is a polygraphic, monoalphabetic substitution cipher - that is, more than one

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].

set of simple rules:

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

accommodate the matrix [in this case "Q"]and a keyword salts the insertion of

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:

an "X" [or any arbitrary letter] to the first letter and then encrypt the new

pair and proceed...

replace those letters with the letter directly to its right

replace those letters with the letter directly beneath

place them with the letters on the same but at the opposite end of the rectangle

In this case we have the following encrypts:

with the resulting cipher text: ZN MO MO GX KD PY SN AP XG DA

appearance of a polyalphabetic substitution cipher - for the fact that "T" is

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].

monoalphabetic substutition cipher. The Hill Cipher uses matrix multipication

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 mod-26 is the key:

Matrix Clear Message [last column]

1	3	5	7	*	7
				*	4
				*	24
				*	23

numbers [1, 3] with the decimal equivalents for "H" and "E" [7, 4]:

(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 (mod-26) = 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 (mod-26) = 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 (mod-26) = V

with the final output: T L P V

substitution cipher. Specifically, it encrypts digraphs [pairs of letters]. The

encrypting device consists of four 5 x 5 squares [omitting an arbitrary letter,

in this case Q] with the two plain alphabets in the top-left and bottom-right

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

you locate the "T" in the upper-left alphabet and the "H" in the lower right

alphabet and find the intersection in the upper-right cipher alphabet "S", then

find the intersection in the bottom-left 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