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

168 CHAPTER 9. DIGRAPHIC CIPHERS
Examples: The algorithm is to consider the letters two-by-two. If the letters
are in alphabetical order, replace both by the letters that follow them. If they
are not, replace them by the letters before them. So et→ FU and of → NE.
(1) Encipher nice doggy.
(2) Decipher EZSBB U. 2
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄
There are a couple of problems. What do we do with double letters (Probably
they are not in alphabetic order.) Or single letters at the end (Add
a meaningless letter to make a pair.) But basically this is a simple digraphic
cipher.
What does a general digraphic cipher look like For our monoalphabetic
ciphers we always ended up with a plaintext alphabet and a ciphertext alphabet.
So for digraph ciphers we’ll have a (really big) chart telling which pairs of letters
got replaced by which pairs of letters.
To develop a very simple such system we start with a 26 × 26 array of all
possible letter–letter pairs. We will use this as our ciphertext pairs. Next we
use some method, say the keyword mixed method, to make two orderings of
the alphabet. We put these alphabets across the top and down the sides of the
chart. To encipher a pair of letters, call them (α, β), find α along the top row
and β down the column. The pair of letters appearing in the α-th column and
β-th row is our ciphertext.
Example: Using the keywords first and second gives the alphabets
fagmuzibknvrcjowsdkpxtelqy and sajryebktzcfluogmvnkpwdiqx. Putting
these above and along the side of the chart gives Figure 9.1.
Then telephones=te-le-ph-on-es is enciphered as VFXFT TOSWA (keeping
the usual 5 letter split.) LBGML OBW is deciphered as railroad. ⋄
There are several drawbacks to this system. Since the chart has 676 = 26×26
entries it is not something a person would like to have to repeatedly produce.
Further, while deciphering is not bad, because the edges are not in order it
is a bit of a pain to encipher. What would be quicker would be to rearrange
the chart so that the top and side alphabets were in order. But this means
rewritting the entire chart, not an attractive idea.
Even if these difficulties were surmounted, this cipher is simply not a good
one. For example, all plaintext pairs f* will be enciphered to A*, and all plaintext
pairs *s will be enciphered to *A, with similar results for the other letters.
This cipher is a monograph-digraph hybrid of some sort, and is not much more
secure than a pure monographic cipher.
2 (1) MHDFE NFFXY, (2) fat cat.