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A valid artifact signature shows that the signed data matches the data covered by the signature and that the corresponding signing key signed it. It supports attribution to an expected publisher only when you also validate the key or certificate and check the signer identity against an identity you trust. It does not, by itself, show that software is safe, correctly built, or free of defects.
What does an artifact signature prove?
A digital signature binds particular data to a signing key. When verification succeeds, it shows that the data covered by the signature has not changed since it was signed, under the signature system’s cryptographic and implementation assumptions. If the artifact changes, verification against the changed content should fail. NIST describes code signing as providing data integrity and source authentication, while its digital-signature glossary explains the underlying mechanism: NIST’s code-signing guidance and digital-signature glossary.
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That is a narrow, useful guarantee: the verified bytes are the bytes associated with a signing key. It applies only to the data actually covered by the signature, not automatically to every file in a download, package, or release.
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The cryptographic operation identifies a key, not a real-world organization by itself. To attribute a signature to a publisher, the verifier needs the correct public key or certificate, must validate it under an accepted trust root, and must compare the signer’s identity with an expected one. A valid signature from an unknown key proves only that the corresponding private key signed the covered data.
For example, a verifier should not accept any certificate that happens to be valid. It should check whether the certificate identifies the expected producer, organization, repository, or workflow. Sigstore’s verification model makes these separate checks explicit: verify the artifact signature, validate the certificate under the Sigstore trust root, compare its identity with the expected identity, and check the transparency-log inclusion. See the Sigstore overview.
What does a code-signing signature not prove?
A signature does not certify that the signed program is benign, bug-free, compliant with a law or policy, or suitable for your system. A malicious or defective program can be signed. Nor does the signature alone show that the artifact was built from the source code you intended to trust, that the build environment was uncompromised, or that its dependencies are safe. Digital signatures also do not provide confidentiality: they do not hide the signed content. These limits follow from the scope of a signature and the additional build checks described in SLSA’s artifact-verification guidance.
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A signature on an attestation or other statement has a similarly bounded meaning. It can show who signed the statement and that the signed statement was not altered, given the relevant trust assumptions; it does not independently prove that the statement is true or adequate. NIST distinguishes first-party statements made by a producer, second-party statements made by a purchaser, and third-party attestations or certifications made by an independent party. The issuer and supporting evidence matter; the distinction is described in NIST’s software supply-chain terminology guidance.
How do you verify a software artifact signature?
Use a verifier and trust policy appropriate to the signing system. A “signature valid” result is not enough if the tool accepted an unexpected signer or checked a different file from the one you plan to use.
- Start with a configured, trusted root of trust for the signing system. Do not treat an arbitrary public key or certificate supplied alongside an artifact as trusted without validating it.
- Check that the signature covers the exact artifact you have, and that the subject or digest recorded by the verification process matches that artifact.
- Compare the validated certificate or signing identity with the producer, organization, repository, or workflow you expected. Reject a valid signature from an unexpected identity.
- If provenance is provided, check its signature and inspect the builder identity, canonical source repository, build type, and externally supplied build parameters against explicit expectations. Do not accept unrecognized external parameters by default.
- Assess dependency information if your policy requires it, while accounting for what the provenance actually guarantees. SLSA v1.0 does not require the
resolvedDependencieslist to be complete or verified.
SLSA’s current main-branch verification guidance describes checking the provenance envelope signature, matching its subject digest to the artifact, checking the predicate type and trusted builder, and comparing package-specific expectations. Because that page tracks the main branch and can change, consult the version relevant to your implementation: SLSA: Build: Verifying artifacts.
What does Sigstore add, and what risks remain?
Sigstore’s documented workflow uses an OpenID Connect (OIDC) identity, a short-lived certificate issued by Fulcio, and an entry in the Rekor transparency log. Together, certificate identity checks and log inclusion can make signing events publicly auditable and reduce dependence on long-lived signing keys. They are additional ways to establish and inspect signing context, not a safety assessment of the signed software. The workflow is described in the Sigstore overview.
Those checks still rely on the identity provider, certificate authority, trust root, transparency log, and monitoring. Sigstore notes that a compromised OIDC identity or provider, or a compromised Fulcio service, could lead to unauthorized certificates. Transparency can help make such events detectable, but detection depends on log publication and monitoring; Sigstore assigns users responsibility for monitoring certificate-transparency entries issued to their identities. See the Sigstore security model.
What does provenance add beyond a signature?
Provenance records claims about how an artifact was built. It can provide evidence about the builder and build process that a bare signature does not contain, but only if a consumer verifies and evaluates those claims. SLSA recommends comparing the artifact and provenance to a preconfigured root of trust, verifying the provenance signature, matching the artifact digest, and checking the builder and relevant provenance fields against package-specific policy. A provenance document that is merely present has not answered those questions.
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Interpret assurance levels within their stated threat model. SLSA Build L3’s protection against some external attacks assumes the build platform itself is trusted; it does not cover a compromised platform, such as one controlled by a malicious insider. Combined with the limitation on dependency completeness, this means a provenance level is not a universal guarantee that software is safe. The scope and verification guidance are in SLSA’s artifact-verification documentation.
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