containers-policy.json - syntax for the signature verification policy file
## DESCRIPTION
Signature verification policy files are used to specify policy, e.g. trusted keys,
applicable when deciding whether to accept an image, or individual signatures of that image, as valid.
By default, the policy is read from `$HOME/.config/containers/policy.json`, if it exists, otherwise from `/etc/containers/policy.json`; applications performing verification may allow using a different policy instead.
## FORMAT
The signature verification policy file, usually called `policy.json`,
uses a JSON format. Unlike some other JSON files, its parsing is fairly strict:
unrecognized, duplicated or otherwise invalid fields cause the entire file,
and usually the entire operation, to be rejected.
The purpose of the policy file is to define a set of *policy requirements* for a container image,
usually depending on its location (where it is being pulled from) or otherwise defined identity.
Policy requirements can be defined for:
- An individual *scope* in a *transport*.
The *transport* values are the same as the transport prefixes when pushing/pulling images (e.g. `docker:`, `atomic:`),
and *scope* values are defined by each transport; see below for more details.
Usually, a scope can be defined to match a single image, and various prefixes of
such a most specific scope define namespaces of matching images.
This requirement requires an image to be signed using “simple signing” with an expected identity, or accepts a signature if it is using an expected identity and key.
Exactly one of `keyPath`, `keyPaths` and `keyData` must be present, containing a GPG keyring of one or more public keys. Only signatures made by these keys are accepted.
The `signedIdentity` field, a JSON object, specifies what image identity the signature claims about the image.
One of the following alternatives are supported:
- The identity in the signature must exactly match the image identity. Note that with this, referencing an image by digest (with a signature claiming a _repository_`:`_tag_ identity) will fail.
```json
{"type":"matchExact"}
```
- If the image identity carries a tag, the identity in the signature must exactly match;
if the image identity uses a digest reference, the identity in the signature must be in the same repository as the image identity (using any tag).
(Note that with images identified using digest references, the digest from the reference is validated even before signature verification starts.)
```json
{"type":"matchRepoDigestOrExact"}
```
- The identity in the signature must be in the same repository as the image identity. This is useful e.g. to pull an image using the `:latest` tag when the image is signed with a tag specifying an exact image version.
```json
{"type":"matchRepository"}
```
- The identity in the signature must exactly match a specified identity.
This is useful e.g. when locally mirroring images signed using their public identity.
```js
{
"type": "exactReference",
"dockerReference": docker_reference_value
}
```
- The identity in the signature must be in the same repository as a specified identity.
This combines the properties of `matchRepository` and `exactReference`.
```js
{
"type": "exactRepository",
"dockerRepository": docker_repository_value
}
```
- Prefix remapping:
If the image identity matches the specified prefix, that prefix is replaced by the specified “signed prefix”
(otherwise it is used as unchanged and no remapping takes place);
matching then follows the `matchRepoDigestOrExact` semantics documented above
(i.e. if the image identity carries a tag, the identity in the signature must exactly match,
if it uses a digest reference, the repository must match).
The `prefix` and `signedPrefix` values can be either host[:port] values
(matching exactly the same host[:port], string),
repository namespaces, or repositories (i.e. they must not contain tags/digests),
and match as prefixes *of the fully expanded form*.
For example, `docker.io/library/busybox` (*not* `busybox`) to specify that single repository,
or `docker.io/library` (not an empty string) to specify the parent namespace of `docker.io/library/busybox`==`busybox`).
The `prefix` value is usually the same as the scope containing the parent `signedBy` requirement.
```js
{
"type": "remapIdentity",
"prefix": prefix,
"signedPrefix": prefix,
}
```
If the `signedIdentity` field is missing, it is treated as `matchRepoDigestOrExact`.
*Note*: `matchExact`, `matchRepoDigestOrExact` and `matchRepository` can be only used if a Docker-like image identity is
provided by the transport. In particular, the `dir:` and `oci:` transports can be only
The `signedIdentity` field has the same semantics as in the `signedBy` requirement described above.
Note that `cosign`-created signatures only contain a repository, so only `matchRepository` and `exactRepository` can be used to accept them (and that does not protect against substitution of a signed image with an unexpected tag).
To use this with images hosted on image registries, the `use-sigstore-attachments` option needs to be enabled for the relevant registry or repository in the client's containers-registries.d(5).
