On 24 March 2014 02:48, Brian Warner <[email protected]> wrote: > A recipient figures out which slots they're interested in for a given > day's dataset (both H(nym+date) and a handful following it), then for > each slot, they compute some number of million-bit vectors, one for each > distributor they're going to use. (you must use at least two, and the > unlinkability holds if at least one of the distributors you use does not > collude with the rest). All but the last vector is randomly generated. > The final vector is created by XORing all the other strings together, > and then flipping the bit of the one slot you're interested in. The > recipient sends one vector to each distributor. > > The distributor computes the XOR of all slots that have a 1-bit in the > vector, and returns the result (equal in size to a single slot, e.g. > 1kB). The recipient XORs together all of the results to get just the > desired slot contents, since everything else XORs into zeros. > > I'm confused on these parts - I'm not able to connect it in my head in a way that doesn't send something the size of the whole database down. Let's say there's 10 slots of 1KB each, and 5 distributors.
I generate 5 * 10 = 50 vectors of 1KB each? 40 are randomly generated, but the last 10 are chosen such that the slots I care about are 1's? That's not right... I generate 5 vectors of 10 bits each? And if vector #1 is 1001000000 the distributor XORs slots 1 and 4 together and sends me the result? That can't be right either, because while the slots I don't care about will XOR out, I'd only have the XOR of the slots I do care about - not the actual slots.... Do I generate 5 vectors of 10 bits, each with 3 1's, and then 3 vectors of 1KB? And if distributor #1 gets 1001001000, it XORs slot #1 with vector #1? That doesn't seem like what you described... Sorry - I really want to 'get' the PIR in Pynchon Gate, but I haven't quite found the lightbulb. I feel like I'm close though. -tom
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