ATAES132A AuthCompute Command

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Hi,  is there anyone here who uses the Auth and AuthCompute Commands? If so, then please describe me the procedure of Inbound Only Authentication. 

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I use Auth in Mutual Authentication mode, which is basically inbound + outbound and it works OK.

I don't use AuthCompute though as I evaluate the MAC myself in C as described in the datasheet in sections  I.3 & I.6

What response code error do you get

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ERROR CODE 0x40 (Bad MAC), can you share your code to calculate MAC?

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Sadly it's not my code, it belongs to work, so I'm afraid I can't let you have a copy.

 

I can tell you a few of the 'gotchas' I stumbled over writing  it...

The mac_flag byte in the ADATA block. Make sure the Random bit and the Input bit are set correctly for your MAC.

Check the endian-ness of all your parameters, and the length fields in the A_0 and B_0 blocks.

Using a constant nonce makes things much easier and will ensure MacCount=1 for the InMAC calculation used in the Auth command immediately after (re)loading the nonce.

 

Auth was the first command I got working because it has no payload and so is the simplest MAC calculation, thoug it still took a week of fiddling to get it going.

 

Good luck.