code: function createPSCrypto(password, callback) {
          crypto.pbkdf2(password, SALT, NUM_ITERATIONS, KEY_LENGTH, DIGEST, function (err, derivedKey) {
              if (err) {
                  callback(err, null);
              } else {
                  callback(null, new PSCrypto(derivedKey));
              }
          });
      }
docs: Factory Functions -----------------