Telegram's Servers use the format described in draft draft-ietf-httpbis-encryption-encoding-04 of RFC8188 to encode the WebPush messages. Telegram's API Documentation about Push Notifications (https://core.telegram.org/api/push-updates) falsely reports that it follows RFC8291, which implies RFC8188 in its final version, not any of the previous drafts.
It'd be better if Telegram updated its implementation of WebPush to support the latest revision of the draft, or at the very least mentioned the draft it's implementing in the documentation
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