While writing an article on cryptographic hashing for programmers, I stumbled across the Rip Van Winkle Cipher in Applied Cryptography:
James Massey and Ingemar Ingemarsson proposed the Rip Van Winkle cipher, so named because the receiver has to receive 2^n bits of ciphertext before attempting decryption. The algorithm, illustrated in Figure 17.10, is simple to implement, provably secure, and completely impractical. Simply XOR the plaintext with the keystream, and delay the keystream by 0 to 20 years – the exact date is part of the key. In Massey’s words: “One can easily guarantee that the enemy cryptanalyst will need thousands of years to read the plaintext, if one is willing to wait millions of years to read the plaintext.” Further work on this idea can be found in [references].