In December 1801, Robert Patterson wrote to Thomas Jefferson, and included a somewhat lengthy statement on the use of cryptology. At the end of the letter he included a sample of what the use of cryptology and codes could be like.
It is believed that Jefferson never cracked this particular code.
In 2007 Dr. Lawren Smithline, a noted cyrptologist, cracked the code. Dr. Smithline's paper on this has finally been published.
You can read the hidden message that Patterson sent to Jefferson on THE WALL STREET JOURNAL.