LEUCHTTONNE?

I'm hoping to find more information about a Bletchley Park anecdote that I
heard a while back, which went something along the lines of:

"In order to effect an attack on a certain cipher, a crib of a special form was
required, namely, that two pairs of repeated, adjacent letters occurred in the
plaintext spaced an odd number of places apart. A German language expert
suggested 'LEUCHTTONNE', with repeated pairs 'TT' and 'NN', meaning
'lightbuoy'. In order to induce this crib, it was arranged for an actual
lightbuoy to be bombed; it was fairly certain that the resulting German
messages would include the word."

I don't suppose anyone has a source for this anecdote, or knows what the
codebreaking technique was which required this property?

-- Matt R.




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