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If an attacker has an unlimited physical access to CPU, but does not have access to memory, including RAM, can he attack and gain access to the user's data?

I heard the opinion that this is impossible according to the laws of physics, even with an electron microscope. It's true?

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  • $\begingroup$ This seems more appropriate for Cryptography. $\endgroup$ Jul 27, 2018 at 17:38
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    $\begingroup$ I don't see what the laws of physics have to do with this question. $\endgroup$ Jul 27, 2018 at 17:39
  • $\begingroup$ What's the basis for that opinion? Have you asked them why they said that? $\endgroup$
    – D.W.
    Jul 27, 2018 at 18:48
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    $\begingroup$ The title question seems different from the body question… $\endgroup$
    – Draconis
    Jul 27, 2018 at 20:04
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    $\begingroup$ This seems largely hypothetical. The CPU and RAM are in the same box. How can I have physical access to one but not the other? $\endgroup$ Jul 31, 2018 at 12:42

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If you have unlimited physical access to the CPU, then you can send your own instructions into it, including instructions to read from arbitrary memory addresses. Looking at the voltages on the data bus then shows you exactly what's in that word of memory.

Remember, any data the computer is able to use, has to be accessible by at least one CPU. That's just how it works. So access to the CPU gives you theoretical access to everything else.

(Note: this is an answer to the question in the body, not the question in the title.)

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