Military Communications and Information Technology: A Trusted ...

486 Military Communications and Information Technology...
generation of binary random sequences with the potential bit rate 100 Mbit/s
eliminates the restriction connected with availability of very long one-time keys
for the perfect ciphering. Unfortunately, other than the trusted couriering key
distribution system still requires an effective and reasonably priced solution.
II. Vernam cipher or One-Time Pad
The one-time pad (OTP), also called Vernam-cipher or the perfect cipher,
is a crypto algorithm where plaintext is combined with a random key. The one-time
pad was developed in 1917 by Gilbert Vernam for the use in telex machines. Each
transmitted 5-bit Baudot code was mixed with a random 5-bit code on a paper
tape. Such tapes contained a large number of these random 5-bit codes and were
called one-time-tape. The one-time tape ran synchronously on both the sender’s
and the receiver’s telex. Vernam’s invention was the basis for several pencil-andpaper
versions. The name one-time pad refers to the notepads on which the keys are
printed as shown in Fig. 1. In general, these pads are small booklets or microfilms
with groups of five numbers or letters.
Figure 1. Example of one time keys in a paper form
A. One-Time Pad in practice
We can only talk about OTP if four important rules are followed. When rules
are applied correctly, the one-time pad can be prove unbreakable (see Claude Shannon’s
“Communication Theory of Secrecy Systems”). However, if only one of these
rules is disregarded, the cipher is no longer unbreakable.
1. The key is as long as the plaintext.
2. The key is truly random (not generated by simple computer Rnd functions
or whatever!).