MFC r201898:   Add comments trying to explain what bad things happen here, i.e.   how hashed MD5/SHA are implemented, abusing Final() for padding and   sw_octx to transport the key from the beginning to the end.
MFC r201898:
  Add comments trying to explain what bad things happen here, i.e.
  how hashed MD5/SHA are implemented, abusing Final() for padding and
  sw_octx to transport the key from the beginning to the end.

  Enlightened about what was going on here by: cperciva
  Reviewed by:  cperciva

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