Chapter 2
Sigstore Architecture and Components
Behind every secure container or binary lies a symphony of cryptographic systems and transparency services working in orchestration. This chapter takes you beneath the abstraction, unraveling Sigstore's architecture layer by layer. Explore how modular design and collaborative verification establish trust at scale, and discover the interplay between Cosign, Fulcio, and Rekor that empowers automated, tamper-evident supply chains.
2.1 Sigstore's Modular Design
Sigstore's architecture embodies a deliberate modularity that strategically enhances security, agility, and extensibility within modern software supply chains. By decomposing responsibilities into discrete, well-defined components, Sigstore enforces strict separation of concerns while maintaining flexible interfaces that accommodate evolving requirements and threat landscapes. This approach ensures each component can be independently developed, audited, or replaced without compromising overall system integrity, thus reinforcing resilience against sophisticated supply chain attacks.
At the highest level, Sigstore consists primarily of three core components: the client tooling, the transparency log (Rekor), and the certificate authority (Fulcio), alongside an optional timestamping service (Tuf). Each component operates as a modular service, communicating through well-established protocols and bounded interfaces, thereby enabling a clear distribution of responsibilities.
The client tooling mediates interaction between developers or CI/CD pipelines and the Sigstore back-end infrastructure. Its main responsibilities include generating ephemeral key pairs for signing, invoking the certificate authority to obtain short-lived X.509 certificates, submitting certificates and signing material to the transparency log, and optionally requesting trusted timestamps. By isolating key generation and signing operations within the client, Sigstore limits the exposure window for private keys and reduces persistent key storage risks. The client acts as the localized trust boundary, enforcing cryptographic best practices and preventing leakage of sensitive material.
The certificate authority, Fulcio, issues ephemeral certificates by validating identity proofs anchored in existing identity providers such as OAuth/OIDC. The modular CA design focuses exclusively on trusted identity attestation and certificate issuance, delegating integrity and accountability assurances to the transparency log. This separation of concerns prevents the CA from evolving into a monolithic trust anchor and simplifies incorporation of other identity providers, enhancing agility.
The transparency log, Rekor, is a cryptographically verifiable append-only ledger that records all issued certificates and signatures. Rekor's modular design encapsulates log storage, hashing, and consistency proofs, enabling independent scaling and auditing. Rekor's API supports inserting new entries and querying proofs of inclusion and consistency, facilitating continuous monitoring and verification by clients or third-party auditors. The transparency log enforces immutable public accountability, significantly elevating supply chain trust by exposing any misbehaviors such as certificate forgery or signature tampering.
The timestamping service, often integrated through the TUF (The Update Framework) ecosystem, adds a temporal dimension to signature validity, protecting against rollback or delay attacks. Its modular role is strictly confined to timestamp provisioning, enabling independent evolution of secure timing sources without impacting core signing and logging functionalities.
Communication flows between these components are asynchronous yet tightly coordinated. Upon signing, the client generates a key pair and requests a certificate from Fulcio, which requires an authenticated identity proof. Fulcio issues a certificate bound to the ephemeral key and returns it to the client, who then submits the certificate and signature material to Rekor. Rekor returns a transparency log entry with an inclusion proof, which the client bundles alongside the signature and certificate into a signed bundle. Optionally, the client requests a timestamp from the timestamping service to embed temporal attestation.
The modular architecture enables several critical design patterns. First, separation of concerns reinforces security by isolating identity verification, certificate issuance, transparency auditing, and cryptographic signing into distinct modules. This reduces the attack surface and simplifies threat modeling for each component. For instance, should the CA become compromised, public verifiability via the transparency log still enforces detection and remediation. Second, pluggability is achieved through well-defined API contracts and abstracted identity assertion layers, allowing the substitution or addition of alternative CAs, logs, or identity providers without systemic redesign. This facilitates rapid adaptation to new standards or regulations.
Furthermore, Sigstore leverages defense in depth by integrating multiple, independently auditable checkpoints. The ephemeral key generation within clients minimizes persistent secrets; the automated certificate issuance ties keys to verified identities; the transparency log ensures accountability and non-repudiation; and the timestamping service preserves freshness guarantees. The modular division between components enables independent scaling strategies and fault tolerance mechanisms, allowing Sigstore to maintain high availability despite diverse operational pressures.
Extensibility is intrinsic to the design, as each component maintains minimal coupling and exposes comprehensive interfaces. For example, alternative transparency logs can be incorporated, provided they guarantee append-only consistency and public verifiability. Similarly, Fulcio's CA module can onboard additional identity proofs or extended validation workflows to support enterprise policies. The client tooling can evolve to support new signing formats, artifact types, or integration points without altering core infrastructure.
This modular, layered design also supports emerging use cases such as revocation transparency, configurable policy enforcement, and multi-stakeholder signing workflows. By disaggregating concerns and enabling interoperability, Sigstore's architecture anticipates future requirements, embedding flexibility directly into the supply chain trust fabric. It establishes an extensible foundation that not only addresses current challenges in securing software provenance but also adapts to the unpredictable contours of next-generation threat vectors.
Sigstore's modular design is a strategic synthesis of architectural principles that collectively enhance software supply chain security. The explicit boundaries and well-defined responsibilities between client tooling, certificate authority, transparency log, and timestamping service exemplify separation of concerns and pluggability, fostering a resilient and adaptable ecosystem. These structural elements underpin Sigstore's ability to deliver transparent, accountable, and trustworthy software signings, making it a robust defense mechanism against the ever-evolving landscape of supply chain threats.
2.2 Cosign: Signer and Verifier
Cosign operates as a dual-purpose tool within secure software supply chains, functioning both as a signer to cryptographically attest artifacts and as a verifier to establish their authenticity and integrity. At its core, Cosign leverages modern public-key cryptography techniques to provide an auditable and automated framework for artifact trust, which is essential in environments where software provenance is crucial.
The signing process in Cosign begins with robust key management capabilities. Cosign supports multiple key storage modalities, including hardware security modules (HSMs), cloud-based key management systems such as AWS KMS, Google Cloud KMS, and Azure Key Vault, and local file-based key pairs. This flexibility allows organizations to tailor their security posture according to their threat model and compliance requirements. The keys are used to produce digital signatures over container images or other supported artifact types, which bind identity and integrity claims cryptographically.
Cosign extends beyond container images to support a wide variety of artifact types, including Helm charts, Software Bill of Materials (SBOMs), and generic files. This multi-artifact support is facilitated by a pluggable signature format and annotation mechanism, enabling the embedding of metadata such as timestamp, subject, and predicate fields. These annotations provide context and enhance the semantic value of signatures, which is crucial for downstream policy enforcement and auditing.
Integration hooks within Cosign are designed for seamless embedding into existing CI/CD pipelines and supply chain workflows. Through its command-line interface (CLI) and APIs, Cosign can be invoked as a step in automated build or deployment processes to attach signatures post-build. It also supports verification stages that can be...
