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Checksum in a Data

  • 22-02-2011 12:55PM
    #1
    RealistSpy
    Registered Users, Registered Users 2 Posts: 1,686 ✭✭✭


    Hi,

    I hope this is the right place to post this but here is a scenario.

    I have a data but every time I modify the data in anyway the data gets corrupted. The data has a crc32 security.

    Is there anyways I can find out the crc32 offset in the data so I can modify the data and fix back the crc32 security after modification?


Comments

  • #2
    JimmyCrackCorn
    Registered Users, Registered Users 2 Posts: 1,689 ✭✭✭JimmyCrackCorn


    If you have enough samples you can work it out.
    Otherwise you have to calculate it and try and work it out yourself.
    Best to write it up in c and work it out that way using brute force.

    The crc is usually at the end but not always.

    Any help on this is situational dependently.

    Algorithm is here.
    http://en.wikipedia.org/wiki/Cyclic_redundancy_check


  • #3
    DublinDilbert
    Registered Users, Registered Users 2 Posts: 5,403 ✭✭✭DublinDilbert


    Are you sure it's a crc? It could also be a simple checksum or a hash of the data, using a hashing algorithm.

    As the poster above says if you have enough samples of data you might be able to brute force it...


  • #4
    Gavin
    Registered Users, Registered Users 2 Posts: 4,660 ✭✭✭Gavin


    RealistSpy wrote: »
    Hi,

    I hope this is the right place to post this but here is a scenario.

    I have a data but every time I modify the data in anyway the data gets corrupted. The data has a crc32 security.

    Is there anyways I can find out the crc32 offset in the data so I can modify the data and fix back the crc32 security after modification?

    Load the application which uses the data file into a debugger. Find the code which accesses the data and then just step through to see where the CRC is being calculated.


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