[Bug 2000276] Re: FIDO2 user verification impossible when using the ssh agent

[Andreas Hasenack]

I believe this issue is correct. I noticed this when writing up the
documentation[1] on how to use openssh with fido2 resident keys:

"""
NOTE
If you used the -O verify-required option when generating the keys, or if that option is set on the SSH server via /etc/ssh/sshd_config’s PubkeyAuthOptions verify-required, then using the agent currently in Ubuntu 22.04 LTS won’t work.
"""

I remember I found an upstream bug about this, but it is a bit muddy
because there is ssh-agent from ssh, and there is one from gnome (and I
think from gpg-agent as well; again, muddy).

If I find those pointers again, I'll post them here.


1. https://ubuntu.com/server/docs/service-openssh

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https://bugs.launchpad.net/bugs/2000276

Title:
  FIDO2 user verification impossible when using the ssh agent

Status in openssh package in Ubuntu:
  New

Bug description:
  I am having trouble setting up FIDO2 ssh keys with my yubikey 5C NFC
  if I want to enable user verification (user presence works fine).

  
  Steps to reproduce:

  = Prep work =

  Client (kinetic):
  * generate a key that requires user verification:
    $ ssh-keygen -t ed25519-sk -f ~/.ssh/id_ed25519_verify_sk -O verify-required -C "this key requires UV"
    [provide your authenticator PIN, touch it, and add an encryption password]

  Server (jammy):
  * add id_ed25519_verify_sk.pub to authorized_keys

  = Symptoms =

  Shell 1 (w/ssh agent):

  $ eval $(ssh-agent)
  Agent pid 3279738

  $ ssh-add ~/.ssh/id_ed25519_verify_sk
  Enter passphrase for /home/aieri/.ssh/id_ed25519_verify_sk: 
  Identity added: /home/aieri/.ssh/id_ed25519_verify_sk (this key requires UV)

  $ ssh ubuntu at 10.35.202.231 -i ~/.ssh/id_ed25519_verify_sk "echo FIDO2 fails\!"
  sign_and_send_pubkey: signing failed for ED25519-SK "/home/aieri/.ssh/id_ed25519_verify_sk" from agent: agent refused operation
  ubuntu at 10.35.202.231: Permission denied (publickey).

  [note that the above is printed immediately, and that the yubikey does
  not light up]

  Shell 2 (no ssh agent):

  $ ssh ubuntu at 10.35.202.231 -i ~/.ssh/id_ed25519_verify_sk "echo FIDO2 works\!"
  Enter passphrase for key '/home/aieri/.ssh/id_ed25519_verify_sk': 
  Enter PIN for ED25519-SK key /home/aieri/.ssh/id_ed25519_verify_sk: 
  Confirm user presence for key ED25519-SK SHA256:nhgS4c2rGtE7XKeez3rAsofrjJvsL6rmBLShZxfTXIY
  User presence confirmed
  FIDO2 works!


  NOTE: 
  * user _presence_ can be validated correctly with or without the ssh-agent: keys generated without `-O verify-required` work as expected (aside from bug 1869897)

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