Re: Using principal wildcards in gpg.ssh.allowedSignersFile

From: Fabian Stelzer <fs@gigacodes.de>
Date: Dec 17, 2021, 09:42 UTC
Message-ID: <20211217094235.i2fwildp7rcjcgtz@fs>
In-Reply-To: <87zgoziwfo.fsf@gentoo.org>

On 17.12.2021 00:20, Matthias Maier wrote:

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>Dear all,
>
>I am experimenting with git version 2.34.1 (and OpenSSH 8.8_p1) a bit
>trying to set up a repository with SSH signatures for commits instead of
>pgp. I have also tested the current "git next" branch.
>
>The straight-forward setup (by having an "allowed_signers" file
>naming individual e-mails and pubkeys) works as anticipated.
>
>However, when trying to combine this with an SSH certificate authority
>(which would be the use case I have in mind) I am not able to use an
>e-mail wildcard in the "allowed_signers" file but have to specify full
>e-mails instead. This, unfortunately, defeats a bit the purpose of
>having an SSH certificate authority in the first place...
>

Thanks for your report. I tested the described behaviour and I think this is a bug in openssh. find-principals will never match on a CA cert with wildcard principals whereas wildcards for non-CA keys work just fine. I've emailed the openssh maintainer about it and will prepare a patch.

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>Steps to reproduce:
>
>====================
>Set up a minimal CA:
>====================
>
>  $ mkdir /tmp/signing-test
>  $ cd /tmp/signing-test
>
>
>A)  Set up two test pubkeys:
>
>  $ ssh-keygen -t ed25519 -C "ca key" -f id_ca
>  [...]
>  $ ssh-keygen -t ed25519 -C "user key" -f id_user
>  [...]
>
>
>B)  Sign user key creating an SSH certificate:
>  [...]
>
>C)  Create allowed signers file:
>
>  $ (printf '*@43-1.org cert-authority,namespaces="file,git" '; cat id_ca.pub) > allowed_signers
>
>  ! Important: I used a wild card "*@43-1.org" for the principal!
>
>
>D) Test setup:
>
>  $ echo this is some random text > test.txt
>  $ ssh-keygen -Y sign -f id_user-cert.pub -n file test.txt
>  Signing file test.txt
>  Write signature to test.txt.sig
>
>  $ ssh-keygen -Y find-principals -f allowed_signers -n file -s test.txt.sig
>  tamiko@43-1.org

Are you sure the allowed_signers file was exactly what you generated before for this command? If I follow your steps this will not produce a principal for me with neither openssh-8.8.1, nor master. Can you run this with `-vvv` which will show a bit more ssh internal output? In the openssh code for find-principals wildcard principals are filtered for CA certs. I'm not sure why and have asked them about it.

By the way, find-principals will not consider the namespace parameter. This has another bug in the current master producing a segfault for which I've already sent a patch. But this should be unrelated to your issue.

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>
>  $ ssh-keygen -Y verify -f allowed_signers -I "tamiko@43-1.org" -n file -s test.txt.sig < test.txt
>  Good "file" signature for tamiko@43-1.org with ED25519-CERT key SHA256:noSSfVeVlrYi6vGgK+jRPvyBnIV4ccVA0iW4IXYdXDQ
>
>
>=======================
>Set up a git repository
>=======================
>
>E) Set up an empty repository somewhere
>
>  $ cd /tmp
>  $ git init signing-test-repo
>  $ cd signing-test-repo
>
>  and modify .git/config to look like this:
>
>        [core]
>                repositoryformatversion = 0
>                filemode = true
>                bare = false
>                logallrefupdates = true
>        [commit]
>                gpgsign = true
>        [user]
>                signingkey = /tmp/signing-test/id_user-cert.pub
>        [gpg]
>                format = ssh
>        [gpg "ssh"]
>                allowedSignersFile = /tmp/signing-test/allowed_signers
>
>
>F) make a commit
>
>  $ git commit -a --allow-empty -m "my shiny new ssh key signed commit"
>
>  $ git log --show-signature
>  Good "git" signature with ED25519-CERT key SHA256:noSSfVeVlrYi6vGgK+jRPvyBnIV4ccVA0iW4IXYdXDQ
>  /tmp/signing-test/allowed_signers:1: no valid principals found
>  No principal matched.
>  Author: Matthias Maier <tamiko@43-1.org>
>  Date:   Mon Dec 13 23:51:03 2021 -0600

Just FYI: if you add GIT_TRACE=1 to the git commands you can see the executed ssh-keygen commands, which can help to see whats going on.

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>
>
>G) modify allowd_signers entry to read "tamiko@43-1.org" instead of the wildcard "*@43-1.org":
>
>  $ git log --show-signature
>  Good "git" signature for tamiko@43-1.org with ED25519-CERT key SHA256:noSSfVeVlrYi6vGgK+jRPvyBnIV4ccVA0iW4IXYdXDQ
>  Author: Matthias Maier <tamiko@43-1.org>
>  Date:   Mon Dec 13 23:51:03 2021 -0600