Paper 2008/496

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

Debra L. Cook, Moti Yung, and Angelos Keromytis


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.

Available format(s)
Secret-key cryptography
Publication info
Published elsewhere. shorter version will appear in Inscrypt 2008
(strong) PRPsblock ciphersvariable-length PRPs
Contact author(s)
dcook @ cs columbia edu
2008-12-02: received
Short URL
Creative Commons Attribution


      author = {Debra L.  Cook and Moti Yung and Angelos Keromytis},
      title = {Constructing Variable-Length PRPs and SPRPs from Fixed-Length PRPs},
      howpublished = {Cryptology ePrint Archive, Paper 2008/496},
      year = {2008},
      note = {\url{}},
      url = {}
Note: In order to protect the privacy of readers, does not use cookies or embedded third party content.