Quantum Key Distribution: the bad idea that won't die...

Steven Bellovin smb at cs.columbia.edu
Thu Apr 22 09:09:22 EDT 2010

While I'm quite skeptical that QKD will prove of practical use, I do think it's worth investigating.  The physics are nice, and it provides an interesting and different way of thinking about cryptography.  I think that there's a non-trivial chance that it will some day give us some very different abilities, ones we haven't even thought of.  My analog is all of the strange and wondrous things our cryptographic protocols can do -- blind signatures, zero knowledge proofs, secure multiparty computation, and more -- things that weren't on the horizon just 35 years ago.  I'm reminded of a story about a comment Whit Diffie once heard from someone in the spook community about public key crypto.  "We had it first -- but we never knew what we had.  You guys have done much more with it than we ever did."  All they knew to do with public key was key distribution or key exchange; they didn't even invent digital signatures.  They had "non-secret encryption"; we had public key cryptography.

Might the same be true for QKD?  I have no idea.  I do suggest that it's worth thinking in those terms, rather than how to use it to replace conventional key distribution.  Remember that RSA's essential property is not that you can use it to set up a session key; rather, it's that you can use it to send a session key to someone with whom you don't share a secret.  

Beyond Perry's other points -- and QKD is inherently point-to-point; you need n^2 connections, since you can't terminate the link-layer crypto at a router without losing your security guarantees -- it's worth reminding people that the security guarantees apply to ideal quantum systems.  If your emitter isn't ideal -- and of course it isn't -- it can (will?) emit more photons; I can play my interception games with the ones your detector doesn't need.
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