F5 A Steganographic Algorithm

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# F5 A Steganographic Algorithm - PowerPoint PPT Presentation

F5 A Steganographic Algorithm. Davang Patel Thomas Schulze. Agenda. Introduction Capacity vs. Security JPEG JSTEG F3 F4 F5. Message. Message. steganographical decryption algorithm. steganographical encryption algorithm. Steganogram. Message should be invisible to an aggressor

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### F5A Steganographic Algorithm

Davang Patel

Thomas Schulze

Agenda

Introduction

Capacity vs. Security

JPEG

JSTEG

F3

F4

F5

Message

Message

steganographicaldecryption

algorithm

steganographical

encryption

algorithm

Steganogram

• Message should be invisible to an aggressor
• Message should be not readable to an aggressor

Attack

Carrier

Carrier

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.

http://blogs.mathworks.com/steve/2006

### Capacity vs. Security

• 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. Jsteg
• Embedding capacity 12 %
• Secure against visual attacks
• Weak against statistical attacks
• 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

### 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.

### 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

### F3 Algorithm

• 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

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

### 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.

F4 embedding example
• Embed the code “01110”
• If LSB and message does not match,
• Increment negative Coefficients
• Decrement positive coefficients
F5 Algorithm
• Overall algorithm the same as F4.
• Extends F4 by adding two distinct features:
• Matrix encoding
• 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.

### Matrix Encoding

459 changes

1157 changes

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.

F5 encoding process
• Permutation is generated using user-defined password.

Random Number Generator

Inverse

Permutation

Discrete CosineTransform

Quantization

Permutation

F5

Embedding

function

Quality

Huffman

Encoder

Bitmap Carrier

Message

Steganogram

(JPEG)

F5 example

Carrier Image

F5 Encrypt/Decrypt

Program

Resultant Steganogram

Secret Message

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).