TypeError: can't pickle HASH objects?

[est]

>>> import md5
>>> a=md5.md5()
>>> import pickle
>>> pickle.dumps(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python25\lib\pickle.py", line 1366, in dumps
    Pickler(file, protocol).dump(obj)
  File "C:\Python25\lib\pickle.py", line 224, in dump
    self.save(obj)
  File "C:\Python25\lib\pickle.py", line 306, in save
    rv = reduce(self.proto)
  File "C:\Python25\lib\copy_reg.py", line 69, in _reduce_ex
    raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle HASH objects

Why can't I pickle a md5 object? Is it because md5 algorithm needs to
read 512-bits at a time?

I need to md5() some stream, pause(python.exe quits), and resume
later.  It seems that the md5 and hashlib in  std module could not be
serialized?

Do I have to implement md5 algorithm again for this special occasion?

Or is there anyway to assige a digest when creating md5 objects?