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Quantum Cryptography Broken, and Fixed

Posted by kdawson on Tue May 13, 2008 06:08 PM
from the knowing-the-unknowable dept.
Security Encryption
schliz writes in with research out of Sweden in which researchers showed that, looking at a quantum cryptographic system as a whole, it was possible for an eavesdropper to extract some information about the QC key, thus reducing the security of the overall system. The team then proposed a cheap and simple fix for the problem. "The advanced technology was thought to be unbreakable due to laws of quantum mechanics that state that quantum mechanical objects cannot be observed or manipulated without being disturbed. But a research team at Linköping University in Sweden claim that it is possible for an eavesdropper to [get around the limitations] without being discovered. In a research paper, published in the international engineering journal IEEE Transactions on Information Theory (abstract), the researchers propose a change in the quantum cryptography process that they expect will restore the security of the technology."
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Submission: Quantum cryptography broken - and fixed by schliz (994115)
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  • So is the cat dead? (Score:5, Funny)

    by EmbeddedJanitor (597831) on Tuesday May 13 2008, @06:11PM (#23397342)
    Quantum stuff is so illogical to us mortals that you'd expect attempting to break it would just make it stronger.

    • Re:So is the cat dead? (Score:5, Funny)

      by Tackhead (54550) on Tuesday May 13 2008, @06:25PM (#23397488)
      > Quantum stuff is so illogical to us mortals that you'd expect attempting to break it would just make it stronger.

      Which is precisely what happened.

      In a research paper, published in the international engineering journal IEEE Transactions on Information Theory (abstract), the researchers propose a change in the quantum cryptography process that they expect will restore the security of the technology.

      By being sufficiently precise about the nature of the insecurity, they changed the probability of its being insecure!

      Furthermore, now that we know it's secure again (that is, we've proven it to be secure, effectively computing the probability of insecurity to be precisely zero), we no longer know anything about the nature of the system's security holes again!

      That was all supposed to be a lead-up to a Heisenberg Uncertainty Principle joke, but it's actually a pretty good description of how computer security works in even the non-quantum world. The more secure you think your system is, the more likely it is you'll get 0wn3d in some completely unexpected way. The known unknowns aren't the ones you've gotta worry about, and nailing them down doesn't do anything about the unknown unknowns, other than to collapse the joke's waveform into something resembling a Don Rumsfeld speech.

      In anything other than a Slashdot quantum crypto discussion, that sort of whiplash-inducing change of joke subjects would be highly improbable. As it stands, I'm going to shift gears a third time and hand it off to Douglas Adams.

      Zaphod: Tackhead, is this sort of thing going to happen every time you post using the Infinite Improbability joke drive?
      Tackhead: Very probably, I'm afraid.

      • Re:So is the cat dead? (Score:5, Funny)

        by NotQuiteReal (608241) on Tuesday May 13 2008, @07:15PM (#23397828) Journal
        You can increase the complexity by using a tri-state cat.

        It can be either alive or dead or both alive and dead.

        We call these three states alive, dead and zombie.

        There, I hope that sheds some photons on the matter.

        • I thought a tri-state cat would be alive, dead and high-impedance.
          • I thought a tri-state cat would be alive, dead and high-impedance.

            Actually, "high-impedance" is how'd I'd describe the alive state for most cats - yikes!

        • *sigh* Dude, the whole point of the bi-state cat is that both alive and dead is exactly the state the cat ends up in. It's a superposition until you measure it. That's why it's so bizarre. Schrodinger's cat is a zombie.

          • To quote one of my favorite games [kingdomofloathing.com]:

            The cat looks up at you and, noticing a certain hungry gleam in your eye that it doesn't like one bit, jumps from the divan and hides in a box under the coffee-table. Just before the lid clicks shut, you see a tiny pendulum inside, and wonder if the cat's going to be alive for much longer. You reason that, since the cat could be either alive or dead, and you can't know which without opening the box, then therefore the cat must be both alive and dead -- or in other words, undead. That must be what funerals are for -- so that everyone knows for certain that the person going into the coffin is definitely dead, and you don't have to worry about quantum uncertainty causing zombies to burst out of the ground.

        • Re:So is the cat dead? (Score:4, Funny)

          by Thanshin (1188877) on Wednesday May 14 2008, @12:52AM (#23399536)
          The tri-state cat should be alive, dead or dog.

        • Re: (Score:3, Funny)

          by caveman (7893)
          There is yet another state that the cat can be in, as alluded to in 'Lords and Ladies' by Terry Pratchett..

          From Wikipedia [wikipedia.org]:

          Greebo had spent an irritating two minutes in that box. Technically, a cat locked in a box may be alive or it may be dead. You never know until you look. In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.

          Shawn dived sideways as Greebo went

  • I know the solution (Score:5, Funny)

    by jollyreaper (513215) on Tuesday May 13 2008, @06:19PM (#23397416)
    They were connecting the computers via cat-5 cable. Everyone knows you're supposed to use Schrödinger's cat-5 cable in that sort of application.

  • There is no such thing as absolute security (Score:4, Insightful)

    by Anonymous Coward on Tuesday May 13 2008, @06:23PM (#23397458)
    If data is stored, with the intent and purpose of actually being retrievable at some time in the future, and a mechanism exists to access said data, then it is not absolutely secure because it has been designed to be retrieved.
    As long as there is even one access method there exists the opportunity to expoloit it somehow.
    • So how do you retrieve something that's not retrievable?

    • As long as there is even one access method there exists the opportunity to expoloit it somehow.

      No. In Mathematics, 1 + 1 = 2. It doesn't just usually equal 2, except in cases that you can't think of right now. Similarly, the computer program:

      x = 0
      x = x + 1

      We know with absolute certainty that x = 1.

      Returning to access methods, you need to parse the requested object and retrieve it from storage. For both of those operations it is possible to break them down into simple, irrefutable steps much like x = x + 1 amd prove conclusivly that the program has no security flaws.

      • In theory, yes, x will always be 1. However, there are a number of practical cases which can screw this up, since the computer is a mechanical device. For example, cosmic radiation can flip one of the bits in the memory location x was being stored in after it's assigned 0 but before the addition takes place, which can cause a dramatically different result. More realistically, you could have multiple threads running at once and you could be preempted anywhere (including in the middle of that addition) betwee

  • That wacky quantum cryptography (Score:5, Funny)

    by Anonymous Coward on Tuesday May 13 2008, @06:23PM (#23397466)
    It was actually broken AND fixed at the SAME TIME!
  • It's quantum right? So there's really just a probability of it being broken or fixed at any given point in time...
  • Wait, so it was both broken AND not broken? Don't open the box! Just leave it as it is and we can have a half-cryptographic solution forever.

  • hype alert (Score:3, Informative)

    by BReflection (736785) * on Tuesday May 13 2008, @06:49PM (#23397664) Homepage
    The title of their paper is "Security Aspects of the Authentication Used in Quantum Cryptography." That would make an awesome title for a book that aimed to cover every single security aspect of the authentication used in QC, but not a paper that simply points out that (duh!) you shouldn't allow the eavesdropper to see the key.

    • Re: (Score:3, Insightful)

      by bh_doc (930270)
      While I agree it seems to be surrounded in hype, and while I'm unfortunately unable to access the paper itself (my university doesn't subscribe to IEEE Transactions on Information Theory, apparently), your comment about evesdroppers seeing the key is without merit. The whole point of quantum cryptography is that by employing superposition and state collapse, a key can be negotiated between two parties with an exponentially high probability that any evesdropper listening in will be detected, due to their bac
  • Just like the last time, the laws of quantum physics still work and it is still impossible to observe a quantum system without altering it. The researchers found that the classical authentication protocols that prevent man-in-the-middle attacks were insufficient.

  • Initialization vector (Score:3, Informative)

    by andrewsb (1142141) on Tuesday May 13 2008, @07:46PM (#23398002)
    This bit from the article sounds like they just added an initialization vector (see wikipedia for definition):

    "The researchers propose an additional, non-quantum exchange of a small amount of random bits that are separate from the quantum key."
  • There was an interesting book on cryptography which I loaned to a friend, that surmised that the law of cryptography which state that every code can be broken is now defunct due to quantum cryptography.
    This in effect means that the science of cryptography has met its end in terms of development.
    Like the game of checkers, there are no more moves to make.
    At the time of publication (2002?), the longest distance an encrypted quantum message sent and received was approximately 50kms and considered to be impossible to break.

    • Re: (Score:2, Insightful)

      by Anonymous Coward
      That book was full of shit. Cryptography is no where near finished. I wish people would stop making such a big God damned fuss about quantum cryptography. All it does is make eavesdropping detectable. In any secure application you still don't want the eavesdropper to be able to understand what they overhear, even if you can immediately detect them overhearing it, so you still need to scramble your message somehow, i.e. using actual cryptography. Furthermore, quantum cryptography works exclusively over

      • Re: (Score:3, Funny)

        by Tmack (593755)

        ...It will never be applied to cat 5 LANs....

        Unless of course (as was pointed out above), that lan is using Shrödinger's cat-5...

        tm

    • No, not really (Score:5, Insightful)

      by Moraelin (679338) on Wednesday May 14 2008, @12:22AM (#23399410) Journal
      No, not really. QC only works over dedicated, point-to-point fibre optic lines.

      Do you understand that one crucial aspect? If I want to talk to you completely securely, with quantum handshake, and able to detect eavesdroppers, I would need one uninterrupted strand of fibre from Germany to wherever you are. Screw 50kms, we're talking potentially tens of thousands of kilometres.

      Or a chain of routers along the way that we both trust blindly to not be compromised, because each breaks that quantum handshake, and each is a point where someone could eavesdrop. You can't tunnel QC over such a hop, so it's a bit like having SSL only from your computer to your ISP, then have it decrypted there and re-encrypted to the next hop, and so on.

      It's also pretty much against the whole idea of a network like the Internet. Since again, it needs dedicated uninterrupted point-to-point connections, not a loose mesh of routing machines. (You _could_ transmit the rest over the internet once you negotiated a key over QC, but: 1. you still need a dedicated connection for that handshake, and 2. you still need normal cryptography for the actual transmission then.)

      For two John Does like us it's already pretty infeasible to go QC all the way.

      Even for someone like the US Army:

      1. Good luck having an all-QC connection from Washington to Baghdad. Even in 50 km segments, you need a lot of basically routers every 50 km on the ocean floor, each of them being a potential eavesdropping point. So if you ditch normal cryptography, you'd need to do... what? Park a couple of submarines near each of them to make damn sure the Russkies and Chinese don't tamper with them? Have permanent manned bases on the ocean floor every 50 km, with a company of soldiers watching each router, and watching each other so none of them can be a double agent and tamper with it?

      2. And what do you do if someone drops a depth charge on one of those? You sure you don't want some regular crypto as backup?

      3. That still doesn't help your communication to your airplanes, tanks, cruise missiles, etc, there. You can't tie a cable from each of them to Washington.

      Etc.

      So basically... well, let me put it mildly: I don't know what book you've read, or by what author, but I'd bet it wasn't written by someone who knows much about cryptography. It sounds more like the kind of predictions made by self-styled "pundits" like Cringely or Dvorak. Or, of course, any other of the many like them.
    • Quantum cryptography is quantum cryptography only in the sense that it is quantum and is used in cryptographic protocols. It is literally no different than having a guaranteed secure line over which to transmit a private key. The protection quantum cryptography lends to you is the guarantee of that line security. Nothing else.

  • Oh, (Score:3, Funny)

    by oliverk (82803) on Tuesday May 13 2008, @09:59PM (#23398766)
    What's really going to bake your noodle later on is, would you still have broken it if I hadn't said anything?
  • As I don't know what I'm supposed to know about quantum cryptography, where can I find Alice and Bob to explain it to me? I feel sorry for them though. I'm always bugging them for an explanation and they always oblige. I'm really pissed off though. Every time, I want a different opinion, there they are in every book - Alice .... and .... Bob. Why must *they* always explain to me the most difficult concept in computing. If they aren't doing their jobs, as is obvious with QC, we need some new instructors. If I were either of them, I'd quit my day job. Since nobody understands QC, and anyone that does can't simplify it for the rest of us, they're setting themselves up for massive overtime or heart attack.

    • [N]obody understands QC, and anyone that does can't simplify it for the rest of us
      You've just summed up the entirety of quantum physics. Really, it's impossible to simplify it enough for the general public to both know what it means (as in, the behaviours it predicts) and "understand" it in any intuitive way. Hell, most physicists don't understand it in that sense. It just isn't intuitive (for common definitions of the word). So some of the time (probably more than we'd like to admit) we just plug in the math. And it works.

  • Broken QC FAQ (Score:3, Informative)

    by jalar (1283742) on Wednesday May 14 2008, @08:00AM (#23401486)
    See http://www.mai.liu.se/~jalar/qkg/faq.html?lang=en [mai.liu.se]

    • Re:Wah? (Score:5, Interesting)

      by mrbluze (1034940) on Tuesday May 13 2008, @06:24PM (#23397476) Journal

      The advanced technology was thought to be unbreakable due to laws of quantum mechanics that state that quantum mechanical objects cannot be observed or manipulated without being disturbed.

      Well the worst thing about an encrypted stream is that you trust it, not really knowing if someone is listening half way down the line. If you get a hint that it's being listened to, you can start sending garbage (or misinformation) down the line so as to confuse the hell out of the eavesdropper, whilst taking up alternative methods of communication or something.

      This makes me wonder if cryptography needs to become cleverer. I mean, depending on the type of data you're sending, might there be a role in padding encrypted streams with 'honeypot' data, like random bits of vaguely interesting crap that the expected listener might want to be interested in. Sort of a live equivalent of Truecrypt's plausible deniability.

      What do people think about that?

        • Re: (Score:3, Interesting)

          by MadnessASAP (1052274)
          It doesn't matter. The moment he tries to read the stream to see whether the data is garbage or not he has changed the quantum properties and the receiver will know someone is listening. It is theoretically impossible to discern anything about the stream without being detected.
          • I don't really get it.. why can't eve read the stream (altering it or destroying it or whatever) and send out the exact same stream- at least one that collapses to the same message (I don't know much about quantum waveforms but that at least seems possible).. a classic man in the middle
            • Eve can read the stream if she already has the key and state of the transaction. She cannot read the stream in a hope to apply brute-force cracking, or similar, to it later. Naturally, if she has the key(s), and access to the medium, she can fake the identity of the sender more or less perfectly.

            • Re:Wah? (Score:4, Informative)

              by something_wicked_thi (918168) on Wednesday May 14 2008, @04:33AM (#23400424)
              1. Alice sends the key to Bob, in the open, unencrypted, but using a random base-4 encoding. There are two states for a 1 bit and two states for a 0 bit.

              2. Bob reads the key, but, due to the random encoding, he can read only half of it (you can read only if the receiver is in the same state as the sender), so Bob sees some random subset of the bits. This random subset is the key. Alice does not know which subset this is.

              3. Bob transmits the configuration he used to read the stream back to Alice. Alice compares the configuration to her own configuration for sending data and derives which bits Bob saw. They now both know the key.

              It is impossible to read the bits without changing them, in which case Bob will see something different from what was sent, so the keys won't match.

              It is also impossible to derive the key from the configuration that is sent back by Bob because it only specifies how the bits were read, not what the bits were.

              This is, of course, vulnerable to a man-in-the-middle attack, however.

            • Re:Wah? (Score:4, Informative)

              by catprog (849688) on Wednesday May 14 2008, @04:38AM (#23400456) Homepage
              The thing is you can only accurately read about 50% of the photons.

              When Eve reads the message changes to 50% correct, 50% incorrect.

              When Bob gets the photons his 50% will consist of 25% correct and 25% incorrect ones. (assuming true randomness)

              When Alice and Bob compare there keys they will see the discrepancy.

              Then the 1 and 0 are XORs with the message and then the result is sent.

              http://en.wikipedia.org/wiki/Quantum_cryptography#Polarized_photons_-_Charles_H._Bennett_and_Gilles_Brassard_.281984.29 [wikipedia.org]

            • Re:Wah? (Score:4, Informative)

              by temcat (873475) on Wednesday May 14 2008, @04:39AM (#23400460)
              Eve cannot read the stream because 0s and 1s are sent, shall we say, in two coordinate systems (bases) randomly chosen by Alice. The receiver, be it Bob or Eve, cannot in principle measure these basis, only guess them (randomly). If you guess right, you correctly receive 0 or 1. If you guess wrong, you receive garbage. After the transmission Alice and Bob tell each other (over a classical channel) the bases they chose for each bit, and they discard the bits for which they chose different bases. Then they check (and discard) some subset of bits for discrepancies. If Eve was measuring the stream during the transmission, she would inevitably introduce errors by wrongly guessing some bases. Therefore, if error rate is higher than a certain threshold, Alice and Bob conclude that their communication was eavesdropped and discard the transmission altogether.

              Then there's the separate question of Eve messing with the classical communication between Alice and Bob, but AFAIK it has also been successfully dealt with.

    • Again in English please?
      It just means that they were using Debian.

    • Re: (Score:2, Informative)

      by maxume (22995)
      Lasing is a quantum effect. If they weren't positively blase, we would probably call them quantum lasers, and then you would be in trouble.
    • Except for the stuff that actually works and they have proven.

      "I don't believe.."
      How about some thinking, eh?
    • >> When did common-sense stop making sense in science?

      If something already makes sense then there is less of a need to study it scientifically. So science will gravitate towards non-intuitive things like neutrinos, recessive genes, bose-einstein condensates, etc.

    • Re: (Score:3, Insightful)

      by kestasjk (933987)

      The whole thing strikes me as a theory in a vacuum, I don't believe that ANY quantum object is invulnerable to observation.
      Someone's beliefs are at odds with well founded, empirically established physical laws?!

      I just hope this doesn't catch on..
      • Someone's beliefs are at odds with well founded, empirically established physical laws?! I just hope this doesn't catch on..

        Too late, it caught on long ago. It's called religion.

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