You "read something" but you don't provide link to the source of information. Why?
A contract for a difference(or CFD) is term from the financial markets. Trader A and Trader B make a CFD about an asset with a base price of 100. When the asset price does from 100 to 130, trader A pays 30 units to Trader B. If the asset price drops down from 100 to 70, trader B pays 30 units to trader A. This is the most oversimplified way for me to explain this. It's basically a way for some traders to hedge the risk of drastic price movements. I don't know anything about "discreet log" contracts for a difference and I don't know anything about this ItchySats project. OP, perhaps you could elaborate more on what you know about ItchySats.