PEP 761: Add notes for Cosign and offline verification (#4055)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

peps/pep-0761.rst

@@ -207,9 +207,57 @@ During the `pre-PEP discussion <https://discuss.python.org/t/pre-pep-discussion-
 there was a question of whether offline verification was supported by
 Sigstore. Using a Sigstore bundle (:file:`.sigstore`) file, `Sigstore clients
 support verifying the artifact completely offline <https://discuss.python.org/t/pre-pep-discussion-stop-providing-gpg-signatures-for-cpython-artifacts/65058/9>`_.
-When in offline mode, Sigstore can't verify whether a signature has been
-revoked. This is a similar restriction to PGP key revocations not being
-detectable during offline verification.
+
+Using offline verification with Sigstore requires disabling root of trust
+updates and "pinning" a root of trust in a file to use during verification.
+
+Pinning a root of trust means signatures made after a *new* root of trust
+is established would no longer be able to verify using a "pinned" previous
+root of trust. New roots of trust are expected to be rare events, such as
+when the root of trust is compromised, and in this case verifiers would
+want signatures to fail to verify.
+
+Offline verification also makes revocation checks impossible, but this
+is similar to PGP's model where revocation of keys requires an online lookup.
+
+Barring rare events like root of trust compromise, using offline verification
+with Sigstore doesn't impose additional operations requirements to verifiers.
+
+Support for a pre-compiled executable for verification
+------------------------------------------------------
+
+During discussion there were requests for a pre-compiled executable that could
+be used for verifying Sigstore bundles without needing to either install
+a Go build toolchain to build `sigstore-go <https://github.com/sigstore/sigstore-go>`_
+from source or already have a working Python installation for
+`sigstore-python <https://github.com/sigstore/sigstore-python/>`_.
+
+`Cosign <https://github.com/sigstore/cosign/>`_ is another Sigstore project
+that provides pre-compiled standalone binaries and supports verifying bundles
+offline:
+
+.. code-block::
+
+    # Download Cosign
+    wget https://github.com/sigstore/cosign/releases/download/v2.4.1/cosign-linux-amd64
+
+    # For offline verification, also need the Root of Trust. Can be grabbed
+    # from GitHub at: https://github.com/sigstore/root-signing/blob/main/targets/trusted_root.json
+    wget https://raw.githubusercontent.com/sigstore/root-signing/refs/heads/main/targets/trusted_root.json
+
+    # Download CPython artifacts
+    wget https://www.python.org/ftp/python/3.13.0/Python-3.13.0.tgz
+    wget https://www.python.org/ftp/python/3.13.0/Python-3.13.0.tgz.sigstore
+
+    ./cosign-linux-amd64 verify-blob \
+      --new-bundle-format \
+      --certificate-oidc-issuer 'https://accounts.google.com' \
+      --certificate-identity 'thomas@python.org' \
+      --bundle ./Python-3.13.0.tgz.sigstore \
+      # --offline and --trust-root optional for offline verification
+      --offline \
+      --trust-root ./trusted_root.json \
+      ./Python-3.13.0.tgz
 
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