Monday, October 9, 2017

Cryptopals Set 3 -- CBC Padding Oracle


Problem 17 is an interesting exercise in implementing the CBC padding oracle attack, described elegantly in Serge Vaudenay's 2012 paper:

https://www.iacr.org/archive/eurocrypt2002/23320530/cbc02_e02d.pdf

The crux of the attack, of course, is that it only requires 1 bit of information (!) -- a yes or no from a padding oracle, indicating whether a given ciphertext's CBC padding is valid or not. It would be interesting to study at some point how the padding oracle is manifested in the real world (in older versions of TLS, etc).

The implementation took longer than expected. I'm fairly certain that I'm missing some functional programing constructs, as my solution is not very elegant... the problem (or point) being that non-mutative data structures are used, so there's quite a bit of generating new arrays (which can be verbose when one is just trying to change a single byte in the middle).

https://github.com/tkuriyama/cryptopals/blob/master/set3/p17.fsx



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