CSE 3482Introduction to Computer Security

Instructor: N. Vlajic, Winter 2017

http://www.marw0rm.com/steganography-what-your-eyes-dont-see/

Learning Objectives

Upon completion of this material, you should be able to:

• Identify various motivations and implementation approaches to information hiding.

• Describe the basic concepts and uses of digitalsteganography.

• Explain the key principles pertaining to four common types of digital steganography: plaintext, image,audio and datagram.

• Enlist the common uses of digital watermarking.

• Explain the difference between digital watermarkingand digital fingerprinting.

Introduction

• WHO Protects Information in Digital Age & WHY? companies: to protect trade secret & intellectual property

governments: to protect classified information

individuals: to hide sensitive information from others or from authorities

Introduction (cont.)

• Information Protection in Digital Age – techniques of digital information protection can be grouped in twomajor categories:

Information Encryption the presence of information is ‘obvious’, but the content is

‘scrambled’ using a crypto-key, so it becomes meaningless

no matter how ‘unbreakable’, encrypted message will arosesuspicion

Information Hiding the goal is not just to prevent others from accessing hidden

information, but to make others unaware of the very existenceof the hidden information

Introduction (cont.)

Example: Encryption vs. Information Hiding

Introduction (cont.)

• Information Protection in Digital Age

Information Protection

Steganography Watermarking Fingerprinting

Information Hiding(data not visible)

Encryption(data visible but encrypted)

Introduction (cont.)

• Techniques of Information Hiding Steganography steganography – Greek word for “concealed writing”

art and science of hiding information in some cover mediafor the purpose of protecting information confidentiality

digital steganography – cover media: image, text, audio, video

Watermarking also aims to make information invisible, but for the purpose

of protection of intellectual property

Fingerprinting embedding user-unique marking to different copies of content

for the purpose of tracking of intellectual property

Introduction (cont.)

Example: Watermarking vs. Digital Fingerprinting

http://itslab.inf.kyushu-u.ac.jp/research/fingerprint.html

http://www.cse.unt.edu/~smohanty/Publications_Others/Mohanty_Masters_Thesis_Spring1999.html

Information Hiding:Steganography

Classical Steganography

• History of Steganography – the need to protectinformation from unsolicited access, by making itobscure, precedes our digital age

in ancient Greece, a message would be tattooed on theshaved head of a messenger; the hair would be grown over

in era of printed press, different typefaces were used to‘encode’ a message

in WW2, the French resistance used invisible ink (e.g., wax)to write messages on the back of regular currier

ahzemllyo

Digital Steganography

• Digital Steganography process of hiding information in digital multimedia files

and in network packets

elements of digital steganography system include cover media (C ) that will hold the hidden data

secret message (M) - may be plain text or any other type of data

stego function (Fe) and its inverse (Fe-1)

an optional stego-key (K) or password to hide and unhide the message

stego object (S) = cover media + secret message

receiver

Digital Steganography (cont.)

Example: Steganography of ‘Image inside an Image’

sender

Digital Steganography (cont.)

• What Makes Steganography Work? digital steganography takes advantage of

1) space redundancy in cover media2) data redundancy in cover media in combination with

inherent weaknesses of human perception

e.g., in computer/text file steganography, information canbe hidden in unused areas of the file/text

e.g., in image steganography, information can be embeddedin the Least Significant Bits (LSBs) of an image (introduced change is insignificant for human eye)

e.g., in audio steganography, information can be embeddedin high frequencies of audio spectrum (human ear is insensitiveto slight variations in high audio frequencies)

Digital Steganography (cont.)

Plaintext Still Imagery

Digital Steganography

• Techniques of Digital SteganographyBased on Type of Cover Media

Audio and Video

Datagram

most popular

Plaintext Steganography

1.1) Plaintext Steganography:Use of Selected Characters in Cover Media sender sends a

1) text message = stego object2) a series of integer number = key

secret message is hidden within the respective positions of subsequent words in cover media

The weather is sunny and wonderful. They have gone running at the beach.2 2 1 1 2 2 1 1 4 1 0 0 2He is not here.

Example: Plaintext Steganography with Selected Characters

Plaintext Steganography (cont.)

1.2) Plaintext Steganography:Line Shifting or Word Shifting in Cover Media e.g., lines are shifted down by a small fraction shift present = 1, shift not present = 0

e.g., words are shifted right by a small fraction shift present = 1, shift not present = 0

encoded bits are extracted and compared againsta predefined Codebook

Example: Plaintext Steganography with Line ShiftingIF you can keep your head when all about youAre losing theirs and blaming it on you,

If you can trust yourself when all men doubt you,But make allowance for their doubting too;If you can wait and not be tired by waiting,Or being lied about, don't deal in lies,Or being hated, don't give way to hating,And yet don't look too good, nor talk too wise:

If you can dream - and not make dreams your master;If you can think - and not make thoughts your aim;If you can meet with Triumph and Disaster

And treat those two impostors just the same;If you can bear to hear the truth you've spokenTwisted by knaves to make a trap for fools,Or watch the things you gave your life to, broken,And stoop and build 'em up with worn-out tools:

If you can make one heap of all your winningsAnd risk it on one turn of pitch-and-toss,And lose, and start again at your beginningsAnd never breathe a word about your loss;If you can force your heart and nerve and sinew

To serve your turn long after they are gone,And so hold on when there is nothing in youExcept the Will which says to them: 'Hold on!‘

If you can talk with crowds and keep your virtue,

Or walk with Kings - nor lose the common touch,If neither foes nor loving friends can hurt you,If all men count with you, but none too much;If you can fill the unforgiving minuteWith sixty seconds' worth of distance run,Yours is the Earth and everything that's in it,And - which is more - you'll be a Man, my son!

Plaintext Steganography (cont.)

Codebook:00000 – a00001 – b00010 – c00011 – d00100 – e00101 – f00111 – g01000 – h01001 – i01011 – j01111 – k10000 – l10001 – m

10010 – n10011 – o10100 – p10101 – q10110 – r10111 – s11000 – t11001 – u11010 – v11011 – w11100 – x11101 – y11110 – z

0100000100100001000010011

hello

Image Steganography

Image Steganography

Example: Digitized Image any image can be digitized – i.e., represented by a discrete

set of same-color points / display elements

How can we improve the quality of digital images?

By decreasing the sizeof display elements

(increase resolution).

Image Steganography (cont.)

• Digital Image –a 2D (NxM) array/grid of m-bit pixels

• Pixel – fundamental same-color display elementin a digital image

each pixel is made up of one or more bits monochrome image: pixel = 1 bit =>

(black/white) grayscale image: pixel = 8 bits =>

256 shades of gray RGB image: pixel = 24 bits =>

8 bits for each – red, green, blue => 16777216 different color shades

Image Steganography (cont.)

Example: Pixels in Grayscale Image

Image Steganography (cont.)

Example: RGB Image

http://www.freeonlinephotoeditor.com/

Image Steganography (cont.)

Example: Image Size

What is the size (in kbytes and KBytes) of a grayscale imagecomprising 200x300 pixels?

200 x 300 x 8 = 480000 bits= 60000 bytes= 60 kbytes= 58.59 KBytes

kbyte = 103 bytes = 1000 bytesKByte = 210 bytes = 1024 bytes

http://jan.ucc.nau.edu/lrm22/pixels2bytes/calculator.htm

Image Steganography (cont.)

• Bits in a Pixel – relative importanceof different pixels is different LSB – least significant bit – last bit MSB – most significant bit – 1st bit

10100011

LSB carries the least information – it changesmost rapidly

MSB carries the most information – it changesleast rapidly

Image Steganography (cont.)

Example: LSB(s) and Human Eye

R = 10010 = 01100100 G = 15010 = 10010110B = 20010 = 11001000

R = 10210 = 01100110 G = 15210 = 10011000B = 20210 = 11001010

R = 13010 = 01111000 G = 15010 = 10010110B = 23010 = 11100110

http://jumk.de/color-calculator/

2.1) Image Steganography:Use of Least Significant Bit (LSB) easiest and surprisingly effective way of hiding

information in an image LSB(s) of each pixel is/are used to hide the most

significant bits of another image algorithm:

(1) load up host image and image to hide(2) choose the number of bits you whish to hide

the secret image in=> better quality of hidden image => more distortion in cover image

(3) to get original image back, pick out the LSBsaccording to the number used in (2)

Image Steganography (cont.)

more bits used

http://cgi.csc.liv.ac.uk/~leszek/COMP526/week4/comp526-3.pdf

Image Steganography (cont.)

Example: Embedding Process using only one LSB

Image Steganography (cont.)

Example: ’Image in Image’ with LSBNumber of LSB used = 4

Encoding:

Decoding:

Cover Secret Stego Image Recovered Image

Image Steganography (cont.)

Number of LSB used = 1

Cover Secret Stego Image Recovered Image

fewer LSB bits used => ‘hiding’ capacity low –better stego-image worse recovered image

Example: ’Image in Image’ with LSB

Image Steganography (cont.)

Number of LSB used = 7

Cover Secret Stego Image Recovered Image

more LSB bits used => ‘hiding’ capacity high –worse stego-image better recovered-image

Example: ’Image in Image’ with LSB

Image Steganography (cont.)

• Pattern of LSB Embedding – secret bits can beembedded in LSBs of cover image in two ways: sequentially simple embedding & extraction of secret bits statistics of cover image abruptly changed - easy to detect

randomly the key to generate pseudorandom numbers must be sent secret bits scattered throughout cover image - hard to detect

Image Steganography (cont.)

What is a better place to hide secret bits:- same-color background- part of image with lots of detail ???