iMessage Encryption Flaw Found and Fixed
Matthew Green and team found and reported a significant iMessage encryption flaw last year.
Green suspected there might be a flaw in iMessage last year after he read an Apple security guide describing the encryption process and it struck him as weak. He said he alerted the firm’s engineers to his concern. When a few months passed and the flaw remained, he and his graduate students decided to mount an attack to show that they could pierce the encryption on photos or videos sent through iMessage.
It took a few months, but they succeeded, targeting phones that were not using the latest operating system on iMessage, which launched in 2011.
To intercept a file, the researchers wrote software to mimic an Apple server. The encrypted transmission they targeted contained a link to the photo stored in Apple’s iCloud server as well as a 64-digit key to decrypt the photo.
Although the students could not see the key’s digits, they guessed at them by a repetitive process of changing a digit or a letter in the key and sending it back to the target phone. Each time they guessed a digit correctly, the phone accepted it. They probed the phone in this way thousands of times.
“And we kept doing that,” Green said, “until we had the key.”
A modified version of the attack would also work on later operating systems, Green said, adding that it would likely have taken the hacking skills of a nation-state.
This flaw is fixed in iOS 9.3. (You should download and install it now.)
I wrote about this flaw in IEEE Security and Privacy earlier this year:
Going back to the new vulnerability that you’ll learn about in mid-February, the lead researcher wrote to me: “If anyone tells you that [the vendor] can just ‘tweak’ the system a little bit to add key escrow or to man-in-the-middle specific users, they need to spend a few days watching the authentication dance between [the client device/software] and the umpteen servers it talks to just to log into the network. I’m frankly amazed that any of it works at all, and you couldn’t pay me enough to tamper with any of it.” This is an important piece of wisdom.
The designers of this system aren’t novices. They’re an experienced team with some of the best security engineers in the field. If these guys can’t get the security right, just imagine how much worse it is for smaller companies without this team’s level of expertise and resources. Now imagine how much worse it would be if you add a government-mandated back door. There are more opportunities to get security wrong, and more engineering teams without the time and expertise necessary to get
Related: A different iOS flaw was reported last week. Called AceDeceiver, it is a Trojan that allows an attacker to install malicious software onto an iOS device, bypassing Apple’s DRM protections. I don’t believe that Apple has fixed this yet, although it seems as if Apple just has to add a certificate revocation list, or make the certs nonreplayable by having some mandatory interaction with the iTunes store.
EDITED (4/14): The paper describing the iMessage flaw.