Apple rejects Signal 2 from the appstore March 2015
EDIT: the tweet has been deleted, no news about what happened
There was a lot of talks about Signal 2, a messaging app that was doing end-to-end encryption on iOS.
It seems like Apple is not going to allow that:
WTF Apple?! They are rejecting Signal 2.0.1 because we are doing privacy-friendly bloom filter contact intersection.
A Bloom filter is a space-efficient probabilistic data structure, conceived by Burton Howard Bloom in 1970, that is used to test whether an element is a member of a set. False positive matches are possible, but false negatives are not, thus a Bloom filter has a 100% recall rate. In other words, a query returns either "possibly in set" or "definitely not in set". Elements can be added to the set, but not removed (though this can be addressed with a "counting" filter). The more elements that are added to the set, the larger the probability of false positives.