Lecture 17: Introduction to Encryption

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 (or other mechanical) copy is needed -- 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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