Vigenere Cypher (Cryptography)

#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;

const string chars = "abcdefghijklmnopqrstuvwxyz !?.";

string encode(string input, const string key)
{
    string output;
    int pos[2];
    for(int c = 0; c < input.length(); c++)
    {
        pos[0] = chars.find( tolower(input[c]) );
        pos[1] = chars.find( tolower(key[c]) );
        if(pos[0] != string::npos && pos[1] != string::npos)
        {
            output.append( chars.substr( ((pos[0] + pos[1]) % 30), 1) );
        }
    }
    return output;
}

string decode(string input, const string key)
{
    string output;
    int pos[2];
    int p;
    for(int c = 0; c < input.length(); c++)
    {
        pos[0] = chars.find( tolower(input[c]) );
        pos[1] = chars.find( tolower(key[c]) );
        if(pos[0] != string::npos && pos[1] != string::npos)
        {
            p = (pos[0] - pos[1]);
            if(p < 0) p = 30+p;
            output.append( chars.substr( p, 1) );
        }
    }
    return output;
}

int main()
{
    string message =    "hello world";
    const string key =  "thelemonade";
    string encoded;

    cout << "Original -> " << message << endl;
    encoded = encode(message, key);
    cout << "Encoded -> " << encoded << endl;
    cout << "Decoded -> " << decode(encoded, key) << endl;

    return 1;
}