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<a href="https://gitlab.com/nmav">Nikos Mavrogiannopoulos</a>
commented on a
discussion on <a href="https://gitlab.com/gnutls/gnutls/merge_requests/1097#note_241321055">lib/auth/vko_gost.c</a>:
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<pre style="margin: 0;">+<span id="LC141" class="line" lang="c"> <span class="k" style="font-weight: 600;">goto</span> <span class="n" style="color: #333;">cleanup</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC142" class="line" lang="c"> <span class="p">}</span></span>
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<td class="diff-line-num new new_line" data-linenumber="143" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
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<pre style="margin: 0;">+<span id="LC143" class="line" lang="c"></span>
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<td class="diff-line-num new new_line" data-linenumber="144" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
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<pre style="margin: 0;">+<span id="LC144" class="line" lang="c"> <span class="k" style="font-weight: 600;">if</span> <span class="p">(</span><span class="n" style="color: #333;">info</span> <span class="o" style="font-weight: 600;">!=</span> <span class="nb" style="color: #0086b3;">NULL</span> <span class="o" style="font-weight: 600;">&&</span> <span class="n" style="color: #333;">info</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333;">ncerts</span> <span class="o" style="font-weight: 600;">!=</span> <span class="mi" style="color: #099;">0</span><span class="p">)</span> <span class="p">{</span></span>
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<td class="diff-line-num new new_line" data-linenumber="145" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
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<pre style="margin: 0;">+<span id="LC145" class="line" lang="c"> <span class="n" style="color: #333;">ret</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">_gnutls_get_auth_info_pcert</span><span class="p">(</span><span class="o" style="font-weight: 600;">&</span><span class="n" style="color: #333;">peer_cert</span><span class="p">,</span></span>
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<td class="diff-line-num new new_line" data-linenumber="146" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">
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<pre style="margin: 0;">+<span id="LC146" class="line" lang="c"> <span class="n" style="color: #333;">session</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333;">security_parameters</span><span class="p">.</span></span>
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<pre style="margin: 0;">+<span id="LC147" class="line" lang="c"> <span class="n" style="color: #333;">server_ctype</span><span class="p">,</span> <span class="n" style="color: #333;">info</span><span class="p">);</span></span>
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<pre style="margin: 0;">+<span id="LC148" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC149" class="line" lang="c"> <span class="k" style="font-weight: 600;">if</span> <span class="p">(</span><span class="n" style="color: #333;">ret</span> <span class="o" style="font-weight: 600;"><</span> <span class="mi" style="color: #099;">0</span><span class="p">)</span> <span class="p">{</span></span>
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<pre style="margin: 0;">+<span id="LC150" class="line" lang="c"> <span class="n" style="color: #333;">gnutls_assert</span><span class="p">();</span></span>
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<pre style="margin: 0;">+<span id="LC151" class="line" lang="c"> <span class="k" style="font-weight: 600;">goto</span> <span class="n" style="color: #333;">cleanup</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC152" class="line" lang="c"> <span class="p">}</span></span>
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<pre style="margin: 0;">+<span id="LC154" class="line" lang="c"> <span class="n" style="color: #333;">has_pcert</span> <span class="o" style="font-weight: 600;">=</span> <span class="mi" style="color: #099;">1</span><span class="p">;</span></span>
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<pre style="margin: 0;">+<span id="LC155" class="line" lang="c"> <span class="p">}</span></span>
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<p dir="auto">There is <code>gnutls_memcmp</code> for it. However going through <code>_gnutls_gost_keytrans_decrypt</code>, it is possible to distinguish with timing the ukm comparison failure vs the key unwrap failure. The key transport as per RSA is an oracle itself which allows anyone (client) to send arbitrary data to the server and time its results or try many inputs until something succeeds and that help the attacker to deduce something about the key. I do not know the particulars of this key exchange but it looks like something risky to add or enable by default (in gost or non-gost setup).</p>
<p dir="auto">As I do not know the algorithm, I do not have an attack, but only hints that such thing may exist. Are there public papers on this algorithm that describe its resistance to RSA-style bleichenbacher oracle attacks? What I'm trying to avoid here is a big rewrite in the future like we did for the RSA.</p>
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