The FBI may have paid a small fortune to unlock an iPhone 5c used by the San Bernardino shooter. But a security researcher has demonstrated a way to do it for less than US $100.
Sergei Skorobogatov at the University of Cambridge used a technique known as NAND mirroring to bypass the passcode retry limit on an iPhone 5c. Using store-bought equipment, he created copies of the phone’s flash memory to generate more tries to guess the passcode.
Skorobogatov detailed the whole process in a new paper that disputes the FBI’s assertion that the San Bernardino shooter’s iPhone couldn’t be accessed with the NAND mirroring technique.
The device in question had been passcode protected with an auto-erase function that would activate after 10 failed tries, deleting all the data inside.
Prior to paying a contractor, the FBI demanded Apple assist in cracking the phone’s passcode, sparking a major legal tussle over privacy and security.
However, earlier in the year, computer experts said that the FBI could possibly crack the phone’s security using the NAND mirroring technique. NAND refers to the phone’s flash memory, which researchers have said can be copied.
Skorobogatov’s paper, published on Thursday, provides a working prototype on how to pull off the hack. The equipment he used consisted of off-the-shelf components, and his tests involved iPhone 5cs updated with the latest 9.3 version of iOS.
However, it took four months of Skorobogatov’s spare time to understand how to successfully copy the phone’s memory and exploit it. "It was a backburner project carried out on my own, because no one believed it would be possible," he said in an email.
Skorobogatov’s method -- which is quite technical -- involves taking apart the phone, desoldering the memory and then creating a copy. “Because I can create as many clones as I want, I can repeat that process many, many times,” he said in a video explaining the process.
The phone’s passcode still needs to be guessed, but Skorobogatov’s paper said the process of testing the codes can be automated.
He estimates that at most it can take 20 hours to guess a four-digit passcode. For a six-digit passcode, it would take about three months.
So far, the FBI and Apple haven’t commented on the paper. But Jonathan Zdziarski, a security expert, called the research “very respectable.” Earlier this year, he also demoed a software-based concept method to use NAND mirroring on a jailbroken iPod Touch.
Skorobogatov’s research does exactly what Zdziarski had previously outlined, the second researcher said in an email. Zdziarski faulted the FBI for spending so much to unlock an iPhone when a security researcher had done so with “almost zero budget.”
“The FBI’s due diligence practices clearly leave room for improvement, and these should be reviewed,” he added.
Skorobogatov’s research might also weaken the FBI’s arguments that Apple should create an alleged backdoor into its products.
“The FBI needs computer-security expertise, not backdoors,” wrote Bruce Schneier, a long-time security guru, in a blog post.