Cryptology - Unofficial St. Mary's College of California Web Site

9.7. EXERCISES 181
(b) The use of “alphabetical” in part (a) didn’t work. Instead, when
the letters are in alphabetical order, add 4 to each. When they are
not, subtract 3. Leave doubles alone. Will this work (Again try to
encipher and decipher some words.)
3. A slightly fancier digraphic cipher is as follows. If the letters are both
vowels, replace each by the vowels before them in the alphabet (ea become
AU.) If the pair is two consonants, replace each by the next consonant in
the alphabet (ns becomes PT). And if the pair is is one each, swap them
(ho becomes OH).
(a) Encipher vowels.
(b) Encipher breakers.
(c) Decipher ROEAP VLA.
(d) Decipher EBUOVZ.
(e) We’ve learned to recognized monoalphabetic ciphers by their odd
appearance – too few good etaoinshr letters, too many bad jkvwxyz
letters. What sort of appearance will this cipher produce
4. To make a combination Caesar-AutoKey Digraphic cipher, encipher each
pair of letters by using a Caesar cipher with key k on the first letter, and
an Auto Key Cipher on the second letter, using the first letter as the key.
Symbolically, for each pair p 1 p 2 of plaintext letters, the ciphertext pair is
(p 1 + k)%26, (p 1 + p 2 )%26. So if k = 3, then to becomes WH, as W is three
letters past t, and o enciphered with key T is H.
(a) Encipher combinations with key 7.
(b) Decipher JCLWH R, if the key was 12.
(c) Encipher primer if the key is 21.
(d) Decipher KZKVE MZAKV, if the key was 6.
5. In a table-based digraph cipher, like Figure 9.2, it is best if each letter
appears in each row exactly once, and similarly in each column exactly
once. If we had a three letter language “ABC” or four letter language
“ABCD” then examples of such tables are
AB BC CA
CC AA BB
BA CB AC
and
DC AD BB CA
CD BC AA DB
AB CA CC BD
BA CB DD AC
These arrangements are called Greek-Latin Squares, and exists for every
size of square, except 2 × 2 and 6 × 6.
(The 6×6 case is often referred to as the 36 Officers Problem: Suppose we
have 36 military officers of 6 ranks from 6 different states, no two of whom