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Classic Cryptography: Caesar

Classic Cryptography: Caesar. Wenyuan Xu Department of Computer Science and Engineering University of South Carolina 2008. Want to have dinner on Saturday night?. Eve. The Basic Secure Communication Scenario. Bob wants to send Alice a message. Eve is evil and eavesdrops . Her goals:

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Classic Cryptography: Caesar

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  1. Classic Cryptography: Caesar Wenyuan Xu Department of Computer Science and Engineering University of South Carolina 2008

  2. Want to have dinner on Saturday night? Eve The Basic Secure Communication Scenario • Bob wants to send Alice a message. • Eve is evil and eavesdrops. Her goals: • Read the message • Determine the keys • Corrupt the message to Bob gets something different than what Alice sent Bob Alice What can you do?

  3. ciphertext Encrypt Encrypt Decrypt Eve The Basic Secure Communication Scenario Bob Alice ZDQW WR KDYH GLQQHU RQ VDWXUGDB QLJKW? Want to have dinner on Saturday night? plaintext Want to have dinner on Saturday night?

  4. VDWXUGDB Saturday ciphertext Encrypt Encrypt Decrypt Eve 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 plaintext 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 ciphertext Caesar Shift Cipher - Encrypt Bob Alice plaintext Shift key = 3

  5. Saturday ciphertext Encrypt Encrypt Decrypt Eve 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 plaintext 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 ciphertext Caesar Shift Cipher - Decrypt Bob Alice VDWXUGDB plaintext How can Alice get the original message? Shift key = 3

  6. Saturday VDWXUGDB ciphertext Encrypt Encrypt Decrypt Eve 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 plaintext 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 ciphertext Caesar Shift Cipher - Decrypt Bob Alice plaintext How can Alice get the original message? Shift key = 3 Shift back letters by 3

  7. ciphertext Encrypt Encrypt Decrypt Eve Caesar Shift Cipher - Attack Bob Alice VDWXUGDB plaintext What Eve can do? Saturday ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? plaintext 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 ciphertext

  8. ciphertext Encrypt Encrypt Decrypt Eve vdwxugdb ucvwtfca tbuvsebz saturday saturday Caesar Shift Cipher - Attack Bob Alice VDWXUGDB plaintext What Eve can do? Saturday plaintext 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 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 ciphertext 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 A C 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 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

  9. What we plan to do today • http://www.simonsingh.net/The_Black_Chamber/caesar.html • Let’s open the file “Readme” again and find out what it is!

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