RoadRunneR is a small and fast bitslice lightweight block cipher for low cost 8-bit processors proposed by Adnan Baysal and Sa ̈hap S ̧ahin in the LightSec 2015 conference. While most software efficient lightweight block ciphers lacking a security proof, RoadRunneR's security is provable against differential and linear attacks. RoadRunneR is a Feistel structure block cipher with 64-bit block size. RoadRunneR-80 is a vision with 80-bit key and 10 rounds, and RoadRunneR-128 is a vision with 128-bit key and 12 rounds. In this paper, we obtain 5-round truncated differentials of RoadRunneR-80 and RoadRunneR-128 with probability 2^{−56}. Using the truncated differentials, we give a truncated differential attack on 7-round RoadRunneR-128 without whitening keys with data complexity of 2^{55} chosen plaintexts, time complexity of 2^{121} encryptions and memory complexity of 2^{68}. This is first known attack on RoadRunneR block cipher.
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