Despite research published last year that demonstrated that Apple has the ability to decrypt users iMessages if it so chooses, Apple CEO Tim Cook said that the company does not hold the encryption key for those messages and couldn t even produce the plaintext in response to a government order. Cook also flatly denied that Apple had given any government agency direct access to user data as part of the much-discussed NSA PRISM program. The iMessage protocol allows users to send messages among iOS devices using the company s PUSH notification service.
Source: https://threatpost.com/apple-ceo-defends-imessage-security/108289/