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

1.1. SAINT CYR SLIDE 13
paper turned sideways and put one letter per space. Leave a couple inches
of empty paper on each end of the letters. Under the first 25 letters put the
corresponding shift number: 1 below B, 2 below C, ... 25 below Z. Write on the
left edge that this is the “Ciphertext Alphabet”.
On a larger rectangular piece of paper write in lower case the letters of the
alphabet
abcdefghijklmnopqrstuvwxyz
Also write the numbers 1 through 26 above the corresponding letters. Make sure
to spread these letters at the same constant width that the ciphertext letters
were spread. This paper should be entitled “Plaintext Alphabet”. Then below
and just to the left of the a, and below and just to the right of the z, make
incisions in the paper of the same height as the cipherstrip. (It also wouldn’t
hurt to add an arrow entitled “key” on the plaintext page pointing to the spot
under the plaintext a to remind us where the key will appear.)
The cipherstrip can then be inserted into the plaintext piece and slid back
and forth, making it easy to form the Caesar alphabet of choice. Simply choose
a key number, slide the ciphertext strip so that this letter is under the plaintext
a, and then read from plaintext to ciphertext and back. This allows us to
relatively quickly and easily encipher and decipher using any key.
Because of the way the Saint Cyr Slide is constructed, we may also use letters
to indicate keys. The key as a number we understand: a key of 5 says move each
letter five letters down the alphabet. It is actually more common to use a key
letter. The key letter is the letter the plaintext a is enciphered into. So a key
of R says that a will become R in the ciphertext (and b becomes S, c becomes
T, etc.). In either case, we line up the key under the plaintext a and use the
plaintext alphabet – ciphertext alphabet pair the Saint Cyr Slide displays.
Examples:
(1) Encipher Roman Holiday using key S. What shift amount is this
To encipher, first line up the cipherstrip’s first S under the a of the plain
alphabet. Under S you should have written 18, so the shift amount is
18. Now encipher the title by finding the letters of roman holiday in the
plaintext alphabet and replace them one by one with the letters of the
ciphertext alphabet (underneath). You should see J under r, G under o
and E under m. If so, keep going. If not, make sure you positioned your
slide correctly and double check to see if you left out any letters of the
alphabet on your slide!
(2) Encipher Marc Antony, key 6.
(3) Decipher PEXANEQO, key W.
Line up the key letter W under the plaintext letter a. Remembering to
read from ciphertext back to plaintext (“up”), you should see t above P
and i above E.