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#1 2016-05-18 09:41:48

Vinnie
Contributor
From: Belgium
Registered: 2016-03-24
Posts: 39

Help needed with a checksum

Hi!

I am currently researching my studentcard with the Proxmark.
This card can be used to buy things out of the vending machines.

I already found where the money is stored but the vending machine also checks 2 bytes which are probably a XOR over some bytes.

Any help is welcome. I already tried many possibilities but couldn't find it.
Information I have about the checksum:

  • The checksum is stored at Block 9&10 at byte 0xe&0xf.

  • It uses the transaction number at Block 9 & 10 0x1

  • Money is also used in the function

  • Most likely it uses the UID of the card

Card layout when money is 0:
Studentcard


Some transactions with the card and the changed values: (Only these values change during transactions.)
Studentcard Transactions


Thanks!

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#2 2016-05-19 12:27:39

Vinnie
Contributor
From: Belgium
Registered: 2016-03-24
Posts: 39

Re: Help needed with a checksum

Any help about how I could find this function would be useful.

I just made a python script that calculates many XOR's with the values on the card.
http://pastebin.com/qkfV7AV0
But didn't find the solution.
Feedback on this script is welcome smile

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#3 2016-05-24 13:39:49

Vinnie
Contributor
From: Belgium
Registered: 2016-03-24
Posts: 39

Re: Help needed with a checksum

Tried bruteforcing XOR and CRC16
But no results.

It could be something else than a checksum.
If somebody has an idea what I should check, let me know.

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#4 2016-05-24 14:11:09

iceman
Administrator
Registered: 2013-04-25
Posts: 9,497
Website

Re: Help needed with a checksum

if could be a part of a hash,   like two bytes from a MD5, SHA1, etc etc

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