1. 15-349 Introduction to Computer and Network Security Iliano Cervesato 24 August 2008 – Introduction to Cryptography

2. 2 Where we are  Course intro  Cryptography  Intro to crypto  Modern crypto  Symmetric encryption  Asymmetric encryption  Beyond encryption  Cryptographic protocols  Attacking protocols  Program/OS security & trust  Networks security  Beyond technology

3. 3 Outline  Basic concepts  Protecting information  Goals of cryptography  Brief history  Cryptographic toolbox (preview)  Cryptanalysis  Traditional attack models  Side-channel attacks  Early ciphers  Substitution ciphers  Transposition ciphers

4. 4 Confidentiality of Communication Implement a virtual trusted channel over an insecure medium ED

5. 5 Confidentiality of storage Implement a virtual trusted safebox over an insecure storage medium E

6. 6 Insecure Channels External observer can  Read traffic  Interception  Inject new traffic  Fabrication  Block traffic … (sometimes)  Interruption  Modify traffic … (sometimes)  Modification Active attack Passive attack

7. 7 Representing Data  Divide data into blocks  Character, records, …  Represent each block by a number  E.g., ASCII  Why?  Cryptography is based on mathematics

8. 8 Encryption and Decryption E, D realize a virtual trusted channel E D Message (cleartext, plaintext) Message (cleartext, plaintext) Encrypted message (ciphertext) Encrypted message (ciphertext) EncryptionDecryption X X

9. 9 Keys What are E and D?  Channel-specific algorithm  Requires a lot of algorithms  Hard  Universal algorithms  Parameterized by key  Easier –1 algorithm –Large space of keys E ms E ms k

10. 10 Classical Cryptography E, D realize a virtual trusted channel, given key E D Message (cleartext, plaintext) Message (cleartext, plaintext) Encrypted message (ciphertext) Encrypted message (ciphertext) EncryptionDecryption key X X

11. 11 Goals of Cryptography Not just about confidentiality!  Integrity  Digital signatures  Hash functions  Non-repudiation, fair exchange  Contract signing  Anonymity  Electronic cash  Electronic voting  … Non-goals  Denial of service

12. 12 A Brief History of Cryptography  ~2000 years ago: Substitution ciphers  A few centuries later: Transposition ciphers  Renaissance: Polyalphabetic ciphers  1844: Mechanization  1976: Public-key cryptography

13. 13 Substitution Ciphers Replace each letter with another  Key: substitution table  How to break it?  Brute force? 26! possibilities (= 4x10 26 )  Count the frequencies of letters, pairs, …  Koran was tabulated by 1412  Ciphertext is enough: ciphertext-only attack  Example: A  C B  E D  F … X  A Y  B Z  C Caesar’s cipher: QVAQBCWZQRLWDVEFW V  X W  M X  T Y  J Z  P O  S P  R Q  I R  D S  U T  Y U  K H  L I  Q J  N K  H L  F M  A N  B A  V B  E C  Z D  C E  W F  G G  O IAMINDECIPHERABLE

14. 14 Renaissance Ciphers Use message and key letters for cipher  Key: a word ( CRYPTO )  Example:  Polyalphabetic cipher:  Encryption of letter is context-dependent  Seed of modern cryptography CRYPTOCRYPTOCRYPT WHATANICEDAYTODAY ZZZJUCLUDTUNWGCQS + (mod 26)

15. 15 Book Ciphers Same thing but with very long key  Key: a poem, a book, … ( TOBEORNOTTOBETHATISTHEQUESTION… )  Example:  … there are not all that many famous books, poems, etc. TOBEORNOTTOBETHAT WHATANICEDAYTODAY PVBXOEVQXWOZXHKAR + (mod 26)

16. 16 One-Time Pad Same thing, but now key is a infinite random string  Example:  This is a perfect cipher  How to remember/transmit the key??  Short key stretched by means of a random number generator  Vernam cipher  Use  (xor) to combine key and message YKSUFTGOARFWPFWEL WHATANICEDAYTODAY ZZZJUCLUDTUNWGCQS + (mod 26)

17. 17 Book Ciphers  Same thing, but now use a very long key

18. 18 Transposition Ciphers Switch letters around by a permutation  Example: HELLOWORLD   Key: permutation  Breakable with ciphertext-only attack 1 2 3 4 5 3 5 4 1 2 k = LOLHERDLWO

19. 19 More transposition  Write code in rows and read it in columns  A very regular type of permutation THEGOAL OFSUBSI TUTIONI SCONFUS IONXXXX THE GOAL OF SUBSITUTION IS CONFUSION TOTSIHFUCOESTONGUINXOBOFXASNUXLIISX

20. 20 Confusion and Diffusion Confusion  Replace symbol with another Diffusion  Mix up symbols WHATANI ZZZJUCL WHATANI ANWIHAT Modern ciphers are a combination

21. 21 Mechanization  1844: invention of telegraph  Beginning of civilian crypto  Rotor machines  Key: initial position of rotors  Culminate in WW II  1975: DES  1996-2000 AES  1976: Public key cryptography We will examine in some detail The Enigma

22. 22 Cryptographic Toolbox  Encryption  Symmetric  Asymmetric  Digests  Hashing  Digital signatures  Certificates

23. 23 Symmetric Encryption D k (E k (m)) = m E D M M X X k Message (cleartext) Message (cleartext) Encrypted message (ciphertext) Encrypted message (ciphertext) Secret key Decryption box Encryption box

24. 24 Asymmetric Encryption D k (E k (m)) = m E D M M X X k Cleartext Ciphertext Public key Decryption box Encryption box k -1 Private key Public data k

25. 25 Digital Signatures V k (m,s) = S V M M M, s k Message Signature signature key Verification box Signature box k -1 Verification key Public data k true if s =S k (m) false otherwise

26. 26 Certificates How do you know this public key is mine?  Certificate  Binding between key and owner  Certified by authority  Who is the authority?  Public-key infrastructure

27. 27 Message Digests  Short message to certify integrity  Un-keyed  Checksums, hashes  No crypto  Anybody can calculate/modify it  Keyed  MACs  Based on a secret key  Only owners can calculate/modify it

28. 28 Cryptanalysis The art  science of breaking a cipher  Try all possible plaintext corresponding to a ciphertext  Plain silly!  Try all possible keys for an encryption algorithm  Algorithm must be known  Enormous space of keys  Exploit weaknesses, regularities, shortcuts  Side-channel attacks  E.g., basic substitution cipher

29. 29 What is “breaking a cipher”?  Recover the key k  Hard  Often not needed!  Decipher a single message  Decipher all messages  Modify messages  “Attack at dawn”  “attack at dusk”  Exploit properties of the cipher

30. 30 Attack Models Good ciphers resist all attack models x Random Ciphertext Only m, x Ek(m)Ek(m) Known Plaintext Random x, m Dk(x)Dk(x) Chosen Ciphertext Chosen m, x Ek(m)Ek(m) Chosen Plaintext Chosen

31. 31 Sneaky Attacks  Obtain the key somehow  Network sniffers, worms, backup tapes, …  Blackmail, bribery, torture, …  Side-channel cryptanalysis  Power consumption  Encryption time  Radiation Be careful!  off-peak computation  random noise  physical shielding Better implementation and design From http://www.cryptography.com/dpa/technical Detail: Round 2 Round 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Differential Power Analysis on DES