To generate token from jwt.io we need public key. But how do we generate a public key for a specific private key

I want to create and test the apple's server to server notification to keep track of the non-renewing subscriptions happening in my backend using php. How do I create that and test the app on phone using sandbox.

I've provided a url in the app information section to get app store server to server notification on non-renewing subscription purchases. But I'm not getting any response from app store even after trying 4-5 test purchases using sandbox account in my device. Any help would be appreciated. On checking this News and Updates - https://developer.apple.com/news/?id=tv3bhra6 they've cleared that now it's possible to get server to server notification on all in-app purchases.

Based on this  News and Updates - https://developer.apple.com/news/?id=tv3bhra6, it means that now it's possible to get server to server notification on all in-app purchases including non-renewing subscriptions right? I've provided a url in the app information section to get app store server to server notification. I've tried all four types of subscriptions. I'm not getting any response from app store even after trying 4-5 test purchases using sandbox account in my device for consumable, non-consumable and non renewing subscription.But if I'm doing an auto-renewing subscription test purchase, then the url is getting hit and I'm getting response from the app store. Why I'm getting notification only for auto renewing subscription and not for others? Is the new update as mentioned in the above link live ?