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proposal: Process for signing charts (establishing provenance) #23
We want to be able to add provenance to all charts in this repo. The
Fundamental Assumption: A chart with an accompanying
The process outlined here is designed to meet each of those three expectations.
When is a Chart Signed?
A chart is signed when it has passed all of its acceptance criteria, and it is ready to be pushed to the chart repository. So the chart should be (a) passing automated tests (CI), (b) approved via LGTM or whatever human process we use, and (c) accepted as ready to release (e.g. the one who opened the request believes it is now ready for release).
At that point, one of the "core maintainers" signs the chart and commits the result to the repo.
$ helm package --sign --key "my-key" mysql $ git add mysql-1.2.3.tgz.prov $ ... # rest of Git workflow
(alternately: we could have the signer load the signature directly into GCS. But then it will not be tracked in revision control)
At this point, the CI/CD toolchain can actually verify that it can rebuild the package and match the
So this process addresses assumptions 1 and 2: It provides cryptographic proof that a chart is unaltered, and it asserts that someone has verified that the process of approving the chart has been completed.
How do we manage cryptographic keypairs?
Every core maintainer of the project may have one or more signing keypairs (ideally just one).
Private keys: In this model, the signer is the only one who needs the private key. So none of the "official" infrastructure needs to store private keys.
Public Keys: The public keys used to sign must be made available by the maintainer. Possible ways to make these available include mundane ways like sending them via email, or other ways like storing them in https://keybase.io (cf. https://keybase.io/technosophos).
How do we say which public keys are trusted for the kubernetes/charts repository?
We need a way to assert something like this: "The following keys are official for the kubernetes/charts repo..."
One way to do this would be to put those public keys in a well documented place that the helm tool itself could look up. It might be necessary to also cryptographically sign that information. with a "root key" or use one of the other chain of trust mechanisms popular in PKI.
As a first step, this proposal suggests that a binary keyring file called
If this is deemed sufficient, we should file an issue at https://github.com/kubernetes/helm to include support for automatically fetching and using this file to verify charts in a given repo.
So it would work like this:
(OPTIONAL STEP 3.5) Helm client verifies that the key used to sign is in some other chain of trust (say, local keyring or keybase.io).
How do we manage compromised/rogue keys?
Say we successfully implement this plan, but at some point a key that was used to sign numerous charts becomes compromised. What do we do?
In addition, we could provide a canonical place to post revoked keys. (perhaps
The following are known weaknesses in this scheme: