On the Round Security of Symmetric-Key Cryptographic Primitives

by Zulfikar Ramzan and Leonid Reyzin

We put forward a new model for understanding the security of symmetric-key primitives, such as block ciphers. The model captures the fact that many such primitives often consist of iterating simpler constructs for a number of rounds, and may provide insight into the security of such designs.

We completely characterize the security of four-round Luby-Rackoff ciphers in our model, and show that the ciphers remain secure even if the adversary is given black-box access to the middle two round functions. A similar result can be obtained for message authentication codes based on universal hash functions.

This work appears in Advances in Cryptology -- Crypto 2000, Mihir Bellare, editor, Lecture Notes in Computer Science 1880, Springer-Verlag, 2000. © IACR.