On Wed, Aug 19, 2015 at 10:49:50PM -0700, Trevor Perrin wrote: > On Wed, Aug 19, 2015 at 6:17 PM, Daniel Reichert <[email protected]> wrote: > > Private Set Intersection has come a long ways since 2009. Just this year a > > paper[1] was published where private contact discovery is a primary use > > case. Detailed benchmarks for varying sizes of the client and server set > > sizes are included. Sadly it's still not practical since the only way to > > prevent a brute force search requires making the protocol O(n1*n2) where n1 > > is the client set size and n2 is the server set size. > > It's nice to see people looking at this, but I agree with Moxie (and I > think you) that a scaleable solution based on PIR or PSI doesn't seem > to exist. From the paper you cite: > > http://eprint.iacr.org/2015/634.pdf > """ > Private contact discovery > [...] > In these cases each user has a small number of records n2, e.g., n2 = > 256, whereas the service has millions of registered users (in our > experiments we use n1 = 2^24). It therefore holds that n2 << n1. In > our best PSI protocol, the client needs only O(n2 * log n1) memory, > O(n2) symmetric cryptographic operations and O(n1) cheap hash table > lookups, and the communication is O(n1 * > log n1). > """ > > In other words, the communication overhead is worse than the trivial > PIR of sending a compressed list of users to the client = O(n1). For > n2=256, n1=2^24, their example protocols send hundreds of MB.
But for numbers that small, why *can't* you do actual PIR? The database will be of size ~1 GB, and if you're looking up 256 records in it, the client will upload 256 * sqrt(2^30) B = 8 MB to the server, and download the same amount. The computation time on the server will be a bit annoying: ~120 (highly parallelizable) core-seconds, but it's only done once per new client, right? (Both the bandwidth and computation will be a little less for Chor-like non-robust multi-server PIR, and a little more for single-server CPIR.) - Ian _______________________________________________ Messaging mailing list [email protected] https://moderncrypto.org/mailman/listinfo/messaging
