The Keyword cipher is identical to the Caesar Cipher with the exception that the substitution alphabet used can be represented with a keyword.
To create a substitution alphabet from a keyword, you first write down the alphabet. Below this you write down the keyword (omitting duplicate letters) followed by the remaining unused letters of the alphabet.
ABCDEFGHIJKLMNOPQRSTUVWXYZ
KEYWORDABCFGHIJLMNPQSTUVXZ
To encipher a plaintext message, you convert all letters from the top row to their correspondng letter on the bottom row (A to K, B to E, etc).
These types of simple substitution ciphers can be easily cracked by using frequency analysis and some educated guessing.
KEYWORD:
keyword
PLAINTEXT:
this is a secret message
CIPHERTEXT:
qabp bp k poynoq hoppkdo
