Whats the most secure way to encrypt a string without the use of a computer?

Question

I want to decrease attack surface by preventing the plaintext string or encryption key from ever being typed or otherwise entered into a computer. Ideally the decryption key would be something that the user can generate and store in their head. But encryption should still be as strong as possible. I don't need to be able to decrypt without a computer, though that would be cool.

Answer 1

A one time pad, the first problem is the key is as big as the message, the second problem is you have to roll some dice a whole bunch of times and do a lot of repetitive math and looking at tables to convert numbers to characters.

The advantage is perfect secrecy. But I think that answers the question as nothing is more secure.