lawrence snyder university of washington, seattle © lawrence snyder 2004 computing is pretty...
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Lawrence SnyderUniversity of Washington, Seattle
© Lawrence Snyder 2004
Computing Is Pretty Strange
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Encryption is the process of “scrambling” data so it is difficult (impossible?) to understand it
We encrypt data to keep it private Every site that you use as https:// is encrypted Familiar example: Caesar cipher:
C: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
E: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
What would Julius be encrypted to?
04/21/23 © 2011 Larry Snyder, CSE 2
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Encryption is the process of “scrambling” data so it is difficult (impossible?) to understand it
We encrypt data to keep it private Every site that you use as https:// is encrypted Familiar example: Caesar cipher:
C: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
E: D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
What would Julius be encrypted to? Mxolxv
04/21/23 © 2011 Larry Snyder, CSE 3
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The fixed shift of an alphabet is easy to break
Alternate: Sender uses a key, k, to multiply
clear byte sequences (recall they’re numbers) by k
-- Send encrypted result – looks like gibberish --
Receiver divides by k to decrypt getting clear
04/21/23 © 2011 Larry Snyder, CSE 4
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04/21/23 © 2011 Larry Snyder, CSE 5
776913=100997698413=090792326413=042432325713=042341
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The problem with “private key” encryption: the two sides have to meet to agree on key
Public Key fixes this: The receiver publishes (on Web site, say) a (very very special) key, K
More importantly, the theory it uses means that NO practical amount of computing can break the code
Here’s what you do …
04/21/23 © 2011 Larry Snyder, CSE 6
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Sender breaks up the message into blocks as before
Sender cubes each block – yup, raises to the 3rd power – and mods it by K, i.e. (<text>3)%K
Transmit results Receiver raises each remainder to a high power
determined by prime numbers & known only to him
Receiver mods by K, too, which are – surprisingly – the original blocks! The receiver assembles the message Thanks to Euler and Diffie & Hellman
04/21/23 © 2011 Larry Snyder, CSE 7
This Is Amazing!!!This Is Amazing!!!
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The process of hiding informationTwo Greek roots meaning:
“stego” == “roof” “stega” == “cover”
04/21/23 © 2011 Larry Snyder, CSE 8
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Most common reason to hide information is to avoid being caught with it Military and spy documents Repressive governments restricting
news/info Avoid others “snooping” – privacy
Hiding is different than encryption … uses the fact that the searcher doesn’t know it’s there
04/21/23 © 2011 Larry Snyder, CSE 9
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The Plan … hide “subversive” protest picture in
“calendar art”
04/21/23 © 2011 Larry Snyder, CSE 10
Guest Image
Host Image
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We don’t need all of the bits in RGB to get a decent picture
04/21/23 © 2011 Larry Snyder, CSE 11
1011 0100 1101 0011 0001 1100 1011 0100 1101 0011 0001 1100
All bits Left 2 bits of each color
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Put guest bits into right 2 bits of host
04/21/23 © 2011 Larry Snyder, CSE 12
1111 0100 1101 0011 1011 1101 1011 0100 1101 0011 0001 1100
1111 0110 1101 0011 1011 1100
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fog.jpg
stegFog.png
Really?Just Do It!
Really?Just Do It!
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… and then we’ll see the details
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Encoding Code
Code To Save Result on Click
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After the pictures are loaded
Clear right 2 bits of hostExtract left 2
bits of guestNew combined color
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04/21/23 © 2011 Larry Snyder, CSE 17
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Read in the file, and then on mouse click, pull out the bits and make a picture
Remove right 2 bits
Make them left 2 bits for each color
New color
04/21/23 © 2011 Larry Snyder, CSE 18
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Run A Test … www.tineye.com
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The OriginalThe Original
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Steganized Steganized