cli: --offline means fully offline

CHANGELOG.md

@@ -8,6 +8,12 @@ All versions prior to 0.9.0 are untracked.
 
 ## [Unreleased]
 
+### Changed
+
+* CLI: When verifying, the `--offline` flag now fully disables all online
+  operations, including routine local TUF repository refreshes
+  ([#1143](https://github.com/sigstore/sigstore-python/pull/1143))
+
 ## [3.3.0]
 
 ### Added

README.md

@@ -29,7 +29,9 @@ else!
   * [Signing with ambient credentials](#signing-with-ambient-credentials)
   * [Signing with an email identity](#signing-with-an-email-identity)
   * [Signing with an explicit identity token](#signing-with-an-explicit-identity-token)
-  * [Verifying against a signature and certificate](#verifying-against-a-signature-and-certificate)
+  * [Verifying against a bundle](#verifying-against-a-bundle)
+  * [Offline verification](#offline-verification)
+  * [Verifying a digest instead of a file](#verifying-a-digest-instead-of-a-file)
   * [Verifying signatures from GitHub Actions](#verifying-signatures-from-github-actions)
 * [Licensing](#licensing)
 * [Community](#community)
@@ -402,7 +404,7 @@ $ python -m sigstore sign --identity-token YOUR-LONG-JWT-HERE foo.txt
 Note that passing a custom identity token does not circumvent Fulcio's requirements,
 namely the Fulcio's supported identity providers and the claims expected within the token.
 
-### Verifying against a signature and certificate
+### Verifying against a bundle
 
 By default, `sigstore verify identity` will attempt to find a `<filename>.sigstore.json`
 or `<filename>.sigstore` in the same directory as the file being verified:
@@ -423,6 +425,50 @@ $ python -m sigstore verify identity foo.txt bar.txt \
     --cert-oidc-issuer 'https://github.com/login/oauth'
 ```
 
+### Offline verification
+
+> [!IMPORTANT]
+> Because `--offline` disables trust root updates, `sigstore-python` falls back
+> to the latest cached trust root or, if none exists, the trust root baked
+> into `sigstore-python` itself. Like with any other offline verification,
+> this means that users may miss trust root changes (such as new root keys,
+> or revocations) unless they separately keep the trust root up-to-date.
+>
+> Users who need to operationalize offline verification may wish to do this
+> by distributing their own trust configuration; see
+> [Configuring a custom root of trust](#configuring-a-custom-root-of-trust-byo-pki).
+
+During verification, there are two kinds of network access that `sigstore-python`
+*can* perform:
+
+1. When verifying against "detached" materials (e.g. separate `.crt` and `.sig`
+   files), `sigstore-python` can perform an online transparency log lookup.
+2. By default, during all verifications, `sigstore-python` will attempt to
+   refresh the locally cached root of trust via a TUF update.
+
+When performing bundle verification (i.e. `.sigstore` or `.sigstore.json`),
+(1) does not apply. However, (2) can still result in online accesses.
+
+To perform **fully** offline verification, pass `--offline` to your
+`sigstore verify` subcommand:
+
+```bash
+$ python -m sigstore verify identity foo.txt \
+    --offline \
+    --cert-identity 'hamilcar@example.com' \
+    --cert-oidc-issuer 'https://github.com/login/oauth'
+```
+
+Alternatively, users may choose to bypass TUF entirely by passing
+an entire trust configuration to `sigstore-python` via `--trust-config`:
+
+```bash
+$ python -m sigstore --trust-config public.trustconfig.json verify identity ...
+```
+
+This will similarly result in fully offline operation, as the trust
+configuration contains a full trust root.
+
 ### Verifying a digest instead of a file
 
 `sigstore-python` supports verifying digests directly, without requiring the artifact to be

sigstore/_cli.py

@@ -996,12 +996,12 @@ def _collect_verification_state(
 
     if args.staging:
         _logger.debug("verify: staging instances requested")
-        verifier = Verifier.staging()
+        verifier = Verifier.staging(offline=args.offline)
     elif args.trust_config:
         trust_config = ClientTrustConfig.from_json(args.trust_config.read_text())
         verifier = Verifier._from_trust_config(trust_config)
     else:
-        verifier = Verifier.production()
+        verifier = Verifier.production(offline=args.offline)
 
     all_materials = []
     for file_or_hashed, materials in input_map.items():

sigstore/_internal/tuf.py

@@ -115,7 +115,11 @@ def __init__(self, url: str, offline: bool = False) -> None:
         _logger.debug(f"TUF targets cache: {self._targets_dir}")
 
         self._updater: None | Updater = None
-        if not offline:
+        if offline:
+            _logger.warning(
+                "TUF repository is loaded in offline mode; updates will not be performed"
+            )
+        else:
             # Initialize and update the toplevel TUF metadata
             self._updater = Updater(
                 metadata_dir=str(self._metadata_dir),

sigstore/verify/verifier.py

@@ -77,23 +77,23 @@ def __init__(self, *, rekor: RekorClient, trusted_root: TrustedRoot):
         self._trusted_root = trusted_root
 
     @classmethod
-    def production(cls) -> Verifier:
+    def production(cls, *, offline: bool = False) -> Verifier:
         """
         Return a `Verifier` instance configured against Sigstore's production-level services.
         """
         return cls(
             rekor=RekorClient.production(),
-            trusted_root=TrustedRoot.production(),
+            trusted_root=TrustedRoot.production(offline=offline),
         )
 
     @classmethod
-    def staging(cls) -> Verifier:
+    def staging(cls, *, offline: bool = False) -> Verifier:
         """
         Return a `Verifier` instance configured against Sigstore's staging-level services.
         """
         return cls(
             rekor=RekorClient.staging(),
-            trusted_root=TrustedRoot.staging(),
+            trusted_root=TrustedRoot.staging(offline=offline),
         )
 
     @classmethod

test/unit/verify/test_verifier.py

@@ -69,16 +69,27 @@ def test_verifier_multiple_verifications(signing_materials, null_policy):
         verifier.verify_artifact(file.read_bytes(), bundle, null_policy)
 
 
+@pytest.mark.online
 @pytest.mark.parametrize(
     "filename", ("bundle.txt", "bundle_v3.txt", "bundle_v3_alt.txt")
 )
-def test_verifier_bundle(signing_bundle, null_policy, mock_staging_tuf, filename):
+def test_verifier_bundle(signing_bundle, null_policy, filename):
     (file, bundle) = signing_bundle(filename)
 
     verifier = Verifier.staging()
     verifier.verify_artifact(file.read_bytes(), bundle, null_policy)
 
 
+@pytest.mark.parametrize(
+    "filename", ("bundle.txt", "bundle_v3.txt", "bundle_v3_alt.txt")
+)
+def test_verifier_bundle_offline(signing_bundle, null_policy, filename):
+    (file, bundle) = signing_bundle(filename)
+
+    verifier = Verifier.staging(offline=True)
+    verifier.verify_artifact(file.read_bytes(), bundle, null_policy)
+
+
 @pytest.mark.staging
 def test_verifier_email_identity(signing_materials):
     verifier = Verifier.staging()