I'll always like seeing good crypto resources. https://cryptopals.com https://cryptopals.com has some nice challenges in this area, progressing to more advance

I'll always like seeing good crypto resources. https://cryptopals.com https://cryptopals.com has some nice challenges in this area, progressing to more advanced things like CBC padding oracle attacks. I also quite like the idea of cyberchef from GCHQ (https://gchq.github.io/CyberChef/ https://gchq.github.io/CyberChef/), although to be honest spinning up a python interpreter with the same libraries is not much harder, so it probably depends more on personal preference for web or CLI.