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Cryptography

Cryptography. CS-103 Chapter 8. History. Humans have been devising systems to encode information for at least 4000 years. The original message is in “plaintext” while the encoded message is in “ciphertext”. History – Some Encoding Schemes. Caesar Shift Substitution Cipher Vigenere Cipher

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Cryptography

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  1. Cryptography CS-103 Chapter 8

  2. History • Humans have been devising systems to encode information for at least 4000 years. • The original message is in “plaintext” while the encoded message is in “ciphertext”.

  3. History – Some Encoding Schemes • Caesar Shift • Substitution Cipher • Vigenere Cipher • One-Time Pad Cipher • Enigma • Public Key Cryptography

  4. First used by Julius Caesar Uses a simple alphabetic shift Plaintext is shifted a certain number of letters forward or backward Example: Plaintext - AUTO Ciphertext – BVUP Alphabet is shifted on letter forward – A=B, B=C, etc. History – Caesar Shift

  5. Caesar Shift (Cont’d) • This type of cipher is relatively easy to break by trial-and-error. • Recipient must only know the number of places the alphabet has been shifted. • Still used during the Civil War to send messages on the battlefield.

  6. Encoded letters are randomly scrambled. Recipient must know the scrambled alphabet. May use a key word to begin scrambling. Alphabet – A B C D E F G H Scrambled – C O D E A B F G Plaintext – FACE Ciphertext – BCDA History – Substitution Cipher

  7. Substitution Cipher (Cont’d) • Because letter frequencies in various languages are well established, this type of cipher is relatively easy to break. • Newspapers often contain an encoded puzzle that uses this method of encryption.

  8. Substitution Cipher (Cont’d) • If the alphabet is randomly scrambled, the recipient must know the entire substitution scheme. • If a keyword is used, the recipient must only know the keyword to complete the substitution scheme.

  9. History – Vigenere Cipher • Modification of the Substitution Cipher • The alphabet is re-scrambled for each letter of the plaintext message • Requires a keyword or phrase to start the substitution sequence • Requires use of a Vigenere Table

  10. Vigenere Cipher (Cont’d) • Considered unbreakable for several centuries. • Eventually Babbage, Kasiski, and Kerchoff devised a method to break the Vigenere cipher. • Be sure you understand how this cipher works. Use the website provided in the on-line text.

  11. History – One-Time Pad Cipher • Modification of the Vigenere Cipher • Message length is limited to some prescribed number of characters • Key is longer than any message • Each key is used only once and then destroyed.

  12. One-Time Pad Cipher (Cont’d) • Recipient must know which “pad” and key were used to encode the message • Once the message is decoded, the key and the “pad” on which it was recorded are destroyed • The “pad” and key are never used again

  13. One-Time Pad Cipher (Cont’d) • This cipher method was used by many governments for diplomatic communications • Since each “pad” was used only once, the Babbage-Kasiski-Kerchoff method could not be used to decipher messages • Drawback – both sender and recipient had to know the number of the “pad” being used to encode the message

  14. History – The Engima Machine • Germany developed this encoding device during WWII • England enlisted some of the greatest mathematicians of the time to attempt to break the code • Alan Turing, the “father” of modern computer science was part of this team • Enigma code was not broken until a German submarine was captured and a machine and code book were found

  15. History – Public-Key Cryptography • Computers ushered in a new age of cryptography • Increased electronic transmission of sensitive data required new levels of security • Requires the use of two keys: a public key and a private key

  16. Public Key Known to everyone who wishes to send an encoded message to a recipient Can’t be used to decode message Private Key Known only to the recipient Can only be used to decode a message Public-Key Cryptography (Cont’d)

  17. Public-Key Cryptography (Cont’d) • Diffie, Hellman, and Merkle discovered and published the key concepts for this method • Rivest, Shamir, and Adleman devised an algorithm that could be used to implement the idea • The RSA algorithm has become the foundation for modern electronic security methods

  18. Public-Key Cryptography (Cont’d) • RSA method is based upon the idea that some mathematical processes are easy to implement but almost impossible to reverse. • Multiplying two large prime numbers is easy • Factoring this product to find the original prime numbers is very difficult • Current methods use products of more than 300 digits.

  19. Public-Key Cryptography (Cont’d) • As computer speeds increase and new methods of factoring large numbers are discovered, perhaps the RSA method will someday be broken.

  20. TheEnd

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