Mimic function
A mimic function changes a file  so it assumes the statistical properties of another file . That is, if  is the probability of some substring  occurring in , then a mimic function , recodes  so that  approximates  for all strings  of length less than some . It is commonly considered to be one of the basic techniques for hiding information, often called steganography. 

The simplest mimic functions use simple statistical models to pick the symbols in the output. If the statistical model says that item  occurs with probability  and item  occurs with probability , then a random number is used to choose between outputting  or  with probability  or  respectively.

Even more sophisticated models use reversible Turing machines.

References
 
 
 

Steganography