Sunday, August 26, 2018

Cryptopals Set 6 -- Bleichenbacher


After another hiatus, I finally managed to wrap up Set 6 with Bleichenbacher's PKCS 1.5 padding oracle attack (original paper from '98 http://archiv.infsec.ethz.ch/education/fs08/secsem/bleichenbacher98.pdf).

https://github.com/tkuriyama/cryptopals/tree/master/set6

All of the padding-based attacks are quite interesting. From first principles, I guess it is difficult to design padding that is cryptographically sound, since by definition padding needs to be deterministic and reasonably easy to parse.

One thing that escapes me is the use of ceiling division for a number of equations in Bleichenbacher's paper -- it doesn't seem to be specified, yet all the implementations I referenced seemed to apply it consistently.

I will probably take a break from the Cryptopals for now (i.e. defer Sets 7 and 8) and tackle some other projects. I feel reasonably comfortable with F# now, but still want to learn Haskell, and meanwhile Python is fully transitioning to Python 3 and Guido has retired as BDFL...

The MOOC landscape seems to have advanced in the past year or two as well, so it will be interesting to take up a few more courses again.

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