Impossible Differential
The impossible differential attack exploits differential characteristics of probability zero (a given input difference can never produce a given output difference) to rule out wrong key guesses.
Purpose
It provides effective key recovery for ciphers whose high-probability characteristics coexist with impossible paths (e.g., AES, Skipjack).
Mechanism
An impossible differential path is built (a meet-in-the-middle contradiction); for each key guess, check whether it produces that path — if it does, the guess is wrong and is eliminated, leaving the correct key.
Use cases
- AES-128 key recovery
- Skipjack attacks
- Block-cipher security analysis