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Classical Ciphers

Classical Ciphers. Cryptography Lecture 1: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University. Cryptography. Why might one need cryptography?. What is a cryptosystem?. A cryptosystem consists of: All messages: called plaintext

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Classical Ciphers

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  1. Classical Ciphers Cryptography Lecture 1: Chantilly Academy Poorvi Vora Department of Computer Science George Washington University

  2. Cryptography Why might one need cryptography? Chantilly Academy Crypto Lecture 1: Spring 07

  3. 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 Chantilly Academy Crypto Lecture 1: Spring 07

  4. Caesar Cipher AN EYE FOR AN EYE dq hbh iru dq hbh Trivial to break Chantilly Academy Crypto Lecture 1: Spring 07

  5. Shift Cipher Chantilly Academy Crypto Lecture 1: Spring 07

  6. 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. Chantilly Academy Crypto Lecture 1: Spring 07

  7. 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 Chantilly Academy Crypto Lecture 1: Spring 07

  8. Vigenère Cipher The key used for each symbol is different. For example: AN EYE FOR AN EYE BA BA BLACK SHEEP (Here: A=0, B=1, C=2, etc.) bn fyf qot k…. Chantilly Academy Crypto Lecture 1: Spring 07

  9. Vigenère Cipher on bits 010111011101 message Chantilly Academy Crypto Lecture 1: Spring 07

  10. 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 Chantilly Academy Crypto Lecture 1: Spring 07

  11. Decrypt Decrypt the message given to you using the key on the back of the card Chantilly Academy Crypto Lecture 1: Spring 07

  12. 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? Chantilly Academy Crypto Lecture 1: Spring 07

  13. Permutation Cipher Encrypt: canwegohomenow Chantilly Academy Crypto Lecture 1: Spring 07

  14. Special Permutation Cipherperhaps the oldest known cipher classisboringtoday ciidlsnaabgysotrsrox What was the permutation? History Chantilly Academy Crypto Lecture 1: Spring 07

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