The term "pen and paper" refers to cryptographic functions, schemes, and procedures that can be handled by humans, without access to computers or other electronic equipment. So, "pen and paper" can also mean "pencil and post-it", "chalkboard and chalk", "wall and spraycan", etc.

To explain it a bit more practically: the term "pen and paper" originates in military cryptography, where soldiers in the field can frequently find themselves in situations where they need to apply cryptography to transport messages in a secure way, but have nothing but a pencil and some paper available to do so.

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