Trapdoors

[Eric Hughes <hughes@soda.berkeley.edu>]

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