Alex Biryukov was so kind as to organize a public mirror of Wiki of ESC 2008 which took place in Echternach, Luxembourg from January 7th to 11th, 2008. I found this workshop extremely interesting and had quite a blast there. Lots of interesting slides for you on the mirror of the Wiki; not all of the talks were on symmetric-key cryptography. I’ve been meaning to write about some of the talks here, but due to my limited time available for such things at the moment I have kept postponing it thus far; there is something in my drafts folder though.

# Category Archives: symmetric

# MiFare’s CRYPTO1 algorithm mostly reverse-engineered

MiFare’s CRYPTO1 stream cipher has captured my attention for a while. However, hardware reverse-engineering is not a field I actively engage in. So I was very happy when Karsten Nohl (University of Virginia), Starbug and Henryk PlÃ¶tz gave a talk at the 24C3 [the 24th Congress of the Chaos Computer Club taking place in Berlin at this very moment] yesterday evening showing that they have reverse-engineered most parts of this cipher. CRYPTO1 uses a 48-bit LFSR-based filter generator to generate key stream.

The filter function – if I understood correctly – uses ~~24~~ 20 taps (this was not mentioned in the talk, I asked Karsten privately about this) however the degree of the boolean function implementing the filter , thus it remains to be seen whether algebraic attacks can be applied.Â Even if no algebraic attacks are applied, a BSW sampling TMTO will break CRYPTO1 completely. This was pretty obvious before they gave their talk, but now vendors actually have to worry about this being out in the wild once the feedback and the filter function have been revealed.

My colleague Erik took photos of the slides which I put up on Zooomr. A video recording of the talk should be available shortly and will be linked here.

__ Update 2008-01-02:__ A recording of the talk now is available (MPEG4, iPod compatible).

# TMTOs and lattice based collision attacks against LASH-x

LASH, the lattice-based hash function designed by Kamel Bentahar, Dan Page, Markku-Juhani O. Saarinen and Nigel Smart has been broken into tiny little pieces. A septumvirate of authors (Scott Contini, Krystian Matusiewicz, Josef Pieprzyk, Ron Steinfeld, Jian Guo, San Ling, and Huaxiong Wang) have submitted the following devastating analysis of LASH-x to FSE 2008:

- Preimages for LASH-x in about
*2*hash evaluations (and memory).^{0.36x} - Collisions for LASH-x in about
*2*work (and storage). LASH-160’s maximum security level thus effectively is about 58 bits, for LASH-256 we get 93 bits.^{0.57x} - Long-message attacks using lattice reduction techniques
- LASH is not a pseudo-random function (control 1 byte of input and distinguish the output from random with probability 2^{-8})
- A TMTO attack on the final compression function working in about
*O(x2*) time and memory^{x/4} - Heuristics for a CVP attack against LASH-160 (using NTL‘s BKZ implementation)

The pre-image and collision attacks presented in the paper exploit the specific fact that LASH uses an all-zero IV, however the authors also demonstrate that tweaking the hash function by changing the IV will not help much. An extended version of the article is available on the IACR’s ePrint server.**Update 2007-12-03:** as correctly pointed out in a comment by *random*, this paper has only been submitted to FSE 2008. It is not clear yet whether it is accepted or not, since notifications of acceptance will only be sent out on December, 10th, 2007. Sorry for the confusion!**Update 2007-12-18:** The list of accepted papers for the FSE 2008 is public. The LASH paper has been accepted.

# Meaningful results against Trivium with reduced key setup

Michael Vielhaber presents an interesting result against a reduced version of the eSTREAM submitted cipher Trivium in a paper on the IACR ePrint archive. By reducing the key setup from 4 full rounds (1152 clock cycles) to two cycles (576 cycles) he is able to mount a chosen IV algebraic attack using a total of 2^6 chosen IV resyncs. This reduced version is called ONE.FIVIUM by him. The impact of the attack against ONE.FIVIUM that 47 of the 80 key bits are claimed to be determined with probability 1 by this attack. The write up needs polishing but most of the results indeed mostly check out. Unfortunately the method by which he arrived at these results is (in my opinion, at least) only superficially described. I strongly suspect this paper is intended to mark a claim. Further results against a clock setup with more cycles are to be expected. Trivium is living dangerously.If you want to check out Michael Vielhaber’s results yourself, I have written Python code for just that. My colleague Andrey Pyshkin initially confirmed the 6 out of the first 7 approximations listed in the table, aidatest.py checks all of them. Below is the output of 256 runs.

approx. for key bits [1] was correct 256 out of 256 times approx. for key bits [2, 65] was correct 256 out of 256 times approx. for key bits [3, 66] was correct 0 out of 256 times approx. for key bits [4] was correct 256 out of 256 times approx. for key bits [5] was correct 256 out of 256 times approx. for key bits [6] was correct 256 out of 256 times approx. for key bits [8] was correct 256 out of 256 times approx. for key bits [9] was correct 256 out of 256 times approx. for key bits [11] was correct 256 out of 256 times approx. for key bits [14] was correct 256 out of 256 times approx. for key bits [16] was correct 256 out of 256 times approx. for key bits [17] was correct 256 out of 256 times approx. for key bits [19] was correct 256 out of 256 times approx. for key bits [25] was correct 256 out of 256 times approx. for key bits [26] was correct 256 out of 256 times approx. for key bits [27] was correct 256 out of 256 times approx. for key bits [36] was correct 256 out of 256 times approx. for key bits [38] was correct 256 out of 256 times approx. for key bits [39] was correct 256 out of 256 times approx. for key bits [55] was correct 256 out of 256 times approx. for key bits [56] was correct 256 out of 256 times approx. for key bits [57, 63] was correct 256 out of 256 times approx. for key bits [59, 65] was correct 256 out of 256 times approx. for key bits [60, 66] was correct 256 out of 256 times approx. for key bits [61] was correct 256 out of 256 times approx. for key bits [62] was correct 256 out of 256 times approx. for key bits [63] was correct 256 out of 256 times approx. for key bits [64] was correct 256 out of 256 times approx. for key bits [65] was correct 256 out of 256 times approx. for key bits [66] was correct 256 out of 256 times approx. for key bits [67] was correct 0 out of 256 times approx. for key bits [68] was correct 0 out of 256 times approx. for key bits [15] was correct 256 out of 256 times approx. for key bits [18] was correct 256 out of 256 times approx. for key bits [20] was correct 256 out of 256 times approx. for key bits [23] was correct 256 out of 256 times approx. for key bits [30] was correct 256 out of 256 times approx. for key bits [32] was correct 256 out of 256 times approx. for key bits [33] was correct 256 out of 256 times approx. for key bits [35] was correct 256 out of 256 times approx. for key bits [58] was correct 166 out of 256 times approx. for key bits [21] was correct 256 out of 256 times approx. for key bits [22] was correct 256 out of 256 times approx. for key bits [10] was correct 115 out of 256 times approx. for key bits [12] was correct 141 out of 256 times approx. for key bits [58] was correct 256 out of 256 times approx. for key bits [69] was correct 256 out of 256 times