Evening star. [volume] (Washington, D.C.) 1854-1972, 1945-09-23

each letter appears. In the message
above P appears 10 times, r 16 times, @
Is times, N IL times, D IL times and
F and L 10 times each. Thus, by the
frequency table P should represent E,
r would be T, @ would be A, N would
be o, D would be I and F or L would
be N or s. 1t would be waste Of time
to write all these above their matching
letters on the work sheet because fire.
quencies are merely guide for expert
ment, and are far from reliable. The
cryptanalyst would approach the mes
sage at this point by suspecting that p
might mean e or T, or even o.
Logical Sequence
A trial application by writing E above
each P would not be too encouraging.
But he has logic to fall back on. He
sees two words that contain a double
P. There is another word with double
Q and one with double r. He goes
through the message undermining all
double letters and any groups OF letters
which appear in the same sequence
more than once, as OR and LOW.
sense of word structure, which is q
turn OF mind rather than a type of
learning or native intelligence, suggests
that ther re are comparatively few double
letters in ordinary words. Double E
probably would be most frequent, with
double T second, maybe double o close
third, and double L trailing. So his
two double Ps in only 81 words is en-
couraging enough to assume that P
represents E and he can find out what
r stands for. There are Is of them,
including a double. 1t also appears
several times in two-letter words.
Starting with the frequency table
again T is written above each r. One
OF the two-letter words used twice
begins with r, so If R stands for T,
then F must stand for o. because the
only two-letter word beginning with T
Is to" There are enough Fs in the
message to make this plausible. And II
F Is really o, then study of the other
tWo-letter words shows N Is either N
or r. Since there is no other two-letter
word ending in r, and another one in
the message begins with a different
letter, FIN must be "or."
Putting these apparently known let
tiers above their companions In the
clpher, the analyst reaches a point
where he has number of reasonable
combinations from which to work. He
then proceeds as he would in the late
stages OF doing crossword puzzle. He
Is assisted by the fact that two of the
words end in the same group of three
letters, although the rest of the words
are different.
Three common endings are "ing,"
lest" and ion" He thinks he knows
the cipher for and o, so LOW prob
ably is ing Thus, altogether he now
has the key to six or seven of the most
frequently used letters in the message
and the rest is merely filling in.
Key Word Cipher
AS has been said, this is a simple ex-
ample OF key word cipher. Its solu.
ton Is assisted by the separation of the
words calling attention to the beginning
and ending letters. Any comprehensive
frequency table would include separate
lists Of frequencies for letters which
begin or end words But in practical
use, the sender doesn't consider the
convenience of the person trying to
break his cipher, SO he doesn't nut
spaces between his words.
To meet this complication the ANALYST
must hunt patiently with his underlin-
ing pencil for double letters and group.
lngs that are repeated. Doubles might
be either within words or the last and
hrst letters Of adjacent words. 1t is a
slow process and often long one.
Several persons might work simultane.
ously, each experimenting with different
possibilities and comparing notes. MANY
times the ordinary table of freouencies
will be found awry. But eventually a
word ending here, grouping there will
give two or three letters which open
the door.
linen the person devising the cipher
may have more complicated ideas He
may devise a square with the cipher
letters above and in the margin. He
may. and probably does, use key word
which can be changed from time to
time In this form the cipher of young
Joseph Williams would be like this:
n z