Securing data on Flash media is harder than most people believe, Peter Jamieson of UL subsidiary Witham Laboratories, told AusCERT delegates.
“The only way to be absolutely certain that data can’t be recovered from a USB key is to hit it with a hammer… and to get medieval about it,” he said. Even encrypted keys, he said, have vulnerabilities that might expose data the user believes is safe.
Jamieson highlighted a combination of how Flash controllers manage the storage device for the longest possible life, the physical behavior of the Flash cells, and how encryption is managed.
“Flash media is portable, robust, ubiquitous – and therefore losable”, he said.
One problem lies in how the Flash controller behaves, with two particular characteristics of the Flash Transfer Layer (FTL) that are designed to cope with some of the shortcomings of Flash.
Flash controllers, Jamieson explained, have a particular lifecycle consideration in mind: each block on the drive can only be physically deleted between 10,000 and 100,000 times before it begins to degrade. Moreover, the block – which is the smallest unit that Flash controllers delete – will have data other than what you’re trying to alter or delete.
An alteration to a small file (say, an e-mail signature) will, he explained, result in the drive’s controller creating a new copy of the block containing that file – but to avoid an unnecessary erasure, the FTL will mark the block as unusable, but only erase it when it’s necessary.
“When you ‘delete’ a file on a USB flash drive,” he said, “you don’t overwrite the file, you just delete the filesystem’s representation of the file.”
The electrical properties of Flash cells are also problematic: “deleting” the contents of the cell only requires that its charge falls below the arbitrary value that represents “zero”. Various experiments, he noted, suggest that operating a Flash drive at lower than its nominal voltage will cause residual charge remaining in the cells to be readable.
He recommends that all data stored on USBs is encrypted (with a preference for software encryption or a combination of hardware and software), and strongly suggests multiple-pass overwrites as a data deletion strategy.