A disadvantage of the general monoalphabetic cipher is that bo 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 CIPHER , write out the keyword followed by unused letters in normal order and match this against the plaintext letters:

The question is to determine the Key word.

plain: abcdefghjk1mnoparstuvwxyz 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: CIPHER A B D F G J K L M N O Q S T U V W x Y Z This yields the sequence: CAK SYIBLTZ PDMUHFNVEGOWRJQ X Such a system is used in the example in Section 2.2 (the one that begins "it was disclosed yesterday", see below if you don't have the book). Determine the keyword. cipher: UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWP FUP ZHMDJUDTMOHMQ plain: it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in moscow