The way for USB key RNGs to combat that is to encrypt the random number stream. That's what Simtec's Entropy Key[1] did. Your adversary would not know the secret that is shared between the USB device and the computer, and so the swap would be immediately detected.
That said, if you have an adversary that has and is devoting the time and resources necessary to arrange to have someone on the plane with you, with a doctored USB RNG, in a seat where they can get to your laptop when you go to the bathroom and mess with it, you probably should not be traveling alone, and your traveling laptop should be one that never contains important secrets and is never connected to a network that has important secrets.
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The way for USB key RNGs to combat that is to encrypt the random number stream. That's what Simtec's Entropy Key[1] did. Your adversary would not know the secret that is shared between the USB device and the computer, and so the swap would be immediately detected.
That said, if you have an adversary that has and is devoting the time and resources necessary to arrange to have someone on the plane with you, with a doctored USB RNG, in a seat where they can get to your laptop when you go to the bathroom and mess with it, you probably should not be traveling alone, and your traveling laptop should be one that never contains important secrets and is never connected to a network that has important secrets.
[1] http://www.entropykey.co.uk