  This paper presents a new general framework of information hiding, in which
the hidden information is embedded into a collection of activities conducted by
selected human and computer entities (e.g., a number of online accounts of one
or more online social networks) in a selected digital world. Different from
other traditional schemes, where the hidden information is embedded into one or
more selected or generated cover objects, in the new framework the hidden
information is embedded in the fact that some particular digital activities
with some particular attributes took place in some particular ways in the
receiver-observable digital world.
  In the new framework the concept of "cover" almost disappears, or one can say
that now the whole digital world selected becomes the cover. The new framework
can find applications in both security (e.g., steganography) and non-security
domains (e.g., gaming). For security applications we expect that the new
framework calls for completely new steganalysis techniques, which are likely
more complicated, less effective and less efficient than existing ones due to
the need to monitor and analyze the whole digital world constantly and in real
time. A proof-of-concept system was developed as a mobile app based on Twitter
activities to demonstrate the information hiding framework works. We are
developing a more hybrid system involving several online social networks.
