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FILTER-ROOM :
A room in which reports of aircraft from a variety of sources are
correlated, the tracks worked out, tracks of (known) friendly air-
craft weeded out, and details of the remaining tracks (which are
presumably of hostile aircraft) sent on to those who deal with such.
FINNERY :
Practice, first observed in Finnish Hagelin, of moving certain of the
wheels forward one or more positions by hand at points (denoted
e.g. by a letter of the indicator) in the encipherment of a message,
designed to prevent or impede breaking.
FISH MACHINE :
FIT :
(in Enigma).
1. A repeat or click involving two messages.
2. Two messages. set in depth on the evidence of the above.
FIVE CUPBOARDS :
A system of simple reciprocal substitution for which the key phrase
is "five cupboards", used especially for sending intercepts or other
'raw material' over all British cable routes for security reasons.
FLAG. n :
Conventional triangular arrangement of all (minor) differences derivable
from a series of code-groups, the groups concerned being written along
the top and down the left hand side and each difference in the same
column as one of the groups from which it is derived, and in the same
line as the other.
FLAG v :
To write out all the minor differences arising from a series of code-
groups or (more usually) the cipher groups forming a column in a depth
of reciphered code, in the manner described above, esp. in a search for
good or repeated differences in the process of key-breaking.
FORM :
(in Transposition). Apparently used for cage.
FORWARD CLICK
(in Enigma). The occurrence of the same letter at the same position in two
messages in depth with each other, associated with the repetition of a
different letter in two cribs.
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