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Maniphest T5931

OpenSSH 8.9, 9.0, and 9.1 can't authenticate with gpg-agent and usb token (Gnuk >= 1.2.16 is required)
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Description

Since updating openssh to 8.9p1-1 (on archlinux), I can't authenticate via gpg-agent using a USB hardware token.

gpg-agent shows the pinentry dialog, but then ssh says:
sign_and_send_pubkey: signing failed for ED25519 "cardno:***" from agent: agent refused operation

Downgrading to openssh-8.8p1-1 fixes it.

Updating gnupg from 2.2.32 to 2.2.33 or 2.2.34 (by manually editing the gnupg PKGBUILD since the archlinux version is out of date) and doing "gpgconf --kill gpg-agent", "gpgconf --launch gpg-agent" did not fix it.

I have this in my environment:
SSH_AUTH_SOCK=/run/user/1000/gnupg/S.gpg-agent.ssh

and enable-ssh-support in ~/.gnupg/gpg-agent.conf
and it has worked fine until now.

I'm guessing from comments on the archlinux bug report that this is related: https://www.openssh.com/agent-restrict.html

I'm reporting here on the assumption that the ABI/API break in openssh's agent API is intended and gnupg needs fixing to handle it (though the breakage on openssh's part clearly sucks for users)!

Details

External Link
https://bugs.archlinux.org/task/74143
Version
2.2.34

Revisions and Commits

rG GnuPG
rG46d62d80a2b8 ssh: Returned faked response for the new session-bind extension.

Related Objects

Event Timeline

amalon created this task.Apr 14 2022, 9:17 AM
amalon created this object in space S1 Public.
• werner added a commit: rG46d62d80a2b8: ssh: Returned faked response for the new session-bind extension..Apr 14 2022, 12:33 PM
• werner triaged this task as High priority.Apr 14 2022, 12:36 PM
• werner added a project: gnupg (gpg23).
• werner added a subscriber: • werner.

I have not yet tested OpenSSH 9 and thus the patch to master is here just as a test. Please better use gnupg 2.3 (stable) instead of 2.2 (LTS) because it is unlikely that we will backport all this new ssh stuff.

• werner mentioned this in T5743: Release GnuPG 2.3.5.Apr 21 2022, 5:59 PM
thesamesam added a subscriber: thesamesam.Apr 25 2022, 6:24 PM
• werner lowered the priority of this task from High to Normal.Apr 28 2022, 8:55 AM
• werner added a project: Restricted Project.
amalon added a comment.Apr 28 2022, 9:16 AM

FYI, I built 2.3.6 using a modified archlinux PKGBUILD (& disabling patches to avoid conflicts), then did:
gpgconf --kill gpg-agent
gpgconf --launch gpg-agent
but ssh still fails as before

dkg added a subscriber: dkg.Apr 29 2022, 6:24 PM

this looks similar to https://dev.gnupg.org/T5935 and https://bugs.debian.org/1008573

• gniibe added a subscriber: • gniibe.May 2 2022, 1:53 AM

Please describe what token is used. For my use cases with rGe8fb8e2b3e66: scd: Don't inhibit SSH authentication for larger data if it can., both of Gnuk (>= 1.2.16) and Yubikey (>= 5) work well.

amalon added a comment.May 2 2022, 10:36 PM

Its a nitrokey start. I gave it another spin just to make sure, and again when updating to openssh 9.0 and "gpg (GnuPG) 2.3.6-unknown", it fails (again with careful gpgconf --kill gpg-agent etc. Double checked the downloaded source code by arch's makepkg, appears to have that patch applied. Also tried adding -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com to the ssh command, which didn't help.

I don't know what other relevant info about the card i can give. It has ed25519 keys on it.

• gniibe added a comment.May 3 2022, 10:43 AM

Nitrokey Start uses Gnuk as its firmware. You need to upgrade its firmware to version 1.2.16 or newer.
Please note that when upgrading the firmware, your keys will be removed.

• gniibe claimed this task.Jun 1 2022, 5:09 AM
• gniibe edited projects, added Documentation; removed Restricted Project.Jul 12 2022, 3:26 AM

Changed the tags and the title.

• gniibe renamed this task from OpenSSH 8.9 and 9.0 can't authenticate with gpg-agent and usb token to OpenSSH 8.9 and 9.0 can't authenticate with gpg-agent and usb token (Gnuk >= 1.2.16 is required).Jul 12 2022, 3:26 AM
• alexk added a project: workaround.Nov 9 2022, 10:50 AM
• alexk added a subscriber: • alexk.

A workaround you can add the following line to ~/.ssh/config or /etc/ssh/ssh_config:

KexAlgorithms -sntrup761x25519-sha512@openssh.com

For me ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com ... does work as well.

amalon added a comment.Nov 9 2022, 7:40 PM
In T5931#165009, @alexk wrote:

A workaround you can add the following line to ~/.ssh/config or /etc/ssh/ssh_config:

KexAlgorithms -sntrup761x25519-sha512@openssh.com

For me ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com ... does work as well.

Neither of these options work for me on openssh-9.1p1-3 (archlinux), I've been sticking to 8.8p1-1 since upgrading the firmware is a faff that I haven't got around to.

• gniibe added a comment.Nov 22 2022, 8:12 AM

I tested with openssh 9.1. When I add -o PubkeyAuthentication=unbound, I can make the length of data smaller.

• gniibe added a comment.Nov 24 2022, 2:22 AM

In my cases (tested with 9.1), here are the length of data to be signed by ssh-agent (emulation by gpg-agent).

  • 164 bytes: Both features disabled by: ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com -o PubkeyAuthentication=unbound
  • 192 bytes: Unbound only by: ssh -o PubkeyAuthentication=unbound
  • 298 bytes: No Post Quantum only by: ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com
  • 330 bytes: Both features enabled (no options)
• gniibe renamed this task from OpenSSH 8.9 and 9.0 can't authenticate with gpg-agent and usb token (Gnuk >= 1.2.16 is required) to OpenSSH 8.9, 9.0, and 9.1 can't authenticate with gpg-agent and usb token (Gnuk >= 1.2.16 is required).Nov 24 2022, 2:38 AM
amalon added a comment.Nov 24 2022, 9:01 PM

Thanks. Adding 'PubkeyAuthentication unbound' to my ~/.ssh/config seems to workaround it for me on openssh-9.1p1-3 (arch). I don't quite follow what the implications of that setting are though.

• gniibe added a comment.Nov 25 2022, 12:54 AM

Implications are... you won't be possible to use new protocols introduced by newer OpenSSH:

  • PubkeyAuthentication: No use of "public-key-hostbound-v00@openssh.com" which is OpenSSH specific extension, but use plain "publickey" authentication
    • With the extension, ssh-agent can see the server's host key when it is asked to generate signature, so, it can reject signing based on the destination.
    • This feature cannot be used when using plain "publickey" authentication
• werner added a project: gnupg24.Dec 13 2022, 11:21 AM