Verification configuration format
Introduction​
The verification-config format is used by:
policy-server
to verify policy modules provenanceverify-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 leastanyOf.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 extensionworkflow_repository
created by the OpenID Connect of Github, and not only theissuer
andsubject
. Hence, it is strongly recommended to use thiskind
if dealing with GitHub Actions.genericIssuer
: for signatures performed with Sigstore's keyless worfklow, where the user needs to validate the certificateissuer
andsubject
on their own. It accepts asubject
, which can be:equal
: the value passed here must match exactly against thesubject
in the signing certificate.urlPrefix
: the value passed here is post-fixed with/
to prevent typo-squatting, and must be a prefix of thesubject
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 theminimumMatches
criterion is met. Above, theminimumMatches
field is 2. So, at least two of the signature requirements must be met. The default value forminimumMatches
field is1
.
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 theIssuer
attribute in the certificate generated by Fulcio. This shows the OIDC used to sign the policy.subject
: field used to match theSubject
attribute in Fulcio's certificate. TheSubject
(Fulcio) field contains the user used to authenticate against the OIDC provider. The verification field,subject
, can have one of two sub fields:equal
: theSubject
(Fulcio) from the certificate must be equal to the value in the signature validation;urlPrefix
: the certificate'sSubject
(Fulcio) field value must be prefixed by the value defined in the signature validation.
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 trustrepo
: 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