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.