sbt-gpg

Simple and secure artifact signing for sbt.

This sbt plugin aims to make artifact signing simple and unobtrusive. It is guided by two core ideas:

1. easy configuration with sane defaults
2. use of standard cryptography tools (gpg)

The motivation is that these principles are both essential for promoting secure builds.

Highlights

• Uses the system command gpg to do all operations. This enables advanced features such as use of smartcards, key splitting, or cutting-edge ciphers.

• Hooks into the publish and publishLocal tasks. All artifacts will be signed; there is no need to run a separate publishSigned task.

• Unobtrusive configuration. Key selection can be done through sbt's credentials mechanism, thus enabling global configuration without the need of adding a global plugin.

• Works out-of-the-box. publishLocal falls back to unsigned artifacts in case key material cannot be found, after emitting an explicit warning. publish will fail the build by default if signing fails to avoid accidentally publishing unsigned artifacts, though you can override this with a setting.

Requirements

• sbt version >= 1.0.0
• gpg installed on user's machine (this requirement won't get in the way of a user's productivity; missing gpg will simply disable the functionality provided by this plugin)

Getting started

Latest available tag:

addSbtPlugin("io.crashbox" % "sbt-gpg" % "<latest_tag>")

Copy the above snippet to an sbt configuration file. E.g.

• project/plugins.sbt to enable the plugin on a per-project basis
• ~/.sbt/1.0/plugins/gpg.sbt to enable the plugin globally (not recommended)

That's it! The autoplugin "SbtGpg" will now be enabled for the given project(s). It will modify the publish and publishLocal tasks to always include signatures of all published artifacts.

The default configuration will pick up local GPG settings. See the next section to find out how to customize the plugin.

Configuration

Signing key

By default, all signing operations will use gpg's default key. A specific key can be used by setting sbt Credentials for the host "gpg".

credentials += Credentials(
  "GnuPG Key ID",
  "gpg",
  "4E7DA7B5A0F86992D6EB3F514601878662E33372", // key identifier
  "ignored" // this field is ignored; passwords are supplied by pinentry
)

The user name (3rd field, "key identifier" in the snippet above) will determine the key to use and can be any valid key id, fingerprint, email or user accepted by GPG.

Other settings

Check out the autoplugin definition for an exhaustive list of settings and tasks that can be customized.

Securely Publishing Locally

Some projects use the "release from the maintainer's laptop" strategy. There's nothing specifically wrong with this strategy from a security standpoint, so long as the maintainer in question practices good security hygiene and protects their signing key appropriately (hint: strongly consider a YubiKey or similar if you maintain OSS projects; it's quite cheap and solves a major security vulnerability).

This plugin is well-optimized for this publication strategy, and in fact it is the secure default! You don't need to do anything to publish using a local key. Simply invoke publish (as described above), securely unlock your key when prompted by pinentry, and all of the signing and key management will be handled for you.

Securely Publishing in Travis (and other CI)

Note: These instructions are written in terms of Travis, since it is probably the most common CI server in use today, but they are easily applicable to any which supports secure key management.

It is very common to configure your CI server (Travis or otherwise) to perform the artifact signing and publication tasks. Conventionally, builds generally rely upon GPG's key encryption functionality to apply a decryption password to the key. The (password-protected) signing key is then checked in with the source code while the decryption password is encrypted and managed by the CI server. At build-time, this password is decrypted and injected into the build using an environment variable, closing the loop and allowing the CI server to securely decrypt the signing key and publish the artifacts.

This scheme works well when SBT manages signing key passwords and decryption (as in sbt-pgp). It works quite poorly when securely delegating to pinentry, as is the case with this plugin.

The solution is to not password-protect the CI signing key and instead encrypt it explicitly using openssl.

To generate a new key without passphrase protection, simply press Enter when prompted for the passphrase and select the option "Continue without passphrase protection".

Next, you should have your CI signing key in your local GPG keyring. Let's assume this key has an ID of 1234ABCD. Run the following commands within your project root:

$ gpg --export-secret-keys -a 1234ABCD > key.asc
$ travis encrypt-file key.asc --add
$ rm key.asc
$ git add key.asc.enc
$ git commit

Replace travis encrypt-file with whatever mechanism is required to securely encrypt files for your CI solution. You may omit the --add switch and manually modify your .travis.yml if you prefer. Travis' file encryption documentation is here.

These steps handle securely materializing a plain-text (not password protected!) secret key on your CI server. The only remaining task is to make it available to gpg on your CI so that it can be picked up by sbt-gpg. If using Travis, add the following to your .travis.yml:

before_script: gpg --import key.asc

Best Practices

Do NOT use your personal GPG key for CI signing! Ever. Your personal private key should never leave your laptop. In fact, it probably shouldn't be on your laptop at all. Strongly consider YubiKey or similar. Never, ever enter the decryption password (or smartcard PIN) for your private key into anything other than pinentry on your local machine. If you're using MacGPG (which you should be, if using macOS), this dialog will look similar to the following:

If you are using your CI server to sign artifacts, your CI server should have its own public/private key pair, generated by you (or someone else on your team). You probably also want to sign that CI key with your own key, establishing a chain of trust (assuming the CI signing key has ID 1234AVCD):

$ gpg --sign-key 1234ABCD
$ gpg --send-key 1234ABCD

This is not the same as using your personal key for CI signing!