[ pycrypto-Bugs-1054714 ] wishlist: variable length CTR mode

Bugs item #1054714, was opened at 2004-10-26 17:13
Message generated for change (Comment added) made by zooko
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Zooko O'Whielacronx (zooko)
Assigned to: Nobody/Anonymous (nobody)
Summary: wishlist: variable length CTR mode

Initial Comment:
I'm using AES in CTR mode.  It objects if I give it a
plaintext whose length isn't an integer multiple of 16.
 So I'll have to check for that, pad, encrypt, then
truncate.

It would be nice if pycrypto would do this itself.

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>Comment By: Zooko O'Whielacronx (zooko)
Date: 2005-07-16 17:59

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I've updated my patch to make CTR mode support variable
length texts so that it applies against 2.0.1.  (We need it
for a product I'm working on.)

I'll attach the current version of the patch to the thread
about the OFB patch in the patch tracker.

I would like to unify the CTR mode patch and the OFB patch,
but not right now as I need to go work on the aforementioned
product...


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Comment By: Zooko O'Whielacronx (zooko)
Date: 2004-10-27 12:58

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details about CTR mode and not-using-padding:

RFC 3711:

http://www.faqs.org/rfcs/rfc3711.html

the default encryption transform is AES-CTR mode.  "None of
the pre-defined encryption transforms uses any padding; for
these, the RTP and SRTP payload sizes match exactly"

Comments to NIST concerning AES Modes of Operations:
CTR-Mode Encryption (2000),  Helger Lipmaa, Phillip Rogaway,
David Wagner:
http://citeseer.ist.psu.edu/lipmaa00comments.html


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Comment By: Zooko O'Whielacronx (zooko)
Date: 2004-10-26 19:51

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Man, the linewrapping of this HTML form thingie is irritating.

I made symlinks so here are the slightly shorter URLs:

list of my patches:

http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zp?c=patches

The test patch:

http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zp/?c=diff&p=20041026183323-92b7f-c777a077c2f516680a548c486d4f3c2730b2b0e7.gz

(If that URL is too long to deal with then just go to the
list above and click on the name of the patch and then click
"diff" to get a unified diff.)

The variable-length-input-for-CTR-mode patch:
http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zp/?c=diff&p=20041026191834-92b7f-72995f81f2b92accfc02254869d2ea84ecdbc960.gz


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Comment By: Zooko O'Whielacronx (zooko)
Date: 2004-10-26 19:24

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CTR mode as typically used doesn't add any padding.

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Comment By: Zooko O'Whielacronx (zooko)
Date: 2004-10-26 19:23

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Okay I implemented it.
Here is a list of my patches to pycrypto:
http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zookopatches?c=patches&t=

The only interesting ones are this one:
http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zookopatches/?c=diff&p=20041026183323-92b7f-c777a077c2f516680a548c486d4f3c2730b2b0e7.gz

which adds a unit test for non-multiple-of-blocksize inputs
and this one:
http://zooko.com/cgi-bin/darcs.cgi/pycrypto/zookopatches/?c=diff&p=20041026191834-92b7f-72995f81f2b92accfc02254869d2ea84ecdbc960.gz

which makes CTR mode handle variable length inputs.



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Comment By: Nobody/Anonymous (nobody)
Date: 2004-10-26 17:24

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I'm almost certian this will never happen.  There are too
many variations on how to pad the block.  Also, truncating
the block will make it undecipherable.  If you want an easy
way to pad your string:

bs = 16
pad = '\x00'
s = 'something that is not of length 16'

s += (bs - (len(s) % bs)) * pad

Alternatively, you can use a stream cipher like RC4, which
doesn't have a block size.

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