Ross Anderson (previously) is one of the world's top cryptographers; the British academic and practitioner was honored by having his classic, Security Engineering, inducted into The Cybersecurity Canon; however, he was not able to attend the awards gala himself because the US government sat on his visa application for months, and ultimately did not grant it in time.
Anderson's not the only one: Israeli cryptography legend Adi Shamir (he's the "S" in "RSA") could not get a visa to visit the USA to participate in the RSA conference (again, he's the "S" in "RSA"). Shamir is a recipient of the Turing Prize — computer science's answer to the Nobel Prize. During the panel that Shamir missed, co-panelist Shafi Goldwasser said that Shamir was just one of many cryptographers who could not attend because of visa issues.
Shamir made a video that was presented at RSA that called for a "rethink" of "the question of how and where we organize our major scientific conferences."
Why Are Cryptographers Being Denied Entry into the US? [Bruce Schneier/Schneierblog]