Picking the right key for DESFire EV2 cards when UIDs are unreliable/unavailable

[Mario Mariani]

Hi, I'm building a software that runs on a PC that authenticates users with cards. I'm using DESFire EV2 for this. At the moment I'm stuck because, since there are multiple cards in the system (possibly dozens, each with a different key), I need to know what card is on the reader in order to pick the right key and to perform the authentication. I thought that UIDs were the way to go, but UIDs in the anti-collision phase could be random and the GetUID function is available only AFTER a successful authentication. Do you have any ideas on how can I achieve my goal? I feel like this is a bit of a rookie question, but I wasn't able to find an answer in the docs, nor in the forum. Thanks

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[TapLinx Support]

Hi Mario,

This is a special configuration setting to set the UID to random in the anti-collision. In this case I would have a file with read permission 0x0E and communication mode “plain” (all other files should be protected with keys). This file can be read without authentication and give some hints about the key. In and of itself, knowing this data this is not a security problem, it is only a piece for subsequent method.

Usually, it is no practicable to manage a lot of keys in the reader terminal. I would prefer using diversified keys. There is a root key for all cards, but with a “unique diversification value” the authentication key is generated. The unique diversification value could be the data from the readable file and the authentication key is different for all cards.

The TapLinx team

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[Mario Mariani]

Thank you very much, your reply was very helpful. I think I will use these techniques.

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[TapLinx Support]

Hi Mario,

An Addendum. What I called as “unique diversification value” is usually the UID of the card or some other constant which is unique for the card in the field. There exists an application note from NXP where the key diversification is explained:

https://www.nxp.com/docs/en/application-note/AN10922.pdf

The TapLinx team

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[Mario Mariani]

Thanks again. That application note is a very valuable resource.

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[fsurleau]

Hi,

The purpose of a random UID is to prevent a badge from being trackable.

If the badge contains an open-access file containing a unique identifier, don't you think that some might consider that this introduces a security flaw ?

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[Allen Roberts]

In principle, you can use this option

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