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<a href="https://gitlab.com/lumag">Dmitry Eremin-Solenikov</a>
commented on a discussion
on <a href="https://gitlab.com/gnutls/gnutls/merge_requests/992#note_167534005">lib/nettle/gost/streebog.c</a>:
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<pre style="margin: 0;"> <span id="LC1219" class="line" lang="c"> <span class="p">}</span></span>
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<pre style="margin: 0;"> <span id="LC1220" class="line" lang="c"> <span class="p">}</span></span>
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<pre style="margin: 0;"> <span id="LC1221" class="line" lang="c"></span>
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<pre style="margin: 0;">+<span id="LC1222" class="line" lang="c"> <span class="n" style="color: #333;">cf</span> <span class="o" style="font-weight: 600;">=</span> <span class="mi" style="color: #099;">0</span><span class="p">;</span></span>
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<td class="diff-line-num new_line" data-linenumber="1223" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">
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<pre style="margin: 0;"> <span id="LC1223" class="line" lang="c"> <span class="n" style="color: #333;">ctx</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333;">sigma</span><span class="p">[</span><span class="mi" style="color: #099;">0</span><span class="p">]</span> <span class="o" style="font-weight: 600;">+=</span> <span class="n" style="color: #333;">M</span><span class="p">[</span><span class="mi" style="color: #099;">0</span><span class="p">];</span></span>
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<pre style="margin: 0;"> <span id="LC1224" class="line" lang="c"> <span class="k" style="font-weight: 600;">for</span> <span class="p">(</span><span class="n" style="color: #333;">i</span> <span class="o" style="font-weight: 600;">=</span> <span class="mi" style="color: #099;">1</span><span class="p">;</span> <span class="n" style="color: #333;">i</span> <span class="o" style="font-weight: 600;"><</span> <span class="mi" style="color: #099;">8</span><span class="p">;</span> <span class="n" style="color: #333;">i</span><span class="o" style="font-weight: 600;">++</span><span class="p">)</span></span>
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<pre style="margin: 0;">-<span id="LC1224" class="line" lang="c"> <span class="k" style="font-weight: 600;"><span class="idiff left">if</span></span><span class="idiff"> </span><span class="p"><span class="idiff">(</span></span><span class="n" style="color: #333;"><span class="idiff">ctx</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-></span></span><span class="n" style="color: #333;"><span class="idiff">sigma</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff">]</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff"><</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff">M</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff right">])</span></span></span>
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<pre style="margin: 0;">-<span id="LC1225" class="line" lang="c"> <span class="n" style="color: #333;"><span class="idiff left">ctx</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-></span></span><span class="n" style="color: #333;"><span class="idiff">sigma</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="p"><span class="idiff">]</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">+=</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff">M</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="p"><span class="idiff">]</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">+</span></span><span class="idiff"> </span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff right">;</span></span></span>
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<pre style="margin: 0;">-<span id="LC1226" class="line" lang="c"><span class="idiff left"> </span><span class="k" style="font-weight: 600;"><span class="idiff right">else</span></span></span>
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<pre style="margin: 0;">-<span id="LC1227" class="line" lang="c"> <span class="n" style="color: #333;">ctx</span><span class="o" style="font-weight: 600;">-></span><span class="n" style="color: #333;">sigma</span><span class="p">[</span><span class="n" style="color: #333;">i</span><span class="p">]</span> <span class="o" style="font-weight: 600;">+=</span> <span class="n" style="color: #333;">M</span><span class="p">[</span><span class="n" style="color: #333;">i</span><span class="p">];</span></span>
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<pre style="margin: 0;">+<span id="LC1225" class="line" lang="c"> <span class="p"><span class="idiff left right">{</span></span></span>
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<pre style="margin: 0;">+<span id="LC1226" class="line" lang="c"> <span class="k" style="font-weight: 600;"><span class="idiff left">if</span></span><span class="idiff"> </span><span class="p"><span class="idiff">(</span></span><span class="n" style="color: #333;"><span class="idiff">ctx</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-></span></span><span class="n" style="color: #333;"><span class="idiff">sigma</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff">]</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">!=</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff">M</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff right">])</span></span></span>
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<pre style="margin: 0;">+<span id="LC1227" class="line" lang="c"><span class="idiff left"> </span><span class="n" style="color: #333;"><span class="idiff">cf</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">=</span></span><span class="idiff"> </span><span class="p"><span class="idiff">(</span></span><span class="n" style="color: #333;"><span class="idiff">ctx</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-></span></span><span class="n" style="color: #333;"><span class="idiff">sigma</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff">]</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff"><</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff">M</span></span><span class="p"><span class="idiff">[</span></span><span class="n" style="color: #333;"><span class="idiff">i</span></span><span class="o" style="font-weight: 600;"><span class="idiff">-</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff right">]);</span></span></span>
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<p dir="auto">No, it will be reset once there is no carried overflow.
In your example (if we use bytes instead of u64, lsb arrays):
sigma = 01 00 00 00
M = ff 00 00 00
0. sigma = <strong>00</strong> 00 00 00</p>
<ol dir="auto">
<li>i = 1, cf = 1, sigma = 00 <strong>01</strong> 00 00</li>
<li>i = 2, cf = 0, sigma = 00 01 <strong>00</strong> 00</li>
<li>i = 3, cf = 0, sigma = 00 01 00 <strong>00</strong>
</li>
</ol>
<p dir="auto">A failing example:
sigma = ff ff ff 00 00
M = ff ff ff 00 00
then:
0. sigma = <strong>fe</strong> ff ff 00 00</p>
<ol dir="auto">
<li>i = 1, cf = 1 (overflow), sigma = fe <strong>ff</strong> ff 00 00</li>
<li>i = 2, cf = 1 (carried), sigma = fe ff <strong>ff</strong> 00 00 (with current code this would be fe ff <strong>fe</strong> 00 00)</li>
<li>i = 3, cf = 1 (carried), sigma = fe ff ff <strong>01</strong> 00</li>
<li>i = 4, cf = 0 (reset, no overflow), sigma = fe ff ff 01 <strong>00</strong>
</li>
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