Dismantling the AUT64 Automotive Cipher

Christopher Hicks, Flavio D. Garcia, David Oswald

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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 immobilisation 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.
Original languageEnglish
Pages (from-to)46-69
Number of pages24
JournalIACR Transactions on Cryptographic Hardware and Embedded Systems
Issue number2
Publication statusPublished - 8 May 2018
EventConference on Cryptographic Hardware and Embedded Systems 2018 - Amsterdam, Netherlands
Duration: 9 Sept 201812 Sept 2018


  • Automotive security
  • Hardware and software reverse engineering


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