Apple Resources - End-to-End Encryption

End-To-End Encryption (EE2E)

  End-to-end encryption (E2EE) is a system of communication
  where only the communicating users can read the messages. In
  principle, it prevents potential eavesdroppers - including
  telecom providers, Internet providers, and even the provider
  of the communication service - from being able to access the
  cryptographic keys needed to decrypt the conversation.
Here is everything you need to know about the End-to-End 
Encryption system

  In End-to-end encryption, the encryption happens at the
  device level. Meaning, that the messages and files are
  encrypted before they leave the phone/computer and aren't
  decrypted until it reaches their destination, which can be
  another phone/computer. This is one of the main reasons that
  hackers cannot access data on the server because they do not
  have the private keys to decrypt the data. The secret keys
  are stored with the individual user on their device which
  makes it much harder to access an individual's data as well.

  The security behind end-to-end encryption is enabled by the
  creation of a public-private key pair. This process is known
  as asymmetric cryptography. Asymmetric or public-key
  cryptography encrypts and decrypts the data using two
  separate cryptographic keys. The public key is used to
  encrypt a message and send it to the public key's owner.
  Thereafter, the message can only be decrypted using a
  corresponding private key, also known as a decryption key.

Apple iMessage And Facetime & Privacy

  We designed iMessage and FaceTime to use end-to-end
  encryption, so there's no way for Apple to decrypt the
  content of your conversations when they are in transit
  between devices. Attachments you send over iMessage (such
  as photos or videos) are encrypted so that no one but the
  sender and receiver(s) can access them.  
  However, if either of the Apple devices gets backed up to 
  iCloud, undeleted iMessages can be decrypted by Apple with
  a lawful court order.

Sending End-to-End Encrypted VoIP Calls   

iCloud security overview