1. Chapter 1 Introduction Cryptography-Principles and Practice Harbin Institute of Technology School of Computer Science and Technology Zhijun Li http://cst.hit.edu.cn/~lizhijun lizhijun_os@hit.edu.cn

2. Zhijun LiS1034040/Autumn08/HIT2 Outline Cryptography Overview Basic Terminology Goals of Cryptography Historical Review of Cryptology

3. Zhijun LiS1034040/Autumn08/HIT3 Cryptography Overview Greek: “krypto” = hide Cryptology – science of hiding = cryptography + cryptanalysis Cryptography – design to hide Cryptanalysis – analyzing (breaking) secrets –Cryptanalysis is what attacker does –Decryption is what legitimate receiver does

4. Zhijun LiS1034040/Autumn08/HIT4 Outline Cryptography Overview Basic Terminology Goals of Cryptography Historical Review of Cryptology

5. Zhijun LiS1034040/Autumn08/HIT5 Basic Terminology Cryptology Cryptography Cryptanalysis Ciphers (Cryptosystems, Cryptoalgorithms) Encryption and Decryption Plaintext(Cleartext) Ciphertext(Cryptogram)

6. Zhijun LiS1034040/Autumn08/HIT6 Cryptography Cryptography deals with the design of algorithms, protocol, or service using some basic tools to achieve the security goals Tools cover encryption algorithms, one-way functions, pseudorandom functions, hashing functions, signature schemas, etc

7. Zhijun LiS1034040/Autumn08/HIT7 Cryptanalysis Description: Cryptanalysis is an (usually vulnerability) analysis of a cryptosystem –Using mathematical methods to prove that the design does (or does not) achieve a security goal under some (new) attacks Description: Attack is an attempt to cryptanalysis Note: a part of cryptography

8. Zhijun LiS1034040/Autumn08/HIT8 Ciphers Alice EncryptionDecryption Bob Secure Channel Oscar M C M K EncipherDecipher C=E K (M); M=D K (C) M is plaintext (cleartext) C is ciphertext (cryptogram) K is key

9. Zhijun LiS1034040/Autumn08/HIT9 Formal Description A cryptosystems is (M,C,K,E,D) –M set of all plaintext –C set of all ciphertext –K set of all keys (keyspace) –E set of encryption rules, e k : M  C –D set of decryption rules d k : C  M –For any k  K, there exists a e k  E and corresponding d k  D such that  m  M, d k (e k (m))=m

10. Zhijun LiS1034040/Autumn08/HIT10 Mappings or Algorithms Mappings –e k and d k are functions –e k is an injection and normally an one-to-one function –If M=C, e k is a permutation –The theory aspect Algorithms –e k and d k should be implemented and analyzed via computer –The practice aspect Mappings and Algorithms

11. Zhijun LiS1034040/Autumn08/HIT11 Cryptography Overview Basic Terminology Goals of Cryptography Historical Review of Cryptology Outline

12. Zhijun LiS1034040/Autumn08/HIT12 The most basic problem: Ensure security of communication over insecure medium In the architecture of information security Goals of Cryptography

13. Zhijun LiS1034040/Autumn08/HIT13 Confidentiality(privacy,secrecy) –Ensures that information flow between the sender and the receiver is unintelligible to outsiders (against eavesdropping) Goal  Confidentiality

14. Zhijun LiS1034040/Autumn08/HIT14 Message integrity –Enables the receiver to verify whether the message has been tampered with by outsiders Goal  Message Integrity

15. Zhijun LiS1034040/Autumn08/HIT15 Goal  Message Authentication Message authentication –Allows the receiver of messages to determine the true identity of the sender (against impersonation)

16. Zhijun LiS1034040/Autumn08/HIT16 Goal  Non-repudiation Non-repudiation –Prevents the sender of a message from claiming that they have not sent the message

17. Zhijun LiS1034040/Autumn08/HIT17 Security Service is something that enhances the security –Make use of one or more security mechanisms Security Mechanism is designed to detect, prevent, or recover from a security attack –One particular element is: cryptographic techniques Attacks are any action that compromises the security of information Security Infrastructure

18. Zhijun LiS1034040/Autumn08/HIT18 Kerckhoff’s Principle The type of Attacks Attacks of Cryptography Known something –The algorithm –Cleartext or Ciphertext or others Want to know (Purpose) –The key –Cleartexts for some Ciphertexts –Others

19. Zhijun LiS1034040/Autumn08/HIT19 Types of Attacks Ciphertext-only attack (COA) –Only know some ciphertexts Known-plaintext attack (KPA) –Know some pairs of message and corresponding cryptogram Chosen-plaintext attack (CPA) –Can choose messages and read the corresponding cryptogram –Can interact the encipher Chosen-ciphertext attack (CCA) –Can choose cryptograms and observe the corresponding messages –Can interact the decipher

20. Zhijun LiS1034040/Autumn08/HIT20 Relationships of Attacks Strengths of Attacks –COA<KPA<CPA<CCA

21. Zhijun LiS1034040/Autumn08/HIT21 Brute Force Attack Key size(bits)Time (1us/test)Time(1us/10 6 test) 3235.8mins2.15 msec 406.4days550 msec 561140yeas10.0 hours 64~500000yeas107 days 128 5  10 24 yeas5  10 18 years Key space enumeration –Ciphertext-only attacks Suppose the correct plaintexts can be identified –Know one pair of message and cryptogram

22. Zhijun LiS1034040/Autumn08/HIT22 Outline Cryptography Overview Basic Terminology Goals of Cryptography Historical Review of Cryptology

23. Zhijun LiS1034040/Autumn08/HIT23 Cryptanalysts 3000BC monoalphabetics 900 al-Kindi - frequency analysis Alberti – first polyalphabetic cipher 1460 Vigenère 1854 Babbage breaks Vigenère; Kasiski (1863) publishes Cryptographers Brief History-First 4000 Years

24. Zhijun LiS1034040/Autumn08/HIT24 Cryptographers Cryptanalysts 18541918 Mauborgne – one-time pad Mechanical ciphers - Enigma 1939 Rejewski repeated message-key attack Turing’s loop attacks, Colossus Enigma adds rotors, stops repeated key 1945 Feistel block cipher, DES Linear, Differential Cryptanalysis ? 1973 Public-Key Quantum Crypto 1978 Brief History-Last 100 Years