func (s *SeriesData) getPrivKeyFor(pubKey *hdkeychain.ExtendedKey) (*hdkeychain.ExtendedKey, error) { for i, key := range s.publicKeys { if key.String() == pubKey.String() { return s.privateKeys[i], nil } } return nil, newError(ErrUnknownPubKey, fmt.Sprintf("unknown public key '%s'", pubKey.String()), nil) }