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();