I send you a cipher [a coding device] to be used between us, which will give you some trouble to understand, but, once understood, is the easiest to use, the most undecipherable, and varied by a new key with the greatest facility of any I have ever known … But why a cipher between us…? … there may be matters merely personal to ourselves, and which require the cover of a cipher more than those of any other character. This last purpose and others, which we cannot foresee, may render it convenient and advantageous to have at hand a mask for whatever may need it.
Thomas Jefferson to Robert R. Livingston, 1802, 1275
Patrick Lee’s Explanation
In a portion of this letter not included here, Jefferson explains how the coding device worked. That explanation might not be obscure to some readers, but it is to me! No doubt, Jefferson loved creating things like this, not only for their practical application but for the intellectual challenge.
Jefferson did not trust the postal carriers to safeguard the confidentiality of his written correspondence. He would not send sensitive material through the post. He often chose to have his correspondence hand delivered by a trusted friend. A cipher would have protected him in these instances.
To what degree Jefferson actually used coded messages I cannot say. He did send these instructions with Meriwether Lewis, should he need to send a secret message while on his exploration of Louisiana. Lewis did not use it.
How “crackable” Jefferson’s code was is also debatable. He thought it secure.
For an delightful article about a Jefferson-era cipher (and practical joke!) that eluded him and everyone else until 2009, read this article from the Wall Street Journal, by Rachel Emma Silverman, “Two Centuries On, a Cryptologist Cracks a Presidential Code,”http://online.wsj.com/article/SB124648494429082661.html