From owning DNS to owning SSL (1/2)

Dan Kaminsky walked away with the Pwnie award for most overhyped bug and being a good sport, appeared in person with a brief acceptance speech. The BlackHat presentation did turn into an over-crowded spectacle as expected, there was nothing new to report. (Even though a section of the deck was prefaced with “Here is something that did not leak…”) The cat had been out of the bag, compliments of earlier speculation by Halvar Flake and a miscue by the folks at Matasano. And that’s just the public disclosure: the presentation itself credited several people who identified the same vulnerability within days independently but decided to remain quiet, in keeping with the unusual request.

The more interesting was the second piece of the talk: the question of “why”, why it is worth subverting DNS and what can be accomplished. Decidedly more speculative in nature, in this section Kaminsky argued that SSL,  most software updates and online identity management services are vulnerable. If these claims hold for real-world implementation, not simply the marginal ones written by careless developers, it would be more remarkable than the original discovery.

SSL and in general PKI were designed to be resilient against an untrusted network. The design of the protocols assumes the transport is completely unreliable. The metaphor this blogger uses to describe it in security orientation classes is two people, say Alice and Bob, trying to communicate but restricted exchanging post-it notes carried by a shady messenger. In this model it is clear the messenger may fail to deliver the note, and the two sides never manage to communicate. No surprise there. But more interestingly, our shady messenger can erase part of the mesage, add forged languge, for that matter replace the entire note by a new one fabricated out of thin air, change the order in which notes are delivered, even replay one person’s note back to him/her as if it originated from the other side. This bizarre threat model is intended to capture the man-in-the-middle (MITM) attack in the abstract, where a malicious adversary is capable of reading and modifying any message sent between two people.

Communication protocols including SSL/TLS are designed to be secure in this model, in the sense that the nefarious messenger can not read a private message intended for Alice, nor convince Alice that Bob sent a message he did not in fact originate. SSL/TLS protocol itself has lived up to this claim so far– there are no known, practical cryptographic attacks against the protocol itself (as opposed to specific implementations, which can have coding issues that are not intrinsic in the protocol)  The closest call was the Bleichenbacher attack against RSA padding first published in 1998 and later refined.



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