1. Cryptography CS-103 Chapter 8

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

3. History – Some Encoding Schemes Caesar ShiftCaesar Shift Substitution CipherSubstitution Cipher Vigenere CipherVigenere Cipher One-Time Pad CipherOne-Time Pad Cipher EnigmaEnigma Public Key CryptographyPublic Key Cryptography

4. History – Caesar Shift First used by Julius CaesarFirst used by Julius Caesar Uses a simple alphabetic shiftUses a simple alphabetic shift Plaintext is shifted a certain number of letters forward or backwardPlaintext 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.

5. Caesar Shift (Cont’d) This type of cipher is relatively easy to break by trial-and-error.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.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.Still used during the Civil War to send messages on the battlefield.

6. History – Substitution Cipher Encoded letters are randomly scrambled.Encoded letters are randomly scrambled. Recipient must know the scrambled alphabet.Recipient must know the scrambled alphabet. May use a key word to begin scrambling.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

7. Substitution Cipher (Cont’d) Because letter frequencies in various languages are well established, this type of cipher is relatively easy to break.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.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 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.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 CipherModification of the Substitution Cipher The alphabet is re-scrambled for each letter of the plaintext messageThe alphabet is re-scrambled for each letter of the plaintext message Requires a keyword or phrase to start the substitution sequenceRequires a keyword or phrase to start the substitution sequence Requires use of a Vigenere TableRequires use of a Vigenere Table

10. Vigenere Cipher (Cont’d) Considered unbreakable for several centuries.Considered unbreakable for several centuries. Eventually Babbage, Kasiski, and Kerchoff devised a method to break the Vigenere cipher.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.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 CipherModification of the Vigenere Cipher Message length is limited to some prescribed number of charactersMessage length is limited to some prescribed number of characters Key is longer than any messageKey is longer than any message Each key is used only once and then destroyed.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 messageRecipient 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 destroyedOnce the message is decoded, the key and the “pad” on which it was recorded are destroyed The “pad” and key are never used againThe “pad” and key are never used again

13. One-Time Pad Cipher (Cont’d) This cipher method was used by many governments for diplomatic communicationsThis 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 messagesSince 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 messageDrawback – 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 WWIIGermany developed this encoding device during WWII England enlisted some of the greatest mathematicians of the time to attempt to break the codeEngland 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 teamAlan 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 foundEnigma 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 cryptographyComputers ushered in a new age of cryptography Increased electronic transmission of sensitive data required new levels of securityIncreased electronic transmission of sensitive data required new levels of security Requires the use of two keys: a public key and a private keyRequires the use of two keys: a public key and a private key

16. Public-Key Cryptography (Cont’d) Public KeyPublic 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

17. Public-Key Cryptography (Cont’d) Diffie, Hellman, and Merkle discovered and published the key concepts for this methodDiffie, 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 ideaRivest, 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 methodsThe 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.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.As computer speeds increase and new methods of factoring large numbers are discovered, perhaps the RSA method will someday be broken.

20. The End