Verification configuration format

Introduction

The verification-config format is used by:

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

See secure supply chain for more info.

Format

The config has 2 root keys:

• allOf: All verification info listed here must be satisfied for accepting a container image as signed.
• anyOf: At least anyOf.minimumMatches of all info listed here must be satisfied for accepting 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 given 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. Hence, it is strongly recommended to use this kind if dealing with GitHub Actions.
• genericIssuer: for signatures performed with Sigstore's keyless worfklow, 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 against 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 optionally accepts an annotations key, with a list of key-values, that must be present in the signature.

Example

Here you can see 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. Here is 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 configuration above contains the two highlighted sections, allOf and anyOf:

• allOf: The policy is trusted only if all signature requirements here are valid.

• anyOf: The policy is trusted if the minimumMatches criterion is met. Above, the minimumMatches field is 2. So, at least two of the signature requirements must be met. The default value for minimumMatches field is 1.

All the signatures requirements from allOf and the minimum number from anyOf must be met.

Public key validation

To check a policy is signed with the correct public key, you specify the key data and the owner of the key. In this example, kind is set 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 we can verify. You can still verify the policy with the 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: 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 sub fields:
  • 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 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 to be 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 by during the signing process.

With Kubewarden, you can ensure policies are signed by trusted users and have specific annotations.

The next validation checks 2 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 config 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