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\section{\module{sha} ---
SHA message digest algorithm}
\declaremodule{builtin}{sha}
\modulesynopsis{NIST's secure hash algorithm, SHA.}
\sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
This module implements the interface to NIST's\index{NIST} secure hash
algorithm,\index{Secure Hash Algorithm} known as SHA. It is used in
the same way as the \refmodule{md5} module:\ use the \function{new()}
to create an sha object, then feed this object with arbitrary strings
using the \method{update()} method, and at any point you can ask it
for the \dfn{digest} of the contatenation of the strings fed to it
so far.\index{checksum!SHA} SHA digests are 160 bits instead of 128
bits.
\begin{funcdesc}{new}{\optional{string}}
Return a new sha object. If \var{string} is present, the method
call \code{update(\var{string})} is made.
\end{funcdesc}
The following values are provided as constants in the module and as
attributes of the sha objects returned by \function{new()}:
\begin{datadesc}{blocksize}
Size of the blocks fed into the hash function; this is always
\code{1}. This size is used to allow an arbitrary string to be
hashed.
\end{datadesc}
\begin{datadesc}{digestsize}
The size of the resulting digest in bytes. This is always
\code{20}.
\end{datadesc}
A sha object has all the methods the md5 objects have, plus one:
\begin{methoddesc}[sha]{hexdigest}{}
Return the digest value as a string of hexadecimal digits. This may
be used to exchange the value safely in email or other non-binary
environments.
\end{methoddesc}
\begin{seealso}
\seetext{The Secure Hash Algorithm is defined by NIST document FIPS
PUB 180-1: \citetitle{Secure Hash Standard}, published in April
of 1995. It is available online as plain text at
\url{http://csrc.nist.gov/fips/fip180-1.txt} (at least one
diagram was omitted) and as PostScript at
\url{http://csrc.nist.gov/fips/fip180-1.ps}.}
\end{seealso}
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