code: func (ch Challenge) ExpectedKeyAuthorization(key *jose.JSONWebKey) (string, error) {
  	if key == nil {
  		return "", fmt.Errorf("Cannot authorize a nil key")
  	}
  
  	thumbprint, err := key.Thumbprint(crypto.SHA256)
  	if err != nil {
  		return "", err
  	}
  
  	return ch.Token + "." + base64.RawURLEncoding.EncodeToString(thumbprint), nil
  }
docs: // ExpectedKeyAuthorization computes the expected KeyAuthorization value for
  // the challenge.