1. A method implemented at least in part by a computing device, the method comprising: determining one or more statistically improbable phrases recognizable by and associated with the user, wherein individual statistically improbable phrases comprise at least two contiguous words that occur in a corpus of text more than a threshold number of times; determining the words that compose the one or more statistically improbable phrases that are associated with the user; inputting, into an index as keywords, the determined words that compose the one or more statistically improbable phrases; determining phrases that are associated with the determined words that compose the one or more statistically improbable phrases; and selecting at least one of the phrases as an identifier for the user.