Thoughts of a geek

18 February 2010

Your fare is Mifare

Filed under: Computers, University — Tags: , , , , , — qwandor @ 9:22 pm

Or rather, your student ID.

I recently bought a Snapper Feeder to have a play with, as I had heard that they were supported by libnfc under all the major operating systems and could talk to a variety of contactless smartcards. I tried any cards I could get my hands on, and other than the Snapper card itself I found that our building access cards were not compatible (using a different frequency, perhaps?) but the Vic student ID cards were recognised. Furthermore, it turns out that these ID cards are MIFARE Classic 4k cards.

Now, this is interesting because encryption scheme used by the Mifare Classic was broken and research published explaining the vulnerabilities as early as 2007, and this is even acknowledged by the manufacturer. Anyway, being the curious sort I am, I went about trying to see what I could see about the Vic student ID cards. I have not yet been able to replicate the card-only crack to recover the keys, but I have had a bit of a look at how the cards are formatted.

Firstly, a bit of background. The Mifare Classic 4k has 40 sectors, each of which has 2 48-bit encryption keys (called A and B) and 12 configuration bits which control which of the keys allow read and write access to the sector’s data and configuration. Each sector is broken down into a number of 16 byte blocks. The first 32 sectors have 4 such blocks (64 bytes total), while the last 8 sectors each have 16 blocks (256 bytes per sector). The last block of each sector is called the sector trailer and contains the encryption keys and configuration bits previously mentioned. Note that the configuration and keys for each sector is independent of all the other sectors. Reading from and writing to the card is done on a block by block basis. Accessing a block is a two step process. First you must authenticate to the sector with either the A or the B key, then you can read or write one of the blocks in that sector.

With this in mind, here is what I have found so far about Vic’s student ID cards. I used the micmd tool, which provides a fairly simple interface to access Mifare Classic cards using libnfc, and a few other bits and pieces. Authentication to all sectors except sector 15 worked using FFFFFFFFFFFF (a common default key) as either key A or key B. However, despite the successful authentication, I was only able to read the blocks of sector 0. (Admittedly I did not try all of the other sectors, but all that I did try failed to read.) This may indicate that these sectors are configured to not to be accessible by either key, as a way of permanently disabling them, or it may be a problem with my reader. The reason I suspect my reader is that it would often lock up after certain operations, not responding at all until I unplugged it and plugged it back in again. I am not sure what is causing this; if anyone has any ideas do say.

Sector 15 appears to be using a proper key, and is probably where the real data of the card is stored.

Sector 0, the one sector I did manage to successfully read, does not appear to hold much of interest. On one card, with a UID of D4 EE 01 6E, the four blocks were

0: D4EE016E55980200648E565165603905
1: 800F0000000000000000000000000000
2: 00000000000000000000000000001248
3: 000000000000787788C1000000000000

Block 0 of sector 0 apparently holds read-only data set by the manufacturer, so is not that exciting. It appears that the first 4 bytes are the UID of the card. The 5th byte also seems to vary between cards (on the 3 cards I tried the values were 0x55, 0x5F and 0x61). The remaining 11 bytes of block 0 were the same on all 3 cards I tried. Blocks 1, 2 and 3 (the trailer block) were also the same on all 3 cards, which suggests that they are unlikely to be interesting.

I did attempt to use the nested authentication attach (I believe) as implemented by mfoc and MFCUK to recover the keys for sector 15, but for some reason both implementations failed, possibly due to the reader ceasing to respond part-way through as mentioned above. Any suggestions on how to get past this are welcomed.

Does anyone have any other interesting smartcards?


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