Taking away the football
Whatever the reason for the lack of visa is really makes no difference. If the guy wants a conference about his encryption system and it is too troublesome to get into the US he can hold the conference wherever he likes.
(I don't know who really invented this form of encryption, but surely this particular implementation was invented by R, S and A?)
I'm voting for Iceland because I have never been there. Nor have I ever been invited to any crypto conferences.