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IS 302: Information Security and Trust Week 3: From DES to AES. 2012. Review. Kerckhoff principle Attacks to cryptosystem Caesar, Vigenere, Zimmerman, Vernam cipher Confusion and diffusion. Modern Symmetric Ciphers. DES AES. Block Ciphers vs Stream Ciphers. Block ciphers DES and AES…
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IS 302: Information Security and TrustWeek 3: From DES to AES 2012
Review • Kerckhoff principle • Attacks to cryptosystem • Caesar, Vigenere, Zimmerman, Vernam cipher • Confusion and diffusion
Modern Symmetric Ciphers • DES • AES
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
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• 1976 US Federal standard • 1990 DES design is optimal • Almost any change to DES weakens it • May 26, 2002, DES was superseded by AES • brute force attack can easily break 56-bit DES key • 1998 3DES valid till 2030 • extensively used in banking industry
Jeff Moser: http://www.moserware.com/2009/09/stick-figure-guide-to-advanced.html
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
Overall Diagram http://en.wikipedia.org/wiki/Data_Encryption_Standard f function
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/
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
DES encryption demo • Cryptoolencrypt/decryptsymmetric (modern) • DES (CBC) • 3-DES (CBC)
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
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
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
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
Four Modes of Block Ciphers • How are multiple blocks processed? • ECB: Electronic Code Book • CBC: Cipher Block Chaining • CFB: Cipher Feedback • OFB: Output Feedback
Electronic codebook (ECB) mode K Enc P3 P1 P2 C3 C1 C2 K Dec C3 C1 C2 P3 P1 P2
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
ECB vs CBC • Which mode would you choose? • Which one is semantically secure? Original image Encrypted with ECB Encrypted with CBC
Hands-On Exercise • AES Encryption and Decryption • OpenSSL • JCE • Download Lab.doc and follow instructions
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
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
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();
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
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();
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
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)