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I read in the answer here by Marcus that ROM is Tamper-proof.

What is the difference between tamper-proof, tamper protected and tamper-resistant?

Now, there are some re-writable memory (eg. eNVM) that can be configured to have some of its pages as write-protected (set as ROM option) (Read-Only VS. Write Protected). In that case, can we consider that the configured ROMs are Tamper-protected? or they are only tamper-proof?

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    Nothing is "tamper-proof". You may say "Oh, ROM can't be re-written." What stops me from removing the ROM altogether and replacing it with a custom one?
    – user163495
    Jan 30, 2020 at 9:56
  • @MechMK1 I see your point. This maybe means that it depends on the emplacement of the ROM. For instance, if the ROM is inside the System on Chip (SoC) this is make it quite challenging to take off the ROM without damaging the SoC it self.
    – Lavender
    Jan 30, 2020 at 10:09
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    Then nothing stops me from taking the whole SoC off, analyzing it, and building a copy with a modified ROM. Yes, it may be expensive, but depending on the potential gain it may be worth it.
    – user163495
    Jan 30, 2020 at 10:46

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About tampering

Dan Kaminsky from "The Motherfucking Professionals" held a great talk at Blackhat 2011 regarding tamper-evident, tamper-resistant and tamper-proof devices. One of the key points he made was that there is no such things as a tamper-proof device! Anything can be tampered with, and it's up to whomever it may concern to see how far one would go to check for tampering.

He explains the meaning of those three words:

  • Tamper-Evident: A tamper-evident device does not stop you from accessing something, but it makes it evident that you did. A good example for tamper-evident devices are those G-force stickers used on packages. They don't stop someone from handling a package roughly, but it will be evident (by the broken glass) that it was handled roughly.
  • Tamper-Resistant: I assume that this is the closest to "tamper-protected" in your original question. A tamper-resistant device will actively attempt to prevent you from tampering with something. So it's a bit of a middle-ground between a lock and a seal.
  • Tamper-Proof: A device that claims to be tamper-proof is a device that is impossible to tamper with. It's effectively as real as a lock that claims to be pick-proof. As I already mentioned in the comments, nothing is tamper-proof.

From this list, you can see that tamper-protected is a stronger claim than tamper-evident, and tamper-resistant is a stronger claim than tamper-resistant.

About Memory

Neither ROM nor other write-protected memory is tamper-resistant or even tamper-evident, at least not by default. While these devices are designed to not be writable, they are not designed to be unique. That means, I can take one ROM chip out of a device, and insert a different ROM chip (of the same model) with my own data, and I have tampered with the device. If my soldering was on point, it will not be evident at all that this has happened. As such, any regular ROM fails at being tamper-evident at all.

Are there ways to make it tamper-evident?

Yes, there are. You could, for example, put a cryptographic signature over the contents of the ROM and a unique serial number. That means that without the knowledge of the private key being used, I will not be able to modify the content of the ROM without the signature check failing (which would make it evident that I tampered with the ROM).

This could be circumvented by replacing the public key that does the verification with my own public key, that I used to sign the tampered ROM contents with. But then it again remains evident that the ROM was modified. It depends on who does the checking, and how thoroughly they do.

You could also attempt some physical tamper-resistance. A simple example would be a solid epoxy blob over the chip. Yes, it will not stop a dedicated attacker, but it makes it harder to tamper with the thing, so it fits our definition for tamper-resistance (and you will now realize that we never specified how well something has to resist).

All in all, any tamper-resistant or tamper-evident device or measure can be defeated by a skilled and dedicated attacker.

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