There&#x27;s a silicon doping attack for CPUs which makes them impossible to verify with current verification techniques (as far as I understand). The idea is that if you dope the silicon in the circuit you can change the circuit and it&#x27;s not possible to figure out if it&#x27;s been changed through forensics (x-rays, etc) or just using it (sufficiently clever backdoor). I&#x27;m unsure if FPGAs are vulnerable to this attack (though I&#x27;ve heard they might not be). This is the paper: <a href="http:&#x2F;&#x2F;sharps.org&#x2F;wp-content&#x2F;uploads&#x2F;BECKER-CHES.pdf" rel="nofollow">http:&#x2F;&#x2F;sharps.org&#x2F;wp-content&#x2F;uploads&#x2F;BECKER-CHES.pdf</a>