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reused key

Vernam's Cipher

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]

Stream Cipher Reuse: A Graphic Example

Take a look at the following image. You should see two different 'messages' here.

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  Two messages

This particular mis-mash of messages reflects the failure of otherwise strong cryptography: the improper implementation of a one-time pad or a stream cipher.

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