Why RSA encryption padding is critical (cool RSA implementation flaw) | Hacker News Reader

Why RSA encryption padding is critical (cool RSA implementation flaw)

rdist.root.org

22 pointstptacek9 comments

A generalization of the attack (and many more attacks) is given in: http://crypto.stanford.edu/~dabo/papers/RSA-survey.pdf

The attack seems to be based on the arbitrary assumption that e=3. Isn't e always a huge number in practice?

Yes. 65537 is commonly used.

The lower your public exponent in RSA, the faster you can verify signatures. So e=3 is still used quite often in embedded applications. It can be secure if used properly, but for most cases, it is safer to use 65537 as that can protect you from actual exploitation even if you have a variety of implementation bugs.

Hell, even if you do send just one message and do pad it, but the padding sucks (i.e. is known to the adversary) and e is small, you can use Coppersmith's theorem to break the encryption.

This is really just reason number 242151205 why you shouldn't attempt to build your own crypto system.

This is really just reason number 242151205 why you shouldn't attempt to build your own crypto system.

No, it's reason 242151205 why you should read the established standards. RFC 3447 says "use RSA-OAEP for encryption and RSA-PSS for signing".

If you have security flaws due to doing something non-standard in any field other than cryptography, people will say you're an idiot. Why does cryptography get a free pass with "oh well, we all know that stuff's hard"?

I don't think that's correct. Coppersmith's attack requires at least 2 messages. You're talking about the paper "Low-Exponent RSA with Related Messages (1996)" right?

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.33.6...

No... what I'm referring to is given in http://crypto.stanford.edu/~dabo/papers/RSA-survey.pdf -- basically using LLL to bring the numbers down to below the RSA modulo. (I know it works, because I wrote Mathematica code to implement it. :) )

slightly OT, but another good anecdote about buggy RSA implementation

http://media.ccc.de/browse/congress/2008/25c3-2799-en-consol...

*edit: it's about Team Tweezer exploiting the RSA implementation on nintendo's Wii to run unsigned code.

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