There is an easy workaround: Append an exclamation mark to the adsk key. This way gpg will only search for this subkey.
An example with my test keys:

isn't T3456 the same issue?

OCB mode (i.e. packet 20) is only used if the keys announce it. Thus only after moving a (private) key from GnuPG to a non-GnuPG compatible implementation you will run into this problem. The compatibility options won't override the preference system.

Things for 2.4 are all done.

For 2.2 we will for now only implement the encryption.

Thanks for the description; this is good for documentation.

Sorry, I mistakenly closed this task. I reopen it.

Could it be the case that your implementation actually used those bits to calculate a public key?

Sorry for a bit late follow up. How do you calculate a public key? RNP's crypto backend, Botan, is calculating public key without taking in account bits which should be tweaked. I.e. both tweaked and non-tweaked secret keys would produce the same public key. The same is with decryption. Could it be the case that your implementation actually used those bits to calculate a public key?

To fix this we also need to fix our key selection test (key-selection.scm) which is can't cope with all combinations. The tests are run with a faked time of 2004-01-01 on all subsets of this ordered list of keys

See also T4713

So here is a redacted CLI-dump of the exact sequence I'm describing in my post. This is with untweaked keys and gpg 2.2.40 and a factory-reset yubikey.

So in case this was not clear... What I'm describing is very similar to the original description, but it is "inverted" - the untweaked key works flawlessly (import and decryption) except for keytocard. And the tweaked key can't be imported - either "Bad Secret Key" or asking for passphrase.

@onickolay Yes, I have. I have used --check-cv25519-bits and it said that it needs patching. I then did --fix-cv25519-bits and exported the key. Looking at the CV25519 private-key bytes produced by my code and by RNP, I confirmed that they did the exact same transformation.
When trying to re-import the exported key into gpg, I got the "Bad Secret Key" error again

@bigmomma Just for a quick check - did you try to use RNP's CLI command --edit-key --fix-cv25519-bits, as it's not clear from the message?

Hi! I would like to chime in on this issue as I am having some weird problems with a CV25519 sub-key and after stumbling upon this thread, I think it is related to this.
Unfortunately, I can't post the key material here, because it is my actual encryption private-key.

Another idea (based on above ideas): We add an optional column named "Valid For" and only list the usages that are currently possible. If the encryption subkey is expired, then the key will probably only be valid for signing and certifying. This should be easier to understand than my fancy color coding idea while still giving the user a hint what a certain certificate can be used for. I'd still abbreviate the usage to a single letter in English. Translators could still use more letters if a single letter would be ambiguous.

Meanwhile I have _some_ doubts that the v5 format is a good idea. It will introduce a lot of problems and thus a more lean way of replacing the fingerprint should be re-considered. Even if that means, we have to live with two kinds of fingerprints for a decade or so.

Given that the OpenPGP WG practically decided to fork OpenPGP I don't see a reason why we should keep this bug open.

We could use single letters or icons (with proper tool tip and accessible name). I'm not sure mentioning the cert usage is that useful.

Another point where this is very problematic are S/MIME certificates for signing and encryption. While the certificate line edit and the certificate combo box filter the usage, Groups are problematic. If you want to create an encryption group and include one "signing only" certificate the whole group is no longer visible for example in Outlook when encrypting. Both me and Eva thought that S/MIME Groups did not work at all in Outlook because of this.

It's in 1.18.0.

This is related to T5950: Allow viewing expired certificates more easily where a user was wondering why some key wasn't offered as encryption key. It turned out that the encryption subkey was expired.

TL;DR: can reproduce, needs fixing

In T5783#153879, @werner wrote:

Sending a private key with just the local protection is not a good idea.

Sending a private key with just the local protection is not a good idea. It is better to export the key and then send it in an encrypted mail - for example in symmetric mode with a strong password.

Fixed in GnuPG 2.3.3.

I should have explained the context.
No, there is no discussion about this in the WG.

Is that really required? Should we wait what the conlusion of the WG will be?

I'm reading RFC5297, which says:

SIV can be used as a drop-in replacement for any specification that uses [RFC3394] or [RFC3217], including the aforementioned use. It is a more general purpose solution as it allows for associated data to be specified.

Fix for this issue landed RNP master, and will be included to the RNP v0.16.0 release.
Within fix:

• new keys will be generated with correctly tweaked bits
• using secret key with non-tweaked bits would issue a warning
• CLI command --edit-key [--check-cv25519-bits | --fix-cv25519-bits] added, allowing to fix older key

As long as we can't replicate this, it does not make sense to keep this bug open. Please re-open it if you run into it again in a replicatable way.

Use of version 5 format for Ed448/X448 was pushed by rG86cb04a23d2b: gpg: Ed448 and X448 are only for v5 (for subkey)..

Bug in creating such a blob is fixed in rG08a3a4db27dc: kbx: A 20 byte fingerprint is right filled in version 2 blob..

Thanks for clarification, indeed attempt to decrypt data returns an error afterwards.

Well, while importing you get the warning:

Yes, for migration from GnuPG 2.0 reasons, a batch import delays the key checking (i.e. converting from OpenPGP to GnuPG internal format) to the first use. Thus you don't see an error immediately. But if you encrypt something , you won't be able to decrypt it again:

Thanks, Werner.
During further work on this got another issue:

Thanks. I meanwhile pushed a fix to 2.3 so that a warning is shown if the low bits are set.

Thanks for the replies, this makes things clear. We'll update RNP to correctly set/unset those bits while saving a generated secret key and a way to fix up previously generated keys.

Right, as long as there is only one format in widespread use (based on a long existing 4880bis draft) only this format should go over the wire.
Thus, it is a matter how the key is exported. In cryptography you should never have several options - one clearly defined format is what you want. We have had enough trouble with PGP5 peculiarities but in that case their implementation had more users and thus GnuPG had to work around it. Not good, but there was no standard at all at this time.

@onickolay No sorry needed. It was me, who cannot answer promptly.

@gniibe sorry for pinging, but this issue gets attention as TB users (with RNP OpenPGP backend) cannot import to GnuPG EdDSA secret key which was generated by RNP since it doesn't tweak bits when storing or exporting a secret key.
Should we update RNP to tweak those bits during storage to be more compatible (given that those bits doesn't make any difference)?

Do I correctly understand that issue will be resolved on GnuPG side by tweaking key bits before private-key import/and/or/operations?

We should not support a different OID or representation of 22519 which will only lead to incompatibilities and trouble existing users. 25519 is in too widespread use than to allow for any changes.

@werner isn't it used just for the public key? The secret x25519 key, exported by GnuPG, looks as following (in the way it is stored in file):

We invented the 0x40 compression flag to declare that as native curve point format. With the introduction of 448 things got more complicated due to the new IETF statdards for this curev. This is the reason for @gniibe's proposal for a Simple Octet String (SOS) as a new data type in OpenPGP.

Investigated it more, and it looks problem is not in incorrect endianness. Exporting x25519 secret subkey from the GnuPG showed up that we still need to change byte order.
After some experiments I ended up with the following self-explaining code piece, which makes RNP-generated keys to work with GnuPG for import:

repeat:
    if (botan_privkey_create(&pr_key, "Curve25519", "", rng_handle(rng))) {
        goto end;
    }
    /* botan returns key in little-endian, while mpi is big-endian */
    if (botan_privkey_x25519_get_privkey(pr_key, keyle.data())) {
        goto end;
    }
    if ((keyle[31] != 0x45) || (keyle[0] != 0x40)) {
        botan_privkey_destroy(pr_key);
        goto repeat;
    }
    if (botan_privkey_export_pubkey(&pu_key, pr_key)) {
        goto end;
    }

Thanks for investigations! Indeed, we do change byte order when storing/loading private key, as MPI should be big-endian, while curve25519 private key is little endian.
Do I correctly understand that we should store it in the MPI as it is (like with Ed25519)? It would be nice to clarify that in the RFC draft.
Another thing is that in my test even if byte order is not reversed in the secret key (including the attached test key), GnuPG still asks for password, reporting "error sending to agent: Bad passphrase".

The problem here appears to be that the "MPI" of the curve25519 secret key is not actually a standard-issue big-endian OpenPGP MPI -- it's an opaque bytestring expected to be passed to the underlying "native" implementation of x25519, in the same way that the secret key is handled for Ed25519.

investigating the subkey in python: