Container Image Signing

What is container image signing?

Container image signing is a cryptographic process that verifies the authenticity and integrity of container images using digital signatures. This security practice ensures that container images haven't been tampered with from creation to deployment, which establishes trust in your containerized applications.

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Cryptographic signatures: The guardians of container security

Cryptographic signatures form the foundation of container image signing. These digital signatures use asymmetric encryption to verify image integrity.

The process works through a two-key system. A private key signs the container image, while a corresponding public key verifies the signature. This system creates a tamper-evident, cryptographically secure seal that's nearly impossible to forge.

Asymmetric encryption: The foundation of trust

Asymmetric encryption is the bedrock of container image signing. You keep the private key confidential, while widely distributing the public key. It's like having a secure password that only you know, while others can verify your identity without it.

Why container image signing is necessary

Container image signing prevents security breaches by ensuring image authenticity and integrity. Without signatures, container images become unverifiable packages with unknown origins, contents, or security statuses.

Unsigned images create significant risks. You have no way to verify if an image comes from a trusted source or if an attacker injected malicious code during transit or storage. NIST guidelines recommend validating image signatures before execution to ensure images are from trusted sources and haven’t been tampered with.

Unsigned images: A risky business

Without signing, you can’t confirm if that container image you're pulling from a container registry is the real deal or a cleverly disguised piece of malware. The risks include security breaches, data loss, compliance failures, and significant financial penalties.

The anonymous nature of container images

Container images are complex and consist of multiple layers controlled by different entities. A base layer might contain a minimal operating system, followed by dependencies and your application. If any layer is compromised without a valid signature, the entire workload is at risk.

Key components of container image signing

Let's unpack the toolbox of container image signing. It's not just about attaching a signature to your images. It’s a complicated dance between technologies and practices to ensure signatures remain trustworthy and verifiable.

Cryptographic keys: The unsung heroes

Cryptographic keys enable container image signing through a two-part system that creates and verifies digital signatures.

Private keys create the digital signature and must remain secure and confidential. Only the image publisher should have access to the private key used for signing.

Public keys verify signatures and remain available for public distribution. Anyone can use the public key to confirm that a signature was created with the corresponding private key, establishing trust without exposing sensitive information.

Generating keys requires precise craftsmanship—done correctly, it results in unbreakable security. Secure methods for key generation remain critical. They generate unpredictable, complex keys that withstand attacks.

Digital signatures: The proof in the pudding

Digital signatures are where the rubber meets the road in container image signing.

• Signature creation uses your private key to generate a unique digital fingerprint for the container image. This process creates cryptographic proof of the image's authenticity and origin.

• Signature verification confirms the validity of a signature using the corresponding public key. This process ensures that no one has modified the image and that it came from the expected source.

Timestamping logs exactly when you sign the image, providing the chronological proof that's essential for compliance and security auditing.

Certificate authorities (CAs): The trusted third parties

Certificate authorities act as digital notaries. They're trusted entities that issue digital certificates to vouch for the legitimacy of your public keys.

CAs establish a trust hierarchy by confirming who owns a public key.

The certificates serve as digital ID cards for your public keys, offering a verifiable trail back to a trusted source.

Trust models: The framework of belief

Trust models in container image signing define how trust is established and maintained.

Centralized trust relies on a single source of truth. Decentralized trust spreads responsibility across a network, similar to a web of trust.

In a web of trust, known entities verify one another, creating a resilient network.

Hash functions: The integrity checkpoint

Hash functions serve as integrity checkpoints of container images by creating a unique digital fingerprint of their contents.

Hash functions ensure that even slight changes in the image are detectable, maintaining the integrity of the container.

Strong hash functions prevent collisions—situations where different images produce the same fingerprint—which would compromise security.

Signature storage: The vault of trust

Storing and distributing signatures and the accompanying public keys requires a secure infrastructure to protect your valuable security assets.

Best practices for storing and distributing signatures ensure that they remain accessible for verification while staying secure from tampering or unauthorized access.

Managing lists of revoked signatures is crucial for maintaining the ecosystem's trustworthiness. These ledgers identify invalid signatures that systems should no longer trust.

These essentials—from the creation and management of cryptographic keys to the role of CAs and trust models—form the foundation of container security.

Container image signing tools and methods

Several widely used tools and approaches exist for signing container images. Each offers unique strengths and ecosystem support, so it's important to choose the right fit for your environment.

• Cosign (Sigstore): This open-source tool simplifies signing, verifying, and storing container image signatures. It integrates with Kubernetes, supports keyless signing via OpenID Connect (OIDC) authentication, and is the standard for cloud-native workflows.

• Notary (Docker Content Trust): Notary manages signing and verification for Docker images using The Update Framework (TUF). Docker Content Trust enables image signing and verification within the Docker CLI and compatible registries.

• Amazon ECR image signing: AWS ECR supports image signing using AWS Signer or integration with external tools like Cosign, allowing teams to enforce signature verification on image pulls.

• Red Hat image signing: Red Hat provides tools for signing and verifying images in OpenShift and RHEL environments, often using GPG-based signatures.

When choosing a tool, consider your registry support, CI/CD integration, and how you will manage signing keys. Most organizations benefit from automating signing in their build pipelines and enforcing verification at deployment time.

How to verify container signatures

Verifying a container image signature ensures that the image remains unaltered and comes from a trusted source. This process typically requires using the public key or certificate associated with the signer. Here are different verification methods by tool:

• Cosign: To verify an image signed with Cosign, execute the cosign verify command along with the relevant public key. For example:

cosign verify --key cosign.pub registry.example.com/myimage:tag

• Docker Content Trust: When enabled, Docker Content Trust automatically verifies signatures when pulling images from a trusted registry.

• Amazon ECR: Configure ECR to require signature verification before allowing image pulls, using either AWS Signer or external verification tools.

Integrate signature verification into your CI/CD and deployment workflows to block unsigned or tampered images from reaching production. Many platforms, including Kubernetes, support admission controllers or policies to enforce signature checks automatically.

K8s Security Cheat Sheet

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Challenges and considerations

Let's review the operational hurdles security teams face when moving from theory to practice.

Key management: The custodian's dilemma

Key management controls the access to and lifecycle of cryptographic keys used for container image signing. Proper key management ensures signatures remain trustworthy while preventing unauthorized access to signing capabilities.

Trust establishment: The fragile supply chain

Trust is the currency of security in container image signing. Establishing and maintaining this trust is essential.

Performance overhead: The speed bump

Performance overhead is often an unavoidable byproduct of security processes, but it doesn't have to be a roadblock.

Cross-platform compatibility: The interoperability gap

The container ecosystem includes diverse orchestrators and platforms, making interoperability critical.

Legal and compliance issues: Navigating the maze

The legal landscape surrounding cryptographic signatures can be as complex as the technology itself.

Technical limitations: The constant challenge

The technology landscape evolves rapidly, introducing new constraints and vulnerabilities that require continuous adaptation.

Rigorous key management, compliance, and interoperability are among the considerations modern security professionals face. The goal is to create a secure, efficient, and scalable signing process that stands the test of time and technology.

Best practices for container image signing

Constructing a robust framework for container image signing requires more than just technical implementation. Adhering to industry best practices instills confidence in the integrity of your software supply chain and enhances your overall security posture.

Key security: The keystone of image signing

The security of cryptographic keys underpins the entire image signing process.

Generate keys in a secure, unpredictable manner. Utilize HSMs or secure software processes to ensure keys remain tamper-proof.

Implement strict access control policies that limit key handling to authorized personnel only. Use secure interfaces that log and audit all operations to maintain transparency.

Policy enforcement: The rulebook

Enforcing signing policies ensures consistency and reliability across the signing process.

Automate policy enforcement within the CI/CD pipeline to minimize human error and streamline operations.

Additionally, set organization-wide policies for signature verification to ensure that every image is checked against the same stringent criteria.

Compliance and standards: The navigator

Staying aligned with industry standards leverages collective wisdom to reinforce security posture.

Follow established standards, such as those set by NIST, to ensure your cryptographic processes remain up to date and secure.

Conduct regular audits to verify that your signing processes are functioning properly and remain compliant with both internal and external policies.

Automation and integration: The efficiency engine

Incorporating image signing into automated workflows is essential for maintaining efficiency.

Integrate image signing seamlessly into your continuous integration and delivery processes to make it a natural part of your software deployment.

Implement mechanisms to automatically revoke compromised or outdated signatures to maintain the trustworthiness of your container images.

Education and training: The enlightened workforce

Understanding the importance and intricacies of image signing is crucial across an organization.

Provide comprehensive training for developers on the technical and procedural aspects of image signing.

Run awareness programs to educate teams about the risks of unsigned images and the significance of maintaining security best practices.

Recovery strategies: The safety net

Prepare for the worst-case scenarios to ensure continuity and security.

Establish a comprehensive disaster recovery plan for incidents like key compromise or loss, ensuring operations can recover swiftly and securely.

Implement backups and redundancy for keys and signing infrastructure to prevent single points of failure and ensure uninterrupted container image integrity.

By focusing on cryptographic key security, stringent policy enforcement, industry standards, automation, and education and recovery strategies, organizations can establish a robust defense against the risks of container image deployment.

Wiz's approach to container image integrity

Wiz's admission controller now validates image integrity, marking a milestone in CNAPP security. Customers can leverage Wiz Guardrails to strengthen their software supply chain by enforcing the deployment of only signed or trusted images. Additionally, WizOS provides secured container images continuously maintained at near-zero CVEs to ensure a trusted foundation for all deployments.

Wiz admission controller acts as the gatekeeper for Kubernetes environments, validating the integrity of container images before deployment. It does this by focusing on three key aspects:

1. Image tampering protection:

• Validates image signatures: Wiz admission controller uses signatures created by tools like Cosign or Notary to cryptographically verify that the container image hasn't been tampered with since it was built. Any changes to the image invalidate the signature, preventing deployment.

• Checks image origin: Wiz admission controller also verifies image origin, ensuring it comes from a trusted source and hasn't been tampered with during the build or delivery process. This helps prevent supply chain attacks where malicious actors inject code into images before they reach your environment.

2. Enforcement of trusted sources:

• Policy-driven access control: Define which developers and teams are authorized to deploy container images to your Kubernetes clusters. Using WizOS-secured images ensures that only verified and hardened base images are deployed, reducing inherited vulnerabilities.

• Centralized management: Wiz admission controller provides a central location to manage all your image signing and verification policies, making it easy to enforce consistent security standards across your organization.

3. Complete visibility and threat detection:

• Audit logs and event tracking: Wiz admission controller monitors all attempts to deploy images, successful or not. This provides a centralized view of all activity and helps you identify potential threats or suspicious patterns. Track deployments using WizOS images to gain visibility into which secured images are in use.

• Real-time threat alerts: Configure Wiz admission controller to send real-time alerts for unauthorized or abnormal activity in your Kubernetes environments. This allows teams to quickly investigate and contain potential threats before they cause damage.

Container image signing establishes cryptographic trust in your containerized applications, preventing tampering and ensuring authenticity throughout the deployment pipeline. By implementing digital signatures, you create verifiable proof of image integrity, protecting against supply chain attacks and unauthorized modifications.

This security practice is essential as container adoption grows and threat actors increasingly target software supply chains. Organizations that implement container image signing gain measurable security improvements and compliance advantages while maintaining deployment velocity.

See for yourself how Wiz and WizOS work together to secure container images by requesting a demo today.

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