NAME

Crypt::Ctr - Encrypt Data in Counter Mode

SYNOPSIS

        use Crypt::Ctr;

        my $cipher = new Crypt::Ctr $key, 'Crypt::Rijndael';

        my $ciphertext = $cipher->encrypt($plaintext);
        my $plaintext = $cipher->decrypt($ciphertext);

        my $cipher2 = new Crypt::Ctr $key, 'Digest::MD5';

        $ciphertext = $cipher->encrypt($plaintext);
        $plaintext = $cipher->decrypt($ciphertext);

DESCRIPTION

Generic Counter Mode implementation in pure Perl. The Counter Mode module constructs a stream cipher from a block cipher or cryptographic hash funtion and returns it as an object. Any block cipher in the "Crypt::" class can be used, as long as it supports the "blocksize" and "keysize" methods. Any hash function in the "Digest::" class can be used, as long as it supports the "add" method.

Note

Counter mode produces the keystream independent from the input. Be sure not to re-use keys in Counter mode. As with Cipher Feedback mode, one should use Counter mode inside authenticated channels, e.g. HMAC.

METHODS

"$cipher = new Crypt::Ctr $key, $algorithm": Constructs a Crypt::Ctr object. If $algorithm is a block cipher, then $key should be of the correct size for that cipher. In most cases you can inquire the block cipher module by invoking the "keysize" method. If $algorithm is a hash function, then $key can be of any size.
"$ciphertext = $cipher->encrypt $plaintext": Encrypts $plaintext. The input is XORed with the keystream generated from the internal state of the Ctr object and that state is updated with the output. $plaintext can be of any length.
"$cipher->reset": Resets the internal state. Remember to do that before decrypting, if you use the same object.
"$plaintext = $cipher->decrypt $ciphertext": Decrypts $ciphertext.

BUGS

This is awfully slow. Some classes in "Digest::" do not provide the "add" method, so they will fail. The internal counter is a Perl integer. This could possibly lead to strange errors when encrypting more than "POSIX::LONG_MAX" bytes and decrypting it on a different architecture.

AUTHOR

Matthias Bauer <matthiasb@acm.org>