Assuming a Truth Machine can be built at all, building one that doesn’t leak classified intel shouldn’t be a problem. Reading someone’s thoughts is very different from downloading everything they’ve ever read. Just build the machine to answer “Yes, this person is being deceptive” or “No, they are not” and provide no other output, discarding all other input at the end of each session.
Assuming a Truth Machine can be built at all, building one that doesn’t leak classified intel shouldn’t be a problem. Reading someone’s thoughts is very different from downloading everything they’ve ever read. Just build the machine to answer “Yes, this person is being deceptive” or “No, they are not” and provide no other output, discarding all other input at the end of each session.