[p2p-hackers] convergent encryption reconsidered

[Ivan Krstić]

On Mar 30, 2008, at 3:12 PM, Leichter, Jerry wrote:
> How would that help?

Unless I'm misunderstanding Zooko's writeup, he's worried about an  
attacker going from a partially-known plaintext (e.g. a form bank  
letter) to a completely-known plaintext by repeating the following  
process:

1. take partially known plaintext
2. make a guess, randomly or more intelligently where possible,
    about the unknown parts
3. take the current integrated partial+guessed plaintext, hash
    to obtain convergence key
4. verify whether that key exists in the storage index
5. if yes, you've found the full plaintext. if not, repeat from '2'.

That's a brute force search. If your convergence key, instead of being  
a simple file hash, is obtained through a deterministic but  
computationally expensive function such as PBKDF2 (or the OpenBSD  
bcrypt, etc), then step 3 makes an exhaustive search prohibitive in  
most cases while not interfering with normal filesystem operation.  
What am I missing?

Cheers,

--
Ivan Krstić <krstic at solarsail.hcs.harvard.edu> | http://radian.org

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