Gradle signArchives unable to read Secret Key

I am trying to publish my Java Library to Maven Central. A part of this involves using the signing gradle plugin to sign the artifacts. I need to sign it without using the keyring file as document here as I cant provide my CI secure access to the key ring file.

However when I do this my build fails with:

FAILURE: Build failed with an exception.

* What went wrong:
Could not evaluate onlyIf predicate for task ':signArchives'.
> Could not read PGP secret key

What am I doing wrong? I presume it is related to my GPG_SIGNING_KEY. I used the full private key from the response of gpg --list-secret-keys --keyid-format LONG. Is this not correct?

---

My build.gradle

apply plugin: 'java'
apply plugin: 'signing'
apply plugin: 'maven'
apply from: 'publish.gradle'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    compile group: 'org.apache.httpcomponents', name: 'httpclient', version: '4.5.3'
    testCompile 'junit:junit:4.11'
}

task Wrapper(type: Wrapper) {
    gradleVersion = '5.6.2'
}

My publish.gradle

apply plugin: 'maven'
apply plugin: 'signing'

def isReleaseBuild() {
    return !VERSION.contains("SNAPSHOT")
}

def getReleaseRepositoryUrl() {
    return 'https://oss.sonatype.org/service/local/staging/deploy/maven2/'
}

def getSnapshotRepositoryUrl() {
    return 'https://oss.sonatype.org/content/repositories/snapshots/'
}

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

                repository(url: getReleaseRepositoryUrl()) {
                    def ossrhUsername = OSSRH_USERNAME
                    def ossrhPassword = OSSRH_PASSWORD

                    authentication(userName: ossrhUsername, password: ossrhPassword)
                }

                snapshotRepository(url: getSnapshotRepositoryUrl()) {
                    def ossrhUsername = OSSRH_USERNAME
                    def ossrhPassword = OSSRH_PASSWORD

                    authentication(userName: ossrhUsername, password: ossrhPassword)
                }

                pom.groupId = GROUP_ID
                pom.artifactId = ARTIFACT_ID
                pom.version = VERSION

                pom.project {
                    name ARTIFACT_ID
                    packaging PROJECT_PACKAGING
                    description PROJECT_DESCRIPTION
                    url PROJECT_URL

                    scm {
                        url SCM_URL
                        connection SCM_CONNECTION
                    }

                    licenses {
                        license {
                            name LICENSE_NAME
                            url LICENSE_URL
                        }
                    }

                    organization {
                        name = ORGANIZATION_NAME
                        url = ORGANIZATION_URL
                    }

                    developers {
                        developer {
                            id DEVELOPER_ID
                            name DEVELOPER_NAME
                            email DEVELOPER_EMAIL
                        }
                    }
                }
            }
        }

        signing {
            required { isReleaseBuild() && gradle.taskGraph.hasTask("uploadArchives") }

            def signingKey = GPG_SIGNING_KEY
            def signingPassword = GPG_SIGNING_PASSWORD

            useInMemoryPgpKeys(signingKey, signingPassword)

            sign configurations.archives
        }

        task javadocJar(type: Jar) {
            classifier = 'javadoc'
            from javadoc
        }

        task sourcesJar(type: Jar) {
            classifier = 'sources'
            from sourceSets.main.allSource
        }

        artifacts {
            archives javadocJar, sourcesJar
        }
    }
}

And with gradle.properties

RELEASE_REPOSITORY_URL='https://oss.sonatype.org/service/local/staging/deploy/maven2/'
SNAPSHOT_REPOSITORY_URL='https://oss.sonatype.org/content/repositories/snapshots/'
GPG_SIGNING_KEY=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
GPG_SIGNING_PASSWORD=the password used to encrypt the key
OSSRH_USERNAME=my ossrh username
OSSRH_PASSWORD=my ossrh password

VERSION=1.0.0
GROUP_ID=com.example
ARTIFACT_ID=project-name

PROJECT_PACKAGING=...
PROJECT_DESCRIPTION=...
PROJECT_URL=...

SCM_URL=...
SCM_CONNECTION=...

LICENSE_NAME=Apache License, Version 2.0
LICENSE_URL=...

ORGANIZATION_NAME=...
ORGANIZATION_URL=...

DEVELOPER_ID=...
DEVELOPER_NAME=...
DEVELOPER_EMAIL=...

As you suspected, it’s the format of the secret PGP key that is wrong here. The useInMemoryPgpKeys method expects an “ascii-armored in-memory PGP secret key”. gpg --list-secret-keys is only meant for human consumption and doesn’t even show the ‘content’ of the secret key.

You can get the key in the correct format using gpg --armor --export-secret-keys foobar@example.com instead. Use your own key ID (as returned by gpg --list-secret-keys) or email address instead of foobar@example.com.

To make use of the exported key in the gradle.properties file, you need to escape the newline characters. For example, you could append a new, working line for your GPG_SIGNING_KEY property like so:

gpg --armor --export-secret-keys foobar@example.com \
    | awk 'NR == 1 { print "GPG_SIGNING_KEY=" } 1' ORS='\\n' \
    >> gradle.properties

(See this answer for an explanation of the main awk magic that is used here.)

With your gradle.properties file updated as described (and using your build scripts), I could successfully sign my dummy JAR files with ./gradlew signArchives.