Small follow-up point re: my post this week on iMessage’s delivery architecture being built atop the Apple Push Notification service:
APNs can only relay messages up to 4 or 16 KB in size, depending
on the iOS or iPadOS version. If the message text is too long or
if an attachment such as a photo is included, the attachment is
encrypted using AES in CTR mode with a randomly generated 256-bit
key and uploaded to iCloud. The AES key for the attachment, its
Uniform Resource Identifier (URI), and an SHA-1 hash of its
encrypted form are then sent to the recipient as the contents of
an iMessage, with their confidentiality and integrity protected
through normal iMessage encryption, as shown in the following
diagram.
This explains why you can often text, but not send or receive images, with iMessage over in-flight Wi-Fi. (Thanks to Adam Shostack for flagging this detail.)
