F5 A Steganographic Algorithm

1
F5A Steganographic Algorithm

• Davang Patel
• Thomas Schulze

2
Agenda
Introduction Capacity vs. Security JPEG JSTEG F
3 F4 F5
3
What is Steganography?

• Steganography is the art and science of embedding
  hidden messages in a carrier medium such that no
  one apart from the sender and intended recipient
  even realizes there is a hidden message.
• Mediums can include images, video, and audio.

4
Steganogram ExampleThe Story Behind the MATLAB
Default Image
More Info http//blogs.mathworks.com/steve/2006
5
Capacity vs. Security
6
JPEG Mechanism
Histogram for JPEG coefficient after quantization

1. The coefficients frequency of occurrence
   decreases with increasing absolute value.
2. The difference between two bars in the middle of
   the histogram is greater than the difference
   between two bars near the ends.

7
JSTEG Algorithm

• After quantization, Jsteg replaces (over writes)
  the least significant bits (LSB) of the frequency
  coefficient by the secret message. The embedding
  mechanism skips all coefficient with the value 0
  and 1.
• Resistant against the visual attacks and good
  capacity with 12.8 of the steganogram's size,
  but the secret message is easily detected by
  statistical attacks. (chi-square test)
• Jsteg influences pairs of the coefficients
  frequency of occurrence !!!

JSTEG
8
F3 Algorithm

1. Does not overwrite bits -- Decrement the non-zero
   coefficients absolute value only if the LSB does
   not match. Zero coefficients are skipped.
2. The LSB of a non-zero coefficient will match the
   secret message after embedding.

Advantage statistical attack (chi-square test)
will not be successful Disadvantage Less
capacity and surplus of even coefficients caused
by shrinking. The surplus
of even coefficients can be detected by
statistical means.
Original
F3 Algorithm
JSTEG
9
F3 Shrinkage

• Shrinkage of coefficients causes a decrease in
  embedding capacity.
• Since the receiver cannot tell between a skipped
  zero and a zero that was generated due to
  shrinkage, repetitive embedding is necessary.

-4 -3 -2 -1 0 1 2 3
4
10
F4 Algorithm

• Mapping negative coefficients to the inverted
  steganographic value.
• Even negative coefficients and odd positive
  coefficients represent a steganographic one.
• Even positive coefficients and odd negative
  coefficients represent a steganographic zero.

11
F4 embedding example

• Embed the code 01110
• If LSB and message does not match,
• Increment negative Coefficients
• Decrement positive coefficients

12
F5 Algorithm

• Overall algorithm the same as F4.
• Extends F4 by adding two distinct features
• Permutative straddling
• Matrix encoding

13
Permutative Straddling

• F4 embeds the data into the next available
  non-zero coefficient.
• F5 will scatter the entire message throughout the
  carrier.
• Uses permutation to equalize the spread of
  embedded data.

F5
F4
carrier
permutation
carrier
carrier message
Inverse permutation
permutation message
Note treat each pixel as if it was a JPEG
coefficient.
14
Matrix Encoding
Improves the embedding efficiency from 1.5 bit to
3.8 bit per change.
Embed 1736 bit message
F4
F5
How does it work?
Consider we want to embed x1 and x2 in LSB
locations a1, a2, and a3.
15
F5 encoding process

• Permutation is generated using user-defined
  password.

16
F5 example
Carrier Image
F5 Encrypt/Decrypt Program
Resultant Steganogram
Secret Message
17
Conclusion

• F5 has high embedding capacity (gt13) but can be
  pushed even further.
• Resistance to both visual and statistical
  attacks.
• Uses a common image format (JPEG).

18
Any Questions?