I think Section 97 is what you're referring to. If so, it's not obstruction of purpose the FCC forbids; it's obstruction of meaning.
Section 97: https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D...
Throughout part 97, there is a repeated prohibition of transmitting "messages encoded for the purpose of obscuring their meaning." If I understand it correctly, there are many accepted ways to encode a message, but those encoding methods are (and must be) published and publicly accessible. I think that encrypting a message so that only certain people could decipher it would fall under the category of "messages encoded for the purpose of obscuring their meaning." Yes, you could encrypt a message with a published standard, but to be legal I think there would have to be some specific exceptions made to allow it because it ultimately runs afoul of the spirit of that rule.