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Robert Bihlmeyer >
gcrypt-0.3 >
GCrypt

Module Version: 0.3
GCrypt - Perl interface to the GNU Crypto library

use GCrypt; $cipher = new GCrypt::Cipher('aes', 'cbc'); $cipher->setkey('a secret'); $cipher->setiv('init vector'); $ciphertext = $cipher->encrypt('plaintext'); $plaintext = $cipher->decrypt($ciphertext);

GCrypt is the Perl interface to the same-named LGPL'd library of cryptographic functions. Currently only symmetric encryption/decryption is supported by this interface.

Note that if you are still confused by the crypto terminology, head over to "BACKGROUND" first.

Symmetric encryption/decryption is done by first obtaining a Cipher object:

$cipher = new GCrypt::Cipher(*ALGORITHM*[, *MODE*[, *FLAGS*]]);

*ALGORITHM* is a string naming the algorithm. At the time of writing, the following choices are available:

**3des**Triple DES, 112 bit key**aes**The Advanced Encryption Standard, a.k.a. Rijndael, 128 bit key**aes192**AES with 192 bit key**aes256**AES with 256 bit key**blowfish****cast5****des**Date Encryption Standard, 56 bit key, too short to thwart brute-forcing**twofish**Successor of Blowfish, 256 bit key**arcfour**stream cipher

*MODE* is a string specifying one of the following encryption/decryption modes:

**stream**only possible mode for stream ciphers**ecb**don't use an IV, encrypt each block on it's own**cbc**the current ciphertext block is encryption of current plaintext block xor-ed with last ciphertext block**cfb**the current ciphertext block is the current plaintext block xor-ed with the current keystream block, which is the encryption of the last ciphertext block**ofb**the current ciphertext block is the current plaintext block xor-ed with the current keystream block, which is the encryption of the last keystream block

Between calls the "last block" is stored in the IV.

If no mode is specified **cbc** is selected for block ciphers, and **stream** for stream ciphers.

*FLAGS* is a string containing zero or more flags seperated by a pipe (`|`

). The possible flags are:

**secure**all data associated with this cipher will be put into non-swappable storage, if possible.**enable_sync**enable the CFB sync operation.-
$cipher->setkey(

*KEY*)Encryption and decryption operations will use

*KEY*until a different one is set. If*KEY*is shorter than the cipher's keylen (see the`keylen`

method) it will be zero-padded, if it is longer it will be truncated.$cipher->setiv([

*IV*])Set the initialisation vector to

*IV*for the next encrypt/decrypt operation. If*IV*is missing a "standard" IV of all zero is used. The same IV is set in newly created cipher objects.$cipher->encrypt(

*PLAINTEXT*)This method encrypts

*PLAINTEXT*with $cipher, returning the corresponding ciphertext. Null byte padding is automatically appended if*PLAINTEXT*'s length is not evenly divisible by $cipher's block size.$cipher->decrypt(

*CIPHERTEXT*)The counterpart to encrypt, decrypt takes a

*CIPHERTEXT*and produces the original plaintext (given that the right key was used, of course).$cipher->keylen()

Returns the number of bytes of keying material this cipher needs.

$cipher->blklen()

As their name implies, block ciphers operate on blocks of data. This method returns the size of this blocks in bytes for this particular cipher. For stream ciphers

`1`

is returned, since this implementation does not support feeding less than a byte into the cipher.$cipher->sync()

Apply the CFB sync operation.

None, as the interface is object-oriented.

*Symmetric ciphers* are basically black boxes that you prime with a *key*. Then you can feed them *plaintext*, which they will munch into the encrypted result called *ciphertext*. They work into the other direction as well (hence the "symmetric"), taking *ciphertext* as input and reconstructing it into *plaintext*.

There are two kind of symmetric ciphers: *block ciphers* like **AES** take their input in chunks of a fixed size (e.g. 256 bit), producing a corresponding block of output (usually of the same size) for each such chunk. If the plaintext length is not evenly divisible by the block size, *padding* (normally a suitable number of null bytes) is appended to the end. This has to be removed again after decryption.

*stream ciphers* take input one bit at a time (you can think of them as special block ciphers with the smallest possible block size), and produce a corresponding output bit. Their advantage is that each bit of plaintext can be immediately encrypted as soon as it is available (think: encryption of an audio stream).

The gcrypt manual should be available via `info gcrypt`

from the shell or `C-h i g (gcrypt)`

from inside emacs.

Robert Bihlmeyer, <robbe@orcus.priv.at>

Copyright 2002 by Robert Bihlmeyer

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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