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<title>15.3 sha -- SHA-1 message digest algorithm</title>
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<H1><A NAME="SECTION0017300000000000000000">
15.3 <tt class="module">sha</tt> --
         SHA-1 message digest algorithm</A>
</H1>

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

<P>
This module implements the interface to NIST's<a id='l2h-4830' xml:id='l2h-4830'></a> secure hash
algorithm,<a id='l2h-4831' xml:id='l2h-4831'></a> known as SHA-1.  SHA-1 is an
improved version of the original SHA hash algorithm.  It is used in
the same way as the <tt class="module"><a href="module-md5.html">md5</a></tt> module: use <tt class="function">new()</tt>
to create an sha object, then feed this object with arbitrary strings
using the <tt class="method">update()</tt> method, and at any point you can ask it
for the <i class="dfn">digest</i> of the concatenation of the strings fed to it
so far.<a id='l2h-4832' xml:id='l2h-4832'></a>  SHA-1 digests are 160 bits instead of
MD5's 128 bits.

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4823' xml:id='l2h-4823' class="function">new</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>string</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Return a new sha object.  If <var>string</var> is present, the method
  call <code>update(<var>string</var>)</code> is made.
</dl>

<P>
The following values are provided as constants in the module and as
attributes of the sha objects returned by <tt class="function">new()</tt>:

<P>
<dl><dt><b><tt id='l2h-4824' xml:id='l2h-4824'>blocksize</tt></b></dt>
<dd>
  Size of the blocks fed into the hash function; this is always
  <code>1</code>.  This size is used to allow an arbitrary string to be
  hashed.
</dd></dl>

<P>
<dl><dt><b><tt id='l2h-4825' xml:id='l2h-4825'>digest_size</tt></b></dt>
<dd>
  The size of the resulting digest in bytes.  This is always
  <code>20</code>.
</dd></dl>

<P>
An sha object has the same methods as md5 objects:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4826' xml:id='l2h-4826' class="method">update</tt></b>(</nobr></td>
  <td><var>arg</var>)</td></tr></table></dt>
<dd>
Update the sha object with the string <var>arg</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-4827' xml:id='l2h-4827' 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 20-byte string which may contain
non-ASCII characters, including null bytes.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-4828' xml:id='l2h-4828' 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 40, 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-4829' xml:id='l2h-4829' class="method">copy</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Return a copy (``clone'') of the sha object.  This can be used to
efficiently compute the digests of strings that share a common initial
substring.
</dl>

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seetitle">
    <dt><em class="citetitle"><a href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2withchangenotice.pdf"
        >Secure Hash Standard</a></em></dt>
    <dd>The Secure Hash Algorithm is defined by NIST document FIPS
     PUB 180-2:
     <em class="citetitle"><a
 href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2withchangenotice.pdf"
 title="Secure Hash Standard"
 >Secure Hash Standard</a></em>, published in August 2002.</dd>
  </dl>

<P>
<dl compact="compact" class="seetitle">
    <dt><em class="citetitle"><a href="http://csrc.nist.gov/encryption/tkhash.html"
        >Cryptographic Toolkit (Secure Hashing)</a></em></dt>
    <dd>Links from NIST to various information on secure hashing.</dd>
  </dl>
</div>

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