crl-monitor

CRL-Monitor monitors CRLs.

It issues certificates, revokes them, and then looks for them to appear in CRLs. Each component runs as an AWS Lambda.

The churner runs periodically. On each run it issues a certificate, fetches and lints the certificate's CRL URL, revokes the certificate, and stores its serial number and metadata for the checker to later verify that it shows up as revoked. It also checks previously seen serials. If they haven't shown up in a CRL after a reasonable amount of time, checker produces an error.

The checker runs in response to the upload of each new CRL shard in S3. It diffs the newly uploaded CRL shard against its previous version and verifies:

• New CRL has a later date and higher CRL number than the previous version.
• New CRL passes lints.
• For any serials removed between the old shard and the new one:
  • The certificate is expired (based on fetching it by serial from Let's Encrypt).
• For any serials added (if the certificate was issued by the churner):
  • The certificate's CRLDistributionPoint matches the CRL shard's IssuingDistributionPoint.

The checker also removes from database any certificates it sees, to indicate that their revocation has been published, so the churner won't alert about them It then marks as completed (deletes) any churner-issued certificates that show up on the new CRL.

Build and Deployment

This repository has two binaries named checker and two binaries named churner. The binaries under cmd are for local use and testing. The binaries under lambda are for deployment to AWS Lambda. The key difference is that the lambda/ binaries register a lambda handler (lambda.StartWithOptions()), which AWS then calls. That handler can return errors, and we have separate Cloudwatch monitoring that alerts when any errors are detected.

The lambda binaries are built by a release workflow on GitHub Actions triggered by uploading a release tag (starting with v). Those binaries are uploaded to S3 under a versioned path. They are then deployed to Lambda using Terraform (in another repository).

Testing

Most of the tests are unittests and can be run with:

go test ./...

There is also an integration test for DynamoDB code. To run this, install Java and run:

./db/run_integration_test.sh

Architecture Diagram

sequenceDiagram
  participant churn as Certificate Churner<br />Lambda
  participant ca as Let's Encrypt<br />Certification Authority
  participant ddb as Pending Certificates<br />DynamoDB
  participant s3 as CRL Storage<br />S3 Bucket
  participant checker as CRL Checker<br />Lambda

  loop timer
    activate churn
    churn->>ca: Issue certificate
    churn->>ca: Fetch CRL
    churn->>ca: Revoke certificate
    churn->>ddb: Store certificate metadata
    ddb->>churn: Get previous revoked serials
    Note over churn: Alert if any<br />expected serials missed<br />inclusion deadline
    deactivate churn
  end

  loop New CRL
    ca->>s3: Publish CRL Shard
    activate checker
    s3->>checker: S3 Event
    checker->>s3: Read current CRL
    checker->>s3: Read previous CRL
    Note over checker: Alert if CRL<br />fails linting
    loop random selection of serials
      checker->>ca: Get Certificate
    end
    Note over checker: Alert if CRL had<br />serials leave early
    checker->>ddb: Get revoked serials
    checker->>ddb: Delete seen serials
    deactivate checker
  end

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