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Are there any algorithms that exist to encode messages written with pen and paper such that the messages can be encrypted/decrypted mentally? For example, Bob thinks of the text "This is my plaintext", writes down "siht si ym txetnialp", and then Larry (who knows the cipher) is able to understand the message "This is my plaintext", just by looking at the cryptotext. The algorithm is simple enough that nothing needs to be written down besides the cryptotext.
This should not be a code, where a specific phrase means something else. I'm looking for an algorithm like a one-time-pad or modular addition method that can be completely contained in one's head.
The strength of the encryption is not as important - it doesn't need to stand up to a complex solver, just be very difficult to decode by someone who doesn't know the cipher. The more important feature here is speed - e.g. encrypting and writing a message can be done simultaneously in (on the order of) twice the time it takes to write it in plain English.