A disadvantage of the general monoalphabetic cipher is that both sender and receiver must commit the permuted cipher sequence to memory. A common technique for avoiding this is to use a keyword from which the cipher sequence can be generated. For example, using the keyword CRYPTO, write out the keyword followed by unused letters in normal order and match this against the plaintext letters:

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

If it is felt that this process does not produce sufficient mixing, write the remaining letters on successive lines and then generate the sequence by reading down the columns:

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

This yields the sequence:

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

Such a system is used in the example in Section 3.2 (the one that begins “it was disclosed yesterday”). Determine the keyword?