Doc/lib/libhashlib.tex

\section{\module{hashlib} ---
         Secure hashes and message digests}

\declaremodule{builtin}{hashlib}
\modulesynopsis{Secure hash and message digest algorithms.}
\moduleauthor{Gregory P. Smith}{greg@users.sourceforge.net}
\sectionauthor{Gregory P. Smith}{greg@users.sourceforge.net}

\versionadded{2.5}

\index{message digest, MD5}
\index{secure hash algorithm, SHA1, SHA224, SHA256, SHA384, SHA512}

This module implements a common interface to many different secure hash and
message digest algorithms.  Included are the FIPS secure hash algorithms SHA1,
SHA224, SHA256, SHA384, and SHA512 (defined in FIPS 180-2) as well as RSA's MD5
algorithm (defined in Internet \rfc{1321}).
The terms secure hash and message digest are interchangable.  Older
algorithms were called message digests.  The modern term is secure hash.

\warning{Some algorithms have known hash collision weaknesses, see the FAQ at the end.}

There is one constructor method named for each type of \dfn{hash}.  All return
a hash object with the same simple interface.
For example: use \function{sha1()} to create a SHA1 hash object.
You can now feed this object with arbitrary strings using the \method{update()}
method.  At any point you can ask it for the \dfn{digest} of the concatenation
of the strings fed to it so far using the \method{digest()} or
\method{hexdigest()} methods.

Constructors for hash algorithms that are always present in this module are
\function{md5()}, \function{sha1()}, \function{sha224()}, \function{sha256()},
\function{sha384()}, and \function{sha512()}.  Additional algorithms may also
be available depending upon the OpenSSL library python uses on your platform.
\index{OpenSSL}

For example, to obtain the digest of the string \code{'Nobody inspects
the spammish repetition'}:

\begin{verbatim}
>>> import hashlib
>>> m = hashlib.md5()
>>> m.update("Nobody inspects")
>>> m.update(" the spammish repetition")
>>> m.digest()
'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
\end{verbatim}

More condensed:

\begin{verbatim}
>>> hashlib.sha224("Nobody inspects the spammish repetition").hexdigest()
'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'
\end{verbatim}

A generic \function{new()} constructor that takes the string name of the
desired algorithm as its first parameter also exists to allow access to the
above listed hashes as well as any other algorithms that your OpenSSL library
may offer.  The named constructors are much faster than \function{new()} and
should be preferred.

Using \function{new()} with an algorithm provided by OpenSSL:

\begin{verbatim}
>>> h = hashlib.new('ripemd160')
>>> h.update("Nobody inspects the spammish repetition")
>>> h.hexdigest()
'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'
\end{verbatim}

The following values are provided as constant attributes of the hash objects
returned by the constructors:

\begin{datadesc}{digest_size}
  The size of the resulting digest in bytes.
\end{datadesc}

A hash object has the following methods:

\begin{methoddesc}[hash]{update}{arg}
Update the hash 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}[hash]{digest}{}
Return the digest of the strings passed to the \method{update()}
method so far.  This is a 16-byte string which may contain
non-\ASCII{} characters, including null bytes.
\end{methoddesc}

\begin{methoddesc}[hash]{hexdigest}{}
Like \method{digest()} except the digest is returned as a string of
double length, containing only hexadecimal digits.  This may 
be used to exchange the value safely in email or other non-binary
environments.
\end{methoddesc}

\begin{methoddesc}[hash]{copy}{}
Return a copy (``clone'') of the hash object.  This can be used to
efficiently compute the digests of strings that share a common initial
substring.
\end{methoddesc}

\begin{seealso}
  \seemodule{hmac}{A module to generate message authentication codes using hashes.}
  \seemodule{base64}{Another way to encode binary hashes for non-binary environments.}
  \seeurl{http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf}
  {The FIPS 180-2 publication on Secure Hash Algorithms.}
  \seeurl{http://www.cryptography.com/cnews/hash.html}
  {Hash Collision FAQ with information on which algorithms have known issues and
   what that means regarding their use.}
\end{seealso}