Why does the maven prompt gpg passphrase when building project on Win 8?

Maven home: F:\Code\apache-maven-3.2.3\bin..

Java version: 1.8.0_25, vendor: Oracle Corporation

Java home: C:\Program Files\Java\jdk1.8.0_25\jre

Default locale: zh_CN, platform encoding: GBK

OS name: "windows 8.1", version: "6.3", arch: "amd64", family: "dos"

I run mvn package install -DcreateChecksum=true

Testing run well without any error. When it comes to Building javadoc.jar, the console prompts to ask me to type in the GPG Passphrase as follow. What secret key I should type in?

GPG Passphrase: *

gpg: skipped "Libin [email protected]": secret key not available

gpg: signing failed: secret key not available