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It was reported today that iMessage was broken (CVE-2016-1788) by a team from John Hopkins around Matthew Green. They said it was kinda to be expected after inspecting the security guide closely enough. The media stories are very vague and the iOS 9.3 patch notes aren't much more concrete either.

How?

How could one of the most used and most trusted protocols be broken in such a way that apparently allows attachment recovery for everybody in a privileged man-in-the-middle position (like Apple)? (iOS 9.3 patch notes)

How can the combination of compressing (error oracle attack?) and "only" signing the ciphertext be leveraged to recover attachments?

  • Is this a question or a rant? I see both in there, but it feels like you're asking how to implement an attack. – Phil Lello Mar 21 '16 at 22:38
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    @PhilLello I'm genuinely interested in how Apple got their crypto wrong. I already do realize that the attacker needs a very powerful position (such as being Apple) so I won't implement it myself but the details of those attacks are always enlighting. – SEJPM Mar 21 '16 at 23:01
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The flaw with iMessage reference aboved was that it was:

A) Vulnerable to a brute force attack This was because of a bug discovered by the researchers that allowed a man-in-the-middle-attack to be setup, due to:

  • Strict certificate pinning was not used until iOS 9 -- so an attacker could create an impostor certificate and hack DNS on their local network to be able to start intercepting data and begin the attack.
  • No MAC is used in the encrypted payload after receiving an ECDSA signature -- thus, the contents of the message/message integrity is not controlled for.
  • The signature can then be tampered with or replaced with another signature. See image provided by researchers: Attack illustration
  • The "error oracle" compression attack you describe allows one to painstakingly decrypt a message bit-by-bit by taking advantage of how CRC checksums are computed over the original data, but not the AES encrypted payload iMessage then creates.

B) Using a static 64-digit symmetric encryption key for message encryption.

  • Rather than use a symmetric key system, an asymmetric system or one utilizing perfect forward secrecy may have been able to prevent this. Once the man in the middle attack was setup, a brute force attack could be launched until, successfully, retrieving the 64-digit key.

How?

Beyond the immediate technical answer, the question as to how this happened is the same for this case as it is for most other commercial, closed-source products; and even many open source packages -- encryption and creating bulletproof cryptosystems is hard. Even given the most valuable company in the world, Apple, with the best engineers and seemingly unlimited resources; mistakes can be and are made.

For more information, I suggest looking at Bruce Schneier's article on the subject, and the researcher's blog post on the subject.

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