A Johns Hopkins team has decrypted iMessage photos by guessing character-by-character the key used to encrypt it, and Apple plans to release a new iOS version today that will fix the flaw.
Upgrading to iOS 9.3 should fix the problem for users of the operating system and iMessage, says Matthew Green, a computer science professor at Johns Hopkins who led a team of grad students that broke the encryption, according to a story in the Washington Post.
The story says he discovered a flaw in the encryption last fall and told Apple about it, but when months went by and nothing was done to patch it, he turned his team loose. Here’s how the Post describes the attack:
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.”
Had the attack been performed outside a lab environment, the user of the phone would have been unaware of it.
Exploiting the vulnerability took months on an earlier version of iOS, but with vast resources of a major country later versions could have been exploited, too, Green says.
The attack was against data in transit not data stored on an iPhone, so it would do nothing to solve the problem the FBI faces in trying to decrypt the iPhone used the San Bernardino terrorists, Green told the Post.
A hearing in the Apple v. FBI case is scheduled for tomorrow.
The exploit by the Hopkins team does point out an issue related to the case: It shows that even without introducing decryption backdoors on purpose, encryption schemes are likely to have some inherent vulnerability that someone will discover and exploit.
Adding intentional chinks to allow lawful decryption would just be an aid to those trying to decrypt information unlawfully; it would give them something specific to look for. “So it scares me that we’re having this conversation about adding back doors to encryption when we can’t even get basic encryption right,” Green told the Post.
For its part, Apple released a statement quoted in part by the Post: “Apple works hard to make our software more secure with every release. We appreciate the team of researchers that identified this bug and brought it to our attention so we could patch the vulnerability. . . . Security requires constant dedication and we’re grateful to have a community of developers and researchers who help us stay ahead.”
Green read a description last year of the iMessage encryption scheme, and he found what he thought was an exploitable flaw. When Apple didn’t fix it after months, he decided to take a run at exploiting it.