"I send you a cipher to be used between us, which will give you some trouble to understand, but, once understood, is the easiest to use." Jefferson wrote United State Minister to France Robert R. Livingston in 1802. Jefferson had used ciphers before with official as well as unofficial correspondence; letters to James Madison, John Adams, James Monroe, Robert Livingston, among others include communication in cipher. It was a way to keep "matters merely personal to ourselves" as well as a way to "have at hand a mask for whatever may need it."
Cognizant of the diplomatically sensitive situation Meriwether Lewis would be in while exploring the northwest, Jefferson prepared a cipher for use during the expedition and sent it to Lewis while he was preparing for the journey in Pennsylvania with astronomer, mathematician, and surveyor Andrew Ellicott.
The cipher, derived from the Vigenere cipher (that was widely used in Europe and was considered unbreakable until the 1830s), was a twenty-eight-column alphanumeric table. The correspondent would write the first line to be ciphered and then write out a keyword above, repeating it for the length of line (for Jefferson and Lewis the keyword was to be "artichokes"). The correspondent would use the up-and-down letter pairs to determine the coded letters, almost as if plotting points on a graph. Knowing the keyword the recipient could then translate the seemingly unintelligible message.