Most devices or device components can be isolated and/or severed from the system. A number of methods that are available to the attacker are: electron microscope, lasers, high powered microscopes and other fine instruments of professional tools. The attacker can remove the solder, or tamper with the entropy generator itself. If that is not possible the attacker will resort to influencing the number of bits generated by the entropy generator. Perhaps the generator will create patterns under extreme or unusual environments. This is where the Bayesian Estimation based attacks will start to dwindle down the number of random bits to something that can be easily brute forced or discovered by a dictionary attack.
Some of the bits can be frozen to be 0 or 1. It doesn't matter as long as the external influence can hold the state of the bit 0/1.
For example, how does extreme temperatures affect the device?
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