Public bug reported:
All of the information contained in this bug report was collected and provided
by Viktor Dukhovni.
I wish to express my thanks to him for digging deep into this issue.
I quote Viktor:
> An Exim MTA SMTP client on Ubuntu 26.04.01 with default settings is unable to
> negotiate a TLS
> connection to [an] SMTP server [presenting both RSA and ML-DSA keys] because
> its GnuTLS library
> advertises ML-DSA-{44,65,87} sigalgs even though the implementation of the
> corresponding
> verification algorithms in libnettle is stubbed out and always fails."
The issue might well be in 'libnettle8t64', but manifests itself in
GnuTLS.
I quote Viktor:
> Running in an ubuntu 26.04.1 container, with debug packages and source I see
> that the GnuTLS
> library in question advertises ML-DSA-{44,65,87} signature algorithms, only
> to perversely hit
> an always failing stub implementation:
>
> (gdb) fr 0
> #0 ml_dsa_verify (algo=GNUTLS_PK_MLDSA65, signature=0x7fff41dbfab0,
> message=0x7fff41dbfac0, raw_pub=0x55c364b1a430)
> at nettle/../../../lib/nettle/pk.c:1849
> 1849 return
> gnutls_assert_val(GNUTLS_E_UNSUPPORTED_SIGNATURE_ALGORITHM);
> (gdb) l -
> 1844 static int ml_dsa_verify(gnutls_pk_algorithm_t algo MAYBE_UNUSED,
> 1845 const gnutls_datum_t *signature
> MAYBE_UNUSED,
> 1846 const gnutls_datum_t *message MAYBE_UNUSED,
> 1847 const gnutls_datum_t *raw_pub MAYBE_UNUSED)
> 1848 {
> 1849 return
> gnutls_assert_val(GNUTLS_E_UNSUPPORTED_SIGNATURE_ALGORITHM);
> 1850 }
A way to see this fail is running this gnutls-cli command:
$ gnutls-cli -d 99 --no-ca-verification --starttls-proto=smtp
--priority NORMAL:-CTYPE-CLI-ALL:+CTYPE-SRV-RAWPK "mx1.imrryr.org:25"
ProblemType: Bug
DistroRelease: Ubuntu 26.04
Package: gnutls-bin 3.8.12-2ubuntu1.1
ProcVersionSignature: Ubuntu 7.0.0-30.30-generic 7.0.12
Uname: Linux 7.0.0-30-generic x86_64
ApportVersion: 2.34.1-0ubuntu0.1
Architecture: amd64
CasperMD5CheckResult: unknown
Date: Sat Sep 5 12:18:16 2026
SourcePackage: gnutls28
UpgradeStatus: Upgraded to resolute on 2026-03-01 (188 days ago)
** Affects: gnutls28 (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug resolute
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https://bugs.launchpad.net/bugs/2166568
Title:
Ubuntu 26.04.01 GnuTLS ML-DSA sigalg friction
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