below shall change or not, viz.-MT x..x..x.xx.x. with Psi .x.x..x is
written MT x..x..x.xx.x. and then the pattern is extended so that the
Psi .x . x .. x
final picture MT x..x..x.xx.x. . The first position of MT is always
Psi'.xxx...xx...xx
x and its second position is always the same as MB. Therefore the sum of
X2 + P5 in the second position must be cross. In the first position its
sum is immaterial but by adopting the convention that the sum must be dot
we compel a cross in MT regardless of the sign or MB. Thus it becomes
unnecessary to have any other special rules for the beginning of the
message. The X2 sign is determined from the pattern, that is, if the X2
pattern starts at 5, the X2 one back pattern starts at 4. If then elements 4 and
5 of X2 are say x. the first two elements of P5 must be xx The easiest
way to think of it is that X2 one back is known and that the first two columns
must be given arbitrary P5 symbols which will combine with the X2 to
produce dot in the first column and cross in the second. The third
column of P5 depends on P5 of the first letter of the actual message.
With these conventions nothing need be discarded and none of the initial
letters of the deciphered text will be incorrect.
5. June Bream was broken very early in the month and is very
favorable with 27 dots Mu37. The Jellyfish wheels also changed on 1
June but efforts to break them by the usual statistical methods were
unsuccessful. However, they have Just been solved by the cribbing method
hereinafter described. Bream went off the air when Rome fell but re-
appeared on 10 June. The new station is near Florence.
6. The new Colossus mentioned in Report #F 22 has been installed
and is in operation. It is designated Colossus # 2. It had been promised for
1 June and was in partial operation on that day although the engineers
and maintenance people had to work the last night until 0300 to carry out
their commitment. Colossus # 3 is expected to be ready by 1 July. 9 more
have been ordered. All subsequent to # 1 are of the same type and represent
a substantial improvement over #1. The. speed or operation is actually
5,000 per second. # 1 could theoretically attain this speed but never
achieved it. The new machines have an extensive switchboard in addition to
a plugboard. They carry a great many more different patterns than the old
and these can be switched in or out with triggers. Furthermore, it is
unnecessary to plug up Delta patterns because the machine itself can
difference if required.
7. In Report #F 36 I mentioned the discovery of re-encodements in
Fish traffic. A statistical method has been developed which is designed
to locate these (and thus solve hitherto~unbroken keys) by sliding a long
crib through a message. About 800 letters of crib are ordinarily needed.
when correctly set this yields a length of key more than suffictent for