Chance of equal checksum and changing blocks

[David de Lama]

Hi @all!

I have two questions:
- First, am I right that the chance of getting the same 32-bit rolling checksum is 1/2^16 and to get the same 128-bit MD5 Hash is 1/2^127?
- Finally I want two know if it is possible to change an amount of blocks manually? 
  e.g. I made a 100 MB file with "dd if=/dev/zero of=/home/test.xyz bs=1M count=100" and know I want to change, lets say, 10 blocks of this file. Is it possible?
  The blocksize above (bs=1M) has nothing to do with the blocksize rsync uses, right?!

Thanks a lot.

Greetings
David


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