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# What is Cryptography?

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Basic Talk. 90 minute talk to an audience of Freshmen and Sophomores of IIT Bombay on 23/02/10 as a part of Science Week. Organised by Web and Coding Club. Place: GG 101 (Elec Department)

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### What is Cryptography?

1. 1. What is Cryptography? Pratik Poddar Senior Undergraduate Computer Science and Engineering IIT Bombay 23th February 2010
2. 2. What is Cryptology? Greek: “krypto” meaning hide Study of: 1) Hiding data 2) Reading Hidden data 3) Signing data Goals: 1) Authentication 2) Integrity 3) Non-repudiation 4) Confidentiality
3. 3. Applications of Cryptography 1) ATM 2) Email - Passwords 3) E-payment 4) Electronic Voting 5) Securing Data
4. 4. Basic Terminology Encryption -> Plaintext to Ciphertext Decryption -> Ciphertext to Plaintext
5. 5. History of Cryptography Greeks used Transposition Cipher “hello world” becomes “ehlol owrdl”
6. 6. “Asimplese ntenceoft ransposit ioncipher” becomes “Anri stao ienn mnsc pcpi leop eosh sfie ettr”
7. 7. History of Cryptography Greeks used Substitution Cipher “hello world” becomes “uryyb jbeyq” 26! (~ 30 digit number) keys possible
8. 8. Caesar Cipher Caesar Substitution Cipher with key 3 “hello world” becomes “khoor zruog” 26 keys possible
9. 9. th Statistical Attacks (9 century) Frequency Analysis in 9th century Armed with statistical knowledge about the plaintext, one can easily break a monoalphabetic substitution cipher: Most frequent characters: e, t, o, a, n, i Most frequent diagrams: th, in, er, re, an Most frequent trigrams: the, ing, and, ion
10. 10. Take the following example cipher text BPMZM WVKM EIA IV COTG LCKSTQVO EQBP NMIBPMZA ITT ABCJJG IVL JZWEV IVL BPM WBPMZ JQZLA AIQL QV AW UIVG EWZLA OMB WCB WN BWEV OMB WCB, OMB WCB, OMB WCB WN BWEV IVL PM EMVB EQBP I YCIKS IVL I EILLTM IVL I YCIKS QV I NTCZZG WN MQLMZLWEV BPIB XWWZ TQBBTM COTG LCKSTQVO EMVB EIVLMZQVO NIZ IVL VMIZ JCB IB MDMZG XTIKM BPMG AIQL BW PQA NIKM VWE OMB WCB, OMB WCB, OMB WCB WN PMZM IVL PM EMVB EQBP I YCIKS IVL I EILLTM IVL I YCIKS IVL I DMZG CVPIXXG BMIZ We need to compare the frequency distribution of this text with standard English
11. 11. Hence the key is probably equal to 8 We can now decrypt the cipher text to reveal: There once was an ugly duckling With feathers all stubby and brown And the other birds said in so many words Get out of town Get out, get out, get out of town And he went with a quack and a waddle and a quack In a flurry of eiderdown That poor little ugly duckling Went wandering far and near But at every place they said to his face Now get out, get out, get out of here And he went with a quack and a waddle and a quack And a very unhappy tear
12. 12. Poly-alphabetic Cipher (1467) Use different ciphers (i.e., substitution alphabets) for various parts of a message The first automatic cipher device Frequency analysis fails
13. 13. KILL HIM TONIGHT with key PRATIK KILL HIM TONIGHT PRAT IKP RATIKPR ------------------------------ ZZLE PSB KOGQQWK
14. 14. Attack on Poly-Alphabetic Cipher If period is known, individual frequency analysis
15. 15. Kerchoff's Principle (1800s) Cryptography always involves: Transformation and Secret Security of the key used should alone be sufficient for a good cipher to maintain confidentiality under an attack The enemy knows the system
16. 16. Cryptography Machines (1900s) Mechanical encryption/decryption devices Rotor machines (electro- mechanical) Enigma machine was used by the German government and military from the late 20s and during World War II
17. 17. Post WW2 Cryptography 1) Cryptography now on bits instead of letters 2) Academic Research began in 1970s 3) Computers helped Cryptography? 4) No more linguistic and lexicographic patterns, but aspects of information theory, computational complexity, statistics, combinatorics, abstract algebra, number theory, and finite mathematics
18. 18. “Modern cryptography is a remarkable discipline. It is a cornerstone of computer and communications security, with end products that are imminently practical. Yet its study touches on branches of mathematics that may have been considered esoteric, and it brings together fields like number theory, computational-complexity theory, and probability theory”
19. 19. Block vs Stream Block Cipher Message is broken into blocks, each of which is then encrypted Stream Cipher Process the message bit by bit ( as a stream )
20. 20. Symmetric Key Cryptosystem
21. 21. Symmetric Key Algorithm - DES
22. 22. Asymmetric Key Cryptosystem
23. 23. Asymmetric Key Cryptosystem - Idea Some modern cryptographic techniques can only keep their keys secret if certain mathematical problems are intractable, such as the integer factorization or the discrete logarithm problems
24. 24. Designing Cipher -- Little Secrets Hide Bigger Secrets -- Open Algorithms and value of Peer-Review -- Mathematical Background -- Unbroken is Not Necessarily Unbreakable What is a good encryption scheme?
25. 25. 1) Complexity of encrypting or decrypting 2) Unbreakable 3) Size of cipher-text relative to plaintext 4) Key exchange should be easy
26. 26. Breaking Cipher Weakness: Length of the key Quality of Random Source Statistical Leaking Faulty Implementation Inadequate Peer-Review Social Engineering and Coercion Attacks: Brute-Force Attack Frequency Analysis Index of Coincidence Linear Cryptanalysis Differential Cryptanalysis Meet in the Middle Attack
27. 27. Research and Education -- Encryption Decryption Algorithms -- Cryptanalysis -- Mathematics -- Proofs of cryptosystem -- Key Exchange Protocols -- Certificates
28. 28. Work being done at IIT Bombay Prof. Bernard Menezes and his 5-6 students Contact: Pratik Poddar pratik.poddar@iitb.ac.in pratik@cse.iitb.ac.in (Topic: IBE and Elliptic-Curve Cryptography)
29. 29. Thank You!