add new OCI manifests for attestations

__tests__/ghcr-client.test.ts

@@ -182,7 +182,7 @@ function configureFetchMock(
   )
 }
 
-const testManifest: ociContainer.Manifest = {
+const testManifest: ociContainer.OCIImageManifest = {
   schemaVersion: 2,
   mediaType: 'application/vnd.oci.image.manifest.v1+json',
   artifactType: 'application/vnd.oci.image.manifest.v1+json',
@@ -526,8 +526,10 @@ function validateRequestConfig(url: string, config: any): void {
   }
 }
 
-function cloneLayers(layers: ociContainer.Layer[]): ociContainer.Layer[] {
+function cloneLayers(
-  const result: ociContainer.Layer[] = []
+  layers: ociContainer.Descriptor[]
+): ociContainer.Descriptor[] {
+  const result: ociContainer.Descriptor[] = []
   for (const layer of layers) {
     result.push({ ...layer }) // this is _NOT_ a deep clone
   }

__tests__/oci-container.test.ts

@@ -1,8 +1,168 @@
-import { createActionPackageManifest, sha256Digest } from '../src/oci-container'
+import {
+  createActionPackageManifest,
+  sha256Digest,
+  sizeInBytes,
+  OCIImageManifest,
+  createSigstoreAttestationManifest,
+  OCIIndexManifest,
+  createReferrerTagManifest
+} from '../src/oci-container'
 import { FileMetadata } from '../src/fs-helper'
 
+const createdTimestamp = '2021-01-01T00:00:00.000Z'
+
 describe('sha256Digest', () => {
   it('calculates the SHA256 digest of the provided manifest', () => {
+    const { manifest } = testActionPackageManifest()
+    const digest = sha256Digest(manifest)
+    const expectedDigest =
+      'sha256:dd8537ef913cf87e25064a074973ed2c62699f1dbd74d0dd78e85d394a5758b5'
+
+    expect(digest).toEqual(expectedDigest)
+  })
+})
+
+describe('size', () => {
+  it('returns the total size of the provided manifest', () => {
+    const { manifest } = testActionPackageManifest()
+    const size = sizeInBytes(manifest)
+    expect(size).toBe(1133)
+  })
+})
+
+describe('createActionPackageManifest', () => {
+  it('creates a manifest containing the provided information', () => {
+    const { manifest, zipFile, tarFile } = testActionPackageManifest()
+
+    const expectedJSON = `{
+            "schemaVersion": 2,
+            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+            "artifactType": "application/vnd.github.actions.package.v1+json",
+            "config": {
+                "mediaType":"application/vnd.oci.empty.v1+json",
+                "size":2,
+                "digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+            },
+            "layers":[
+                {
+                  "mediaType":"application/vnd.oci.empty.v1+json",
+                  "size":2,
+                  "digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+                },
+                {
+                    "mediaType":"application/vnd.github.actions.package.layer.v1.tar+gzip",
+                    "size":${tarFile.size},
+                    "digest":"${tarFile.sha256}",
+                    "annotations":{
+                        "org.opencontainers.image.title":"test-org-test-repo_1.2.3.tar.gz"
+                    }
+                },
+                {
+                    "mediaType":"application/vnd.github.actions.package.layer.v1.zip",
+                    "size":${zipFile.size},
+                    "digest":"${zipFile.sha256}",
+                    "annotations":{
+                        "org.opencontainers.image.title":"test-org-test-repo_1.2.3.zip"
+                    }
+                }
+            ],
+            "annotations":{
+                "org.opencontainers.image.created":"${createdTimestamp}",
+                "action.tar.gz.digest":"${tarFile.sha256}",
+                "action.zip.digest":"${zipFile.sha256}",
+                "com.github.package.type":"actions_oci_pkg",
+                "com.github.package.version":"1.2.3",
+                "com.github.source.repo.id":"123",
+                "com.github.source.repo.owner.id":"456",
+                "com.github.source.commit":"abc"
+            }
+        }`
+
+    const manifestJSON = JSON.stringify(manifest)
+    expect(manifestJSON).toEqual(expectedJSON.replace(/\s/g, ''))
+  })
+})
+
+describe('createSigstoreAttestationManifest', () => {
+  it('creates a manifest containing the provided information', () => {
+    const manifest = testAttestationManifest()
+
+    const expectedJSON = `{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.manifest.v1+json",
+  "artifactType": "application/vnd.dev.sigstore.bundle.v0.3+json",
+  "config": {
+    "mediaType": "application/vnd.oci.empty.v1+json",
+    "size": 2,
+    "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+  },
+  "layers": [
+    {
+      "mediaType": "application/vnd.dev.sigstore.bundle.v0.3+json",
+      "size": 10,
+      "digest": "bundleDigest"
+    }
+  ],
+  "subject": {
+    "mediaType": "application/vnd.oci.image.manifest.v1+json",
+    "size": 100,
+    "digest": "subjectDigest"
+  },
+  "annotations": {
+    "dev.sigstore.bundle.content": "dsse-envelope",
+    "dev.sigstore.bundle.predicateType": "https://slsa.dev/provenance/v1",
+    "com.github.package.type": "actions_oci_pkg_attestation",
+    "org.opencontainers.image.created": "2021-01-01T00:00:00.000Z"
+  }
+}
+`
+
+    const manifestJSON = JSON.stringify(manifest)
+
+    expect(manifestJSON).toEqual(expectedJSON.replace(/\s/g, ''))
+  })
+})
+
+describe('createReferrerIndexManifest', () => {
+  it('creates a manifest containing the provided information', () => {
+    const manifest = testReferrerIndexManifest()
+
+    const expectedJSON = `
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.index.v1+json",
+  "manifests": [
+    {
+      "mediaType": "application/vnd.oci.image.manifest.v1+json",
+      "artifactType": "application/vnd.dev.sigstore.bundle.v0.3+json",
+      "size": 100,
+      "digest": "attDigest",
+      "annotations": {
+        "com.github.package.type": "actions_oci_pkg_attestation",
+        "org.opencontainers.image.created": "2021-01-01T00:00:00.000Z",
+        "dev.sigstore.bundle.content": "dsse-envelope",
+        "dev.sigstore.bundle.predicateType": "https://slsa.dev/provenance/v1"
+      }
+    }
+  ],
+  "annotations": {
+    "com.github.package.type": "actions_oci_pkg_referrer_tag",
+    "org.opencontainers.image.created": "2021-01-01T00:00:00.000Z"
+  }
+}
+            `
+
+    const manifestJSON = JSON.stringify(manifest)
+
+    expect(manifestJSON).toEqual(expectedJSON.replace(/\s/g, ''))
+  })
+})
+
+function testActionPackageManifest(): {
+  manifest: OCIImageManifest
+  tarFile: FileMetadata
+  zipFile: FileMetadata
+} {
   const date = new Date('2021-01-01T00:00:00Z')
   const repo = 'test-org/test-repo'
   const version = '1.2.3'
@@ -31,98 +191,28 @@ describe('sha256Digest', () => {
     date
   )
 
-    const digest = sha256Digest(manifest)
+  return {
-    const expectedDigest =
+    manifest,
-      'sha256:dd8537ef913cf87e25064a074973ed2c62699f1dbd74d0dd78e85d394a5758b5'
+    tarFile,
-
+    zipFile
-    expect(digest).toEqual(expectedDigest)
-  })
-})
-
-describe('createActionPackageManifest', () => {
-  it('creates a manifest containing the provided information', () => {
-    const date = new Date()
-    const repo = 'test-org/test-repo'
-    const sanitizedRepo = 'test-org-test-repo'
-    const version = '1.2.3'
-    const repoId = '123'
-    const ownerId = '456'
-    const sourceCommit = 'abc'
-    const tarFile: FileMetadata = {
-      path: '/test/test/test.tar.gz',
-      sha256: 'tarSha',
-      size: 123
   }
-    const zipFile: FileMetadata = {
-      path: '/test/test/test.zip',
-      sha256: 'zipSha',
-      size: 456
 }
 
-    const expectedJSON = `{
+function testAttestationManifest(): OCIImageManifest {
-            "schemaVersion": 2,
+  return createSigstoreAttestationManifest(
-            "mediaType": "application/vnd.oci.image.manifest.v1+json",
+    10,
-            "artifactType": "application/vnd.github.actions.package.v1+json",
+    'bundleDigest',
-            "config": {
+    100,
-                "mediaType":"application/vnd.oci.empty.v1+json",
+    'subjectDigest',
-                "size":2,
+    new Date(createdTimestamp)
-                "digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
-            },
-            "layers":[
-                {
-                  "mediaType":"application/vnd.oci.empty.v1+json",
-                  "size":2,
-                  "digest":"sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
-                },
-                {
-                    "mediaType":"application/vnd.github.actions.package.layer.v1.tar+gzip",
-                    "size":${tarFile.size},
-                    "digest":"${tarFile.sha256}",
-                    "annotations":{
-                        "org.opencontainers.image.title":"${sanitizedRepo}_${version}.tar.gz"
-                    }
-                },
-                {
-                    "mediaType":"application/vnd.github.actions.package.layer.v1.zip",
-                    "size":${zipFile.size},
-                    "digest":"${zipFile.sha256}",
-                    "annotations":{
-                        "org.opencontainers.image.title":"${sanitizedRepo}_${version}.zip"
-                    }
-                }
-            ],
-            "annotations":{
-                "org.opencontainers.image.created":"${date.toISOString()}",
-                "action.tar.gz.digest":"${tarFile.sha256}",
-                "action.zip.digest":"${zipFile.sha256}",
-                "com.github.package.type":"actions_oci_pkg",
-                "com.github.package.version":"1.2.3",
-                "com.github.source.repo.id":"123",
-                "com.github.source.repo.owner.id":"456",
-                "com.github.source.commit":"abc"
-            }
-        }`
-
-    const manifest = createActionPackageManifest(
-      {
-        path: 'test.tar.gz',
-        size: tarFile.size,
-        sha256: tarFile.sha256
-      },
-      {
-        path: 'test.zip',
-        size: zipFile.size,
-        sha256: zipFile.sha256
-      },
-      repo,
-      repoId,
-      ownerId,
-      sourceCommit,
-      version,
-      date
   )
+}
 
-    const manifestJSON = JSON.stringify(manifest)
+function testReferrerIndexManifest(): OCIIndexManifest {
-    expect(manifestJSON).toEqual(expectedJSON.replace(/\s/g, ''))
+  return createReferrerTagManifest(
-  })
+    'attDigest',
-})
+    100,
+    new Date(createdTimestamp),
+    new Date(createdTimestamp)
+  )
+}

badges/coverage.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="116" height="20" role="img" aria-label="Coverage: 97.17%"><title>Coverage: 97.17%</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="116" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="63" height="20" fill="#555"/><rect x="63" width="53" height="20" fill="#4c1"/><rect width="116" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="325" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="530">Coverage</text><text x="325" y="140" transform="scale(.1)" fill="#fff" textLength="530">Coverage</text><text aria-hidden="true" x="885" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">97.17%</text><text x="885" y="140" transform="scale(.1)" fill="#fff" textLength="430">97.17%</text></g></svg>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="116" height="20" role="img" aria-label="Coverage: 97.39%"><title>Coverage: 97.39%</title><linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="r"><rect width="116" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#r)"><rect width="63" height="20" fill="#555"/><rect x="63" width="53" height="20" fill="#4c1"/><rect width="116" height="20" fill="url(#s)"/></g><g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" text-rendering="geometricPrecision" font-size="110"><text aria-hidden="true" x="325" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="530">Coverage</text><text x="325" y="140" transform="scale(.1)" fill="#fff" textLength="530">Coverage</text><text aria-hidden="true" x="885" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">97.39%</text><text x="885" y="140" transform="scale(.1)" fill="#fff" textLength="430">97.39%</text></g></svg>

dist/index.js

@@ -107754,25 +107754,40 @@ var __importStar = (this && this.__importStar) || function (mod) {
 };
 Object.defineProperty(exports, "__esModule", ({ value: true }));
 exports.createActionPackageManifest = createActionPackageManifest;
+exports.createSigstoreAttestationManifest = createSigstoreAttestationManifest;
+exports.createReferrerTagManifest = createReferrerTagManifest;
 exports.sha256Digest = sha256Digest;
+exports.sizeInBytes = sizeInBytes;
 const crypto = __importStar(__nccwpck_require__(6113));
+const imageIndexMediaType = 'application/vnd.oci.image.index.v1+json';
+const imageManifestMediaType = 'application/vnd.oci.image.manifest.v1+json';
+const actionsPackageMediaType = 'application/vnd.github.actions.package.v1+json';
+const actionsPackageTarLayerMediaType = 'application/vnd.github.actions.package.layer.v1.tar+gzip';
+const actionsPackageZipLayerMediaType = 'application/vnd.github.actions.package.layer.v1.zip';
+const sigstoreBundleMediaType = 'application/vnd.dev.sigstore.bundle.v0.3+json';
+const ociEmptyMediaType = 'application/vnd.oci.empty.v1+json';
+const actionPackageAnnotationValue = 'actions_oci_pkg';
+const actionPackageAttestationAnnotationValue = 'actions_oci_pkg_attestation';
+const actionPackageReferrerTagAnnotationValue = 'actions_oci_pkg_referrer_tag';
+const emptyConfigSize = 2;
+const emptyConfigSha = 'sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a';
 // Given a name and archive metadata, creates a manifest in the format expected by GHCR for an Actions Package.
-function createActionPackageManifest(tarFile, zipFile, repository, repoId, ownerId, sourceCommit, version, created) {
+function createActionPackageManifest(tarFile, zipFile, repository, repoId, ownerId, sourceCommit, version, created = new Date()) {
     const configLayer = createConfigLayer();
     const sanitizedRepo = sanitizeRepository(repository);
     const tarLayer = createTarLayer(tarFile, sanitizedRepo, version);
     const zipLayer = createZipLayer(zipFile, sanitizedRepo, version);
     const manifest = {
         schemaVersion: 2,
-        mediaType: 'application/vnd.oci.image.manifest.v1+json',
+        mediaType: imageManifestMediaType,
-        artifactType: 'application/vnd.github.actions.package.v1+json',
+        artifactType: actionsPackageMediaType,
         config: configLayer,
         layers: [configLayer, tarLayer, zipLayer],
         annotations: {
             'org.opencontainers.image.created': created.toISOString(),
             'action.tar.gz.digest': tarFile.sha256,
             'action.zip.digest': zipFile.sha256,
-            'com.github.package.type': 'actions_oci_pkg',
+            'com.github.package.type': actionPackageAnnotationValue,
             'com.github.package.version': version,
             'com.github.source.repo.id': repoId,
             'com.github.source.repo.owner.id': ownerId,
@@ -107781,6 +107796,59 @@ function createActionPackageManifest(tarFile, zipFile, repository, repoId, owner
     };
     return manifest;
 }
+function createSigstoreAttestationManifest(bundleSize, bundleDigest, subjectSize, subjectDigest, created = new Date()) {
+    const configLayer = createConfigLayer();
+    const sigstoreAttestationLayer = {
+        mediaType: sigstoreBundleMediaType,
+        size: bundleSize,
+        digest: bundleDigest
+    };
+    const subject = {
+        mediaType: imageManifestMediaType,
+        size: subjectSize,
+        digest: subjectDigest
+    };
+    const manifest = {
+        schemaVersion: 2,
+        mediaType: imageManifestMediaType,
+        artifactType: sigstoreBundleMediaType,
+        config: configLayer,
+        layers: [sigstoreAttestationLayer],
+        subject,
+        annotations: {
+            'dev.sigstore.bundle.content': 'dsse-envelope',
+            'dev.sigstore.bundle.predicateType': 'https://slsa.dev/provenance/v1',
+            'com.github.package.type': actionPackageAttestationAnnotationValue,
+            'org.opencontainers.image.created': created.toISOString()
+        }
+    };
+    return manifest;
+}
+function createReferrerTagManifest(attestationDigest, attestationSize, attestationCreated, created = new Date()) {
+    const manifest = {
+        schemaVersion: 2,
+        mediaType: imageIndexMediaType,
+        manifests: [
+            {
+                mediaType: imageManifestMediaType,
+                artifactType: sigstoreBundleMediaType,
+                size: attestationSize,
+                digest: attestationDigest,
+                annotations: {
+                    'com.github.package.type': actionPackageAttestationAnnotationValue,
+                    'org.opencontainers.image.created': attestationCreated.toISOString(),
+                    'dev.sigstore.bundle.content': 'dsse-envelope',
+                    'dev.sigstore.bundle.predicateType': 'https://slsa.dev/provenance/v1'
+                }
+            }
+        ],
+        annotations: {
+            'com.github.package.type': actionPackageReferrerTagAnnotationValue,
+            'org.opencontainers.image.created': created.toISOString()
+        }
+    };
+    return manifest;
+}
 // Calculate the SHA256 digest of a given manifest.
 // This should match the digest which the GitHub container registry calculates for this manifest.
 function sha256Digest(manifest) {
@@ -107791,17 +107859,21 @@ function sha256Digest(manifest) {
     const hexHash = hash.digest('hex');
     return `sha256:${hexHash}`;
 }
+function sizeInBytes(manifest) {
+    const data = JSON.stringify(manifest);
+    return Buffer.byteLength(data, 'utf8');
+}
 function createConfigLayer() {
     const configLayer = {
-        mediaType: 'application/vnd.oci.empty.v1+json',
+        mediaType: ociEmptyMediaType,
-        size: 2,
+        size: emptyConfigSize,
-        digest: 'sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a'
+        digest: emptyConfigSha
     };
     return configLayer;
 }
 function createZipLayer(zipFile, repository, version) {
     const zipLayer = {
-        mediaType: 'application/vnd.github.actions.package.layer.v1.zip',
+        mediaType: actionsPackageZipLayerMediaType,
         size: zipFile.size,
         digest: zipFile.sha256,
         annotations: {
@@ -107812,7 +107884,7 @@ function createZipLayer(zipFile, repository, version) {
 }
 function createTarLayer(tarFile, repository, version) {
     const tarLayer = {
-        mediaType: 'application/vnd.github.actions.package.layer.v1.tar+gzip',
+        mediaType: actionsPackageTarLayerMediaType,
         size: tarFile.size,
         digest: tarFile.sha256,
         annotations: {

src/ghcr-client.ts

@@ -11,7 +11,7 @@ export async function publishOCIArtifact(
   semver: string,
   zipFile: FileMetadata,
   tarFile: FileMetadata,
-  manifest: ociContainer.Manifest
+  manifest: ociContainer.OCIImageManifest
 ): Promise<{ packageURL: URL; publishedDigest: string }> {
   const b64Token = Buffer.from(token).toString('base64')
 
@@ -81,7 +81,7 @@ export async function publishOCIArtifact(
 }
 
 async function uploadLayer(
-  layer: ociContainer.Layer,
+  layer: ociContainer.Descriptor,
   file: FileMetadata,
   registryURL: URL,
   checkBlobEndpoint: string,

src/oci-container.ts

@@ -1,19 +1,46 @@
 import { FileMetadata } from './fs-helper'
 import * as crypto from 'crypto'
 
-export interface Manifest {
+const imageIndexMediaType = 'application/vnd.oci.image.index.v1+json'
+const imageManifestMediaType = 'application/vnd.oci.image.manifest.v1+json'
+const actionsPackageMediaType = 'application/vnd.github.actions.package.v1+json'
+const actionsPackageTarLayerMediaType =
+  'application/vnd.github.actions.package.layer.v1.tar+gzip'
+const actionsPackageZipLayerMediaType =
+  'application/vnd.github.actions.package.layer.v1.zip'
+const sigstoreBundleMediaType = 'application/vnd.dev.sigstore.bundle.v0.3+json'
+const ociEmptyMediaType = 'application/vnd.oci.empty.v1+json'
+
+const actionPackageAnnotationValue = 'actions_oci_pkg'
+const actionPackageAttestationAnnotationValue = 'actions_oci_pkg_attestation'
+const actionPackageReferrerTagAnnotationValue = 'actions_oci_pkg_referrer_tag'
+
+const emptyConfigSize = 2
+const emptyConfigSha =
+  'sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a'
+
+export interface OCIImageManifest {
   schemaVersion: number
   mediaType: string
   artifactType: string
-  config: Layer
+  config: Descriptor
-  layers: Layer[]
+  layers: Descriptor[]
+  subject?: Descriptor
   annotations: { [key: string]: string }
 }
 
-export interface Layer {
+export interface OCIIndexManifest {
+  schemaVersion: number
+  mediaType: string
+  manifests: Descriptor[]
+  annotations: { [key: string]: string }
+}
+
+export interface Descriptor {
   mediaType: string
   size: number
   digest: string
+  artifactType?: string
   annotations?: { [key: string]: string }
 }
 
@@ -26,24 +53,24 @@ export function createActionPackageManifest(
   ownerId: string,
   sourceCommit: string,
   version: string,
-  created: Date
+  created: Date = new Date()
-): Manifest {
+): OCIImageManifest {
   const configLayer = createConfigLayer()
   const sanitizedRepo = sanitizeRepository(repository)
   const tarLayer = createTarLayer(tarFile, sanitizedRepo, version)
   const zipLayer = createZipLayer(zipFile, sanitizedRepo, version)
 
-  const manifest: Manifest = {
+  const manifest: OCIImageManifest = {
     schemaVersion: 2,
-    mediaType: 'application/vnd.oci.image.manifest.v1+json',
+    mediaType: imageManifestMediaType,
-    artifactType: 'application/vnd.github.actions.package.v1+json',
+    artifactType: actionsPackageMediaType,
     config: configLayer,
     layers: [configLayer, tarLayer, zipLayer],
     annotations: {
       'org.opencontainers.image.created': created.toISOString(),
       'action.tar.gz.digest': tarFile.sha256,
       'action.zip.digest': zipFile.sha256,
-      'com.github.package.type': 'actions_oci_pkg',
+      'com.github.package.type': actionPackageAnnotationValue,
       'com.github.package.version': version,
       'com.github.source.repo.id': repoId,
       'com.github.source.repo.owner.id': ownerId,
@@ -54,9 +81,83 @@ export function createActionPackageManifest(
   return manifest
 }
 
+export function createSigstoreAttestationManifest(
+  bundleSize: number,
+  bundleDigest: string,
+  subjectSize: number,
+  subjectDigest: string,
+  created: Date = new Date()
+): OCIImageManifest {
+  const configLayer = createConfigLayer()
+
+  const sigstoreAttestationLayer: Descriptor = {
+    mediaType: sigstoreBundleMediaType,
+    size: bundleSize,
+    digest: bundleDigest
+  }
+
+  const subject: Descriptor = {
+    mediaType: imageManifestMediaType,
+    size: subjectSize,
+    digest: subjectDigest
+  }
+
+  const manifest: OCIImageManifest = {
+    schemaVersion: 2,
+    mediaType: imageManifestMediaType,
+    artifactType: sigstoreBundleMediaType,
+    config: configLayer,
+    layers: [sigstoreAttestationLayer],
+    subject,
+
+    annotations: {
+      'dev.sigstore.bundle.content': 'dsse-envelope',
+      'dev.sigstore.bundle.predicateType': 'https://slsa.dev/provenance/v1',
+      'com.github.package.type': actionPackageAttestationAnnotationValue,
+      'org.opencontainers.image.created': created.toISOString()
+    }
+  }
+
+  return manifest
+}
+
+export function createReferrerTagManifest(
+  attestationDigest: string,
+  attestationSize: number,
+  attestationCreated: Date,
+  created: Date = new Date()
+): OCIIndexManifest {
+  const manifest: OCIIndexManifest = {
+    schemaVersion: 2,
+    mediaType: imageIndexMediaType,
+    manifests: [
+      {
+        mediaType: imageManifestMediaType,
+        artifactType: sigstoreBundleMediaType,
+        size: attestationSize,
+        digest: attestationDigest,
+        annotations: {
+          'com.github.package.type': actionPackageAttestationAnnotationValue,
+          'org.opencontainers.image.created': attestationCreated.toISOString(),
+          'dev.sigstore.bundle.content': 'dsse-envelope',
+          'dev.sigstore.bundle.predicateType': 'https://slsa.dev/provenance/v1'
+        }
+      }
+    ],
+    annotations: {
+      'com.github.package.type': actionPackageReferrerTagAnnotationValue,
+      'org.opencontainers.image.created': created.toISOString()
+    }
+  }
+
+  return manifest
+}
+
 // Calculate the SHA256 digest of a given manifest.
 // This should match the digest which the GitHub container registry calculates for this manifest.
-export function sha256Digest(manifest: Manifest): string {
+export function sha256Digest(
+  manifest: OCIImageManifest | OCIIndexManifest
+): string {
   const data = JSON.stringify(manifest)
   const buffer = Buffer.from(data, 'utf8')
   const hash = crypto.createHash('sha256')
@@ -65,12 +166,18 @@ export function sha256Digest(manifest: Manifest): string {
   return `sha256:${hexHash}`
 }
 
-function createConfigLayer(): Layer {
+export function sizeInBytes(
-  const configLayer: Layer = {
+  manifest: OCIImageManifest | OCIIndexManifest
-    mediaType: 'application/vnd.oci.empty.v1+json',
+): number {
-    size: 2,
+  const data = JSON.stringify(manifest)
-    digest:
+  return Buffer.byteLength(data, 'utf8')
-      'sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a'
+}
+
+function createConfigLayer(): Descriptor {
+  const configLayer: Descriptor = {
+    mediaType: ociEmptyMediaType,
+    size: emptyConfigSize,
+    digest: emptyConfigSha
   }
 
   return configLayer
@@ -80,9 +187,9 @@ function createZipLayer(
   zipFile: FileMetadata,
   repository: string,
   version: string
-): Layer {
+): Descriptor {
-  const zipLayer: Layer = {
+  const zipLayer: Descriptor = {
-    mediaType: 'application/vnd.github.actions.package.layer.v1.zip',
+    mediaType: actionsPackageZipLayerMediaType,
     size: zipFile.size,
     digest: zipFile.sha256,
     annotations: {
@@ -97,9 +204,9 @@ function createTarLayer(
   tarFile: FileMetadata,
   repository: string,
   version: string
-): Layer {
+): Descriptor {
-  const tarLayer: Layer = {
+  const tarLayer: Descriptor = {
-    mediaType: 'application/vnd.github.actions.package.layer.v1.tar+gzip',
+    mediaType: actionsPackageTarLayerMediaType,
     size: tarFile.size,
     digest: tarFile.sha256,
     annotations: {