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<title>15.1 hmac -- Keyed-Hashing for Message Authentication</title>
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<H1><A NAME="SECTION0017100000000000000000">
15.1 <tt class="module">hmac</tt> --
         Keyed-Hashing for Message Authentication</A>
</H1>

<P>
<A NAME="module-hmac"></A>

<P>

<span class="versionnote">New in version 2.2.</span>

<P>
This module implements the HMAC algorithm as described by <a class="rfc" id='rfcref-92061' xml:id='rfcref-92061'
href="http://www.faqs.org/rfcs/rfc2104.html">RFC 2104</a>.

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4807' xml:id='l2h-4807' class="function">new</tt></b>(</nobr></td>
  <td><var>key</var><big>[</big><var>, msg</var><big>[</big><var>, digestmod</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Return a new hmac object.  If <var>msg</var> is present, the method call
  <code>update(<var>msg</var>)</code> is made. <var>digestmod</var> is the digest
  module for the HMAC object to use. It defaults to the
  <tt class="module"><a href="module-md5.html">md5</a></tt> module.
</dl>

<P>
An HMAC object has the following methods:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4808' xml:id='l2h-4808' class="method">update</tt></b>(</nobr></td>
  <td><var>msg</var>)</td></tr></table></dt>
<dd>
  Update the hmac object with the string <var>msg</var>.  Repeated calls
  are equivalent to a single call with the concatenation of all the
  arguments: <code>m.update(a); m.update(b)</code> is equivalent to
  <code>m.update(a + b)</code>.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4809' xml:id='l2h-4809' class="method">digest</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Return the digest of the strings passed to the <tt class="method">update()</tt>
  method so far.  This is a 16-byte string (for <tt class="module"><a href="module-md5.html">md5</a></tt>) or a
  20-byte string (for <tt class="module"><a href="module-sha.html">sha</a></tt>) which may contain non-ASCII
  characters, including NUL bytes.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4810' xml:id='l2h-4810' class="method">hexdigest</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Like <tt class="method">digest()</tt> except the digest is returned as a string of
  length 32 for <tt class="module"><a href="module-md5.html">md5</a></tt> (40 for <tt class="module"><a href="module-sha.html">sha</a></tt>), containing
  only hexadecimal digits.  This may be used to exchange the value
  safely in email or other non-binary environments.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4811' xml:id='l2h-4811' class="method">copy</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  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.
</dl>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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