CSE 100 Lab

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CSE 100 Lab

• Decrypting a simple cipher commands

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What is a Cipher?

• This one is a Simple Cipher
• Weve replaced letters with other letters

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How are we going to crack it?

• Very Simple Statistical Analysis

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Lets do it.
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Login to eniac
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Bash

• make sure we all have the same environment
• you can use the up arrow to see and edit past
  commands

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cat /home/c/cse100/lab/x.cipher

• x is 1,2, or 3. Choose which one you want to
  decrypt
• Read the file

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cat /home/c/cse100/lab/x.cipher more

• see it in a controllable manner
• press q to exit

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cat /home/c/cse100/lab/x.cipher tr 'A-Z'
'a-z' more

• make all lowercase

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cat /home/c/cse100/lab/x.cipher tr 'A-Z'
'a-z' tr -d '\040-\140''\173-\377' more

• get rid of characters other than lower case
  letters

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sed 's/a-z/ /g'

• add a space after each character a-z

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tr ' ' '\012'

• make the spaces in to new lines

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sort

• sort the lines

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uniq c

• count the number of each line (i.e. number of
  each letter)

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sort nr

• sort by most frequent

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compare with the sample from 1,000,000 words,
and replace most common for most common
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cat /home/c/cse100/lab/x.cipher tr 'A-Z'
'a-z' gt/temp

• get lowercase text

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sed s/a/B/g temp

• Replace a with Capital B(no capitals in the text)
• Repeat for each match in the files

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Caveats

• Variance in frequency
• Use two and three letter combinations
• Short messages