Star Trek Season 3 Episode 2

Spock: It is regrettable that you were made an unwilling passenger. It was not intentional. All the Federation wanted was the cloaking device.
Romulan Commander: The Federation? And what did you want?
Spock: It was my only interest when I boarded your vessel.
Romulan Commander: And that's exactly all you came away with.
Spock: You underestimate yourself, Commander.
Romulan Commander: You realize that very soon we will learn to penetrate the cloaking device you stole.
Spock: Obviously. Military secrets are the most fleeting of all. I hope that you and I exchanged something more permanent.
Romulan Commander: It was your choice.
Spock: It was the only choice possible. You would not respect any other.
Romulan Commander: It will be our secret.