Something- sec? I'm sure I've heard the term before.
Trotskyism isn't looking to legitimize the org though, they want to transform the org into a Trotskyish party.
Opsec is different, opsec is operational security. Don't reveal too much information about yourself and don't let yourself be singled out.
Triple Sec
(more seriously, maybe you're thinking of false flag, but infiltration / sabotage are perfectly reasonable words for this)
Entryism is different, entryism is when you try to seize levers of power from within institutions that already wield power. Bernie Sanders was arguably doing entryism with the whole idea of a "political revolution" but the liberal establishment is too adept at blunting and coopting any left populism within its own ranks. OP is talking about people joining organizations to sabotage their efforts to work outside those power structures.
