The LOCAL Attack: Cryptanalysis of the Authenticated Encryption Scheme ALE

We show how to produce a forged (ciphertext,tag) pair for the scheme ALE with data and time complexity of 2 ALE encryptions of short messages and the same number of authentication attempts. We use a differential attack based on a local collision, which exploits the availability of extracted state bytes to the adversary. Our approach allows for a time-data complexity tradeoff, with an extreme case of a forgery produced after 2 attempts and based on a single authenticated message. Our attack is further turned into a state recovery and a universal forgery attack with a time complexity of 2 verification attempts using only a single authenticated 48-byte message.