Cannot specify filename when encrypting + signing

[WordsmithCreativity]

In both v2 and v3 of the library there is no possibility to specify a filename when both signing and encrypting a file.

This would mimic the behavior of gpg as such:

$ gpg --encrypt --sign -r XXX test.txt
$ gpg --list-packets test.txt.gpg
...
:literal data packet:
	mode b (62), created 1721297550, name="test.txt",
	raw data: 9 bytes
...
:signature packet: algo 1, keyid XXX
...

I've altered the https://github.com/ProtonMail/gopenpgp/blob/main/helper/sign_detached.go#L14 function slightly to allow my use-case as such:

func SignAndEncryptWithFileName(publicKey string, privateKey string, passphrase []byte, plaintext string, filename string) (string, error) {
	var privateKeyObj, unlockedKeyObj *crypto.Key
	var publicKeyRing, privateKeyRing *crypto.KeyRing
	var pgpMessage *crypto.PGPMessage
	var ciphertext string

	var message = crypto.NewPlainMessageFromFile([]byte(plaintext), filename, uint32(crypto.GetUnixTime()))

	publicKeyObj, err := crypto.NewKeyFromArmored(publicKey)
	if err != nil {
		return "", err
	}

	publicKeyRing, err = crypto.NewKeyRing(publicKeyObj)
	if err != nil {
		return "", err
	}

	if privateKeyObj, err = crypto.NewKeyFromArmored(privateKey); err != nil {
		return "", errors.New("gopenpgp: unable to read key")
	}

	if unlockedKeyObj, err = privateKeyObj.Unlock(passphrase); err != nil {
		return "", errors.New("gopenpgp: unable to unlock key")
	}
	defer unlockedKeyObj.ClearPrivateParams()

	if privateKeyRing, err = crypto.NewKeyRing(unlockedKeyObj); err != nil {
		return "", errors.New("gopenpgp: unable to create new keyring")
	}

	if pgpMessage, err = publicKeyRing.Encrypt(message, privateKeyRing); err != nil {
		return "", errors.New("gopenpgp: unable to encrypt message")
	}

	if ciphertext, err = pgpMessage.GetArmored(); err != nil {
		return "", errors.New("gopenpgp: unable to armor ciphertext")
	}

	return ciphertext, nil
}

Can this be made possible via the exposed functions?

[inputvalidation]

I'm also experiencing this issue, any chance this could be implemented? Or do you otherwise welcome a PR to introduce this?

[clncy]

+1 I'm also interested in this feature

[inputvalidation]

I'm also experiencing this issue, any chance this could be implemented? Or do you otherwise welcome a PR to introduce this?

I would appreciate some feedback on this if you have a minute?

[lubux]

Hi 👋 The name field in the literal data packet is not included in signatures and ,thus, is malleable. GopenPGP follows RFC 9580's recommendation:

It SHOULD set the filename to the empty string (encoded as a single zero octet) and the timestamp to zero (encoded as four zero octets).

[clncy]

Hi @lubux, thank you for your response. Despite the inherent problems with this field, it would be useful to be able to populate this field to maintain compatibility with clients accustomed to the output of gnupg. Would it be possible to support this feature ONLY when the RFC4880 profile is specified?

Per RFC4880:

An implementation MAY consider the file name in the Literal packet to be a more authoritative name than the actual file name.