PEP: 272
Title: API for Secret-Key Encryption Algorithms
Version: $Revision$
Author: A.M. Kuchling <akuchlin@mems-exchange.org>
Status: Draft
Type: Informational
Created: 18-Sep-2001
Post-History:

Abstract

    This document specifies a standard API for secret-key encryption
    algorithms such as DES or Rijndael, making it easier to switch
    between different algorithms and implementations.  The API is
    intended to be suitable for both block and stream ciphers.


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.

    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         ECB           Electronic Code Book     
        2         CBC           Cipher Block Chaining
        3         CFB           Cipher FeedBack 
        4         PGP           Variant of CFB

    See _Applied Cryptography_ for descriptions of the first three
    feedback modes.  The PGP feedback mode is described in the OpenPGP
    RFC.

    In a strict formal sense, stream ciphers encrypt data bit-by-bit;
    practically, stream ciphers work on a character-by-character
    basis.  Stream ciphers use exactly the same interface as block
    ciphers, with a block length that will always be 1; this is how
    block and stream ciphers can be distinguished.  The only feedback
    mode available for stream ciphers is ECB mode.


Specification

    All cipher algorithms share a common interface.  

    Secret-key encryption modules 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 following table.  

    If 'mode' is CBC or 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 XXXError exception being raised.
    (what exception?  ValueError?  ciphermodule.error?)    

    Depending on the algorithm, a module may support additional
    keyword arguments to this function.  The most common keyword
    argument will likely be 'rounds', to set the number of rounds to
    be used.

    Secret-key encryption modules define two variables:

    block_size

        An integer value; the size of the blocks encrypted by this
        module.  For all feedback modes, the length of strings passed to
        the encrypt() and decrypt() must be a multiple of the block size.
        For stream ciphers, \code{block_size} will be 1.

    key_size

        An integer value; the size of the keys required by this
        module.  If key_size is None, then the algorithm accepts
        arbitrary-length keys.  You cannot pass a key of length 0
        (that is, the null string '') as such a variable-length key.

    Cipher objects require 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.
        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; for stream ciphers, the string can be of any
        length.  Returns a string containing the ciphertext.

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

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


References

    RFC2440: "OpenPGP Message Format" (http://rfc2440.x42.com,
    http://www.faqs.org/rfcs/rfc2440.html)

    Applied Cryptography


Copyright

    This document has been placed in the public domain.



Local Variables:
mode: indented-text
indent-tabs-mode: nil
End: