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add oci bundle spec #3622

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add oci bundle spec #3622

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add oci bundle spec
bdehamer Mar 22, 2024
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Update specs/BUNDLE_SPEC.md
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Update specs/BUNDLE_SPEC.md
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Update specs/BUNDLE_SPEC.md
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clarify annotation scheme
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add signer annotation
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271 changes: 271 additions & 0 deletions specs/BUNDLE_SPEC.md
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# Cosign Bundle Specification

This document aims to describe how `cosign` attaches Sigstore attestation
[bundles](https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_bundle.proto)
to container images.

The goal is to specify the behavior well enough to promote other implementations
and enable interoperability. Attestations attached with `cosign` should be
retrievable in other tools, and vice-versa.

This document focuses on the layout of attestations within an
[OCI Image Manifest V1.1](https://github.com/opencontainers/image-spec/blob/v1.1.0/manifest.md)
object.

This document makes no assumptions about the contents of the Sigstore bundle.
Any attestation which can be represented as a Sigstore bundle (message
signatures, DSSE-wrapped in-toto statements, etc) can be attached to a container
image stored in an OCI registry.

Multiple Attestations may be "attached" to one image.

Attestations attached to a container image are generally assumed to refer to
that image in some way.

## Storage

The approach for storing Sigstore bundles in an OCI registry follows the
[guidelines for artifact usage](https://github.com/opencontainers/image-spec/blob/main/manifest.md#guidelines-for-artifact-usage)
in the OCI
[image spec](https://github.com/opencontainers/image-spec/blob/main/README.md).

### Publishing

First, the bundle itself is stored in its JSON-serialized form as a blob in the
registry:

```
POST /v2/foo/blobs/uploads/?digest=cafed00d...
Content-Type: application/octet-stream

{"mediaType":"application/vnd.dev.sigstore.bundle+json;version=0.2", ...}
```

In this example “foo” is the name of the repository within the registry to which
the artifact is being uploaded. The digest included as part of the POST is the
hex-encoded SHA-256 digest of the raw bytes of the bundle itself.

Once the blob has been created, the next step is to create a manifest that
associates the bundle blob with the image it describes:

```
PUT /v2/foo/manifests/sha256:badf00d...
Content-Type: application/vnd.oci.image.manifest.v1+json

{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"schemaVersion": 2,
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
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"config": {
"mediaType": "application/vnd.oci.empty.v1+json",
"digest": "sha256:44136fa3...",
"size": 2
},
"layers": [
{
"digest": "sha256:cafed00d...",
"mediaType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
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"size": 4971
}
],
"subject": {
"digest": "sha256:c00010ff...",
"mediaType": "application/vnd.oci.image.index.v1+json"
}
}
```

The manifest must have an `artifactType` field which identifies the type of the
artifact being referenced -- in this case, it's the Sigstore bundle media type.

The `layers` collection will have a single entry that points to the bundle's
blob by referencing its size, digest and media type.

The `subject` field associates this artifact with some other artifact which
already exists in this repository (in this case, an image with the digest
`c00010ff`)

Sigstore bundles don't require any additional configuration data, so the
`config` field references the
[empty descriptor](https://github.com/opencontainers/image-spec/blob/f5f87016de46439ccf91b5381cf76faaae2bc28f/manifest.md#guidance-for-an-empty-descriptor).

At this point, any registry which supports the
[referrers API](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers)
will automatically associate this manifest with the listed subject and make it
available in the referrers index for that subject.

If the registry DOES NOT support the referrers API, a referrers list must be
manually created/updated using the
[referrers tag scheme](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#referrers-tag-schema).

```
PUT /v2/foo/manifests/sha256-c00010ff...
Content-Type: application/vnd.oci.image.index.v1+json

{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"digest": "sha256:badf00d..",
"size": 779,
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2"
}
]
}
```

This index is uploaded with a tag that references the digest of the image to
which all of the listed artifacts are associated. Each of the items in the
`manifests` collection points to some other related artifact.

### Retrieval

When a client wants to locate Sigstore bundles which may be associated with a
given image, they would first make a request to
[referrers API](https://github.com/opencontainers/distribution-spec/blob/main/spec.md#listing-referrers)
with the image's digest:

```
GET /v2/foo/referrers/sha256:c000100ff...
```

A `404 Not Found` response indicates that the registry does not support the
referrers API and the referrers tag scheme should be used as a fallback:

```
GET /v2/foo/manifests/sha256-c000100ff...
```

A `404` here would indicate that there are no artifacts associated with the
image.

Assuming there are artifacts present, one of the two above calls will return an
image index listing the artifacts which have been associated with the specified
image:

```
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"digest": "sha256:badf00d..",
"size": 779,
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2"
}
]
}
```

From this the client can identify any Sigstore bundles by looking at the
`artifactType` field.

Using the `digest` listed in the image index, the next step is to retrieve the
manifest for the bundle:

```
GET /v2/foo/manifests/sha256:badf00d..
```

```
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"schemaVersion": 2,
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"config": {
"mediaType": "application/vnd.oci.empty.v1+json",
"digest": "sha256:44136fa3...",
"size": 2
},
"layers": [
{
"digest": "sha256:cafed00d...",
"mediaType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"size": 4971
}
],
"subject": {
"digest": "sha256:c00010ff...",
"mediaType": "application/vnd.oci.image.index.v1+json"
}
}
```

The final step is to use the `digest` from the first of the `layers` to retrieve
the bundle blob:

```
GET /v2/foo/blobs/uploads/?digest=cafed00d...
```

```
{
"mediaType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"verificationMaterial": {...},
"messageSignature": {...}
}
```

## Annotations

For any given image, there may be any number of attached attestation bundles.
When there are multiple Sigstore bundles associated with an image it may be
difficult to identify which artifact is which in the image index:

```json
{
"mediaType": "application/vnd.oci.image.index.v1+json",
"schemaVersion": 2,
"manifests": [
{
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"digest": "sha256:facefeed",
"mediaType": "application/vnd.oci.image.manifest.v1+json"
},
{
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"digest": "sha256:d0d0caca",
"mediaType": "application/vnd.oci.image.manifest.v1+json"
}
]
}
```

To help disambiguate attestations, clients may add annotations to the items
in the `manifests` list which indicate what is contained within each bundle:
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Other fields I'd recommend including are the creation/signing date, and the identity of the signer. This is useful for quickly finding the most recently signed content from a trusted identity. OCI already has an annotation for the creation date that I'd recommend reusing: org.opencontainers.image.created.

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@sudo-bmitch I like the idea of an annotation to identify the signer. Is there an pre-defined annotation key you'd recommend for this or should we define our own?

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I'm not aware of anything predefined, so making your own under the sigstore namespace makes sense to me.

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@bdehamer bdehamer Mar 26, 2024 •
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Update the list of recommended annotations to include:

  • org.opencontainers.image.created

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Ideally, would stick to OCI annotations - just makes it more portable. If you expect images and their signatures to move around, then a arbitrary registry implementation doesn't need to know about dev.sigstore.* namespace.


- `dev.sigstore.cosign.bundle.content` - Must be one "message-signature" or
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"dsse-envelope" and should match the type of content embedded in the Sigstore
bundle.
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- `dev.sigstore.cosign.bundle.predicateType` - When the bundle contains a
DSSE-wrapped in-toto statement, the statement's predicate can be reflected
here.

```json
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Just jotting this down here. Wondering if it would be useful to have a an example out there demonstrating a fully compliant spec?

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@bdehamer bdehamer Mar 28, 2024 •
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I don't know that I want to reference this in the spec, but I do have an example at index.docker.io/bdehamer/hello:latest.

You can poke at this with the oras CLI. Look-up an referring artifacts:

oras discover index.docker.io/bdehamer/hello:latest

Discovered 1 artifact referencing latest
Digest: sha256:01b2325c7cae9939e4484061c37d36e0b95fb3f5e66f80ff924582ba5939e831

Artifact Type                                   Digest
application/vnd.dev.sigstore.bundle.v3.0+json   sha256:30bb112189b0070d8c440fb0c9ef13d4ff25014ccc64d90e5839f90b99c81779

Fetch the bundle manifest:

oras manifest fetch index.docker.io/bdehamer/hello@sha256:30bb112189b0070d8c440fb0c9ef13d4ff25014ccc64d90e5839f90b99c81779

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "artifactType": "application/vnd.dev.sigstore.bundle.v3.0+json",
  "config": {
    "mediaType": "application/vnd.oci.empty.v1+json",
    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a",
    "size": 2,
    "data": "e30="
  },
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.layer.v1.tar",
      "digest": "sha256:0e6245a3f020384b3b10aa2f4fe838ce5e87a71038e710938875cc22d6adb917",
      "size": 10097,
      "annotations": {
        "org.opencontainers.image.title": "bundle.json"
      }
    }
  ],
  "subject": {
    "mediaType": "application/vnd.oci.image.index.v1+json",
    "digest": "sha256:01b2325c7cae9939e4484061c37d36e0b95fb3f5e66f80ff924582ba5939e831",
    "size": 855
  },
  "annotations": {
    "dev.sigstore.bundle.predicateType": "https://slsa.dev/provenance/v1",
    "dev.sigstore.cosign.bundle.content": "dsse-envelope",
    "org.opencontainers.image.created": "2024-03-28T19:44:17Z"
  }
}

Fetch the bundle:

oras blob fetch index.docker.io/bdehamer/hello@sha256:0e6245a3f020384b3b10aa2f4fe838ce5e87a71038e710938875cc22d6adb917 --output -

{"mediaType":"application/vnd.dev.sigstore.bundle.v3.0+json"...}

{
"mediaType": "application/vnd.oci.image.index.v1+json",
"schemaVersion": 2,
"manifests": [
{
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"digest": "sha256:facefeed",
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"annotations": {
"dev.sigstore.cosign.bundle.content": "message-signature"
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}
},
{
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"digest": "sha256:d0d0caca",
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"annotations": {
"dev.sigstore.cosign.bundle.content": "dsse-envelope",
"dev.sigstore.cosign.bundle.predicateType": "https://slsa.dev/provenance/v1"
}
}
]
}
```
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