Blog: Kubernetes 1.26: We're now signing our binary release artifacts!

sorangutan

Author: Sascha Grunert

The Kubernetes Special Interest Group (SIG) Release is proud to announce that we are digitally signing all release artifacts, and that this aspect of Kubernetes has now reached beta.

Signing artifacts provides end users a chance to verify the integrity of the downloaded resource. It allows to mitigate man-in-the-middle attacks directly on the client side and therefore ensures the trustfulness of the remote serving the artifacts. The overall goal of out past work was to define the used tooling for signing all Kubernetes related artifacts as well as providing a standard signing process for related projects (for example for those in kubernetes-sigs).

We already signed all officially released container images (from Kubernetes v1.24 onwards). Image signing was alpha for v1.24 and v1.25. For v1.26, we've added all binary artifacts to the signing process as well! This means that now all client, server and source tarballs, binary artifacts, Software Bills of Material (SBOMs) as well as the build provenance will be signed using cosign. Technically speaking, we now ship additional *.sig (signature) and *.cert (certificate) files side by side to the artifacts for verifying their integrity.

To verify an artifact, for example kubectl, you can download the signature and certificate alongside with the binary. I use the release candidate rc.1 of v1.26 for demonstration purposes because the final has not been released yet:

curl -sSfL https://dl.k8s.io/release/v1.26.0-rc.1/bin/linux/amd64/kubectl -o kubectl
curl -sSfL https://dl.k8s.io/release/v1.26.0-rc.1/bin/linux/amd64/kubectl.sig -o kubectl.sig
curl -sSfL https://dl.k8s.io/release/v1.26.0-rc.1/bin/linux/amd64/kubectl.cert -o kubectl.cert

Then you can verify kubectl using cosign:

COSIGN_EXPERIMENTAL=1 cosign verify-blob kubectl --signature kubectl.sig --certificate kubectl.cert

tlog entry verified with uuid: 5d54b39222e3fa9a21bcb0badd8aac939b4b0d1d9085b37f1f10b18a8cd24657 index: 8173886
Verified OK

The UUID can be used to query the rekor transparency log:

rekor-cli get --uuid 5d54b39222e3fa9a21bcb0badd8aac939b4b0d1d9085b37f1f10b18a8cd24657

LogID: c0d23d6ad406973f9559f3ba2d1ca01f84147d8ffc5b8445c224f98b9591801d
Index: 8173886
IntegratedTime: 2022-11-30T18:59:07Z
UUID: 24296fb24b8ad77a5d54b39222e3fa9a21bcb0badd8aac939b4b0d1d9085b37f1f10b18a8cd24657
Body: {
"HashedRekordObj": {
"data": {
"hash": {
"algorithm": "sha256",
"value": "982dfe7eb5c27120de6262d30fa3e8029bc1da9e632ce70570e9c921d2851fc2"
}
},
"signature": {
"content": "MEQCIH0e1/0svxMoLzjeyhAaLFSHy5ZaYy0/2iQl2t3E0Pj4AiBsWmwjfLzrVyp9/v1sy70Q+FHE8miauOOVkAW2lTYVug==",
"publicKey": {
"content": "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"
}
}
}
}

The HashedRekordObj.signature.content should match the content of the file kubectl.sig and HashedRekordObj.signature.publicKey.content should be identical with the contents of kubectl.cert. It is also possible to specify the remote certificate and signature locations without downloading them:

COSIGN_EXPERIMENTAL=1 cosign verify-blob kubectl \
 --signature https://dl.k8s.io/release/v1.26.0-rc.1/bin/linux/amd64/kubectl.sig \
 --certificate https://dl.k8s.io/release/v1.26.0-rc.1/bin/linux/amd64/kubectl.cert

tlog entry verified with uuid: 5d54b39222e3fa9a21bcb0badd8aac939b4b0d1d9085b37f1f10b18a8cd24657 index: 8173886
Verified OK

All of the mentioned steps as well as how to verify container images are outlined in the official documentation about how to Verify Signed Kubernetes Artifacts. In one of the next upcoming Kubernetes releases we will working making the global story more mature by ensuring that truly all Kubernetes artifacts are signed. Beside that, we are considering using Kubernetes owned infrastructure for the signing (root trust) and verification (transparency log) process.

Getting involved

If you're interested in contributing to SIG Release, then consider applying for the upcoming v1.27 shadowing program (watch for the announcement on k-dev) or join our weekly meeting to say hi.

We're looking forward to making even more of those awesome changes for future Kubernetes releases. For example, we're working on the SLSA Level 3 Compliance in the Kubernetes Release Process or the Renaming of the kubernetes/kubernetes default branch name to main.

Thank you for reading this blog post! I'd like to use this opportunity to give all involved SIG Release folks a special shout-out for shipping this feature in time!

Feel free to reach out to us by using the SIG Release mailing list or the #sig-release Slack channel.

Additional resources

Signing Release Artifacts Enhancement Proposal

https://kubernetes.io/blog/2022/12/12/kubernetes-release-artifact-signing/