[GreenKeys] [External] Two-tape TTY for crypto

[Jones, Douglas W]

From: Dave Horsfall [dave at horsfall.org] -- Monday, October 12, 2020 2:05 PM
>  Teletype made a box that took two tapes: one was the plain/cipher text, and the other was the key, and the two were combined with no-carry addition on the individual bits.
>  Anyone know any more about this?

This is called an XOR cypher.  It's in Wikipedia.  The good thing about it is that, if you never reuse your cryptographic keys and if the keys are genuinely random, it really is unbreakable.

The nice thing about it is that the same machine you use for encryption can be used for decryption.  You and your correspondent both need to have the same key in hand.  You use this box to combine your key tape with your message tape, then transmit the output.  At the far end of the line, your correspondent takes the message tape and the key tape as inputs to the box and combines them to reconstruct the plaintext.  You can put the key on either tape reader, it doesn't matter.

Of course, your plaintext must begin with a synchronization sequence, and this must also be present on the key tape and you must also transmit this synchronization sequence.  Otherwise, you'll get your message thoroughly scrambled.

The NSA used to punch miles of random tape (using things like radioactive decay as a source of randomness) in duplicate.  One copy, carefully labeled, would go to the state department, and the other copy of the key would go to an overseas embassy.  So long as each tape was used just once, this is an unbreakable code and as secure as it gets.  Of course, there's always the temptation to reuse a tape.  The same scheme was used for VLF communication with submarines and encrypted RTTY traffic.

In fact, the same scheme remains very useful, minus the paper tape, plus computers.  Instead of containing miles of tape, today's diplomatic pouches contain electronic media with the key streams on them, although I imagine that they're also using public-key cryptography for lots of traffic.  I have no inside information on that.

        Doug Jones
        jones at cs.uiowa.edu