Cosign

Overview

Cosign is an open-source tool for signing and verifying OCI containers and other software artifacts. It is best suited to teams that can work through a CLI, especially open-source package managers building artifact-signing workflows. Its broad signing options are useful, but its legacy status and lack of API stability make it a poor fit for new application integrations.

Key features

Cosign’s default keyless signing uses Sigstore’s public-good Fulcio certificate authority and Rekor transparency log. It creates ephemeral keys held in memory, obtains short-lived certificates, and records signatures in the log. That avoids managing a persistent signing key for this path, but signing can publish identity information such as an account email in a public log, where it cannot later be removed.

Teams that need control over key custody can use hardware or KMS signing, Cosign-generated encrypted keypairs, or bring their own PKI. Cosign generates ECDSA-P256 keys and uses SHA256 hashes for ephemeral keyless and managed-key signing, so those limits matter when a workflow requires other algorithms or hashes.

Container signatures can be stored alongside images in an OCI registry. Cosign also provides utilities for publishing generic artifacts through OCI and supports in-toto attestations whose payloads are signed with DSSE. Supported targets include container images, blobs, binaries, scripts, configuration files, SBOMs, WASM modules, Tekton bundles, eBPF modules, and in-toto attestations. This breadth makes it relevant beyond container release pipelines, though teams should expect to operate it as a CLI tool rather than depend on it as a stable application API.

Tested registry integrations include AWS ECR, Google Artifact Registry, Docker Hub, Azure Container Registry, GitLab Container Registry, and GitHub Container Registry. Installation guidance covers GitHub Actions and GitLab CI/CD pipelines. Offline verification is possible when the image and signature materials are local and a trusted root is supplied. The installation guide also recommends verifying downloaded Cosign binaries; releases are signed with keyless signing and an artifact key.

Pricing

Cosign is free open-source software: the Cosign plan is free, billed Free. There is no free trial, and no hosted service or usage limits are stated. That makes it a practical option for teams willing to run and integrate the tool themselves, rather than buyers seeking a hosted signing service.

Platforms

Cosign supports Linux, macOS, Windows, and self-hosted use. The project links Linux and macOS release binaries and documents installation through Go, Homebrew, Arch, Alpine, Nix, GitHub Actions, GitLab, and container images. The range gives teams several routes to add it to development and CI environments, while the documented release binaries are for Linux and macOS.

Who it's for

Cosign is a strong fit for open-source package managers and release teams that need to sign and verify artifacts across OCI registries, CI pipelines, or offline environments. Its multiple key-management paths suit teams with different custody requirements. It is less suitable for developers who need a supported API for embedding signing or verification in an application: Cosign’s functions were designed for its CLI, the project makes no API stability guarantees, and it recommends Sigstore-go for verification integrations.

Pros and cons

• Pros: Keyless, hardware, KMS, encrypted-keypair, and bring-your-own-PKI options let teams choose a signing approach suited to their key-management needs.
• Pros: OCI registry storage, generic artifact publishing, and in-toto attestations cover container and broader software-supply-chain workflows.
• Pros: Offline verification is available when signature materials and a trusted root are available locally.
• Cons: Public transparency logging can expose signer identity information that cannot later be removed.
• Cons: CLI-focused design and the absence of API stability guarantees make Cosign a weak choice for new application integrations.
• Cons: Signing is constrained to ECDSA-P256 keys and SHA256 hashes for ephemeral keyless and managed-key signing.
• Cons: The project describes Cosign as a legacy system: it remains intended for signing, while Sigstore-go is recommended for verification integrations.

Alternatives

Compare code signing software if you want to weigh tools across the category. Sigstore is the direct alternative to consider for a broader Sigstore choice; its free plan is available to all developers and software providers.

Choose SignPath if an eligible open-source project wants a free code-signing option; its open-source plan has eligibility conditions. SignPath Foundation is another free option for eligible, actively maintained open-source projects released under an OSI-approved license, with manual release requirements.

SignServer may suit teams looking for community signing and timestamping with source-code or container deployment; its community plan covers basic code, document, and container signing. Consider Bamboo Deploy if its freemium offering fits: Premium costs $15 a month, billed $45 every 3 months, and includes up to 50 apps and 1GB cloud hosting.

DigiCert Software Trust Manager is a paid alternative with custom pricing. Red Hat Trusted Artifact Signer is another paid option with custom pricing. SSL.com Certificate Lifecycle Management is a freemium alternative without a free plan.

Verdict

Choose Cosign if your team needs free, flexible signing and verification for containers and other artifacts and is comfortable operating a CLI-centered tool. Its registry support, key options, attestations, and offline verification give it meaningful breadth. Look elsewhere if you need a stable API for a new application integration, or if the public-log identity exposure and signing algorithm limits do not suit your security requirements.