Proxmark3 community

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:

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

Thanks!

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

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.

iceman

Administrator

Registered: 2013-04-25

Posts: 9,538

Website

Re: Help needed with a checksum

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