Embed SCTs in issued certificates

[haydentherapper]

Summary

This adds support for embedding SCTs in certificates instead of
returning a header with a detached SCTs. This is done by implementing an
SCT interface for a signer. For example, GCP CA Service will not
support embedded SCTs, but KMS will.

This heavily leverages the Go CT library. I've removed the custom
client in favor of the CT library client, which includes more
verification and retry logic. Note that there's a TODO to include the
public key of the CT log in Fulcio so that the SCT is checked before
returning a response.

A certificate is signed twice, which adds an extra remote call to KMS.
The first certificate is added to the CT log via AddPreChain instead of
AddChain.

The Cosign client will need to be updated to support embedded SCTs.

Signed-off-by: Hayden Blauzvern hblauzvern@google.com

Ticket Link

Fixes #42, #310

Release Note

Added support for embedded SCTs for intermediate CA implementation. The CA will not return the SCT detached in a header. The client must verify the SCT using the SCT embedded in the certificate.

[haydentherapper]

If you want to take a look, please only review the second commit! Waiting until we submit #496.

[codecov-commenter]

Codecov Report

Merging #507 (36849e6) into main (4db32b6) will increase coverage by 0.58%.
The diff coverage is 58.67%.

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
+ Coverage   43.32%   43.91%   +0.58%     
==========================================
  Files          18       16       -2     
  Lines        1221     1273      +52     
==========================================
+ Hits          529      559      +30     
- Misses        617      633      +16     
- Partials       75       81       +6     

Impacted Files	Coverage Δ
pkg/api/ca.go	46.63% <28.12%> (-5.02%)	⬇️
pkg/ca/intermediateca/intermediateca.go	72.51% <64.70%> (-8.44%)	⬇️
pkg/ctl/utils.go	85.71% <85.71%> (ø)

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[dlorenc]

The Cosign client will need to be updated to support embedded SCTs.

How will this rollout work? Can cosign support both for awhile?

[haydentherapper]

#42 (comment) - Cosign will need to be updated to support both. It'll conditionally check if the SCT is embedded or in a header, and verify based on that. Then we can rollout this change, but it'll be a breaking change for anyone who hasn't updated Cosign. It's not possible for the current version of Cosign to successfully verify a certificate with an embedded SCT (because an SCT is signed using the To-Be-Signed portion of a certificate and removing either the poison ext or the SCT ext, and you need to set a flag for the verifier to remove an ext when verifying).

[dlorenc]

Cool! OK, so we can change cosign to support both, get a release out with a version bump (1.8?), then deploy this change.

[dlorenc]

Do we have an issue or PR in flight to add support to cosign for verifying both?

[haydentherapper]

sigstore/cosign#1730 for the issue, sigstore/cosign#1731 for the WIP PR - The logic is all there, just working on testing (generating SCTs in tests is fun!)

[haydentherapper]

Any other comments, or can this be submitted?

[bobcallaway]

generally LGTM, a couple questions inline

[dlorenc]

I'll defer on submitting this one to @priyawadhwa and @k4leung4.

The code is fine, but if we submit this before the cosign 1.8 release we won't be able to deploy the main branch to prod, making it a bit unsafe.

[haydentherapper]

We can deploy the main branch to prod, this only affects the intermediate CA implementation, which is not what’s rolled out in prod. The CA Service CA will continue to return detached SCTs.

[priyawadhwa]

Is the plan to switch to the intermediate CA implementation when we migrate clusters in prod?

[haydentherapper]

I wanted to switch once we cut 1.8. Though if you prefer, I’m happy to do testing in staging first.

[dlorenc]

We can deploy the main branch to prod, this only affects the intermediate CA implementation, which is not what’s rolled out in prod. The CA Service CA will continue to return detached SCTs.

Ahh, got it.

[dlorenc]

This can merge now then @bobcallaway can rebase :)