This Stealthy USB Drive Hides Encrypted Data in Plain Sight

People who are concerned about data privacy might use encryption or specialized hardware for additional peace of mind. But there is a cost associated with this approach: it makes your data look like it is valuable. Someone with malicious intent might pass by a thumb drive that fell out of a backpack, but a hardware-encrypted drive is an immediate signal that whatever is stored inside is worth protecting — and worth hacking.

A new open source project called Phantomdrive hopes to solve that problem by making an encrypted drive look completely ordinary. Developed by Ryan Walker of Rootkit Labs, Phantomdrive presents itself to a computer as nothing more than a standard, unencrypted USB flash drive. The rest of the storage is invisible, with no indication to the operating system that a hidden encrypted partition even exists.

Unlocking that hidden storage doesn't require a special procedure that might give the device away. Instead, the user creates a text file called unlock.txt containing a single line in the format "password:<your_password>". When Phantomdrive detects that pattern, it disconnects the visible partition, mounts the hidden one in its place, and transparently encrypts and decrypts data using AES-256. It just appears as though one USB drive has been replaced with another.

At the core of Phantomdrive is a WCH CH569 microcontroller, a chip that includes USB interfaces, SD/eMMC support, and dedicated cryptographic hardware for AES acceleration. Storage comes from a removable microSD card, and a pair of Texas Instruments switching regulators generate the voltages required by the processor. The board also includes an external crystal oscillator, a firmware update button, and UART debug pads. It is housed inside a custom enclosure.

A microSD card might not be the ideal choice, but Walker went this route anyway because eMMC prices have climbed dramatically. The enclosure is glued together with epoxy, so accessing that card requires destroying the case. Even if someone does remove it, they won't be able to read its contents without the correct encryption key.

That key is derived using more than just the user's password. Phantomdrive mixes the password with the CH569's unique hardware identifier, which acts as a device-specific salt, before running the result through 100,000 rounds of key derivation. Because every device has a unique hardware identifier, moving the microSD card into another Phantomdrive won't produce the same encryption key, even if the password is identical.

If you’re worried about saving the password to a file, Walker has that covered as well. The device watches the raw USB write traffic for the "password:" string. When it finds it, the password is copied into RAM and immediately erased from the outgoing data buffer before anything is written to the storage device, preventing the password from ever being saved to disk.

The Phantomdrive is available now for $58 in the Rootkit Labs store, but you’ll have to supply your own SD card (and optional epoxy).