The Alphabet Cipher

Lewis Carroll published The Alphabet-Cipher in 1868, possibly in a children's magazine. It describes what is known as a Vigenère cipher, a well-known scheme in cryptography. While Carroll calls this cipher "unbreakable", Kasiski had published a volume describing how to break such ciphers from five years earlier, and Charles Babbage had secretly found ways to break polyalphabetic ciphers during the Crimean War.

The piece begins with a tabula recta.

The Alphabet-Cipher, Lewis Carroll, 1868

   ABCDEFGHIJKLMNOPQRSTUVWXYZ
 A abcdefghijklmnopqrstuvwxyz A
 B bcdefghijklmnopqrstuvwxyza B
 C cdefghijklmnopqrstuvwxyzab C
 D defghijklmnopqrstuvwxyzabc D
 E efghijklmnopqrstuvwxyzabcd E
 F fghijklmnopqrstuvwxyzabcde F
 G ghijklmnopqrstuvwxyzabcdef G
 H hijklmnopqrstuvwxyzabcdefg H
 I ijklmnopqrstuvwxyzabcdefgh I
 J jklmnopqrstuvwxyzabcdefghi J
 K klmnopqrstuvwxyzabcdefghij K
 L lmnopqrstuvwxyzabcdefghijk L
 M mnopqrstuvwxyzabcdefghijkl M
 N nopqrstuvwxyzabcdefghijklm N
 O opqrstuvwxyzabcdefghijklmn O
 P pqrstuvwxyzabcdefghijklmno P
 Q qrstuvwxyzabcdefghijklmnop Q
 R rstuvwxyzabcdefghijklmnopq R
 S stuvwxyzabcdefghijklmnopqr S
 T tuvwxyzabcdefghijklmnopqrs T
 U uvwxyzabcdefghijklmnopqrst U
 V vwxyzabcdefghijklmnopqrstu V
 W wxyzabcdefghijklmnopqrstuv W
 X xyzabcdefghijklmnopqrstuvw X
 Y yzabcdefghijklmnopqrstuvwx Y
 Z zabcdefghijklmnopqrstuvwxy Z
   ABCDEFGHIJKLMNOPQRSTUVWXYZ

Explanation

Each column of this table forms a dictionary of symbols representing the alphabet: thus, in the A column, the symbol is the same as the letter represented; in the B column, A is represented by B, B by C, and so on.

To use the table, some word or sentence should be agreed on by two correspondents. This may be called the 'key-word', or 'key-sentence', and should be carried in the memory only.

In sending a message, write the key-word over it, letter for letter, repeating it as often as may be necessary: the letters of the key-word will indicate which column is to be used in translating each letter of the message, the symbols for which should be written underneath: then copy out the symbols only, and destroy the first paper. It will now be impossible for anyone, ignorant of the key-word, to decipher the message, even with the help of the table.

For example, let the key-word be vigilance, and the message 'meet me on Tuesday evening at seven', the first paper will read as follows—

v i g i l a n c e v i g i l a n c e v i g i l a n c e v i
m e e t m e o n t u e s d a y e v e n i n g a t s e v e n
h m k b x e b p x p m y l l y r x i i q t o l t f g z z v

The second will contain only 'h m k b x e b p x p m y l l y r x i i q t o l t f g z z v'.

The receiver of the message can, by the same process, retranslate it into English.

If this table is lost, it can easily be written out from memory, by observing that the first symbol in each column is the same as the letter naming the column, and that they are continued downwards in alphabetical order. It would only be necessary to write out the particular columns required by the key-word, but such a paper would afford an adversary the means for discovering the key-word.