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<p style="color: #777777;">
<a href="https://gitlab.com/smuellerDD">Stephan Mueller</a>
commented on a discussion
on <a href="https://gitlab.com/gnutls/gnutls/merge_requests/990#note_171709007">lib/nettle/pk.c</a>:
</p>
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<pre style="margin: 0;"> <span id="LC269" class="line" lang="c"> <span class="k" style="font-weight: 600;">goto</span> <span class="n" style="color: #333;">dh_cleanup</span><span class="p">;</span></span>
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<pre style="margin: 0;"> <span id="LC270" class="line" lang="c"> <span class="p">}</span></span>
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<pre style="margin: 0;"> <span id="LC271" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC272" class="line" lang="c"> <span class="cm" style="color: #998; font-style: italic;">/* if we have Q check that y ^ q mod p == 1 */</span></span>
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<pre style="margin: 0;">+<span id="LC273" class="line" lang="c"> <span class="k" style="font-weight: 600;">if</span> <span class="p">(</span><span class="n" style="color: #333;">q</span> <span class="o" style="font-weight: 600;">!=</span> <span class="nb" style="color: #0086b3;">NULL</span><span class="p">)</span> <span class="p">{</span></span>
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<p dir="auto">Hi Nikos,</p>
<blockquote dir="auto" style="color: #7f8fa4; border-left-width: 3px; border-left-color: #eaeaea; border-left-style: solid; margin: 0; padding: 0 0 0 15px;">
<p>Nikos Mavrogiannopoulos commented on a discussion on lib/nettle/pk.c:
<a href="https://gitlab.com/gnutls/gnutls/merge_requests/990#note_171694887" data-original="https://gitlab.com/gnutls/gnutls/merge_requests/990#note_171694887" data-link="false" data-link-reference="true" data-project="179611" data-merge-request="28830049" data-project-path="gnutls/gnutls" data-iid="990" data-mr-title="DH and ECDH keys tests" data-reference-type="merge_request" data-container="body" data-placement="bottom" title="" class="gfm gfm-merge_request">!990 (comment 171694887)</a></p>
<blockquote style="color: #7f8fa4; border-left-width: 3px; border-left-color: #eaeaea; border-left-style: solid; margin: 0; padding: 0 0 0 15px;">
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true" style="background-color: #fff; font-family: monospace; font-size: 90%; -premailer-cellpadding: 0; -premailer-cellspacing: 0; -premailer-width: 100%; margin: 0;"><code><span id="LC1" class="line" lang="plaintext"> goto dh_cleanup;</span>
<span id="LC2" class="line" lang="plaintext"></span>
<span id="LC3" class="line" lang="plaintext"> }</span></code></pre>
<ul>
<li>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true" style="background-color: #fff; font-family: monospace; font-size: 90%; -premailer-cellpadding: 0; -premailer-cellspacing: 0; -premailer-width: 100%; margin: 0;"><code><span id="LC1" class="line" lang="plaintext"> /* if we have Q check that y ^ q mod p == 1 */</span></code></pre>
</li>
<li>
<pre class="code highlight js-syntax-highlight plaintext" lang="plaintext" v-pre="true" style="background-color: #fff; font-family: monospace; font-size: 90%; -premailer-cellpadding: 0; -premailer-cellspacing: 0; -premailer-width: 100%; margin: 0;"><code><span id="LC1" class="line" lang="plaintext"> if (q != NULL) {</span></code></pre>
</li>
</ul>
</blockquote>
<p>If we disable DHE in TLS1.2 there is no practical value from the new test
using Q because this catches incorrect public keys, but only in the
non-safe groups (SP800-56A acknowledges that). I'd expect the ACVP/CAVS
tests to be different depending on whether <code>SP800-56A 5.6.2.2.2 case 1</code> or
case (2) is claimed. Let's see what Stephan thinks on that.</p>
</blockquote>
<p dir="auto">The following is NOT yet official, but should guide your considerations.</p>
<p dir="auto">We just had a discussion with the responsible persons within NIST defining the
crypto requirements. The following conclusion came out of the discussion that
yet needs to be poured into a FIPS IG.</p>
<ul dir="auto">
<li>
<p>If we have Sophie-Germain primes, we must have a check of the remote key.
For TLS, you will always be able to get the Q from the communicated P ( Q = (P</p>
</li>
<li>
<ol>
<li>/ 2) ) or via a lookup-table.</li>
</ol>
</li>
<li>
<p>If you get a random prime via TLS, you can dispense with the remote key
check.</p>
</li>
<li>
<p>We have to expect that the ACVP testing will be updated to allow testing of
DH with Sophie-Germain primes.</p>
</li>
<li>
<p>For the existing ACVP testing providing a random prime, the key check must
be performed.</p>
</li>
</ul>
<p dir="auto">That said, the check above should be updated to check that P references a
Sophie-Germain prime and obtain Q if this is the case. Only if the Q value is
not found after that P lookup, the q != NULL is good.</p>
<p dir="auto">Though, this solution is yet to be turned into an official statement. I expect
an official statement in the not too far future.</p>
<p dir="auto">Until that happens, please leave the check in the code above for now.</p>
<p dir="auto">Ciao
Stephan</p>
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