Running key cipher implementation in C++

runningkey.cpp

/*  
 The Running Key  
 ----------------  
 The running key cryptographic algorithm is a classical cipher that uses a long key  
 such as random sentences from books to encrypt another message. It uses a table for  
 mapping the cipher text using the key and the plain-text.  
   
 get more info here  
   
   
 This is a simple implementation of the running key algorithm.  
   
 Author : Navin Mohan  
  e-mail: navinmohan81@gmail.com  
  
  Visit: http://www.coderew.com/c_implementation_of_runnig_key_cipher/
   
 */  
 #include <iostream>  
 #include <cctype>  
 #include <string>  
 #include <algorithm>  
   
 using namespace std;  
   
 void encrypt(string&,string&,string&); // encryption function accepting plaintext key and ciphertext references  
 char getCipher(char p,char k); //the table mapping  
 void getKeyIndex(int &i,string &key); //to repeat the key sequentially  
   
   
 int main()  
 {  
  string plaintext,key,ciphertext;  
  cout << "Enter the text:";  
  getline(cin,plaintext);  
  cout << "Enter the key(should be longer than text):";  
  getline(cin,key);  
  key.erase(remove(key.begin(), key.end(), ' '), key.end()); // remove spaces from the key text  
  if (key.length() < plaintext.length()) return 1; // exit with status 1 if the key is shorter than plaintext  
  encrypt(plaintext,key,ciphertext);  
  cout << "\nEncrypted Text is:" <<endl;  
  cout << ciphertext << endl;  
  return 0;  
 }  
   
 void encrypt(string &plaintext,string &key,string &cipher)  
 {  
  string::iterator it1,it2; // string iterators  
  cipher = ""; // intitialize the ciphertext variable to NULL  
  for(it1 = key.begin(),it2 = plaintext.begin();it2 < plaintext.end();){  
   if (!isalpha(*it2)) { // omit the non alphabetic chars in plain text  
    it2++;  
    continue;  
   }  
   cipher += getCipher(tolower(*it2),tolower(*it1)); // get encrypted char  
   it1++;  
   it2++;  
  }  
 }  
   
   
 char getCipher(char p,char k)  
 {  
  int cipher;  
  cipher = p + k;  
  if (cipher >= 219) return (char) (cipher - 123);  
  return (char)(cipher-97);  
 }