Crypto MakeUp 1 Regular Session 2 BITS Pilani

Crypto Make-Up #1 Regular Session #2 BITS Pilani

Prereqs n Mathematics Back-ground Groups , Rings and Fields n n Primitive Boolean Operations Discrete Mathematics Probability / Number Theory Almost all of Computer Networks !!

History n n Evolution of Secret Writing For thousands of years , Kings , queens and generals have relied on efficient communication in order to govern their countries and command their armies. At the same time , they have all been aware of the consequences of their messages falling in the wrong hands. Some of the earliest accounts of secret writing date back to Herodotus , ‘the father of history’ according to the Roman philosopher and statesman Cicero. Greece and Persia 5 th Century BC.

Other Techniques n n n Shaving the head of a messenger , writing on his scalp and then waiting for the hair to re-grow. Tiny balls of wax , invisible ink ( sap of the thithymalus plant ), organic fluids A mixture of one ounce of alum and a pint of vinegar on a hard-boiled egg. n n STEGANOGRAPHY Steganos meaning ‘covered’ and graphein meaning ‘to write’

n n The longevity of steganography illustrates that it certainly offers a means of security , but it suffers from a fundamental weakness. ? ? ? If the messenger is searched and the message discovered, the contents of the intended secret communication are revealed at once. Interception of the message immediately compromises all security. A thorough guard might routinely search any person crossing the border, scraping any wax tablets , heating blank sheets of paper, shelling boiled eggs, shaving people’s heads , and so on , and inevitably there will be occasions where message is uncovered. n That’s where CRYPTOGRAPHY came into picture

Cryptography n n n Derived from the Greek word “kryptos” meaning “hidden” and “graphien” meaning “to write”. Idea and advantage ? ? Cryptography + Steganography !!! Microdot !!!! The first Microdot to be spotted was in 1941 by the FBI, following a tip-off that the Americans should look for a tiny gleam from the surface of a letter.

SECRET WRITING STEGANOGRAPHY CRTPTOGRAPHY Substitution Cipher Code Transposition

n n n n Assassinate = D Blackmail= p Capture = j Protect = Z general = 3 immediately = 08 king = @ today =73 minister = # tonight = 28 prince = !!! Tomorrow=43 Plain message= assassinate the king tonight Encoded message= D-@-28 Technically a code is defined as a substitution at the level of words or phrases , whereas a cipher is the substitution at the level of letters.

Goals of cryptography n n Privacy ( Confidentiality) Integrity Authentication Non-Repudiations !!!

• Privacy has to do with keeping your info out of unauthorized hands • Authentication deals with whom you are talking to before revealing sensitive info • Integrity Control…. . no manipulation in data { message received == message sent } • Non Repudiations… no Impersonification ( seeming sender == actual sender , No denial)

Terminology ( sender, receiver , plain text etc)

n n n n P = Plain text C = Cipher text E = Encryption Algorithm D = Decryption Algorithm C = E(P) P = D(C) P = D ( E ( P ))

An Encryption process involves an algorithm and a key n n n C = Ek ( P ) P = Dk ( C ) P = Dk ( Ek ( P )) n n CRYPTANALYSIS The Process of breaking the encryption when key is not known in an attempt to : Discover the Plain text Discover the Key

Terminology Contd … n Unconditionally Secure If the cipher text generated does not contain enough information to get the plain text , no matter how much cipher text is available. n Computationally Secure If the cost of breaking the Cipher text exceeded the value of encrypted information. The time required to break the cipher exceeds the useful information life-time. . Period !!!

Some Other Ciphers n n n Caesar Square Cipher Rail Fence Cipher Ottendorf Cipher At. Bash Cipher Play-Fair Cipher Hill Cipher

n n n The first recorded description of the Playfair cipher was in a document signed by Wheatstone on 26 March 1854. However, the scheme eventually came to be known by the name of Wheatstone's friend Lord Playfair, who popularized it The Playfair cipher uses a 5 by 5 table containing a keyword or phrase. To generate the key table, one would first fill in the spaces in the table with the letters of the keyword (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (usually omitting "Q" to reduce the alphabet to fit, other versions put both "I" and "J" in the same space).

Rules: n n n Break message into digraphs If both letters are the same (or only one letter is left), add a "X" after the first letter. Encrypt the new pair and continue. If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively If the letters appear on the same column of your table, replace them with the letters immediately below. If the letters are not on the same row or column, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair.

n n n PLAYF IREXM BCDGH JKNOS TUVWZ Hide the Gold in the tree stump HI DE TH EG OL DI NT HE TR EX ES TU MP

n n n n HI -> BM DE -> ND Th -> ZB EG -> XD OL -> KY DI -> BE NT -> JV HE –> DM TR -> UI EX -> XM ES -> MN TU -> UV MP -> IF

Security Revisited !!! n What is a Secure Cryptographic Algorithm ? ? ?

n Some other Finer points !!!!

ANY QUESTIONS? ? ?