24 March 1999. Thanks to Dianelos, Vin and Dan.
See report of Day 1: http://jya.com/aes2-day1.htm
This is a second bulletin from the Rome AES2 conference,
as posted to the sci.crypt newsgroup by the perceptive Dianelos
Georgoudis, one of the designers of FROG, an AES candidate.
The NIST website offers most of the AES2 papers at:
Subject: Re: Live from the Second AES Conference Date: Wed, 24 Mar 1999 14:18:38 GMT From: email@example.com Newsgroups: sci.crypt Here is my report on the second and last day of AES2. First came five papers with analysis of several algorithms (in general it was felt that the analytic results of the first round were relatively few): Sean Murphy described properties of Twofish pertaining to its key schedule. First it was shown that the whitening keys (XORed with the data before and after the main cipher) are not uniformly distributed, i.e. take not all possible values. In a similar vain, each 8-byte round subkey also cannot take all possible values. Ideally, of course, all key material within a cipher should be as close to random as possible, otherwise an attacker gains some predictive power. Later in the afternoon workers from the Twofish team explained exactly how much entropy was lost: the whitening key has 117 bits of entropy (instead of the optimal 128) and each round key has 63.2 bits of entropy (instead of 64). They claimed this is not a useful cryptanalytic property which does seems reasonable. John Kelsey, also from the Twofish design team, described a weakness in Safer+'s key schedule (256 bit key variant) that led to a possible attack requiring 2^200 encryptions - clearly a very theoretical attack. He finished suggesting an improvement to the cipher's key schedule. Later in the afternoon, Safer+'s designer, James Massey, conceded that his main attention was invested in the 128 bit variant and described a similar fix. Vincent Rijmen, one of Rijndael's designers, explained the attack on LOKI97, the first serious published attack against an AES candidate: a differential attack that needs 2^56 chosen plaintexts and a lineal attack that need 2^56 known plaintexts but works only on 2^(-25) of the keys. Then came David Wagner's exposition of the attack on Frog. David, by the way is still only a student in California. I liked his description of Frog as a cipher where the "wires" are parametrized to the key. This means that Frog looks like a large collection of individual ciphers with different internal wiring. The basic point is that the attacker can search and choose the instances where this wiring defines a weaker cipher, and then mount an attack on the weak key that gave rise to this configuration. (For those familiar with Frog the weak wiring happens when the bombpermu pointer addresses the previous byte.) It turns out that 2^(-33) of the keys are now weak and that in these cases the key can be broken with 2^58 chosen plaintexts. The decryption function of Frog has much slower diffusion (something first noticed by John Savard, a frequent participant in sci.crypt) which allows for a more successful attack that requires 2^36 chosen ciphertexts and works for 2^(-29.3) of the keyspace. A linear attack on Frog was also described. Frog-like ciphers do have this kind of problem: by putting some or most of the functional definition of the cipher within the key, the possibility exists that some of these functions will be weak. I suppose there are ways to fix Frog in order to strengthen it against the differential and lineal characteristics discovered by Wagner and achieve a much lower or zero number of weak keys, trivially: suppress the weak wiring configurations. (I did ask him to propose a fix but he was not forthcoming - I wonder whether using ADDs instead of XORs for diffusion would cut it). The really interesting question though is to find out if a generalized version of Frog that just follows its design principles to a higher degree would increase or decrease the frequency of Wagner weak keys. The whole idea in Frog is to do nothing designed specifically as a defense against a particular attack. So if Generalized Frog turns out to be weaker against Wagner's attack then it is bad news for the basic idea. But, if it turns out that "purer" (even less structured) forms of Frog are more resistant, then confidence in Frog's design principle will increase. So Wagner's attack can be used as a platform to evaluate the design methodology, something even more important than the candidate cipher itself. Finally Eli Biham described the attack against Magenta. The attack works with 2^64 chosen plaintexts and 2^64 complexity or 2^33 known plaintexts but 2^97 complexity. In the afternoon, Magenta's Klaus Huber gave a very short rebuttal, explaining that this weakness can easily be fixed and that Magenta does have some interesting theoretical and implementational properties. In software it is relatively slow though. Next came a series of theoretical papers about various candidates: Jaques Stern of DFC described an update of the cipher that is faster and can be made scalable to allow for different block sizes (a useful property of several other candidates). The DFC team worked really hard to have decorrelation theory serve as the theoretical underpinning of their cipher. (Also it seems that decorrelation modules can be inserted into other ciphers such as E2.) He explained that the proofs apply to an idealized version of the cipher that DFCs approximates. By the way, later this afternoon somebody asked whether "proofs" are really important in cipher design. Bruce Schneier's reasonable comment was that any proven property in a cipher increases the quality of its analysis and hence its security. He also made clear that proofs discuss a specific thread model and that general proofs for all possible threats are not really possible. Well, I tried to go the other way and asked Jaques whether DFC's designers had put any thought in defending against unknown attacks (the proofs on DFC pertain only to first order differential attacks). He clearly disliked the question and his answer was somehow patronizing in the sense that it is meaningless to discuss unknowns. There was laughter around the room but I really think this is no laughing matter: Every ten years or so a new powerful cryptanalytic technique is discovered. In AES's life span maybe five new types of attack will be published. Only in the last year unforeseen and extremely powerful attacks against smart cards were demonstrated. Also, several ciphers including Mars, E2 and Twofish (not to mention Frog) did include in their documentation at least some reasoning about their strength against unknown attacks. I understand that mathematicians prefer to work with well defined problems and consider vague problems as problems not worth thinking about, but ciphers are machines to be used in the real world where the givens are not always clear-cut. Kazukuni Kobara next discussed a very theoretical paper on pseudorandomness. After her talk somebody had to ask her how this work relates to E2, to which she answered, as if just remembering, that E2 does indeed achieve optimal pseudorandomness after two rounds. It is interesting to observe how cultural characteristics have an impact in this international process. The designers of the Asian candidates (E2 and Crypton) are really far too modest and this, I think, hurts their chances. American designers, of course, are the most crass, and I think this is a winning strategy. Next, James Massey explained why Safer+ diffusion is optimal. Safer+ is a mathematician's kind of cipher and its weakness were discovered in the key scheduler, precisely the place where math does not help a lot. RSA's Scott Contini quantified the differential properties of data-depending rotations followed by multiplications as used in MARS and RC6 - a technique that appears to be powerful and is also under the cloud of a RSA patent. In general, I am in favor of patents but not in favor of being able to patent such broad and obvious ideas as using data-depending rotations in ciphers; after all this is an available machine instruction. My very first cipher design done several years ago, GodSave, uses data-depending rotations like crazy and I had no idea that RSA was using them too. Finally, Dough Whiting described some new results on Twofish. Even better speeds and better implementation flexibility (even though it appears that the very fastest code is self-modifying - this may look "unpure" to some but I think that it is a valid optimization technique). Even more carefully crafted cryptanalytic results were given. Next came lunch. Today I sat with a guy who works with a large organization that builds encryption hardware for the government! Sorry, not NSA; I never met any of these guys even though they were there - they appeared to be playing No Such Agency. I certainly would have enjoyed talking to somebody from the NSA. No, my table companion worked for a private company and the government in question is UK's. I asked him why government ciphers are still implemented in hardware and not in software working in dedicated one chip computers. Apparently speed is not the answer - very high security communication is really low bandwidth. I didn't find his answers very convincing: He said that an alpha particle from space can knock out a transistor in the chip and modify the cipher. Then again it is a trivial matter to have not one but many CPUs computing the same cipher in parallel to account for such type of events. Also he claimed that software correctness was more costly to verify than a hardware design. Anyhow, I suspect that for all the mathematical image of cryptography, tradition and the inertia of ideas does play an important role. Later, I happened to speak to one of the top cryptographers of the world and I asked him one of my favourite questions: First assume two very good ciphers A and B, say two of the better AES candidates. If your life depended on it and you had to choose between A.A (doubling A's rounds), B.B (doubling B's rounds) or A.B (sequentially executing both ciphers), what would you do? The standard answer is that mixing ciphers is a bad idea because the resulting cipher could be weaker than each individual one, that the resulting cipher would be a completely new cipher which would require fresh analysis which is the only way to gain trust, etc. Well my companion's answer was "A.B", and he gave precisely the reasons I expected: a future attack that may work against A could very well stop at B's gate. After lunch came a long session with open discussions. Supposedly candidates would slug it out between themselves while seated in front of the audience - you know the Christians and lions metaphor that NIST's Miles had used to explain the choice of Rome for the second AES conference. Well, to play it safe I sat at the very back of the room close to the exit. Thankfully, nothing dramatic happened. A few designers talked for a few minutes defending their ciphers and that was it. By the way all candidate ciphers, with the exception of HPC and LOKI97, were represented at this conference. Then came a long discussion about intellectual rights (IP) issues. Miles really seemed unhappy about the whole situation. Here is the problem: All candidates have renounced their property rights on their own ciphers if they are declared winners. This was a condition to enter the competition. So far so good, but even in the first conference the issue was raised about what would happen if a looser candidate claimed IP rights on some parts of the winner algorithm: this certainly could result in a very messy litigious situation that would defeat one of the basic ideas behind the AES: that it should be a global royalty-free algorithm. A few weeks back NIST sent to all candidates an informal question trying to measure how they stood in this matter. Several of them (CAST-256, Crypton, Deal, Frog, LOKI97, Rijndael, Serpent and Twofish) gave a clear positive answer. The others (DFC, E2, HPC, Magenta, MARS, RC6 and Safer) were not that forthcoming. The situation is not as bad as it looks. For example, one of the favorite ciphers, RC6, in only asking for some kind of honorific recognition if some of its IP is used by the AES winner. Another favorite, IBM's MARS, had a more convoluted position. They claimed the NIST's clarification question was not very clear itself and that it literally said something very different - therefore their convoluted answer which striked me also as somehow ambiguous. Overall, my feeling is that NIST will apply gentle arm twisting behind the scenes before announcing the finalists for the second round and that it will succeed in getting all the important designers to agree to play it clean. After that a lively discussion followed with the participation of many people from the audience. The main themes: Smart cards. Several people expressed that the importance that was given to implementation issues of the AES on a low end smart card was not reasonable. The standard argument in favor of very tight implementations on smart cards was that there are cases where millions of smart cards are needed where even a small price difference has a large impact. Against that, somebody from the audience presented the following strong argument: cases where the AES is going to be used in a smart card are cases where you want to protect information that is valuable - otherwise you don't really need a 128 bit key and can use any of well known medium range ciphers. If you are protecting information that is valuable, then spending one dollar more for each card is a reasonable investment. Miles explained that originally NIST had not intended to require implementations on 8-bit platforms but that the initial comments supplied by the public were strongly in favor that NIST should add smart card flexibility to the AES. So, even though it will be judged positively if a candidate does operate efficiently on a smart card, he felt that the whole issue was becoming overrated. My feeling is that the PDA attacks against smart cards have really confused the situation here - possibly security for smart cards will have to follow completely novel design paradigms, maybe even hardware designs. (At the coffee break I approached a small group of people that included Adi Shamir who had talked about power analysis attacks the previous day. The normal implementation of Generalized Frog takes the secret key and compiles it to actual code that can then be loaded in a smart card. So I asked Adi if a situation where each smart card actually holds a different program processing only data would be an improvement. As expected the answer is no, because one of the easiest things to read from a card is the "signature" of the instructions being executed. My other idea, to have a card hold a string of keys that will be used only once might work with some applications, such as high security home banking. On the whole though the situation with smart cards is considered hopeless). Another big theme was to have not one but several candidates declared AES. The previous day Miles had explained the main ideas of Don Johnson's paper (who unfortunately was not present) and Bruce Schneier had presented a passionate case in favor of a unique AES. Today several participants spoke in favor of more than one AES (the main arguments were insurance against a weakness being found in the main winner and allowing for more specialized ciphers); I think these arguments won the day. Miles said that NIST would very carefully consider the advantages that more than one winner would represent. Having more than one NIST approved cipher does have another advantage nobody mentioned: it would quiet those paranoid minds that might suspect that the AES winner could be a NSA implanted mole. The question of number of rounds came up. NIST said it would consider a cipher secure if it could not be broken with 2^64 chosen texts. A thoughtful counter-argument was that different amounts of analysis result in different apparent minimal secure rounds. For example, complicated ciphers have an advantage here, because simpler ciphers allow for more elegant analysis and therefore attract more cryptanalysis. This gave me the idea that instead of trying to normalize security (a difficult proposition) and then comparing speeds, a better methodology could be to normalize speed (best assembly implementation on one or two reference platforms) and then compare security. After all, optimized speed on a given platform gives a good idea about an algorithm's workload. Also, speed could be normalized at such a high level that ALL competing ciphers would be pushed into insecurity which would then allow more or less reasonable quantitative comparisons to be made about their relative "structural" strenght. It is clearly too late to use this methodology in the first round but maybe it could be used in the second. I feel like sending NIST an official comment in this sense. I wonder what the reader thinks about this? At the very end of the session Miles explained what the future of the AES competition looks like: Comment period for Round I ends April 15. So if the reader wants to influence the process, the time to send in a comment is now. The email address for that is firstname.lastname@example.org April 19, the official comments will be published. May 15, NIST will accept "tweaks" on the algorithms. NIST is the final judge about what qualifies for a tweak. Mid-summer, probably by July 99 the finalists will be announced (probably no more than five but possibly more). At the same time the justification will be given for their choice. In this point of time, Round 2 begins. Now the finalists may submit new optimized code for any platforms the feel like. CD-ROMs will be distributed 2-3 months after the start of Round 2 with the new code and most relevant information about the AES process. Miles expressed the hope that in Round 2 more serious cryptanalytic work will be presented. Efficiency will now be tested also for the 192 and 256 bit variants. Hardware performance estimation for all finalists will by supplied by NSA. IP issues will be clarified. January 15, 2000 is closing day for presenting papers for AES3. Week of April 10, 2000, AES3 will be held in New York. May 15, 2000 comment period of Round 2 closes. Approximately August 2000 the winner(s) are announced. The "(s)" was on NIST's slide. So who will the winner(s) be? No idea. Many people expressed the opinion that this may indeed not be a fully technical choice. For example, it is difficult to imagine the main AES chosen by NIST for the US government could be a foreign cipher. (By the way, this makes another point in favor of more than one winner. Jaques Stern did mention something rather cryptical about a possible European cipher competition.) Who will make to the second round? I do feel there is some general consensus. Here are my choices: RC6 and MARS will make it for their pedigree not to mention the fact that they are fine ciphers with probably a lot of work invested in them. Rijndael should make on merit alone. After that, things become somehow less clear. Serpent is a favorite. It is considered a very conservative design and everybody agrees that security is indeed the preeminent concern. Its designers have some of the sharpest minds in the field and I think NIST will choose Serpent just for keeping Shamir and Biham in the competition, not to mention motivate them to attack everybody else. Twofish is all over the place; it would be cruel to kill it off so soon. Also it is a very valiant effort and certainly it is a fine cipher. Extracting the full key out a smart card with Twofish is widely considered an implementation issue rather than a weakness of the cipher itself. If you really want to be logical here, this makes no sense. On the other hand if you would kick Twofish out for this reason alone then there would a blood bath with most of the well considered candidates being thrown overboard too. Twofish does have some good pedigree (it came out of Blowfish), is fast and flexible on many platforms and appears to be strong (results to date are not really significant). Finally, as a nod to Asia, I think NIST will pick E2 too. It is certainly a very fine cipher considering its analysis, its theoretical underpinning and its speed. The Japanese presenters are probably the most sympathetic and choosing E2 would include some ladies in the finalist makeup. Also nobody wants to miss the nice accents. So here is my guess: MARS, RC6, Serpent and Twofish from the US, Rijndael from the EU and E2 from Japan. -30- -------------------------- <First Response on sci.crypt> Subject: Re: Live from the Second AES Conference Date: 24 Mar 1999 16:14:17 +0000 From: Alan Braggins <email@example.com> Organization: nCipher Corporation Newsgroups:sci.crypt firstname.lastname@example.org writes: > it appears that the very fastest code is self-modifying - this may > look "unpure" to some but I think that it is a valid optimization > technique). [...] > improvement. As expected the answer is no, because one of the > easiest things to read from a card is the "signature" of the > instructions being executed. Any ideas whether self-modifying code would make reading instruction signatures easier or more difficult? ----- Vin McLellan + The Privacy Guild + <email@example.com> 53 Nichols St., Chelsea, MA 02150 USA <617> 884-5548 -- <@><@> --