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<a href="https://gitlab.com/gstrauss">Glenn Strauss</a> created an issue: <a href="https://gitlab.com/gnutls/gnutls/-/work_items/1828">#1828</a>
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<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">gnutls server negotiates X25519 instead of X25519MLKEM768 unless FFDHE also configured in priority string</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">Test client:
<code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>$ openssl s_client -connect 127.0.0.1:443 2>&1 | grep -i x25519</code></p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">With priority string:<br style="margin-top: 0px;">
<code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>"NORMAL:-GROUP-ALL:+GROUP-X25519-MLKEM768:+GROUP-X25519"</code><br>
the test client result is
<code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>Peer Temp Key: X25519, 253 bits</code></p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">With priority string:<br style="margin-top: 0px;">
<code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>"NORMAL:-GROUP-ALL:+GROUP-X25519-MLKEM768:+GROUP-X25519:+GROUP-FFDHE2048"</code><br>
the test client result is
<code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>Negotiated TLS1.3 group: X25519MLKEM768</code></p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">The expected result for both cases is <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>Negotiated TLS1.3 group: X25519MLKEM768</code></p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">System: Fedora 43<br style="margin-top: 0px;">
GnuTLS: gnutls 3.8.12</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">This issue was originally reported 9 months ago as part of <a href="https://gitlab.com/gnutls/gnutls/-/work_items/1713" title="Inconsistent/illogical behaviour in GnuTLS server when selecting group for the connection" class="gfm gfm-work_item" data-original="https://gitlab.com/gnutls/gnutls/-/work_items/1713" data-link="false" data-link-reference="true" data-work-item="168402013" data-project="179611" data-iid="1713" data-namespace-path="gnutls/gnutls" data-project-path="gnutls/gnutls" data-work-item-type="issue" data-container="body" data-placement="top" data-reference-type="work_item" style="margin-top: 0px;">#1713</a> but I believe this issue is worth filing separately.</p>
<p dir="auto" style="color: #3a383f; margin: 0px 0px 1rem;" align="initial">I am a lighttpd developer. In the upcoming release of lighttpd 1.4.83, lighttpd mod_gnutls default priority string includes <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>"-GROUP-ALL:+GROUP-X25519-MLKEM768:+GROUP-X25519:P-256:P-384:X448"</code> and a TLS connection uses X25519 instead of X25519MLKEM768 for the key exchange. That is not what is intended for clients supporting PQC hybrid KEM.</p>
<p dir="auto" style="color: #3a383f; margin: 0px;" align="initial">The behavior with GnuTLS is different than with other TLS libraries. With other TLS libraries using an equivalent OpenSSL syntax like <code style='font-size: 90%; color: #18171d; word-wrap: break-word; background-color: #ececef; border-radius: .25rem; margin-top: 0px; font-weight: inherit; overflow-wrap: break-word; white-space: break-spaces; font-family: "GitLab Mono", "JetBrains Mono", "Menlo", "DejaVu Sans Mono", "Liberation Mono", "Consolas", "Ubuntu Mono", "Courier New", "andale mono", "lucida console", monospace; font-variant-ligatures: none; word-break: keep-all; padding: 0.125rem 0.25rem;'>"X25519MLKEM768:X25519:P-256:P-384:X448"</code>, the upcoming lighttpd 1.4.83 properly negotiates X25519MLKEM768 with the client. These other TLS libraries include OpenSSL, BoringSSL, AWS-LC, NSS, WolfSSL.</p>
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