Simple Substitution Cypher

function substitutionCipher()
{
    var str = prompt("Enter a string to encrypt","").toUpperCase().replace(/[^A-Z]*/g,'');
    var pwd = prompt("Enter you Little Orphan Annie secret password","").toUpperCase().replace(/[^A-Z]*/g,'');
    var endecrypted = '';
    var alphabet = cypherbet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    var charBox = '';
    var i = len = 0;
    var encryptFlag = confirm('To encrypt, click OK. To decrypt click CANCEL.');

    /* strip duplicate letters from the password */
    for(i = 0, len = pwd.length; i < len; i++)
    {
        if(!charBox || charBox.indexOf(pwd.charAt(i)) < 0 )
        {
            charBox += pwd.charAt(i);
            cypherbet = cypherbet.replace(pwd.charAt(i),'');  /*remove the password character from the cyphered alphabet for now*/
        }
    }
    pwd = charBox;   /*replace the password with the result of stripping redundant characters from the password*/



    /*prepend the password to the cyphered alphabet*/
    cypherbet = pwd + cypherbet;


    for(i = 0, len = str.length; i < len; i++)
    {
        if(encryptFlag)
        {
            if(endecrypted && i % 5 == 0)
            {
                endecrypted += ' ';
            }
            endecrypted += cypherbet.charAt(alphabet.indexOf(str.charAt(i)));
        }else{
            endecrypted += alphabet.charAt(cypherbet.indexOf(str.charAt(i)));
        }
    }

    alert(endecrypted);
}

substitutionCipher();