I've had the same situation before, and I don't think I've ever had to read them the entire thing. Usually we did something like this:

Rep: "Tell me the answer to this question."

Me: "Ok, let's see.....ah. So, it looks like a random string of gibberish, right?"

Rep: "Um, well...(unsure if he's allowed to say Yes or No)"

Me: "Yeah, I use a password manager for all my stuff, so all my passwords are randomly generated. I didn't think I'd ever have to read it over the phone. Sorry about that! I can read it out for you, but it might take awhile. If I read you the first three characters and the last three characters, is that sufficient to demonstrate for you that I know the Answer?

Rep: "Yes, I think that would be fine."

Me: "Alright, then! First three, 'F', 'caret', 'capital O'. Last three, 'capital G', 'lowercase l', 'dollar sign'.

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As I said, I've never had anyone challenge me to read the full thing out. When I explain why it is that way and give them the bookends, they are usually convinced that I'm me.