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<a href="https://gitlab.com/rufferson">Ruslan Marchenko</a> created an issue <a href="https://gitlab.com/gnutls/gnutls/-/issues/1041">#1041</a>:
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<h2 dir="auto">
<a id="user-content-description-of-problem" class="anchor" href="#description-of-problem" aria-hidden="true"></a>Description of problem:</h2>
<p dir="auto">When using TLS1.3 connection and attempting to retrieve tls-unique binding data the
gnutls_session_channel_binding() call succeeds however returns empty data (zero size buffer).
Looking at the code - the call intercepts finished message only on lib/handshake.c but not in lib/tls13/finished.c
Technically you don't need to store finished in tls13 as it could always be computed, which is not done in this call.</p>
<h2 dir="auto">
<a id="user-content-version-of-gnutls-used" class="anchor" href="#version-of-gnutls-used" aria-hidden="true"></a>Version of gnutls used:</h2>
<p dir="auto">3.6.14-1</p>
<h2 dir="auto">
<a id="user-content-distributor-of-gnutls-eg-ubuntu-fedora-rhel" class="anchor" href="#distributor-of-gnutls-eg-ubuntu-fedora-rhel" aria-hidden="true"></a>Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)</h2>
<p dir="auto">ArchLinux</p>
<h2 dir="auto">
<a id="user-content-how-reproducible" class="anchor" href="#how-reproducible" aria-hidden="true"></a>How reproducible:</h2>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext"> gnutls_datum_t cb;</span>
<span id="LC2" class="line" lang="plaintext"> int ret = gnutls_session_channel_binding (priv->session, GNUTLS_CB_TLS_UNIQUE, &cb);</span>
<span id="LC3" class="line" lang="plaintext"></span>
<span id="LC4" class="line" lang="plaintext"> if (ret == GNUTLS_E_SUCCESS)</span>
<span id="LC5" class="line" lang="plaintext"> {</span>
<span id="LC6" class="line" lang="plaintext"> if (data != NULL)</span>
<span id="LC7" class="line" lang="plaintext"> {</span>
<span id="LC8" class="line" lang="plaintext"> g_tls_log_debug (gnutls, "tls-unique binding size %d", cb.size);</span>
<span id="LC9" class="line" lang="plaintext"> g_free (g_byte_array_steal (data, NULL));</span>
<span id="LC10" class="line" lang="plaintext"> g_byte_array_append (data, cb.data, cb.size);</span>
<span id="LC11" class="line" lang="plaintext"> }</span>
<span id="LC12" class="line" lang="plaintext"> g_free (cb.data);</span>
<span id="LC13" class="line" lang="plaintext"> return TRUE;</span>
<span id="LC14" class="line" lang="plaintext"> }</span></code></pre>
<p dir="auto">Steps to Reproduce:</p>
<ul dir="auto">
<li>run with G_TLS_GNUTLS_PRIORITY='NORMAL:%COMPAT:!VERS-TLS1.3' - result is</li>
</ul>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">(/home/ruff/co/glib-networking/_build/tls/tests/connection-gnutls:430951): GLib-Net-DEBUG: 11:27:43.445: CLIENT[0x55b24b896440]: tls-unique binding size 12</span>
<span id="LC2" class="line" lang="plaintext">(/home/ruff/co/glib-networking/_build/tls/tests/connection-gnutls:430951): GLib-Net-DEBUG: 11:27:43.445: SERVER[0x55b24b8a29d0]: tls-unique binding size 12</span></code></pre>
<ul dir="auto">
<li>run with defaults (which prefers TLS1.3)</li>
</ul>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true"><code><span id="LC1" class="line" lang="plaintext">(/home/ruff/co/glib-networking/_build/tls/tests/connection-gnutls:430949): GLib-Net-DEBUG: 11:27:43.398: CLIENT[0x564054f58440]: tls-unique binding size 0</span>
<span id="LC2" class="line" lang="plaintext">(/home/ruff/co/glib-networking/_build/tls/tests/connection-gnutls:430949): GLib-Net-DEBUG: 11:27:43.398: SERVER[0x564054f649d0]: tls-unique binding size 0</span></code></pre>
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<a id="user-content-actual-results" class="anchor" href="#actual-results" aria-hidden="true"></a>Actual results:</h2>
<p dir="auto">see above</p>
<h2 dir="auto">
<a id="user-content-expected-results" class="anchor" href="#expected-results" aria-hidden="true"></a>Expected results:</h2>
<p dir="auto">tls-unique binding size 48 (at least that's what I get with OpenSSL)</p>
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