I am looking for - or considering the design of a new - encryption algorithm that does not use variable keys, such that it is easy for a computer to encrypt and decrypt any message regardless of its source, yet it is possible but impractical for a human using their brain and a pencil and paper to decrypt for any non-trivial length message.
Does anyone know of such an algorithm or have any ideas as to how to design one?
EDIT: The people using this algorithm have computers, but may need to hand-write the cipher text due to a lack of a printer, and may lack networking capability, so the message would have to be physically transmitted and scanned or retyped, thus, easy readability and hand-copyability is an advantage.
The persons attacking this cipher do not have computers, but may know the algorithm. The goal is to make the attackers waste human resources decrypting the messages.