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iCloud is not an "endpoint". iCloud is Apple, the server. It's not E2E with the key stored on the server. That's just TLS with more steps.

• giantrobot 5y ago
  You have a misunderstanding of Messages. There's no iCloud "endpoint". A message between users are encrypted with a session key encrypted with the recipient's device public key. The private keys are not stored on Apple's servers. They live only on your device. When you sign into Messages on a device it generates a key pair and sends the public key to Apple. When you first send someone a message your device gets their list of device public keys. Messages are encrypted with session keys that are encrypted with the public keys. Each recipient device gets a copy of the message and uses its device private key to decrypt the message. The sender and receiver will receive copies on all their signed in devices. The encryption "hole" with Messages is when the local device database is backed up to iCloud. This is the same hole that exists for any messenger app that has its local database backed up to iCloud. iCloud backups are encrypted at rest on Apple's servers but with keys they can access.