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Breaking The FF3 Format-Preserving Encryption Standard Over Small Domains


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Tuesday, May 30, 2017, 02:00pm


The National Institute of Standards and Technology (NIST) recently published a Format-Preserving Encryption standard accepting two Feistel structure based schemes called FF1 and FF3. Particularly, FF3 is a tweakable block cipher based on an 8-round Feistel network. In CCS~2016, Bellare et. al. gave an attack to break FF3 (and FF1) with time and data complexity $O(N^5log(N))$, which is much larger than the code book (using many tweaks), where $N^2$ is domain size to the Feistel network. In this work, we give a new practical total break attack to the FF3 scheme (also known as BPS scheme). Our FF3 attack requires $O(N^{frac{11}{6}})$ chosen plaintexts with time complexity $O(N^{5})$. Our attack is practical when the message domain is small with less than $2^9$. It is a slide attack (using two tweaks) that exploits the bad domain separation of the FF3 design. Due to this weakness, we reduced the FF3 attack to an attack on 4-round Feistel network. Biryukov et. al. already gave a 4-round Feistel structure attack in SAC~2015. However, it works with chosen plaintexts and ciphertexts whereas we need a known-plaintext attack. Therefore, we developed a new generic known-plaintext attack to 4-round Feistel network that reconstructs the entire tables for all round functions. It works with $N^{frac{3}{2}} left( frac{N}{2} right)^{frac{1}{6}}$ known plaintexts and time complexity $O(N^{3})$. Our 4-round attack is simple to extend to five and more rounds. Finally, we provide an easy and intuitive fix to prevent the FF3 scheme from our attack.

Speaker: Fatma Betül Durak



Location : CoRE B (305)


Prof. David Cash (Chair), Prof. Rebecca Wright, Prof. Shubhangi Saraf, Prof. Thomas Ristenpart (Cornell Tech)

Event Type: Pre-Defense



Department of Computer Science