A one-time pad is an encryption method that uses a key (the pad) that is as long as the message. It provides perfect secrecy but at the cost of having as much key data as payload. Not to be confused with one-time passwords.
A one-time pad is an encryption method that consists of encrypting each character in the message independently. The nth character of the pad specifies how to encrypt the nth character of the message: ciphertext[n] = pad[n] xor plaintext[n]. As the name indicates, the pad may only be used once.
This (or variations thereof) is the only encryption method that provides theoretical perfect secrecy (revealing only the length of the message). However, as it requires a key (the pad) that as long as the message, that must be perfectly random, that must be shared between the sender and the recipient, and that may not be reused, one-time pads are often not practical.
Not to be confused with OTP as in one-time-password.
