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https://github.com/sigstore/cosign

Code signing and transparency for containers and binaries
https://github.com/sigstore/cosign

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Code signing and transparency for containers and binaries

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  Cosign logo




# cosign



Signing OCI containers (and other artifacts) using [Sigstore](https://sigstore.dev/)!



[![Go Report Card](https://goreportcard.com/badge/github.com/sigstore/cosign)](https://goreportcard.com/report/github.com/sigstore/cosign)

[![e2e-tests](https://github.com/sigstore/cosign/actions/workflows/e2e-tests.yml/badge.svg)](https://github.com/sigstore/cosign/actions/workflows/e2e-tests.yml)

[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5715/badge)](https://bestpractices.coreinfrastructure.org/projects/5715)

[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/sigstore/cosign/badge)](https://securityscorecards.dev/viewer/?uri=github.com/sigstore/cosign)



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`](https://sigstore.dev) project.

We also use a [slack channel](https://sigstore.slack.com)!

Click [here](https://join.slack.com/t/sigstore/shared_invite/zt-mhs55zh0-XmY3bcfWn4XEyMqUUutbUQ) for the invite link.



## Installation



For Homebrew, Arch, Nix, GitHub Action, and Kubernetes installs see the [installation docs](https://docs.sigstore.dev/cosign/system_config/installation/).



For Linux and macOS binaries see the [GitHub release assets](https://github.com/sigstore/cosign/releases/latest).



:rotating_light: If you are downloading releases of cosign from our GCS bucket - please see more information on the July 31, 2023 [deprecation notice](https://blog.sigstore.dev/cosign-releases-bucket-deprecation/) :rotating_light:



## Developer Installation



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



```shell

$ 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](./CONTRIBUTING.md).



Future Cosign development will be focused the next major release which will be based on

[sigstore-go](https://github.com/sigstore/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](https://github.com/sigstore/sigstore-go/issues) 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:



```shell

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



# Source: https://github.com/chainguard-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](https://docs.sigstore.dev/cosign/signing/overview/))

* verify the container image



### 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!



```shell

 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.



```shell

$ 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



Cosign can do completely offline verification by verifying a [bundle](./specs/SIGNATURE_SPEC.md#properties) 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 --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

```



### What ** is not ** production ready?



While parts of `cosign` are stable, we are continuing to experiment and add new features.

The following feature set is not considered stable yet, but we are committed to stabilizing it over time!



#### Formats/Specifications



While the `cosign` code for uploading, signing, retrieving, and verifying several artifact types is stable,

the format specifications for some of those types may not be considered stable yet.

Some of these are developed outside of the `cosign` project, so we are waiting for them to stabilize first.



These include:



* The SBOM specification for storing SBOMs in a container registry

* The In-Toto attestation format



## 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](https://docs.sigstore.dev/cosign/signing/other_types/) for more information.



### Blobs



You can publish an artifact with `cosign upload blob`:



```shell

$ 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:



```shell

$ 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:



```shell

$ cat artifact-fetched | shasum -a 256

c69d72c98b55258f9026f984e4656f0e9fd3ef024ea3fac1d7e5c7e6249f1626  -

```



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



```shell

$ 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](https://tekton.dev) bundles can be uploaded and managed within an OCI registry.

The specification is [here](https://tekton.dev/docs/pipelines/tekton-bundle-contracts/).

This means they can also be signed and verified with `cosign`.



Tekton Bundles can currently be uploaded with the [tkn cli](https://github.com/tektoncd/cli), but we may add this support to

`cosign` in the future.



```shell

$ 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](https://github.com/solo-io/wasm/tree/master/spec).



Cosign can upload these using the `cosign wasm upload` command:



```shell

$ 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](https://ebpf.io) modules can also be stored in an OCI registry, using this [specification](https://github.com/solo-io/bumblebee/tree/main/spec).



The image below was built using the `bee` tool. More information can be found [here](https://github.com/solo-io/bumblebee/)



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



```shell

$ 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](https://in-toto.io) attestations.

The specification for these is defined [here](https://github.com/in-toto/attestation).



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



```shell

$ cosign attest --predicate  --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](https://github.com/secure-systems-lab/dsse).



To verify:



```shell

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

```



## Detailed Usage



See the [Usage documentation](https://docs.sigstore.dev/cosign/signing/overview/) for more information.



## Hardware-based Tokens



See the [Hardware Tokens documentation](https://docs.sigstore.dev/cosign/key_management/hardware-based-tokens/) for information on how to use `cosign` with hardware.



## Registry Support



`cosign` uses [go-containerregistry](https://github.com/google/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](https://github.com/sigstore/cosign/blob/main/specs/SIGNATURE_SPEC.md), one may need to use `COSIGN_DOCKER_MEDIA_TYPES` to fall back to legacy equivalents. For example:



```shell

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](https://github.com/sigstore/cosign/issues/40).



## 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](https://www.redhat.com/en/blog/container-image-signing)

format for payloads.

That looks like:



```json

{

    "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/nv2/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:



```shell

$ 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](https://cloud.google.com/container-registry), a registry name

  like `gcr.io/$REPO` is sufficient, as in the example above.

* To use [Artifact Registry](https://cloud.google.com/artifact-registry),

  specify a full image name like

  `$LOCATION-docker.pkg.dev/$PROJECT/$REPO/$STORAGE_IMAGE`, not just a

  repository. For example,



  ```shell

  $ 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](https://jedisct1.github.io/minisign/) and

[signify](https://www.openbsd.org/papers/bsdcan-signify.html).



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`:



```shell

-----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](#registry-details) 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!



See the [KMS docs](https://docs.sigstore.dev/cosign/key_management/overview/) for more details.



### OCI Artifacts



Push an artifact to a registry using [oras](https://github.com/deislabs/oras) (in this case, `cosign` itself!):



```shell

$ 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:



```shell

$ 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:



```shell

$ 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](https://medium.com/@dlorenc/notary-v2-and-cosign-b816658f044d)



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:



```shell

$ 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`:



```shell

$ 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](https://github.com/opencontainers/image-spec/blob/master/manifest.md) 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.



```json

{

  "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.



```shell

$ 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:



```shell

$ 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:



```shell

$ 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](https://github.com/sigstore/.github/blob/main/SECURITY.md)



## PEM files in GitHub Release Assets



The GitHub release assets for cosign contain a PEM file produced by [GoReleaser](https://github.com/sigstore/cosign/blob/ac999344eb381ae91455b0a9c5c267e747608d76/.goreleaser.yml#L166) 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.



By default, cosign output these PEM files in [base64 encoded format](https://github.com/sigstore/cosign/blob/main/doc/cosign_sign-blob.md#options), this approach might be good for air-gapped environments where the PEM file is stored in a file system. So, you should decode these PEM files before using them to verify the blobs.