New installation:

Install GPG at http://www.gpgtools.org/gpgmail/index.html
Generate new key with gpg --gen-key
build-with-maven
gpg --list-keys
Send the public key:
  gpg --keyserver hkp://pool.sks-keyservers.net --send-keys <public_key>
  instructions at https://docs.sonatype.org/display/Repository/How+To+Generate+PGP+Signatures+With+Maven
Configure ~/.m2/settings.xml with Nexus user/password:
  <settings>
    <servers>
      <server>
        <id>sonatype-nexus-staging</id>
        <username>xxx</username>
        <password>xxx</password>
      </server>
    </servers>
  </settings>