Breaking NLM-MAC Generator

NLM generator, designed by HoonJae Lee, SangMin Sung, HyeongRag Kim, is the strengthened version of the LM-type summation generator with two memory bits; which uses non-linear combination of linear feedback shift register and non-linear feedback shift register. Recently, the cipher along with a massage authenticate function have been proposed for a lightweight communication framework in wireless sensor networks. Also, the generator has been used in two different RFID mutual authentication protocols and a protocol to secure access in internet. This paper indicates some critical cryptographic weak points leading to the key recovery and forgery attack. We prove the internal state of NLMn can be recovered with time complexity about nlog7×2 where the total length of internal state is 2 · n + 2 bits. The attack needs about n keystream bits. We also show attacker is able forge any MAC tag in real time by having only one pair (MAC tag, cipher-text). The proposed attacks are completely practical and break the scheme with negligible error probability.