DECIPHER: A Compilation of codes and ciphers

DECIPHER 8 : KEYWORD CIPHER

‣ Keyword cipher is a form of monoalphabetic substitution cipher. A keyword is used as the key, and it determines the letter matchings of the cipher alphabet to the plain alphabet.

In the keyword cipher, repeats of letters in the word are removed, then the cipher alphabet is generated with the keyword matching to A,B,C etc. until the keyword is used up, whereupon the rest of the ciphertext letters are used in alphabetical order, excluding those already used in the key.

Tip:
↳ The best way to crack keyword cipher without knowing the keyword are through known-plaintext attack, frequency analysis and discovery of the keyword (often a cryptanalyst will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.

Encryption & Decryption:
↳ In encryption of Keyword cipher, you need to have a "keyword" or "password" to your plaintext. In this example, we'll choose "AGENT" as the keyword.

The first step is to write down your keyword without repeating any letters. The next step is to write down all the letters of the alphabet, in order, skipping any letters in the keyword. The first thing you will realize is that we've already written down "A," so we write "B" and continue. Then, match your plaintext letters against the regular alphabet. The letters labeled "plaintext" are the ones in your message, and the letters labeled "ciphertext" are the coded letters.

Plaintext = Ciphertext:

A = A

B = G

C = E

D = N

E = T

F = B

G = C

H = D

I = F

J = H

K = I

L = J

M = K

N = L

O = M

P = O

Q = P

R = Q

S = R

T = S

U = U

V = V

W = W

X = X

Y = Y

Z = Z

Encryption and decryption is often easier if the letters are arranged vertically (as shown above).

Let's encrypt "BEHOLD OUR LOYALTY AND INTELLECT" using the keyword of "AGENT".

↳ Message: BEHOLD OUR LOYALTY AND INTELLECT

Keyword: Agent

Encrypted: GTDMJN MUQ JMYAJSY ALN FLSTSSES