At 11:48 PM 10/8/97 +0100, Adam Back wrote:
I wonder if we can come up with a way to formalise this technique and automate it. I think it was James voiced something similar earlier in the thread.
I think that's the problem: once it's formalized and automated, it's spoofable by a MITM. The informal, "Hi Adam, my first digit's the third digit of your phone number today, how's your end?" type of query, perhaps made before or after the entire "I've got 78F, what's yours?" sequence, is going to remain secure. If we assume the MITM can spoof your digits, he can also spoof a whole query without weird digit splicing in the middle. It's like the cartoon cat looking the cartoon dog image in the mirror, waving and trying to trick the image. You can attempt to outmaneuver a MITM with trick questions which will almost certainly expose him, but if the stakes are high enough...where there's a will, there was a dead body. Without some external reference, be it the knowledge of the public key of the other participant, or using a trusted arbitrator, keyserver or whatever, I don't think you *can* mathematically authenticate the other end. The MITM with full knowledge will always be able to reconstruct the proper replies. Your best defenses will be: 1) External (out-of-band) authentication, as in a PGP key signing party, or even just having Bob's key fingerprint from a piece of e-mail posted to cypherpunks a year ago. 2) Wetware outclevering the MITM. Re: your noise introduction protocol. Cute idea, but what if Mallory simply audio-couples his two phones together along with a noise compander circuit a la Dolby? That kind of noise is precisely what the simple electronic filters can remove, and the digital portion is passed via the coupling computer system (the one that spoofs the voices.) "Hey, Bob, I think we've got a MITM! This connection is too clear!" :-) I think the hardest to defeat will be the case where Mallory hires Alicia and Bobby to impersonate Alice and Bob to each other. Alice <---------------------------> Bob (what they think is happening) Alice <---> Bobby______Alicia <---> Bob (what actually is happening) / \ under Mallory's control where Bobby and Alicia have the translator's gift of speaking while listening. They could fill in the cracks with idle chatter, have their own personalities, and basically run both shows. They could even run offline from each other, being intelligent humans. As long as Alice and Bob only communicate through Mallory's intermediaries (i.e. never meet in real life, out-of-band), Mallory owns the show. (Tell me you've never seen something like that on a rerun of _Mission_Impossible_! :-) Would you know if you were talking to the REAL John Deters? All I could assume about you is that if my phone rang and a voice claimed to be Adam Back, I'd expect to hear it in a British accent, and with just a bit of time delay so I'd know it was coming across the Atlantic. About 50 million people could pass themselves off as you today, and I'd never know the difference. I can't even tell a Liverpool accent from a London accent! And if I had a voice recognition system, well, then I'd hope if I was taking a voice sample from you that I'd be smart enough to exchange public keys so we could do our authentication the right way. Again, my position is: o internal authentication is *not* information-theoretically possible; o out-of-band authentication *can be* theoretically secure; o MITM attacks are *almost* infeasible, and *always* expensive; and o if you still suspect a MITM after trying to outwit one, you're just being paranoid. Stop it. John P.S. Even if I exchanged PGP keys with you face-to-face, compared picture ID's, checked your fingerprints, read a signed letter from your Mom (moms don't lie,) and *knowing* that you are indeed Adam Back, I still won't know if you work for MI-5 or not! Nothing's perfect, so lets take what we can get. -- J. Deters "Don't think of Windows programs as spaghetti code. Think of them as 'Long sticky pasta objects in OLE sauce'." +--------------------------------------------------------------------+ | NET: mailto:jad@dsddhc.com (work) mailto:jad@pclink.com (home) | | PSTN: 1 612 375 3116 (work) 1 612 894 8507 (home) | | ICBM: 44^58'36"N by 93^16'27"W Elev. ~=290m (work) | | For my public key, send mail with the exact subject line of: | | Subject: get pgp key | +--------------------------------------------------------------------+