Guide how to actually encrypt JSON Token for APNS - json

Question

Hope somebody can get me past this point... because I spend pretty much time on it and still not working.
Short story is that I want to use Azure Notification Hub for my Xamarin.Forms app.
It want's these info to work:
That's all good and I got all of them under control, expect the Token one.
Ok, so I follow the Microsoft docs on the subject:
https://learn.microsoft.com/en-us/azure/notification-hubs/notification-hubs-push-notification-http2-token-authentification
I follow along and got things under controls I think, until I get to:
"Authentication token"
Here it says:
The authentication token can be downloaded after you generate a token for your application. For details on how to generate this token, refer to Apple’s Developer documentation.
Like it's no big deal and then it links to this page, which is suppose to help me. Read through it, clicked the links etc. read stuff.
I end up on this page: Establishing a Token-Based Connection to APNs
And the the craziness and confusion really kicks off for me, because, it then says, like it's the most common thing in the world:
Encrypt the resulting JSON data using your authentication token signing key and the specified algorithm
It doesn't really explain much, other than link to the jwt.io tool.
Well, that would have been great if I could make the tool work...
On the surface it's pretty easy, as the docs explains what to put in where, so I do that:
So the "header" and the "payload" is filled in and I assume it's correct - however, at the bottom I clearly need to put in some keys for this to be able to decrypted correctly on the other end...the question what do I put in here?
When I created my key in the Apple Developer portal, I of cause downloded the .p8 file, which I can see contains my PRIVATE key...but I have 2 problems.
Putting that into this jwt.io tool, result in a "invalid signature" right away, and I have no idea what to put into the "PUBLIC KEY" part.
So, what am I doing wrong?
Thanks in advance and really hope somebody can help me, as I'm starting to go crazy over this, "tiny" step in the development that have been taking WAY too long now.

Answer 1

At the bottom of jwt.io there are libraries you can use to encrypt the token on your server. For example, this php library: https://github.com/lcobucci/jwt/blob/3.3/README.md
About public key. I think it's the KeyID that is the public key that APNs uses to verify. You only need the private key to generate the token. It goes like this in this php sample:
$token = (new Builder())->issuedBy('http://example.com') // Configures the issuer (iss claim)
->permittedFor('http://example.org') // Configures the audience (aud claim)
->identifiedBy('4f1g23a12aa', true) // Configures the id (jti claim), replicating as a header item
->issuedAt($time) // Configures the time that the token was issue (iat claim)
->canOnlyBeUsedAfter($time + 60) // Configures the time that the token can be used (nbf claim)
->expiresAt($time + 3600) // Configures the expiration time of the token (exp claim)
->withClaim('uid', 1) // Configures a new claim, called "uid"
->getToken($signer, $privateKey); // Retrieves the generated token

Answer 2

Just to whoever stumbles upon this question.
The token field in the Azure Notification Hubs Settings is the private key which you will find inside the .p8 file you downloaded from Apple Developer Account for Universal APN.
As for the JWT encryption, you need that when you sending a request to apple's apn server directly. You will need to send a Bearer token by encrypting the header and payload ( specifications are in apple's website). The encryption is done by crypto libraries, using algorithm ES256 ( only one supported for APN ) and the signing key is the token we mentioned above, that is the private key in the .p8. This creates a JWT that you include in your Authorization header for the request to APN server