Digest(3p) Perl Programmers Reference Guide Digest(3p)NAMEDigest - 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 "impossi-
ble" 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 inter-
face. A functional interface for simple use, as well as an
object oriented interface that can handle messages of arbi-
trary 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
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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 mes-
sage 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 con-
tains 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 refer-
ence 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 mes-
sage we calculate the digest for. The return value is
the $ctx object itself.
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$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 calcu-
late 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)", that is
$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 des-
tructive, read-once operation. Once it has been per-
formed, 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 hex-
adecimal form.
$ctx->b64digest
Same as $ctx->digest, but will return the digest as a
base64 encoded string.
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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 origi-
nally 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.
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Copyright 1998-2001,2003-2004 Gisle Aas.
Copyright 1995-1996 Neil Winton.
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