encrypting your laptop while you discuss drug deals is like wearing gloves while you steal the Hope Diamond.
Disagree. The encrypted laptop contains evidence, in the form of emails, chat logs, or whatever, generated in the course of the discussion. Wearing gloves doesn't destroy evidence; it simply prevents its creation. Not logging your chats about drug deals would be the equivalent of wearing gloves while burgling; logging to an encrypted drive is something entirely different.
You can "lie by omission" while providing testimony
Not always. If you don't mention that you were watching your neighbor in the shower when you saw the defendant break into her apartment and kill her, no-one's going to bat an eye. If the omission is materially relevant to the matter at hand, and made with an intent to mislead, however, that's still perjury.