Security expert Matthew Green decided to check whether Apple has an unauthorized access to iMessages. The company notes that it doesn’t have such access, because iMessages and messages in FaceTime are « protected by end-to-end encryption so no one but the sender and receiver can see or read them».
However, Green questioned the company's claims. He said that iMessages are copied to iCloud. This function is enabled by many users. According to the expert, Apple may not have access to personal messages only if iMessage user’s backups are encrypted with the user key, which every device has, or if they are password protected. None of these options works with Apple.
Green conducted an experiment: after allegedly losing the device, he reset the password using iForgot service, and then bought a new phone. After that, the expert restored the old iMessages from backup of iCloud. Thus, the company itself, NSA, and anyone can recover the messages.
This information does not negate the fact that Apple uses cryptography, however, clearly indicates that a copy of the «secret key» user is stored somewhere else.