Thank God for the constitution; I expect any reasonable court to toss Trump out on his ear if he tries this in about a minute.
Trump certainly can't block Americans from selling cryptographic algorithims (or anything which implements those algorithms) if the consumer is also american.
But preventing the communication of those algorithms to foreign nationals is something that was done for several decades.
I think its a terrible thing for him to try to do, but he would have some legal grounds to stand on, provided the restrictions are related to foreign nationals only.
Unfortunately, you can't put the cat back in the bag. It's no good putting export restrictions on encryption when the tightest encryption we have at present is already available all round the world.
Talking about judges as being "assigned" is both inaccurate and betrays an ignorance of the American system.