Sherlock describes Devon Gaspar's message to Moriarity as written using a technique called the Ave Maria Cipher (a type of steganography). The Ave Maria Cipher uses a word, from a predetermined list, to represent a letter. The letters are replaced by the corresponding word in the message so only certain words have any significance. Only the recipient knows the hidden information is there. Devon's message is 75 letters long (not counting spaces and punctuation) so the ad in the newspaper for a used car would need to contain 75 seemingly random words, in addition to those actually about the used car.
Holmes: (yells in French into an old rotary phone and slams the receiver down)
Watson: (gesturing to the phone) What's with the...
Holmes: Telephone? I felt compelled to hang up on a good few many people today, I dug it out of the closet so I could do it with more emphasis.
Moriarty: Your letters have meant a great deal to me. I find them influencing my decisions in the most surprising ways.
Holmes: You could have killed Agent Mattoo. And from your perspective, it would have been the expedient thing.
Moriarty: And yet, to you, it would have been repugnant. Tell me, is that how you learned to be one of them? By learning to care how your actions seemed in the eyes of another?
Holmes: I'm not sure I am one of them.
Holmes: Did you say everything you wanted to say today? To Moriarty? That is why you accompanied her to the Fuller residence, is it not? So you would have a private dialogue with her?
Watson: Well, you have been hogging her for the last few months.
Holmes: I told you. I have merely been--
Watson: Studying her. Right. You have feelings for her, and that is perfectly normal.
Holmes: I have nothing of the sort.
Watson: Irene was the love of your life. That is why, in spite of everything, you can't quite give her up. I get that. But I also know that you're only gonna get hurt in the long run because there is no Irene. There is only Moriarty. And Moriarty is never gonna change.
Watson: Is that why you've been writing him? Because you want to... understand him?
Moriarty: Part of it, I suppose.
Watson: And the rest?
Moriarty: What do you think?
Watson: You think you're in love with him. Only you can't be sure, because as much as you claim to know about the world, love is something you don't quite get.
Moriarty: You'd be surprised what I'd do for love.
Watson: Nothing crazy people do surprises me.
Moriarty: I write to Sherlock because he's the only person on the planet I can really talk to. He writes to me because I'm the only one he can talk to. The only one he can ever truly relate to. If you still don't understand that, you will someday. I promise.
Moriarty: You look a bit tired.
Holmes: You look a bit evil.
"The Crooked Kind" by Radical Face (last scene)