Mimic function
Encyclopedia
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.
More sophisticated mimic functions use grammar-based models of
to structure the output. One example, "SpamMimic", uses a context free grammar to model the structure of a typical spam email message. It will convert the input file A to read like a spam message.
Even more sophisticated models use reversible Turing machine
s.
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 informationInformation hiding
In computer science, information hiding is the principle of segregation of the design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed...
, often called steganography
Steganography
Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity...
.
The simplest mimic functions use simple statistical models to pick the symbols in the output. If the statistical model says that item
occurs with probabilityProbability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
and item occurs with probability , then a random number is used to choose between outputting or with probability or respectively.More sophisticated mimic functions use grammar-based models of
to structure the output. One example, "SpamMimic", uses a context free grammar to model the structure of a typical spam email message. It will convert the input file A to read like a spam message.Even more sophisticated models use reversible Turing machine
Turing machine
A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...
s.