**8** 0

CALMER_THAN_YOU_ The enigma machine really was unbreakable for its time. It was only broken because of the human factor involved (identifying consistent patterns from German messages meant that we could make the problem smaller). This equates to say you change your 8 digit password every day but I find out you always put 1945 as the last 4 characters. If I’m going to brute force attack your password, I only have to try 10^4 tries instead of 10^8. I effectively made your problem easier because The weakness in your password was the human factor.

**5** 0

iranoutofspacehere Most modern cryptosystems use modular arithmetic with upwards of hundreds of digits. They rely on being able to select arbitrarily large prime numbers and multiplying them... The sheer size of the numbers prevents brute force attacks.
Storing numbers this large without electronic memory and manipulating them without modern computers is not practical.
Basically, any system that could have been implemented before modern computers would be brute forced by modern technology.
For reference, a group called the M4 Project was able to use distributed computing on a volunteer basis to brute force some WW2 messages encrypted with 4 rotor Enigma 1-2 months.

**6** 0

Lalaithion42 If you had modern knowledge, you could probably build a machine that multiplied large numbers; this would be enough to allow for Diffie-Hellman key exchange. You could also probably build a deterministic random number generator, and use the DH key to seed it. This would allow you to construct a fairly basic stream cipher.
This would reduce the chances of operator error from many to exactly one: the operator must choose a number randomly between 1 and the maximum, which would likely have been around a dozen digits. If operators reused numbers, it would become extremely easy to break the system.