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Lincoln D. Stein


Crypt::CBC - Encrypt Data with Cipher Block Chaining Mode


  use Crypt::CBC;
  $cipher = Crypt::CBC->new( {'key'             => 'my secret key',
                              'cipher'          => 'Blowfish',
                              'iv'              => '$KJh#(}q',
                              'regenerate_key'  => 0,   # default true
                              'padding'         => 'space',
                              'prepend_iv'      => 0,
                              'pcbc'            => 1  #default 0
  $ciphertext = $cipher->encrypt("This data is hush hush");
  $plaintext = $cipher->decrypt($ciphertext);
  while (read(F,$buffer,1024)) {
      print $cipher->crypt($buffer);
  print $cipher->finish;


This module is a Perl-only implementation of the cryptographic cipher block chaining mode (CBC). In combination with a block cipher such as DES or IDEA, you can encrypt and decrypt messages of arbitrarily long length. The encrypted messages are compatible with the encryption format used by SSLeay.

To use this module, you will first create a Crypt::CBC cipher object with new(). At the time of cipher creation, you specify an encryption key to use and, optionally, a block encryption algorithm. You will then call the start() method to initialize the encryption or decryption process, crypt() to encrypt or decrypt one or more blocks of data, and lastly finish(), to pad and encrypt the final block. For your convenience, you can call the encrypt() and decrypt() methods to operate on a whole data value at once.


  $cipher = Crypt::CBC->new( {'key'             => 'my secret key',
                              'cipher'          => 'Blowfish',
                              'iv'              => '$KJh#(}q',
                              'regenerate_key'  => 0,   # default true
                              'padding'         => 'space',
                              'prepend_iv'      => 0
  # or (for compatibility with earlier versions)
  $cipher = new Crypt::CBC($key,$algorithm);

The new() method creates a new Crypt::CBC object.

You must provide an encryption/decryption key, which can be any series of characters of any length. If regenerate_key is not specified as a false value, the actual key used is derived from the MD5 hash of the key you provide. The cipher is optional and will default to DES unless specified otherwise. You may use any compatible block encryption algorithm that you have installed. Currently, this includes Crypt::DES, Crypt::DES_EDE3, Crypt::IDEA, Crypt::Blowfish, and Crypt::Rijndael. You may refer to them using their full names ("Crypt::IDEA") or in abbreviated form ("IDEA").

An initialization vector may be specified, either by passing in a key of 'iv' as an option to new, or by calling $cipher->set_initialization_key($iv) before calling $cipher->start(). The IV will be ignored in decryption if the ciphertext is prepended by text which matches the regex /^RandomIV.{8}/, in which case the 8 characters following "RandomIV" will be used as the IV. When encrypting, by default the ciphertext will be prepended with "RandomIV<IV>" (16 bytes). To disable this, set 'prepend_iv' to a false value. The padding method can be specified by the 'padding' option. If no padding method is specified, PKCS#5 ("standard") padding is assumed.

Instead of the default cipher-block-chaining mode a modified algorithm PCBC can be used. It provides better error propagation characteristics than CBC encryption. To switch it on you have to set 'pcbc' to a true value. The PCBC mode is part of the des library and required e.g. in Kerberos4 authentication procedures as mentioned in RFC 2222 and other documents.



The start() method prepares the cipher for a series of encryption or decryption steps, resetting the internal state of the cipher if necessary. You must provide a string indicating whether you wish to encrypt or decrypt. "E" or any word that begins with an "e" indicates encryption. "D" or any word that begins with a "d" indicates decryption.


   $ciphertext = $cipher->crypt($plaintext);

After calling start(), you should call crypt() as many times as necessary to encrypt the desired data.


   $ciphertext = $cipher->finish();

The CBC algorithm must buffer data blocks inernally until they are even multiples of the encryption algorithm's blocksize (typically 8 bytes). After the last call to crypt() you should call finish(). This flushes the internal buffer and returns any leftover ciphertext.

In a typical application you will read the plaintext from a file or input stream and write the result to standard output in a loop that might look like this:

  $cipher = new Crypt::CBC('hey jude!');
  print $cipher->crypt($_) while <>;
  print $cipher->finish();


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

This convenience function runs the entire sequence of start(), crypt() and finish() for you, processing the provided plaintext and returning the corresponding ciphertext.


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

This convenience function runs the entire sequence of start(), crypt() and finish() for you, processing the provided ciphertext and returning the corresponding plaintext.

encrypt_hex(), decrypt_hex()

  $ciphertext = $cipher->encrypt_hex($plaintext)
  $plaintext  = $cipher->decrypt_hex($ciphertext)

These are convenience functions that operate on ciphertext in a hexadecimal representation. encrypt_hex($plaintext) is exactly equivalent to unpack('H*',encrypt($plaintext)). These functions can be useful if, for example, you wish to place the encrypted


  $iv = $cipher->get_initialization_vector()

This function will return the IV used in encryption and or decryption. This function may be useful to determine the random IV used when encrypting if none is specified in new(). The IV is not guaranteed to be set when encrypting until start() is called, and when decrypting until crypt() is called the first time.



This function sets the IV used in encryption and/or decryption. This function may be useful if the IV is not contained within the ciphertext string being decrypted, or if a particular IV is desired for encryption. If not set, a random IV will be generated. The IV is not guaranteed to be set when encrypting until start() is called, and when decrypting until crypt() is called the first time.

Padding methods

Use the 'padding' option to change the padding method.

When the last block of plaintext is shorter than the block size, it must be padded. Padding methods include: "standard" (i.e., PKCS#5), "oneandzeroes", "space", and "null".

   standard: (default) Binary safe
      pads with the number of bytes that should be truncated. So, if 
      blocksize is 8, then "0A0B0C" will be padded with "05", resulting
      in "0A0B0C0505050505". If the final block is a full block of 8 
      bytes, then a whole block of "0808080808080808" is appended.

   oneandzeroes: Binary safe
      pads with "80" followed by as many "00" necessary to fill the
      block. If the last block is a full block and blocksize is 8, a
      block of "8000000000000000" will be appended.

   null: text only
      pads with as many "00" necessary to fill the block. If the last 
      block is a full block and blocksize is 8, a block of 
      "0000000000000000" will be appended.

   space: text only
      same as "null", but with "20".

Both the standard and oneandzeroes paddings are binary safe. The space and null paddings are recommended only for text data. Which type of padding you use depends on whether you wish to communicate with an external (non Crypt::CBC library). If this is the case, use whatever padding method is compatible.

You can also pass in a custom padding function. To do this, create a function that takes the arguments:

   $padded_block = function($block,$blocksize,$direction);

where $block is the current block of data, $blocksize is the size to pad it to, $direction is "e" for encrypting and "d" for decrypting, and $padded_block is the result after padding or depadding.

When encrypting, the function should always return a string of <blocksize> length, and when decrypting, can expect the string coming in to always be that length. See _standard_padding(), _space_padding(), _null_padding(), or _oneandzeroes_padding() in the source for examples.

Standard and oneandzeroes padding are recommended, as both space and null padding can potentially truncate more characters than they should.


Two examples, des.pl and idea.pl can be found in the eg/ subdirectory of the Crypt-CBC distribution. These implement command-line DES and IDEA encryption algorithms.


The encryption and decryption process is about a tenth the speed of the equivalent SSLeay programs (compiled C). This could be improved by implementing this module in C. It may also be worthwhile to optimize the DES and IDEA block algorithms further.


Please report them.


Lincoln Stein, lstein@cshl.org

This module is distributed under the ARTISTIC LICENSE using the same terms as Perl itself.


perl(1), Crypt::DES(3), Crypt::IDEA(3), rfc2898 (PKCS#5)