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<li class="toctree-l1 current"><a class="current reference internal" href="#">Example use case</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#grab-asn-1-schema-for-ssh-keys">Grab ASN.1 schema for SSH keys</a></li>
<li class="toctree-l2"><a class="reference internal" href="#compile-asn-1-schema-into-python">Compile ASN.1 schema into Python</a></li>
<li class="toctree-l2"><a class="reference internal" href="#read-your-ssh-id-rsa">Read your ~/.ssh/id_rsa</a></li>
<li class="toctree-l2"><a class="reference internal" href="#play-with-the-keys">Play with the keys</a></li>
<li class="toctree-l2"><a class="reference internal" href="#transform-to-built-ins">Transform to built-ins</a></li>
<li class="toctree-l2"><a class="reference internal" href="#write-it-back">Write it back</a></li>
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<section id="example-use-case">
<h1>Example use case<a class="headerlink" href="#example-use-case" title="Permalink to this headline">¶</a></h1>
<div class="toctree-wrapper compound">
</div>
<p>To briefly explain how to approach pyasn1, consider a quick workflow example.</p>
<section id="grab-asn-1-schema-for-ssh-keys">
<h2>Grab ASN.1 schema for SSH keys<a class="headerlink" href="#grab-asn-1-schema-for-ssh-keys" title="Permalink to this headline">¶</a></h2>
<p>ASN.1 is widely used in many Internet protocols. Frequently, whenever ASN.1 is employed,
data structures are described in ASN.1 schema language right in the RFC.
Take <a class="reference external" href="https://www.ietf.org/rfc/rfc2437.txt">RFC2437</a> for example – we can look into
it and weed out data structures specification into a local file:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># pkcs-1.asn</span>
<span class="n">PKCS</span><span class="o">-</span><span class="mi">1</span> <span class="p">{</span><span class="n">iso</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="n">member</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="n">us</span><span class="p">(</span><span class="mi">840</span><span class="p">)</span> <span class="n">rsadsi</span><span class="p">(</span><span class="mi">113549</span><span class="p">)</span> <span class="n">pkcs</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="n">pkcs</span><span class="o">-</span><span class="mi">1</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="n">modules</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="n">pkcs</span><span class="o">-</span><span class="mi">1</span><span class="p">(</span><span class="mi">1</span><span class="p">)}</span>
<span class="n">DEFINITIONS</span> <span class="n">EXPLICIT</span> <span class="n">TAGS</span> <span class="p">:</span><span class="o">:=</span> <span class="n">BEGIN</span>
<span class="n">RSAPrivateKey</span> <span class="p">:</span><span class="o">:=</span> <span class="n">SEQUENCE</span> <span class="p">{</span>
<span class="n">version</span> <span class="n">Version</span><span class="p">,</span>
<span class="n">modulus</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">publicExponent</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">privateExponent</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">prime1</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">prime2</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">exponent1</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">exponent2</span> <span class="n">INTEGER</span><span class="p">,</span>
<span class="n">coefficient</span> <span class="n">INTEGER</span>
<span class="p">}</span>
<span class="n">Version</span> <span class="p">:</span><span class="o">:=</span> <span class="n">INTEGER</span>
<span class="n">END</span>
</pre></div>
</div>
</section>
<section id="compile-asn-1-schema-into-python">
<h2>Compile ASN.1 schema into Python<a class="headerlink" href="#compile-asn-1-schema-into-python" title="Permalink to this headline">¶</a></h2>
<p>In the best case, you should be able to automatically compile ASN.1 spec into
Python classes. For that purpose we have the <a class="reference external" href="https://github.com/kimgr/asn1ate">asn1ate</a>
tool:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>$ asn1ate pkcs-1.asn > rsakey.py
</pre></div>
</div>
<p>Though it may not work out as, as it stands now, asn1ate does not support
all ASN.1 language constructs.</p>
<p>Alternatively, you could check out the <a class="reference external" href="https://github.com/etingof/pyasn1-modules">pyasn1-modules</a>
package to see if it already has the ASN.1 spec you are looking for compiled and shipped
there. Then just install the package, import the data structure you need and use it:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>$ pip install pyasn1-modules
</pre></div>
</div>
<p>As a last resort, you could express ASN.1 in Python by hand. The end result
should be a declarative Python code resembling original ASN.1 syntax like
this:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># rsakey.py</span>
<span class="k">class</span> <span class="nc">Version</span><span class="p">(</span><span class="n">Integer</span><span class="p">):</span>
<span class="k">pass</span>
<span class="k">class</span> <span class="nc">RSAPrivateKey</span><span class="p">(</span><span class="n">Sequence</span><span class="p">):</span>
<span class="n">componentType</span> <span class="o">=</span> <span class="n">NamedTypes</span><span class="p">(</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'version'</span><span class="p">,</span> <span class="n">Version</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'modulus'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'publicExponent'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'privateExponent'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'prime1'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'prime2'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'exponent1'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'exponent2'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">()),</span>
<span class="n">NamedType</span><span class="p">(</span><span class="s1">'coefficient'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">())</span>
<span class="p">)</span>
</pre></div>
</div>
</section>
<section id="read-your-ssh-id-rsa">
<h2>Read your ~/.ssh/id_rsa<a class="headerlink" href="#read-your-ssh-id-rsa" title="Permalink to this headline">¶</a></h2>
<p>Given we’ve put our Python classes into the <cite>rsakey.py</cite> module, we could import
the top-level object for SSH keys container and initialize it from our
<cite>~/.ssh/id_rsa</cite> file (for sake of simplicity here we assume no passphrase is
set on the key file):</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">base64</span> <span class="kn">import</span> <span class="n">b64decode</span>
<span class="kn">from</span> <span class="nn">pyasn1.codec.der.decoder</span> <span class="kn">import</span> <span class="n">decode</span> <span class="k">as</span> <span class="n">der_decoder</span>
<span class="kn">from</span> <span class="nn">rsakey</span> <span class="kn">import</span> <span class="n">RSAPrivateKey</span>
<span class="c1"># Read SSH key from file (assuming no passphrase)</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'.ssh/id_rsa'</span><span class="p">)</span> <span class="k">as</span> <span class="n">key_file</span><span class="p">:</span>
<span class="n">b64_serialisation</span> <span class="o">=</span> <span class="s1">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">key_file</span><span class="o">.</span><span class="n">readlines</span><span class="p">()[</span><span class="mi">1</span><span class="p">:</span><span class="o">-</span><span class="mi">1</span><span class="p">])</span>
<span class="c1"># Undo BASE64 serialisation</span>
<span class="n">der_serialisation</span> <span class="o">=</span> <span class="n">b64decode</span><span class="p">(</span><span class="n">b64_serialisation</span><span class="p">)</span>
<span class="c1"># Undo DER serialisation, reconstruct SSH key structure</span>
<span class="n">private_key</span><span class="p">,</span> <span class="n">rest_of_input</span> <span class="o">=</span> <span class="n">der_decoder</span><span class="p">(</span><span class="n">der_serialisation</span><span class="p">,</span> <span class="n">asn1Spec</span><span class="o">=</span><span class="n">RSAPrivateKey</span><span class="p">())</span>
</pre></div>
</div>
<p>Once we have Python ASN.1 structures initialized, we could inspect them:</p>
<div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">private_key</span><span class="p">)</span>
<span class="go">RSAPrivateKey:</span>
<span class="go"> version=0</span>
<span class="go"> modulus=280789907761334970323210643584308373...</span>
<span class="go"> publicExponent=65537</span>
<span class="go"> privateExponent=1704567874679144879123080924...</span>
<span class="go"> prime1=1780178536719561265324798296279384073...</span>
<span class="go"> prime2=1577313184995269616049017780493740138...</span>
<span class="go"> exponent1=1193974819720845247396384239609024...</span>
<span class="go"> exponent2=9240965721817961178848297404494811...</span>
<span class="go"> coefficient=10207364473358910343346707141115...</span>
</pre></div>
</div>
</section>
<section id="play-with-the-keys">
<h2>Play with the keys<a class="headerlink" href="#play-with-the-keys" title="Permalink to this headline">¶</a></h2>
<p>As well as use them nearly as we do with native Python types:</p>
<div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">pk</span> <span class="o">=</span> <span class="n">private_key</span>
<span class="go">>>></span>
<span class="gp">>>> </span><span class="n">pk</span><span class="p">[</span><span class="s1">'prime1'</span><span class="p">]</span> <span class="o">*</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'prime2'</span><span class="p">]</span> <span class="o">==</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'modulus'</span><span class="p">]</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">pk</span><span class="p">[</span><span class="s1">'prime1'</span><span class="p">]</span> <span class="o">==</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'modulus'</span><span class="p">]</span> <span class="o">//</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'prime2'</span><span class="p">]</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">pk</span><span class="p">[</span><span class="s1">'exponent1'</span><span class="p">]</span> <span class="o">==</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'privateExponent'</span><span class="p">]</span> <span class="o">%</span> <span class="p">(</span><span class="n">pk</span><span class="p">[</span><span class="s1">'prime1'</span><span class="p">]</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="n">pk</span><span class="p">[</span><span class="s1">'exponent2'</span><span class="p">]</span> <span class="o">==</span> <span class="n">pk</span><span class="p">[</span><span class="s1">'privateExponent'</span><span class="p">]</span> <span class="o">%</span> <span class="p">(</span><span class="n">pk</span><span class="p">[</span><span class="s1">'prime2'</span><span class="p">]</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Technically, pyasn1 classes <a class="reference external" href="https://docs.python.org/3/reference/datamodel.html#emulating-container-types">emulate</a>
Python built-in types.</p>
</section>
<section id="transform-to-built-ins">
<h2>Transform to built-ins<a class="headerlink" href="#transform-to-built-ins" title="Permalink to this headline">¶</a></h2>
<p>ASN.1 data structures exhibit a way more complicated behaviour compared to
Python types. You may wish to simplify things by turning the whole tree of
pyasn1 objects into an analogous tree made of base Python types:</p>
<div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">pyasn1.codec.native.encoder</span> <span class="kn">import</span> <span class="n">encode</span>
<span class="gp">>>> </span><span class="o">...</span>
<span class="gp">>>> </span><span class="n">py_private_key</span> <span class="o">=</span> <span class="n">encode</span><span class="p">(</span><span class="n">private_key</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">py_private_key</span>
<span class="go">{'version': 0, 'modulus': 280789907761334970323210643584308373, 'publicExponent': 65537,</span>
<span class="go"> 'privateExponent': 1704567874679144879123080924, 'prime1': 1780178536719561265324798296279384073,</span>
<span class="go"> 'prime2': 1577313184995269616049017780493740138, 'exponent1': 1193974819720845247396384239609024,</span>
<span class="go"> 'exponent2': 9240965721817961178848297404494811, 'coefficient': 10207364473358910343346707141115}</span>
</pre></div>
</div>
<p>You can do vice-versa: initialize ASN.1 structure from a dict:</p>
<div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">pyasn1.codec.native.decoder</span> <span class="kn">import</span> <span class="n">decode</span>
<span class="gp">>>> </span><span class="n">py_private_key</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'modulus'</span><span class="p">:</span> <span class="mi">280789907761334970323210643584308373</span><span class="p">}</span>
<span class="gp">>>> </span><span class="n">private_key</span> <span class="o">=</span> <span class="n">decode</span><span class="p">(</span><span class="n">py_private_key</span><span class="p">,</span> <span class="n">asn1Spec</span><span class="o">=</span><span class="n">RSAPrivateKey</span><span class="p">())</span>
</pre></div>
</div>
</section>
<section id="write-it-back">
<h2>Write it back<a class="headerlink" href="#write-it-back" title="Permalink to this headline">¶</a></h2>
<p>Possibly not that applicable to the SSH key example, but you can of course modify
any part of the ASN.1 data structure and serialise it back into the same or other
wire representation:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">pyasn1.codec.der.encoder</span> <span class="kn">import</span> <span class="n">encode</span> <span class="k">as</span> <span class="n">der_encoder</span>
<span class="c1"># Serialise SSH key data structure into DER stream</span>
<span class="n">der_serialisation</span> <span class="o">=</span> <span class="n">der_encoder</span><span class="p">(</span><span class="n">private_key</span><span class="p">)</span>
<span class="c1"># Serialise DER stream into BASE64 stream</span>
<span class="n">b64_serialisation</span> <span class="o">=</span> <span class="s1">'-----BEGIN RSA PRIVATE KEY-----</span><span class="se">\n</span><span class="s1">'</span>
<span class="n">b64_serialisation</span> <span class="o">+=</span> <span class="n">b64encode</span><span class="p">(</span><span class="n">der_serialisation</span><span class="p">)</span>
<span class="n">b64_serialisation</span> <span class="o">+=</span> <span class="s1">'-----END RSA PRIVATE KEY-----</span><span class="se">\n</span><span class="s1">'</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'.ssh/id_rsa.new'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span> <span class="k">as</span> <span class="n">key_file</span><span class="p">:</span>
<span class="n">key_file</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">b64_serialisation</span><span class="p">)</span>
</pre></div>
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