Trapdoors

Jim writes:

Interestingly enough, V9 SPARC has a population count (number of bits set in word) instruction that was put in specificly at the request of No Such Agency.

Does anybody have a good idea what applications this is useful for? My first thought is that it's a very quick way to do linear error detection codes, since this instruction directly computes the Hamming weight of a code word. I can also see it being useful to detect correlations between sequences, such as a trial random stream and a known pseudorandom number generator. One would XOR the streams together and then count bits to calculate a correlation frequency. Other ideas? Eric