PEP: 272
Title: API for Block Encryption Algorithms v1.0
Author: A.M. Kuchling <amk@amk.ca>
Status: Final
Type: Informational
Created: 18-Sep-2001
Post-History: 17-Apr-2002, 29-May-2002


Abstract
========

This document specifies a standard API for secret-key block
encryption algorithms such as DES or Rijndael, making it easier to
switch between different algorithms and implementations.


Introduction
============

Encryption algorithms transform their input data (called
plaintext) in some way that is dependent on a variable key,
producing ciphertext.  The transformation can easily be reversed
if and only if one knows the key.  The key is a sequence of bits
chosen from some very large space of possible keys.  There are two
classes of encryption algorithms: block ciphers and stream ciphers.

Block ciphers encrypt multibyte inputs of a fixed size (frequently
8 or 16 bytes long), and can be operated in various feedback
modes.  The feedback modes supported in this specification are:

======  ========  =====================
Number  Constant  Description
======  ========  =====================
1       MODE_ECB  Electronic Code Book
2       MODE_CBC  Cipher Block Chaining
3       MODE_CFB  Cipher Feedback
5       MODE_OFB  Output Feedback
6       MODE_CTR  Counter
======  ========  =====================

These modes are to be implemented as described in NIST publication
SP 800-38A [1]_.  Descriptions of the first three feedback modes can
also be found in Bruce Schneier's book *Applied Cryptography* [2]_.

(The numeric value 4 is reserved for MODE_PGP, a variant of CFB
described in :rfc:`2440`: "OpenPGP Message Format". This mode
isn't considered important enough to make it worth requiring it
for all block encryption ciphers, though supporting it is a nice
extra feature.)

In a strict formal sense, stream ciphers encrypt data bit-by-bit;
practically, stream ciphers work on a character-by-character
basis.  This PEP only aims at specifying an interface for block
ciphers, though stream ciphers can support the interface described
here by fixing 'block_size' to 1.  Feedback modes also don't make
sense for stream ciphers, so the only reasonable feedback mode
would be ECB mode.


Specification
=============

Encryption modules can add additional functions, methods, and
attributes beyond those described in this PEP, but all of the
features described in this PEP must be present for a module to
claim compliance with it.

Secret-key encryption modules should define one function::

    new(key, mode, [IV], **kwargs)

Returns a ciphering object, using the secret key contained in the
string 'key', and using the feedback mode 'mode', which must be
one of the constants from the table above.

If 'mode' is MODE_CBC or MODE_CFB, 'IV' must be provided and must
be a string of the same length as the block size.  Not providing a
value of 'IV' will result in a ``ValueError`` exception being raised.

Depending on the algorithm, a module may support additional
keyword arguments to this function.  Some keyword arguments are
specified by this PEP, and modules are free to add additional
keyword arguments.  If a value isn't provided for a given keyword,
a secure default value should be used.  For example, if an
algorithm has a selectable number of rounds between 1 and 16, and
1-round encryption is insecure and 8-round encryption is believed
secure, the default value for 'rounds' should be 8 or more.
(Module implementors can choose a very slow but secure value, too,
such as 16 in this example.  This decision is left up to the
implementor.)

The following table lists keyword arguments defined by this PEP:

============  ============================================
Keyword       Meaning
============  ============================================
counter       Callable object that returns counter blocks
              (see below; CTR mode only)

rounds        Number of rounds of encryption to use

segment_size  Size of data and ciphertext segments,
              measured in bits (see below; CFB mode only)
============  ============================================

The Counter feedback mode requires a sequence of input blocks,
called counters, that are used to produce the output.  When 'mode'
is MODE_CTR, the 'counter' keyword argument must be provided, and
its value must be a callable object, such as a function or method.
Successive calls to this callable object must return a sequence of
strings that are of the length 'block_size' and that never
repeats.  (Appendix B of the NIST publication gives a way to
generate such a sequence, but that's beyond the scope of this
PEP.)

The CFB mode operates on segments of the plaintext and ciphertext
that are 'segment_size' bits long.  Therefore, when using this
mode, the input and output strings must be a multiple of
'segment_size' bits in length.  'segment_size' must be an integer
between 1 and block_size\*8, inclusive.  (The factor of 8 comes
from 'block_size' being measured in bytes and not in bits).  The
default value for this parameter should be block_size\*8.
Implementors are allowed to constrain 'segment_size' to be a
multiple of 8 for simplicity, but they're encouraged to support
arbitrary values for generality.

Secret-key encryption modules should define two variables:

- block_size

  An integer value; the size of the blocks encrypted by this
  module, measured in bytes.  For all feedback modes, the length
  of strings passed to the encrypt() and decrypt() must be a
  multiple of the block size.

- key_size

  An integer value; the size of the keys required by this
  module, measured in bytes.  If key_size is None, then the
  algorithm accepts variable-length keys.  This may mean the
  module accepts keys of any random length, or that there are a
  few different possible lengths, e.g. 16, 24, or 32 bytes.  You
  cannot pass a key of length 0 (that is, the null string '') as
  a variable-length key.


Cipher objects should have two attributes:

- block_size

  An integer value equal to the size of the blocks encrypted by
  this object.  For algorithms with a variable block size, this
  value is equal to the block size selected for this object.

- IV

  Contains the initial value which will be used to start a
  cipher feedback mode; it will always be a string exactly one
  block in length.  After encrypting or decrypting a string,
  this value is updated to reflect the modified feedback text.
  It is read-only, and cannot be assigned a new value.


Cipher objects require the following methods:

- decrypt(string)

  Decrypts 'string', using the key-dependent data in the object
  and with the appropriate feedback mode.  The string's length
  must be an exact multiple of the algorithm's block size or, in
  CFB mode, of the segment size.  Returns a string containing
  the plaintext.

- encrypt(string)

  Encrypts a non-empty string, using the key-dependent data in
  the object, and with the appropriate feedback mode.  The
  string's length must be an exact multiple of the algorithm's
  block size or, in CFB mode, of the segment size.  Returns a
  string containing the ciphertext.

Here's an example, using a module named 'DES'::

    >>> import DES
    >>> obj = DES.new('abcdefgh', DES.MODE_ECB)
    >>> plaintext = "Guido van Rossum is a space alien."
    >>> len(plaintext)
    34
    >>> obj.encrypt(plaintext)
    Traceback (innermost last):
      File "<stdin>", line 1, in ?
    ValueError: Strings for DES must be a multiple of 8 in length
    >>> ciphertext = obj.encrypt(plain+'XXXXXX')   # Add padding
    >>> ciphertext
    '\021,\343Nq\214DY\337T\342pA\372\255\311s\210\363,\300j\330\250\312\347\342I\3215w\03561\303dgb/\006'
    >>> obj.decrypt(ciphertext)
    'Guido van Rossum is a space alien.XXXXXX'


References
==========

.. [1] NIST publication SP 800-38A, "Recommendation for Block Cipher
       Modes of Operation" (http://csrc.nist.gov/encryption/modes/)

.. [2] Applied Cryptography


Changes
=======

2002-04: Removed references to stream ciphers; retitled PEP;
prefixed feedback mode constants with ``MODE_``; removed PGP feedback
mode; added CTR and OFB feedback modes; clarified where numbers
are measured in bytes and where in bits.

2002-09: Clarified the discussion of key length by using
"variable-length keys" instead of "arbitrary-length".


Acknowledgements
================

Thanks to the readers of the python-crypto list for their comments on
this PEP.


Copyright
=========

This document has been placed in the public domain.
