Re: AES MAC security question

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Date: Sun, 03 Jul 2005 22:20:26 GMT

Rein Anders Apeland wrote:
> Hi all.
>
> I am working on a remote keyless entry system, e.g. for
> use with car immobilizers. The car owner carries a
> key-sized RF transmitter, a fob, and the car has the
> receiver. The RF packet has the following fields:
>
> - Unique transmitter ID
> - Sequence counter
> - Command
> - MAC
>
> The transmitter IDs are unique from the car manufacturer.
> The sequence counter is incremented with every message transmitter,
> so that the car reject repeated commands. The command itself
> could be 'lock', 'unlock', 'panic' and so on. Not important.
>
> I am planning on using AES for generating the MAC, since for other
> reasons the transmitter software already implements AES for other
> puproses, and I cannot affort an increase in code size for adding
> e.g. a secure hash algorithm for use as a MAC.
>
> Since the RF packet is so small that it fits inside one AES block,
> I am thinking of padding the packet to full 128 bits, encrypt it
> and then truncate the result to e.g. 32 bits and use that as a MAC.
> Bacause of the low bandwidth of the system, I believe truncating is
> ok in this case.

   What protects against a man-in-the-middle attack?

> My questions are:
>
> - Is this a good solution, given that I would like to use AES ?
>
> - Since very few bits change from message to message,
> would this solution leak too much information on the
> encryption key if an attacker snaps several RF packets?
>
> - Alternatives?
>
> I also thought of extending each of the three fields by padding or
> repeating to 128 bits and use AES-CBC-MAC, but I don't know if that
> helps at all.
>
> And finally, if you want to know, all fobs have different encryption
> keys, and the receiver/car has a list of ID/key pairs of accepted fobs.

  Is your method of adding a new remote-keyless-entry-key for a
receiver/car better than the current method for getting a conventional
remote to work on a particular car?

--Mike Amling

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