code: func (k *Kite) AuthenticateSimpleKiteKey(key string) (string, error) {
  	claims := &kitekey.KiteClaims{}
  
  	token, err := jwt.ParseWithClaims(key, claims, k.verify)
  	if err != nil {
  		return "", err
  	}
  
  	if !token.Valid {
  		return "", errors.New("Invalid signature in token")
  	}
  
  	if claims.Subject == "" {
  		return "", errors.New("token has no username")
  	}
  
  	return claims.Subject, nil
  }
docs: // AuthenticateSimpleKiteKey authenticates user from the given kite key and
  // returns the authenticated username. It's the same as AuthenticateFromKiteKey
  // but can be used without the need for a *kite.Request.