Gilbert Vernam was a digital systems designer from the early 20th century. He invented the stream cipher
, what browsers often use today to encrypt messages exchanged with protected web sites. In his days, however, the mechanism of choice was the relay: an electromagnetic switch. Vernam also described the one-time pad
, and noted the danger in reusing the key stream
What, then is a Vernam cipher
? Is it a stream cipher or a one-time pad? I've seen the term used both ways.
Now we can check the source. Steve Bellovin recently blogged on Vernam's work
, and posted a PDF of Vernam's original paper
. Vernam wrote the paper for an AIEE conference (that's one of the precursors of today's IEEE - Bellovin negotiated permission to post the historic paper).
If we look at the historical description, Vernam does not restrict his cipher to the one-time pad case. Thus, a Vernam cipher in practice might - or might not - be a one-time pad. [revised 9/7/09]