Question

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

This yields the sequence:
C A K S Y I B L T Z P D M U H F N V E G O W R J Q X
Such a system is used in the example in Section 2.2 (the one that begins “it was disclosed yesterday”). Determine the keyword.

Answer 1

Answer:

polyalphabetic cipher

Explanation:

A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.