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One-Time Pads
Another approach (although rarely used) is the one-time
pad, (or Vernam Cipher) where a simple
algorithm is used in conjunction with a key of the same
length as the message, and employing a brand new key for
every message transmitted.
For example, convert both the plaintext and the key to bit strings
(which will be, necessarily, of the same length). Apply the
XOR function function bitwise between the
strings, giving the ciphertext. The recipient can then apply the
same key to the ciphertext using XOR function
and thus recover the original plaintext.
This is, in every respect, unbreakable, but rather impractical for
real-world use in most cases. Some reasons:
- The key is the same length as the message, which means that it
will probably be too long to memorise, hence a written copy is
needed, which is potentially dangerous.
- The length of the message is limited by the amount of key
available.
- If synchronisation is lost, subsequent messages will be
garbled.
Neverthless, see s/key for a practical,
working example of a one-time pad system.
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