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by ComputerGuru 382 days ago
The (compromised) Linux decides whether to load the “compromised mode” code or the mp1 secure system? Sounds like an avenue to explore. It says the bootloader itself is secure, but that doesn’t mean much if it’s being loaded into a compromised environment, depending on where it is actually being executed. I guess the coprocessor could be considered a Secure Enclave of Sorts, but the fact that Linux could load a separate bootloader and run that (somehow) is of concern.
1 comments
stgl 382 days ago
No, it cannot load a separate bootloader. I tried to tamper with the loadercode (the "secure" bootloader), but it wouldn't boot. So I am guessing there is some third party (boot ROM) that verifies it.

Also, I think Linux always loads loadercode + mp1.img, regardless of the tamper state. The different code paths depending on tamper state are taken within the (integrity protected) loadercode.

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ComputerGuru 382 days ago
Keep in mind that no tamper mode will be set if you use the external debug interface. If Linux is used for networking then maybe you could MITM payments.
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_djo_ 382 days ago
The card details and payment are encrypted and then signed by the firmware running in the secure/trusted zone, using public keys provided by the acquirer.
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