[cryptography] RSA OAEP (Was: Explaining crypto to engineers)

Tom Ritter tom at ritter.vg
Thu Mar 8 13:38:45 EST 2012


On 26 February 2012 14:21, Ondrej Mikle <ondrej.mikle at nic.cz> wrote:
> I've just found an article about the OAEP padding oracle (that I couldn't recall
> before):
>
> http://ritter.vg/blog-mangers_oracle.html
>
> Reportedly there is no major implementation that would suffer from error
> side-channel, although there is an interesting experiment with timing side-channel.

Hey that's me! An error-based side channel does seem to prevented in
all libraries, and I got a good timing side channel out of libgcrypt -
but smaller timing side channels may be present in others.  Pascal
Junod shows that even OpenSSL that tried to prevent this attack
specifically may be vulnerable to a timing side channel on local or
embedded systems: http://crypto.junod.info/hashdays10_talk.pdf because
it uses a very short branch (slide 59 abouts).

On 26 February 2012 22:25, Peter Gutmann <pgut001 at cs.auckland.ac.nz> wrote:
> Ondrej Mikle <ondrej.mikle at nic.cz> writes:
>
>>I've just found an article about the OAEP padding oracle (that I couldn't
>>recall before):
>
> There's another one that was published about a year ago that looks at things
> like side-channel attacks via the integer-to-octet-string conversion
> primitives and other really low-bandwidth channels, I think it was "Manger's
> Attack Revisited".  At the time I was thinking of doing a writeup on generalised
> defences (via randomisation) against this sort of thing because as Revisited
> points out, you're always going to get timing channels somewhere if you look
> hard enough and a generalised defence would be better than the penetrate-and-
> patch approah to stopping timing channels.

Interesting, this appears to be it:
http://www.cdc.informatik.tu-darmstadt.de/reports/reports/mangers_attack_revisited.pdf

-tom



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