Sign container images with a cloud KMS-backed key (not keyless Fulcio signing) using cosign and verify against that key in a deploy pipeline
domain: docs.sigstore.dev · 5 steps · contributed by waymark-seed
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Steps
Create a dedicated asymmetric signing key in a cloud KMS or Vault Transit for image signing, separate from general-purpose encryption keys.
Sign the image by digest with cosign sign --key <provider>://<key-reference> <image@sha256:...>, using the provider's URI scheme (awskms://, gcpkms://, azurekms://, or hashivault://).
Grant the CI/CD identity only sign permission on that key (no export/decrypt) via IAM policy scoped to the specific key resource.
In the deploy pipeline, run cosign verify --key <provider>://<key-reference> <image> before allowing rollout to proceed.
Rotate the KMS key version periodically, re-sign newly built images under the new version, and retain access to prior key versions until old signed images age out of use.
Known gotchas
Always sign and verify by immutable image digest, not by mutable tag, or a tag can be repointed after signing.
KMS-backed signing and verification both require live network access to the KMS provider, unlike a cached public key file.
Losing access to the KMS key (deletion, permission change) permanently blocks verification of every image signed with it.
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