posts: Add guide to verify a commit from a tarball

When generating archives, git-archive(1) puts the commit ID in the
global extended pax header. Therefore, an interesting side effect of
signing tarballs generated with git-archive(1) is that we also
implicitly sign a commit. Whilst we don't expect anyone to go through
the hurdles needed to verify such a commit signature, make sure to
document this case anyway.

1 files changed, 26 insertions, 0 deletions

diff --git a/posts/verify-with-signify.md b/posts/verify-with-signify.md
index 5103ccc..0fd22c4 100644
--- a/posts/verify-with-signify.md
+++ b/posts/verify-with-signify.md
@@ -56,4 +56,30 @@ the `sha256sum` tool to only verify the integrity of the download:
 	$ tail -n1 <snapshot>.asc | sha256sum -c
 	<snapshot>: OK
 
+### Verification of the corresponding commit
+
+Every tarball hosted here contains its corresponding commit ID in the global
+extended pax header[^2]. Therefore, if a tarball is signed, it may be used to verify
+a commit and all of its ancestors.
+
+To do so, place the tarball in the cloned git repository first. If you already
+have the detached signature, you may verify the tarball normally like shown
+above. Otherwise, you can fetch the signatures directly from the repository:
+
+	$ git fetch origin refs/notes/signatures/tar.gz:refs/notes/signatures/tar.gz
+
+Now you can verify the tarball like so:
+
+	$ git notes --ref=signatures/tar.gz show <version> | signify -Cp release.pub -x -
+	Signature Verified
+	<snapshot>: OK
+
+Once verified, you can extract the commit using
+[`git-get-tar-commit-id(1)`](https://git-scm.com/docs/git-get-tar-commit-id) and
+show it:
+
+	$ gzip -d -c <snapshot> | git get-tar-commit-id | xargs git show
+
 [^1]: If you are interested in how this is put together, check out [this](signify-cgit) post.
+[^2]: [`git-archive(1)`](https://git-scm.com/docs/git-archive#_description) puts
+  it there.