[ pycrypto-Bugs-1364920 ] bug in Zooko's fast-CTR-mode patch

Bugs item #1364920, was opened at 2005-11-23 19:07
Message generated for change (Comment added) made by zooko
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https://sourceforge.net/tracker/?func=detail&atid=120937&aid=1364920&group_id=20937

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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: bug in Zooko's fast-CTR-mode patch

Initial Comment:
My patch for fast CTR mode (all in C -- no Python) has
a bug in that it isn't "restartable" -- if you encrypt
or decrypt some data which is not itself a multiple of
the block length, then subsequent calls to
encrypt/decrypt will give incorrect results.

I'll make fixes for this soon.  In the meantime, here's
a unit test showing how it doesn't work.

https://yumyum.zooko.com:19144/cgi-bin/darcs.cgi/pycrypto-zookopatches/?c=patches

Wed Nov 23 15:00:57 AST 2005  [email protected]
  * add unit test that shows that CTR mode is not
restartable -- this is a bug
diff -rN -u
old-pycrypto-2.0.1-varlength-cctr/pycrypto-2.0.1/Util/test.py
new-pycrypto-2.0.1-varlength-cctr/pycrypto-2.0.1/Util/test.py
---
old-pycrypto-2.0.1-varlength-cctr/pycrypto-2.0.1/Util/test.py
2005-11-23 15:06:39.000000000 -0400
+++
new-pycrypto-2.0.1-varlength-cctr/pycrypto-2.0.1/Util/test.py
2005-11-23 15:06:39.000000000 -0400
@@ -146,6 +146,23 @@
     print_timing(256, end-start, verbose)
     del obj1, obj2
 
+    if verbose: print '  CTR mode, variable length,
restart:',
+    strv = str[:-1]
+    obj1=ciph.new(password, ciph.MODE_CTR,
counterstart=long_to_bytes(startctr, ciph.block_size))
+    obj2=ciph.new(password, ciph.MODE_CTR,
counterstart=long_to_bytes(startctr, ciph.block_size))
+    start=time.time()
+    tempstrv = strv
+    while tempstrv:
+        chunksiz = random.randrange(1, len(tempstrv)+1)
+        ciphertext = obj1.encrypt(tempstrv[:chunksiz])
+        tempstrv = tempstrv[chunksiz:]
+    plaintext=obj2.decrypt(ciphertext)
+    end=time.time()
+    if (plaintext!=strv):
+        die('Error in resulting plaintext from CTR
mode, variable length, restart')
+    print_timing(256, end-start, verbose)
+    del obj1, obj2
+
     # Test the IV handling
     if verbose: print '  Testing IV handling'
     obj1=ciph.new(password, ciph.MODE_CBC, IV)

----------------------------------------------------------------------

>Comment By: Zooko O'Whielacronx (zooko)
Date: 2005-12-05 20:12

Message:
Logged In: YES 
user_id=52562

The latest version fixes this bug in "restarting" ctr mode
usage by preserving the offset-within-the-block between
calls to crypt().

https://yumyum.zooko.com:19144/cgi-bin/darcs.cgi/pycrypto-zookopatches/?c=patches
https://yumyum.zooko.com:19144/pub/repos/pycrypto-zookopatches/
http://zooko.com/repos/pycrypto/

----------------------------------------------------------------------

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