Comprehensive Guide to Image Signing Tools for Kubernetes for Devsecops implementation practices

 

Table of Contents

1. Introduction
2. Image Signing Tools
   1. Cosign
   2. Notary
   3. Docker Content Trust (DCT)
   4. GPG
   5. Sigstore
3. Cloud Provider Solutions
   1. AWS Signer
   2. Azure Container Registry Content Trust
   3. Google Container Analysis
4. Comparison
5. Integration with Kubernetes
6. Conclusion

Introduction

Image signing is a crucial security measure in container ecosystems, especially for Kubernetes deployments. It ensures that only trusted and unaltered container images are deployed in your production environment. This guide focuses on various image signing tools, including those provided by major cloud providers, and compares their features, advantages, and disadvantages.

Image Signing Tools

Cosign

Cosign is a relatively new tool developed by the Sigstore project, aimed at making software signing and verification easier and more accessible.

Advantages:

• Simple and easy to use
• Integrates well with other Sigstore projects (Fulcio, Rekor)
• Supports various key management options (file-based, hardware tokens, cloud KMS)
• Can sign and verify any artifact, not just container images

Disadvantages:

• Relatively new, so the ecosystem is still maturing
• May require additional setup for full PKI integration

Sample Implementation:

# Install Cosign
go install github.com/sigstore/cosign/cmd/cosign@latest

# Generate a key pair
cosign generate-key-pair

# Sign an image
cosign sign --key cosign.key myregistry.azurecr.io/myimage:tag

# Verify an image
cosign verify --key cosign.pub myregistry.azurecr.io/myimage:tag

Notary

Notary is an older and more established tool for signing and verifying container images, part of the CNCF.

Advantages:

• Well-established and widely used
• Provides a comprehensive trust model with multiple roles
• Integrates well with Docker and other container registries

Disadvantages:

• More complex to set up and use compared to newer tools like Cosign
• Requires a separate Notary server for full functionality

Sample Implementation:

# Install Notary
docker run -d --name notary-server -p 4443:4443 theupdateframework/notary-server:latest

# Sign an image
notary init myregistry.azurecr.io/myimage
notary publish myregistry.azurecr.io/myimage

# Verify an image
notary verify myregistry.azurecr.io/myimage:tag

Docker Content Trust (DCT)

Docker Content Trust is built on top of Notary and provides a way to sign and verify Docker images.

Advantages:

• Integrated with Docker CLI
• Works well with Docker registries
• Provides a straightforward way to sign and verify images

Disadvantages:

• Limited to Docker ecosystem
• Requires Docker Enterprise for advanced features

Sample Implementation:

# Enable Docker Content Trust
export DOCKER_CONTENT_TRUST=1

# Sign an image (automatic when pushing)
docker push myregistry.azurecr.io/myimage:tag

# Verify an image
docker trust inspect --pretty myregistry.azurecr.io/myimage:tag

GPG

GNU Privacy Guard (GPG) can be used for signing container images, although it's not specifically designed for this purpose.

Advantages:

• Widely used and understood
• Flexible and can be used for various signing needs
• Doesn't require additional infrastructure

Disadvantages:

• Not container-specific, so integration can be more complex
• Key management can be challenging at scale

Sample Implementation:

# Sign an image
gpg --sign myimage.tar

# Verify an image
gpg --verify myimage.tar.gpg

Sigstore

Sigstore is a project that aims to improve software supply chain security by making it easy to sign, verify, and protect software.

Advantages:

• Provides a suite of tools (including Cosign) for comprehensive signing and verification
• Supports keyless signing using OpenID Connect
• Integrates with transparency logs for better auditability

Disadvantages:

• Relatively new and evolving
• May require changes to existing workflows

Sample Implementation:

# Sign an image using Sigstore's keyless signing
cosign sign --identity-token $(gcloud auth print-identity-token) myregistry.azurecr.io/myimage:tag

# Verify an image
cosign verify myregistry.azurecr.io/myimage:tag

Cloud Provider Solutions

AWS Signer

AWS Signer is a fully managed code-signing service to ensure the trust and integrity of your code.

Advantages:

• Seamless integration with AWS services
• Supports code signing for AWS Lambda and IoT
• Managed service with built-in security and compliance features

Disadvantages:

• Limited to AWS ecosystem
• May not be suitable for multi-cloud environments

Sample Implementation:

# Sign a Lambda deployment package
aws signer sign-payload \
    --profile MySigningProfile \
    --payload fileb://MyLambdaFunction.zip \
    --output-file fileb://MySignedLambdaFunction.zip

# Verify a signed payload
aws signer verify-signature \
    --profile MySigningProfile \
    --payload fileb://MySignedLambdaFunction.zip

Azure Container Registry Content Trust

Azure Container Registry (ACR) supports Docker Content Trust for image signing and verification.

Advantages:

• Integrated with Azure ecosystem
• Uses Docker Notary for signing
• Supports Azure Key Vault for key management

Disadvantages:

• Limited to Azure Container Registry
• Requires additional setup for key management

Sample Implementation:

# Enable Content Trust for an Azure Container Registry
az acr config content-trust update --status enabled --registry MyRegistry

# Sign and push an image
docker trust sign myregistry.azurecr.io/myimage:tag

# Pull and verify a signed image
docker pull myregistry.azurecr.io/myimage:tag

Google Container Analysis

Google Container Analysis provides vulnerability scanning and metadata storage for containers, including support for Binary Authorization.

Advantages:

• Integrated with Google Cloud Platform
• Provides comprehensive container analysis and policy enforcement
• Supports attestations for image verification

Disadvantages:

• Limited to Google Cloud Platform
• Requires setup of Binary Authorization policies

Sample Implementation:

# Create an attestation
gcloud container binauthz attestations sign-and-create \
    --artifact-url=gcr.io/project-id/image@sha256:digest \
    --attestor=projects/project-id/attestors/attestor-name \
    --keyversion=projects/project-id/locations/global/keyRings/keyring-name/cryptoKeys/key-name/cryptoKeyVersions/1

# Verify an attestation
gcloud container binauthz attestations list \
    --artifact-url=gcr.io/project-id/image@sha256:digest \
    --attestor=projects/project-id/attestors/attestor-name

Comparison

Integration with Kubernetes

To enforce image signing in Kubernetes, you'll need to use admission controllers or policy engines. Some options include:

1. Portieris: An admission controller that integrates with Notary and supports various signing backends.
2. Kritis: A Google-developed solution for software supply chain security in Kubernetes.
3. Connaisseur: An admission controller that supports multiple signature verification backends.
4. OPA Gatekeeper: A policy engine that can be configured to enforce image signing policies.
5. Kyverno: An admission controller and policy engine specifically designed for Kubernetes. It supports multiple signature verification backends and can enforce image signing policies.

These tools can be configured to work with the signing solutions discussed above to ensure only signed images are deployed in your Kubernetes cluster.

Conclusion

Choosing the right image signing solution depends on your specific requirements, existing infrastructure, and cloud provider preferences:

• For a simple, modern solution that's easy to implement, Cosign or Sigstore might be your best bet.
• If you're already using Docker heavily, Docker Content Trust could be a natural fit.
• For cloud-specific deployments, the respective cloud provider's solution (AWS Signer, Azure Container Registry Content Trust, or Google Container Analysis) might offer the tightest integration.
• For maximum flexibility and control, GPG might be suitable, although it requires more manual configuration.

Consider factors such as ease of use, maturity, cloud integration, flexibility, and multi-cloud support when making your decision. Remember that image signing is just one part of a comprehensive container security strategy, and should be combined with other security measures for robust protection of your Kubernetes environment.

Labels:

Kubernetes, Container Security, Image Signing, Admission Controllers, DevSecOps, Cloud Native, Portieris, Kritis, Connaisseur, OPA Gatekeeper, Kyverno, Supply Chain Security, Policy Enforcement, Docker, Cosign, Notary, GPG, GitOps, CI/CD, Compliance