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It's worth noting that Cyph came up in an earlier discussion about HPKP being removed from Chrome:

https://news.ycombinator.com/item?id=15572888

As far as I'm aware that means their approach no longer works (across browsers), and, though it was a fantastic idea, it did require you to trust that they really were throwing their keys away (a trust model slightly weaker than TOFU).




We (Cyph) have been pretty disappointed in the Chrome team's decision to kill HPKP.

Paraphrasing, but IIRC the reasoning pretty much boiled down to "it's a pain to maintain and Expect-CT is kind of similar anyway" — which I think is a really weak justification for harming end user security and breaking established APIs that people depend on in production. Fingers crossed that Firefox keeps it alive!

That said, it doesn't entirely break WebSign in Chrome, just weakens a bit further below strict TOFU. https://www.cyph.com/websign goes into detail, but WebSign has some client-side logic to validate its own hash against a signed whitelist. The major downsides to relying on this are:

1. It depends on a caching layer, not a security feature. This means that any guarantees are potentially out the window if a browser vendor decides to do something crazy for performance reasons or whatever.

2. It opens up an attack vector where it can be forcibly unpinned by filling up the user's disk and making the browser evict the cached WebSign instance.

All in all I think it's still basically fine, but shipping an optional browser extension for hardening WebSign is now a higher priority because of this.




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