Cryptology - Unofficial St. Mary's College of California Web Site

1.4. LINQUIST’S METHOD 21
the ciphertext. In this example, the only letters occurring more than once are
A, P and T, twice each, and M, which appears three times. Even for such a short
message (when the frequencies can be quite messed up) it many of these letters
must come from etaoinshr. Linquist’s idea is to see if there is one shift amount
that would produce all or most of the most common ciphertext letters from the
common plaintext letters.
To do this, make a chart with the common plaintext letters across the top
and common ciphertext letters down the side. For each of the etaoinshr letters
determine what key produces the ciphertext letter from this plaintext letter. For
instance, e must be shifted by 22 letters to become A, so we enter 22 in the e-th
column and A-th row. To find the shift amount using the Saint Cyr slide, line up
A under e and then see what the key is. Similarly, lining up M under e produces
an 8, so 8 is the second entry in the first column.
e t a o n i r s h
A 22 7 0 12 13 18 9 8 19
M 8 19 12 24 25 4 21 20 5
P 11 22 15 1 2 7 24 23 8
T 15 0 19 5 6 11 2 1 12
We are looking for one shift amount that would cause four of etaoinshr to
become AMPT. So we are searching for numbers that appear in every row, or,
failing that, occur in at least in three of the four rows. No number appears in
each row, but 8, 12 and 19 appear in appear in three of the four rows. It is most
likely that one of these three is the proper shift amount. We simply decipher
the message using each to see which one it is. 16
Example: Decrypt WZRVM ZOCSD YZNGA HVMXC using Linquist’s method.
The only letters than appear more than once are C, M, V and Z, so our chart
is set up as
e t a o n i r s h
C 24 11
M 19 4
V 8 3
Z 25 17
Some of the chart has been filled in for you. Complete it, find the most common
letter (i.e., the possible keys) and try them to determine the key. Finally,
decipher the message. 17
⋄
16 It is 8. Deciphering gives Rome has seven hills.
17 Beware the Ides of March, key = 17.