Vigenere

#include <bits/stdc++.h>

using namespace std;
ifstream fin("vigenere.in");
ofstream fout("vigenere.out");
string text, cod;
void encrypt_vigenere(const string& plaintext, const string& key)
{
    string ciphertext;
    unsigned int i, j, ch;
    for(i = 0, j = 0; i < plaintext.length(); ++i, j++) {
        if(j >= key.length())
            j = 0;
        if(plaintext[i] >= 'A' && plaintext[i] <= 'Z')
            ch = ((plaintext[i] - 'A') + (key[j] - 'A')) % 26;
        else
            ch = ((plaintext[i] - 'a') + (key[j] - 'a')) % 26;
        if(isupper(ch))
            ciphertext += (char)(ch + 'A');
        else
            ciphertext += (char)(ch + 'a');
     }
     fout<< ciphertext;
}
int main()
{
    fin>>text>>cod;
    encrypt_vigenere(text, cod);
    return 0;
}