Came across this today. Especially the collection highlights on Wikipedia [0] really made my day.

[0]: https://en.wikipedia.org/wiki/Museum_of_Bad_Art#Collection_h...

> The case I was making, is that weakhash(stronghash(m)) has the security of weakhash, no matter how strong stronghash is.

I'll have to disagree. There are no known collision attacks against SHA-1(SHA-3(M)), so in the applied case, a combination can be more secure for some properties, even if it isn't in the theoretical case.

Museum of Bad Art

I have to wonder how a company at the scale of GitHub can be so bad at keeping track of their status.

Now 4 out of 10 services are marked as "Incident", yet most of the others are also completely dead.

> H0(H1(m)) has the security of just H0. Hashes are not made to protect the content of m, but instead made to test the integrity of m. As such, a flaw in H0 will break the security guarantee, no matter how secure H1 is.

But this isn't true for all flaws. For example, even with the collision attacks against SHA-1, I don't think they're even remotely close to enabling a collision for SHA-1(some_other_hash(M)).

Similarly, HMAC-SHA-1 is still considered secure, as it's effectively SHA-1-X(SHA-1-Y(M)), where SHA-1-X and SHA-1-Y are just SHA-1 with different starting states.

So there's some value to be found in nesting hashes.

[1]: https://en.wikipedia.org/wiki/HMAC#Definition