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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, 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. 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, ... )
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->add_bits( $data, $nbits )
$ctx->add_bits( $bitstring )
The bits provided are appended to the message we calculate the digest
for. The return value is the $ctx object itself.
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)", i.e.
$nbits is silently ignored.
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))".
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. For those 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 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.
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
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 1998-2001,2003-2004 Gisle Aas.
Copyright 1995-1996 Neil Winton.
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