code: func (c *CertChecker) Authenticate(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
  	err := validate(key)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	perms, err := c.CertChecker.Authenticate(conn, key)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	return perms, nil
  }
docs: // Authenticate checks the validity of a user certificate.
  // a value for ServerConfig.PublicKeyCallback.