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.

The Continued Threat to Personal Data: Key Factors Behind the 
2023 Increase

  The technology industry is increasingly adopting innovative
  solutions that implement end-to-end encryption such as
  iCloud's Advanced Data Protection to reduce the amount of
  vulnerable data stored by organizations and the risk to
iMessage with PQ3:  (Feb.21, 2024)
The new state of the art in quantum-secure messaging at scale

  Today we are announcing the most significant cryptographic
  security upgrade in iMessage history with the introduction
  of PQ3, a groundbreaking post-quantum cryptographic protocol
  that advances the state of the art of end-to-end secure
  messaging. With compromise-resilient encryption and
  extensive defenses against even highly sophisticated quantum
  attacks, PQ3 is the first messaging protocol to reach what
  we call Level 3 security — providing protocol protections
  that surpass those in all other widely deployed messaging
  apps. To our knowledge, PQ3 has the strongest security
  properties of any at-scale messaging protocol in the world.
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 advances user security with powerful new data protections

iCloud security overview -- Data categories and encryption

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.  
  Make sure Apple's Advanced Data Protection is turned on.

Sending End-to-End Encrypted VoIP Calls