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

7.10. EXERCISES 133
(a) Construct a device on which to do the Telegraph Cipher.
(b) Encipher Through the Looking Glass with key ALICE.
(c) Decipher KHDKD WLDGG R using key HOLE.
(d) The Saint Cyr slide uses the alphabet three times, whereas Dodgson’s
device needs only two. Why
(e) Explain why the Telegraph Cipher is the same as the Beaufort Cipher.
(Dodgson was likely aware of Beaufort’s 1857 cipher. That they were
equivalent was not pointed out until 1883 by Auguste Kerckhoffs in
his La Cryptographi Militaire.)
26. Charles Dodgson also (see Exercise 25) invented two ciphers on February
23rd and 26th of 1858. From his diary entry on the 26th [Abeles3]:
Invented another cypher, far better than the last: it has these advantages.
(a) The system is easily carried in the head.
(b) The key-word is the only thing necessarily kept secret.
(c) Even one knowing the system cannot possibly read the cipher
without knowing the key-word.
(d) Even with the English to the cipher given, it is impossible to
discover the key-word.
To use this cipher, Dodgson writes the (latin) alphabet in form of square
A F L Q W
B G M R X
C H N S Y
D I O T Z
E K P V *
(so I=J, U=V, and * just fills the square.)
With keyword of GROUND he enciphers send as follows
Measuring from G to S we find it to be “2nd column 1st line,” and
write 21. In re-translating [deciphering] we begin at G, & go “2
columns to the right, & 1 line further down,” and this gives us S
again.
Measuring from R to E gives 23. from O to N - 04. from U to D -
24 .... we write 21.23.04.24.
So we always read to the right and then down. If we need to move off the
end of the square, simply re-enter on the top or left, as appropriate.
Dodgson added some complications:
i) Putting pairs in parentheses, such as (2.5), would mean to restart the
keyword at its fifth letter N. Or (2.5)(1.2) says restart with the letter that
is 1.2 from the 5th letter: N + (1.2) = V.