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.

       To know what arguments (if any) the constructor takes (the
       "$args,..." above) consult the docs for the specific digest
       implementation.

       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 can just ignore this
       method since typical applications involve only whole-byte data.

       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 without padding.

   $ctx->base64_padded_digest
       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 <[email protected]>

   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.

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