Classical Ciphers Cryptography Lecture 1 Chantilly Academy Poorvi

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

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

Cryptography Why might one need cryptography? 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07

Cryptography Why might one need cryptography? 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 2

What is a cryptosystem? A cryptosystem consists of: • • • All messages: called

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 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 3

Caesar Cipher A B C D E F G H I J d m

Caesar Cipher A B C D E F G H I J d m n e f g h i j k l K AN EYE FOR AN EYE dq hbh iru dq hbh Trivial to break 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 5

Shift Cipher Key A B C D E F 0 a b c d

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 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 6

Encrypt your own messages • Choose a key • Choose a short message for

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. 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 7

Decrypt your messages • If you got a message, decrypt it, using the key

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 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 8

Vigenère Cipher The key used for each symbol is different. For example: AN EYE

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…. 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 9

Vigenère Cipher on bits 01011101 message 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07

Vigenère Cipher on bits 01011101 message 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 10

Encrypt with Vigenère Cipher Use a coin to generate the bits of the key.

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 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 11

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

Decrypt the message given to you using the key on the back of the card 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 12

HW Assignment Decrypt the following texts: – etarq tqemu encrypted using the shift cipher

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? 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 13

Permutation Cipher x 1 2 3 4 5 (x) 3 2 5 1 4

Permutation Cipher x 1 2 3 4 5 (x) 3 2 5 1 4 x 1 2 3 4 5 -1(x) Encrypt: canwegohomenow 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 14

Special Permutation Cipher perhaps the oldest known cipher classisboringtoday ciidlsnaabgysotrsrox What was the permutation?

Special Permutation Cipher perhaps the oldest known cipher classisboringtoday ciidlsnaabgysotrsrox What was the permutation? History 9/16/2020 Chantilly Academy Crypto Lecture 1: Spring 07 15