Cryptographic History of Work on the German Naval Enigma
Chapter X
Cribbing.
The Nature of the Problem.
1. There are three main varieties of cribs (1) depth cribs (2) R.E's (re-encodements) (3) routine cribs. The first of these came up in our work mainly in connection with Banburismus and this account of it should be read in conjunction with Chapter IX.
Depth Cribbing.
2. Go back to the example in Chap. [IX,] Para. 4, where the messages with trigrams TYB and TYQ are written out in depth; suppose that we know the probable content of TYB (because it is a routine message of some sort). Then we have the following position:
3. The letters written in the clear text of TYQ must be correct if the depth is true because at any particular position of the machine a letter can only encode in one, so if (taking the "N" written in the TYQ clear text) N encyphers as M in one message so it must in the other and therefore we must [have] an M in the TYQ clear text as written in. Such implied letters, or "consequences", are of two types (1) "repeats" or "clicks" where the cypher texts agree with each other - these are underlined (2) "reciprocals" where the clear text of one message is the same as the cypher text of the other.
4. Now looking at the consequences very little experience is needed to see that they must be correct. Numerals, and especially strings of numerals are extremely common in German Naval traffic so ..NS.REI almost certainly comes from EINS DREI; moreover the scattered letters produced are on the whole very good - 3 E's and XX (punctuation)
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