PLAYFAIR CRYPTANALYSIS
Our preliminary step is to perform individual letter
frequency and digraphic counts. The former because high
frequency ciphertext letters follow closely the high
frequency letters they represent and will be located in
the upper rows; similarly, low frequency letters follow
their plain counterparts (UVWXYZ) and may be located at
the last row of the square. A digraph count is useful
because cipher digraphs follow closely the frequency of
their plaintext digraphs. i.e. TH = HM. The frequency of
HM must be high for a normal length message. Also
tetragraphs may be tested THAT, TION, THIS for
corresponding their frequencies in the square.
All the authors agree that a probable word is need for
entry into the Playfair. Due to its inherent
characteristics, Playfair cipher words will follow the
same pattern as their plaintext equivalents; they carry
their pattern into the cipher.
Given: Tip "er one day entere" Hampian. 10/1952
EU SM FV DO VC PB FC GX DZ SQ DY BA AQ OB
ZD AC OC ZD ZC UQ HA FK MH KC WD QC MH DZ
BF NT BP OF HA SI KE QA KA NH EC WN HT CX
SU HZ CS RF QS CX DB SF SI KE FP (106)
We set up a combined frequency tally with letters to the
right and left of the reference letter shown:
K Q H H B . A . Q C
D O P . B . A F P
E Q K Z O A F V . C . X S X
W Z Z . D . O Z Y Z B
K K . E . U C
S R O B . F . V C K P
. G . X
N M M . H . A A T Z
S S . IJ.
F . K . C E A E
. L .
S . M . H H
W . N . T H
D . O . B C F
F B . P . B
U A S . Q . C A S
. R . F
Q C . S . M Q I U F I
H N . T .
S E . U . Q
F . V . C
. W . D N
C C G . X .
D . Y .
H D D . Z . D D C
This particular message has no significant repeats.
Cipher GX DZ SQ DY BA AQ OB ZD AC
Plain .. ER ON ED AY EN TE RE ..
Note the first and last pair reversal.
It is necessary to take each set of these pair
equalities and establish the position of the four
letters with respect to each other. They must conform to
the above three rules for row, column, and rectangle.
The six different sets of pairs of know equalities are
set up:
1 2 3 4 5
er = DZ on = SQ ed = DY ay = BA en = AQ
------ ------- ------ ------- -------
E D R Z O S N Q E D Y Y A B E A N Q
D S D A A
R E D N O S Y B N E A
Z Z R Q Q N Q Q N
6
te = OB
-------
T O E B
O
E T O
B B E
The three possible relations of the letters are labeled
Vertical (v), Horizontal (h), Diagonal (d). Our object
is to combine the letters in each of the set of pairs.
Combine 1 and 3: E R D Z Y
1/v - 3/v 1/h - 3/h 1/d - 3/h
--------- --------- ---------
E E D Y R Z E D Y
D Z R
Y
R
Z
Combine 2 and 5: O N S Q E A
2/h - 5/d 2/d - 5/h 2/d - 5/d
--------- --------- ---------
O S N Q E A N Q S O
A E S O N Q
A E
Note that all the equalities hold for all letters.
Set number 6 combines only with the last combination: T
E O B N S Q A
2/d - 5/d - 6/v 2/d - 5/d - 6/d
---------------- ---------------
T S O T
S O N Q
A E A E B
B
N Q
which we now combine with 4:
2/d - 5/d - 6/d - 4/h
---------------------
S T O
Y A E B (rearranged and
N Q equalities hold)
only one combination of 1 and 3 will combine with the
above: S T O Y A B E D N Q Z R
1/d - 2/d - 3/h - 4/h - 5/d - 6/d
---------------------------------
S T O
Y A E B D
N Q
Z R
Arranged in a 5 X 5 square:
. . S T O
D Y A B E
. . . . .
. . N . Q
R . . . Z
We see that O is in the keyword, the sequence NPQ
exists, the letters S T Y are in the keyword, and three
of the letters U V W X are in needed to fill the bottom
row.
----------
. . S T O| C
D Y A B E|
. . . . .|
. . N P Q|
R . . . Z| U V W X
With the exception of F G H I K L M which must in order
fill up the 3rd and 4th rows, the enciphering square is
found as:
C U S T O
D Y A B E
F G H I K
L M N P Q
R V W X Z
Our plaintext message starts off: YOUNG RECRUIT DRIVER
ONE DAY ENTERED STORE ROOM ....
Sumber: Bahan Kuliah Pak Rinaldi Munir
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