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Digest(3)	       Perl Programmers Reference Guide		     Digest(3)

NAME
       Digest - Modules that calculate message digests

SYNOPSIS
	 $md5  = Digest->new("MD5");
	 $sha1 = Digest->new("SHA-1");
	 $sha256 = Digest->new("SHA-256");
	 $sha384 = Digest->new("SHA-384");
	 $sha512 = Digest->new("SHA-512");

	 $hmac = Digest->HMAC_MD5($key);

DESCRIPTION
       The "Digest::" modules calculate digests, also called "fingerprints" or
       "hashes", of some data, called a message.  The digest is (usually) some
       small/fixed size string.	 The actual size of the digest depend of the
       algorithm used.	The message is simply a sequence of arbitrary bytes or
       bits.

       An important property of the digest algorithms is that the digest is
       likely to change if the message change in some way.  Another property
       is that digest functions are one-way functions, that is it should be
       hard to find a message that correspond to some given digest.
       Algorithms differ in how "likely" and how "hard", as well as how
       efficient they are to compute.

       Note that the properties of the algorithms change over time, as the
       algorithms are analyzed and machines grow faster.  If your application
       for instance depends on it being "impossible" to generate the same
       digest for a different message it is wise to make it easy to plug in
       stronger algorithms as the one used grow weaker.	 Using the interface
       documented here should make it easy to change algorithms later.

       All "Digest::" modules provide the same programming interface.  A
       functional interface for simple use, as well as an object oriented
       interface that can handle messages of arbitrary length and which can
       read files directly.

       The digest can be delivered in three formats:

       binary  This is the most compact form, but it is not well suited for
	       printing or embedding in places that can't handle arbitrary
	       data.

       hex     A twice as long string of lowercase hexadecimal digits.

       base64  A string of portable printable characters.  This is the base64
	       encoded representation of the digest with any trailing padding
	       removed.	 The string will be about 30% longer than the binary
	       version.	 MIME::Base64 tells you more about this encoding.

       The functional interface is simply importable functions with the same
       name as the algorithm.  The functions take the message as argument and
       return the digest.  Example:

	 use Digest::MD5 qw(md5);
	 $digest = md5($message);

       There are also versions of the functions with "_hex" or "_base64"
       appended to the name, which returns the digest in the indicated form.

OO INTERFACE
       The following methods are available for all "Digest::" modules:

       $ctx = Digest->XXX($arg,...)
       $ctx = Digest->new(XXX => $arg,...)
       $ctx = Digest::XXX->new($arg,...)
	   The constructor returns some object that encapsulate the state of
	   the message-digest algorithm.  You can add data to the object and
	   finally ask for the digest.	The "XXX" should of course be replaced
	   by the proper name of the digest algorithm you want to use.

	   The two first forms are simply syntactic sugar which automatically
	   load the right module on first use.	The second form allow you to
	   use algorithm names which contains letters which are not legal perl
	   identifiers, e.g. "SHA-1".  If no implementation for the given
	   algorithm can be found, then an exception is raised.

	   If new() is called as an instance method (i.e. $ctx->new) it will
	   just reset the state the object to the state of a newly created
	   object.  No new object is created in this case, and the return
	   value is the reference to the object (i.e. $ctx).

       $other_ctx = $ctx->clone
	   The clone method creates a copy of the digest state object and
	   returns a reference to the copy.

       $ctx->reset
	   This is just an alias for $ctx->new.

       $ctx->add( $data )
       $ctx->add( $chunk1, $chunk2, ... )
	   The string value of the $data provided as argument is appended to
	   the message we calculate the digest for.  The return value is the
	   $ctx object itself.

	   If more arguments are provided then they are all appended to the
	   message, thus all these lines will have the same effect on the
	   state of the $ctx object:

	     $ctx->add("a"); $ctx->add("b"); $ctx->add("c");
	     $ctx->add("a")->add("b")->add("c");
	     $ctx->add("a", "b", "c");
	     $ctx->add("abc");

	   Most algorithms are only defined for strings of bytes and this
	   method might therefore croak if the provided arguments contain
	   chars with ordinal number above 255.

       $ctx->addfile( $io_handle )
	   The $io_handle is read until EOF and the content is appended to the
	   message we calculate the digest for.	 The return value is the $ctx
	   object itself.

	   The addfile() method will croak() if it fails reading data for some
	   reason.  If it croaks it is unpredictable what the state of the
	   $ctx object will be in. The addfile() method might have been able
	   to read the file partially before it failed.	 It is probably wise
	   to discard or reset the $ctx object if this occurs.

	   In most cases you want to make sure that the $io_handle is in
	   "binmode" before you pass it as argument to the addfile() method.

       $ctx->add_bits( $data, $nbits )
       $ctx->add_bits( $bitstring )
	   The add_bits() method is an alternative to add() that allow partial
	   bytes to be appended to the message.	 Most users should just ignore
	   this method as partial bytes is very unlikely to be of any
	   practical use.

	   The two argument form of add_bits() will add the first $nbits bits
	   from $data.	For the last potentially partial byte only the high
	   order "$nbits % 8" bits are used.  If $nbits is greater than
	   "length($data) * 8", then this method would do the same as
	   "$ctx->add($data)".

	   The one argument form of add_bits() takes a $bitstring of "1" and
	   "0" chars as argument.  It's a shorthand for
	   "$ctx->add_bits(pack("B*", $bitstring), length($bitstring))".

	   The return value is the $ctx object itself.

	   This example shows two calls that should have the same effect:

	      $ctx->add_bits("111100001010");
	      $ctx->add_bits("\xF0\xA0", 12);

	   Most digest algorithms are byte based and for these it is not
	   possible to add bits that are not a multiple of 8, and the
	   add_bits() method will croak if you try.

       $ctx->digest
	   Return the binary digest for the message.

	   Note that the "digest" operation is effectively a destructive,
	   read-once operation. Once it has been performed, the $ctx object is
	   automatically "reset" and can be used to calculate another digest
	   value.  Call $ctx->clone->digest if you want to calculate the
	   digest without resetting the digest state.

       $ctx->hexdigest
	   Same as $ctx->digest, but will return the digest in hexadecimal
	   form.

       $ctx->b64digest
	   Same as $ctx->digest, but will return the digest as a base64
	   encoded string.

Digest speed
       This table should give some indication on the relative speed of
       different algorithms.  It is sorted by throughput based on a benchmark
       done with of some implementations of this API:

	Algorithm      Size    Implementation		       MB/s

	MD4	       128     Digest::MD4 v1.3		      165.0
	MD5	       128     Digest::MD5 v2.33	       98.8
	SHA-256	       256     Digest::SHA2 v1.1.0	       66.7
	SHA-1	       160     Digest::SHA v4.3.1	       58.9
	SHA-1	       160     Digest::SHA1 v2.10	       48.8
	SHA-256	       256     Digest::SHA v4.3.1	       41.3
	Haval-256      256     Digest::Haval256 v1.0.4	       39.8
	SHA-384	       384     Digest::SHA2 v1.1.0	       19.6
	SHA-512	       512     Digest::SHA2 v1.1.0	       19.3
	SHA-384	       384     Digest::SHA v4.3.1	       19.2
	SHA-512	       512     Digest::SHA v4.3.1	       19.2
	Whirlpool      512     Digest::Whirlpool v1.0.2	       13.0
	MD2	       128     Digest::MD2 v2.03		9.5

	Adler-32	32     Digest::Adler32 v0.03		1.3
	CRC-16		16     Digest::CRC v0.05		1.1
	CRC-32		32     Digest::CRC v0.05		1.1
	MD5	       128     Digest::Perl::MD5 v1.5		1.0
	CRC-CCITT	16     Digest::CRC v0.05		0.8

       These numbers was achieved Apr 2004 with ActivePerl-5.8.3 running under
       Linux on a P4 2.8 GHz CPU.  The last 5 entries differ by being pure
       perl implementations of the algorithms, which explains why they are so
       slow.

SEE ALSO
       Digest::Adler32, Digest::CRC, Digest::Haval256, Digest::HMAC,
       Digest::MD2, Digest::MD4, Digest::MD5, Digest::SHA, Digest::SHA1,
       Digest::SHA2, Digest::Whirlpool

       New digest implementations should consider subclassing from
       Digest::base.

       MIME::Base64

       http://en.wikipedia.org/wiki/Cryptographic_hash_function

AUTHOR
       Gisle Aas <gisle@aas.no>

       The "Digest::" interface is based on the interface originally developed
       by Neil Winton for his "MD5" module.

       This library is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

	   Copyright 1998-2006 Gisle Aas.
	   Copyright 1995,1996 Neil Winton.

perl v5.10.0			  2007-12-18			     Digest(3)
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