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Version: Next 🚧

Verification configuration format

Introduction​

You can use the verification-config format with:

  • policy-server to verify policy modules provenance
  • verify-image-signatures policy to verify cluster images provenance

Refer to secure supply chain for more information.

Format​

The configuration has 2 root keys:

  • allOf: You should satisfy all verification information listed here to verify container images as signed.
  • anyOf: You must satisfy at least anyOf.minimumMatches of all listed verification information to accept a container image as signed.

These two root keys accept an array of keys of type kind. A full list of accepted keys based on different use cases is below:

  • pubKey: For signatures performed with traditional public/private key cryptography.

  • githubAction: For signatures performed with Sigstore's keyless workflow inside GitHub Actions. Kubewarden checks this information against the x509 certificate extension workflow_repository created by the OpenID Connect of GitHub, and not only the issuer and subject. You should use this kind if dealing with GitHub Actions.

  • genericIssuer: For signatures performed with Sigstore's keyless workflow, where the user needs to validate the certificate issuer and subject on their own. It accepts a subject, which can be:

    • equal: The value passed here must match exactly the subject in the signing certificate.
    • urlPrefix: The value passed here is post-fixed with / to prevent typo-squatting, and must be a prefix of the subject in the signing certificate.

The kind key accepts an optional annotations key, with a list of key-values, that must be present in the signature.

Example​

This is an example of a configuration for verifying signatures using the Sigstore workflow:

---
apiVersion: v1

allOf:
- kind: githubAction
owner: kubewarden # mandatory
repo: policy-server # optional
annotations: # optional
env: prod

anyOf: # at least `anyOf.minimumMatches` are required to match
minimumMatches: 2 # default is 1
signatures:
- kind: pubKey
owner: alice # optional
key: | # mandatory
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEQiTy5S+2JFvVlhUwWPLziM7iTM2j
byLgh2IjpNQN0Uio/9pZOTP/CsJmXoUNshfpTUHd3OxgHgz/6adtf2nBwQ==
-----END PUBLIC KEY-----
annotations: # optional
env: prod
- kind: genericIssuer
issuer: https://github.com/login/oauth
subject:
equal: alice@example.com
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
equal: https://github.com/bob/app-example/.github/workflows/release.yml@refs/heads/main
annotations: # optional
env: prod
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/bob # <- it will be post-fixed with `/` for security reasons

Signature configuration reference​

You can validate signature requirements contained in a file. Expand for an example:

A file of signature requirements

apiVersion: v1

allOf:
- kind: githubAction
owner: kubewarden # mandatory
annotations:
env: prod

anyOf: # at least `anyOf.minimumMatches` are required to match
minimumMatches: 2 # default is 1
signatures:
- kind: pubKey
owner: flavio # optional
key: .... # mandatory
annotations: # optional
env: prod
foo: bar
- kind: pubKey
owner: victor # optional
key: .... # mandatory
- kind: genericIssuer
issuer: https://github.com/login/oauth
subject:
equal: alice@example.com
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
equal: https://github.com/flavio/policy-secure-pod-images/.github/workflows/release.yml@refs/heads/main
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/flavio/
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/kubewarden # <- it will be post-fixed with `/` for security reasons
- kind: githubAction
owner: flavio # mandatory
repo: policy1 # optional
- kind: pubKey
owner: alice # optional
key: .... # mandatory

Signature validation​

The previous configuration contains the two sections, allOf and anyOf:

  • allOf: You trust the policy only if all signature requirements are valid.
  • anyOf: You trust the policy if the minimumMatches criterion are valid.

In the example, the minimumMatches field is 2. So, you need to meet at least two of the signature requirements. The default value for minimumMatches field is 1.

For signature validation, you need to meet all the signature's requirements from allOf and the minimum number from anyOf.

Public key validation​

To check a policy has the correct public key signature, you specify the key data and the owner of the key. In this example, you set kind to pubKey and the key has the public key. The owner field is optional, but can be useful to clarify who owns the key.

  - kind: pubKey
owner: bob # optional
key: |
-----BEGIN PUBLIC KEY-----
MBFKHFDGHKIJH0CAQYIKoZIzj0DAQcDQgAEX0HFTtCfTtPmkx5p1RbDE6HJSGAVD
BVDF6SKFSF87AASUspkQsN3FO4iyWodCy5j3o0CdIJD/KJHDJFHDFIu6sA==
-----END PUBLIC KEY-----

Keyless signature validation​

A policy signed in keyless mode doesn't have a public key you can verify. You can still verify the policy with the OpenID Connect (OIDC) data used during the signing process. For that, it's necessary to define the signature validation as genericIssuer.

It's possible to verify information from the signature:

  • issuer(mandatory): This matches the Issuer attribute in the certificate generated by Fulcio. This shows the OIDC used to sign the policy.
  • subject: The field used to match the Subject attribute in Fulcio's certificate. The Subject (Fulcio) field contains the user used to authenticate against the OIDC provider. The verification field, subject, can have one of two subfields:
    • equal: The Subject (Fulcio) from the certificate must be equal to the value in the signature validation;
    • urlPrefix: The certificate's Subject (Fulcio) field value must be prefixed by the value defined in the signature validation.
note

Both the cosign verify and the kwctl inspect commands can show information about keyless signatures.

For example, this configuration means the policy must have a keyless signature from Alice using the GitHub OIDC:

- kind: genericIssuer
issuer: https://github.com/login/oauth
subject:
equal: alice@example.com

This configuration needs the policy signed in GitHub actions, from a repository owned by the GitHub user flavio:

- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/flavio

GitHub actions signature verification​

The "kind" githubAction is to validate policies signed in GitHub Actions. You can do this with the genericIssuer kind as well. To simplify the signature requirement process, use two extra fields for githubAction:

  • owner (mandatory): GitHub ID of the user or organization to trust.
  • repo: The name of the repository to trust.

For example, the last snippet, using genericIssuer, could be rewritten as:

- kind: githubAction
owner: flavio

Signature annotations validation​

All signature types can have other optional validation fields, annotations. These fields are key/value data added during the signing process.

With Kubewarden, you can verify policy signatures from trusted users and have specific annotations.

The next validation checks two conditions for the policy:

  • that it's signed with a specific key
  • it has a production environment annotation
- kind: pubKey
key: |
-----BEGIN PUBLIC KEY-----
MBFKHFDGHKIJH0CAQYIKoZIzj0DAQcDQgAEX0HFTtCfTtPmkx5p1RbDE6HJSGAVD
BVDF6SKFSF87AASUspkQsN3FO4iyWodCy5j3o0CdIJD/KJHDJFHDFIu6sA==
-----END PUBLIC KEY-----
annotations:
environment: production

Using a signature verification configuration file to check a policy OCI artifact​

You can test if a policy passes verification using the verification configuration file. Use the --verification-config-path flag of the kwctl verify command.

$ cat signatures_requirements.yaml
apiVersion: v1
allOf:
- kind: pubKey
key: |
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE5Q+cN1Jj2S7N05J4AXnqwP2DyzSg
Mc+raYce2Wthrd30MSgFtoh5ADAkCd/nML2Nx8UD9KBuASRb0gG5jXqgMQ==
-----END PUBLIC KEY-----

$ kwctl verify --verification-config-path signatures_requirements.yaml ghcr.io/kubewarden/policies/user-group-psp:latest
2022-03-29T17:34:37.847169Z INFO kwctl::verify: Policy successfully verified

This last example tests if a given policy came from the Kubewarden organization:

$ cat kubewarden_signatures.yaml
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden

$ kwctl verify --verification-config-path kubewarden_signatures.yaml ghcr.io/kubewarden/policies/user-group-psp:latest
2022-03-29T18:07:39.062292Z INFO kwctl::verify: Policy successfully verified