[2] It describes what is known as a Vigenère cipher, a well-known scheme in cryptography.
While Carroll calls this cipher "unbreakable", Friedrich Kasiski had already published in 1863 a volume describing how to break such ciphers and Charles Babbage had secretly found ways to break polyalphabetic ciphers in the previous decade during the Crimean War.
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— 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'.
Continue till you code or decode the entire message.