DKIM failures for gitlab mail

[Bryan Richter]

Thanks for noticing, Joachim!

Ben Gamari is still the primary contact for GitLab configuration...
Ben, maybe you know something about this?

On Wed, Nov 30, 2022 at 7:12 PM Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:
>
> On Wed, Nov 30, 2022 at 05:33:44PM +0100, Joachim Breitner wrote:
>
> > I noticed that a small number of Gitlab notification emails end up in
> > my spamfilter. While there is not much you can do about triggering some
> > bayesian style spam filter at my email provider (mailbox.org), I did
> > notice this in the headers:
> >
> > X-Spam-Status: No, score=2.704 tagged_above=2 required=6
> >         tests=[DKIM_INVALID=0.1, DKIM_SIGNED=0.1, HS_RSPAMD_10_11=2.5,
> >         HTML_MESSAGE=0.001, SPF_HELO_NONE=0.001, SPF_NONE=0.001,
> >         URIBL_BLOCKED=0.001] autolearn=no autolearn_force=no
> > Authentication-Results: spamfilter01.heinlein-hosting.de (amavisd-new);
> >         dkim=fail (1024-bit key) reason="fail (bad RSA signature)"
> >         header.d=gitlab.haskell.org
> > DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
> > d=gitlab.haskell.org;
> >         s=mail; t=1669733134;
> >         bh=D0NUcHiskEnwSP99umP3zo8Fz8fl74OgAJ8NRDKCsp4=;
> >         h=Date:From:Reply-To:To:In-Reply-To:References:Subject:List-Id:
> >          List-Unsubscribe;
> >         b=R+WMLfhRZZdYxMd6K6w+iodDe8EHzwONNArNyboqsU5NnafPRhKZ1UeGxO/BCMvEK
> >          M7XHRRrBsPfRYpTph7xSGY427KGXieASVg1GDhAiwKSLBCiqDdkBaoJLLUIfUD02NS
> >          ouI3tvQ9mddNdaEK7retq8N+29hzs/ezf9cpgy+Q=
>
> Indeed the signature in "b=" was not made by the key at
> mail._domainkey.gitlab.haskell.org.  Running the below:
>
>     sig=$(
>         printf "%s\n%s\n%s\n" \
>             R+WMLfhRZZdYxMd6K6w+iodDe8EHzwONNArNyboqsU5NnafPRhKZ1UeGxO/BCMvE \
>             KM7XHRRrBsPfRYpTph7xSGY427KGXieASVg1GDhAiwKSLBCiqDdkBaoJLLUIfUD0 \
>             2NSouI3tvQ9mddNdaEK7retq8N+29hzs/ezf9cpgy+Q=
>         )
>
>     pkey=$(
>         dig +short -t txt mail._domainkey.gitlab.haskell.org |
>         perl -MMIME::Base64 -ne '
>             /^"v=DKIM1;/ or next;
>             print decode_base64($1) if m{;\s*p=(\S+?)(?:;|$)}
>             ' |
>         openssl pkey -pubin -inform DER
>         )
>
>     openssl rsautl -raw -encrypt -pubin \
>         -inkey <( printf "%s\n" "$pkey" ) \
>         -in <(printf "%s\n" "$sig" | openssl base64 -d) |
>         xxd -p
>
> the output is:
>
>     509bfc93a492f1b5328308e51624d9a7ed1378861f577b11413c5034bc0c
>     673d61660434d4bc30844e7648da0f9605923805973a313a8c3bc82215cc
>     ac447e47551087c544a0592ac3ae48474584bad7d9ca5b850a67493a7977
>     d28aaa3a9a7580d165dc4f31ff484bdbc40e94a2be1750e71c51c555b5c1
>     6bc051947bb07ae4
>
> Which is not a PKCS#1.5 padded signature block.  So either the
> "b=" value was corrupted in transit, or it was signed by a key
> that is different from what is published in DNS.
>
> > but maybe Postfix  is not using the right key?
>
> Strictly speaking that's not Postfix itself, but some DKIM milter, but
> nits aside, more likely a stale public key is published in DNS.
>
> --
>     Viktor.
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