Chapter

Information Security and Cryptology

Volume 5487 of the series Lecture Notes in Computer Science pp 157-180

Constructing Variable-Length PRPs and SPRPs from Fixed-Length PRPs

  • Debra L. CookAffiliated withColumbia University
  • , Moti YungAffiliated withGoogle, Inc. and Columbia University
  • , Angelos KeromytisAffiliated withDepartment of Computer Science, Columbia University

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Abstract

We create variable-length pseudorandom permutations (PRPs) and strong PRPs (SPRPs) accepting any input length chosen from the range of b to 2b bits from fixed-length, b-bit PRPs. We utilize the elastic network that underlies the recently introduced concrete design of elastic block ciphers, exploiting it as a network of PRPs. We prove that three and four-round elastic networks are variable-length PRPs and five-round elastic networks are variable-length SPRPs, accepting any input length that is fixed in the range of b to 2b bits, when the round functions are independently chosen fixed-length PRPs on b bits. We also prove that these are the minimum number of rounds required.

Keywords

(strong) pseudorandom permutations block ciphers variable-length PRPs