cosign

Signing OCI containers (and other artifacts) using Sigstore!

Cosign aims to make signatures invisible infrastructure.

Cosign supports:

• “Keyless signing” with the Sigstore public good Fulcio certificate authority and Rekor transparency log (default)
• Hardware and KMS signing
• Signing with a cosign generated encrypted private/public keypair
• Container Signing, Verification and Storage in an OCI registry.
• Bring-your-own PKI

Info

Cosign is developed as part of the sigstore project. We also use a slack channel! Click here for the invite link.

Installation

For Homebrew, Arch, Nix, GitHub Action, and Kubernetes installs see the installation docs.

For Linux, macOS and Windows binaries, see the GitHub release assets.

🚨 If you are downloading releases of cosign from our GCS bucket - please see more information on the July 31, 2023 deprecation notice 🚨

Developer Installation

If you have Go 1.22+, you can setup a development environment:

$ git clone https://github.com/sigstore/cosign
$ cd cosign
$ go install ./cmd/cosign
$ $(go env GOPATH)/bin/cosign

Contributing

If you are interested in contributing to cosign, please read the contributing documentation.

Future Cosign development will be focused the next major release which will be based on sigstore-go. Maintainers will be focused on feature development within sigstore-go. Contributions to sigstore-go, particularly around bring-your-own keys and signing, are appreciated. Please see the issue tracker for good first issues.

Cosign 2.x is a stable release and will continue to receive periodic feature updates and bug fixes. PRs that are small in scope and size are most likely to be quickly reviewed.

PRs which significantly modify or break the API will not be accepted. PRs which are significant in size but do not introduce breaking changes may be accepted, but will be considered lower priority than PRs in sigstore-go.

Dockerfile

Here is how to install and use cosign inside a Dockerfile through the ghcr.io/sigstore/cosign/cosign image:

FROM ghcr.io/sigstore/cosign/cosign:v2.4.1 as cosign-bin

# Source: https://github.com/chainguard-images/images/tree/main/images/static
FROM cgr.dev/chainguard/static:latest
COPY --from=cosign-bin /ko-app/cosign /usr/local/bin/cosign
ENTRYPOINT [ "cosign" ]

Quick Start

This shows how to:

• sign a container image with the default identity-based “keyless signing” method (see the documentation for more information)
• verify the container image
• explore broader keyless blob signing/verification flows in the Sigstore Cosign Quickstart

Sign a container and store the signature in the registry

Note that you should always sign images based on their digest (@sha256:...) rather than a tag (:latest) because otherwise you might sign something you didn’t intend to!

 cosign sign $IMAGE

Generating ephemeral keys...
Retrieving signed certificate...

    Note that there may be personally identifiable information associated with this signed artifact.
    This may include the email address associated with the account with which you authenticate.
    This information will be used for signing this artifact and will be stored in public transparency logs and cannot be removed later.

By typing 'y', you attest that you grant (or have permission to grant) and agree to have this information stored permanently in transparency logs.
Are you sure you would like to continue? [y/N] y
Your browser will now be opened to:
https://oauth2.sigstore.dev/auth/auth?access_type=online&client_id=sigstore&code_challenge=OrXitVKUZm2lEWHVt1oQWR4HZvn0rSlKhLcltglYxCY&code_challenge_method=S256&nonce=2KvOWeTFxYfxyzHtssvlIXmY6Jk&redirect_uri=http%3A%2F%2Flocalhost%3A57102%2Fauth%2Fcallback&response_type=code&scope=openid+email&state=2KvOWfbQJ1caqScgjwibzK2qJmb
Successfully verified SCT...
tlog entry created with index: 12086900
Pushing signature to: $IMAGE

Cosign will prompt you to authenticate via OIDC, where you’ll sign in with your email address. Under the hood, cosign will request a code signing certificate from the Fulcio certificate authority. The subject of the certificate will match the email address you logged in with. Cosign will then store the signature and certificate in the Rekor transparency log, and upload the signature to the OCI registry alongside the image you’re signing.

Verify a container

To verify the image, you’ll need to pass in the expected certificate subject and certificate issuer via the --certificate-identity and --certificate-oidc-issuer flags:

cosign verify $IMAGE --certificate-identity=$IDENTITY --certificate-oidc-issuer=$OIDC_ISSUER

You can also pass in a regex for the certificate identity and issuer flags, --certificate-identity-regexp and --certificate-oidc-issuer-regexp.

Verify a container against a public key

This command returns 0 if at least one cosign formatted signature for the image is found matching the public key. See the detailed usage below for information and caveats on other signature formats.

Any valid payloads are printed to stdout, in json format. Note that these signed payloads include the digest of the container image, which is how we can be sure these “detached” signatures cover the correct image.

$ cosign verify --key cosign.pub $IMAGE_URI:1h
The following checks were performed on these signatures:
  - The cosign claims were validated
  - The signatures were verified against the specified public key
{"Critical":{"Identity":{"docker-reference":""},"Image":{"Docker-manifest-digest":"sha256:87ef60f558bad79beea6425a3b28989f01dd417164150ab3baab98dcbf04def8"},"Type":"cosign container image signature"},"Optional":null}

Verify a container in an air-gapped environment

Note: This section is out of date.

Note: Most verification workflows require periodically requesting service keys from a TUF repository. For airgapped verification of signatures using the public-good instance, you will need to retrieve the trusted root file from the production TUF repository. The contents of this file will change without notification. By not using TUF, you will need to build your own mechanism to keep your airgapped copy of this file up-to-date.

Cosign can do completely offline verification by verifying a bundle which is typically distributed as an annotation on the image manifest. As long as this annotation is present, then offline verification can be done. This bundle annotation is always included by default for keyless signing, so the default cosign sign functionality will include all materials needed for offline verification.

To verify an image in an air-gapped environment, the image and signatures must be available locally on the filesystem.

An image can be saved locally using cosign save (note, this step must be done with a network connection):

cosign initialize # This will pull in the latest TUF root
cosign save $IMAGE_NAME --dir ./path/to/dir

Now, in an air-gapped environment, this local image can be verified:

cosign verify \
  --certificate-identity $CERT_IDENTITY \
  --certificate-oidc-issuer $CERT_OIDC_ISSUER \
  --offline=true \
  --new-bundle-format=false \ # for artifacts signed without the new protobuf bundle format
  --trusted-root ~/.sigstore/root/tuf-repo-cdn.sigstore.dev/targets/trusted_root.json \ # default location of trusted root
  --local-image ./path/to/dir

You’ll need to pass in expected values for $CERT_IDENTITY and $CERT_OIDC_ISSUER to correctly verify this image. If you signed with a keypair, the same command will work, assuming the public key material is present locally:

cosign verify --key cosign.pub --offline --local-image ./path/to/dir

Identity-based blob signing and verification

Use keyless blob signing (cosign sign-blob without --key) and verify against the expected signer identity:

$ cosign sign-blob artifact --bundle artifact.sigstore.json --yes
$ cosign verify-blob artifact \
  --bundle artifact.sigstore.json \
  --certificate-identity "https://github.com/ORG/REPO/.github/workflows/release.yml@refs/heads/main" \
  --certificate-oidc-issuer "https://token.actions.githubusercontent.com"

Troubleshooting

If you encounter issues with Cosign, first make sure you are using a recent release: The Cosign project actively supports the most recent release as well as the last release in the v2 series.

Common issues and remedies

1. Verification fails with failed to verify timestamps: threshold not met for verified log entry integrated timestamps: 0 < 1: You may be verifying a signature that requires RFC3161 timestamp support
   • Upgrade to most recent Cosign or
   • With Cosign 2.6.x, use --use-signed-timestamps
2. Verification fails with no signatures found: You may be verifying an image signature that requires support for Rekor v2 transparency log
   • Upgrade to most recent Cosign
3. Signing fails with HTTP errors: Signing with Cosign depends on multiple Sigstore services. Retrying on failure may be a useful workaround if any of these services fail – filing issues for specific failures is also appreciated

My problem is something else

Please open an issue or ask in the slack channel.

Working with Other Artifacts

OCI registries are useful for storing more than just container images! Cosign also includes some utilities for publishing generic artifacts, including binaries, scripts, and configuration files using the OCI protocol.

This section shows how to leverage these for an easy-to-use, backwards-compatible artifact distribution system that integrates well with the rest of Sigstore.

See the documentation for more information.

Blobs

You can publish an artifact with cosign upload blob:

$ echo "my first artifact" > artifact
$ BLOB_SUM=$(shasum -a 256 artifact | cut -d' ' -f 1) && echo "$BLOB_SUM"
c69d72c98b55258f9026f984e4656f0e9fd3ef024ea3fac1d7e5c7e6249f1626
$ BLOB_NAME=my-artifact-$(uuidgen | head -c 8 | tr 'A-Z' 'a-z')
$ BLOB_URI=ttl.sh/$BLOB_NAME:1h

$ BLOB_URI_DIGEST=$(cosign upload blob -f artifact $BLOB_URI) && echo "$BLOB_URI_DIGEST"
Uploading file from [artifact] to [ttl.sh/my-artifact-f42c22e0:5m] with media type [text/plain]
File [artifact] is available directly at [ttl.sh/v2/my-artifact-f42c22e0/blobs/sha256:c69d72c98b55258f9026f984e4656f0e9fd3ef024ea3fac1d7e5c7e6249f1626]
Uploaded image to:
ttl.sh/my-artifact-f42c22e0@sha256:790d47850411e902aabebc3a684eeb78fcae853d4dd6e1cc554d70db7f05f99f

Your users can download it from the “direct” url with standard tools like curl or wget:

$ curl -L ttl.sh/v2/$BLOB_NAME/blobs/sha256:$BLOB_SUM > artifact-fetched

The digest is baked right into the URL, so they can check that as well:

$ cat artifact-fetched | shasum -a 256
c69d72c98b55258f9026f984e4656f0e9fd3ef024ea3fac1d7e5c7e6249f1626  -

You can sign it with the normal cosign sign command and flags:

$ cosign sign --key cosign.key $BLOB_URI_DIGEST
Enter password for private key:
Pushing signature to: ttl.sh/my-artifact-f42c22e0

As usual, make sure to reference any images you sign by their digest to make sure you don’t sign the wrong thing!

Tekton Bundles

Tekton bundles can be uploaded and managed within an OCI registry. The specification is here. This means they can also be signed and verified with cosign.

Tekton Bundles can currently be uploaded with the tkn cli, but we may add this support to cosign in the future.

$ tkn bundle push us.gcr.io/dlorenc-vmtest2/pipeline:latest -f task-output-image.yaml
Creating Tekton Bundle:
        - Added TaskRun:  to image

Pushed Tekton Bundle to us.gcr.io/dlorenc-vmtest2/pipeline@sha256:124e1fdee94fe5c5f902bc94da2d6e2fea243934c74e76c2368acdc8d3ac7155
$ cosign sign --key cosign.key us.gcr.io/dlorenc-vmtest2/pipeline@sha256:124e1fdee94fe5c5f902bc94da2d6e2fea243934c74e76c2368acdc8d3ac7155
Enter password for private key:
tlog entry created with index: 5086
Pushing signature to: us.gcr.io/dlorenc-vmtest2/demo:sha256-124e1fdee94fe5c5f902bc94da2d6e2fea243934c74e76c2368acdc8d3ac7155.sig

WASM

Web Assembly Modules can also be stored in an OCI registry, using this specification.

Cosign can upload these using the cosign wasm upload command:

$ cosign upload wasm -f hello.wasm us.gcr.io/dlorenc-vmtest2/wasm
$ cosign sign --key cosign.key us.gcr.io/dlorenc-vmtest2/wasm@sha256:9e7a511fb3130ee4641baf1adc0400bed674d4afc3f1b81bb581c3c8f613f812
Enter password for private key:
tlog entry created with index: 5198
Pushing signature to: us.gcr.io/dlorenc-vmtest2/wasm:sha256-9e7a511fb3130ee4641baf1adc0400bed674d4afc3f1b81bb581c3c8f613f812.sig

eBPF

eBPF modules can also be stored in an OCI registry, using this specification.

The image below was built using the bee tool. More information can be found here

Cosign can then sign these images as they can any other OCI image.

$ bee build ./examples/tcpconnect/tcpconnect.c localhost:5000/tcpconnect:test
$ bee push localhost:5000/tcpconnect:test
$ cosign sign  --key cosign.key localhost:5000/tcpconnect@sha256:7a91c50d922925f152fec96ed1d84b7bc6b2079c169d68826f6cf307f22d40e6
Enter password for private key:
Pushing signature to: localhost:5000/tcpconnect
$ cosign verify --key cosign.pub localhost:5000/tcpconnect:test

Verification for localhost:5000/tcpconnect:test --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - The signatures were verified against the specified public key

[{"critical":{"identity":{"docker-reference":"localhost:5000/tcpconnect"},"image":{"docker-manifest-digest":"sha256:7a91c50d922925f152fec96ed1d84b7bc6b2079c169d68826f6cf307f22d40e6"},"type":"cosign container image signature"},"optional":null}]

In-Toto Attestations

Cosign also has built-in support for in-toto attestations. The specification for these is defined here.

You can create and sign one from a local predicate file using the following commands:

$ cosign attest --predicate <file> --key cosign.key $IMAGE_URI_DIGEST

All of the standard key management systems are supported. Payloads are signed using the DSSE signing spec, defined here.

To verify:

$ cosign verify-attestation --key cosign.pub $IMAGE_URI

Detailed Usage

See the Usage documentation for more information.

Hardware-based Tokens

See the Hardware Tokens documentation for information on how to use cosign with hardware.

Registry Support

cosign uses go-containerregistry for registry interactions, which has generally excellent compatibility, but some registries may have quirks.

Today, cosign has been tested and works against the following registries:

• AWS Elastic Container Registry
• GCP’s Artifact Registry and Container Registry
• Docker Hub
• Azure Container Registry
• JFrog Artifactory Container Registry
• The CNCF distribution/distribution Registry
• GitLab Container Registry
• GitHub Container Registry
• The CNCF Harbor Registry
• Digital Ocean Container Registry
• Sonatype Nexus Container Registry
• Alibaba Cloud Container Registry
• Red Hat Quay Container Registry 3.6+ / Red Hat quay.io
• Elastic Container Registry
• IBM Cloud Container Registry
• Cloudsmith Container Registry
• The CNCF zot Registry
• OVHcloud Managed Private Registry

We aim for wide registry support. To sign images in registries which do not yet fully support OCI media types, one may need to use COSIGN_DOCKER_MEDIA_TYPES to fall back to legacy equivalents. For example:

COSIGN_DOCKER_MEDIA_TYPES=1 cosign sign --key cosign.key legacy-registry.example.com/my/image@$DIGEST

Please help test and file bugs if you see issues! Instructions can be found in the tracking issue.

Caveats

Intentionally Missing Features

cosign only generates ECDSA-P256 keys and uses SHA256 hashes, for both ephemeral keyless signing and managed key signing. Keys are stored in PEM-encoded PKCS8 format. However, you can use cosign to store and retrieve signatures in any format, from any algorithm.

Things That Should Probably Change

Payload Formats

cosign only supports Red Hat’s simple signing format for payloads. That looks like:

{
    "critical": {
           "identity": {
               "docker-reference": "testing/manifest"
           },
           "image": {
               "Docker-manifest-digest": "sha256:20be...fe55"
           },
           "type": "cosign container image signature"
    },
    "optional": {
           "creator": "Bob the Builder",
           "timestamp": 1458239713
    }
}

Note: This can be generated for an image reference using cosign generate $IMAGE_URI_DIGEST.

I’m happy to switch this format to something else if it makes sense. See https://github.com/notaryproject/notation/issues/40 for one option.

Registry Details

cosign signatures are stored as separate objects in the OCI registry, with only a weak reference back to the object they “sign”. This means this relationship is opaque to the registry, and signatures will not be deleted or garbage-collected when the image is deleted. Similarly, they can easily be copied from one environment to another, but this is not automatic.

Multiple signatures are stored in a list which is unfortunately a race condition today. To add a signature, clients orchestrate a “read-append-write” operation, so the last write will win in the case of contention.

Specifying Registry

cosign will default to storing signatures in the same repo as the image it is signing. To specify a different repo for signatures, you can set the COSIGN_REPOSITORY environment variable.

This will replace the repo in the provided image like this:

$ export COSIGN_REPOSITORY=gcr.io/my-new-repo
$ cosign sign --key cosign.key $IMAGE_URI_DIGEST

So the signature for gcr.io/dlorenc-vmtest2/demo will be stored in gcr.io/my-new-repo/demo:sha256-DIGEST.sig.

Note: different registries might expect different formats for the “repository.”

• To use GCR, a registry name like gcr.io/$REPO is sufficient, as in the example above.

• To use Artifact Registry, specify a full image name like $LOCATION-docker.pkg.dev/$PROJECT/$REPO/$STORAGE_IMAGE, not just a repository. For example,

  $ export COSIGN_REPOSITORY=us-docker.pkg.dev/my-new-repo/demo
  $ cosign sign --key cosign.key $IMAGE_URI_DIGEST

  where the sha256-DIGEST will match the digest for gcr.io/dlorenc-vmtest2/demo. Specifying just a repo like $LOCATION-docker.pkg.dev/$PROJECT/$REPO will not work in Artifact Registry.

Signature Specification

cosign is inspired by tools like minisign and signify.

Generated private keys are stored in PEM format. The keys encrypted under a password using scrypt as a KDF and nacl/secretbox for encryption.

They have a PEM header of ENCRYPTED SIGSTORE PRIVATE KEY:

-----BEGIN ENCRYPTED SIGSTORE PRIVATE KEY-----
...
-----END ENCRYPTED SIGSTORE PRIVATE KEY-----

Public keys are stored on disk in PEM-encoded standard PKIX format with a header of PUBLIC KEY.

-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAELigCnlLNKgOglRTx1D7JhI7eRw99
QolE9Jo4QUxnbMy5nUuBL+UZF9qqfm/Dg1BNeHRThHzWh2ki9vAEgWEDOw==
-----END PUBLIC KEY-----

Storage Specification

cosign stores signatures in an OCI registry, and uses a naming convention (tag based on the sha256 of what we’re signing) for locating the signature index.

reg.example.com/ubuntu@sha256:703218c0465075f4425e58fac086e09e1de5c340b12976ab9eb8ad26615c3715 has signatures located at reg.example.com/ubuntu:sha256-703218c0465075f4425e58fac086e09e1de5c340b12976ab9eb8ad26615c3715.sig

Roughly (ignoring ports in the hostname): s/:/-/g and s/@/:/g to find the signature index.

See Race conditions for some caveats around this strategy.

Alternative implementations could use transparency logs, local filesystem, a separate repository registry, an explicit reference to a signature index, a new registry API, grafeas, etc.

Signing subjects

cosign only works for artifacts stored as “manifests” in the registry today. The proposed mechanism is flexible enough to support signing arbitrary things.

KMS Support

cosign supports using a KMS provider to generate and sign keys. Right now cosign supports Hashicorp Vault, AWS KMS, GCP KMS, Azure Key Vault and we are hoping to support more in the future!

Additional KMS providers are available as external plugins, such as OVHcloud KMS.

See the KMS docs for more details.

OCI Artifacts

Push an artifact to a registry using oras (in this case, cosign itself!):

$ oras push us-central1-docker.pkg.dev/dlorenc-vmtest2/test/artifact ./cosign
Uploading f53604826795 cosign
Pushed us-central1-docker.pkg.dev/dlorenc-vmtest2/test/artifact
Digest: sha256:551e6cce7ed2e5c914998f931b277bc879e675b74843e6f29bc17f3b5f692bef

Now sign it! Using cosign of course:

$ cosign sign --key cosign.key us-central1-docker.pkg.dev/dlorenc-vmtest2/test/artifact@sha256:551e6cce7ed2e5c914998f931b277bc879e675b74843e6f29bc17f3b5f692bef
Enter password for private key:
Pushing signature to: us-central1-docker.pkg.dev/dlorenc-vmtest2/test/artifact:sha256-551e6cce7ed2e5c914998f931b277bc879e675b74843e6f29bc17f3b5f692bef.sig

Finally, verify cosign with cosign again:

$ cosign verify --key cosign.pub  us-central1-docker.pkg.dev/dlorenc-vmtest2/test/artifact@sha256:551e6cce7ed2e5c914998f931b277bc879e675b74843e6f29bc17f3b5f692bef
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - The claims were present in the transparency log
  - The signatures were integrated into the transparency log when the certificate was valid
  - The signatures were verified against the specified public key
  - The code-signing certificate was verified using trusted certificate authority certificates

{"Critical":{"Identity":{"docker-reference":""},"Image":{"Docker-manifest-digest":"sha256:551e6cce7ed2e5c914998f931b277bc879e675b74843e6f29bc17f3b5f692bef"},"Type":"cosign container image signature"},"Optional":null}

FAQ

Why not use Notary v2

It’s hard to answer this briefly. This post contains some comparisons:

Notary V2 and Cosign

If you find other comparison posts, please send a PR here and we’ll link them all.

Why not use containers/image signing

containers/image signing is close to cosign, and we reuse payload formats. cosign differs in that it signs with ECDSA-P256 keys instead of PGP, and stores signatures in the registry.

Why not use TUF?

I believe this tool is complementary to TUF, and they can be used together. I haven’t tried yet, but think we can also reuse a registry for TUF storage.

Design Requirements

• No external services for signature storage, querying, or retrieval
• We aim for as much registry support as possible
• Everything should work over the registry API
• PGP should not be required at all.
• Users must be able to find all signatures for an image
• Signers can sign an image after push
• Multiple entities can sign an image
• Signing an image does not mutate the image
• Pure-go implementation

Future Ideas

Registry API Changes

The naming convention and read-modify-write update patterns we use to store things in a registry are a bit, well, “hacky”. I think they’re the best (only) real option available today, but if the registry API changes we can improve these.

Other Types

cosign can sign anything in a registry. These examples show signing a single image, but you could also sign a multi-platform Index, or any other type of artifact. This includes Helm Charts, Tekton Pipelines, and anything else currently using OCI registries for distribution.

This also means new artifact types can be uploaded to a registry and signed. One interesting type to store and sign would be TUF repositories. I haven’t tried yet, but I’m fairly certain TUF could be implemented on top of this.

Tag Signing

cosign signatures protect the digests of objects stored in a registry. The optional annotations support (via the -a flag to cosign sign) can be used to add extra data to the payload that is signed and protected by the signature. One use-case for this might be to sign a tag->digest mapping.

If you would like to attest that a specific tag (or set of tags) should point at a digest, you can run something like:

$ docker push $IMAGE_URI
The push refers to repository [dlorenc/demo]
994393dc58e7: Pushed
5m: digest: sha256:1304f174557314a7ed9eddb4eab12fed12cb0cd9809e4c28f29af86979a3c870 size: 528
$ TAG=sign-me
$ cosign sign --key cosign.key -a tag=$TAG $IMAGE_URI_DIGEST
Enter password for private key:
Pushing signature to: dlorenc/demo:1304f174557314a7ed9eddb4eab12fed12cb0cd9809e4c28f29af86979a3c870.sig

Then you can verify that the tag->digest mapping is also covered in the signature, using the -a flag to cosign verify. This example verifies that the digest $TAG which points to (sha256:1304f174557314a7ed9eddb4eab12fed12cb0cd9809e4c28f29af86979a3c870) has been signed, and also that the tag annotation has the value sign-me:

$ cosign verify --key cosign.pub -a tag=$TAG $IMAGE_URI | jq .
{
  "Critical": {
    "Identity": {
      "docker-reference": ""
    },
    "Image": {
      "Docker-manifest-digest": "97fc222cee7991b5b061d4d4afdb5f3428fcb0c9054e1690313786befa1e4e36"
    },
    "Type": "cosign container image signature"
  },
  "Optional": {
    "tag": "sign-me"
  }
}

Timestamps could also be added here, to implement TUF-style freeze-attack prevention.

Base Image/Layer Signing

Again, cosign can sign anything in a registry. You could use cosign to sign an image that is intended to be used as a base image, and include that provenance metadata in resulting derived images. This could be used to enforce that an image was built from an authorized base image.

Rough Idea:

• OCI manifests have an ordered list of layer Descriptors, which can contain annotations. See here for the specification.
• A base image is an ordered list of layers to which other layers are appended, as well as an initial configuration object that is mutated.
  • A derived image is free to completely delete/destroy/recreate the config from its base image, so signing the config would provided limited value.
• We can sign the full set of ordered base layers, and attach that signature as an annotation to the last layer in the resulting child image.

This example manifest manifest represents an image that has been built from a base image with two layers. One additional layer is added, forming the final image.

{
  "schemaVersion": 2,
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 7023,
    "digest": "sha256:b5b2b2c507a0944348e0303114d8d93aaaa081732b86451d9bce1f432a537bc7"
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "size": 32654,
      "digest": "sha256:9834876dcfb05cb167a5c24953eba58c4ac89b1adf57f28f2f9d09af107ee8f0"
    },
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "size": 16724,
      "digest": "sha256:3c3a4604a545cdc127456d94e421cd355bca5b528f4a9c1905b15da2eb4a4c6b",
      "annotations": {
        "dev.cosign.signature.baseimage": "Ejy6ipGJjUzMDoQFePWixqPBYF0iSnIvpMWps3mlcYNSEcRRZelL7GzimKXaMjxfhy5bshNGvDT5QoUJ0tqUAg=="
      }
    },
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar+gzip",
      "size": 73109,
      "digest": "sha256:ec4b8955958665577945c89419d1af06b5f7636b4ac3da7f12184802ad867736"
    }
  ],
}

Note that this could be applied recursively, for multiple intermediate base images.

Counter-Signing

Cosign signatures (and their protected payloads) are stored as artifacts in a registry. These signature objects can also be signed, resulting in a new, “counter-signature” artifact. This “counter-signature” protects the signature (or set of signatures) and the referenced artifact, which allows it to act as an attestation to the signature(s) themselves.

Before we sign the signature artifact, we first give it a memorable name so we can find it later.

$ cosign sign --key cosign.key -a sig=original $IMAGE_URI_DIGEST
Enter password for private key:
Pushing signature to: dlorenc/demo:sha256-97fc222cee7991b5b061d4d4afdb5f3428fcb0c9054e1690313786befa1e4e36.sig
$ cosign verify --key cosign.pub dlorenc/demo | jq .
{
  "Critical": {
    "Identity": {
      "docker-reference": ""
    },
    "Image": {
      "Docker-manifest-digest": "97fc222cee7991b5b061d4d4afdb5f3428fcb0c9054e1690313786befa1e4e36"
    },
    "Type": "cosign container image signature"
  },
  "Optional": {
    "sig": "original"
  }
}

Now give that signature a memorable name, then sign that:

$ crane tag $(cosign triangulate $IMAGE_URI) mysignature
2021/02/15 20:22:55 dlorenc/demo:mysignature: digest: sha256:71f70e5d29bde87f988740665257c35b1c6f52dafa20fab4ba16b3b1f4c6ba0e size: 556
$ cosign sign --key cosign.key -a sig=counter dlorenc/demo:mysignature
Enter password for private key:
Pushing signature to: dlorenc/demo:sha256-71f70e5d29bde87f988740665257c35b1c6f52dafa20fab4ba16b3b1f4c6ba0e.sig
$ cosign verify --key cosign.pub dlorenc/demo:mysignature
{"Critical":{"Identity":{"docker-reference":""},"Image":{"Docker-manifest-digest":"71f70e5d29bde87f988740665257c35b1c6f52dafa20fab4ba16b3b1f4c6ba0e"},"Type":"cosign container image signature"},"Optional":{"sig":"counter"}}

Finally, check the original signature:

$ crane manifest dlorenc/demo@sha256:71f70e5d29bde87f988740665257c35b1c6f52dafa20fab4ba16b3b1f4c6ba0e
{
  "schemaVersion": 2,
  "config": {
    "mediaType": "application/vnd.oci.image.config.v1+json",
    "size": 233,
    "digest": "sha256:3b25a088710d03f39be26629d22eb68cd277a01673b9cb461c4c24fbf8c81c89"
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.descriptor.v1+json",
      "size": 217,
      "digest": "sha256:0e79a356609f038089088ec46fd95f4649d04de989487220b1a0adbcc63fadae",
      "annotations": {
        "dev.sigstore.cosign/signature": "5uNZKEP9rm8zxAL0VVX7McMmyArzLqtxMTNPjPO2ns+5GJpBeXg+i9ILU+WjmGAKBCqiexTxzLC1/nkOzD4cDA=="
      }
    }
  ]
}

Release Cadence

We cut releases as needed. Patch releases are cut to fix small bugs. Minor releases are cut periodically when there are multiple bugs fixed or features added. Major releases will be released when there are breaking features.

Security

Should you discover any security issues, please refer to sigstore’s security process

Bundle files in GitHub Release Assets

The GitHub release assets for cosign contain Sigstore bundle files produced by GoReleaser while signing the cosign blob that is used to verify the integrity of the release binaries. This file is not used by cosign itself, but is provided for users who wish to verify the integrity of the release binaries.