I have a question about how they store the contacts. Can't they encrypt each of the phone numbers before they get sent to the server? This way there's no breach of privacy and the friend suggestion feature still works for everyone.
That wouldn't add any real protection. Phone numbers is a very small set (100 million possible in the U.S. and Canada). A rainbow table of all possible combinations can be created in only a week or two.
Hashing phonenumber+userid does absolutely nothing for them, though.
The purpose of uploading your contacts is so that if Jack's phone number is (555) 555-5555, and Sam uploads a contact list saying that he is friends with a guy whose phone number is (555) 555-5555, Path can match up those two phone numbers (or hashed versions of them) and tell Sam that Jack is a member. That match-up doesn't work if the phone number is stored as (a hashed version of) 5555555555jack and 5555555555sam.
They could take the phone numbers, sort them and hash them together. So if Sam is 5 and Jack is 6, they both upload the hashed social relationship 56 to the system and it can match them up.
It wouldn't keep someone with access from checking if a social relationship existed in the database, but it should make recovering phone numbers and the like from the hashes quite a lot harder.
That only works though if you have both Jack & Sam's phone books (and they have each other in their books) so the hit rate would go down, possibly significantly.
a cryptographic hash of a phone number on their server should match a cryptographic hash of a phone number in a contact list on a phone. The app sends the hash to the server, the server looks up users via the hash and responds with user data for matches.
To be honest this should be a third party service, since it sounds like every major social networking app is doing the same exact thing.
In my opinion, giving out your number, along with the hash of each phone number in your address book to an authority with millions of such hashes isn't appreciably better than giving them in plaintext.
But you wouldn't give out your number. I haven't completely thought it through but the service provider would provide an api for common platforms. All it would do is 2-way encrypt contact numbers (SSL?). Then the service would do a basic lookup using the encrypted data as a key. If there is a hit for this particular platform it'll return the platform specific data (in this case, like a path specific user id).
Of course the other side would be maintaining users in this service, which again is pretty straight forward.
(Hi David?... I'm the OTHER DJB, probably not the one you are thinking of)
