# Functional style
use Digest::HMAC qw(hmac hmac_hex);
$digest = hmac($data, $key, \&myhash);
print hmac_hex($data, $key, \&myhash);
# OO style
$hmac = Digest::HMAC->new($key, "Digest::MyHash");
$digest = $hmac->digest;
$digest = $hmac->hexdigest;
$digest = $hmac->b64digest;
DESCRIPTIONHMAC is used for message integrity checks between two parties that share a secret key, and works in combination with some other Digest algorithm, usually MD5 or SHA-1. The HMAC mechanism is described in RFC 2104.
HMAC follow the common "Digest::" interface, but the constructor takes the secret key and the name of some other simple "Digest::" as argument.
The hmac() and hmac_hex() functions and the Digest::HMAC->new() constructor takes an optional $blocksize argument as well. The HMAC algorithm assumes the digester to hash by iterating a basic compression function on blocks of data and the $blocksize should match the byte-length of such blocks.
The default $blocksize is 64 which is suitable for the MD5 and SHA-1 digest functions. For stronger algorithms the blocksize probably needs to be increased.