CODEWITHSUNDEEP
c#.netmd5
Sreekanth Vaddepati
Sreekanth Vaddepati

Reputation: 7

How to combine md5 hashes to of byte array to get equalent md5 hash of combined array

I have two byte arrays lets say A and B. And I computed MD5 Hash AH and BH for them respectively. Now if I Combine these arrays A and B to AB, and compute MD5 hash as ABH what whould be the way to combine AH and BH to equate ABH?

Upvotes: 0

Views: 764

Answers (2)

RenniePet
RenniePet

Reputation: 11658

Based on your comments that you don't need a cryptographic hash, but just some kind of "fingerprint" to give a certain degree of hash = data assurance, then here's one possibility:

The simplest hash of all is to just XOR (exclusive-or) the data against itself. So for an 8-byte hash, you just walk over the data and XOR every 8 bytes together, producing an 8-byte hash. This provides a very high degree of certainty, unless the "bad guys" know how you do the hashing, then it is easy to fake.

If you concatenate two data arrays, A and B, then as long as A is a multiple of 8 in length, or if it's acceptable to place zeros padding between A and B so A + padding is a multiple of 8, then the combined hash will be (hash of A) XOR (hash of B). (At least, that's what I learned 50 years ago, if I remember right.)

Edit:

Just found this: Why is XOR the default way to combine hashes?

Upvotes: 1

Steve Ruble
Steve Ruble

Reputation: 3895

It's possible that cryptography experts may have some ability to use partial MD5 hashes of a value to get some information about the MD5 hash for the complete value, but for practical purposes the only way to get the MD5 hash of a value is to compute the MD5 hash from that value.

Upvotes: 0

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