Related-Key Attacks on the Py-Family of Ciphers and an Approach to Repair the Weaknesses

Abstract

The stream cipher TPypy has been designed by Biham and Seberry in January 2007 as the strongest member of the Py-family ciphers, after weaknesses in the other members Py, Pypy, Py6 were discovered. One main contribution of the paper is the detection of related-key weaknesses in the Py-family of ciphers including the strongest member TPypy. Under related keys, we show a distinguishing attack on TPypy with data complexity 2 193.7 which is lower than the previous best known attack on the cipher by a factor of 2 88. It is shown that the above attack also works on the other members TPy, Pypy and Py. A second contribution of the paper is design and analysis of two fast ciphers RCR-64 and RCR-32 which are derived from the TPy and the TPypy respectively. The performances of the RCR-64 and the RCR-32 are 2.7 cycles/byte and 4.45 cycles/byte on Pentium III (note that the speeds of the ciphers Py, Pypy and RC4 are 2.8, 4.58 and 7.3 cycles/byte). Based on our security analysis, we conjecture that no attacks lower than brute force are possible on the RCR ciphers.

New Analysis on Security of Stream Cipher HC-256

“…Related-key attack is wide range of applications in block cipher, but is rarely applied in stream cipher. The existing results are applied in RC4 and Py Series of stream ciphers [3,4]. In ACISP2008, Yuseop Lee et al [5] combined related-key attack with chosen IV attacks and applied this new attack on Grain v1 and Grain-128.…”

Related Key Chosen IV Attack on Grain-128a Stream Cipher

“…Application of related-key attacks to stream ciphers was not very common in the literature, mainly due to the heavy operations executed in one-time key-scheduling algorithms com- pared to the operations performed in iterative block ciphers, see [7]. However, there is a growing tendency by cryptanalysts nowadays to attack stream ciphers using related key attacks.…”

“…However, there is a growing tendency by cryptanalysts nowadays to attack stream ciphers using related key attacks. Sekar, Paul and Preneel [7] presented related-key weaknesses in the Py-family of ciphers including the strongest member TPypy. The result had been improved significantly by Ding and Guan in [8].…”

Cryptanalysis of block-wise stream ciphers suitable for the protection of multimedia and ubiquitous systems

“…It transforms a 16-byte master key into an 8-byte block of output at a time. The main cryptanalytic result [8] obtained on TWOPRIME so far is the analysis 1 In the literature, there have been several examples of the application for the related-key attack on stream ciphers [15,41], however, no general framework of related-key attack on stream ciphers has been published. exploiting structural weaknesses of TWOPRIME.…”