Questions about hash functions Let $H$ be a secure hash function (e.g. SHA256), is it possibile to find $H(M1)$ given $H(M1||M2)$ if the length of $M1$ and/or $M2$ is exact the block size of the hash (that is, 64 bytes for SHA family hashes)?
I think this is formalized as non mallability of hash functions. However, you do not find it as a requirement e.g. in the SHA-3 competition and I think that practical hash functions are not analyzed in this direction. For non-mallability you might want to take a look at 
http://eprint.iacr.org/2009/065