[cross-posted to tahoe-...@allmydata.org and cryptogra...@metzdowd.com]


It doesn't look like I'm going to get time to write a long post about this bundle of issues, comparing Cleversafe with Tahoe-LAFS (both use erasure coding and encryption, and the encryption and key-management part differs), and arguing against the ill-advised Fear, Uncertainty, and Doubt that the Cleversafe folks have posted. So, I'm going to try to throw out a few short pieces which hopefully each make sense.

First, the most important issue in all of this is the one that my programming partner Brian Warner already thoroughly addressed in [1] (see also the reply by Jason Resch [2]). That is the issue of access control, which is intertwined with the issues of key management. The other issues are cryptographic details which are important to get right, but the access control and key management issues are the ones that directly impact every user and that make or break the security and usefulness of the system.

Second, the Cleversafe documents seem to indicate that the security of their system does not rely on encryption, but it does. The data in Cleversafe is encrypted with AES-256 before being erasure-coded and each share stored on a different server (exactly the same as in Tahoe-LAFS). If AES-256 is crackable, then a storage server can learn information about the file (exactly as in Tahoe-LAFS). The difference is that Cleversafe also stores the decryption key on the storage servers, encoded in such a way that any K of the storage servers must cooperate to recover it. In contrast, Tahoe-LAFS manages the decryption key separately. This added step of including a secret-shared copy of the decryption key on the storage servers does not make the data less vulnerable to weaknesses in AES-256, as their documents claim. (If anything, it makes it more vulnerable, but probably it has no effect and it is just as vulnerable to weaknesses in AES-256 as Tahoe-LAFS is.)

Third, I don't understand why Cleversafe documents claim that public key cryptosystems whose security is based on "math" are more likely to fall to future advances in cryptanalysis. I think most cryptographers have the opposite belief -- that encryption based on bit-twiddling such as block ciphers or stream ciphers is much more likely to fall to future cryptanalysis. Certainly the history of modern cryptography seems to fit with this -- of the original crop of public key cryptosystems founded on a math problem, some are still regarded as secure today (RSA, DH, McEliece), but there has been a long succession of symmetric crypto primitives based on bit twiddling which have then turned out to be insecure. (Including, ominously enough, AES-256, which was regarded as a gold standard until a few months ago.)

Fourth, it seems like the same access control/key management model that Cleversafe currently offers could be achieved by encrypting the data with a random AES key and then using secret sharing to split the key and store on share of the key with each server. I *think* that this would have the same cryptographic properties as the current Cleversafe approach of using an All-Or-Nothing-Transform followed by erasure coding. Both would qualify as "computation secret sharing" schemes as opposed to "information-theoretic secret sharing" schemes. I would be curious if there are any significant differences between these two constructions.

I don't think there is any basis to the claims that Cleversafe makes that their erasure-coding ("Information Dispersal")-based system is fundamentally safer, e.g. these claims from [3]: "a malicious party cannot recreate data from a slice, or two, or three, no matter what the advances in processing power." ... "Maybe encryption alone is 'good enough' in some cases now - but Dispersal is 'good always' and represents the future."

Fifth, as I've already mentioned, the emphasis on cryptography being defeated due to advances in processing power e.g. reference to Moore's Law is confused. Advances in processing power would not be sufficient to crack modern cryptosystems and in many cases would not be necessary either.

Okay I think that's it. I hope these notes are not so terse as to be confusing or inflammatory.


Zooko Wilcox-O'Hearn

[1] http://allmydata.org/pipermail/tahoe-dev/2009-July/002482.html
[2] http://allmydata.org/pipermail/tahoe-dev/2009-August/002514.html
[3] http://dev.cleversafe.org/weblog/?p=63

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