Manually unbreakable cryptography

Suppose you were able to go back in time, to an era before computers, and give someone contemporary cryptography. Encryption methods that are essentially unbreakable now would certainly be unbreakable then. But there’s a catch: not only do attackers not have computers, neither do users. If you told someone about RSA encryption, for example, you’d lose them right after you said “First you find a couple 1000-digit primes.” But there’s no need for using 1000-digit primes if no attacker has a computer. You could use 100 digit primes. Could you use 10 digit primes? If you chose primes just big enough to make the method unbreakable by hand, could someone implement it by hand? Kirchoff’s principle says the strength of an encryption method should depend only on keeping the key secret, not the method. But if you could keep the method secret, RSA would be unbreakable because nobody thought of anything like it before computers. World War II was a time of transition from manual cryptography to…