letsencrypt / letsencrypt/boulder

CA: Add lint/check that SCTs validate before issuing final cert

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Dominant language
Go
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Description

This idea came out of remediation for https://bugzilla.mozilla.org/show_bug.cgi?id=1838667

Recently we've seen two reasons that the SCTs included in a final cert might not actually validate:

  • A GTS incident where they used the SCTs from the wrong precert
  • Our incident where the SCT signatures were calculated over different bytes (because the precert didn't match the final cert)

However, we have no lint that checks this. The certificate-transparency-go library checks that the SCTs returned by CT logs actually validate over the precert contents, but we don't have a check that they also validate over the final cert contents.

We should add a lint-like check which confirms this. It will need the CT log public keys in order to successfully validate the SCT signatures. https://github.com/zmap/zlint/issues/728 may handle this for us, in which case this bug should track updating and enabling that lint.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reviewing certificate-transparency-go's SCT validation behavior and the requirements for obtaining CT log public keys. Check whether zlint issue #728 provides the needed lint, then determine how it could be updated and enabled for Boulder. Done means a lint confirms that SCTs in a final certificate validate against that certificate's contents.

Written by the indexing model from the issue text.

Assessment

Tech stack
go
Domain
cryptography, security
Issue type
Feature
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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