code: func (g *GPG) SigningKeys() []openpgp.Key {
  	keys := []openpgp.Key{}
  	for _, e := range g.secring {
  		for _, subKey := range e.Subkeys {
  			if subKey.PrivateKey != nil && (!subKey.Sig.FlagsValid || subKey.Sig.FlagSign) {
  				keys = append(keys, openpgp.Key{
  					Entity:        e,
  					PublicKey:     subKey.PublicKey,
  					PrivateKey:    subKey.PrivateKey,
  					SelfSignature: subKey.Sig,
  				})
  			}
  		}
  	}
  	return keys
  }
docs: // SigningKeys returns a list of signing keys