From: siliconshaman	Date: July 13th, 2012 12:15 pm (UTC)		(Link)

There is a letter to a friend in the war office, posted the day before he died, in which Turing claimed he was working on 'something' [no-one knows what] that would not only render all current cyphers obsolete, but the very idea of encryption itself pointless. [more-or-less his words].

To date, no other hint has been found of this thing, but then Turning rarely wrote down what he was thinking of in the initial stages.