How to encrypt private key for app signing?

Question

Android

The instruction reads as follows:
You do not store the application signing key in the Java Keystore
Encrypt and upload the application's private signing key.
1. Export your application signing private key to a PEM text file.
2. To encrypt the private key for signing the application, use this code with the public hex key (4 bytes of identification and 64 bytes of the P256 point) below. The code performs EC-P256+AES-GCM hybrid encryption.
Under the phrase "this code" is a link to the pepk-src.jar file ( https://www.gstatic.com/play-apps-publisher-rapid/... ). But how to use this code? Unclear. Has anyone come across?