code: func (h *AuthHandlers) IsUserAuthority(cert ssh.PublicKey) bool {
  	if _, err := h.authorityForCert(services.UserCA, cert); err != nil {
  		return false
  	}
  
  	return true
  }
docs: // IsUserAuthority is called during checking the client key, to see if the
  // key used to sign the certificate was a Teleport CA.