An F in cryptography is aes-js / pyaes, as this Trail of Bits blog by Opal Wright explains:

Mistakes in cryptography are not a sin, even if they can have a serious impact. They’re simply a fact of life. As somebody once said, “cryptography is nightmare magic math that cares what color pen you use.” We’re all going to get stuff wrong if we stick around long enough to do something interesting, and there’s no reason to deride somebody for making a mistake.

What matters—what separates carelessness from craftsmanship—is the response to a mistake. A careless developer will write off a mistake as no big deal or insist that it isn’t really a problem—yadda, yadda, yadda. A craftsman will respond by fixing what’s broken, examining their tools and processes, and doing what they can to prevent it from happening again.

Does this sound familiar?