Cryptography Lecture 1: Chantilly Academy

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Classical Ciphers

• Cryptography Lecture 1 Chantilly Academy
• Poorvi Vora
• Department of Computer Science
• George Washington University

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Cryptography

• Why might one need cryptography?

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What is a cryptosystem?

• A cryptosystem consists of
• All messages called plaintext
• All encrypted messages called ciphertext
• A set of all keys
• A procedure for encryption
• A procedure for decryption

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(No Transcript)
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Caesar Cipher
A B C D E F G H I J K
d e f g h i j k l m n
AN EYE FOR AN EYE dq hbh iru dq hbh Trivial to
break
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Shift Cipher
Key A B C D E F
0 a b c d e f
1 b c d e f g
2 c d e f g h
3 d e f g h i
4 e f g h i j




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Encrypt your own messages

• Choose a key
• Choose a short message for the person to your
  right.
• Use the scratch paper to encrypt the message
  using the table and the key
• Write the encrypted message on one side of an
  index card. Write the key you used on the other.
• Give it to the person.

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Decrypt your messages

• If you got a message, decrypt it, using the key
  and the table
• If not, decrypt
• rfym nx httq
• using key 5

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Vigenère Cipher

• The key used for each symbol is different.
• For example
• AN EYE FOR AN EYE
• BA BA BLACK SHEEP
• (Here A0, B1, C2, etc.)
• bn fyf qot k.

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Vigenère Cipher on bits

• 010111011101 message

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Encrypt with Vigenère Cipher

• Use a coin to generate the bits of the key.
• Encrypt the following string
• 01001011
• Encrypted message on index card, key on back,
  pass it to person next to you

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Decrypt

• Decrypt the message given to you using the key on
  the back of the card

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HW Assignment

• Decrypt the following texts
• etarq tqemu encrypted using the shift cipher
• hjko me rtds encrypted using the Vigenère Cipher
• One is impossible to break without knowing the
  key. Which one? Why?

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Permutation Cipher
x 1 2 3 4 5
?(x) 3 2 5 1 4
x 1 2 3 4 5
?-1(x)
Encrypt canwegohomenow
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Special Permutation Cipherperhaps the oldest
known cipher

• classisboringtoday
• ciidlsnaabgysotrsrox
• What was the permutation?
• History