Hacking Hardware Bitcoin Wallets: Extracting The Cryptographic Seed From A Trezor

It’s long been common wisdom that one of the safest places to keep your cryptocurrency holdings is in a hardware wallet. These are small, portable devices that encrypt your keys and offer a bit more peace of mind than holding your coins in a soft or web wallet.

But of course, as we know, nothing is totally secure.

And we were reminded of this fact by Kraken Security Labs, when they showed us how they bypassed all of the safeguards in a popular wallet, the Trezor, to dump and decrypt it’s seed.

It’s worth noting that the hack does require physical access to the wallet — albeit only about fifteen minutes worth. And by “physical access” we mean that the hack leaves the device thoroughly mutilated. The Kraken team started by desoldering the heart of the wallet, a STM32 processor. They then dropped it into a socket on an interface board, and got to glitching.

The hack relies on an attack known as voltage glitching. Essentially, at a precisely-timed moment during the device’s boot sequence, the supply voltage is fluctuated. This enables the chip’s factory bootloader, which can read out the contents of it’s onboard flash memory. The memory is read-protected, but can be accessed 256 bytes at a time through a second voltage glitch. Neither of these attacks work 100% of the time, so if the device fails to boot or the memory remains locked, the FPGA performing the attacks simply tries again. After enough iterations, the Kraken team was able to fully dump the chip’s flash memory.

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A Close Eye On Power Exposes Private Keys

Hardware wallets are devices used exclusively to store the highly sensitive cryptographic information that authenticates cryptocurrency transactions. They are useful if one is worried about the compromise of a general purpose computer leading to the loss of such secrets (and thus loss of the funds the secrets identify). The idea is to move the critical data away from a more vulnerable network-connected machine and onto a device without a network connection that is unable to run other software.¬†When designing a security focused hardware devices like hardware wallets it’s important to consider what threats need to be protected against. More sophisticated threats warrant more sophisticated defenses and at the extreme end these precautions can become highly involved. In 2015 when [Jochen] took a look around his TREZOR hardware wallet he discovered that maybe all the precautions hadn’t been considered.

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