COSIGN_EXPERIMENTAL and offline mode

[znewman01]

COSIGN_EXPERIMENTAL was introduced AFAICT for two reasons:

1. The Sigstore idea, interface, and implementation were still experimental
2. The Sigstore infrastructure wasn't reliable/didn't have guarantees.

So to start, anything that touched Sigstore infrastructure (TUF roots, Fulcio, Rekor) was gated: users needed to set COSIGN_EXPERIMENTAL=1 to use them.

Over time, we've blurred the line a little; without turning on experimental mode:

• You can pass a Rekor bundle and cosign will check it.
• To do that, you need the Rekor public key, which you get via TUF (this goes over the network!)
• Similarly, you can provide a cert signed by Fulcio, and cosign will check it against Fulcio's key
• It gets that key via TUF

There's a question about "what is the point of cosign verify-blob without COSIGN_EXPERIMENTAL but with a provided certificate?"

I don't want to answer that, at least not here. It's hopefully going to be an obsolete question soon: we're relatively comfortable with the idea of Sigstore, and we're targeting general availability (GA) for the infrastrcucture soon: sigstore/fulcio#766 and sigstore/rekor#1005 . But, at least some folks have been deliberately not providing this flag to indicate "don't try to hit the network." That's sort-of what it does right now: "I don't try to query Rekor/Fulcio, but I know about them, and also I'll get their keys via TUF."

That suggests to me that, post-dropping-COSIGN_EXPERIMENTAL, there should be some mechanism to verify offline. Maybe a --offline flag, or even a verify-offline command (which is nice because now we can only show users the relevant flags). If we do that, we probably want to have some requirement on how out-of-date your TUF roots can be (maybe user-configurable?) and instructions on how to grab them from cosign initialize.

[haydentherapper]

+1 to an offline flag! Here's a copy paste of a comment I left on a doc when we were looking into the Cosign CVE:

With no flag, I'd propose the behavior be:

• Default to fetching from Rekor without any flags
• To avoid a call to Rekor, either
  --- provide a bundle
  --- add a flag explicitly opting out of the rekor call

[znewman01]

+1 to that as the default behavior.

Then, the additional changes for an offline mode:

• use cached TUF state (after checking their age), which gives you Rekor/Fulcio root certs
• fail the command immediately if the bundle or opt-out flag isn't provided
• we never hit Fulcio in verification anyway AFAICT (though maybe eventually we should check the CT log by default unless an SCT is provided embedded in the cert)

I think that covers it?

[znewman01]

Additional note: we need to give more guidance about offline settings, especially w/r/t TUF (do you mirror it? do you risk stale root certs?). I think this documentation would be really valuable.

[haydentherapper]

@asraa's design doc for the Sigstore TUF client also mentioned supporting offline TUF. IIRC it was configurable how to handle expired metadata.

[vaikas]

this may be relevant for the discussion here too:
theupdateframework/go-tuf#397

[haydentherapper]

cc @priyawadhwa