WASHINGTON (Reuters) - The National Security Agency has a challenge for hackers who think they're hot stuff: prove it by working on the "hardest problems on Earth." (http://feeds.feedburner.com/~ff/reuters/topNews?d=yIl2AUoC8zA) (http://feeds.feedburner.com/~ff/reuters/topNews?i=iRAcx5OGafc:Uow8VU5NpKk:V_sGLiPBpWU) (http://feeds.feedburner.com/~ff/reuters/topNews?i=iRAcx5OGafc:Uow8VU5NpKk:-BTjWOF_DHI) (http://feeds.feedburner.com/~r/reuters/topNews/~4/iRAcx5OGafc)