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)
}