In May 1804, Meriwether Lewis and William Clark set off into the great unknown of the Louisiana Territory, far from help and far from home. Ahead lay vast prairies, endless mountain ranges, uncharted streams, untold dangers.

Their mission: lead the Corps of Discovery across the continent, establish relations with Native peoples they met along the way, document plant and animal life and, most importantly, find a practical water route to the Pacific.

President Thomas Jefferson, who had ordered the expedition, expected no regular communication from the corps. But he did hope that traders or Natives might help get occasional messages back to Washington. Some of those communications, he believed, might contain sensitive information best kept secret.

Long fascinated by encryption, Jefferson devised a special cipher for use by the expedition and sent it to Lewis. Only they would understand any messages encoded with it.

“Avail yourself of these means to communicate to us, at seasonable intervals, a copy of your journal, notes & observations of every kind,” Jefferson wrote to Lewis on June 20, 1803, “putting into cypher whatever might do injury if betrayed.”

Two versions of the cipher, handwritten by the president, are preserved in the Jefferson papers held by the Library’s Manuscript Division. Both used grids of letters, numbers and symbols to encrypt and decode messages.