The way where people think they learn cryptography by making their own implementation is, indeed, wrong. Cryptography is about ensuring specific requirements in the face of active adversaries.
Self-implemented crypto misses many well-known caveats, causing them to be easily breakable. As such, it is not reasonable to consider them as something that aims learn about ensuring security properties in the face of active adversaries... but then that effort is not actually teaching about crypto, which is exactly that.
This is also the reason that the established wisdom for learning cryptography is to learn to break systems first. Almost everyone can make a cryptographic system they cannot break. For most folks, that means little. For those skilled at breaking crypto, that carries weight.
It's akin to saying, "you're not allowed to build your own smoke detector because it will be unsafe!". Of course I know that, I want to understand the differences between a photoelectric and ionization smoke detector, how they work in practice, because reading some PDF schematics just doesn't cut it for me.
I honestly don't understand the line of reasoning of all this crypto gatekeeping.
Fun fact: while I was doing my crypto deep dive in 2015, my language of choice being Haskell, I found issues in several libraries, specifically around entropy, and even one library with modulo bias [1]. They were acknowledged and addressed. It was a super fun learning exercise, and seeing all these comments how it's supposedly almost illegal to do this misses the point of people exploring and learning in their own ways.
https://github.com/vincenthz/hs-crypto-numbers/commit/bceb54...
This is exactly what the other people in this thread mean when they say "learn by breaking other crypto", assuming you didn't write those libraries.
It’s all about intent: my intent is understanding “how does this work?”, “how do I do this the right way?”, not breaking, and that’s how I found these things.
I don’t care about breaking stuff, it’s just not appealing to me. My whole frustration with this discussion is that for some reason, my intentions are not the True Way of the Crypto Experts, and then I’m not allowed to proceed.
It’s such utter nonsense, and this whole discussion made me realise the situation is actually worse than I thought. If these commenters here are representative of the wider crypto community, they’re really a bunch of elitist gatekeepers that cannot understand the difference between being the NSA and a mere mortal trying to learn things in their own way.
On a related note, Beldin here just contributed a new entry into cryptopals: the Gatekeeper hash, but only certain people can see the link.
I honestly think, as a community, we’re being far too dogmatic about our advice here, and we should recognise that there is value in learning things by example, and that more people having a better understanding of crypto is generally a Good Thing.
Using it in production, of course, is an entirely different matter, but that’s not what I’m contesting.