code: function yubikey__genNonceAsync(callback) {
      // Create a secure, random nonce value
      crypto.randomBytes(40, function(err, buf) {
        if (err) return callback(err);
        return callback(null, buf.toString('hex').slice(0,40));
      });
    }
docs: /*
  The nonce should be a 16- to 40-character-long string containing
  random, unique data.