Cryptography CS103 Chapter 8 History Humans have been

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 • One-Time Pad Cipher • Enigma • Public Key Cryptography

History – Caesar Shift • 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.

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.

History – Substitution Cipher • Encoded letters are randomly scrambled. • Recipient must know the scrambled alphabet. • May use a key word to begin scrambling. • Alphabet – – ABCDEFGH • Scrambled – – CODEABFG • Plaintext – – FACE • Ciphertext – – BCDA

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.

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.

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

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.

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.

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

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

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

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

Public-Key Cryptography (Cont’d) • 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) • 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

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.

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.

The End