Cryptology ePrint Archive: Report 2008/496

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

Debra L. Cook and Moti Yung and Angelos Keromytis

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.

Category / Keywords: secret-key cryptography / (strong) PRPs, block ciphers, variable-length PRPs

Publication Info: shorter version will appear in Inscrypt 2008

Date: received 25 Nov 2008

Contact author: dcook at cs columbia edu

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Version: 20081202:015330 (All versions of this report)

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