Classic Cryptography Caesar Wenyuan Xu Department of Computer

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Classic Cryptography: Caesar Wenyuan Xu Department of Computer Science and Engineering University of South

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

The Basic Secure Communication Scenario Bob Alice Want to have dinner on Saturday night?

The Basic Secure Communication Scenario Bob Alice Want to have dinner on Saturday night? Eve What can you do? n n 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 ¨ 2008

The Basic Secure Communication Scenario Bob ZDQW WR KDYH GLQQHU Want to have dinner.

The Basic Secure Communication Scenario Bob ZDQW WR KDYH GLQQHU Want to have dinner. RQ on. VDWXUGDB Saturday night? QLJKW? ciphertext Encrypt Decrypt plaintext Alice Want to have dinner on Saturday night? Eve 2008

Caesar Shift Cipher - Encrypt Bob Alice VDWXUGDB ciphertext Saturday plaintext Encrypt Shift key

Caesar Shift Cipher - Encrypt Bob Alice VDWXUGDB ciphertext Saturday plaintext Encrypt Shift key = 3 Decrypt Eve 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 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 2008

Caesar Shift Cipher - Decrypt Bob Saturday VDWXUGDB ciphertext plaintext Encrypt Shift key =

Caesar Shift Cipher - Decrypt Bob Saturday VDWXUGDB ciphertext plaintext Encrypt Shift key = 3 Alice Decrypt Eve How can Alice get the original message? 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 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 2008

Caesar Shift Cipher - Decrypt Bob Alice Saturday VDWXUGDB ciphertext plaintext Encrypt Shift key

Caesar Shift Cipher - Decrypt Bob Alice Saturday VDWXUGDB ciphertext plaintext Encrypt Shift key = 3 Decrypt Eve How can Alice get the original message? Shift back letters by 3 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 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 2008

Caesar Shift Cipher - Attack Bob Alice VDWXUGDB ciphertext plaintext Encrypt Decrypt What Eve

Caesar Shift Cipher - Attack Bob Alice VDWXUGDB ciphertext plaintext Encrypt Decrypt What Eve can do? Saturday Eve plaintext ? ? ? ? ? ? ? ciphertext 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 2008

Caesar Shift Cipher - Attack Bob Alice VDWXUGDB ciphertext plaintext Encrypt Decrypt What Eve

Caesar Shift Cipher - Attack Bob Alice VDWXUGDB ciphertext plaintext Encrypt Decrypt What Eve can do? Saturday Eve 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 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 vdwxugdb BCDEFGHIJKLMNOPQRSTUVWXYZ A CDEFGHIJKLMNOPQRSTUVWXYZA B DEFGHIJKLMNOPQRSTUVWXYZAB C ucvwtfca tbuvsebz saturday 2008

What we plan to do today n http: //www. simonsingh. net/The_Black_Ch amber/caesar. html n

What we plan to do today n http: //www. simonsingh. net/The_Black_Ch amber/caesar. html n Let’s open the file “Readme” again and find out what it is! 2008