This cipher, which Jefferson may have drafted earlier than the preceding one, establishes coordinates using the left column and bottom row of the matrix.1 Similarly, it uses all 26 letters of the alphabet, plus the ampersand (&) symbol.
The partially legible writing at the bottom of the page shows the encryption of Jefferson's sample plaintext, which reads: "I am at the head of the Missouri. All well and the Indians so far friendly."
Jefferson then illustrates the encoding of the plaintext. The letters of the keyword artichoke are distributed, in order, beneath the letters of the plaintext. Using the procedure applied to the other sample plaintext, "The man whose mind . . ." etc., Find the first letter of artichoke in the column at extreme left. Find the first letter of the plaintext in the bottom row of characters. The letter at which the two coordinates intersect, j, is the first letter of the encryption. Find the second letter of the key word in the left column. Find the second letter of the plaintext in the bottom row. The letter at which the two coordinates intersect is s, which is the second letter in the encryption.
Jefferson concluded: "then copy fair thus":
The message is decoded by the recipient in the following way: Write the letters of the keyword, artichoke, above the successive letters in the encoded message. Locate the keyword letter a (the first letter in artichoke) in the bottom row; trace that column upward until arriving at the code letter j. Follow that row to the left to locate the plaintext letter I in the first column. Next, find the keyword letter r in the bottom row; trace that column up to the code letter s; follow that row to the first column at left, which yields the plaintext letter a. And so on.
1. Donald Jackson, ed., Letters of the Lewis and Clark Expedition with Related Documents, 1783-1854 (2d ed., 2 vols., Urbana: University of Illinois Press, 1978), 1:10.
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