Relationship Between two Hex strings... programming challenge
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  1. #1
    Join Date
    Aug 2013
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    2

    Question Relationship Between two Hex strings... programming challenge

    Hi All!

    I'm trying to make an application for ELM327 trouble code scanning device for my own educational purpose. Most of my app code is done. however, I am stuck at one point. And I need someone to help me finding relationship between two hex strings.

    I have logged some communication between ECU and a chinese code scanning device for carrying out car's fuel pressure test. However there is one message (that is sent from the user side) that keeps on changing each time which is dependent upon the last message received from the ECU which is also varying every time.
    I'm posting different responses from logged data for carrying out fuel pressure test at different times.

    Code:
    RECEIVED (07E8)			     SEND (07E0)
    05 67 03 0C 17 6A 00 00	     05 27 04 8F 45 37 00 00
    05 67 03 0C DB 68 00 00	     05 27 04 B0 70 6B 00 00
    05 67 03 10 3B 87 00 00	     05 27 04 3B BC 10 00 00
    05 67 03 12 CC E7 00 00	     05 27 04 5A 50 5B 00 00
    05 67 03 13 A4 88 00 00	     05 27 04 34 E7 63 00 00
    05 67 03 15 D9 CB 00 00	     05 27 04 5D 76 7D 00 00
    05 67 03 1A 2F 05 00 00	     05 27 04 57 81 33 00 00
    05 67 03 1B 45 AE 00 00	     05 27 04 E8 10 E7 00 00
    05 67 03 29 BC B3 00 00	     05 27 04 85 65 10 00 00
    05 67 03 31 09 97 00 00	     05 27 04 E1 FB A6 00 00
    05 67 03 32 C5 6B 00 00	     05 27 04 D1 EC 83 00 00
    05 67 03 33 1E 9E 00 00	     05 27 04 7A A4 AF 00 00
    05 67 03 3A F6 2F 00 00	     05 27 04 71 26 98 00 00
    05 67 03 3B 75 AB 00 00	     05 27 04 E9 76 0C 00 00
    05 67 03 3E 50 51 00 00	     05 27 04 59 2B 67 00 00
    05 67 03 3F D9 94 00 00	     05 27 04 91 2B D3 00 00
    05 67 03 42 58 A9 00 00	     05 27 04 19 E7 CB 00 00
    05 67 03 43 F9 A0 00 00	     05 27 04 55 81 ED 00 00
    05 67 03 48 82 DD 00 00	     05 27 04 F3 2D 41 00 00
    05 67 03 4E 25 DE 00 00	     05 27 04 DE F2 99 00 00
    05 67 03 51 40 DE 00 00	     05 27 04 DB B6 DF 00 00
    as it can be seen 3 bytes are always changing with each test, both, in the message received from the ECU, and the one sent back to the ECU from the diagnostic scanner. Can anyone help me trying to figure out relationship between these two strings?

    P.S: about 16 million combinations for 3 byte data are possible... there must be some link between them...

  2. #2
    Join Date
    Apr 1999
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    27,426

    Re: Relationship Between two Hex strings... programming challenge

    Quote Originally Posted by akn10 View Post
    However there is one message (that is sent from the user side) that keeps on changing each time
    Possibly a time, time stamp, clock, etc?

    Regards,

    Paul McKenzie

  3. #3
    Join Date
    Aug 2013
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    55

    Re: Relationship Between two Hex strings... programming challenge

    Quote Originally Posted by akn10 View Post
    for my own educational purpose.
    Could you be more specific?

  4. #4
    Join Date
    Aug 2013
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    2

    Re: Relationship Between two Hex strings... programming challenge

    Quote Originally Posted by Paul McKenzie View Post
    Possibly a time, time stamp, clock, etc?

    Regards,

    Paul McKenzie
    maybe.... but can't say anything... logs don't seem that easy...

    Quote Originally Posted by zizz View Post
    Could you be more specific?
    well I think its clear... this app is for my private learning purpose... and not for distribution

  5. #5
    Join Date
    Nov 2013
    Posts
    1

    Re: Relationship Between two Hex strings... programming challenge

    What you are seeing is a security seed and key.

    The full message would be

    Send (7E0) Receive (7E8)
    7E0 02 27 03 00 00 00 00 00
    7E8 05 67 03 0C 17 6A 00 00

    The seed from the module is 0C 17 6A

    This is then put through a public seed/key calculation to give a key (8F 45 37) and this is sent to the module

    7E0 05 27 04 8F 45 37 00 00
    7E0 02 67 04 00 00 00 00 00

    Without knowing the fixed security bytes associated with the control module and the algorythm used you don't have a hope of finding the data yourself.

    Security level 03 is generally used for output control or settings updating.

    PH.

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