W.: Leaked-state-forgery attack against the authenticated encryption algorithm ale

Abstract. ALE is a new authenticated encryption algorithm published at FSE 2013. The authentication component of ALE is based on the strong Pelican MAC, and the authentication security of ALE is claimed to be 128-bit. In this paper, we propose the leaked-state-forgery attack (LSFA) against ALE by exploiting the state information leaked from the encryption of ALE. The LSFA is a new type of differential cryptanalysis in which part of the state information is known and exploited to improve the differential probability. Our attack shows that the authentication security of ALE is only 97-bit. And the results may be further improved to around 93-bit if the whitening key layer is removed. We implemented our attacks against a small version of ALE (using 64-bit block size instead of 128-bit block size). The experimental results match well with the theoretical results. Key words: authenticated encryption, forgery attack, ALE