Representation Supplemental

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Representation - Supplemental
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Changing Representations

• The ROT13 transformation
• each letter goes 13 later
• maintain capitalization
• Rapelcgvba
• What is ROT13 good for?

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Changing Representations

• The ROT13 transformation
• each letter goes 13 later
• maintain capitalization
• Rapelcgvba --gt Encryption
• (But its not good for encryption)

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(No Transcript)
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Guessing The Representation Scheme

• Can you interpret data if you dont know the
  representation scheme?
• No, but you might be able to guess what the
  representation scheme is

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Just Data
01010000100000100101001100000000100101001100000000
0110010101001110

• Suppose someone gave you this bit-string and told
  you it was important. Could you interpret it?
• What if the person told you the original message
  was a meaningful phrase in English?

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A Few Guesses
01010000100000100101001100000000100101001100000000
0110010101001110

• Evaluate each of these possible representation
  schemes.
• Can you produce a text string using the scheme?
• Is the scheme likely to be correct?

2-bit codes 00 a 01 b 10 c 11 space
6-bit codes 000000 space 000001 a 000010
b 000011 c 011000 x 011001 y 011010 z
6-bit codes 000000 a 000001 b 000010
c 010111 x 011000 y 011001 z 011010
space
Note It may help to write out all of the 6-bit
tables.
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A Few Guesses
01010000100000100101001100000000100101001100000000
0110010101001110

• Can you produce a text string using the scheme?
• Sure, it starts bbaacaac
• Is the scheme likely to be correct?
• If I didnt know anything about the structure of
  the original, then I couldnt say
• If the original was English, then this is
  probably wrong.
• I dont even have to decode it to say this. Why
  not?

2-bit codes 00 a 01 b 10 c 11 space
Not many meaningful English phrases can be made
with a, b, c, and space.
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A Few Guesses
01010000100000100101001100000000100101001100000000
0110010101001110

• Can you produce a text string using the scheme?
• Sure, it starts uijtagvo
• Is the scheme likely to be correct?
• If I didnt know anything about the structure of
  the original, then I couldnt say
• If the original was English, then this is
  probably wrong.

6-bit codes 000000 a 000001 b 000010
c 010111 x 011000 y 011001 z 011010
space
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A Few Guesses
01010000100000100101001100000000100101001100000000
0110010101001110

• Is the scheme likely to be correct?

• Did anyone finish decoding this? What is it?

6-bit codes 000000 space 000001 a 000010
b 000011 c 011000 x 011001 y 011010 z