You can't, if you're that serious/paranoid about it. It's possible that the HSM maker wasn't approached by the NSA and is secure, but there are very few of the

You can't, if you're that serious/paranoid about it. It's possible that the HSM maker wasn't approached by the NSA and is secure, but there are very few of them in the US so chances of the NSA having missed one is very low. Plus, without a STM to inspect the silicon and reverse-engineer it, how would you know? So what if you buy one made outside the US? Say, China. Well, there's the obvious possibility that the Chinese authorities have backdoored the silicon. But my guess is that the Chinese maker just cloned one of the US vendors, including the portions inserted by the NSA...