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 computerized cryptography. Encryption machines were attacked by cryptanalysis machines though these machines were general-purpose computers. If you could implement a symmetric encryption method like AES in a mechanical device, no mechanical device could break it.

You could even use something like DES. It’s only considered broken because now you could throw enough compute power at it to break it by brute force. But it’s safe to say it would not be possible break it with mechanical devices.

My hunch is that the best approach would be stream ciphers. Maybe it would be practical to implement one of these by hand or with the aid of simple calculating machines. Something like PCG, which is not cryptographically secure today [1], would have been then, though I don’t know how practical it would have been to carry out PCG, say, in the 1940s.

Pre-computer cryptography posts

• Vigenère cipher
• Playfair cipher
• ADFGV

[1] In 2020, Charles Bouillaguet, Florette Martinez, and Julia Sauvage were able to break PCG using 20,000 CPU-hours. Practical seed-recovery for the PCG Pseudo-Random Number Generator. IACR Transactions on Symmetric Cryptology. ISSN 2519-173X, Vol. 2020, No. 3, pp. 175–196.