Researchers publish iMessage report and recommend service replacement

The researchers, led by Matthew Green, who identified a vulnerability in Apple’s iMessage service have published their research. In it they advise to replace the service in its entirety, despite the recent patch from Apple.

Matthew Green previously told The Washington Post that the research would be published if Apple released a patch for the vulnerability with the release of iOS 9.3. That happened on Monday evening and the investigation can now be viewed. In it, Green and his collaborators argue that iMessage is a widespread service with end-to-end encryption, which, however, has never been thoroughly researched. The authors, after their own extensive analysis, conclude that iMessage has some serious flaws and the best solution is to replace the service with a well-designed and formally verified system.

Part of that solution could be the implementation of a new encryption protocol, they write. This includes an existing protocol that uses forward secrecy and message authentication, such as OTR or TextSecure. The latter protocol is present, for example, in the crypto chat app Signal. The researchers indicate that it is difficult to replace the entire iMessage service, given the number of users. They therefore also provide a number of patches for the short term, such as the forced destruction and re-creation of all key pairs by Apple.

The vulnerability discovered by Green’s team allowed a sophisticated attacker to decrypt attachments in iMessage messages by impersonating an Apple server and repeatedly guessing parts of an encryption key. The research shows that the team was able to retrieve 232 of the key’s 256 bits within 35 hours with an average delay of 309 milliseconds. In doing so, they initially took advantage of the fact that iMessage does not correctly authenticate the encrypted part of a message. Secondly, they could modify the encrypted text and repeatedly present it to the sender or recipient for decryption.