OP's employer does not own them - they have a right to a private life.
While they certainly could not want OP identifying themselves as an agent of their employer when giving a talk, OP is entitled to use their free time / vacation days as they see fit.
The IEEE and similar bodies would not look very kindly on an employer that barred its members from CPD.
The only time an employer reasonably might not want to have the attendee use his employers name are TLA's or some companies at Blackhat conferences (if you work for a phone company for example)
She wanted to do an external presentation on cutting edge security testing (nothing proprietary, just cutting edge) and the idiot manager wouldn't let her.
She asked why and was given bullshit reasons and so she asked her manager's manager which esentially let her do the presentation and agreed with her she was given bullshit reasons.
Nevertheless her direct manager kept a grudge and she eventually had to leave the company, I'm sure she went to a much better place after that.
If he is just giving a generic talk about lessons learned without explaining the intricate war stories, thats a dull talk. Those are commodities.
OP can go to conference, in his/her "own time". OP is an adult and does not need an OK from manager to go to the conference in his/her private life.
But, OP cannot expect the manager to give the OK. OP is not "entitled" to an automatic OK. The manager does not owe OP an OK.
Unless OP is at risk of exposing privileged information about their company, the manager has no business trying to interfere with OP.
