ChangeKey Command Example in AES Mode

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  • 18. September 2017 at 16:10
    Hi,

    I am trying to understand the Case 1 of ChangeKey Command Example in AES Mode of the AN094533 document but I have some questions.
    - How can I calculate the CMAC? (Step 15)
    - How can I get the last IV? (Step 16)
    - If I want to change again the same key, what will be the previous key?

    Thanks in advance,
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    Re: ChangeKey Command Example in AES Mode

    19. September 2017 at 10:03
    Hi Cales,

    As described in the documentation, the CMAC is defined in “NIST Special Publication 800-38B”. If you google it, you will find the documentation.

    IV is updated in every ciphered operation.

    The TapLinx team

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    Re: ChangeKey Command Example in AES Mode

    19. September 2017 at 10:20
    Hi,

    I know it but I don't know which is the Key, the message and the IV to get the CMAC of the step 15.
    I know that the IV is updated in every ciphered operation but I don't know how can get the second part of the iv of the step 16.

    Thanks,
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    Re: ChangeKey Command Example in AES Mode

    31. January 2018 at 10:25
    Hi Cales,

    In step 15 the CMAC is calculated over the status byte, in this case only 00, therefore CMAC(00,IV). Take in mind, you have to use the current IV in CMAC calculation and update the IV with the last CMAC result. The card send only the upper 8 bytes of the CMAC for verifying.

    The TapLinx team

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