code: func (pc *PublicCertificates) CheckSignature(key string, signed, signature []byte) error {
  	cert, err := pc.CertificateForKey(key)
  	if err != nil {
  		return err
  	}
  	return cert.CheckSignature(x509.SHA256WithRSA, signed, signature)
  }
docs: // CheckSignature returns nil if `signed` was indeed signed by given key.