[TLS] Re: FYI: Ubuntu 26.04.01 GnuTLS ML-DSA sigalg friction

[Dmitry Belyavsky <[e-mail]>]

I'll try to ask GnuTLS people

SY, Dmitry Belyavsky

Dne st 2. 9. 2026 20:13 uživatel Viktor Dukhovni <[e-mail]>
napsal:

> An Exim MTA SMTP client on Ubuntu 26.04.01 with default settings is
> unable to negotiate a TLS connection to my SMTP server 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.
>
> My server has both RSA and ML-DSA keys and selects the latter when
> both are offered by the client.
>
> For the specific case reported, Exim falls back to cleartext (the sending
> system has not enabled client-side DANE) so I am still able to receive
> email from that system.  Other Ubuntu 26.04.01 configurations may in
> fact be unable to send me mail without explicitly overriding the sigalg
> list. :-(
>
> I hope this combination will not become entrenched.
>
> --
>     Viktor.  🇺🇦 Слава Україні!
>
>     (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    }
>
> The full backtrace is:
>
>     (gdb) bt
>     #0  ml_dsa_verify (algo=GNUTLS_PK_MLDSA65, signature=0x7fff41dbfab0,
> message=0x7fff41dbfac0, raw_pub=0x55c364b1a430)
>         at nettle/../../../lib/nettle/pk.c:1849
>     #1  _wrap_nettle_pk_verify (algo=GNUTLS_PK_MLDSA65,
> vdata=0x7fff41dbfac0, signature=0x7fff41dbfab0, pk_params=0x55c364b1a398,
>         sign_params=0x7fff41dbfa00) at nettle/../../../lib/nettle/pk.c:2719
>     #2  0x00007fb19bbb0639 in pubkey_verify_data (se=se@entry=0x7fb19bd2d830
> <sign_algorithms.lto_priv+2800>,
>         me=me@entry=0x7fb19bd2cbf0 <hash_algorithms.lto_priv+1456>,
> data=data@entry=0x7fff41dbfac0,
>         signature=signature@entry=0x7fff41dbfab0, params=params@entry=0x55c364b1a398,
> sign_params=sign_params@entry=0x7fff41dbfa00,
>         flags=33619968) at ../../lib/pubkey.c:2695
>     #3  0x00007fb19bbb32af in gnutls_pubkey_verify_data2
> (pubkey=0x55c364b1a390, algo=<optimized out>, flags=33619968,
>         data=0x7fff41dbfac0, signature=0x7fff41dbfab0) at
> ../../lib/pubkey.c:2209
>     #4  0x00007fb19bbc7e42 in _gnutls13_handshake_verify_data
> (session=0x55c364b20b90, verify_flags=33619968, cert=0x7fff41dbfad0,
>         context=0x7fb19bd277c0 <srv_ctx>, signature=0x7fff41dbfab0,
> se=<optimized out>) at ../../lib/tls13-sig.c:132
>     #5  _gnutls13_recv_certificate_verify (session=0x55c364b20b90) at
> ../../lib/tls13/certificate_verify.c:128
>     #6  0x00007fb19bb5f0f8 in _gnutls13_handshake_client
> (session=0x55c364b20b90) at ../../lib/handshake-tls13.c:131
>     #7  0x00007fb19bb77bcb in handshake_client (session=0x55c364b20b90) at
> ../../lib/handshake.c:3094
>     #8  gnutls_handshake (session=0x55c364b20b90) at
> ../../lib/handshake.c:2914
>     #9  0x000055c35a22ea0c in do_handshake (socket=socket@entry=0x7fff41dc5200)
> at ../../src/cli.c:1856
>     #10 0x000055c35a22f97c in socket_open_int (hd=hd@entry=0x7fff41dc5200,
> hostname=0x7fff41dc7826 "mx1.imrryr.org",
>         service=service@entry=0x55c35a2b6ea0 <service> "25",
> app_proto=0x7fff41dc77ef "smtp", app_hostname=0x0,
>         flags=flags@entry=68, rdata=0x0, edata=0x0, server_trace=0x0,
> client_trace=0x0, msg=0x55c35a238750 "Connecting to")
>         at ../../src/socket.c:627
>     #11 0x000055c35a228e4e in main (argc=<optimized out>, argv=<optimized
> out>) at ../../src/cli.c:1366
>
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