EMAC(3) User Contributed Perl Documentation EMAC(3)NAMEDigest::EMACEXAMPLE 1
use Digest::EMAC qw(emac hexdigest base64digest);
print "Enter key: ";
my $key = <STDIN>;
chomp $key;
print "Enter cipher: ";
my $cipher = <STDIN>;
chomp $cipher;
print "Enter string: ";
my $str = <STDIN>;
chomp $str;
my $mac = emac($key, $cipher, $str);
print hexdigest($mac), "\n";
print base64digest($mac), "\n";
EXAMPLE 2
use Digest::EMAC qw(emac hexdigest base64digest);
print "Enter key: ";
my $key = <STDIN>;
chomp $key;
print "Enter cipher: ";
my $cipher = <STDIN>;
chomp $cipher;
print "Enter file: ";
my $infile = <STDIN>;
chomp $infile;
local $/ = undef;
open F, $infile;
my $data = <F>;
my $mac = emac($key, $cipher, $data);
close F;
print hexdigest($mac), "\n";
print base64digest($mac), "\n";
EXAMPLE 3
use Digest::EMAC qw(emac hexdigest base64digest);
print "Enter key: ";
my $key = <STDIN>;
chomp $key;
print "Enter cipher: ";
my $cipher = <STDIN>;
chomp $cipher;
local $/ = undef;
while (<>) {
my $mac = emac($key, $cipher, $_);
print hexdigest($mac), "\n";
}
DESCRIPTION
This is Encrypted MAC (EMAC), also known as Double MAC (DMAC). Unlike
HMAC, which reuses an existing one-way hash function, such as MD5,
SHA-1 or RIPEMD-160, EMAC reuses an existing block cipher to produce a
secure message authentication code (MAC).
Using the block cipher, a message is encrypted in CBC mode. The last
block is taken as the MAC of the message. For fixed-length messages,
this method is provably secure. In reality, however, messages have
arbitrary lengths, and this method is not secure. To make secure MACs
for variable length messages, the last block is encrypted once more
with a different key. The security of this construction has been proved
in the paper, ``CBC MAC for Real-Time Data Sources'' by Erez Petrank
and Charles Rackoff. The security can be proved on the assumption that
the underlying block cipher is pseudo-random.
The performance and key-agility of EMAC are reasonable. EMAC is prefer-
able for short messages because the block length is smaller compared to
the schemes based on a hash function. EMAC is also chosen as one of the
NESSIE winners for Message Authentication Codes, along with UMAC, TTMAC
and HMAC. The current NESSIE specification chooses the AES as block
cipher.
Also specified in the paper by Petrank and Rackoff is the construction
of two encryption keys from a single key. The first subkey is derived
by encrypting `0' (zero) using the original secret key. To produce the
second subkey, a `1' (one) is encrypted using the original secret key.
The first subkey is used in CBC mode to encrypt the entire message. The
last ciphertext block is then re-encrypted with the second subkey. The
result is a MAC whose length is equal to the block length of the cipher
used.
PREREQUISITES
The module Crypt::CBC is required, plus any block cipher that is capa-
ble of returning its block size when queried. RC5 is not supported,
however, because its block size is variable.
MIME::Base64 is also required for base64 encoding of output.
LICENSE
This program is free software; you can redistribute it or modify it
under the terms of the GNU General Public License.
AUTHOR
Julius C. Duque <jcduque (AT) lycos (DOT) com>
perl v5.8.8 2004-07-03 EMAC(3)
