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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>14.2. hmac — Keyed-Hashing for Message Authentication — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="14.3. md5 — MD5 message digest algorithm" href="md5.html" /> <link rel="prev" title="14.1. hashlib — Secure hashes and message digests" href="hashlib.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/library/hmac.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/library/hmac.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="md5.html" title="14.3. md5 — MD5 message digest algorithm" accesskey="N">next</a> |</li> <li class="right" > <a href="hashlib.html" title="14.1. hashlib — Secure hashes and message digests" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="crypto.html" accesskey="U">14. Cryptographic Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <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 notranslate"><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 notranslate"><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 notranslate"><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 notranslate"><span class="pre">m.update(a);</span> <span class="pre">m.update(b)</span></code> is equivalent to <code class="docutils literal notranslate"><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 notranslate"><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 notranslate"><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 notranslate"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal notranslate"><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 notranslate"><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 notranslate"><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 notranslate"><span class="pre">compare_digest()</span></code></a> function instead of the <code class="docutils literal notranslate"><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 notranslate"><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 notranslate"><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 notranslate"><span class="pre">hashlib</span></code></a></dt> <dd>The Python module providing secure hash functions.</dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="hashlib.html" title="previous chapter">14.1. <code class="docutils literal notranslate"><span class="pre">hashlib</span></code> — Secure hashes and message digests</a></p> <h4>Next topic</h4> <p class="topless"><a href="md5.html" title="next chapter">14.3. <code class="docutils literal notranslate"><span class="pre">md5</span></code> — MD5 message digest algorithm</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/library/hmac.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="md5.html" title="14.3. md5 — MD5 message digest algorithm" >next</a> |</li> <li class="right" > <a href="hashlib.html" title="14.1. hashlib — Secure hashes and message digests" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="crypto.html" >14. 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