> How do both parties determine the keys used during a conversation? They don’t: public key cryptography is not initially used. The sender generates a random

> How do both parties determine the keys used during a conversation? They don’t: public key cryptography is not initially used. The sender generates a random AES-256 key, applies it in CTR mode and uploads the encrypted blob to GCS. Every receiving device gets a message with the key, the URI, and the SHA-1 of the blob. These messages are encrypted as usual and sent via APNS (<n>-courier.push.apple.com:5223) > you’d think they would wrap it so at minimum they could tweak the underlying infrastructure without requiring every client to update Apple does this: two other endpoints are *.blobstore.apple.com and the Chinese Guizhou-Cloud Big Data. In my logs blobstore is used less than 1% of the time.