vigenere cipher

[robdb]

can someone tell me how this program works? I'm trying to learning C++ by reading some examples, and this one is one of them.

#include <iostream>
#include <string>
using namespace std;
 
class Vigenere
{
public:
  string key;
 
  Vigenere(string key)
  {
    for(int i = 0; i < key.size(); ++i)
    {
      if(key[i] >= 'A' && key[i] <= 'Z')
        this->key += key[i];
      else if(key[i] >= 'a' && key[i] <= 'z')
        this->key += key[i] + 'A' - 'a';
    }
  }
 
  string encrypt(string text)
  {
    string out;
 
    for(int i = 0, j = 0; i < text.length(); ++i)
    {
      char c = text[i];
 
      if(c >= 'a' && c <= 'z')
        c += 'A' - 'a';
      else if(c < 'A' || c > 'Z')
        continue;
 
      out += (c + key[j] - 2*'A') % 26 + 'A'; 
      j = (j + 1) % key.length();
    }
 
    return out;
  }
 
  string decrypt(string text)
  {
    string out;
 
    for(int i = 0, j = 0; i < text.length(); ++i)
    {
      char c = text[i];
 
      if(c >= 'a' && c <= 'z')
        c += 'A' - 'a';
      else if(c < 'A' || c > 'Z')
        continue;
 
      out += (c - key[j] + 26) % 26 + 'A'; 
      j = (j + 1) % key.length();
    }
 
    return out;
  }
};
 
int main()
{
  Vigenere cipher("VIGENERECIPHER");
 
  string original = "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!";
  string encrypted = cipher.encrypt(original);
  string decrypted = cipher.decrypt(encrypted);
 
  cout << original << endl;
  cout << "Encrypted: " << encrypted << endl;
  cout << "Decrypted: " << decrypted << endl;
}

thanks

[Cross213]

This is a very complex example if you are only learning. I'd suggest very basic programs to start out with like the infamous "Hello world". Also read some programmer books for beginners.

[gusano79]

For reference, this is what it's supposed to be doing: Vigenère cipher

What programming background do you have?

--smg

[robdb]

I've made only some ns2 scripts. Anyway, I know the vigenere cipher, but i don't understand how the algorithm works.

[gusano79]

I've made only some ns2 scripts. Anyway, I know the vigenere cipher, but i don't understand how the algorithm works.

I'm not sure what exactly you mean by that; the cipheris the algorithm. Is there something specific about the code that is giving you trouble?

Here's a quick summary of what each function in the Vigenere class is doing: Vigenere constructs a new cipher with the given key. All that if/else business makes it ignore parts of the key that aren't letters, and capitalizes everything.
encrypt does what it says on the box. More if/else to ignore characters that aren't letters and capitalize the ones that are. Note that the actual cipher is happening in the out += ... line; this is the algebraic version of the cipher.
Same goes for decrypt ; it's the algebraic version again.