PE Checksum Computation

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Hi!

For the case that a checksum algorithm with more than 8 bits is used, all 
fields over which the checksum is computed must be padded to the bit size of 
the checksum algorithm, i.e. 32 bits. Otherwise, bytes can be misinterpreted 
as the following example shows:

Example:
- - PE 0x12345678 in pool "Pool1"
- - PE 0x33445566 in pool "PoolX2"
=> Checksum over: Pool1(0x12345678)PoolX20x33445566
1. 32 bits: "Pool"
2. 32 bits: "1"0x123456
3. 32 bits: 0x78"Poo"
4. 32 bits: "lX2"0x33
5. 32 bits: 0x445566(0x00 Padding)

Here, the PH is not padded to 32 bits, so when you compute the checksum of the 
second PE alone:
1. 32 bits: "Pool"
2. 32 bits: "X2"0x3344
3. 32 bits: 0x5566(0x0000 Padding)

That is, the meaning of the first byte has shifted and checksum(Pool1
(0x12345678)PoolX20x33445566) != checksum(Pool1(0x12345678)) + 
checksum(PoolX20x33445566).

To avoid this problem, the draft should explicitly say that each field over 
which the checksum is computed - especially the PH, since it is 
variable-length - MUST be padded to full 32 bits before the checksum can be 
computed.


Best regards
- -- 
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 Dipl.-Inform. Thomas Dreibholz

 University of Essen,                            Room ES210
 Inst. for Experimental Mathematics              Ellernstraße 29
 Computer Networking Technology Group            D-45326 Essen/Germany
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 E-Mail:     [email protected]
 Homepage:   http://www.exp-math.uni-essen.de/~dreibh
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