Django string signing in JavaScript (using String object prototypes)

django-string-signing.js

/// Makes use of “hashes.js” -- q.v. https://git.io/fjsmQ

(function (hashes, undefined) {
    
    var SIGNATURE_LENGTH = 27,
        SIGNATURE_SALT = 'django.core.signing.Signer',
        SIGNATURE_SECRET = 'YO DOGG',
        SIGNATURE_SEPARATOR = ':',
        SIGNATURE_DJANGO_SALT = 'signer';
    
    String.prototype.chomp = (function (regex, undefined) {
        return function () {
            var arg = arguments[0] || undefined,
                chomper = (arg === undefined)
                    ? regex
                    : new RegExp("(" + arg + ")+$");
            return this.replace(chomper, '');
        };
    })(/(\s|\n|\r)+$/);
    
    if (hashes !== undefined) {
        /// Django-compatible string signatures
        
        String.prototype.signature = (function (SHA1, padding) {
            return function (key_secret, salt) {
                var sha1 = new SHA1({ utf8: false, b64pad: padding }),
                    key = sha1.raw(salt + key_secret),
                    hmac64 = sha1.setPad(padding).b64_hmac(key, this).chomp();
                return hmac64.replace(/\//g, '_').replace(/\+/g, '-'); /// Replacements are a Django thing
            };
        })(hashes.SHA1, ' ');
        
        String.prototype.sign = function () {
            var key_secret = arguments[0] || SIGNATURE_SECRET,
                key_salt = arguments[1] || SIGNATURE_SALT,
                sep = arguments[2] || SIGNATURE_SEPARATOR,
                salt = key_salt + SIGNATURE_DJANGO_SALT,
                signature = this.signature(key_secret, salt);
            return (this + sep + signature);
        };
        
        String.prototype.unsign = function () {
            var key_secret = arguments[0] || SIGNATURE_SECRET,
                key_salt = arguments[1] || SIGNATURE_SALT,
                sep = arguments[2] || SIGNATURE_SEPARATOR,
                salt = key_salt + SIGNATURE_DJANGO_SALT,
                split = this.split(sep),
                sig = split.pop(),
                orig = split.join(sep),
                signature = orig.signature(key_secret, salt);
            return sig == signature ? orig : false;
        };
        
        String.prototype.voidsign = function () {
            var sep = arguments[0] || SIGNATURE_SEPARATOR,
                split = this.split(sep),
                sig = split.pop(),
                orig = split.join(sep);
            return (this.indexOf(sep) !== -1 && sig.length === SIGNATURE_LENGTH) ? orig : this.toString();
        };
        
    }
    
})(window['Hashes']);