ADR-013 Phase 2: enforcement + signed shelf + adopt

Completes the marketplace security model. Verification stops being advisory:
a plugin only loads or migrates when its tree matches a trusted signature, and
plugins are pulled from a signed shelf with anti-rollback and revocation.

Enforcement (default OFF - existing deploys unchanged):
- verification.py PluginVerifier, shared by the loader (verify-at-load, before
  plugin.py is imported) and the migration manager (verify-at-migrate, before
  any DDL). Fail-closed: an unsigned/tampered/wrong-key plugin does not run.
- Gated by PLUGIN_REQUIRE_SIGNED. PLUGIN_DEV_TRUST_DIRS exempts named dirs but
  only under DEBUG/TESTING; production ignores it.
- flask plugin stamp-bundled writes provenance into in-tree plugins so
  verify-at-load applies to bundled plugins too (image build step).
- tier:core manifest guard: uninstall/disable refuse a core-tier plugin.

Shelf (shelf.py):
- Signed shelf-index.json (+ .sig): monotonic serial (a site refuses an older
  index - anti-rollback), revoked list carried across builds, per-entry
  version/tier/core_version for browse. Index is a browse layer only; adopt
  reads security-bearing fields from the verified artifact.
- flask plugin shelf-build / shelf-list / adopt / audit. adopt verifies index +
  artifact (signature + every file hash), unpacks to staging, re-verifies, then
  atomically moves into place and installs+enables the closure. Refuses a
  downgrade without --force-downgrade. Anti-rollback serial stored in
  instance/shelf-state.json.
- config PLUGIN_SHELF_DIR; the app only reads the folder, never speaks a
  network. .env.example + docs/PLUGIN-SIGNING.md document the flow.

22 tests: verifier policy (off / no-keys / signed / tampered / wrong-key /
dev-exempt), verify-at-load + verify-at-migrate integration, tier guard, index
sign/verify + tamper/wrong-key, serial state, revocation, version resolution,
verified atomic unpack + tamper refusal. Live-smoked keygen->pack->shelf-build
->list->adopt->audit + serial guard. 1050 pass, naming green.

.env.example

@@ -66,6 +66,19 @@ ZABBIX_TOKEN=
 # to verify signed plugin artifacts. Delivered with this config, NEVER from the
 # plugin shelf. Empty on a site that does not adopt marketplace plugins.
 # PLUGIN_TRUSTED_KEYS=/etc/shopdb/keys/curator.pub
+#
+# Enforce signatures: a plugin only loads/migrates if its tree matches a
+# trusted signature. Default off. Turn on only after stamping plugins
+# (flask plugin stamp-bundled) and pinning keys above.
+# PLUGIN_REQUIRE_SIGNED=false
+#
+# Dev-only: directories whose UNSIGNED plugins are trusted, honored ONLY under
+# DEBUG/TESTING (external-repo/symlink dev). Production ignores this.
+# PLUGIN_DEV_TRUST_DIRS=/home/dev/my-plugin-repo
+#
+# Read-only folder the app pulls plugin artifacts from (a SharePoint-synced or
+# copied shelf). The app reads this folder; it never speaks any network.
+# PLUGIN_SHELF_DIR=/srv/shopdb/plugin-shelf
 
 # ---- Employee directory database (optional, read-only) ----
 # Separate HR/employee lookup DB consumed by the notifications plugin and the

docs/PLUGIN-SIGNING.md

@@ -67,9 +67,60 @@ Keys are read only from this deployed config, never from the shelf - a folder an
 attacker could write must not also carry the keys that authenticate it. With no
 keys set, `validate` on an artifact fails closed (unverifiable).
 
-## What is NOT in this phase
+## Enforce signatures (Phase 2)
 
-Verification is available but not yet enforced automatically at plugin load or
+By default nothing is enforced - plugins load unsigned, as before. To require
-migrate time, and there is no `adopt` command that pulls from a shelf yet. Those
+signatures on a site:
-land in Phase 2 (verify-at-load, verify-at-migrate, the signed shelf index).
+
-For now, verify artifacts manually with `validate` before installing.
+1. Stamp the plugins the image ships with, so verify-at-load applies to them
+   too (run at image build with the site/build key):
+
+   ```
+   flask plugin stamp-bundled --key ./keys/curator.key
+   ```
+
+   This writes `PROVENANCE.json` + `PROVENANCE.sig` into each in-tree plugin.
+
+2. Pin the public key(s) and turn enforcement on (site config):
+
+   ```
+   PLUGIN_TRUSTED_KEYS=/etc/shopdb/keys/curator.pub
+   PLUGIN_REQUIRE_SIGNED=true
+   ```
+
+Now a plugin only loads (verify-at-load) or migrates (verify-at-migrate) when
+its tree matches a trusted signature. An unsigned, tampered, or wrong-key
+plugin is refused - fail-closed. `PLUGIN_DEV_TRUST_DIRS` exempts named
+directories, but ONLY under DEBUG/TESTING (the external-repo dev workflow);
+production ignores it.
+
+## The shelf and adopt (Phase 2)
+
+A shelf is a read-only folder of artifacts plus a signed index. The app reads
+`PLUGIN_SHELF_DIR`; it never talks to SharePoint - a sync (or robocopy/USB)
+populates that folder, and the signature makes the transport untrusted and
+interchangeable.
+
+Publish (curator, after packing artifacts into the shelf folder):
+
+```
+flask plugin shelf-build --dir /srv/shelf --key ./keys/curator.key --serial 3
+```
+
+The index carries a monotonic `serial` (a site refuses an index older than the
+last it saw) and a `revoked` list (carried forward across builds). Bump
+`--serial` on every publish.
+
+On a site:
+
+```
+flask plugin shelf-list                 # browse (verifies index + serial)
+flask plugin adopt printers             # or printers==1.2.0
+flask plugin audit                      # warn if an installed version is revoked
+```
+
+`adopt` verifies the shelf index and the artifact (signature + every file
+hash), unpacks into a staging area, re-verifies, then atomically moves it into
+place and installs + enables it with its dependency closure. It refuses a
+downgrade unless `--force-downgrade`. Run `flask plugin upgrade-all` and restart
+afterward so migrations apply and routes register.

shopdb/config.py

@@ -70,6 +70,25 @@ class Config:
         if path.strip()
     ]
 
+    # When true, a plugin only loads/migrates if its tree matches a trusted
+    # signature (verify-at-load, verify-at-migrate). Default false = existing
+    # behavior. Turn on only after stamping plugins + pinning keys.
+    PLUGIN_REQUIRE_SIGNED = os.environ.get(
+        'PLUGIN_REQUIRE_SIGNED', 'false').lower() == 'true'
+
+    # Directories whose unsigned plugins are trusted - honored ONLY under
+    # DEBUG/TESTING (the external-repo/symlink dev workflow). Production ignores.
+    PLUGIN_DEV_TRUST_DIRS = [
+        path.strip()
+        for path in os.environ.get('PLUGIN_DEV_TRUST_DIRS', '').split(os.pathsep)
+        if path.strip()
+    ]
+
+    # Read-only folder the app pulls plugin artifacts from (a SharePoint-synced
+    # or copied shelf). Empty = no shelf configured. The app never speaks any
+    # network protocol; it reads this folder.
+    PLUGIN_SHELF_DIR = os.environ.get('PLUGIN_SHELF_DIR', '')
+
     ZABBIX_ENABLED = os.environ.get('ZABBIX_ENABLED', 'false').lower() == 'true'
     ZABBIX_URL = os.environ.get('ZABBIX_URL', '')
     ZABBIX_TOKEN = os.environ.get('ZABBIX_TOKEN', '')

shopdb/plugins/__init__.py

@@ -39,6 +39,7 @@ class PluginManager:
         self.registry: Optional[PluginRegistry] = None
         self.loader: Optional[PluginLoader] = None
         self.migration_manager: Optional[PluginMigrationManager] = None
+        self.verifier = None
         self._app: Optional[Flask] = None
         self._db = None
         # API prefixes already claimed by a registered plugin blueprint, to
@@ -66,6 +67,19 @@ class PluginManager:
             app.config.get('SQLALCHEMY_DATABASE_URI')
         )
 
+        # One trust policy, shared by verify-at-load and verify-at-migrate.
+        from .verification import PluginVerifier
+        verifier = PluginVerifier(
+            plugins_dir,
+            require_signed=app.config.get('PLUGIN_REQUIRE_SIGNED', False),
+            trusted_key_paths=app.config.get('PLUGIN_TRUSTED_KEYS', []),
+            dev_trust_dirs=app.config.get('PLUGIN_DEV_TRUST_DIRS', []),
+            is_dev=bool(app.config.get('DEBUG') or app.config.get('TESTING')),
+        )
+        self.verifier = verifier
+        self.loader.verifier = verifier
+        self.migration_manager.verifier = verifier
+
         # Load enabled plugins
         self._load_enabled_plugins()
 
@@ -257,6 +271,18 @@ class PluginManager:
                     "Seeded %d permission(s) for plugin %s",
                     created, plugin.meta.name)
 
+    def _is_core_tier(self, name: str) -> bool:
+        """True when the plugin's manifest marks it tier=core (mandatory).
+
+        A core-tier plugin refuses uninstall/disable so a site cannot remove a
+        plugin its deployment depends on. No plugin ships tier=core today; the
+        guard makes that a manifest edit, not a code change (ADR-013).
+        """
+        try:
+            return self.loader.load_manifest(name).get('tier') == 'core'
+        except PluginError:
+            return False
+
     def _installed_dependents(self, name: str,
                               enabled_only: bool = False) -> List[str]:
         """Installed plugins that declare `name` as a hard dependency.
@@ -293,6 +319,10 @@ class PluginManager:
             logger.warning(f"Plugin {name} is not installed")
             return False
 
+        if self._is_core_tier(name):
+            logger.error(f"Cannot uninstall {name}: it is a core-tier plugin")
+            return False
+
         # Refuse if any INSTALLED plugin (enabled or not) depends on this one.
         dependents = self._installed_dependents(name)
         if dependents:
@@ -372,6 +402,10 @@ class PluginManager:
             logger.info(f"Plugin {name} is already disabled")
             return True
 
+        if self._is_core_tier(name):
+            logger.error(f"Cannot disable {name}: it is a core-tier plugin")
+            return False
+
         # Refuse if any ENABLED plugin depends on this one (a disabled dependent
         # is not running, so it does not block). Manifests read from disk.
         dependents = self._installed_dependents(name, enabled_only=True)

shopdb/plugins/cli.py

@@ -553,6 +553,283 @@ def apply_profile(profile: str):
         "restart so new blueprints/routes register.", fg='yellow'))
 
 
+def _shelf_keys(pubkeys):
+    """Trusted keys for shelf/artifact verification: explicit --pubkey first,
+    else the site's PLUGIN_TRUSTED_KEYS."""
+    from .signing import load_trusted_keys
+    paths = list(pubkeys) or current_app.config.get('PLUGIN_TRUSTED_KEYS', [])
+    return load_trusted_keys(paths), paths
+
+
+@plugin_cli.command('stamp-bundled')
+@click.option('--key', 'key_path', required=True, type=click.Path(exists=True),
+              help='Private signing key PEM')
+@click.option('--publisher', default='', help='Publisher id in the provenance')
+@click.argument('name', required=False)
+@with_appcontext
+def stamp_bundled(key_path, publisher, name):
+    """Write PROVENANCE.json/.sig into in-tree plugin dirs (image build step).
+
+    Stamps one plugin (NAME) or every discovered plugin, so verify-at-load
+    applies to bundled plugins identically to adopted ones. Run at image build
+    with the site/build key, then set PLUGIN_REQUIRE_SIGNED.
+    """
+    from .signing import (load_private_key, build_provenance,
+                          serialize_provenance, sign,
+                          PROVENANCE_NAME, PROVENANCE_SIG)
+
+    pm = current_app.extensions.get('plugin_manager')
+    if not pm:
+        click.echo(click.style("Plugin manager not initialized", fg='red'))
+        raise SystemExit(1)
+
+    private_key = load_private_key(Path(key_path).read_bytes())
+    names = [name] if name else pm.loader.discover_plugins()
+    plugins_dir = Path(pm.loader.plugins_dir)
+
+    for plugin_name in names:
+        plugin_dir = plugins_dir / plugin_name
+        manifest = pm.loader.load_manifest(plugin_name)
+        provenance = build_provenance(
+            plugin_dir, manifest['name'], manifest['version'], publisher)
+        provenance_bytes = serialize_provenance(provenance)
+        (plugin_dir / PROVENANCE_NAME).write_bytes(provenance_bytes)
+        (plugin_dir / PROVENANCE_SIG).write_bytes(
+            sign(private_key, provenance_bytes))
+        click.echo(click.style(f"  stamped {plugin_name}", fg='green'))
+
+
+@plugin_cli.command('shelf-build')
+@click.option('--dir', 'shelf_dir', required=True,
+              type=click.Path(exists=True, file_okay=False),
+              help='Shelf directory holding the artifacts')
+@click.option('--key', 'key_path', required=True,
+              type=click.Path(exists=True), help='Private signing key PEM')
+@click.option('--serial', type=int, required=True,
+              help='Index serial (must exceed the current published serial)')
+@click.option('--publisher', default='')
+def shelf_build(shelf_dir, key_path, serial, publisher):
+    """Build + sign the shelf index from the artifacts in DIR.
+
+    Carries forward the existing revoked list and refuses a serial that does not
+    advance past the published one (a lower serial would let a site's
+    anti-rollback reject the new index).
+    """
+    import json as _json
+    from .signing import load_private_key
+    from .shelf import build_index, INDEX_NAME
+
+    shelf = Path(shelf_dir)
+    revoked = []
+    existing = shelf / INDEX_NAME
+    if existing.exists():
+        try:
+            prev = _json.loads(existing.read_text())
+            revoked = prev.get('revoked', [])
+            if serial <= int(prev.get('serial', -1)):
+                click.echo(click.style(
+                    f"serial {serial} must exceed current "
+                    f"{prev.get('serial')}", fg='red'))
+                raise SystemExit(1)
+        except (ValueError, TypeError):
+            pass
+
+    private_key = load_private_key(Path(key_path).read_bytes())
+    index = build_index(shelf, private_key, serial,
+                        publisher=publisher, revoked=revoked)
+    total = sum(len(v) for v in index['plugins'].values())
+    click.echo(click.style(
+        f"Shelf index signed: serial {serial}, {total} artifact(s), "
+        f"{len(revoked)} revoked.", fg='green'))
+
+
+@plugin_cli.command('shelf-list')
+@click.option('--pubkey', 'pubkeys', multiple=True, type=click.Path(exists=True))
+@with_appcontext
+def shelf_list(pubkeys):
+    """Browse the configured shelf (verifies index signature + serial)."""
+    from shopdb import __contract_version__
+    from packaging.specifiers import SpecifierSet
+    from packaging.version import Version
+    from .shelf import load_index, read_last_serial, write_last_serial, is_revoked
+
+    shelf_dir = current_app.config.get('PLUGIN_SHELF_DIR')
+    if not shelf_dir:
+        click.echo(click.style("PLUGIN_SHELF_DIR not configured", fg='red'))
+        raise SystemExit(1)
+
+    keys, _ = _shelf_keys(pubkeys)
+    index, errors = load_index(shelf_dir, keys)
+    for err in errors:
+        click.echo(click.style(f"  {err}", fg='red'))
+    if index is None:
+        raise SystemExit(1)
+
+    last = read_last_serial(current_app.instance_path)
+    if index['serial'] < last:
+        click.echo(click.style(
+            f"REFUSING shelf: index serial {index['serial']} < last seen "
+            f"{last} (rollback?)", fg='red'))
+        raise SystemExit(1)
+    if errors:
+        raise SystemExit(1)
+    write_last_serial(current_app.instance_path, index['serial'])
+
+    click.echo("")
+    click.echo(click.style(
+        f"Shelf (serial {index['serial']}):", fg='cyan', bold=True))
+    for name in sorted(index.get('plugins', {})):
+        for entry in index['plugins'][name]:
+            compat = 'ok'
+            spec = entry.get('core_version', '')
+            if spec and Version(__contract_version__) not in SpecifierSet(spec):
+                compat = f'needs core {spec}'
+            flags = []
+            if is_revoked(index, name, entry['version']):
+                flags.append('REVOKED')
+            if compat != 'ok':
+                flags.append(compat)
+            tag = ('  [' + ', '.join(flags) + ']') if flags else ''
+            click.echo(f"  {name:20} {entry['version']:10} "
+                       f"({entry.get('tier', 'optional')}){tag}")
+
+
+@plugin_cli.command('adopt')
+@click.argument('spec')
+@click.option('--pubkey', 'pubkeys', multiple=True, type=click.Path(exists=True))
+@click.option('--force-downgrade', is_flag=True,
+              help='Allow adopting a version lower than the installed one')
+@with_appcontext
+def adopt_plugin(spec, pubkeys, force_downgrade):
+    """Pull a plugin from the shelf, verify it, and install + enable it.
+
+    SPEC is a plugin name or name==version. Verifies the shelf index and the
+    artifact signature + hashes, unpacks atomically only after verification,
+    then installs + enables the plugin and its dependency closure.
+
+    Usage: flask plugin adopt printers        (or printers==1.2.0)
+    """
+    from packaging.version import Version
+    from ..exceptions import PluginError
+    from .shelf import (load_index, read_last_serial, write_last_serial,
+                        resolve_version, unpack_verified)
+
+    pm = current_app.extensions.get('plugin_manager')
+    if not pm:
+        click.echo(click.style("Plugin manager not initialized", fg='red'))
+        raise SystemExit(1)
+
+    name, _, version = spec.partition('==')
+    name = name.strip()
+    version = version.strip() or None
+
+    shelf_dir = current_app.config.get('PLUGIN_SHELF_DIR')
+    if not shelf_dir:
+        click.echo(click.style("PLUGIN_SHELF_DIR not configured", fg='red'))
+        raise SystemExit(1)
+
+    keys, _ = _shelf_keys(pubkeys)
+    index, errors = load_index(shelf_dir, keys)
+    for err in errors:
+        click.echo(click.style(f"  {err}", fg='red'))
+    if index is None or errors:
+        raise SystemExit(1)
+
+    last = read_last_serial(current_app.instance_path)
+    if index['serial'] < last:
+        click.echo(click.style(
+            f"REFUSING shelf: serial {index['serial']} < last seen {last} "
+            f"(rollback?)", fg='red'))
+        raise SystemExit(1)
+
+    try:
+        resolved_version, artifact_name = resolve_version(index, name, version)
+    except (KeyError, ValueError) as e:
+        click.echo(click.style(f"{e}", fg='red'))
+        raise SystemExit(1)
+
+    state = pm.registry.get(name)
+    if state and not force_downgrade:
+        try:
+            if Version(resolved_version) < Version(state.version):
+                click.echo(click.style(
+                    f"Refusing downgrade {state.version} -> {resolved_version} "
+                    f"(use --force-downgrade)", fg='red'))
+                raise SystemExit(1)
+        except Exception:
+            pass
+
+    artifact_path = Path(shelf_dir) / artifact_name
+    plugins_dir = Path(pm.loader.plugins_dir)
+    click.echo(f"Verifying + unpacking {name} {resolved_version} ...")
+    try:
+        unpack_verified(artifact_path, keys, plugins_dir, name)
+    except ValueError as e:
+        click.echo(click.style(f"  verification failed: {e}", fg='red'))
+        raise SystemExit(1)
+
+    # Clear any cached manifest so the freshly unpacked one is read.
+    pm.loader._manifests.pop(name, None)
+    try:
+        result = pm.apply_profile([name])
+    except PluginError as e:
+        click.echo(click.style(f"  {e}", fg='red'))
+        click.echo(click.style(
+            "  (adopt the missing dependency plugins first)", fg='yellow'))
+        raise SystemExit(1)
+
+    write_last_serial(current_app.instance_path, index['serial'])
+    click.echo("")
+    click.echo(click.style(
+        f"Adopted {name} {resolved_version}.", fg='green'))
+    if result['installed']:
+        click.echo(f"  installed: {', '.join(result['installed'])}")
+    if result['enabled']:
+        click.echo(f"  enabled:   {', '.join(result['enabled'])}")
+    click.echo(click.style(
+        "Run 'flask plugin upgrade-all' then restart so routes register.",
+        fg='yellow'))
+
+
+@plugin_cli.command('audit')
+@click.option('--pubkey', 'pubkeys', multiple=True, type=click.Path(exists=True))
+@with_appcontext
+def audit_plugins(pubkeys):
+    """Warn about installed plugins whose version is revoked on the shelf."""
+    from .shelf import load_index, is_revoked
+
+    pm = current_app.extensions.get('plugin_manager')
+    if not pm:
+        click.echo(click.style("Plugin manager not initialized", fg='red'))
+        raise SystemExit(1)
+
+    shelf_dir = current_app.config.get('PLUGIN_SHELF_DIR')
+    if not shelf_dir:
+        click.echo(click.style("PLUGIN_SHELF_DIR not configured", fg='red'))
+        raise SystemExit(1)
+
+    keys, _ = _shelf_keys(pubkeys)
+    index, errors = load_index(shelf_dir, keys)
+    for err in errors:
+        click.echo(click.style(f"  {err}", fg='red'))
+    if index is None:
+        raise SystemExit(1)
+
+    flagged = []
+    for name, state in pm.registry.get_all().items():
+        if is_revoked(index, name, state.version):
+            flagged.append(f"{name} {state.version}")
+
+    click.echo("")
+    if flagged:
+        click.echo(click.style(
+            "REVOKED plugins installed:", fg='red', bold=True))
+        for item in flagged:
+            click.echo(click.style(f"  {item}", fg='red'))
+        raise SystemExit(1)
+    click.echo(click.style("No installed plugin is revoked.", fg='green'))
+
+
 @plugin_cli.command('migrate')
 @click.argument('name')
 @click.option('--revision', default='head', help='Target revision')

shopdb/plugins/loader.py

@@ -45,6 +45,8 @@ class PluginLoader:
         self._loaded_plugins: Dict[str, BasePlugin] = {}
         self._plugin_classes: Dict[str, Type[BasePlugin]] = {}
         self._manifests: Dict[str, dict] = {}
+        # Set by PluginManager.init_app; a PluginVerifier or None (no policy).
+        self.verifier = None
 
     def _is_strict_mode(self, app: Optional[Flask]) -> bool:
         """Return True if loader should re-raise instead of isolating failures."""
@@ -198,6 +200,17 @@ class PluginLoader:
             manifest = self.load_manifest(name)
             self.check_contract_version(name, __contract_version__)
 
+            # verify-at-load: refuse to import plugin.py unless the tree matches
+            # a trusted signature (when the site enforces it). Runs BEFORE any
+            # plugin code is imported, so a tampered/unsigned plugin never runs.
+            if self.verifier is not None:
+                ok, reason = self.verifier.check(name)
+                if not ok:
+                    raise PluginError(
+                        f'Plugin {name} failed signature verification: {reason}',
+                        plugin_name=name,
+                    )
+
             for dep in manifest.get('dependencies', []):
                 if not self.registry.is_enabled(dep):
                     raise PluginDependencyError(

shopdb/plugins/migrations.py

@@ -18,6 +18,8 @@ class PluginMigrationManager:
     def __init__(self, plugins_dir: Path, database_url: str):
         self.plugins_dir = plugins_dir
         self.database_url = database_url
+        # Set by PluginManager.init_app; a PluginVerifier or None (no policy).
+        self.verifier = None
 
     def get_migrations_dir(self, plugin_name: str) -> Optional[Path]:
         """Get migrations directory for a plugin."""
@@ -36,6 +38,16 @@ class PluginMigrationManager:
 
         Uses flask db upgrade with the plugin's migrations directory.
         """
+        # verify-at-migrate: never run a plugin's DDL (full DB rights) from an
+        # unverified tree when the site enforces signing.
+        if self.verifier is not None:
+            ok, reason = self.verifier.check(plugin_name)
+            if not ok:
+                logger.error(
+                    "Refusing migrations for %s: signature verification failed "
+                    "(%s)", plugin_name, reason)
+                return False
+
         migrations_dir = self.get_migrations_dir(plugin_name)
 
         if not migrations_dir:

shopdb/plugins/shelf.py

@@ -0,0 +1,185 @@
+"""Signed plugin shelf: index build/verify, anti-rollback state, adopt mechanics
+(ADR-013 Phase 2).
+
+The shelf is a read-only folder of .shopdbplugin artifacts plus a signed
+shelf-index.json (+ .sig). The index carries a monotonic serial (a site refuses
+an index older than the last it saw) and a revoked list. Browse reads the index;
+ADOPT always re-verifies the artifact's own signed manifest, never trusting the
+index for anything security-bearing.
+"""
+
+import json
+import shutil
+import zipfile
+from pathlib import Path
+
+from packaging.version import Version, InvalidVersion
+
+from . import signing, packaging
+
+INDEX_NAME = 'shelf-index.json'
+INDEX_SIG = 'shelf-index.sig'
+STATE_NAME = 'shelf-state.json'
+STAGING_DIR = '.staging'
+
+
+def _read_artifact_manifest(artifact_path) -> dict:
+    with zipfile.ZipFile(artifact_path) as archive:
+        return json.loads(archive.read('manifest.json'))
+
+
+def build_index(shelf_dir, private_key, serial: int,
+                publisher: str = '', revoked=None) -> dict:
+    """Scan the shelf for artifacts, build and SIGN the index. Caller supplies a
+    serial that must exceed any previously published one."""
+    shelf_dir = Path(shelf_dir)
+    plugins = {}
+    for artifact in sorted(shelf_dir.glob(f'*{packaging.ARTIFACT_SUFFIX}')):
+        manifest = _read_artifact_manifest(artifact)
+        plugins.setdefault(manifest['name'], []).append({
+            'version': manifest['version'],
+            'tier': manifest.get('tier', 'optional'),
+            'core_version': manifest.get('core_version', ''),
+            'artifact': artifact.name,
+        })
+
+    index = {
+        'serial': int(serial),
+        'publisher': publisher or '',
+        'plugins': plugins,
+        'revoked': list(revoked or []),
+    }
+    index_bytes = signing.serialize_provenance(index)  # canonical, sorted bytes
+    (shelf_dir / INDEX_NAME).write_bytes(index_bytes)
+    (shelf_dir / INDEX_SIG).write_bytes(signing.sign(private_key, index_bytes))
+    return index
+
+
+def load_index(shelf_dir, public_keys):
+    """(index_or_None, errors). Verifies the index signature. Serial-vs-state is
+    the caller's check (needs the instance path)."""
+    shelf_dir = Path(shelf_dir)
+    index_path = shelf_dir / INDEX_NAME
+    sig_path = shelf_dir / INDEX_SIG
+    if not index_path.exists() or not sig_path.exists():
+        return None, ['shelf has no signed index (shelf-index.json/.sig)']
+
+    index_bytes = index_path.read_bytes()
+    signature = sig_path.read_bytes()
+
+    errors = []
+    if not public_keys:
+        errors.append('no trusted keys - shelf index unverifiable')
+    elif not signing.verify(public_keys, index_bytes, signature):
+        errors.append('shelf index signature does not match any trusted key')
+
+    try:
+        index = json.loads(index_bytes)
+    except json.JSONDecodeError as exc:
+        return None, errors + [f'shelf index is not valid JSON: {exc}']
+    return index, errors
+
+
+def read_last_serial(instance_path) -> int:
+    state = Path(instance_path) / STATE_NAME
+    if state.exists():
+        try:
+            return int(json.loads(state.read_text()).get('last_serial', -1))
+        except (json.JSONDecodeError, ValueError, TypeError):
+            return -1
+    return -1
+
+
+def write_last_serial(instance_path, serial: int) -> None:
+    state = Path(instance_path) / STATE_NAME
+    state.parent.mkdir(parents=True, exist_ok=True)
+    state.write_text(json.dumps({'last_serial': int(serial)}, indent=2))
+
+
+def is_revoked(index: dict, name: str, version: str) -> bool:
+    for entry in index.get('revoked', []):
+        if entry.get('name') == name and entry.get('version') == version:
+            return True
+    return False
+
+
+def _safe_version(value):
+    try:
+        return Version(value)
+    except InvalidVersion:
+        return None
+
+
+def resolve_version(index: dict, name: str, version: str = None):
+    """Pick the artifact filename for name[==version] from the index.
+
+    With no version, picks the highest non-revoked version. Returns
+    (version, artifact_filename) or raises KeyError/ValueError.
+    """
+    candidates = index.get('plugins', {}).get(name)
+    if not candidates:
+        raise KeyError(f'plugin {name} not on the shelf')
+
+    if version is not None:
+        for entry in candidates:
+            if entry['version'] == version:
+                if is_revoked(index, name, version):
+                    raise ValueError(f'{name} {version} is revoked')
+                return version, entry['artifact']
+        raise KeyError(f'{name} {version} not on the shelf')
+
+    live = [e for e in candidates if not is_revoked(index, name, e['version'])]
+    if not live:
+        raise ValueError(f'every published version of {name} is revoked')
+    best = max(live, key=lambda e: (_safe_version(e['version']) or Version('0'),))
+    return best['version'], best['artifact']
+
+
+def unpack_verified(artifact_path, public_keys, plugins_dir, name: str):
+    """Verify an artifact, then atomically place it at plugins_dir/name.
+
+    Verifies signature + every file hash in a staging area BEFORE it can be
+    imported; only a fully verified tree is moved into place. Returns the final
+    plugin directory. Raises ValueError with all problems on any failure.
+    """
+    plugins_dir = Path(plugins_dir)
+
+    manifest, errors = packaging.verify_artifact(artifact_path, public_keys)
+    if errors:
+        raise ValueError('; '.join(errors))
+    if manifest.get('name') != name:
+        raise ValueError(
+            f"artifact manifest name '{manifest.get('name')}' != '{name}'")
+
+    staging = plugins_dir / STAGING_DIR
+    staging.mkdir(parents=True, exist_ok=True)
+    staged = staging / name
+    if staged.exists():
+        shutil.rmtree(staged)
+
+    with zipfile.ZipFile(artifact_path) as archive:
+        archive.extractall(staged)
+
+    # Re-verify the unpacked tree before it goes live (defense against an
+    # extract that did not land what we verified).
+    _, staged_errors = packaging.verify_dir(staged, public_keys)
+    if staged_errors:
+        shutil.rmtree(staged, ignore_errors=True)
+        raise ValueError('staged tree failed re-verification: '
+                         + '; '.join(staged_errors))
+
+    target = plugins_dir / name
+    backup = staging / f'{name}.backup'
+    if target.exists():
+        if backup.exists():
+            shutil.rmtree(backup)
+        target.rename(backup)
+    try:
+        staged.rename(target)
+    except OSError:
+        # cross-device or rename race: fall back to copy, keep backup on failure
+        shutil.copytree(staged, target)
+        shutil.rmtree(staged, ignore_errors=True)
+    if backup.exists():
+        shutil.rmtree(backup, ignore_errors=True)
+    return target

shopdb/plugins/verification.py

@@ -0,0 +1,64 @@
+"""Plugin signature enforcement policy (ADR-013 Phase 2).
+
+One PluginVerifier is built per app from config and shared by the loader
+(verify-at-load) and the migration manager (verify-at-migrate). A plugin only
+loads or migrates when it verifies against a trusted key - so signing stops
+being advisory and becomes enforced everywhere plugin code executes.
+
+Default is OFF (PLUGIN_REQUIRE_SIGNED unset), so existing deploys are
+unchanged. A site opts into hardening by stamping its plugins
+(`flask plugin stamp-bundled`), pinning keys (PLUGIN_TRUSTED_KEYS), and setting
+PLUGIN_REQUIRE_SIGNED. Fail-closed: an unverifiable plugin does NOT run.
+"""
+
+import logging
+from pathlib import Path
+
+from . import signing, packaging
+
+logger = logging.getLogger(__name__)
+
+
+class PluginVerifier:
+    """Decides whether a plugin may load/migrate under the site's trust policy."""
+
+    def __init__(self, plugins_dir, require_signed=False, trusted_key_paths=None,
+                 dev_trust_dirs=None, is_dev=False):
+        self.plugins_dir = Path(plugins_dir)
+        self.require_signed = bool(require_signed)
+        self.is_dev = bool(is_dev)
+        # dev trust dirs only mean anything in dev/test (ADR-013); prod ignores.
+        self.dev_trust_dirs = [Path(p).resolve() for p in (dev_trust_dirs or [])]
+        self._keys = signing.load_trusted_keys(trusted_key_paths or [])
+
+    def _dev_exempt(self, plugin_dir: Path) -> bool:
+        # unsigned dev/external-repo plugins pass, but ONLY under DEBUG/TESTING
+        if not self.is_dev or not self.dev_trust_dirs:
+            return False
+        resolved = Path(plugin_dir).resolve()
+        for trusted in self.dev_trust_dirs:
+            try:
+                resolved.relative_to(trusted)
+                return True
+            except ValueError:
+                continue
+        return False
+
+    def check(self, plugin_name: str):
+        """(ok, reason). ok True = plugin may load/migrate."""
+        if not self.require_signed:
+            return True, 'enforcement off'
+
+        plugin_dir = self.plugins_dir / plugin_name
+        if self._dev_exempt(plugin_dir):
+            return True, 'dev-trusted'
+
+        if not self._keys:
+            # require-signed with nothing to verify against = refuse everything.
+            return False, ('PLUGIN_REQUIRE_SIGNED is set but no trusted keys '
+                           'are configured')
+
+        _, errors = packaging.verify_dir(plugin_dir, self._keys)
+        if errors:
+            return False, '; '.join(errors)
+        return True, 'verified'

tests/test_plugin_lifecycle.py

@@ -58,18 +58,21 @@ def test_registry_migrates_equipment_to_machines(tmp_path):
 
 # --- Manager enable/disable dependency guards + permission seeding -----------
 
-def _write_plugin(plugins_dir, name, deps=None, permissions=None):
+def _write_plugin(plugins_dir, name, deps=None, permissions=None, tier=None):
     """Write a minimal valid synthetic plugin (manifest.json + plugin.py)."""
     pdir = plugins_dir / name
     pdir.mkdir()
-    (pdir / 'manifest.json').write_text(json.dumps({
+    manifest = {
         'name': name,
         'version': '1.0.0',
         'description': f'synthetic {name}',
         'dependencies': deps or [],
         'core_version': '>=0.1.0,<1.0.0',
         'api_prefix': f'/api/{name}',
-    }))
+    }
+    if tier:
+        manifest['tier'] = tier
+    (pdir / 'manifest.json').write_text(json.dumps(manifest))
     (pdir / '__init__.py').write_text('')
     (pdir / 'plugin.py').write_text(
         'from shopdb.plugins.base import BasePlugin, PluginMeta\n\n'
@@ -286,6 +289,28 @@ def test_validate_bundled_plugin_passes(app):
     assert 'valid' in result.output
 
 
+def test_core_tier_plugin_cannot_be_disabled_or_uninstalled(app, db, tmp_path):
+    """A tier=core plugin refuses disable and uninstall (ADR-013)."""
+    plugins_dir = tmp_path / 'plugins'
+    plugins_dir.mkdir()
+    _write_plugin(plugins_dir, 'kernel', tier='core')
+
+    registry = PluginRegistry(tmp_path / 'plugins.json')
+    registry.register('kernel', '1.0.0', enabled=True)
+
+    pm = PluginManager()
+    pm._app = app
+    pm._db = db
+    pm.registry = registry
+    pm.loader = PluginLoader(plugins_dir, registry)
+    pm.migration_manager = None
+
+    assert pm.disable_plugin('kernel') is False
+    assert pm.registry.is_enabled('kernel')
+    assert pm.uninstall_plugin('kernel') is False
+    assert pm.registry.is_installed('kernel')
+
+
 # --- Seed idempotency -------------------------------------------------------
 
 def test_seed_settings_is_idempotent(app, db):

tests/test_plugin_shelf.py

@@ -0,0 +1,180 @@
+"""Signed shelf index + adopt-mechanics tests (ADR-013 Phase 2).
+
+Covers: a signed index round-trips and a tampered/wrong-key index is rejected;
+serial anti-rollback state; revocation; version resolution; and unpack_verified
+placing only a fully verified tree (refusing a tampered artifact).
+"""
+
+import json
+import zipfile
+
+import pytest
+
+from shopdb.plugins import signing, packaging, shelf
+
+
+def _write_plugin(plugin_dir, name='demo', version='1.0.0', tier=None):
+    plugin_dir.mkdir(parents=True, exist_ok=True)
+    manifest = {
+        'name': name, 'version': version, 'description': 'demo',
+        'dependencies': [], 'core_version': '>=0.1.0,<1.0.0',
+        'api_prefix': f'/api/{name}',
+    }
+    if tier:
+        manifest['tier'] = tier
+    (plugin_dir / 'manifest.json').write_text(json.dumps(manifest))
+    (plugin_dir / 'plugin.py').write_text('# demo\n')
+    return plugin_dir
+
+
+@pytest.fixture
+def keypair(tmp_path):
+    private_pem, public_pem = signing.generate_keypair()
+    return (signing.load_private_key(private_pem),
+            signing.load_public_key(public_pem))
+
+
+def _build_shelf(tmp_path, private_key, specs, serial=1, revoked=None):
+    """specs: list of (name, version). Packs each into a shelf dir + index."""
+    shelf_dir = tmp_path / 'shelf'
+    shelf_dir.mkdir(parents=True, exist_ok=True)
+    for name, version in specs:
+        src = tmp_path / 'src' / f'{name}-{version}'
+        _write_plugin(src, name=name, version=version)
+        packaging.pack(src, private_key, out_dir=shelf_dir)
+    shelf.build_index(shelf_dir, private_key, serial, revoked=revoked)
+    return shelf_dir
+
+
+# --- index sign/verify ------------------------------------------------------
+
+def test_index_round_trip(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(tmp_path, priv, [('demo', '1.0.0')])
+    index, errors = shelf.load_index(shelf_dir, [pub])
+    assert errors == []
+    assert index['serial'] == 1
+    assert 'demo' in index['plugins']
+
+
+def test_tampered_index_rejected(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(tmp_path, priv, [('demo', '1.0.0')])
+    # change the index bytes but keep the old signature
+    path = shelf_dir / shelf.INDEX_NAME
+    data = json.loads(path.read_text())
+    data['serial'] = 999
+    path.write_bytes(signing.serialize_provenance(data))
+    _, errors = shelf.load_index(shelf_dir, [pub])
+    assert any('signature does not match' in e for e in errors)
+
+
+def test_index_wrong_key_rejected(tmp_path, keypair):
+    priv, _ = keypair
+    shelf_dir = _build_shelf(tmp_path, priv, [('demo', '1.0.0')])
+    wrong_pub = signing.load_public_key(signing.generate_keypair()[1])
+    _, errors = shelf.load_index(shelf_dir, [wrong_pub])
+    assert any('signature does not match' in e for e in errors)
+
+
+def test_index_no_keys_unverifiable(tmp_path, keypair):
+    priv, _ = keypair
+    shelf_dir = _build_shelf(tmp_path, priv, [('demo', '1.0.0')])
+    _, errors = shelf.load_index(shelf_dir, [])
+    assert any('unverifiable' in e for e in errors)
+
+
+# --- anti-rollback state ----------------------------------------------------
+
+def test_serial_state_round_trip(tmp_path):
+    assert shelf.read_last_serial(tmp_path) == -1
+    shelf.write_last_serial(tmp_path, 5)
+    assert shelf.read_last_serial(tmp_path) == 5
+
+
+# --- revocation + version resolution ----------------------------------------
+
+def test_resolve_highest_non_revoked(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(
+        tmp_path, priv, [('demo', '1.0.0'), ('demo', '1.2.0')])
+    index, _ = shelf.load_index(shelf_dir, [pub])
+    version, artifact = shelf.resolve_version(index, 'demo')
+    assert version == '1.2.0'
+    assert artifact == 'demo-1.2.0.shopdbplugin'
+
+
+def test_resolve_skips_revoked(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(
+        tmp_path, priv, [('demo', '1.0.0'), ('demo', '1.2.0')],
+        revoked=[{'name': 'demo', 'version': '1.2.0'}])
+    index, _ = shelf.load_index(shelf_dir, [pub])
+    version, _ = shelf.resolve_version(index, 'demo')
+    assert version == '1.0.0'
+
+
+def test_resolve_explicit_revoked_raises(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(
+        tmp_path, priv, [('demo', '1.0.0')],
+        revoked=[{'name': 'demo', 'version': '1.0.0'}])
+    index, _ = shelf.load_index(shelf_dir, [pub])
+    with pytest.raises(ValueError):
+        shelf.resolve_version(index, 'demo', '1.0.0')
+
+
+def test_resolve_unknown_raises(tmp_path, keypair):
+    priv, pub = keypair
+    shelf_dir = _build_shelf(tmp_path, priv, [('demo', '1.0.0')])
+    index, _ = shelf.load_index(shelf_dir, [pub])
+    with pytest.raises(KeyError):
+        shelf.resolve_version(index, 'nope')
+
+
+# --- verified atomic unpack -------------------------------------------------
+
+def test_unpack_verified_places_tree(tmp_path, keypair):
+    priv, pub = keypair
+    src = _write_plugin(tmp_path / 'src' / 'demo')
+    artifact = packaging.pack(src, priv, out_dir=tmp_path / 'dist')
+    plugins_dir = tmp_path / 'live'
+    plugins_dir.mkdir()
+
+    target = shelf.unpack_verified(artifact, [pub], plugins_dir, 'demo')
+    assert target == plugins_dir / 'demo'
+    assert (target / 'manifest.json').exists()
+    assert (target / signing.PROVENANCE_NAME).exists()
+    # staging cleaned up
+    assert not (plugins_dir / shelf.STAGING_DIR / 'demo').exists()
+
+
+def test_unpack_verified_refuses_tampered(tmp_path, keypair):
+    priv, pub = keypair
+    src = _write_plugin(tmp_path / 'src' / 'demo')
+    artifact = packaging.pack(src, priv, out_dir=tmp_path / 'dist')
+
+    # tamper a member, keep provenance + sig
+    with zipfile.ZipFile(artifact) as zin:
+        members = {n: zin.read(n) for n in zin.namelist()}
+    members['manifest.json'] += b'tampered'
+    with zipfile.ZipFile(artifact, 'w', zipfile.ZIP_DEFLATED) as zout:
+        for name, data in members.items():
+            zout.writestr(name, data)
+
+    plugins_dir = tmp_path / 'live'
+    plugins_dir.mkdir()
+    with pytest.raises(ValueError):
+        shelf.unpack_verified(artifact, [pub], plugins_dir, 'demo')
+    # nothing placed
+    assert not (plugins_dir / 'demo').exists()
+
+
+def test_unpack_verified_wrong_name_refused(tmp_path, keypair):
+    priv, pub = keypair
+    src = _write_plugin(tmp_path / 'src' / 'demo')
+    artifact = packaging.pack(src, priv, out_dir=tmp_path / 'dist')
+    plugins_dir = tmp_path / 'live'
+    plugins_dir.mkdir()
+    with pytest.raises(ValueError):
+        shelf.unpack_verified(artifact, [pub], plugins_dir, 'somethingelse')

tests/test_plugin_verification.py

@@ -0,0 +1,167 @@
+"""Verify-at-load / verify-at-migrate enforcement tests (ADR-013 Phase 2).
+
+The security model rests on: with PLUGIN_REQUIRE_SIGNED set, a plugin only
+loads or migrates when its tree matches a trusted signature; unsigned, tampered,
+or wrong-key plugins are refused; and the dev-trust exemption only relaxes this
+under DEBUG/TESTING.
+"""
+
+import json
+
+import pytest
+
+from shopdb.exceptions import PluginError
+from shopdb.plugins import signing
+from shopdb.plugins.loader import PluginLoader
+from shopdb.plugins.migrations import PluginMigrationManager
+from shopdb.plugins.registry import PluginRegistry
+from shopdb.plugins.verification import PluginVerifier
+
+
+def _write_loadable_plugin(plugins_dir, name='demo'):
+    """A minimal plugin whose plugin.py actually imports + instantiates."""
+    pdir = plugins_dir / name
+    pdir.mkdir(parents=True, exist_ok=True)
+    (pdir / 'manifest.json').write_text(json.dumps({
+        'name': name, 'version': '1.0.0', 'description': 'demo',
+        'dependencies': [], 'core_version': '>=0.1.0,<1.0.0',
+        'api_prefix': f'/api/{name}',
+    }))
+    (pdir / '__init__.py').write_text('')
+    (pdir / 'plugin.py').write_text(
+        'from shopdb.plugins.base import BasePlugin, PluginMeta\n\n'
+        'class ThePlugin(BasePlugin):\n'
+        '    @property\n'
+        '    def meta(self):\n'
+        f'        return PluginMeta(name={name!r}, version="1.0.0",\n'
+        '                          description="demo")\n'
+        '    def get_blueprint(self):\n'
+        '        return None\n'
+        '    def get_models(self):\n'
+        '        return []\n'
+    )
+    return pdir
+
+
+def _stamp(pdir, private_key, name='demo', version='1.0.0'):
+    provenance = signing.build_provenance(pdir, name, version, 'publisher')
+    provenance_bytes = signing.serialize_provenance(provenance)
+    (pdir / signing.PROVENANCE_NAME).write_bytes(provenance_bytes)
+    (pdir / signing.PROVENANCE_SIG).write_bytes(
+        signing.sign(private_key, provenance_bytes))
+
+
+@pytest.fixture
+def keypair(tmp_path):
+    private_pem, public_pem = signing.generate_keypair()
+    pub_path = tmp_path / 'curator.pub'
+    pub_path.write_bytes(public_pem)
+    return signing.load_private_key(private_pem), str(pub_path)
+
+
+# --- PluginVerifier policy --------------------------------------------------
+
+def test_enforcement_off_allows_unsigned(tmp_path):
+    _write_loadable_plugin(tmp_path / 'plugins')
+    verifier = PluginVerifier(tmp_path / 'plugins', require_signed=False)
+    ok, _ = verifier.check('demo')
+    assert ok
+
+
+def test_require_signed_without_keys_refuses_all(tmp_path):
+    _write_loadable_plugin(tmp_path / 'plugins')
+    verifier = PluginVerifier(tmp_path / 'plugins', require_signed=True,
+                              trusted_key_paths=[])
+    ok, reason = verifier.check('demo')
+    assert not ok
+    assert 'no trusted keys' in reason
+
+
+def test_signed_plugin_passes(tmp_path, keypair):
+    private_key, pub_path = keypair
+    pdir = _write_loadable_plugin(tmp_path / 'plugins')
+    _stamp(pdir, private_key)
+    verifier = PluginVerifier(tmp_path / 'plugins', require_signed=True,
+                              trusted_key_paths=[pub_path])
+    ok, reason = verifier.check('demo')
+    assert ok, reason
+
+
+def test_tampered_plugin_refused(tmp_path, keypair):
+    private_key, pub_path = keypair
+    pdir = _write_loadable_plugin(tmp_path / 'plugins')
+    _stamp(pdir, private_key)
+    (pdir / 'plugin.py').write_text('# tampered after signing\n')
+    verifier = PluginVerifier(tmp_path / 'plugins', require_signed=True,
+                              trusted_key_paths=[pub_path])
+    ok, reason = verifier.check('demo')
+    assert not ok
+    assert 'hash mismatch' in reason
+
+
+def test_wrong_key_refused(tmp_path, keypair):
+    private_key, _ = keypair
+    other_pub = tmp_path / 'attacker.pub'
+    other_pub.write_bytes(signing.generate_keypair()[1])
+    pdir = _write_loadable_plugin(tmp_path / 'plugins')
+    _stamp(pdir, private_key)
+    verifier = PluginVerifier(tmp_path / 'plugins', require_signed=True,
+                              trusted_key_paths=[str(other_pub)])
+    ok, reason = verifier.check('demo')
+    assert not ok
+    assert 'signature does not match' in reason
+
+
+def test_dev_trust_dir_only_relaxes_in_dev(tmp_path, keypair):
+    _, pub_path = keypair
+    _write_loadable_plugin(tmp_path / 'plugins')  # unsigned
+    plugins_dir = tmp_path / 'plugins'
+
+    dev = PluginVerifier(plugins_dir, require_signed=True,
+                         trusted_key_paths=[pub_path],
+                         dev_trust_dirs=[str(plugins_dir)], is_dev=True)
+    ok, reason = dev.check('demo')
+    assert ok and reason == 'dev-trusted'
+
+    prod = PluginVerifier(plugins_dir, require_signed=True,
+                          trusted_key_paths=[pub_path],
+                          dev_trust_dirs=[str(plugins_dir)], is_dev=False)
+    ok2, _ = prod.check('demo')
+    assert not ok2
+
+
+# --- verify-at-load and verify-at-migrate integration -----------------------
+
+def test_loader_refuses_unsigned_when_enforced(tmp_path, app, db):
+    plugins_dir = tmp_path / 'plugins'
+    _write_loadable_plugin(plugins_dir)
+    registry = PluginRegistry(tmp_path / 'plugins.json')
+    loader = PluginLoader(plugins_dir, registry)
+    loader.verifier = PluginVerifier(plugins_dir, require_signed=True,
+                                     trusted_key_paths=[])
+    # TESTING app is strict -> the refusal surfaces as a raise.
+    with pytest.raises(PluginError):
+        loader.load_plugin('demo', app, db)
+
+
+def test_loader_loads_signed_when_enforced(tmp_path, app, db, keypair):
+    private_key, pub_path = keypair
+    plugins_dir = tmp_path / 'plugins'
+    pdir = _write_loadable_plugin(plugins_dir)
+    _stamp(pdir, private_key)
+    registry = PluginRegistry(tmp_path / 'plugins.json')
+    loader = PluginLoader(plugins_dir, registry)
+    loader.verifier = PluginVerifier(plugins_dir, require_signed=True,
+                                     trusted_key_paths=[pub_path])
+    plugin = loader.load_plugin('demo', app, db)
+    assert plugin is not None
+    assert plugin.meta.name == 'demo'
+
+
+def test_migrate_refused_when_unverified(tmp_path):
+    plugins_dir = tmp_path / 'plugins'
+    _write_loadable_plugin(plugins_dir)
+    manager = PluginMigrationManager(plugins_dir, 'sqlite://')
+    manager.verifier = PluginVerifier(plugins_dir, require_signed=True,
+                                      trusted_key_paths=[])
+    assert manager.run_plugin_migrations('demo') is False