We are going to use a new form of encryption... we will use Sarcasm in our message, then say in public what we can't say... by saying the opposite, sarcastically.
What effect does the sarcastic tone have in the speech?
Because it's a tone rather than a rhetorical fact, sarcasm by definition can only be used in dialogue. It would describe a way that one character talks to another. This can have all kinds of effects, but generally it shows that the speaker is being impatient or contemptuous.
