One-time pad

A one-time pad is a key used by a cipher that is as long as the message that is being encrypted.

It is only used once (hence the name) and then we never use it again. That’s because if we reuse the same pad for multiple messages, it becomes much easier to crack the encryption.

We can then use each character in the pad to encrypt the corresponding character in the plaintext message, using whatever encryption function the cipher uses.
For example, the Vernam cipher uses a one-time pad where each character in the pad is XORed with the plaintext character to produce the ciphertext character.

The fact that each character is encoded with a different key makes one-time pads very secure, as long as the pad is truly random and kept secret. However, the main challenge with one-time pads is that they can be difficult to actually distribute - how does the receiver get the pad without it being intercepted by an attacker?