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Paper: Dismantling the AUT64 Automotive Cipher

Authors:
Christopher Hicks , School of Computer Science, University of Birmingham, United Kingdom
Flavio D. Garcia , School of Computer Science, University of Birmingham United Kingdom
David Oswald , School of Computer Science, University of Birmingham United Kingdom
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DOI: 10.13154/tches.v2018.i2.46-69
URL: https://tches.iacr.org/index.php/TCHES/article/view/874
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Abstract: AUT64 is a 64-bit automotive block cipher with a 120-bit secret key used in a number of security sensitive applications such as vehicle immobilization and remote keyless entry systems. In this paper, we present for the first time full details of AUT64 including a complete specification and analysis of the block cipher, the associated authentication protocol, and its implementation in a widely-used vehicle immobiliser system that we have reverse engineered. Secondly, we reveal a number of cryptographic weaknesses in the block cipher design. Finally, we study the concrete use of AUT64 in a real immobiliser system, and pinpoint severe weaknesses in the key diversification scheme employed by the vehicle manufacturer. We present two key-recovery attacks based on the cryptographic weaknesses that, combined with the implementation flaws, break both the 8 and 24 round configurations of AUT64. Our attack on eight rounds requires only 512 plaintext-ciphertext pairs and, in the worst case, just 237.3 offline encryptions. In most cases, the attack can be executed within milliseconds on a standard laptop. Our attack on 24 rounds requires 2 plaintext-ciphertext pairs and 248.3 encryptions to recover the 120-bit secret key in the worst case. We have strong indications that a large part of the key is kept constant across vehicles, which would enable an attack using a single communication with the transponder and negligible offline computation.
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BibTeX
@article{tches-2018-28954,
  title={Dismantling the AUT64 Automotive Cipher},
  journal={Transactions on Cryptographic Hardware and Embedded Systems},
  publisher={Ruhr-Universit├Ąt Bochum},
  volume={2018, Issue 2},
  pages={46-69},
  url={https://tches.iacr.org/index.php/TCHES/article/view/874},
  doi={10.13154/tches.v2018.i2.46-69},
  author={Christopher Hicks and Flavio D. Garcia and David Oswald},
  year=2018
}