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FCS in Authentication

Posted on 2011-05-04
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Hello Experts,

Can FCS in the Ethernet frames be utilized in developing an authentication check for data sent across network ? If possible, then how can this be done ? If not, then why ?

Many thanks
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Question by:mshankarak
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by:giltjr
ID: 35697200
This question along with your question about LLC and MAC sounds like you may be in a class.

So instead of just out right answering your question I will ask you questions to see if you can figure out the answer on your own.  Even if this is not a class related question, this will help you to get thinking about what you are asking.

Do you really understand what FCS is?  If so, explain so we know that you do.
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by:mshankarak
ID: 35703483
I appreciate your approach.

FCS is frame check sequence which is usually a CRC-32 code used for error detection. In the Ethernet frame format it is the final 4 bytes (32 bits). The error detection is carried out following the principles of CRC.
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by:giltjr
giltjr earned 2000 total points
ID: 35703686
O.K., so:

How is the value derive?

Would all frames from the same client always have the same value?

Could two frames from two different computers have the same value?



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Author Comment

by:mshankarak
ID: 35704052
As CRC functions, the message is divided by a polynomial function (n+1) bits to produce a CRC/FCS of n bits which is appended to the message.

I don't think all the frames from the same client will have the same value as the message being sent across would be different every time and the message is crucial in deriving the CRC.

2 frames from two different computers MAY have the same FCS value.
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giltjr earned 2000 total points
ID: 35705364
O.K., so you have established:

1) The FCS value is based on the data content of a specific frame.
2) The FCS value has nothing to do with the user that is logged on to the computer or the user id that the process that is sending the data may be running under.
3) The FCS value has nothing to do with the computer the data was sent from.
4) The FCS value in a packet coming from the same user/computer will most likely be different for each packet.
5) The FCS value in a packet coming from different users on the same computer could be the same.
6) The FCS value in a packet coming from different users on different computers could be the same.


Now, think about.  So in short the FCS value is a predictable value.  For a given data stream it will always be the same no matter what.  It has nothing to do with what device or user sent the data, it is 100% based on the data.

So how could you use it to identify a specific user or device if it has nothing to do with a specific user or device?
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Author Comment

by:mshankarak
ID: 35708280
Thanks. Correct me if I am wrong, as FCS doesn't have any user or device info it cannot authenticate a message being sent over the network.
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Author Comment

by:mshankarak
ID: 35708298
Hey, I have just added a new comment on the LLC question. Would you mind checking that out for me ?
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Assisted Solution

by:giltjr
giltjr earned 2000 total points
ID: 35708591
You are correct, FCS has nothing related to the user or device, so it can't be used for anything other than its designed purpose.  Which is to try and prevent data corruption along the network path.
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