one-time pad

A manual encryption system in the form of a paper pad for one-time use.

An encryption algorithm in which the key is a random sequence of symbols and each symbol is used for encryption only one time -- i.e., used to encrypt only one plaintext symbol and thus produce only one ciphertext symbol -- and a copy of the key is used similarly for decryption.

Tutorial: To ensure one-time use, the copy of the key used for encryption is destroyed after use, as is the copy used for decryption. This is the only encryption algorithm that is truly unbreakable, even given unlimited resources for cryptanalysis [Schn], but key management costs and synchronization problems make it impractical except in special situations.