IS 302: Information Security and Trust Week 3: From DES to AES

1. IS 302: Information Security and TrustWeek 3: From DES to AES 2012

2. Review • Kerckhoff principle • Attacks to cryptosystem • Caesar, Vigenere, Zimmerman, Vernam cipher • Confusion and diffusion

3. Modern Symmetric Ciphers • DES • AES

4. Block Ciphers vs Stream Ciphers • Block ciphers • DES and AES… • For each block, perform multiple rounds of confusion and diffusion operations • Stream ciphers • Vernam,… • Bit by bit operations

5. DES • 1972-1974 NBS call for proposal • IBM’s DES  Horst Feistel’s Lucifer cipher • 1976 US Federal standard • 1990  DES design is optimal • Almost any change to DES weakens it • May 26, 2002, DES was superseded by AES • brute force attack can easily break 56-bit DES key • 1998 3DES  valid till 2030 • extensively used in banking industry

6. Jeff Moser: http://www.moserware.com/2009/09/stick-figure-guide-to-advanced.html

11. One Round in Encoding • 56-bits key  16 48-bits sub-key • 64-bits blocks • Right half  left half • Left half mixed with encrypted right half  right half A round of encoding a block in DES (repeat 16 times) Left half block Right half block substitution Sub key f permutation New right half block New left half block

12. Overall Diagram http://en.wikipedia.org/wiki/Data_Encryption_Standard f function

13. DES procedure visualization • Cryptool: • Indiv. procedures  visualization of algorithms  DES • You need Java runtime environment at least version 1.6 http://java.sun.com • You also need to download and install the current version of ANIMAL animation software http://www.algoanim.info/Animal2/

14. Multiple DESes • Two-Key DES • Total key size is 56x2=112 bits; but the effective key size is only 57 bits only! • Triple DES (3DES) • This is a secure solution with effective key size of 112 bits Data E E C K1 K2 Data C E D E K1 K2 K1

15. DES encryption demo • Cryptoolencrypt/decryptsymmetric (modern) • DES (CBC) • 3-DES (CBC)

16. Security Concerns 56 bit key is too short • Can be broken on average in 2^55 ≈3.6*10^16 trials • Moore’s law: speed of processor doubles per 1.5 yr • 1997: 3500 machines broke DES in about 4 months • 1998: 1M dollar machine broke DES in about 4 days

25. AES • 1997 NIST call • Final five • Rijndael(Joan Daemen and Vincent Rijmen), • Serpent(Ross Anderson), • Twofish(Bruce Schneier), • RC6(Don Rivest, Lisa Yin), • MARS (Don Coppersmith, IBM) • 2000 Rijndael won • 2002 Rijndael became AES

26. AES vs DES

27. High-Level Cipher Algorithm • KeyExpansion (one 128/192/256-biy key to 10/12/14 128-bit subkeys) • Initial Round • AddRoundKey (cipher key) • Steps in each of 9/11/13 rounds (state: 4*4=16-array of bytes = 128 bit-block) • SubBytes — a non-linear substitution step where each byte is replaced with another according to a lookup table. • ShiftRows — a transposition step where each row of the state is shifted cyclically a certain number of steps. • MixColumns — a mixing operation which operates on the columns of the state, combining the four bytes in each column • AddRoundKey — each byte of the state is combined with the round key; each round key is derived from the cipher key using a key schedule. • Final Round (no MixColumns) • SubBytes • ShiftRows • AddRoundKey • How many rounds in total? How many round keys? http://en.wikipedia.org/wiki/Advanced_Encryption_Standard

32. Follow Me • Cryptool: AES procedures • Invid. Procedures  visualization of algorithms  AES • Web version: http://www.ccna-security.net/wp-content/uploads/2008/10/rijndael_ingles2004.swf • AES-128-CBC encryption • Encrypt/decrypt  symmetric (modern)  AES

33. Four Modes of Block Ciphers • How are multiple blocks processed? • ECB: Electronic Code Book • CBC: Cipher Block Chaining • CFB: Cipher Feedback • OFB: Output Feedback

34. Electronic codebook (ECB) mode K Enc P3 P1 P2 C3 C1 C2 K Dec C3 C1 C2 P3 P1 P2

35. Cipher-block Chaining (CBC) Mode K Enc P3 C2 P1 IV P2 C1 C3 C1 C2 K Dec C3 C1 C2 P3 C2 P1 IV P2 C1 IV C1 C2 P3 P1 P2

36. ECB vs CBC • Which mode would you choose? • Which one is semantically secure? Original image Encrypted with ECB Encrypted with CBC

37. Hands-On Exercise • AES Encryption and Decryption • OpenSSL • JCE • Download Lab.doc and follow instructions

38. Introduction:Javax.crypto.Cipher • This class provides the functionality of a cryptographic cipher for encryption and decryption • Methods: • getInstance(String algorithm ) • Generates a Cipher object that implements the specified algorithm. • init(int opmode, Key key ) • The cipher is initialized with a key for either encryption or decryption. • doFinal(byte[] input ) • Encrypts or decrypts data depending on how this cipher was initialized. Further details: http://java.sun.com/j2se/1.4.2/docs/api/javax/crypto/Cipher.htmlhttp://java.sun.com/j2se/1.5.0/docs/api/javax/crypto/Cipher.html

39. Example: AES Encryption • The following sample encrypts a file “clear.txt”and save the output as a file named “encryptedfile”. // Create a cipher object with algorithm “AES”. Cipher cipher = Cipher.getInstance("AES"); //Set the Cipher object to ENCRYPT MODE //Initialise it with the encryption key “mykey”. //Previous example demostrated how to retrieve this key from a keystore. cipher.init(Cipher.ENCRYPT_MODE, mykey); //Create an input stream to read the file. File clr=new File(“clear.txt”); FileInputStream fi = new FileInputStream(clr); //Get the size of the file. long length = clr.length(); …..cont’d

40. Example: AES Encryption cont’d //Create a byte array with the size of the file. byte[] plaintext = new byte[(int) length]; //Read data into the byte array fi.read(plaintext); //Close file fi.close(); // Now encrypt the text and store it in the byte array ciphertext byte[] ciphertext = cipher.doFinal(plaintext); //Write encrypted text into the output file File enc=new File(“encryptedfile”) FileOutputStream fo = new FileOutputStream(enc); fo.write(ciphertext); fo.close();

41. Example: AES Decryption • The following sample decrypts a file “encryptedfile”and save the output as a file named “decryptedfile”. // Create a cipher object with algorithm “AES” Cipher cipher = Cipher.getInstance("AES"); //Set the Cipher object to DECRYPT MODE //Initialise it with the decryption key “mykey”. //Previous example demostrated how to retrieve this key from a keystore. cipher.init(Cipher.DECRYPT_MODE, mykey); //Create an input stream to read the file. File enc=new File(“encryptedfile”); FileInputStream fi = new FileInputStream(enc); //Get the size of the file. long length = enc.length(); …..cont’d

42. Example: AES Decryption cont’d //Create a byte array with the size of the file. byte[] ciphertext = new byte[(int) length]; //Read data into the byte array fi.read(ciphertext); //Close file fi.close(); // Now decrypt the text and store it in the byte array ciphertext byte[] plaintext = cipher.doFinal(ciphertext); //Write encrypted text into the output file File dec=new File(“decryptedfile”) FileOutputStream fo = new FileOutputStream(dec); fo.write(plaintexttext); fo.close();

43. Review Questions • Which of the following is stream cipher • DES 2) AES 3) Vernam • What is effective key size for 4-DES • 112 bits 2) 113 bits 3) 168 bits • Increasing key size from 56 bits to 128 bits, how many times more effort an attacker needs to spend in brute force attack? • 72 2) 2^72 3) 2^184

44. Individual Assignment 1 (5%) • Due in week 4 (please submit hardcopy during week 4 class) • Textbook 1.11 Exercises 3, 12, 15 (pages 32-33 in 3rd edition, pages 34-36 in 4th edition) • Textbook 2.13 Exercises 1, 13, 17, 19 (pages 91-93 in 3rd edition, pages 94-97 in 4th edition)