Top Banner
F5 A Steganographic Algorithm Davang Patel Thomas Schulze
18

F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Dec 14, 2015

Download

Documents

Jordyn Kier
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F5A Steganographic Algorithm

Davang Patel Thomas Schulze

Page 2: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Agenda

Introduction

Capacity vs. Security

JPEG

JSTEG

F3

F4

F5

Page 3: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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

steganographicalencryptionalgorithm

steganographicaldecryptionalgorithm

Message

CarrierAttack Message should be

invisible to an aggressor

Message should be not readable to an aggressor

Steganogram

Page 4: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Steganogram ExampleThe Story Behind the MATLAB Default Image

More Info:http://

blogs.mathworks.com/steve/2006

Page 5: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Capacity vs. Security

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

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)

F5 algorithm combines BOTH advantages!!

Great capacity for steganographic messages Strong against statistical and visual attacks 13.4% embedding capacity

Page 6: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

JPEG Mechanism

Histogram for JPEG coefficient after quantization

1. The coefficient’s 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.

Page 7: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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

Page 8: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F3 Algorithm

1. Does not overwrite bits -- Decrement the non-zero coefficient’s 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

JSTEG

F3 Algorithm

Page 9: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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

Page 10: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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.

Page 11: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F4 embedding example

Embed the code “01110” If LSB and message does not match,

Increment negative Coefficients Decrement positive coefficients

Original Coefficients 5 0 0 2 3 -1 0 -3 0 1 -3Binary of Coefficients 0101 0000 0000 0010 0011 1111 0000 1101 0000 0001 1101

Inverted LSB Binary (Negative Coefficients) 1110 1100 1100

Message 0 skip skip 1 1 1 skip 1 skip 0 0

Operation 5-1 nothing nothing 2-1 same -1+1 nothing -3+1 nothing 1-1 sameFinal Result 4 0 0 1 3 0 0 -2 0 0 -3

shrinkage shrinkage

Page 12: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F5 Algorithm

Overall algorithm the same as F4. Extends F4 by adding two distinct

features: Permutative straddling Matrix encoding

Page 13: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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.

Permutative Straddling

F4

carrier carrier+ message

carrier permutation

permutation +message

InversepermutationNote: treat each pixel as if it

was a JPEG coefficient.

F5

Page 14: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Matrix Encoding

Improves the embedding efficiency from 1.5 bit to 3.8 bit per change.

Embed 1736 bit message

F4 F5459 changes

How does it work?

1157 changes

Consider we want to embed x1 and x2 in LSB locations a1, a2, and a3.

Page 15: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F5 encoding process

Permutation is generated using user-defined password.

Discrete CosineTransform

Quantization PermutationF5

Embeddingfunction

InversePermutation

HuffmanEncoder

Steganogram(JPEG)

Bitmap Carrier Message

Random Number GeneratorPassword

Quality

Page 16: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

F5 example

Carrier Image

Secret Message

F5 Encrypt/DecryptProgram Resultant

Steganogram

Page 17: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

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).

Page 18: F5 A Steganographic Algorithm Davang Patel Thomas Schulze.

Any Questions?