[Cryptography] Brainstorming for encrypted text messaging ideas...with a twist

[Ray Dillinger]

Considering this again and getting back to the simplicity of the
one-time pad.... There is a classic construction for a simple
mechanical device to assist the use of a one-time pad. Tabletop versions
were in use in WWII, but a refined and convenient version could easily
be made shirt-pocket size, with a 20-character photo-friendly display.

It consists of 20 sliding strips, each having the alphabet printed
vertically (in type about the size of watch numbers for a shirt-pocket
device) with a hole drilled next to each letter.  They fit together into
a rack where each strip slides vertically behind a slot where the
letters aren't visible but a stylus can be inserted through the slot
into the holes.  Each slot also has a window in the center where one
letter is visible.  Finally each vertical slot is marked with the
alphabet twice: once above and once below the windows. So each letter in
a strip will occur either under a letter on the face of the device, or
under the window.

Alice takes the device and uses a stylus to shove all the strips to the
top end (this sets the display to all blanks).  Then, strip by strip,
she inserts the stylus into the hole next to the plaintext letter above
the window (the letter now is both the marked location on the slot and
the value of the underlying letter on the strip), and slides it down to
the window. This sets the display to the plaintext.  Strip by strip, she
then inserts the stylus into the hole next to each key letter (the
marked location on the slot, which now does not match the underlying
strip letter) and slides them either up, or down, to the windows.  This
sets the display to the ciphertext.  She snaps a pic of the device,
sends it to Bob, and goes on to the next 20-character line.

At the far end, Bob sets his device to the displayed ciphertext, then
inserts his stylus into the strip at each ciphertext window and slides
it up or down to each key letter to reveal the corresponding ciphertext
letter in the window.

This is easy and cheap to make, mechanically obvious enough to reassure
the paranoid, has a display that can be photographed, doesn't leak key
information, and doesn't contain anything with a battery, much less
anything that can run invisible software.  It implements a one-time pad
mechanically and with instructions simple enough for people to
understand. It can be made entirely out of wood or plastic, so as to
pass metal detectors and some kinds of x-rays unremarked.

But it is still a one-time pad.  And in practice, people will use short
repeating pads (a 'keyword cipher' broken via its index of coincidence)
or non-random one-time pads (a 'book cipher', broken by covariant
frequency analysis) or use one-time pads more than once (a 'two-time
pad', broken by simple subtraction). So while it's easy, simple,
convenient, and at least potentially secure, people will, very
predictably, fail to distribute the required keying material and will
use it in insecure ways.

If someone wants to make it, it should probably be packaged with some
mechanical entropy device (30-sided dice 26 sides of which are marked
with letters, or whatever) to help generate one-time pads that are
actually random.

				Bear

PS:  Did you know that the classic "30-sided dice shape" is technically
called a rhombic triacontahedron?


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