> On 04.11.26 11:21, A. Schulze via mailop wrote:
> >accedently I found a message having 8 DKIM signatures. My DKIM
> >verifier (OpenDKIM) wrote a log "ignoring 2 signatures". Challenge
> >accepted! I also found "ignoring 53 signatures"! *)
> >
> >The number of messages having *many* DKIM signatures is very low. But
> >the exist.
> >And for that reason it's good to have a limit like this:
> >https://manpages.debian.org/trixie/opendkim/opendkim.conf.5.en.html#MaximumSignaturesToVerify
>
> >This is also mentioned in RFC 6376, Section 4.2
> >
> >>To limit potential denial-of-service attacks, Verifiers MAY limit the total
> >>number of signatures they will attempt to verify
> >
> >Have a minute to check that your DKIM validation implementation have
> >limits implemtent and activated ...
>
> I just want to point out that the default for this is according to opendkim
> manpage "3" which I believe should be enough most of the time.
>
> Also, the same doc indicates that signature for domain in From: is always
> verified.
To add to this good information (above) is that two common reasons
for having multiple DKIM keys is:
1. to support re-signing existing DKIM keys in parallel with the old
ones (usually only for a short during)
2. to support newer ED-signed DKIM keys in parallel with RSA-signed
DKIM keys so as not to exclude older systems that don't support these
newer algorithms
RSA 2048-bit DKIM keys are the most common, although many systems are
still using only RSA 1024-bit DKIM keys (these DKIM keys should be
upgraded to at least 2048-bit as soon as possible).
ED25519 and ED448 keys are newer, but not as widely supported,
although I hope this will cease to be a problem soon. While ED25519
has the smallest key size, ED448 has a slightly larger key size with
the benefit of better security (both are a lot smaller than the RSA
keys, which is also much nicer for DNS record maintenance).
I suspect that as more mail systems begin to transition to supporting
ED-signed DKIM keys that the presence of multiple DKIM keys will also
become more common as long as there is a significant quantity of mail
SMTP servers that only support checking RSA-signed DKIM keys.
The following Linux script I wrote generates all these keys, just in
case anyone is interested in seeing the differences, and also
possibly deploying these:
#!/bin/bash
# DKIM key generation script by Randolf Richardson
# in Beautiful British Columbia, Canada
export DATE=`date +'%Y%m%d'`
# RSA (modern OpenSSL versions default to 2048-bit)
echo openssl genpkey -algorithm rsa -out dkim_private_${DATE}rsa.pem
echo openssl pkey -in dkim_private_${DATE}rsa.pem -pubout -out
dkim_public_${DATE}rsa.pem
# ED25519
echo openssl genpkey -algorithm ed25519 -out dkim_private_${DATE}ed25519.pem
echo openssl pkey -in dkim_private_${DATE}ed25519.pem -pubout -out
dkim_public_${DATE}ed25519.pem
# ED448
echo openssl genpkey -algorithm ed448 -out dkim_private_${DATE}ed448.pem
echo openssl pkey -in dkim_private_${DATE}ed448.pem -pubout -out
dkim_public_${DATE}ed448.pem
--
Postmaster - [email protected]
Randolf Richardson, CNA - [email protected]
Inter-Corporate Computer & Network Services, Inc.
Vancouver, Beautiful British Columbia, Canada
https://www.inter-corporate.com/
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