Typically, hand-computed ciphers are very simple and easy to break. They are interesting only from a historical perspective.

LC4 (ElsieFour) is different. This low-tech stream cipher is well designed to be difficult to break. The cipher allows to encrypt and decrypt messages without any computing device, using only pen and paper.

A simple handmade wooden appliance can greatly simplify the routines required for encryption and decryption.

LC4 can be used not only to encrypt a message but also to authenticate the sender.

#math #cryptography #lowtech