Message digest
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Posted By:   Alessandro_Brawerman
Posted On:   Saturday, December 6, 2003 08:40 AM

Hi all, I'd like to know how to recombine previously calculated digest in one final value. I'll explain better: I have a binary file and I want to divide this file into 4 equally parts. Each of these parts go to a different processor (my machine has 4) and the message digest (currently using MD5) is calculated for that part in each processor. After that I need to recombine the digests into a final value, so that when my client calculates the digest of the entire binary file it obtains the same digest I have. How to recombine the 4 values I got from the processors? Is it possible? Also, would it make any difference in the method to recombine if instead of MD5 I use SHA-1 or SHA-256? Thanks a lot for the help. Alessandro.

Re: Message digest

Posted By:   Christopher_Koenigsberg  
Posted On:   Monday, December 8, 2003 07:50 AM

Google for resources ...

My naive guess is that your requirement, for additivity (e.g. you want that the fingerprint of the sum (of parts) is derivable from sum of the fingerprints (of parts)) would conflict fundamentally with the requirements, of uniqueness and non-invertibility, of message digest/fingerprint algorithms, so I would look elsewhere for your kind of function.

Otherwise, I think the objection would go, that one could basically split a file repeatedly into sub-parts, down to the minimum block size, where the number of possible fingerprint values for each block is much smaller and hence the number of possible original different blocks is smaller too, so the original can be guessed (inverted) by brute force or something, and then be able to work back up from there, using your combinatorial/additivity property to reconstruct the fingerprint for larger and larger parts of the file, eventually compromising the secureness in message digest terms (e.g. uniqueness and non-invertibility) of the algorithm.

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