input : text (cipher/plain), key; output : (cipher/plain) text [plaintext: lowercase, ciphertext: UPPERCASE]

shift_cipher.cpp

#include <iostream>
#include <vector>

using namespace std;

string shift_cipher_encrypt (string plaintext, int shift) {
      string ciphertext;
      for (int i = 0; i < plaintext.size(); i ++) {
            //lower case letters
            if (plaintext[i] >= 97 && plaintext[i] <= 122) {
                  if ((plaintext[i] + shift) > 122)
                        ciphertext.push_back((((plaintext[i] + shift) % 122) + 96) - 32 );
                  else 
                        ciphertext.push_back((plaintext[i] + shift) - 32);
            }
            else if (isdigit(plaintext[i])) ciphertext.push_back(plaintext[i]);
      }
      return ciphertext;
}

string shift_cipher_decrypt (string ciphertext, int shift) {
      string plaintext;
      for (int i = 0; i < ciphertext.size(); i ++) {
            //upper case letters
            if (ciphertext[i] >= 65 && ciphertext[i] <= 90) {
                  if ((ciphertext[i] - shift) < 65)
                        plaintext.push_back((91 - (65 - (ciphertext[i] - shift))) + 32);
                  else 
                        plaintext.push_back ((ciphertext[i] - shift) + 32);
            }

            else if (isdigit(ciphertext[i])) plaintext.push_back(ciphertext[i]);

      }
      return plaintext;
}

int main () {
      string plaintext, ciphertext;
      int shift, choice;
      cout << "Enter 1 to Encrypt, 2 to Decrypt: ";
      cin >> choice;
      if (choice == 1) {
            cout << "Enter plaintext: ";
            cin >> plaintext;
            cout << "Enter Shift: ";
            cin >> shift;
            cout << shift_cipher_encrypt (plaintext, shift) << "\n";
      }
      else if (choice == 2) {
            cout << "Enter ciphertext: ";
            cin >> ciphertext;
            cout << "Enter Shift: ";
            cin >> shift;
            cout << shift_cipher_decrypt (ciphertext, shift) << "\n";
      }

      else {
            cout << "Enter valid choice\n";
      }
      
      return 0;
}