Cryptography Reference
In-Depth Information
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The left contact surfaces are scanned by 26 sliding contacts, similarly the
right ones. The sliding contacts correspond to the letters of the alphabet.
Applying voltage to one of the left sliding contacts by the push of a
button, this voltage arrives at another right sliding contact and can cause
one of 26 small lamps to light up. This approach is nothing but a simple
substitution. It offers two minor benefits, compared with rigid schemes:
we can replace the disk by another one, and we can specify an arbi-
trary start position. We already know that the method is one of the most
insecure despite this.
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Next, we turn the disk forward by one step after each character. This
produces a polyalphabetic substitution with period 26 (the 27th character
in the plaintext is then encrypted like the first one). This corresponds to
the Vigenere cipher in the historical sense.
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Another idea uses several such disks arranged adjacently, each one with a
different inner wiring (i.e., substitution). Between each pair of two disks
we attach sliding contacts that connect the right contact surface of the
left disk with the opposite, left contact surface of the right disk.
As we know from the section on 'multiple encryption', this won't initially give
us anything new: again, only a simple substitution will be produced at the far
right.
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Let's combine the last two ideas: after each encrypted character, we turn
each disk a little further, but each by a different amount. The substitu-
tions that are permanently wired in the disks will now produce a totally
different substitution at the right-hand end each time. This arrangement
serves the purpose of keeping the period very large with minimum effort
(which makes cryptanalysis much harder).
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For example, we can move three disks like in a counter. After each
character, the right disk turns forward by one step; after each 26th char-
acter, the middle disk will also turn one step forward, and once the
latter has completed one full rotation, the left disk will also turn. In a
general case, this results in a period of 26
3
=
17 576. Arbitrarily com-
plicated approaches are conceivable to make cryptanalysis as hard as
possible.
Whatever the case, the algorithm of the disk movement is fixed, and only the
ground settings and the disk arrangement are variable.
