I'm with you except for: "a useless diversion".
From the description in your article, I might think that if I have really short messages, I'm okay. I might think that if I reversed my strings, so any additional state is prepended instead of appended, I'd be okay. I might think that if I truncate my hashes (say, used the first 256 bits of SHA-512), I'd be okay. I'd be really, really wrong.
> Point being, although hash functions might work most of the time, their general construction does not make them safe for this purpose. You can't say something is 'mostly secure' because it works 'most of the time'. The times is doesn't work will affect someone, and for them the system is not secure at all.
That's the wrong point though. It's not that hash functions "might work most of the time". Uncompromised secure hash functions work all of the time for their intended purpose. For this other purpose, they work exactly none of the time.
The aspects I described that might make you think SHA-1 actually has this covered except for a few corner cases are trivial challenges for an attacker to overcome, as are all of the above "remedies" I mentioned (all of which resemble idiot moves that have in fact been made by naive developers who thought they could be clever and save themselves... I don't know.. maybe some CPU time?).
I do think the post was some great writing that illustrated an idea quite clearly. I think it could be quite informative for a lot of people. I just think what you wrote would benefit from being framed appropriately. I'd suggest a preface... something like, "HMAC's were invented to address the fact that you can't just use a hash for authentication. Somehow people miss this point, possibly because HMAC's seem like a trivial layer on top of a secure hash. In practice, there are a surprising variety of issues. To give you an idea of the kinds of problems you might run in to, and hopefully a sense that you can't just naively tweak your use of a hash to address them, I'm going to simplify a common problem with using secure hashes for authentication that has probably never occurred to you. I assure you that this problem is actually more complex, and there are many more problems."
