I am trying to reverse engineer some data received over serial port.
The hardware sending the data consists of a gps, compass and a power monitoring unit (which sends information about the battery).
I received three types of messages of varying length. These are given below:
55 BB 55 BB 33 00 E7 29 B4 BD D7 82 61 39 23 C4 04 74 04 38 6B 02 12 8E 8D F3 E3 D7 4B 59 91 9C 01 98 0F C5 43 95 D2 03 D2 CC 4B B1 9A A2 54 8C 99 4A 00 DD 02 AB CD 55 BB 55 BB 38 00 85 1F EA BD DE D0 87 7F 14 9A 5C 46 78 7A D1 E7 29 56 2E 7D F9 A9 FF DE E4 6C F2 F5 26 0D EF CD 6A ED B0 28 1D 9E DB 9D 8E BA 5F 58 26 3F B3 DD 03 00 A6 73 AB CD 55 BB 55 BB 24 00 F3 8B FD F6 BF D0 B0 06 05 8D 7C 40 19 CF 29 72 0E 88 B5 0F 4E 56 67 1C 56 AE F8 A3 01 00 DE 05 AB CD
What i have understood so far is:
Each message starts with 55 BB 55 BB which i assume is the Message Header.
The field (byte 5) is the length of the message from 5th till the last byte.
It is then followed by 00 which might mark the start of payload.
The last two bytes are always AB CD which might be a trailing header.
The fourth last and third last bytes might be checksum for the message.
The fifth last byte is always zero which might indicate the end of payload.
The checksum is obtained by using The 8-bit Fletcher Checksum Algorithm described in the TCP alternate checksum options document (rfc1145).
The data in the payload doesn't make sense. I have to figure out the order as well any encryption that might be occurring. It seems as they are Xoring the data and the Xor mask changes everytime. But they are sending the Xor mask in the data. I tried Xoring the whole string with each byte but couldn't obtain something similar to known values for Lat/Long.
Can somebody provide some guidance on how should i proceed?
Some work has been done here before. Rcgroups Post#15
Here is a rft file containing the rest of the data. 4shared Link RTF File