🔓 A $5 gadget opened BitLocker — the key crosses an open wire

Tutorials & Methods
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:unlocked: A $5 gadget opened BitLocker — the key crosses an open wire

A board-level view: the TPM chip on the left, the CPU on the right, four copper traces between them, a glowing golden key travelling along the traces and a brass probe clip biting the wire — the key crosses an open wire, BitLocker is fine the bus is not, five dollars
A board-level view: the TPM chip on the left, the CPU on the right, four copper traces between them, a glowing golden key travelling along the traces and a brass probe clip biting the wire — the key crosses an open wire, BitLocker is fine the bus is not, five dollars3072×1714 404 KB

Your disk is locked by the strongest thing Windows has. Two researchers opened one with a conference badge.

The encryption is not broken — BitLocker’s maths is fine. The flaw is the wire the key crosses on its way out of the chip, and no Windows update can fix a wire.

:bullseye: Why it matters: on a machine with a separate TPM chip, the key that unlocks your disk travels in the clear between two components, and a cheap probe can read it during boot.
:sparkles: What makes it unique: most BitLocker stories are about a password or a software bug. This one is about the hardware layout underneath the operating system.
:heart: Why you should care: it changes what you do with a laptop that leaves the house.

:key: The trick in one picture

      power on
         |
    [ TPM chip ] --------> CPU
         |      key crosses
         |      in the clear
    [ your disk ]

The Volume Master Key — the key that unlocks your drive — leaves the TPM and heads for the CPU so the machine can start. On a discrete TPM, a separate chip soldered to the board, that trip happens over an SPI bus that is unencrypted, and slow enough to read.

Clip a logic analyser to those traces, power the machine on, capture the boot. The key is in your hand, and the drive opens.

:test_tube: None of this is brand new — it keeps getting cheaper

  • 2019 · Pulse Security — sniffable in the default configuration, from TPM 1.2 or 2.0, with a dirt-cheap FPGA. Their advice still stands: add pre-boot authentication.
  • 2020 · Reversec Labs — a full write-up plus a released Saleae Logic 2 toolkit that decodes TPM traffic and pulls the key out.
  • 2024 · Post-Cyber Labs — pulled it off with BitLocker PIN enabled; their test machine was an HP EliteBook 840 G6.
  • 2024 · Dell’s own advisory — confirms interception on the CPU-to-discrete-TPM bus on certain models, and notes Dell commercial PCs use a discrete TPM.

:desktop_computer: Which machines are exposed

  • Discrete TPM — a separate chip on the board: the exposed design. Common in business laptops and desktops.
  • Firmware TPM — built into the CPU: a different case, where the attack needed fault injection rather than a probe on a wire.
  • Physical access is required. This is a stolen-laptop attack, not a remote one.

:shield: What to do about it

1 · Turn on the pre-boot PIN. The oldest advice, still the first line of defence — the machine waits for you before it boots.
2 · Know the caveat. The 2024 work read the PIN from the same bus. A PIN raises the bar; physical security carries the rest.
3 · Treat a lost laptop as lost. Encryption protects data at rest, not a determined person holding the machine.

:link: Go deeper

:gear: The takeaway: hardware choices protect you as much as software ones. BitLocker is only as strong as the wire it trusts.

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