Support private PKI

[ianlewis]

It would be useful to support private PKI.

• Private sigstore instances ([feature] Add support for using a custom Sigstore deployment #3607)
• Private keys obtained from vault or something else? (Using key-based signing #326)

[steiza]

Yeah, so when I heard about this project I thought of it as made up of two steps:

1. Taking workload identity information (like from GitHub Actions) and encoding it into an in-toto document with SLSA predicates

2. Generating a signed attestation of the in-toto document, so it can be verified downstream

For (2), there are many advantages of using public sigstore infrastructure: individual projects don't have to manage PKI, there's a public transparency log, ... etc. The expectation is that this is what most open source projects would do.

But there might be cases where people don't want to use public sigstore infrastructure, maybe because the don't want their transparency logs to be public, or because they have their own PKI they way to use. In that situation, it would be nice if this project supported just generating the in-toto document (i.e. just step 1 - which they could then sign however they liked), or maybe even different options for signing other than public sigstore infrastructure (i.e. step 2).

That said, there is a lot of value in having an opinionated paved path, and maybe having different options for (2) adds more confusion than it's worth. There's nothing stopping people from implementing support for their own PKI as they need it!

[ianlewis]

I think it's definitely something that could be considered and supported. In order to keep it simpler, maybe it's something we should wait on implementing until folks are specifically asking for it?

If we did implement it, we could allow folks to specify their own address for rekor/fulcio servers in a config file at the base of the repo like the one slsa-github-generator-go has.

[asraa]

we could allow folks to specify their own address for rekor/fulcio servers in a config file at the base of the repo like the one slsa-github-generator-go has.

The biggest challenge around here is verifying against the correct TUF root: folks would need to specify addresses for rekor/fulcio but also the trusted root CA and rekor public key they expect. That information would need to be pushed to the SLSA verifier as well.

I don't know the best solution for this yet. Do we specify the whole TUF root? On the generator side I suppose we only need to specify the rekor/fulcio, and the end-users need to know the TUF root.

[ianlewis]

The biggest challenge around here is verifying against the correct TUF root

How do sigstore tools handle this currently?