[Openbeacon-openamd] Decrypting 25C3 data

[Manuel P. <ha...@cc...>]

Hi folks,

I wrote a small C program using libpcap to extract the beacon data of the
25C3 raw packet dataset. This works pretty well so far, meaning I can
fetch the payload (16 bytes of beacon data) but unfortunately I am not
able to decrypt it using the XX-TEA algorithm and the key published on the
wiki. I do not know where the problem is exactly, so I'd to check with you
if my algoritm is right (pseudo-code):

 1. Extract payload from packet (16 bytes)
 2. Convert payload from network to host byte-order in 4-byte blocks
 3. Decrypt payload with XX-TEA and published key

I tried several ways of changing the byte-order (or not changing it at
all) and different keys but I was not able to get correct results. I
verify the result by the flags field in byte 6 (should be 0 or 2) and byte
1 (protocol version?) which should be constant. Unfortunately, I just get
random numbers.

It would be really cool if you could point to me what I did wrong or if
you could verify the key is correct. I spent quite some time with
debugging already and am totally out of ideas. TIA!

Best regards
Manuel