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<h1>Chapter 3: Witness</h1>
<!-- Table of contents -->
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<h2> Contents </h2>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#setup">Setup</a></li>
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<section class="tex2jax_ignore mathjax_ignore" id="chapter-3-witness">
<h1>Chapter 3: Witness<a class="headerlink" href="#chapter-3-witness" title="Link to this heading">#</a></h1>
<p>Now, we will generate multiple witnesses to test the soundness of our circuit constraints. Note that we only intend to accept the following set of values for signals “a”, “b”, and “c”. “Soundness” in this context refers to faulty witness successfully verified against the constraints (false positives), so any set of witness assignments that is different from the table below but still passes the constraints incurs a “soundness” error.</p>
<div class="pst-scrollable-table-container"><table class="table">
<thead>
<tr class="row-odd"><th class="head text-center"><p>Step Type</p></th>
<th class="head text-center"><p>Step Instance Index</p></th>
<th class="head text-center"><p></p></th>
<th class="head text-center"><p>Signals</p></th>
<th class="head text-center"><p></p></th>
<th class="head text-center"><p></p></th>
<th class="head text-center"><p>Setups</p></th>
<th class="head text-center"><p></p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p>a</p></td>
<td class="text-center"><p>b</p></td>
<td class="text-center"><p>c</p></td>
<td class="text-center"><p>constraint 1</p></td>
<td class="text-center"><p>constraint 2</p></td>
<td class="text-center"><p>constraint 3</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p>fibo step</p></td>
<td class="text-center"><p>0</p></td>
<td class="text-center"><p>1</p></td>
<td class="text-center"><p>1</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>a + b == c</p></td>
<td class="text-center"><p>b == a.next</p></td>
<td class="text-center"><p>c == b.next</p></td>
</tr>
<tr class="row-even"><td class="text-center"><p>fibo step</p></td>
<td class="text-center"><p>1</p></td>
<td class="text-center"><p>1</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>3</p></td>
<td class="text-center"><p>a + b == c</p></td>
<td class="text-center"><p>b == a.next</p></td>
<td class="text-center"><p>c == b.next</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p>fibo step</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>3</p></td>
<td class="text-center"><p>5</p></td>
<td class="text-center"><p>a + b == c</p></td>
<td class="text-center"><p>b == a.next</p></td>
<td class="text-center"><p>c == b.next</p></td>
</tr>
<tr class="row-even"><td class="text-center"><p>fibo step</p></td>
<td class="text-center"><p>3</p></td>
<td class="text-center"><p>3</p></td>
<td class="text-center"><p>5</p></td>
<td class="text-center"><p>8</p></td>
<td class="text-center"><p>a + b == c</p></td>
<td class="text-center"><p>b == a.next</p></td>
<td class="text-center"><p>c == b.next</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
</tr>
</tbody>
</table>
</div>
<section id="setup">
<h2>Setup<a class="headerlink" href="#setup" title="Link to this heading">#</a></h2>
<p>We setup the same circuit and witness in Chapter 2 which were successfully verified:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">chiquito.dsl</span> <span class="kn">import</span> <span class="n">Circuit</span><span class="p">,</span> <span class="n">StepType</span>
<span class="kn">from</span> <span class="nn">chiquito.cb</span> <span class="kn">import</span> <span class="n">eq</span>
<span class="kn">from</span> <span class="nn">chiquito.util</span> <span class="kn">import</span> <span class="n">F</span>
<span class="k">class</span> <span class="nc">FiboStep</span><span class="p">(</span><span class="n">StepType</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">setup</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">c</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">internal</span><span class="p">(</span><span class="s2">&quot;c&quot;</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">constr</span><span class="p">(</span><span class="n">eq</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">a</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">b</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">c</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">transition</span><span class="p">(</span><span class="n">eq</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">b</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">a</span><span class="o">.</span><span class="n">next</span><span class="p">()))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">transition</span><span class="p">(</span><span class="n">eq</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">c</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">b</span><span class="o">.</span><span class="n">next</span><span class="p">()))</span>
<span class="k">def</span> <span class="nf">wg</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>
<span class="n">a_value</span><span class="p">,</span> <span class="n">b_value</span> <span class="o">=</span> <span class="n">args</span>
<span class="bp">self</span><span class="o">.</span><span class="n">assign</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">a</span><span class="p">,</span> <span class="n">F</span><span class="p">(</span><span class="n">a_value</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">assign</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">circuit</span><span class="o">.</span><span class="n">b</span><span class="p">,</span> <span class="n">F</span><span class="p">(</span><span class="n">b_value</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">assign</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">c</span><span class="p">,</span> <span class="n">F</span><span class="p">(</span><span class="n">a_value</span> <span class="o">+</span> <span class="n">b_value</span><span class="p">))</span>
<span class="k">class</span> <span class="nc">Fibonacci</span><span class="p">(</span><span class="n">Circuit</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">setup</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">a</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">forward</span><span class="p">(</span><span class="s2">&quot;a&quot;</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">b</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">forward</span><span class="p">(</span><span class="s2">&quot;b&quot;</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">fibo_step</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">step_type</span><span class="p">(</span><span class="n">FiboStep</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="s2">&quot;fibo_step&quot;</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pragma_num_steps</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">trace</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">args</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fibo_step</span><span class="p">,</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">))</span>
<span class="n">a</span> <span class="o">=</span> <span class="mi">1</span>
<span class="n">b</span> <span class="o">=</span> <span class="mi">2</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fibo_step</span><span class="p">,</span> <span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">))</span>
<span class="n">prev_a</span> <span class="o">=</span> <span class="n">a</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">b</span>
<span class="n">b</span> <span class="o">+=</span> <span class="n">prev_a</span>
<span class="n">fibo</span> <span class="o">=</span> <span class="n">Fibonacci</span><span class="p">()</span>
<span class="n">fibo_witness</span> <span class="o">=</span> <span class="n">fibo</span><span class="o">.</span><span class="n">gen_witness</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="n">fibo</span><span class="o">.</span><span class="n">halo2_mock_prover</span><span class="p">(</span><span class="n">fibo_witness</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>------ Visiting map -------
key = annotations
key = exposed
key = first_step
key = fixed_assignments
key = fixed_signals
key = forward_signals
key = id
key = last_step
key = num_steps
key = q_enable
key = shared_signals
key = step_types
------ Visiting step_types -------
step_types = Some(
{
106216022312130389015997492066735753738: StepType {
id: 106216022312130389015997492066735753738,
signals: [
InternalSignal {
id: 106216029284208690269478589595931773450,
annotation: &quot;c&quot;,
},
],
constraints: [
Constraint {
annotation: &quot;((a + b) == c)&quot;,
expr: (a + b + (-c)),
},
],
transition_constraints: [
TransitionConstraint {
annotation: &quot;(b == next(a))&quot;,
expr: (b + (-next(a))),
},
TransitionConstraint {
annotation: &quot;(c == next(b))&quot;,
expr: (c + (-next(b))),
},
],
lookups: [],
},
},
)
Ok(
(),
)
</pre></div>
</div>
</div>
</div>
<p>Now we swap the first step instance from <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">1,</span> <span class="pre">2)</span></code> to <code class="docutils literal notranslate"><span class="pre">(0,</span> <span class="pre">2,</span> <span class="pre">2)</span></code>. We use the <code class="docutils literal notranslate"><span class="pre">evil_witness_test</span></code> function to swap step index 0 assignment index 0 to <code class="docutils literal notranslate"><span class="pre">F(0)</span></code> and step index 0 assignment index 0 to <code class="docutils literal notranslate"><span class="pre">F(2)</span></code>.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">evil_witness</span> <span class="o">=</span> <span class="n">fibo_witness</span><span class="o">.</span><span class="n">evil_witness_test</span><span class="p">(</span><span class="n">step_instance_indices</span><span class="o">=</span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="n">assignment_indices</span><span class="o">=</span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">],</span> <span class="n">rhs</span><span class="o">=</span><span class="p">[</span><span class="n">F</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">2</span><span class="p">)])</span>
</pre></div>
</div>
</div>
</div>
<p>According to the three constraints <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">+</span> <span class="pre">b</span> <span class="pre">==</span> <span class="pre">c</span></code>, <code class="docutils literal notranslate"><span class="pre">b</span> <span class="pre">==</span> <span class="pre">a.next</span></code>, and <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">==</span> <span class="pre">b.next</span></code>, we swap the second step instance from <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">2,</span> <span class="pre">3)</span></code> to <code class="docutils literal notranslate"><span class="pre">(2,</span> <span class="pre">2,</span> <span class="pre">4)</span></code> and modify the third and fourth step instances likewise, resulting in a new witness displayed below:</p>
<div class="pst-scrollable-table-container"><table class="table">
<thead>
<tr class="row-odd"><th class="head text-center"><p></p></th>
<th class="head text-center"><p>Signals</p></th>
<th class="head text-center"><p></p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td class="text-center"><p>a</p></td>
<td class="text-center"><p>b</p></td>
<td class="text-center"><p>c</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p>0</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>2</p></td>
</tr>
<tr class="row-even"><td class="text-center"><p>2</p></td>
<td class="text-center"><p>2</p></td>
<td class="text-center"><p>4</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p>2</p></td>
<td class="text-center"><p>4</p></td>
<td class="text-center"><p>6</p></td>
</tr>
<tr class="row-even"><td class="text-center"><p>4</p></td>
<td class="text-center"><p>6</p></td>
<td class="text-center"><p>10</p></td>
</tr>
<tr class="row-odd"><td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
<td class="text-center"><p></p></td>
</tr>
</tbody>
</table>
</div>
<p>We use the <code class="docutils literal notranslate"><span class="pre">evil_witness_test</span></code> function to further modify the <code class="docutils literal notranslate"><span class="pre">evil_witness</span></code>:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">evil_witness</span> <span class="o">=</span> <span class="n">evil_witness</span><span class="o">.</span><span class="n">evil_witness_test</span><span class="p">(</span><span class="n">step_instance_indices</span><span class="o">=</span><span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="n">assignment_indices</span><span class="o">=</span><span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">],</span> <span class="n">rhs</span><span class="o">=</span><span class="p">[</span><span class="n">F</span><span class="p">(</span><span class="mi">2</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">4</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">4</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">6</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">4</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">6</span><span class="p">),</span> <span class="n">F</span><span class="p">(</span><span class="mi">10</span><span class="p">)])</span>
</pre></div>
</div>
</div>
</div>
<p>Print the <code class="docutils literal notranslate"><span class="pre">evil_witness</span></code> to confirm that the swap was successful:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">evil_witness</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>TraceWitness(
step_instances={
StepInstance(
step_type_uuid=106216022312130389015997492066735753738,
assignments={
a = 0,
b = 2,
c = 2
},
),
StepInstance(
step_type_uuid=106216022312130389015997492066735753738,
assignments={
a = 2,
b = 2,
c = 4
},
),
StepInstance(
step_type_uuid=106216022312130389015997492066735753738,
assignments={
a = 2,
b = 4,
c = 6
},
),
StepInstance(
step_type_uuid=106216022312130389015997492066735753738,
assignments={
a = 4,
b = 6,
c = 10
},
)
},
)
</pre></div>
</div>
</div>
</div>
<p>Now, generate and verify the proof with <code class="docutils literal notranslate"><span class="pre">evil_witness</span></code>:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">fibo</span><span class="o">.</span><span class="n">halo2_mock_prover</span><span class="p">(</span><span class="n">evil_witness</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Ok(
(),
)
</pre></div>
</div>
</div>
</div>
<p>Surprisingly, <code class="docutils literal notranslate"><span class="pre">evil_witness</span></code> generated a proof that passed verification. This constitutes a soundness error, because the first step instance isnt <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">1,</span> <span class="pre">2)</span></code> as we initially specified, so why can the witness still pass the constraints?</p>
<p>The answer is simple, because in the first step instance, we never constrained the values of “a” and “b” to 1 and 1 in <code class="docutils literal notranslate"><span class="pre">setup</span></code> of <code class="docutils literal notranslate"><span class="pre">FiboStep</span></code>.</p>
<p>You might be wondering: in <code class="docutils literal notranslate"><span class="pre">trace</span></code>, didnt we set “a” and “b” to <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">1)</span></code> and added <code class="docutils literal notranslate"><span class="pre">FiboStep</span></code> as the first step instance? In fact, <code class="docutils literal notranslate"><span class="pre">trace</span></code> and <code class="docutils literal notranslate"><span class="pre">wg</span></code> are really helper functions for the prover to easily generate a witness, whose data can be tampered with as shown in <code class="docutils literal notranslate"><span class="pre">evil_witness_test</span></code>. The only conditions enforced are defined in circuit and step type <code class="docutils literal notranslate"><span class="pre">setup</span></code>. Therefore, to fix the soundness error, we need to add more constraints, in Chapter 4.</p>
</section>
</section>
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