First of all, it would be very unsafe to have an amount of challenge codes in a database. These challenges are random generated numbers, and are generated when the client opens the web banking page.

The user (all of us humble home bankers) receives a challenge from his bank and feeds it to the Vasco Digipass, then the Digipass asks for your pin. If this is correct, the device has access to a number stored on the card. This number is unique and unknown to anybody, except the issuer (bank) and the card itself. This is of huge importance as otherwise it would be very easy to hack this system. Suppose that entering the PIN-code correctly would be the only thing to happen to have the correct challenge reply, then the response from every card to the MCU would always be the same (it would be: “Pin code correct”). If it’s always the same, it would imply a very easy to hack system.

Actually this is a nice example of Message Authentication in standard cryptography: everybody (!!) can see the challenge to start communicating to the web banking server, and everybody can enter a challenge, yet only one unique device will be able to give the right answer (and thus to authenticate itself).

Lastly, the protocol isn’t really a secret. It’s based on known cryptographic protocols (such as AES (aka Rijndael) or ECC or Grain or any other publicly known crypto. The worst choice would have been a “secret” protocol (it’s called having security by obscurity), once the secret has been discovered: all hope is lost. Known (public) protocols have been tested and proofed for years! That is why cryptography is such a nice thing to study!

Tu génère la communication structurée et tu la joint au numéro de facture/numéro de client dans ta base de données.
Un grep de la comm dans tes extraits de compte et tu sais si ton client a payé.

Tu n’as pas besoin de savoir qui paie.

If it work for car batteries, why not on such device ?

If you fry it in the process, well… At least you tried to save it.
In my case, i just asked for a new digipass from my bank. No question or money asked.

Let’s say that the almost empty batteries provide a voltage of 5V instead of 6V (as fully charged). Then I must set my power supply to 5V or less. In this case the 2 CR2032 will not be charged hence will not explode. Correct? Then they could be removed and replaced by new ones…

Or am I overlooking something?

If you keep the device powered, it will not notice the battery exchange, thus the RAM will keep it’s content.

Something to test, but watch the power supply voltage carefully to avoid charging full batteries. Lithium cells are unforgiving and may blow up in your face.

Hope you have a steady hand.