Signing Project Releases | Articles - Fabien Potencier

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About a year ago, I started to sign all my Open-Source project releases. I briefly mentioned it during my SymfonyCon keynote in Warsaw, but this post is going to give you some more details.

Whenever I release a new version of a project, I sign the Git tag with my PGP key: DD4E C589 15FF 888A 8A3D D898 EB8A A69A 566C 0795.

Checking Git Tag Signatures#

If you want to verify a specific release, you need to install PGP first, and then get my PGP key:

$ gpg --keyserver pgp.mit.edu --recv-keys 0xeb8aa69a566c0795

Then, use git tag to check the related tag. Here is how to check the Symfony 2.4.2 tag (from a Symfony clone):

$ git tag -v v2.4.2

Verification worked if the output contains the key used to sign the tag (566C0795) and contains a text starting with "Good signature from ...". Because of how Git works, having a good signature on a tag also means that all commits reachable from that tag are covered by this signature (that's why signing all commits/merges is not needed.)

You can see the PGP signature by using the following command:

$ git show --show-signature v2.4.2

For the curious ones, I'm going to take Symfony 2.4.2 as an example to explain how it works. First, Git does not sign the contents of a commit itself (which is empty anyway for tags), but its headers. Let's display the headers for the Symfony v2.4.2 tag:

$ git cat-file -p v2.4.2

You should get the following output:

object b70633f92ff71ef490af4c17e7ca3f3bf3d0f304
type commit
tag v2.4.2
tagger Fabien Potencier 
      
   
   
        
   
     1392233223 +0100 created tag 2.4.2 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.13 (Darwin) iF4EABEIAAYFAlL7ywcACgkQ64qmmlZsB5W1cAEAtZOVz5OT7i8vAEiLqnMYyM5n +XMbyTMVXyYfBqjqkmUA/AxAFTp7oTeHY3yepx/uuxF91+DOnvbxf4b2BqSCx0dq =sv1G -----END PGP SIGNATURE----- 
   

The PGP signature is calculated on all lines up to the beginning of the signature:

object b70633f92ff71ef490af4c17e7ca3f3bf3d0f304
type commit
tag v2.4.2
tagger Fabien Potencier 
      
   
   
        
   
     1392233223 +0100 created tag 2.4.2 
   

You can try it by yourself by saving those lines in a test file, and create a test.sig file with the PGP signature:

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.13 (Darwin)

iF4EABEIAAYFAlL7ywcACgkQ64qmmlZsB5W1cAEAtZOVz5OT7i8vAEiLqnMYyM5n
+XMbyTMVXyYfBqjqkmUA/AxAFTp7oTeHY3yepx/uuxF91+DOnvbxf4b2BqSCx0dq
=sv1G
-----END PGP SIGNATURE-----

Then, check that the signature matches the Git headers with the following command:

$ gpg --verify test.asc test

So, when signing a tag, you sign the commit sha1 (and so all reachable commits), but also the tag name (and so the version you expect to get).

Signing Github Archives#

That's great, but when using Composer, you can get the code either as a Git clone (--prefer-source) or as an archive (--prefer-dist). If Composer uses the latter, you cannot use the signature coming from the tag, so how can you check the validity of what Composer just downloaded?

Whenever I make a new release, I also publish a file containing a sha1 for the zip file as returned by the Github API (https://api.github.com/repos/XXX/XXX/zipball/VERSION) but also a sha1 calculated on the file contents from the zip (the exact same files installed by Composer.) Those files are hosted on a dedicated checksums repository on Github.

As an example, let's say I have a project using Symfony 2.4.2 (you can check the version installed by Composer by running composer show -i). The sha1s are available here: https://raw.githubusercontent.com/sensiolabs/checksums/master/symfony/symfony/v2.4.2.txt.

This file is signed, so you first need to verify it:

$ curl -O https://raw.githubusercontent.com/sensiolabs/checksums/master/symfony/symfony/v2.4.2.txt
$ gpg --verify v2.4.2.txt

Now, you can check the validity of the files downloaded and installed by Composer:

$ cd PATH/TO/vendor/symfony/symfony
$ find . -type f -print0 | xargs -0 shasum | shasum

The sha1 displayed should match the one from the file you've just downloaded (the one under the files_sha1 entry.)

To make it easier, you can also check all your dependencies via a simple script provided in the repository. From your project root directory (where the composer.json file is stored), run the following

$ PATH/TO/check-vendors.sh

It will output something along the lines of:

symfony/swiftmailer-bundle@v2.2.6                        OK  files signature
symfony/symfony@v2.5.2                                   KO  files signature
twig/extensions@v1.0.1                                   OK  files signature
twig/twig@v1.15.0                                        OK  files signature
white-october/pagerfanta-bundle@dev-master               --  unknown package
willdurand/hateoas@1.0.x-dev                             --  unknown package

 1 packages are potentially corrupted.
 Check that your did not add/modify/delete some files.

Consider the checksum feature as experimental and as such, any feedbacks would be much appreciated.