[WIP] Support Provider Authentication Tokens (JWT)

[nathany]

continuing from #66 to solve #63.

Provider Authentication Tokens documentation is now public. https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/APNsProviderAPI.html#//apple_ref/doc/uid/TP40008194-CH101-SW21

[nathany]

I have a signing key, which is a .p8 file that looks like PEM format.

Same token for all apps on a Team. The signing key doesn't expire.

You should construct a token with header containing a 10 character Key ID (kid). The token claims portion contains Issuer (iss) which is a 10 character Team ID.

The token must be signed with the Elliptic Curve Digital Signature Algorithm (ECDSA) using the P-256 curve and the SHA-256 hash algorithm (ES256)

Presumably a JWT library will take care of this, but can also look at https://github.com/gtank/cryptopasta.

tokens can be reused but need to be periodically regenerated:

APNs will reject push messages with an Expired Provider Token error if the token issue timestamp is not within the last hour.

But not too often 429 TooManyProviderTokenUpdates

When you connect to APNs without a provider certificate, only one stream is allowed on the connection until you send a push message with valid token.

403 MissingProviderToken, InvalidProviderToken, ExpiredProviderToken

[codecov-io]

Current coverage is 53.68% (diff: 11.11%)

Merging #88 into master will decrease coverage by 2.48%

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- Misses          116        132    +16   
  Partials         19         19          

Powered by Codecov. Last update d6d71af...d4fd0bb

[nathany]

For concurrent use, it would be useful to have access to maxConcurrentStreams golang/go#17265 instead of guessing how many workers to use (1, 500, something else).

• Need to test causing a GOAWAY/hangup and creation of a new connection while requests are already in flight. Presumably this will reset maxConcurrentStreams back to 1 for a moment, causing several requests to fail.

Blocking when Transport hits max concurrent streams could prevent the error situations, whether in x/net/http2 or Buford. golang/go#13774 Still would be guessing at the optimal number of workers (500?).

[nathany]

This is blocked on the same thing as apns2:

sideshow/apns2#43 (comment)

[froodian]

golang/go#13774 has been closed, I wonder if this is solvable now? I don't have a lot of the context on the literals of what's required to support JWT here yet, but I do have a fair amount of demand signal for this in my use-case - could you let me know if this is something you might continue work on in the nearish future (or if something is still blocking it)? Otherwise I'll probably work to try to carve out time to get up to speed and implement this in the next few months. Thank you!

[felipejfc]

APNS2 maintainer has merged something similar to this:
sideshow/apns2#43 (comment)

regards

[nathany]

Sorry for the lack of maintenance and responsiveness on this.

The client that was sponsoring this work decided to use Elixir for their project instead, so it hasn't received much attention. Happy to look at a fresh pull request if someone has a chance to fully implement this.

[nathany]

NOTE: https://go-review.googlesource.com/c/net/+/85855

[nathany]

Related: golang/go#27753

https://go-review.googlesource.com/c/net/+/151857/ "http2: revert Transport's strict interpretation of MAX_CONCURRENT_STREAMS"