Accept, verify, and expose Sigstore package attestations

app/lib/frontend/handlers/pubapi.dart

@@ -17,6 +17,7 @@ import 'package:shelf_router/shelf_router.dart';
 import '../../account/consent_backend.dart';
 import '../../admin/backend.dart';
 import '../../package/backend.dart' hide InviteStatus;
+import '../../package/models.dart' show AssetKind;
 import '../../publisher/backend.dart';
 import '../../shared/exceptions.dart';
 import '../../shared/handlers.dart';
@@ -80,6 +81,31 @@ class PubApi {
     );
   }
 
+  /// Fetches the Sigstore attestation bundle for a specific (package, version) pair.
+  @EndPoint.get('/api/packages/<package>/versions/<version>/attestation')
+  Future<Response> getPackageVersionAttestation(
+    Request request,
+    String package,
+    String version,
+  ) async {
+    checkPackageVersionParams(package, version);
+    final asset = await packageBackend.lookupPackageVersionAsset(
+      package,
+      version,
+      AssetKind.attestation,
+    );
+    if (asset == null || asset.textContent == null) {
+      throw NotFoundException.resource('attestation for $package $version');
+    }
+    return Response.ok(
+      asset.textContent,
+      headers: {
+        'content-type': 'application/json; charset="utf-8"',
+        ...CacheControl.clientApi.headers,
+      },
+    );
+  }
+
   /// Downloading package.
   ///
   /// This is the endpoint we link to from the version listing.

app/lib/package/backend.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:_pub_shared/data/account_api.dart' as account_api;
@@ -29,6 +30,7 @@ import 'package:pub_dev/task/backend.dart';
 import 'package:pub_package_reader/pub_package_reader.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:pubspec_parse/pubspec_parse.dart' as pubspec_parse;
+import 'package:sigstore/sigstore.dart';
 
 import '../account/agent.dart';
 import '../account/backend.dart';
@@ -1208,12 +1210,66 @@ class PackageBackend {
         throw PackageRejectedException.dependencyDoesNotExists(name);
       }
 
+      // Check for an accompanying Sigstore attestation bundle in the incoming bucket.
+      String? attestationContent;
+      final attestationObjectName =
+          '${tmpObjectName(uploadGuid)}.sigstore.json';
+      final attestationInfo = await _incomingBucket.tryInfo(
+        attestationObjectName,
+      );
+      if (attestationInfo?.length != null) {
+        _logger.info('Verifying package attestation ($uploadGuid).');
+        final attestationFilename =
+            '${dir.absolute.path}/attestation.sigstore.json';
+        await _incomingBucket.readWithRetry(
+          attestationObjectName,
+          (input) => _saveTarballToFS(input, attestationFilename),
+        );
+        attestationContent = await File(attestationFilename).readAsString();
+        try {
+          final attestationJson =
+              jsonDecode(attestationContent) as Map<String, dynamic>;
+          final bundle = SigstoreBundle.fromJson(attestationJson);
+          final verifier = AttestationVerifier();
+          final archiveBytes = await file.readAsBytes();
+          final verificationResult = verifier.verify(
+            packageName: pubspec.name,
+            packageVersion: Version.parse(versionString),
+            archiveBytes: archiveBytes,
+            bundle: bundle,
+            pubspecRepository: pubspec.repository?.toString(),
+          );
+          if (!verificationResult.isValid) {
+            throw PackageRejectedException(
+              'Invalid package attestation: '
+              '${verificationResult.errors.join(', ')}',
+            );
+          }
+          _logger.info(
+            'Attestation verified successfully for ${pubspec.name} $versionString '
+            'from repository ${verificationResult.repository}.',
+          );
+        } on PackageRejectedException {
+          rethrow;
+        } catch (e, st) {
+          _logger.warning(
+            'Failed to parse or verify attestation bundle.',
+            e,
+            st,
+          );
+          throw PackageRejectedException(
+            'Failed to verify package attestation: $e',
+          );
+        }
+      }
+
       sw.reset();
       final entities = await _createUploadEntities(
         db,
         agent,
         archive,
         sha256Hash: sha256Hash,
+        attestationContent: attestationContent,
       );
       final (version, uploadMessages) = await _performTarballUpload(
         entities: entities,
@@ -1229,6 +1285,9 @@ class PackageBackend {
       sw.reset();
       await _incomingBucket.deleteWithRetry(uploadObjectName);
       await _incomingBucket.deleteWithRetry(workObjectName);
+      if (attestationInfo?.length != null) {
+        await _incomingBucket.deleteWithRetry(attestationObjectName);
+      }
       _logger.info('Temporary object removed in ${sw.elapsed}.');
       return [
         'Successfully uploaded '
@@ -2368,6 +2427,7 @@ Future<_UploadEntities> _createUploadEntities(
   AuthenticatedAgent agent,
   PackageSummary archive, {
   required List<int> sha256Hash,
+  String? attestationContent,
 }) async {
   final pubspec = Pubspec.fromYaml(archive.pubspecContent!);
   final packageKey = db.emptyKey.append(Package, id: pubspec.name);
@@ -2387,6 +2447,7 @@ Future<_UploadEntities> _createUploadEntities(
   final derived = derivePackageVersionEntities(
     archive: archive,
     versionCreated: version.created!,
+    attestationContent: attestationContent,
   );
 
   // TODO: verify if assets sizes are within the transaction limit (10 MB)
@@ -2397,6 +2458,7 @@ Future<_UploadEntities> _createUploadEntities(
 DerivedPackageVersionEntities derivePackageVersionEntities({
   required PackageSummary archive,
   required DateTime versionCreated,
+  String? attestationContent,
 }) {
   final pubspec = Pubspec.fromYaml(archive.pubspecContent!);
   final key = QualifiedVersionKey(
@@ -2455,6 +2517,15 @@ DerivedPackageVersionEntities derivePackageVersionEntities({
         path: archive.licensePath,
         textContent: capContent(archive.licenseContent),
       ),
+    if (attestationContent != null)
+      PackageVersionAsset.init(
+        package: key.package,
+        version: key.version,
+        kind: AssetKind.attestation,
+        versionCreated: versionCreated,
+        path: '${key.package}-${key.version}.sigstore.json',
+        textContent: capContent(attestationContent),
+      ),
   ];
 
   final versionInfo = PackageVersionInfo()

app/lib/package/models.dart

@@ -765,6 +765,7 @@ abstract class AssetKind {
   static const changelog = 'changelog';
   static const example = 'example';
   static const license = 'license';
+  static const attestation = 'attestation';
 }
 
 /// A derived entity that holds extracted asset of a [PackageVersion] archive.

app/lib/tool/utils/pub_api_client.dart

@@ -174,7 +174,10 @@ bool _retryIf(Exception e) {
 
 extension PubApiClientExt on PubApiClient {
   @visibleForTesting
-  Future<String> preparePackageUpload(List<int> bytes) async {
+  Future<String> preparePackageUpload(
+    List<int> bytes, {
+    List<int>? attestationBytes,
+  }) async {
     final uploadInfo = await getPackageUploadUrl();
 
     final request = http.MultipartRequest('POST', Uri.parse(uploadInfo.url))
@@ -197,12 +200,34 @@ extension PubApiClientExt on PubApiClient {
     final callbackUri = Uri.parse(
       uploadInfo.fields!['success_action_redirect']!,
     );
-    return callbackUri.queryParameters['upload_id']!;
+    final uploadId = callbackUri.queryParameters['upload_id']!;
+
+    if (attestationBytes != null) {
+      final baseKey = uploadInfo.fields!['key']!;
+      final attestationFields = Map<String, String>.from(uploadInfo.fields!);
+      attestationFields['key'] = '$baseKey.sigstore.json';
+      attestationFields.remove('success_action_redirect');
+      final attRequest =
+          http.MultipartRequest('POST', Uri.parse(uploadInfo.url))
+            ..headers[fakeClockHeaderName] = clock.now().toIso8601String()
+            ..fields.addAll(attestationFields)
+            ..files.add(http.MultipartFile.fromBytes('file', attestationBytes))
+            ..followRedirects = false;
+      await attRequest.send();
+    }
+
+    return uploadId;
   }
 
   @visibleForTesting
-  Future<SuccessMessage> uploadPackageBytes(List<int> bytes) async {
-    final uploadId = await preparePackageUpload(bytes);
+  Future<SuccessMessage> uploadPackageBytes(
+    List<int> bytes, {
+    List<int>? attestationBytes,
+  }) async {
+    final uploadId = await preparePackageUpload(
+      bytes,
+      attestationBytes: attestationBytes,
+    );
     return await finishPackageUpload(uploadId);
   }
 }

app/pubspec.yaml

@@ -55,6 +55,11 @@ dependencies:
   ulid: '2.0.1'
   tar: '2.0.2'
   api_builder:
+  sigstore:
+    git:
+      url: https://github.com/dart-lang/labs.git
+      ref: add-package-sigstore
+      path: pkgs/sigstore
 
 dev_dependencies:
   build_runner: '^2.0.0'