Rule of crypto #1 - never implement your own crypto.
If you want to know what they are talking about, here are two free courses that go over all of that stuff and more. I particularly liked error oracle decryption exercise from coursera crypto class:
The Coursera one is amazing. I've been waiting for the second part for years. It's a twist a minute ("so how can you break X?" "okay yeah there is NO WAY to break this at all, I'm positive" "pretty simply: do Y" "goddamnit").
Yes, but they did it wrong. But someone else caught the bug and fixed it. But the fix was wrong, but yet another person caught that bug and fixed it. But their fix was wrong, but it got fixed, but that was wrong... and the next, and the next... Repeat for as long as the library has been in active use and development.
That's the value of using established libraries. It's not necessarily that those library authors are so much smarter than the rest of us (although maybe a few are). It's that they already know a lot of the ways that their code was wrong.
If you write it yourself, who's going to catch your bugs? Then who's going to catch theirs?
This is not specific to crypto code, but crypto code is probably harder than most to get right, and more likely to be used on serious stuff.
"those library authors are so much smarter than the rest of us"
Aside from your point (which is valid itself), it's not even necessarily "smarter". Comparably smart people more specialized at task X are likely to be better at task X.
What says any particular person will write said bugs? We have a word for automatically assuming that any particular individual holds any particular trait of the group of which they are a member without first confirming that person has that trait. It's called prejudice. And yes, you can be prejudiced towards your own in-group.
Good crypto code needs a good understanding of crypto and a good understanding of code. The person who has both is rare. Far too frequently, the situation you've described comes about because the only people writing crypto code are the people who understand crypto.
And I think the admonishment "don't write your own crypto code" is peer-pressuring people who have a good understanding of code from abstaining from gaining a good understanding of crypto.
When it comes to putting bugs in software, I am prejudiced against the human race. :-)
Folks saying "don't write your own crypto" are really talking about production systems that will serve customers, employees, governments, etc. Crypto is not going to be a market differentiator; it just needs to work. So why not stand on the shoulders of many others and use a library that is already well tested and patched up?
But in terms of spending your own time to learn, I doubt many folks would say don't do that. Matasano even provides a bunch of freely available materials to do just that.
Elon Musk thought it was a good idea to design and build his own car. Should Albert Parcelle, the creator of the first wheel-hub motor, also have abstained from "reinventing the wheel" in 1890? How about J. Grabowiecki with the Omni Wheel in 1919? How about Bengt Ilon forgoing his Mecanum Wheel in 1973?
See, this is why I hate these sorts of statements. They assume it's impossible to be innovative in well-established fields. Or they assume the person they are saying it to is incapable.
It's a sort of arrogant condescension that I don't think should have any place among polite people
When people say things like "Don't reinvent the wheel" and "Never implement your own crypto", it's generally understood that they mean "unless you have a really good reason for doing so and the budget and expertise to do it right." We don't actually say those things, because we don't want to turn casual conversations into legal documents; instead we trust in the readers' common sense.
But any time any of these sorts of projects come up, the posts are not "what need are you trying to fulfill?", they're just yelling, "don't reinvent the wheel!" That's what I'm talking about, this culture of "I, having spent 5 minutes skimming your README, know better than you."
you can also take matasano's own practical crypto course http://cryptopals.com. If you're taking a vacation, for example, it can be fun to do some of the exercises, write stuff on a notepad, go back to doing exercises and so forth. highly recommended and lighthearted.
If you want to know what they are talking about, here are two free courses that go over all of that stuff and more. I particularly liked error oracle decryption exercise from coursera crypto class:
https://www.coursera.org/course/crypto
https://www.udacity.com/course/cs387