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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 \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 concatenation of the strings fed to it
so far.\index{checksum!SHA} SHA digests are 160 bits instead of
MD5's 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}
An sha object has the same methods as md5 objects:
\begin{methoddesc}[sha]{update}{arg}
Update the sha object with the string \var{arg}. Repeated calls are
equivalent to a single call with the concatenation of all the
arguments: \code{m.update(a); m.update(b)} is equivalent to
\code{m.update(a+b)}.
\end{methoddesc}
\begin{methoddesc}[sha]{digest}{}
Return the digest of the strings passed to the \method{update()}
method so far. This is a 20-byte string which may contain
non-\ASCII{} characters, including null bytes.
\end{methoddesc}
\begin{methoddesc}[sha]{hexdigest}{}
Like \method{digest()} 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.
\end{methoddesc}
\begin{methoddesc}[sha]{copy}{}
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.
\end{methoddesc}
\begin{seealso}
\seetitle[http://csrc.nist.gov/fips/fip180-1.txt]{Secure Hash Standard}{
The Secure Hash Algorithm is defined by NIST document FIPS
PUB 180-1:
\citetitle[http://csrc.nist.gov/fips/fip180-1.txt]{Secure
Hash Standard}, published in April of 1995. It is
available online as plain text (at least one diagram was
omitted) and as PDF at
\url{http://csrc.nist.gov/fips/fip180-1.pdf}.}
\end{seealso}
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