Digest - Modules that calculate message digests
$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);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:
This is the most compact form, but it is not well suited for printing or embedding in places that can't handle arbitrary data.
A twice as long string of lowercase hexadecimal digits.
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.
The following methods are available for all Digest:: modules:
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.
To know what arguments (if any) the constructor takes (the $args,... above) consult the docs for the specific digest implementation.
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).
The clone method creates a copy of the digest state object and returns a reference to the copy.
This is just an alias for $ctx->new.
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.
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.