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Steganography

By :Uday Deep Singh (IT-2 / 7th Sem)

“The Art Of Hiding Content In Images”

Steganography

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Discussion

• What steganography essentially is ?• How it works ?• Feasibility and constraints.• Present day implementations.

Steganography

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What is it ?

• Practice of hiding private or sensitive information within something that appears ordinary.

• Name derived from greek words• “steganos” meaning “cover”• “graphia” meaning “writing”

• Lesser known elder brother of cryptography

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Basic terms

• Carrier/ Cover File – File which has hidden information inside of it.

• Steganalysis – Process of detecting hidden information inside of a file.

• Redundant Bits – Information that can be expended

• Payload – Information which is to be concealed.

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Steganography Carrier files

• bmp• jpeg• gif• wav• mp3• mpeg• Amongst others…

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Basic Model of Steganography

Encoder

Decoder

Cover Image

Secret Data

Key

Stego Object

Original Cover

SecretData Communications

Channel

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Basic Types of steganography

• Substitutional Steganography • Elements of the redundant covering medium are

substituted by the elements of the stego medium.

• Selecting Steganography• Elements of the covering medium are selected to carry

the hidden information.• The relevant information is hidden in a narrow-band

region of a wide band noise.

• Constructional Steganography• The Stego message is made similar to the structure of the

covering medium.

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Techniques of steganography

• Altering LSB (most commonly used)• Masking• Filtering• Transformations

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Altering LSB

• Uses LSB of each pixel in the image• Neccessary to use a lossless

compression format• A 24 bit color image, 3 bits (one each

from RGB) available for each pixel• So, a 800 × 600 pixel image supports

180.000 bytes of secret data

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Altering LSB

For example, (00100111 11101001 11001000)(00100111 11001000 11101001)(11001000 00100111 11101001)

When the character A (10000001), is inserted, the followinggrid results:

(00100111 11101000 11001000)(00100110 11001000 11101000)(11001000 00100111 11101001)

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Masking and Filtering• Use luminence part of image• Suitable for compression• Robust and efficient

Transformations• Direct Cosine Transform is applied to

8x8 blocks• Complex to create and decode

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Feasibility and Constraints

• Visual percievable differences• Amount of data that can be hidden• Vulnerabilty

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Visual percievable differences

• Type of cover image e.g. grids• Size of hidden data• Type of hidden data and the bits it is

dedicated

Cover Image Hidden Image ( Originals )Cover Image Hidden Image

(7 bits) (1 bit)Cover Image Hidden Image

(4 bits) (4 bits)Cover Image Hidden Image

(1 bit) (7 bits)

5 % text 20 % text

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Amount of Hidden Data

• Depends on number of bits being distorted in the spatial domain

• Tradeoff between space available and visual difference• Proportional to the enclosed spaces availaible

Default1 Bit2 Bits3 Bits4 Bits5 Bits6 Bits7 Bits8 Bits

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LSB hiding in 24 bit bmp picture

• Sufficient space available for most purposes

After stegno ( contains 4862 hidden letters)Original

Original Containing hidden information

Bitmap 308 614 bytes 308 614 bytes

JPG 21 795 bytes 21 952 bytes

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Applications

• Security implementations• Digital Watermarking• Storing content metadata

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Security implementations

• Better than cryptography as it doesn’t attract any suspicion

• Posting secret communications on the Web to avoid transmission

• Hiding data on the network in case of a breach

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Digital Watermarking

• Copyright protection• Link to owner’s webite to buy product• Broadcast monitoring• Source tracking• Covert communication

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Storing content metadata

• Data can be stored as XML, Exif, XMP, PLUS etc

• Used to store date photo taken, location, camera info, exposure

• Keywords help in searching for images• Geotagging allows better sorting

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Threatening Implementations• Covert and subversive operations• Terrorism• Weapons and narcotics traffickers• Money laundering• Insider stock trading• People who want to circumvent

encryption export rules.

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Tools

• Steganos• S-Tools (GIF, JPEG)• StegHide (WAV, BMP)• Invisible Secrets (JPEG)• JPHide• Camouflage• Hiderman

Many others…

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References

• http://www.computerworld.com/s/article/71726/Steganography_Hidden_Data

• Paper on Information hiding by Soldatov Nikolay• Paper on Steganography, Steganalysis, &

Cryptanalysis by Michael T. Raggo, CISSP, Principal Security Consultant, VeriSign

• Case Study: An Implementation of a Secure Steganographic System by Xuan-Hien Dang and Krishna C. S. Kota

• http://www.sans.org/reading_room/whitepapers/covert/detailed-steganographic-techniques-open-systems-environment_677