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

Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Encrypted: K R Y P T O S A B C D E F G H I J L M N Q U V W X Z

With KRYPTOS as the keyword, all As become Ks, all Bs become Rs and so on. Encrypting the message "knowledge is power" using the keyword "kryptos":

Plaintext: K N O W L E D G E I S P O W E R
Encoded: D G H V E T P S T B M I H V T L

Only one alphabet is used here, so the cipher is monoalphabetic.

The best ways to attack a keyword cipher without knowing the keyword are through known-plaintext attack

Known-plaintext attack

The known-plaintext attack is an attack model for cryptanalysis where the attacker has samples of both the plaintext , and its encrypted version . These can be used to reveal further secret information such as secret keys and code books...

, frequency analysis

Frequency analysis

In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers....

and discovery of the keyword (often a cryptanalist will combine all three techniques). Keyword discovery allows immediate decryption since the table can be made immediately.