"Unicity distance" is discussed, as I recall, in some of the more
technical sections of Kahn's "The Codebreakers". That wouldn't be
a bad place to start. Note that you must be reading the hardcover
edition -- the paperback is abridged!
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry