[ericgorr]

I'm sure this is not possible, but I thought I would ask in case I am wrong.

Let's say I have two pieces of data - A & B. I then get the MD5 hash over both pieces of data and end up with two hashes: Ah & Bh.

Now, let's say that I simply append B to A so I have single piece of data C and get the MD5 hash of C: Ch.

Is there a way to combine Ah & Bh to get Ch.

The issue I have is that to obtain the very large pieces of data is difficult and might even be separated in time, but I would like to have a hash value computed over the entire set of data without keeping the data collected in the past around.

I would also be curious about whether this would work for the SHA-1 hash.

As I said, I am pretty sure this is not possible, but I always like to leave open the possibility that I am wrong.

Thank you.

[E-Oreo]

It is not possible with either MD5 or SHA. There might be a checksum algorithm that has this property, but I don't know of any off the top of my head.