In fairness, I'm probably over-generalizing from a few examples. For example, my biggest inspiration from the field of crypto is Daniel J. Bernstein, a cryptographer who's in part known for building qmail, which has an impressive security track record & guarantee. He discusses principles for secure software engineering in this paper, which I found pretty helpful for my own thinking.
To your point about hashing the results of several different hash functions, I'm actually kind of surprised to hear that this might to protect against the sorts of advances I'd expect to break hash algorithms. I was under the very amateur impression that basically all modern hash functions relied on the same numerical algorithmic complexity (and number-theoretic results). If there are any resources you can point me to about this, I'd be interested in getting a basic understanding of the different assumptions hash functions can depend on.
This post is a container for my short-form writing. See this post for meta-level discussion about shortform as an upcoming site feature.