Respectfully, I don't think this is how you should learn cryptography (certainly: you shouldn't call this kind of work "cryptographic engineering").
I'm talking my own book here a little (but just a little, since it's not like I make a dime from this) when I say that the better way to learn and understand cryptography is stuff like the Matasano Cryptopals challenges:
https://cryptopals.com/
Even after you've undergone our cryptogaphic Ludovico Process, you still won't be a "cryptographic engineer". I've been testing and building exploits for random cryptosystems for over a decade and I'm nowhere close. The simple, blunt reality of it is that if you're going to build anything close to new with cryptography, you really do have to understand the math, and anyone who claims you can get to "securing a banking interface" without a detour through abstract algebra is, I think, doing you a disservice.
Another good resource for this stuff is LVH's Crypto 101: https://www.crypto101.io/
https://gist.github.com/arkadiyt/5b33bed653ce1dc26e1df9c249d...
Compared to OP's tutorial, this is certainly a better way. I very much doubt this is the best way however, unless of course you actually want to do cryptanalysis.
Breaking stuff for real takes time. Learning that stuff can be broken is much quicker. I don't need to forge messages to be afraid of ciphertext malleability. Once I understand how XOR works, of course I'll run away screaming into the night at the sight of unauthenticated encryption. That said, I reckon doing the challenges is very good for street cred.
Also, some things just have to be taught. Forward secrecy for instance, is either like "I don't have the key, can't break anything", or "duh, you leaked your long term key, of course I can read everything". Exploiting breaches can help someone plug the leaks, but they won't teach them to secure their users' messages after law enforcement went for their long term keys.
And dammit, I don't aspire to be a crypto engineer. I just want to build a secure system. That said:
> if you're going to build anything close to new with cryptography, you really do have to understand the math
Oh yes. That alone warrants my upvote.
Monocypher uses DJB's curves, which are naturally immune against pretty much anything (assuming constant time primitive operations). Invalid curves don't reveal anything (though one needs to check against non contributory behaviour in key exchange), and the whole thing is constant time whether the public key/point was on the curve or not.
Maybe I would take a look at invalid curves attack if I ever try my hand at ECDSA or something, but (i) I don't plan to in the first place, and (ii) even if I did, it would be quicker to just learn about those attacks and how to avoid them.
Same about GCM nonce repeat. Useless. I have read that nonce repeat is catastrophic for GCM, and I trust that. Chacha20/Poly1305 is also vulnerable to nonce reuse (reveals the authentication key and the XOR of 2 plaintext messages), but I don't need to mount an attack against it to learn anything useful. Sure, this might give me further insight about how Wegman-Carter hashes work, but I'm not inventing anything here, so I don't need to understand that part in depth.
Even if I was inventing anything, I still believe being able to mount relevant attacks would still be mostly useless. I don't want to attack flawed systems, I want to build a flawless system. I would have to prove the system is flawless. Making sure the proof doesn't have an error is different from mounting an attack if there is.
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Some people may need to perform the attack to really believe in their core that it is possible after all. I don't. Seeing the math is enough to send shivers down my spine.
I see one thing for which I expect cryptanalysis is genuinely useful: inventing new primitives that we cannot prove secure. Symmetric cypher and hashes, elliptic curves… Those require a deeper understanding, and I do expect one has to know how to break the bad stuff to come up with good stuff. There's just no way I try to elevate myself up to that level. I have no comparative advantage, and I'm not going to spend the 10 years required to have one.
Makes me wonder if the "example" is getting pwned by a Git repo and the lesson I'll learn is to keep my system updated.
That said, browsing the repo online, it looks like a pretty useful tutorial.