code: func (c *clientIdentityImpl) GetID() (string, error) {
  	// The leading "x509::" distinguishes this as an X509 certificate, and
  	// the subject and issuer DNs uniquely identify the X509 certificate.
  	// The resulting ID will remain the same if the certificate is renewed.
  	id := fmt.Sprintf("x509::%s::%s", getDN(&c.cert.Subject), getDN(&c.cert.Issuer))
  	return base64.StdEncoding.EncodeToString([]byte(id)), nil
  }
docs: // GetID returns a unique ID associated with the invoking identity.