I think an important point Snowden makes is that the words used in the passphrase should be unlikely to be in a dictionary. In other words, the phrase should be

I think an important point Snowden makes is that the words used in the passphrase should be unlikely to be in a dictionary. In other words, the phrase should be one to which a language model[1] would assign low probability, as there are hugely many of such phrases, whereas higher probability phrases are less numerous. [1] http://en.wikipedia.org/wiki/Language_model http://en.wikipedia.org/wiki/Language_model