1. F5 A Steganographic Algorithm
Davang Patel
Thomas Schulze

2. Agenda
Introduction
Capacity vs. Security
JPEG
JSTEG F3 F4 F5

3. steganographical decryption
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.
Message
Carrier
steganographical
encryption
algorithm
steganographical decryption
Attack
Message should be invisible to an aggressor
Message should be not readable to an aggressor
Steganogram

4. Steganogram Example The Story Behind the MATLAB Default Image
More Info:

5. Capacity vs. Security F5 algorithm combines BOTH advantages!!
e.g. Jsteg
Embedding capacity 12 %
Secure against visual attacks
Weak against statistical attacks
First algorithm
great Capacity for steganographic messages
Weak against attacks (visual or statistic)‏
Second algorithm
small Capacity for steganographic messages
Strong against attacks (visual or statistic)‏
e.g. MP3Stego
Embedding capacity less then 1 %
Secure against visual attacks
Secure against statistical attacks
F5 algorithm combines BOTH advantages!!
Great capacity for steganographic messages
Strong against statistical and visual attacks
13.4% embedding capacity

6. Histogram for JPEG coefficient after quantization
JPEG Mechanism
Histogram for JPEG coefficient after quantization
The coefficient’s frequency of occurrence decreases with increasing absolute value.
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 Original F3 Algorithm JSTEG
Does not overwrite bits -- Decrement the non-zero coefficient’s absolute value only if the LSB does not match. Zero coefficients are skipped.
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.

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
permutation +
message
Inverse
permutation
Note: treat each pixel as if it was a JPEG coefficient.

14. Improves the embedding efficiency from 1.5 bit to 3.8 bit per change.
Matrix Encoding
Improves the embedding efficiency from 1.5 bit to 3.8 bit per change.
Embed 1736 bit message
459 changes
1157 changes 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.
Discrete Cosine Transform
Quantization
Permutation F5
Embedding
function
Inverse
Huffman
Encoder
Steganogram
(JPEG)
Bitmap Carrier
Message
Random Number Generator
Password
Quality

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

17. Conclusion
F5 has high embedding capacity (>13%) but can be pushed even further.
Resistance to both visual and statistical attacks.
Uses a common image format (JPEG).

18. Any Questions?