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Pencil and paper encryption revisited (long)
From: David Farber <farber () central cis upenn edu>
Date: Sat, 6 Nov 1993 19:54:45 -0500
Happy to send the full article to anyone interested From: richardr () netcom com (Richard L. Robertson) Newsgroups: sci.crypt Subject: Pencil and paper encryption revisited (long) Date: 4 Nov 93 23:29:21 GMT Organization: NETCOM On-line Communication Services (408 241-9760 guest) Lines: 822 Apparently-To: farber@linc Bruce Schneier <schneier () chinet chinet com> in Message-ID: <CEtKr4.7B7 () chinet chinet com> Date: Wed, 13 Oct 1993 05:04:13 GMT Subject: Pencil and paper encryption algorithm proposed a pencil-and-paper encryption algorithm that could be used without computers, but was still secure against computer-aided attacks. I answered with what I felt were several practical usage problems with his proposed methodology that made it infeasible to reliably encrypt and decrypt messages in a finite time. During a much needed vacation from the practical realities of work and life, I have attempted to come up with a simplified message encryption algorithm that meets Bruce's criteria and is practical in use. I took as design constraints that an inexpensive (< $30) pocket calculator was acceptable for performing any necessary calculations, but that something as big and complex as an HP-48 or an Apple Newton was unacceptable. I also changed the requirement from "secure against computer-aided attacks" to "highly resistant against computer aided attacks". My first attempt used a simple, multiple memory, non-programmable Radio Shack checkbook pocket calculator. While the methodology met the "resistance" criterion, it failed the practical test of error- free calculation in a finite time. It turned out to be possible to get reliable encryption and decryption by applying the result cross-checking techniques used in hand pencil-and-paper calculation, however the time required for error-free encryption was exorbitant. By relaxing the design constraints to allow limited programmability in the pocket calculator, I was able to adequately address the problem of speed of error-free encryption calculations. The constraint that I adopted was that the calculator's program steps must be simple and compact enough for the user to be able to memorize and to be able to re-enter the program into the calculator each time that it was used to encrypt or decrypt a message. I believe that this satisfies the reasonable requirement that there be no incriminating evidence left lying around in the calculator between encryption sessions. The following encryption procedure was tested using an $18 Radio Shack Model EC-4021 programmable scientific calculator. The algorithms were modified as necessary to conform to the practical limitations of the calculator keypad and limited programming capabilities. With only moderate training time (a couple of hours) I was able to reliably encrypt and decrypt messages at a rate of 8-10 characters per minute. The primary speed limitation was the actual tran- scription on the results by pencil onto paper. I would appreciate any and all comments, criticisms, error corrections and suggestions for improvements. Richard Robertson richardr () netcom com
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- Pencil and paper encryption revisited (long) David Farber (Nov 06)