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Welcome to the Computer Science Department Dr. Ben Schafer

Welcome to the Computer Science Department Dr. Ben Schafer. So what does this have to do with computer science?. The news story talks about terrorists using a technique called “stegonography” which involves hiding text messages inside of other things.

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Welcome to the Computer Science Department Dr. Ben Schafer

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  1. Welcome to the Computer Science DepartmentDr. Ben Schafer

  2. So what does this have to do with computer science? • The news story talks about terrorists using a technique called “stegonography” which involves hiding text messages inside of other things. • A process easily easily done with a computer

  3. But let’s start with a “magic” trick • Pick a number from 1-31 • Tell me which of the following cards this number appears on

  4. Explaining the “magic” trick • Suppose you picked 25. • You would tell me that your number was on cards 1, 2, and 5 • I just add up the numbers from the upper left hand corners of those cards 16 + 8 + 1 = 25

  5. The concept of Binary • In the decimal system (base ten) each position in a number can hold one of ten digits • 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 • In the binary system (base two) each position in a number can hold one of two digits • 0, 1

  6. The concept of Binary • In the decimal system (base ten) we treat positions as powers of ten • 1’s place, 10’s place, 100’s place, etc. • which is 100 place, 101 place, 102 place, etc. • In the binary system (base two) we treat positions as powers of two • 1’s place, 2’s place, 4’s place, 8’s place… • which is 20 place, 21 place, 22 place, 23 place, etc.

  7. The concept of Binary • In decimal, when we get to 9, we’ve run out of digits • next number is 10 • after 9999 is 10000 • In binary, when we get to 1, we’ve run out of digits • next number is 10 • after 1111 is 10000

  8. The concept of Binary 0  00000000 (8-bit) 1  00000001 (8-bit) 2  00000010 (8-bit) 3  00000011 (8-bit) (1 + 2) 4  00000100 (8-bit) 127  01111111 (8-bit) (1 + 2 + 4 + 8 + 16 + 32 + 64)

  9. Your turn • What decimal numbers are represented by the following binary numbers? Example: 0101 = 5 0100 1101 110011 • Answers [4, 13, 51]

  10. How big is a byte? • The standard unit of a computer is a Byte which is 8 bits • How big is that? • How many different numbers are stored in a byte? 2x2x2x2x2x2x2x2 = 28 = 256 possible combinations

  11. So then how do we store letters? ASCII code - A simple conversion of the common American characters and computer commands to numbers. ‘A’ = 65 ‘a’ = 97 ‘1’ = 49 See www.asciitable.com for more information

  12. How Does Color Vision Work? • Our eyes and brain work together to make sense of what we see • The cones in our eyes are what allow us to see in color • The rods allow us to see black, white, and shades of gray • Our cones are sensitive to red, green, and blue light • All other colors are combinations of these

  13. Red, Green and Blue Light • Black is the absence of all light • No red, green or blue light • White light is a combination of red, green, and blue • Full intensity red, green, and blue combined • All other colors are combinations • Of red, green, and blue • Of different intensities

  14. Color Exercise • Type pickAColor() • Select the RGB tab • Use the Red, Green and Blue slider/boxes to create some colors such as white, black, red, blue, green, yellow, violet, and orange

  15. Pictures are made up of Pixels • Digital cameras record light at pixels • Monitors display pictures using pixels • Our limited vision acuity helps us to see the discrete pixels as a smooth picture • If we blow up the picture we can see the pixels

  16. Digital Pictures • Capture the intensity of the red, green, and blue colors at each pixel • Stored as a bunch of numbers • 8 bits for red, 8 bits for green, 8 bits for blue • Need nearly 1 million bytes to store a 640 x 480 picture • Need 3 million bytes to store an image from a 1 megapixel (million pixel) camera • Displayed as red, green, and blue colors on the computer display • Lots of them close together • Our brain sees a smooth color image

  17. So how would we store a message inside of a picture? • The “easy” way • To store a message n characters long, change the red bit of each of the first n pixels to store the ASCII value of one of the characters. • [Demo simpleEncode() for both message.txt and hamlet.txt] • Unfortunately, this looks pretty obvious if the message is long

  18. So how would we store a message inside of a picture? • Pixel[0,0] = (255,30,30) • Pixel[1,0] = (30,30,255) • Pixel[2,0] = (30,255,30) • Pixel[3,0] = (0,0,0) • Pixel[4,0] = (255,128,128) • …

  19. So how would we store a message inside of a picture? • Char#1 = H = 072 • Char#2 = e = 101 • Char#3 = l = 108 • Char#4 = l = 108 • Char#5 = o = 111 • Pixel[0,0] = (255,30,30) • Pixel[1,0] = (30,30,255) • Pixel[2,0] = (30,255,30) • Pixel[3,0] = (0,0,0) • Pixel[4,0] = (255,128,128) • …

  20. So how would we store a message inside of a picture? • Char#1 = H = 072 • Char#2 = e = 101 • Char#3 = l = 108 • Char#4 = l = 108 • Char#5 = o = 111 • Pixel[0,0] = (072,30,30) • Pixel[1,0] = (101,30,255) • Pixel[2,0] = (108,255,30) • Pixel[3,0] = (108,0,0) • Pixel[4,0] = (111,128,128) • …

  21. So how would we store a message inside of a picture? • The “easy” way • To store a message n characters long, change the red bit of each of the first n pixels to store the ASCII value of one of the characters. • [Demo simpleEncode() for both message.txt and hamlet.txt] • Unfortunately, this looks pretty obvious if the message is long

  22. So how would we store a message inside of a picture? • The “harder” way • To store a message n characters long, split the ASCII value of each character into three pieces and change the “ones” part of the red, green, and blue bits to store one of these pieces. • fancyEncode()

  23. So how would we store a message inside of a picture? • Char#1 = H = 072 • Char#2 = e = 101 • Char#3 = l = 108 • Char#4 = l = 108 • Char#5 = o = 111 • Pixel[0,0] = (255,30,30) • Pixel[1,0] = (30,30,255) • Pixel[2,0] = (30,255,30) • Pixel[3,0] = (0,0,0) • Pixel[4,0] = (255,128,128) • …

  24. So how would we store a message inside of a picture? • Char#1 = H = 072 • Char#2 = e = 101 • Char#3 = l = 108 • Char#4 = l = 108 • Char#5 = o = 111 • Pixel[0,0] = (255,30,30) • Pixel[1,0] = (30,30,255) • Pixel[2,0] = (30,255,30) • Pixel[3,0] = (0,0,0) • Pixel[4,0] = (255,128,128) • …

  25. So how would we store a message inside of a picture? • Char#1 = H = 072 • Char#2 = e = 101 • Char#3 = l = 108 • Char#4 = l = 108 • Char#5 = o = 111 • Pixel[0,0] = (250,37,32) • Pixel[1,0] = (31,30,251) • Pixel[2,0] = (31,250,38) • Pixel[3,0] = (1, 0, 8) • Pixel[4,0] = (251,120,121) • …

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