The Playfair Cipher [actually invented by Charles Wheatstone]
plain text letter is encrypted at a time [in this case two, making it di
graphic]. into two cipher text letters, or symbols, at a time [making it
uniliteral].
K

E

Y

A

B

C

D

F

G

H

I

J

L

M

N

O

P

R

S

T

U

V

W

X

Z

the alphabet [in this case "KEY"  thus omitting the occurrence of those letters
in the rest of the matrix].
Using our clear text message: T H I S I S A S E C R E T M E S S A G E
The message is broken into pairs of two: TH IS IS AS EC RE TM ES SA GE
We apply the following rules:
pair and proceed...
In this case we have the following encrypts:
IS = MO [rule #4]
GE = DA [rule #4]
encrypted as "Z" at one point but "S" in another point. However, this would be
a false lead  since no polyalphabetic cipher alphabet was used [nor a corre
sponding key] you would be at loss to find a consistent decode. This particular
cipher would require digraph analysis [albeit, the missing "Q" would throw a
monkey wrench in the process].
The Hill Cipher, invented by Lester Hill, is a polygraphic,
to encipher.
Clear Message: H E Y X ["X" appended as a pad]
converted to decimal (mod 26): 7 4 24
and the matrix [N x N where N equals the number of characters] of mod26 is the key:
Matrix Clear Message [last column]
1

3

5

7

*

7





*

4





*

24





*

23

(1 * 7) + (3 * 4) = 19 = T
and then summing the products of the second two key numbers [5, 7] with the
decimal equivalents for "H" and "E" [7,4]:
(5 * 7) + (7 * 4) = 63 = 11 (mod26) = L
so "HE" encrypts to "TL"...
"YX" is encrypted by summing the products of the first two key numbers [1, 3]
with the decimal equivalents for "Y" and "X" [24, 23]:
(1 * 24) + (3 * 23) = 93 = 15 (mod26) = P
and then summing the products of the second two key numbers [5, 7] with the
decimal equivalents fo "Y" and "X" [24,23]:
(5 * 24) + (7 * 23) = 281 = 21 (mod26) = V
with the final output: T L P V
The Four Square Cipher is also a polygraphic, monoalphabetic
encrypting device consists of four 5 x 5 squares [omitting an arbitrary letter,
in this case Q] with the two plain alphabets in the topleft and bottomright
corners:
A

B

C

D

E


C

R

A

K

B

F

G

H

I

J


D

E

F

G

H

K

L

M

N

O


I

J

L

M

N

P

R

S

T

U


O

P

S

T

U

V

W

X

Y

Z


V

W

X

Y

Z












T

H

I

S

A


A

B

C

D

E

B

C

D

E

F


F

G

H

I

J

G

J

K

L

M


K

L

M

N

O

N

O

P

R

U


P

R

S

T

U

V

W

X

Y

Z


V

W

X

Y

Z

alphabet and find the intersection in the upperright cipher alphabet "S", then
find the intersection in the bottomleft cipher alphabet "E", so the first part
of the message encrypts to "SE". This is repeated yielding the following cipher
text: SE FR FR AN AA UH SL AU OI HH