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

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Presentation transcript:

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

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 Want to have dinner on Saturday night? AliceBob Eve What can you do?

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

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

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

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

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

2008 Caesar Shift Cipher - Attack AliceBob Eve plaintext ciphertext Encrypt Decrypt 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 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 Saturday What Eve can do? 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 vdwxugdb ucvwtfca saturday tbuvsebz

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