Rip van Winkle cipher
Encyclopedia
In cryptography
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

, the Rip van Winkle cipher is a provably secure
Provable security
In cryptography, a system has provable security if its security requirements can be stated formally in an adversarial model, as opposed to heuristically, with clear assumptions that the adversary has access to the system as well as enough computational resources...

cipher
Cipher
In cryptography, a cipher 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. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...

with a finite key
Key (cryptography)
In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...

, assuming the attacker has only finite storage.

The cipher requires a broadcaster (perhaps a numbers station
Numbers station
A numbers station is a shortwave radio station of uncertain origin. In the 1950s, Time magazine reported that the numbers stations first appeared shortly after World War II and were using a format that had been used to send weather data during that war.Numbers stations generally broadcast...

) publicly transmitting a series of random numbers.

The sender encrypts a plaintext 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.

## Source

J.L. Massey and I. Ingemarsson. The Rip van Winkle cipher - a simple and provably computationally secure cipher with a finite key. In Proc. IEEE
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

Int. Symp. Information Theory (Abstracts), page 146, 1985.