Are there any strong manual (i.e. performed only by pen and paper) ciphers for practical use?
It should be:
- no less strong than modern cryptography algorithms used in computers,
- at least relatively fast/easy to encrypt/decrypt with just a pen and paper (say, no more than 10 minutes per character),
- key should be memorable (i.e. it shouldn't be a 256 random character string, because it's nearly impossible for an ordinary human to memorize it and remember for a long time).
- not necessary, but if possible, it should be quantum resistant.
I believe that latest progress in mathematics and cryptography made all this possible, but we are just unaware of it, as it stands in the "shadow" of easily accessible and powerful computer cryptography. So, cryptography experts, please enlighten us, lay people.