28. The first, and perhaps the most important, lesson we learnt was that cribbing is primarily a cryptographic and not an intelligence job. What matters is not what you think the enemy ought to say because he is doing or about to do so-and-so, but what in fact he did say yesterday and the day before and the day before that .... ; in fact, by far the best cribs are those based on analysis of traffic rather than on an analysis of operations. There are some exceptions or partial exceptions to this, e.g. B bars (Chap. IV), where an analysis of both traffic and operations was necessary, but even in these cases it is better to have the final control with cryptographers since only they can appreciate fully the way in which the crib is finally to be used and the amount of work of one kind and another that will have to be expended on cribs of different types ("work" in our case being bombe time and testing time).
29. It is however essential to have the closest possible cooperation between the cribbing section and all other sections handling the traffic. We received constant and most valuable help from Naval Section - the Z Watch (more than a [yono?]), the intelligence sections (Hinsley and N.S.V), the breakers of Werft and R.H.V. hand cyphers (N.S.II) the U-boat room (N.S.IV), N.S.V in control of interception and the Met. Section all helped us over and over again. The story in Chapter IV. of the W W's and B bars illustrates this very clearly.
30. Secondly, cribbing is a high grade job. It is essential that if the most is to be got out of the traffic (sometimes it is not necessary to squeeze every drop as the key may be easy) it should be examined by the cryptographer himself and a really first class person will find out incomparably more than a second rater. The type of person who makes a first class cribster is not altogether the same type who makes a good research cryptographer (this of course is not to say that the same man may not combine the qualities needed for all three jobs.). The most