[Python-checkins] CVS: python/nondist/peps pep-0272.txt,NONE,1.1

[A.M. Kuchling]

Update of /cvsroot/python/python/nondist/peps
In directory usw-pr-cvs1:/tmp/cvs-serv25751

Added Files:
	pep-0272.txt 
Log Message:
Initial check-in of PEP 272, "API for Secret-Key Encryption Algorithms"


--- NEW FILE: pep-0272.txt ---
PEP: XXX
Title: API for Secret-Key Encryption Algorithms
Version: $Revision: 1.1 $
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 (we hope).  The key is a sequence
    of bits chosen from some very large space of possible keys.

    Block ciphers take multibyte inputs of a fixed size (frequently 8
    or 16 bytes long) and encrypt them.  Block ciphers 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 used by the OpenPGP standard

    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.  After importing a
    given module, there is exactly one function and two variables
    available.

    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:

    blocksize

        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{blocksize} will be 1.

    keysize

        An integer value; the size of the keys required by this
        module.  If keysize is zero, 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.

    All cipher objects have at least three attributes:

    blocksize

        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.

    keysize (XXX this is in mxCrypto, but do we actually need this?  
        I can't remember why it was there, and it seems stupid.)

        An integer value equal to the size of the keys used by this
        object.  If keysize is zero, then the algorithm accepts
        arbitrary-length keys.  For algorithms that support variable
        length keys, this will be 0.  Identical to the module variable
        of the same name.  It does *not* contain the size of the key
        actually

    The methods for secret-key encryption objects are as follows:

    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-null 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.



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