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Describe the segment_size and counter keyword arguments. 

Another bunch of rewrites and clarifications.

I now think this PEP is complete enough to implement.  My next step
will be to implement it and see if any parts turn out to be horribly
complicated or inefficient.  If nothing turns up, I'll finalize the
PEP; if there are problems, the PEP will be revised in light of
implementation experience
This commit is contained in:
Andrew M. Kuchling 2002-04-16 13:33:44 +00:00
parent bef8c5c6dd
commit c33c266f9f
1 changed files with 82 additions and 37 deletions
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119
pep-0272.txt
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@ -36,28 +36,32 @@ Introduction
6 MODE_CTR Counter
These modes are to be implemented as described in NIST publication
SP-800A[1]. Descriptions of the first three feedback modes can
SP-800A [1]. Descriptions of the first three feedback modes can
also be found in Bruce Schneier's book _Applied
Cryptography_ [2].
(The value of 4 is reserved for MODE_PGP, a variant of CFB
described in RFC 2440: "OpenPGP Message Format"[3]. This mode
(The numeric value 4 is reserved for MODE_PGP, a variant of CFB
described in RFC 2440: "OpenPGP Message Format" [3]. This mode
isn't considered important enough to make it worth requiring it
for all block encryption ciphers.)
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. Stream ciphers can use exactly the same interface as block
ciphers, fixing the block length at 1; this is how block and
stream ciphers can be distinguished. The only feedback mode
available for stream ciphers is ECB mode.
basis. Stream ciphers can support the API described here by using
fixing the block length at 1; this is how block and stream ciphers
could be distinguished. The only feedback mode available for
stream ciphers is ECB mode.
Specification
All cipher algorithms share a common interface.
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 define one function:
Secret-key encryption modules should define one function:
new(key, mode, [IV], **kwargs)
@ -65,33 +69,73 @@ Specification
string 'key', and using the feedback mode 'mode', which must be
one of the constants from the table above.
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?)
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. The most common keyword
argument will likely be 'rounds', to set the number of rounds to
be used.
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.)
Secret-key encryption modules define two variables:
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. 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, block_size will be 1.
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. For stream ciphers, 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.
module, measured in bytes. 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:
Cipher objects should have two attributes:
block_size
@ -111,34 +155,36 @@ Specification
decrypt(string)
Decrypts 'string, using the key-dependent data in the object,
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.
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; for stream ciphers, the string can be of any
length. Returns a string containing the ciphertext.
block size or, in CFB mode, of the segment 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.MODE_ECB)
>>> plain="Guido van Rossum is a space alien."
>>> len(plain)
>>> plaintext = "Guido van Rossum is a space alien."
>>> len(plaintext)
34
>>> obj.encrypt(plain)
>>> obj.encrypt(plaintext)
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
>>> 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(ciph)
>>> obj.decrypt(ciphertext)
'Guido van Rossum is a space alien.XXXXXX'
@ -153,7 +199,6 @@ References
http://www.faqs.org/rfcs/rfc2440.html)
Copyright
This document has been placed in the public domain.