cryptography Cryptography, or cryptology (from "hidden, secret"; and ''graphein'', "to write", or ''-logy, -logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of Adversary (cryptography), ...

, the Rip van Winkle cipher is a

provably secure Provable security refers to any type or level of computer security that can be proved. It is used in different ways by different fields. Usually, this refers to mathematical proofs, which are common in cryptography. In such a proof, the capabilit ...

cipher In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...

with a finite key, assuming the attacker has only finite storage. The cipher requires a broadcaster (perhaps a

numbers station A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to voca ...

) publicly transmitting a series of random numbers. The sender encrypts a

plaintext In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of comp ...

message by XORing it with the random numbers, then holding it some length of time T. At the end of that time, the sender finally transmits the encrypted message. The receiver holds the random numbers the same length of time T. As soon as the receiver gets the encrypted message, he XORs it with the random numbers he remembers were transmitted T ago, to recover the original plaintext message. The delay T represents the "key" and must be securely communicated only once. Ueli Maurer says the original Rip van Winkle cipher is completely impractical, but it motivated a new approach to provable security. "A Provably-Secure Strongly-Randomized Cipher" Ueli M. Maure

Sources

J.L. Massey and I. Ingemarsson. ''The Rip van Winkle cipher - a simple and provably computationally secure cipher with a finite key.'' In Proc. Institute of Electrical and Electronics Engineers, IEEE Int. Symp. Information Theory (Abstracts), page 146, 1985. Cryptographic algorithms {{crypto-stub