code: func (x *XC) findDecryptionKey(hdr *xcpb.Header) (*keyring.PrivateKey, error) {
  	for _, pk := range x.secring.KeyIDs() {
  		if _, found := hdr.Recipients[pk]; found {
  			return x.secring.Get(pk), nil
  		}
  	}
  	return nil, fmt.Errorf("no decryption key found for: %+v", hdr.Recipients)
  }
docs: // findDecryptionKey tries to find a suiteable decryption key from the available
  // decryption keys and the recipients