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<div class="section" id="module-hmac">
<span id="hmac-keyed-hashing-for-message-authentication"></span><h1>14.2. <a class="reference internal" href="#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation"><code class="xref py py-mod docutils literal"><span class="pre">hmac</span></code></a> — Keyed-Hashing for Message Authentication<a class="headerlink" href="#module-hmac" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.2.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/hmac.py">Lib/hmac.py</a></p>
<hr class="docutils" />
<p>This module implements the HMAC algorithm as described by <span class="target" id="index-0"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc2104.html"><strong>RFC 2104</strong></a>.</p>
<dl class="function">
<dt id="hmac.new">
<code class="descclassname">hmac.</code><code class="descname">new</code><span class="sig-paren">(</span><em>key</em><span class="optional">[</span>, <em>msg</em><span class="optional">[</span>, <em>digestmod</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.new" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new hmac object. If <em>msg</em> is present, the method call <code class="docutils literal"><span class="pre">update(msg)</span></code>
is made. <em>digestmod</em> is the digest constructor or module for the HMAC object to
use. It defaults to the <code class="xref py py-data docutils literal"><span class="pre">hashlib.md5</span></code> constructor.</p>
</dd></dl>
<p>An HMAC object has the following methods:</p>
<dl class="method">
<dt id="hmac.HMAC.update">
<code class="descclassname">HMAC.</code><code class="descname">update</code><span class="sig-paren">(</span><em>msg</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.update" title="Permalink to this definition">¶</a></dt>
<dd><p>Update the hmac object with the string <em>msg</em>. Repeated calls are equivalent to
a single call with the concatenation of all the arguments: <code class="docutils literal"><span class="pre">m.update(a);</span>
<span class="pre">m.update(b)</span></code> is equivalent to <code class="docutils literal"><span class="pre">m.update(a</span> <span class="pre">+</span> <span class="pre">b)</span></code>.</p>
</dd></dl>
<dl class="method">
<dt id="hmac.HMAC.digest">
<code class="descclassname">HMAC.</code><code class="descname">digest</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the digest of the strings passed to the <a class="reference internal" href="#hmac.HMAC.update" title="hmac.HMAC.update"><code class="xref py py-meth docutils literal"><span class="pre">update()</span></code></a> method so far.
This string will be the same length as the <em>digest_size</em> of the digest given to
the constructor. It may contain non-ASCII characters, including NUL bytes.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">When comparing the output of <a class="reference internal" href="#hmac.HMAC.digest" title="hmac.HMAC.digest"><code class="xref py py-meth docutils literal"><span class="pre">digest()</span></code></a> to an externally-supplied
digest during a verification routine, it is recommended to use the
<a class="reference internal" href="#hmac.compare_digest" title="hmac.compare_digest"><code class="xref py py-func docutils literal"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal"><span class="pre">==</span></code> operator
to reduce the vulnerability to timing attacks.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="hmac.HMAC.hexdigest">
<code class="descclassname">HMAC.</code><code class="descname">hexdigest</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.hexdigest" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#hmac.HMAC.digest" title="hmac.HMAC.digest"><code class="xref py py-meth docutils literal"><span class="pre">digest()</span></code></a> except the digest is returned as a string twice the length
containing only hexadecimal digits. This may be used to exchange the value
safely in email or other non-binary environments.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">When comparing the output of <a class="reference internal" href="#hmac.HMAC.hexdigest" title="hmac.HMAC.hexdigest"><code class="xref py py-meth docutils literal"><span class="pre">hexdigest()</span></code></a> to an externally-supplied
digest during a verification routine, it is recommended to use the
<a class="reference internal" href="#hmac.compare_digest" title="hmac.compare_digest"><code class="xref py py-func docutils literal"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal"><span class="pre">==</span></code> operator
to reduce the vulnerability to timing attacks.</p>
</div>
</dd></dl>
<dl class="method">
<dt id="hmac.HMAC.copy">
<code class="descclassname">HMAC.</code><code class="descname">copy</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#hmac.HMAC.copy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a copy (“clone”) of the hmac object. This can be used to efficiently
compute the digests of strings that share a common initial substring.</p>
</dd></dl>
<p>This module also provides the following helper function:</p>
<dl class="function">
<dt id="hmac.compare_digest">
<code class="descclassname">hmac.</code><code class="descname">compare_digest</code><span class="sig-paren">(</span><em>a</em>, <em>b</em><span class="sig-paren">)</span><a class="headerlink" href="#hmac.compare_digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></code>. This function uses an approach designed to prevent
timing analysis by avoiding content-based short circuiting behaviour,
making it appropriate for cryptography. <em>a</em> and <em>b</em> must both be of the
same type: either <a class="reference internal" href="functions.html#unicode" title="unicode"><code class="xref py py-class docutils literal"><span class="pre">unicode</span></code></a> or a <a class="reference internal" href="../glossary.html#term-bytes-like-object"><span class="xref std std-term">bytes-like object</span></a>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If <em>a</em> and <em>b</em> are of different lengths, or if an error occurs,
a timing attack could theoretically reveal information about the
types and lengths of <em>a</em> and <em>b</em>—but not their values.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 2.7.7.</span></p>
</div>
</dd></dl>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal"><span class="pre">hashlib</span></code></a></dt>
<dd>The Python module providing secure hash functions.</dd>
</dl>
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