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Sigstore is an open-source software-signing ecosystem that links an artifact to an authenticated identity, records the signing event in a public transparency log, and lets anyone check the result. The Linux Foundation announced the service on March 9, 2021 as a free way for developers and software providers to sign release files, container images, and binaries without maintaining long-lived private keys. Fulcio, Rekor, Cosign and OpenID Connect form the core workflow.
What the Linux Foundation announced
The March 9, 2021 announcement described Sigstore as a free signing service for open-source developers and software providers. It covered release files, container images and binaries, with signing materials published in a tamper-resistant public log. Red Hat, Google and Purdue University were named as founding members.
Red Hat security engineering lead Luke Hinds described the goal as combining “provenance, integrity and discoverability” into a transparent, auditable software supply chain. Josh Aas, executive director of ISRG (the organization behind Let’s Encrypt), said deployment security should begin by confirming that the software being run is the software users intended to run.
Sigstore is not a single signing command or a replacement registry. It is a set of services, clients and policy tools that connect an identity to a particular artifact and leave public evidence of that connection.
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How keyless Sigstore signing works
Traditional signing normally requires a maintainer to generate, protect, rotate and eventually revoke a persistent private key. Sigstore’s keyless flow uses a short-lived certificate and an ephemeral key instead.
- Cosign creates an ephemeral keypair. The private key exists in memory for the signing operation rather than being kept as a long-term credential.
- OpenID Connect authenticates the signer. The signer obtains an OIDC identity token from a trusted identity provider, such as the identity used by an approved source-control or CI workflow.
- Fulcio issues a temporary certificate. Fulcio, Sigstore’s certificate authority, binds the ephemeral public key to the authenticated identity. The certificate is valid for a short period.
- Cosign signs the artifact. The signature covers the exact container image, release file or binary being published.
- Rekor records the event. Rekor stores a timestamped entry containing the information needed to connect the artifact, signature, certificate and identity. Its append-only design makes later alteration detectable.
- A consumer verifies the complete set. Verification checks that the downloaded artifact matches the signature, that the certificate chains to the trusted Fulcio root, that the identity is the expected one, and that the signing event appears in Rekor.
The trust anchors for this process include Fulcio’s root CA certificate and Rekor’s public key. Sigstore distributes those roots through The Update Framework (TUF), so clients can obtain and update them in a controlled way.
The Sigstore components
Cosign
Cosign is the client used to sign and verify container images and other artifacts. It integrates with OCI registries, allowing signatures and related metadata to travel with container delivery workflows.
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Fulcio
Fulcio is Sigstore’s free certificate authority. It issues temporary certificates to an authorized identity and publishes those certificates to Rekor. Because certificates are short-lived, a maintainer does not have to operate a permanent signing key merely to establish identity for a release.
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Rekor
Rekor is a searchable, append-only transparency and timestamping ledger. The log supplies a public record that can be audited and makes silent rewriting of signing history more difficult.
OpenID Connect
OpenID Connect supplies the authenticated identity information used by Fulcio. The identity provider is therefore part of the trust boundary: a stolen or incorrectly issued identity token can result in an unauthorized certificate.
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Policy Controller
Sigstore’s Policy Controller enforces container-signing policy at Kubernetes admission. A cluster can require images to carry acceptable signatures and identity claims before allowing workloads to run.
How to sign a container without managing a long-lived key
The following is the practical keyless sequence with Cosign. Exact prompts and flags can differ between Cosign releases, so use the syntax documented by the version installed in your build environment.
- Publish the image. Push the image to an OCI-compatible registry and record its immutable digest, for example
registry.example/app@sha256:…. A tag can move; a digest identifies the exact bytes. - Authenticate to your identity provider. Start the OIDC login requested by Cosign. Use the account or CI identity that should appear in the signing certificate.
- Sign the image. Run the keyless signing operation, such as
cosign sign registry.example/app:1.4.0. Cosign obtains an ephemeral key, requests a Fulcio certificate, signs the image and uploads the transparency record. - Save the release reference. Distribute the registry reference and, where possible, the immutable digest alongside release notes. Consumers should verify the digest they will actually deploy.
- Verify before deployment. A basic check uses
cosign verify registry.example/app:1.4.0. Production policy should additionally require the expected certificate identity, issuer and repository or workflow claims rather than accepting any valid Sigstore signature.
Keyless signing removes storage and rotation of a permanent private key; it does not remove the need to protect the OIDC account or CI job that is allowed to obtain a certificate.
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What verification proves
A successful verification gives evidence that the artifact matched the signed bytes and that the signature was made by the identity bound to the Fulcio certificate while that certificate was valid. Rekor’s matching entry supplies a public timestamp and an auditable record of the event.
To decide whether a release came from the expected maintainer, verify all of these attributes:
- the artifact’s digest or exact file bytes;
- the cryptographic signature;
- the Fulcio certificate chain to the trusted root;
- the certificate identity and issuer, including any repository or workflow claims your policy requires;
- the corresponding Rekor entry and its timestamp; and
- the policy that says which identities are authorized for that project.
A valid signature alone is not the same as “approved by this project.” It is valid evidence only when the signer identity matches the identity your release policy expects.
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Sigstore compared with conventional signing
| Area | Sigstore keyless model | Conventional long-lived-key model |
|---|---|---|
| Key management | Ephemeral keypair; no persistent private signing key for the normal flow | Maintainer or organization stores, protects, rotates and recovers a persistent private key |
| Identity binding | OIDC identity is bound to a short-lived Fulcio certificate | Identity is represented by the holder of the private key and whatever certificate or registry metadata accompanies it |
| Certificate lifetime and revocation | Short-lived Fulcio certificates limit the useful lifetime of an issued credential | Long-lived keys require an operational revocation, replacement and compromise-response process |
| Transparency | Rekor provides a searchable, append-only public record with a timestamp | Transparency depends on whether the organization operates and exposes a separate audit log |
| CI/CD and registry integration | Cosign connects signing and verification to OCI registries and automated identity providers | Automation must be given access to the persistent key or to a signing service that holds it |
| Kubernetes enforcement | Policy Controller can enforce signature and identity requirements at admission | Enforcement requires a separate admission or policy implementation |
| Availability | Rekor and Fulcio general availability was announced in 2022 with a 99.5% uptime SLO and round-the-clock pager support | Availability is determined by the organization’s own key and signing infrastructure |
| Residual trust | Trust still includes the OIDC provider, Fulcio, Rekor, TUF-distributed roots and log monitoring | Trust concentrates more heavily in the private-key holder and its storage controls |
Security limits and failure cases
A compromised identity can authorize a certificate
Sigstore proves what identity was bound to a certificate; it cannot make a compromised identity trustworthy. If an attacker controls an approved OIDC account or CI workflow, that attacker may obtain a certificate that appears legitimate. Restrict signing identities, protect workflow credentials and review certificate claims.
Services and roots remain part of the trust boundary
Fulcio, Rekor, trusted OIDC providers and the TUF-distributed roots must behave as expected. An outage can prevent signing or verification that depends on a live service, while a service compromise could affect issuance or logging.
Logs require monitoring
The append-only design makes unauthorized changes easier to detect, but detection is not automatic protection. The security model notes that undetected behavior remains possible when nobody monitors transparency logs. Organizations should alert on unexpected identities, repositories and signing times.
A signature does not prove software quality
Sigstore establishes origin evidence and integrity of the signed artifact. It does not prove that the code is safe, free of vulnerabilities or built from the source a project intended. Those questions require build provenance, code review, testing and vulnerability management.
Availability and project maturity
On October 25, 2022, Sigstore announced general availability for Rekor and Fulcio, including v1.0.0 releases, a stated 99.5% uptime service-level objective, round-the-clock pager support and a third-party security audit whose findings were reported as addressed. The announcement recommended Cosign, sigstore-python and sigstore-java for signing without user-managed long-lived keys.
A practical adoption path
- Start with one release type. Sign a container or release archive in CI and publish its digest with the release.
- Define the expected identity. Write down which maintainer, repository, workflow or issuer is authorized to sign.
- Verify in a separate environment. Check the artifact, certificate claims and Rekor record before deploying it.
- Enforce policy. Add admission controls with Policy Controller for Kubernetes workloads, or equivalent verification gates in other deployment systems.
- Monitor the log and identity provider. Investigate unexpected signing events instead of treating a mathematically valid signature as automatically approved.
Developers, DevOps engineers and security engineers who need structured practice can use the Linux Foundation’s LFS182 course, which covers Cosign, Fulcio, Rekor, Policy Controller, Gitsign, trusted timestamping and hands-on labs.
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