ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

[Christian Noé Ramos López]

Two things that, together, mean an SSH signing key cannot be reliably
stopped from being trusted. git 2.47.3, OpenSSH 10.0p2, Debian 13;
source read at v2.47.3 and at master (c46c1e37724f).

1. valid-before is checked at a date the signer writes.

SSH signatures carry no time of their own, so git passes -Overify-time
from the committer or tagger line (gpg-interface.c,
parse_payload_metadata). alice's key is in the allowed signers file
with valid-before="20260101":

    ssh-old        %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
    ssh-backdated  %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
    ssh-honest     %G?=U 2026-10-08 02:15:15 -0400 verify-commit=1 merge=128

ssh-backdated was signed today, with only the committer and author
dates set to 2025-06-01. Nothing distinguishes it from ssh-old except
when it was made, which only its author knows. ssh-honest, signed and
dated today, is refused: "key has expired: verify time ... >
valid-before 2026-01-01T00:00:00".

The GPG backend refuses both:

    gpg-old        %G?=Y verify-commit=1 merge=128
    gpg-backdated  %G?=Y verify-commit=1 merge=128

The documentation for gpg.ssh.allowedSignersFile says "Git will mark
signatures as valid if the signing key was valid at the time of the
signature's creation", which is the intent, but does not say the time
comes from the commit. So valid-before rotates a key; it does not
retire one.

2. A configured revocation file that does not exist fails open.

gpg-interface.c:568-574 at v2.47.3 (579-586 at master): if the
revocation file exists, pass -r; otherwise warn and verify without it.
The same file, present and listing alice's key, refuses:

    S2-revoked       %G?=B verify-commit=1 merged=no
    S3-revfile-gone  %G?=G verify-commit=0 merged=yes
                     warning: ssh signing revocation file configured
but not found

S3 merged under `git merge --ff-only --verify-signatures`. An
unreadable file and a directory both fail closed:

    S4-revfile-0000  %G?=B verify-commit=1 merged=no
    S6-revfile-dir   %G?=B verify-commit=1 merged=no

ssh-keygen, given the same missing path, refuses: exit 255, "Could not
verify signature". git avoids that by not passing -r. OpenSSH's
RevokedKeys says, in sshd_config(5), "Note that if this file is not
readable, then public key authentication will be refused for all
users."

No test in git exercises gpg.ssh.revocationFile; it appears only in
Documentation/config/gpg.adoc and gpg-interface.c.

Controls for both runs, fixed beforehand: a good signature gives G and
merges; an unsigned commit gives N and is refused; the revocation file
present and listing the key gives B and is refused; a commit with its
message changed and the signature kept gives B and is refused.

Together: the two ways to stop trusting an SSH signing key are
valid-before, which the signer can date around, and revocationFile,
which does nothing if its path is wrong. Either would be enough on its
own if it held.

What I would ask for: refuse when the revocation file is configured and
missing, as ssh-keygen and sshd do, or say in the documentation that it
is ignored; and say, under valid-before, where the time compared
against it comes from.

On prior art: the ssh signing series (Fabian Stelzer, 2021) carried the
warning from before v4, and the review raised the config name's case,
not what a missing file should do. The key-lifetime series (RFC
2021-10-15 to v6 2021-12-09) passes the commit date to the check, and
the replies are about style. The N for an unconfigured allowed signers
file is already on the list (Grayson Tinker, 2026-06-25) and is not
part of this.

Christian Ramos
Norte Software
chris@nortesoftware.dev

Re: ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

[Phillip Wood]

Hi Christian

Having waded through this here is a human readable summary:

(1) Our documentation implies that we check the expiry date of the key 
(which is recorded in the allowed signers file) against the date the 
commit was signed, but we actually use the committer date which can 
easily be faked.

(2) If the revocation file does not exist we print a warning rather than 
failing the operation like the gpg backend does.

For (1) I'd be happy to see a patch that tightens the wording, but we 
should also note that the timestamp in the gpg signature can also be faked.

For (2) I agree failing seems like the safer option.

Thanks

Phillip

On 08/10/2026 07:56, Christian Noé Ramos López wrote:
> Two things that, together, mean an SSH signing key cannot be reliably
> stopped from being trusted. git 2.47.3, OpenSSH 10.0p2, Debian 13;
> source read at v2.47.3 and at master (c46c1e37724f).
> 
> 1. valid-before is checked at a date the signer writes.
> 
> SSH signatures carry no time of their own, so git passes -Overify-time
> from the committer or tagger line (gpg-interface.c,
> parse_payload_metadata). alice's key is in the allowed signers file
> with valid-before="20260101":
> 
>      ssh-old        %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
>      ssh-backdated  %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
>      ssh-honest     %G?=U 2026-10-08 02:15:15 -0400 verify-commit=1 merge=128
> 
> ssh-backdated was signed today, with only the committer and author
> dates set to 2025-06-01. Nothing distinguishes it from ssh-old except
> when it was made, which only its author knows. ssh-honest, signed and
> dated today, is refused: "key has expired: verify time ... >
> valid-before 2026-01-01T00:00:00".
> 
> The GPG backend refuses both:
> 
>      gpg-old        %G?=Y verify-commit=1 merge=128
>      gpg-backdated  %G?=Y verify-commit=1 merge=128
> 
> The documentation for gpg.ssh.allowedSignersFile says "Git will mark
> signatures as valid if the signing key was valid at the time of the
> signature's creation", which is the intent, but does not say the time
> comes from the commit. So valid-before rotates a key; it does not
> retire one.
> 
> 2. A configured revocation file that does not exist fails open.
> 
> gpg-interface.c:568-574 at v2.47.3 (579-586 at master): if the
> revocation file exists, pass -r; otherwise warn and verify without it.
> The same file, present and listing alice's key, refuses:
> 
>      S2-revoked       %G?=B verify-commit=1 merged=no
>      S3-revfile-gone  %G?=G verify-commit=0 merged=yes
>                       warning: ssh signing revocation file configured
> but not found
> 
> S3 merged under `git merge --ff-only --verify-signatures`. An
> unreadable file and a directory both fail closed:
> 
>      S4-revfile-0000  %G?=B verify-commit=1 merged=no
>      S6-revfile-dir   %G?=B verify-commit=1 merged=no
> 
> ssh-keygen, given the same missing path, refuses: exit 255, "Could not
> verify signature". git avoids that by not passing -r. OpenSSH's
> RevokedKeys says, in sshd_config(5), "Note that if this file is not
> readable, then public key authentication will be refused for all
> users."
> 
> No test in git exercises gpg.ssh.revocationFile; it appears only in
> Documentation/config/gpg.adoc and gpg-interface.c.
> 
> Controls for both runs, fixed beforehand: a good signature gives G and
> merges; an unsigned commit gives N and is refused; the revocation file
> present and listing the key gives B and is refused; a commit with its
> message changed and the signature kept gives B and is refused.
> 
> Together: the two ways to stop trusting an SSH signing key are
> valid-before, which the signer can date around, and revocationFile,
> which does nothing if its path is wrong. Either would be enough on its
> own if it held.
> 
> What I would ask for: refuse when the revocation file is configured and
> missing, as ssh-keygen and sshd do, or say in the documentation that it
> is ignored; and say, under valid-before, where the time compared
> against it comes from.
> 
> On prior art: the ssh signing series (Fabian Stelzer, 2021) carried the
> warning from before v4, and the review raised the config name's case,
> not what a missing file should do. The key-lifetime series (RFC
> 2021-10-15 to v6 2021-12-09) passes the commit date to the check, and
> the replies are about style. The N for an unconfigured allowed signers
> file is already on the list (Grayson Tinker, 2026-06-25) and is not
> part of this.
> 
> Christian Ramos
> Norte Software
> chris@nortesoftware.dev

Re: ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

[Patrick Steinhardt]

On Thu, Oct 08, 2026 at 02:42:42PM +0100, Phillip Wood wrote:
> Hi Christian
> 
> Having waded through this here is a human readable summary:

thanks a lot for the summary, I really appreciate it as I already lost
interest after having read the first sentence.

> (1) Our documentation implies that we check the expiry date of the key
> (which is recorded in the allowed signers file) against the date the commit
> was signed, but we actually use the committer date which can easily be
> faked.

Right. I think there isn't even a proper fix for this as we have no way
to establish the actual time the data was signed. I think this is a
simple fact in a distributed system, as without coordination there is
basically nothing that the contributor can give us that would make us
trust the claimed signature time.

You may be able to create upper bounds if there are subsequent signed
commits that you trust and that have the untrusted commit as child. But
that is not going to be always useful.

> (2) If the revocation file does not exist we print a warning rather than
> failing the operation like the gpg backend does.
> 
> For (1) I'd be happy to see a patch that tightens the wording, but we should
> also note that the timestamp in the gpg signature can also be faked.

Yeah, agreed. This mode is only safe if the signing key has never
leaked, but once it has leaked you can basically not guarantee anything
via "valid-before" and "valid-after". And documenting that would be a
good idea to not give a sense of false trustworthiness.

> For (2) I agree failing seems like the safer option.

Maybe this is another usecase where we can use the ":(optional)" prefix
that we introduced recently for some of the other pathname options? So
we'd fail by default, but give the user an escape hatch if they really
want one.

Patrick

Re: ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

[Phillip Wood]

Hi Patrick

On 09/10/2026 13:19, Patrick Steinhardt wrote:
> On Thu, Oct 08, 2026 at 02:42:42PM +0100, Phillip Wood wrote:
>>
>> Having waded through this here is a human readable summary:
> 
> thanks a lot for the summary, I really appreciate it as I already lost
> interest after having read the first sentence.
> 
>> (1) Our documentation implies that we check the expiry date of the key
>> (which is recorded in the allowed signers file) against the date the commit
>> was signed, but we actually use the committer date which can easily be
>> faked.
> 
> Right. I think there isn't even a proper fix for this as we have no way
> to establish the actual time the data was signed. I think this is a
> simple fact in a distributed system, as without coordination there is
> basically nothing that the contributor can give us that would make us
> trust the claimed signature time.

Indeed

> You may be able to create upper bounds if there are subsequent signed
> commits that you trust and that have the untrusted commit as child. But
> that is not going to be always useful.

Yes, on its own checking the signature just tells you that the commit 
was signed by that key, it does not tell you when it was signed.

>> (2) If the revocation file does not exist we print a warning rather than
>> failing the operation like the gpg backend does.
>>
>> For (1) I'd be happy to see a patch that tightens the wording, but we should
>> also note that the timestamp in the gpg signature can also be faked.
> 
> Yeah, agreed. This mode is only safe if the signing key has never
> leaked, but once it has leaked you can basically not guarantee anything
> via "valid-before" and "valid-after". And documenting that would be a
> good idea to not give a sense of false trustworthiness.
> 
>> For (2) I agree failing seems like the safer option.
> 
> Maybe this is another usecase where we can use the ":(optional)" prefix
> that we introduced recently for some of the other pathname options? So
> we'd fail by default, but give the user an escape hatch if they really
> want one.

Oh, I'd not though of that - that's a good idea

Thanks

Phillip

Re: ssh signing: valid-before is checked at the signer's own date, and a missing revocationFile fails open

[Phillip Wood]

Hi Chirstian

I've add back the the mailing list cc so others can comment as well.

On 08/10/2026 20:40, Christian Noé Ramos López wrote:
> 
>> Having waded through this here is a human readable summary:
> 
> Fair -- your summary is the shape I should have sent. Next time.
> 
> (1) You are right, and I did not test it. gpg takes the signature's own
> creation time, so a manipulated clock moves that too. What I measured is
> narrower: backdating the committer and author dates does not move it --
> ssh-backdated is accepted, gpg-backdated is refused. A difference in cost,
> not in kind.
> 
> So the wording should say two things: that the time compared against
> valid-before comes from the commit, and that a signature timestamp is not
> evidence of when the signing happened. That covers both backends. I will
> send that patch.

That sounds sensible - a valid signature doesn't tell us anything about 
when the commit was signed.

> (2) One thing before I write it. Failing closed changes behaviour for
> anyone whose configured path is already wrong, from a warning to a failed
> verification. I think that is the right trade, but it is a behaviour
> change and not only a fix. There is no test for gpg.ssh.revocationFile
> today, so the patch should add one either way.

A test would be very welcome. Patrick had a good suggestion for allowing 
the path to be optional if the user wanted.

Thanks

Phillip

> Christian Ramos
> Norte Software
> chris@nortesoftware.dev
> 
> 
> El jue, 8 oct 2026 a la(s) 7:42 a.m., Phillip Wood
> (phillip.wood123@gmail.com) escribió:
>>
>> Hi Christian
>>
>> Having waded through this here is a human readable summary:
>>
>> (1) Our documentation implies that we check the expiry date of the key
>> (which is recorded in the allowed signers file) against the date the
>> commit was signed, but we actually use the committer date which can
>> easily be faked.
>>
>> (2) If the revocation file does not exist we print a warning rather than
>> failing the operation like the gpg backend does.
>>
>> For (1) I'd be happy to see a patch that tightens the wording, but we
>> should also note that the timestamp in the gpg signature can also be faked.
>>
>> For (2) I agree failing seems like the safer option.
>>
>> Thanks
>>
>> Phillip
>>
>> On 08/10/2026 07:56, Christian Noé Ramos López wrote:
>>> Two things that, together, mean an SSH signing key cannot be reliably
>>> stopped from being trusted. git 2.47.3, OpenSSH 10.0p2, Debian 13;
>>> source read at v2.47.3 and at master (c46c1e37724f).
>>>
>>> 1. valid-before is checked at a date the signer writes.
>>>
>>> SSH signatures carry no time of their own, so git passes -Overify-time
>>> from the committer or tagger line (gpg-interface.c,
>>> parse_payload_metadata). alice's key is in the allowed signers file
>>> with valid-before="20260101":
>>>
>>>       ssh-old        %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
>>>       ssh-backdated  %G?=G 2025-06-01 12:00:00 +0000 verify-commit=0 merge=0
>>>       ssh-honest     %G?=U 2026-10-08 02:15:15 -0400 verify-commit=1 merge=128
>>>
>>> ssh-backdated was signed today, with only the committer and author
>>> dates set to 2025-06-01. Nothing distinguishes it from ssh-old except
>>> when it was made, which only its author knows. ssh-honest, signed and
>>> dated today, is refused: "key has expired: verify time ... >
>>> valid-before 2026-01-01T00:00:00".
>>>
>>> The GPG backend refuses both:
>>>
>>>       gpg-old        %G?=Y verify-commit=1 merge=128
>>>       gpg-backdated  %G?=Y verify-commit=1 merge=128
>>>
>>> The documentation for gpg.ssh.allowedSignersFile says "Git will mark
>>> signatures as valid if the signing key was valid at the time of the
>>> signature's creation", which is the intent, but does not say the time
>>> comes from the commit. So valid-before rotates a key; it does not
>>> retire one.
>>>
>>> 2. A configured revocation file that does not exist fails open.
>>>
>>> gpg-interface.c:568-574 at v2.47.3 (579-586 at master): if the
>>> revocation file exists, pass -r; otherwise warn and verify without it.
>>> The same file, present and listing alice's key, refuses:
>>>
>>>       S2-revoked       %G?=B verify-commit=1 merged=no
>>>       S3-revfile-gone  %G?=G verify-commit=0 merged=yes
>>>                        warning: ssh signing revocation file configured
>>> but not found
>>>
>>> S3 merged under `git merge --ff-only --verify-signatures`. An
>>> unreadable file and a directory both fail closed:
>>>
>>>       S4-revfile-0000  %G?=B verify-commit=1 merged=no
>>>       S6-revfile-dir   %G?=B verify-commit=1 merged=no
>>>
>>> ssh-keygen, given the same missing path, refuses: exit 255, "Could not
>>> verify signature". git avoids that by not passing -r. OpenSSH's
>>> RevokedKeys says, in sshd_config(5), "Note that if this file is not
>>> readable, then public key authentication will be refused for all
>>> users."
>>>
>>> No test in git exercises gpg.ssh.revocationFile; it appears only in
>>> Documentation/config/gpg.adoc and gpg-interface.c.
>>>
>>> Controls for both runs, fixed beforehand: a good signature gives G and
>>> merges; an unsigned commit gives N and is refused; the revocation file
>>> present and listing the key gives B and is refused; a commit with its
>>> message changed and the signature kept gives B and is refused.
>>>
>>> Together: the two ways to stop trusting an SSH signing key are
>>> valid-before, which the signer can date around, and revocationFile,
>>> which does nothing if its path is wrong. Either would be enough on its
>>> own if it held.
>>>
>>> What I would ask for: refuse when the revocation file is configured and
>>> missing, as ssh-keygen and sshd do, or say in the documentation that it
>>> is ignored; and say, under valid-before, where the time compared
>>> against it comes from.
>>>
>>> On prior art: the ssh signing series (Fabian Stelzer, 2021) carried the
>>> warning from before v4, and the review raised the config name's case,
>>> not what a missing file should do. The key-lifetime series (RFC
>>> 2021-10-15 to v6 2021-12-09) passes the commit date to the check, and
>>> the replies are about style. The N for an unconfigured allowed signers
>>> file is already on the list (Grayson Tinker, 2026-06-25) and is not
>>> part of this.
>>>
>>> Christian Ramos
>>> Norte Software
>>> chris@nortesoftware.dev
>>
> 
>