Hash algorithm for dynamic growing/streaming data?

Question

Are there any algorithms that you can continue hashing from a known hash digest? For example, the client upload a chunk of file to ServerA, I can get a

Answer 1

I was facing this problem too, and found no existing solution, so I wrote a library that uses ctypes to deconstruct the OpenSSL data structure holding the hasher state: https://github.com/kislyuk/rehash. Example:

import pickle, rehash
hasher = rehash.sha256(b"foo")
state = pickle.dumps(hasher)

hasher2 = pickle.loads(state)
hasher2.update(b"bar")

assert hasher2.hexdigest() == rehash.sha256(b"foobar").hexdigest()