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).

Usage

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

Example 1. Using the Signing Plugin
build.gradle.kts
plugins {
    signing
}
build.gradle
plugins {
    id 'signing'
}

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 (The last 8 symbols of the keyId. You can use gpg -K to get it).

  • The absolute path to the secret key ring file containing your private key. (Since gpg 2.1, you need to export the keys with command gpg --keyring secring.gpg --export-secret-keys > ~/.gnupg/secring.gpg).

  • The passphrase used to protect your private key.

These items must be supplied as the values of the signing.keyId, signing.secretKeyRingFile, and signing.password properties, respectively.

Given the personal and private nature of these values, a good practice is to store them in the gradle.properties file in the user’s Gradle home directory (described in System properties) instead of in the project directory itself. 
signing.keyId=24875D73
signing.password=secret
signing.secretKeyRingFile=/Users/me/.gnupg/secring.gpg

If specifying this information (especially signing.password) in the user gradle.properties file is not feasible for your environment, you can supply the information via the command line:

> gradle sign -Psigning.secretKeyRingFile=/Users/me/.gnupg/secring.gpg -Psigning.password=secret -Psigning.keyId=24875D73

Using in-memory ascii-armored keys

In some setups it is easier to use environment variables to pass the secret key and password used for signing. For instance, when using a CI server to sign artifacts, securely providing the keyring file is often troublesome. On the other hand, most CI servers provide means to securely store environment variables and provide them to builds. Using the following setup, you can pass the secret key (in ascii-armored format) and the password using the ORG_GRADLE_PROJECT_signingKey and ORG_GRADLE_PROJECT_signingPassword environment variables, respectively:

build.gradle.kts
signing {
    val signingKey: String? by project
    val signingPassword: String? by project
    useInMemoryPgpKeys(signingKey, signingPassword)
    sign(tasks["stuffZip"])
}
build.gradle
signing {
    def signingKey = findProperty("signingKey")
    def signingPassword = findProperty("signingPassword")
    useInMemoryPgpKeys(signingKey, signingPassword)
    sign stuffZip
}

Using in-memory ascii-armored OpenPGP subkeys

To prevent sharing of the master key and to keep it secure it is also possible to use in-memory ascii-armored subkeys. The main difference between using in-memory ascii-armored keys and subkeys is that it is necessary to specify key identifier as well. Using the following setup, you can pass the key identifier, secret key (in ascii-armored format) and the password using the ORG_GRADLE_PROJECT_signingKeyId, ORG_GRADLE_PROJECT_signingKey and ORG_GRADLE_PROJECT_signingPassword environment variables respectively:

build.gradle.kts
signing {
    val signingKeyId: String? by project
    val signingKey: String? by project
    val signingPassword: String? by project
    useInMemoryPgpKeys(signingKeyId, signingKey, signingPassword)
    sign(tasks["stuffZip"])
}
build.gradle
signing {
    def signingKeyId = findProperty("signingKeyId")
    def signingKey = findProperty("signingKey")
    def signingPassword = findProperty("signingPassword")
    useInMemoryPgpKeys(signingKeyId, signingKey, signingPassword)
    sign stuffZip
}

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.

Using gpg-agent

By default the Signing Plugin uses a Java-based implementation of PGP for signing. This implementation cannot use the gpg-agent program for managing private keys, though. If you want to use the gpg-agent, you can change the signatory implementation used by the Signing Plugin:

Example 2. Sign with GnuPG
build.gradle.kts
signing {
    useGpgCmd()
    sign(configurations.runtimeElements.get())
}
build.gradle
signing {
    useGpgCmd()
    sign configurations.runtimeElements
}

This tells the Signing Plugin to use the GnupgSignatory instead of the default PgpSignatory. The GnupgSignatory relies on the gpg2 program to sign the artifacts. Of course, this requires that GnuPG is installed.

Without any further configuration the gpg (on Windows: gpg.exe) executable found on the PATH will be used. The password is supplied by the gpg-agent and the default key is used for signing.

Gnupg signatory configuration

The GnupgSignatory supports a number of configuration options for controlling how gpg is invoked. These are typically set in gradle.properties:

Example: Configure the GnupgSignatory

gradle.properties
signing.gnupg.executable=gpg
signing.gnupg.useLegacyGpg=true
signing.gnupg.homeDir=gnupg-home
signing.gnupg.optionsFile=gnupg-home/gpg.conf
signing.gnupg.keyName=24875D73
signing.gnupg.passphrase=gradle
signing.gnupg.executable

The gpg executable that is invoked for signing. The default value of this property depends on useLegacyGpg. If that is true then the default value of executable is "gpg" otherwise it is "gpg2".

signing.gnupg.useLegacyGpg

Must be true if GnuPG version 1 is used and false otherwise. The default value of the property is false.

signing.gnupg.homeDir

Sets the home directory for GnuPG. If not given the default home directory of GnuPG is used.

signing.gnupg.optionsFile

Sets a custom options file for GnuPG. If not given GnuPG’s default configuration file is used.

signing.gnupg.keyName

The id of the key that should be used for signing. If not given then the default key configured in GnuPG will be used.

signing.gnupg.passphrase

The passphrase for unlocking the secret key. If not given then the gpg-agent program is used for getting the passphrase.

All configuration properties are optional.

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.

Signing Publications

When publishing artifacts, you often want to sign them so the consumer of your artifacts can verify their signature. For example, the Java plugin defines a component that you can use to define a publication to a Maven (or Ivy) repository using the Maven Publish Plugin (or the Ivy Publish Plugin, respectively). Using the Signing DSL, you can specify that all of the artifacts of this publication should be signed.

Example 3. Signing a publication
build.gradle.kts
signing {
    sign(publishing.publications["mavenJava"])
}
build.gradle
signing {
    sign publishing.publications.mavenJava
}

This will create a task (of type Sign) in your project named signMavenJavaPublication that will build all artifacts that are part of the publication (if needed) and then generate signatures for them. The signature files will be placed alongside the artifacts being signed.

Example: Signing a publication output

Output of gradle signMavenJavaPublication
> gradle signMavenJavaPublication
> Task :compileJava
> Task :processResources
> Task :classes
> Task :jar
> Task :javadoc
> Task :javadocJar
> Task :sourcesJar
> Task :generateMetadataFileForMavenJavaPublication
> Task :generatePomFileForMavenJavaPublication
> Task :signMavenJavaPublication

BUILD SUCCESSFUL in 0s
9 actionable tasks: 9 executed

In addition, the above DSL allows to sign multiple comma-separated publications. Alternatively, you may specify publishing.publications to sign all publications, or use publishing.publications.matching { … } to sign all publications that match the specified predicate.

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 runtimeElements configuration. Using the Signing DSL, you can specify that all of the artifacts of this configuration should be signed.

Example 4. Signing a configuration
build.gradle.kts
signing {
    sign(configurations.runtimeElements.get())
}
build.gradle
signing {
    sign configurations.runtimeElements
}

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

Example: Signing a configuration output

Output of gradle signRuntimeElements
> gradle signRuntimeElements
> Task :compileJava
> Task :processResources
> Task :classes
> Task :jar
> Task :signRuntimeElements

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

Signing Task Output

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 5. Signing a task output
build.gradle.kts
tasks.register<Zip>("stuffZip") {
    archiveBaseName = "stuff"
    from("src/stuff")
}

signing {
    sign(tasks["stuffZip"])
}
build.gradle
tasks.register('stuffZip', Zip) {
    archiveBaseName = '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: Signing a task output

Output of gradle signStuffZip
> gradle signStuffZip
> Task :stuffZip
> Task :signStuffZip

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

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

Conditional Signing

A common usage pattern is to require the signing of build artifacts only under certain conditions. For example, you may not need to sign artifacts for non-release versions. To achieve this, you can specify the condition as an argument of the required() method.

Example 6. Specifying when signing is required
build.gradle.kts
version = "1.0-SNAPSHOT"
extra["isReleaseVersion"] = !version.toString().endsWith("SNAPSHOT")

signing {
    setRequired({
        (project.extra["isReleaseVersion"] as Boolean) && gradle.taskGraph.hasTask("publish")
    })
    sign(publishing.publications["main"])
}
build.gradle
version = '1.0-SNAPSHOT'
ext.isReleaseVersion = !version.endsWith("SNAPSHOT")

signing {
    required = { isReleaseVersion && gradle.taskGraph.hasTask("publish") }
    sign publishing.publications.main
}

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.

If the required condition does not hold true, artifacts will only be signed if signatory credentials are configured. Alternatively, you may want to skip signing entirely whether or not signatory credentials are available. If so, you can configure the Sign tasks to be skipped, for example by attaching a predicate using the onlyIf() method shown in the following example:

Example 7. Specifying when signing is skipped
build.gradle.kts
tasks.withType<Sign>().configureEach {
    onlyIf("isReleaseVersion is set") { project.extra["isReleaseVersion"] as Boolean }
}
build.gradle
tasks.withType(Sign) {
    onlyIf("isReleaseVersion is set") { isReleaseVersion }
}

Publishing the signatures

When signing publications, the resultant signature artifacts are automatically added to the corresponding publication. Thus, when publishing to a repository, e.g. by executing the publish task, your signatures will be distributed along with the other artifacts without any additional configuration.

When signing configurations and tasks, the resultant signature artifacts are automatically added to the signatures dependency configuration.