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Combining MD5 hash values

Asked 2010-02-06T18:45:34.410
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When calculating a single MD5 checksum on a large file, what technique is generally used to combine the various MD5 values into a single value? Do you just add them together? I'm not really interested in any particular language, library or API which will do this; rather I'm just interested in the technique behind it. Can someone explain how it is done?

Given the following algorithm in pseudo-code:

MD5Digest X
for each file segment F
   MD5Digest Y = CalculateMD5(F)
   Combine(X,Y)

But what exactly would Combine do? Does it add the two MD5 digests together, or what?

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1 Answer

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Most digest calculation implementations allow you to feed them the data in smaller blocks. You can't combine multiple MD5 digests in a way that the result will be equal to the MD5 of the entire input. MD5 does some padding and uses the number of proccessed bytes in the final stage which makes the original engine state unrecoverable from the final digest value.

answered 2010-02-06T18:55:56.910

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