[tahoe-dev] [pycryptopp] #18: AES-CTR: easy way to modify the counter for random-access decryption

[pycryptopp]

#18: AES-CTR: easy way to modify the counter for random-access decryption
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Reporter:  warner       |           Owner:       
    Type:  enhancement  |          Status:  new  
Priority:  major        |         Version:  0.5.1
Keywords:               |   Launchpad_bug:       
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 I'd like to improve Tahoe's download process to allow random-access
 decryption. To do this, I need to be able to tell the AES decryptor object
 to process data from arbitrary points in the keystream.

 I think it's possible to do this with the current API, by creating a new
 AES instance every time the counter jumps, and passing it an {{{iv=}}}
 argument which is a string into which we've packed the current offset. But
 this is too hard, and the {{{iv=}}} argument accepts arbitrary-length
 strings, making it difficult to confirm that we're supposed to pass in a
 string whose length is the same as the AES block size.

 I'd like to have an extra argument to {{{process()}}}, which will reset
 the counter value. This argument should take a positive number (an int or
 long). If this argument is not provided, the counter should use the normal
 self-incrementing value. So:

 {{{
  a = AES(key)
  data1 = AES.process("abcde")
  data2 = AES.process("fghij")
  data3 = AES.process("klmno")
 }}}

 should produce the same "data1", "data2", and "data3" as:

 {{{
  a = AES(key)
  data2 = AES.process("fghij", counter=5)
  data3 = AES.process("klmno")
  data1 = AES.process("abcde", counter=0)
 }}}

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Ticket URL: <http://allmydata.org/trac/pycryptopp/ticket/18>
pycryptopp <http://allmydata.org/trac/pycryptopp>
Python bindings for the Crypto++ library