Did a certain cryptography method get abandoned due to security flaws in the past? I am researching how quantum computers affect current encryption methods (RSA and more).

However, I remember learning in a course that there used to be a particular encryption method which was popular but suddenly had a very bad vulnerability in the past (like quantum computers and Shor's algorithm would be for RSA) and everybody had to move on (is that right?).

I cannot find anything about this in the past, does anybody have any idea which cryptosystem I am talking about?
You could be thinking about the Merkle-Hellman knapsack cryptosystem. It was invented in 1978 and everything seemed well and good until it was completely broken six years later in 1984 by Shamir - it was a complete and total break, i.e. the cryptosystem became unusable overnight.

That said I don't know if the knapsack cryptosystem was ever "popular" in the modern sense (it was 1978 after all) but it was certainly an early public key cryptosystem that met an unfortunate end.



On the other hand, for an example of a progressively worn-down algorithm (as opposed to one being broken immediately) there is of course MD5's collision resistance.