[Bug 1812247] [NEW] ssh-agent fails for ssh-add -c: agent refused operation

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Ubuntu uses ssh-agent from OpenSSH which supports adding keys by means
of `ssh-add -c` indicating that keys "should be subject to confirmation
before being used for authentication. In Ubuntu 18.10 this fails with
the error

  sign_and_send_pubkey: signing failed: agent refused operation

To reproduce I used a Ubuntu 18.10 Live "CD", apt-get update && apt-get
upgrade, log out and log back in (these steps are not required but we
want to use an up-to-date system). Then:

$ sudo apt-get install ssh-askpass-gnome
(...)
$ # verify that ssh-askpass shows a popup, confirm with Enter
$ ssh-askpass ; echo $?

0
$ ssh-keygen
(...)
$ ssh-add -D
All identities removed.
$ ssh-copy-id $sshuser@$sshserver
(...)
Number of key(s) added: 1
(...)
$ ssh $sshuser@$sshserver uname -a
Linux server 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux
$ ssh-add -d
Identity removed: /home/ubuntu/.ssh/id_rsa (ubuntu at ubuntu)
$ ssh-add -c
Enter passphrase for /home/ubuntu/.ssh/id_rsa (will confirm each use): 
Identity added: /home/ubuntu/.ssh/id_rsa (/home/ubuntu/.ssh/id_rsa)
The user must confirm each use of the key
$ ssh $sshuser@$sshserver uname -a
sign_and_send_pubkey: signing failed: agent refused operation
sshuser at server's password: [^C'ed]

$ ssh-add -l
2048 SHA256:yvAFsTpkNWnlrQyCp+tWV83dIF8Je3AksM0o+Ajvyyc /home/ubuntu/.ssh/id_rsa (RSA)

So, our key is loaded, ssh-askpass is working (also confirmed with `ssh-
add -c </dev/null`), but authentication fails with
"sign_and_send_pubkey: signing failed: agent refused operation".

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: gnome-session-bin 3.30.0-0ubuntu4
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CasperVersion: 1.399
CurrentDesktop: ubuntu:GNOME
Date: Thu Jan 17 17:14:35 2019
ExecutablePath: /usr/lib/gnome-session/gnome-session-binary
LiveMediaBuild: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-session
UpgradeStatus: No upgrade log present (probably fresh install)

** Affects: openssh (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug cosmic
-- 
ssh-agent fails for ssh-add -c: agent refused operation
https://bugs.launchpad.net/bugs/1812247
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