Improving Upon the TET Mode of Operation

Naor and Reingold had proposed the construction of a strong pseudo-random permutation (SPRP) by using a layer of ECB encryption between two layers of invertible block-wise universal hash functions. At Crypto 2007, Halevi presented constructions of invertible block-wise universal hash functions and a new mode of operation (called TET) based on them. In this paper, we present a new mode of operation called HEH using the Naor-Reingold approach. This is built using a new construction of invertible block-wise universal hash function. The new construction improves over Halevi’s construction by removing restrictions on the hashing key. This in turn, leads to HEH improving over TET by allowing more efficient encryption and decryption of variable length messages as well as supporting better key agility. For the important application of disk encryption, we present a variant called HEHfp which has better key agility than TET.