You may have heard of sigstore and its container image verification tool, cosign. This blog post introduces a policy-driven workflow, Enterprise Contract, built on those technologies.
Note: Red Hat’s Emerging Technologies blog includes posts that discuss technologies that are under active development in upstream open source communities and at Red Hat. We believe in sharing early and often the things we’re working on, but we want to note that unless otherwise stated the technologies and how-tos shared here aren’t part of supported products, nor promised to be in the future.
Before starting…
Let’s say you have an image, quay.io/lucarval/demo:ec, and you want to verify this image was signed and attested by a known and trusted build system. This image was built via Tekton and was signed and attested by Tekton Chains.
When verifying an image, it is recommended to first resolve the image reference to a digest. This ensures the same image is being verified across all steps preventing TOCTOU attacks. There are various tools that can do this. I’ll use skopeo since I’m familiar with it:
$ skopeo inspect --no-tags docker://quay.io/lucarval/demo:ec | jq '.Digest' "sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25"
The “pinned” image reference is then:
quay.io/lucarval/demo:ec@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25
(You may choose to keep the tag in the image reference as well. When both the tag and the digest are present, the digest should be used by clients.)
For the purpose of this blog post, it is assumed that the image has been signed and attested with a long-lived key instead of using the keyless workflow. The public key is:
-----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEZP/0htjhVt2y0ohjgtIIgICOtQtA naYJRuLprwIv6FDhZ5yFjYUEtsmoNcW7rx2KM6FOXGsCX3BNc7qhHELT+g== -----END PUBLIC KEY-----
Let’s save that into a file called “cosign.pub”.
Verifying with cosign
The most common way to verify an image is to use cosign. This section highlights the steps involved in doing so.
We first verify the image has been signed with the expected public key. The build system that built this image does not integrate with Rekor yet, so we need to skip the transparency log checks.
$ cosign verify --key cosign.pub $IMAGE --insecure-ignore-tlog Verification for quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25 -- The following checks were performed on each of these signatures: - The cosign claims were validated - The signatures were verified against the specified public key […]
Great, this tells us that the image was indeed built by the expected build system because the image signature matches the provided public key.
Next, we verify the image contains the expected SLSA Provenance attestation.
$ cosign verify-attestation --type slsaprovenance --key cosign.pub $IMAGE --insecure-ignore-tlog Verification for quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25 -- The following checks were performed on each of these signatures: - The cosign claims were validated - The signatures were verified against the specified public key […]
The SLSA Provenance contains a lot of useful information about the build process. The cosign verify-attestation command does offer some support for evaluating its contents with a policy engine. The current options are Rego and Cue. Applying policies directly with cosign is possible, but let’s introduce a tool to make it easier and more convenient.
Verifying with the Enterprise Contract
The steps on the previous section work wonderfully when verifying an image. But what if you want to validate a group of images? And confirm that they all meet the requirements of a certain policy? What if you want a configuration that can be shared across different teams?
This is where the Enterprise Contract comes into play!
The Enterprise Contract can be evaluated via the ec-cli. The simplest example involves using an empty policy:
$ ec validate image --policy '' --rekor-url '' --public-key cosign.pub --image quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25 --output yaml
components:
- containerImage: quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25
name: Unnamed
signatures:
- keyid: SHA256:IhiN7gY+Z3uSSd7tmj6w5Zfhqafzdhm3DZjIvGc6iYY
metadata:
predicateBuildType: tekton.dev/v1beta1/TaskRun
predicateType: https://slsa.dev/provenance/v0.2
type: https://in-toto.io/Statement/v0.1
sig: MEUCIQDcgZIwEkLFqD7U9HrobgEC8Jo7wm+xJ5AoyO3qg+aj8QIgb9xDpjYGRMmpVk+QATeVKlHonzBiu51HtT3J+lQXPXc=
- keyid: SHA256:IhiN7gY+Z3uSSd7tmj6w5Zfhqafzdhm3DZjIvGc6iYY
metadata:
predicateBuildType: tekton.dev/v1beta1/PipelineRun
predicateType: https://slsa.dev/provenance/v0.2
type: https://in-toto.io/Statement/v0.1
sig: MEYCIQDKSihaAR/zAhJhR5GCqleDvfUUtvRw61vk0YeTBAnOSQIhAKa09B4yEfaSJronmWBFbu5cVPNxm17CMl/PElEz1POa
success: true
ec-version: v0.1.1224-41b7027
key: |
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEZP/0htjhVt2y0ohjgtIIgICOtQtA
naYJRuLprwIv6FDhZ5yFjYUEtsmoNcW7rx2KM6FOXGsCX3BNc7qhHELT+g==
-----END PUBLIC KEY-----
policy:
publicKey: cosign.pub
success: true
The ec-cli has different output formats. I chose the one that displays the full report in YAML format. From its success, we can tell that the image signature and the image attestations match the provided public key. We can also see some metadata information about the found attestations.
Let’s take a step further and say we want to validate the SLSA Provenance attestations with a certain set of policy rules. The ec-policies repo has a set of useful rego policies, so let’s use that.
Create a policy file called policy.yaml with the following content:
sources: - name: policies data: - git::github.com/enterprise-contract/ec-policies.git//data?ref=bca7d72 policy: - git::github.com/enterprise-contract/ec-policies.git//policy/lib?ref=bca7d72 - git::github.com/enterprise-contract/ec-policies.git//policy/release?ref=bca7d72 configuration: include: - slsa_source_version_controlled publicKey: | -----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEZP/0htjhVt2y0ohjgtIIgICOtQtA naYJRuLprwIv6FDhZ5yFjYUEtsmoNcW7rx2KM6FOXGsCX3BNc7qhHELT+g== -----END PUBLIC KEY----- rekorUrl: ""
The sources attribute specifies a list of rego policy rules and corresponding data sources. Each data and policy source can be specified via a different set of transports. Here we choose to use them directly from git.
In configuration, we specify what to include from the sources. (Omit this to include all!) In this example, the policy rules from the slsa_source_version_controlled package are included. Check out the docs for more information.
We can also specify the public key and rekor URL directly in this file. This helps consolidate all the input parameters required for validating images.
Let’s run the ec-cli again with this policy in place:
$ ec validate image --policy policy.yaml --image quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25 --output yaml --info
components:
- containerImage: quay.io/lucarval/demo@sha256:304040ca1911aa4d911bd7c6d6d07193c57dc49dbc43e63828b42ab204fb1b25
name: Unnamed
signatures:
- keyid: SHA256:IhiN7gY+Z3uSSd7tmj6w5Zfhqafzdhm3DZjIvGc6iYY
metadata:
predicateBuildType: tekton.dev/v1beta1/TaskRun
predicateType: https://slsa.dev/provenance/v0.2
type: https://in-toto.io/Statement/v0.1
sig: MEUCIQDcgZIwEkLFqD7U9HrobgEC8Jo7wm+xJ5AoyO3qg+aj8QIgb9xDpjYGRMmpVk+QATeVKlHonzBiu51HtT3J+lQXPXc=
- keyid: SHA256:IhiN7gY+Z3uSSd7tmj6w5Zfhqafzdhm3DZjIvGc6iYY
metadata:
predicateBuildType: tekton.dev/v1beta1/PipelineRun
predicateType: https://slsa.dev/provenance/v0.2
type: https://in-toto.io/Statement/v0.1
sig: MEYCIQDKSihaAR/zAhJhR5GCqleDvfUUtvRw61vk0YeTBAnOSQIhAKa09B4yEfaSJronmWBFbu5cVPNxm17CMl/PElEz1POa
success: true
successes:
- metadata:
code: slsa_source_version_controlled.material_non_git_uri
collections:
- minimal
- slsa2
- slsa3
description: Each entry in the predicate.materials array of the attestation uses a git URI.
title: Material from a git repository
msg: Pass
- metadata:
code: slsa_source_version_controlled.material_without_git_commit
collections:
- minimal
- slsa2
- slsa3
description: Each entry in the predicate.materials array of the attestation includes a SHA1 digest which corresponds to a git commit.
title: Material with git commit digest
msg: Pass
- metadata:
code: slsa_source_version_controlled.missing_materials
collections:
- minimal
- slsa2
- slsa3
description: 'At least one entry in the predicate.materials array of the attestation contains the expected attributes: uri and digest.sha1.'
title: Material format
msg: Pass
ec-version: v0.1.1224-41b7027
key: |
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEZP/0htjhVt2y0ohjgtIIgICOtQtA
naYJRuLprwIv6FDhZ5yFjYUEtsmoNcW7rx2KM6FOXGsCX3BNc7qhHELT+g==
-----END PUBLIC KEY-----
policy:
configuration:
include:
- slsa_source_version_controlled
publicKey: |
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEZP/0htjhVt2y0ohjgtIIgICOtQtA
naYJRuLprwIv6FDhZ5yFjYUEtsmoNcW7rx2KM6FOXGsCX3BNc7qhHELT+g==
-----END PUBLIC KEY-----
sources:
- data:
- git::github.com/enterprise-contract/ec-policies.git//data?ref=bca7d72
name: policies
policy:
- git::github.com/enterprise-contract/ec-policies.git//policy/lib?ref=bca7d72
- git::github.com/enterprise-contract/ec-policies.git//policy/release?ref=bca7d72
success: true
Notice the additional information in the output. It contains a list of policy rules that were successfully evaluated as well as the details for the policy used.
To improve reusability, the policy.yaml file can be stored in your Kubernetes clusters via the EnterpriseContractPolicy custom resource, see example.
Say now that you have a group of images, an “application snapshot”, that must all pass validation. We can do this in one shot by creating the components.yaml file:
components: - containerImage: quay.io/example/one:latest@sha256:... - containerImage: quay.io/example/two:latest@sha256:... - containerImage: quay.io/example/three:latest@sha256:...
Then run the ec-cli:
$ ec validate image --policy policy.yaml --file-path components.yaml --output yaml
The output of this command is a full report that includes the evaluation results of each image, as well as whether all the images pass the validation.
The Enterprise Contract helps us verify that images were built in the expected build system, and that they were built in the expected way. By leveraging OPA’s Rego as its policy engine, it’s easy to create custom rules that apply whatever security and build policies are appropriate for your use case.
What’s next?
As the Enterprise Contract continues to be actively worked on, and used to meet users’ needs, we plan on improving it even more. An important feature currently being worked on is support for keyless workflows which will increase the number of supported use cases.
If you want to learn more, check out our docs and browse the source!
