a' && c <= 'z')
            c += 'A' - 'a';
         else if (c < 'A' || c > 'Z')
            continue;
         output += (c + k[j] - 2 * 'A') % 26 + 'A'; //added 'A' to bring it in range of ASCII alphabet [ 65-90 | A-Z ]
         j = (j + 1) % k.length();
      }
      return output;
   }
   string decryption(string t) {
      string output;
      for (int i = 0, j = 0; i < t.length(); ++i) {
         char c = t[i];
         if (c >= 'a' && c <= 'z')
            c += 'A' - 'a';
         else if (c < 'A' || c > 'Z')
            continue;
         output += (c - k[j] + 26) % 26 + 'A';//added 'A' to bring it in range of ASCII alphabet [ 65-90 | A-Z ]
         j = (j + 1) % k.length();
      }
      return output;
   }
};
int main() {
   Vig v("WELCOME");
   string ori ="Thisistutorialspoint";
   string encrypt = v.encryption(ori);
   string decrypt = v.decryption(encrypt);
   cout << "Original Message: "<<ori<< endl;
   cout << "Encrypted Message: " << encrypt << endl;
   cout << "Decrypted Message: " << decrypt << endl;
}
Original Message: Thisistutorialspoint
Encrypted Message: PLTUWEXQXZTWMPOTZKBF
Decrypted Message: THISISTUTORIALSPOINT