[sitkack]

You need to xray the die and confirm the designs are the same, but it doesn't guarantee that extra or bad transistors aren't inserted that leak data.

You never really know.

https://www.cl.cam.ac.uk/~sps32/ches2012-backdoor.pdf

https://www.usenix.org/legacy/event/leet08/tech/full_papers/...

http://en.wikipedia.org/wiki/Hardware_Trojan