code: func (k *Keyring) LookupBoxSecretKey(kids [][]byte) (int, saltpack.BoxSecretKey) {
  	for i, kid := range kids {
  		if sk, ok := k.encKeys[kidToPublicKey(kid)]; ok {
  			return i, sk
  		}
  	}
  	return -1, nil
  }
docs: // LookupBoxSecretKey tries to find one of the secret keys in its keyring
  // given the possible key IDs. It returns the index and the key, if found, and -1
  // and nil otherwise.