Rip van Winkle cipher

The Rip Van Winkle cipher is a simple substitution cipher named after the character Rip Van Winkle from Washington Irving's story, who fell into a long sleep. In this cipher, each letter of the alphabet is shifted by a fixed number of places down or up the alphabet, similar to a Caesar cipher. However, the unique aspect of the Rip Van Winkle cipher lies in its method of shifting, which changes the shift periodically.