This document aims to describe how cosign attaches Sigstore attestation
bundles
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 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.
The approach for storing Sigstore bundles in an OCI registry follows the guidelines for artifact usage in the OCI image spec.
First, the bundle itself is stored in it's 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",
"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 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.
At this point, any registry which supports the referrers API 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.
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.
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 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": {...}
}
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:
{
"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 help disambiguate attestations, clients may add annotations to the items
in the manifests list which indicate what is contained within each bundle:
dev.sigstore.cosign.bundle.content- Must be one "message-signature" or "dsse-envelope" and should match the type of content embedded in the Sigstore bundle.dev.sigstore.cosign.bundle.predicateType- When the bundle contains a DSSE-wrapped in-toto statement, the statement's predicate can be reflected here.
{
"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.bundle.content": "message-signature"
}
},
{
"artifactType": "application/vnd.dev.sigstore.bundle+json;version=0.2",
"digest": "sha256:d0d0caca",
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"annotations": {
"dev.sigstore.bundle.content": "dsse-envelope",
"dev.sigstore.bundle.predicateType": "https://slsa.dev/provenance/v1"
}
}
]
}