Digest:: - Modules that calculate message digests

NAME

Digest:: - Modules that calculate message digests

SYNOPSIS

  $md2 = Digest->MD2;
  $md5 = Digest->MD5;

  $sha1 = Digest->SHA1;
  $sha1 = Digest->new("SHA-1");

  $hmac = Digest->HMAC_MD5($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.

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, i.e. 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.

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. the MIME::Base64 manpage 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 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,...): The $data provided as argument are appended to the message we calculate the digest for. The return value is the $ctx object itself.
$ctx->addfile($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.
$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 reseting 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.

AUTHOR

Gisle Aas <gisle@aas.no>