Table of Contents
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).
To use the Signing plugin, include the following in your build script:
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 gradle.properties file (described in Section 11.2, “Gradle properties and system properties”).
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 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"
}
}
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.
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.
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.
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 32.3. Signing a configuration output
Output of gradle signArchives
> gradle signArchives :compileJava :processResources :classes :jar :signArchives BUILD SUCCESSFUL Total time: 1 secs
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 32.4. Signing a task
build.gradle
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 32.5. Signing a task output
Output of gradle signStuffZip
> gradle signStuffZip :stuffZip :signStuffZip BUILD SUCCESSFUL 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.
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 32.6. Conditional signing
build.gradle
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.
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.
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 32.7. Signing a POM for deployment
build.gradle
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.