Chapter 33. The Signing Plugin

Table of Contents

33.1. Usage
33.2. Signatory credentials
33.3. Specifying what to sign
33.4. Publishing the signatures
33.5. Signing POM files

The signing plugin adds the ability to digitally sign built files and artifacts. These digital signatures can then be used to prove who built the artifact the signature is attached to as well as other information such as when the signature was generated.

The signing plugin currently only provides support for generating OpenPGP signatures (which is the signature format required for publication to the Maven Central Repository).

33.1. Usage

To use the Signing plugin, include the following in your build script:

Example 33.1. Using the Signing plugin


apply plugin: 'signing'

33.2. Signatory credentials

In order to create OpenPGP signatures, you will need a key pair (instructions on creating a key pair using the GnuPG tools can be found in the GnuPG HOWTOs). You need to provide the signing plugin with your key information, which means three things:

  • The public key ID (an 8 character hexadecimal string).

  • The absolute path to the secret key ring file containing your private key.

  • The passphrase used to protect your private key.

These items must be supplied as the values of properties signing.keyId, signing.secretKeyRingFile, and signing.password respectively. Given the personal and private nature of these values, a good practice is to store them in the user file (described in Section 12.2, “Gradle properties and system properties”).


If specifying this information (especially signing.password) in the user file is not feasible for your environment, you can source the information however you need to and set the project properties manually.

import org.gradle.plugins.signing.Sign

gradle.taskGraph.whenReady { taskGraph ->
    if (taskGraph.allTasks.any { it instanceof Sign }) {
        // Use Java 6's console to read from the console (no good for
        // a CI environment)
        Console console = System.console()
        console.printf "\n\nWe have to sign some things in this build." +
                       "\n\nPlease enter your signing details.\n\n"

        def id = console.readLine("PGP Key Id: ")
        def file = console.readLine("PGP Secret Key Ring File (absolute path): ")
        def password = console.readPassword("PGP Private Key Password: ")

        allprojects { ext."signing.keyId" = id }
        allprojects { ext."signing.secretKeyRingFile" = file }
        allprojects { ext."signing.password" = password }

        console.printf "\nThanks.\n\n"

33.2.1. Using OpenPGP subkeys

OpenPGP supports subkeys, which are like the normal keys, except they're bound to a master key pair. One feature of OpenPGP subkeys is that they can be revoked independently of the master keys which makes key management easier. A practical case study of how subkeys can be leveraged in software development can be read on the Debian wiki.

The signing plugin supports OpenPGP subkeys out of the box. Just specify a subkey ID as the value in the signing.keyId property.

33.3. Specifying what to sign

As well as configuring how things are to be signed (i.e. the signatory configuration), you must also specify what is to be signed. The Signing plugin provides a DSL that allows you to specify the tasks and/or configurations that should be signed.

33.3.1. Signing Configurations

It is common to want to sign the artifacts of a configuration. For example, the Java plugin configures a jar to build and this jar artifact is added to the archives configuration. Using the Signing DSL, you can specify that all of the artifacts of this configuration should be signed.

Example 33.2. Signing a configuration


signing {
    sign configurations.archives

This will create a task (of type Sign) in your project named “signArchives”, that will build any archives artifacts (if needed) and then generate signatures for them. The signature files will be placed alongside the artifacts being signed.

Example 33.3. Signing a configuration output

Output of gradle signArchives

> gradle signArchives


Total time: 1 secs

33.3.2. Signing Tasks

In some cases the artifact that you need to sign may not be part of a configuration. In this case you can directly sign the task that produces the artifact to sign.

Example 33.4. Signing a task


task stuffZip (type: Zip) {
    baseName = "stuff"
    from "src/stuff"

signing {
    sign stuffZip

This will create a task (of type Sign) in your project named “signStuffZip”, that will build the input task's archive (if needed) and then sign it. The signature file will be placed alongside the artifact being signed.

Example 33.5. Signing a task output

Output of gradle signStuffZip

> gradle signStuffZip


Total time: 1 secs

For a task to be “signable”, it must produce an archive of some type. Tasks that do this are the Tar, Zip, Jar, War and Ear tasks.

33.3.3. Conditional Signing

A common usage pattern is to only sign build artifacts under certain conditions. For example, you may not wish to sign artifacts for non release versions. To achieve this, you can specify that signing is only required under certain conditions.

Example 33.6. Conditional signing


version = '1.0-SNAPSHOT'
ext.isReleaseVersion = !version.endsWith("SNAPSHOT")

signing {
    required { isReleaseVersion && gradle.taskGraph.hasTask("uploadArchives") }
    sign configurations.archives

In this example, we only want to require signing if we are building a release version and we are going to publish it. Because we are inspecting the task graph to determine if we are going to be publishing, we must set the signing.required property to a closure to defer the evaluation. See SigningExtension.setRequired(java.lang.Object) for more information.

33.4. Publishing the signatures

When specifying what is to be signed via the Signing DSL, the resultant signature artifacts are automatically added to the signatures and archives dependency configurations. This means that if you want to upload your signatures to your distribution repository along with the artifacts you simply execute the uploadArchives task as normal.

33.5. Signing POM files

When deploying signatures for your artifacts to a Maven repository, you will also want to sign the published POM file. The signing plugin adds a signing.signPom() (see: SigningExtension.signPom(org.gradle.api.artifacts.maven.MavenDeployment, groovy.lang.Closure)) method that can be used in the beforeDeployment() block in your upload task configuration.

Example 33.7. Signing a POM for deployment


uploadArchives {
    repositories {
        mavenDeployer {
            beforeDeployment { MavenDeployment deployment -> signing.signPom(deployment) }

When signing is not required and the POM cannot be signed due to insufficient configuration (i.e. no credentials for signing) then the signPom() method will silently do nothing.