Digest(3) User Contributed Perl Documentation 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 AT 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 v5.16.3 2011-10-02 Digest(3)
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