Ubuntu Manpage: Digest - Modules that calculate message digests

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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.

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