Schemes involving lookups into some reasonably sized random table/matrix are probably the most secure thing that is practical with only your head or only pen an

Schemes involving lookups into some reasonably sized random table/matrix are probably the most secure thing that is practical with only your head or only pen and paper. Similar system is/was used by various armed forces for both authentication and encryption of short messages (with the matrices being larger and rotated pretty often and thus not memorized).