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WUSTL CSE 571S - Secret Key Cryptography

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5-1©2007 Raj JainCSE571SWashington University in St. LouisSecret Key Secret Key CryptographyCryptographyRaj Jain Washington University in Saint LouisSaint Louis, MO [email protected]/Video recordings of this lecture are available at:http://www.cse.wustl.edu/~jain/cse571-07/5-2©2007 Raj JainCSE571SWashington University in St. LouisOverviewOverview1. Data Encryption Standard (DES)2. International Data Encryption Algorithm (IDEA)3. Advanced Encryption Standard (AES)4. Ron’s Cipher 4 (RC4)Ref: Chapter 3 of the textbook.5-3©2007 Raj JainCSE571SWashington University in St. LouisEncryption SchemesEncryption Schemes! Symmetric = 1 Key/2 users = Secret Key! Asymmetric = Public Key = Public and Private Keys! Block: Message broken in to fixed size blocks! Synchronous: Key stream depends on the key and IV! Asynchronous: Key stream depends on key, IV, and previous cipher textAsymmetric CiphersBlock Ciphers Stream CiphersSynchronous Self-SynchronousSymmetric Ciphers5-4©2007 Raj JainCSE571SWashington University in St. LouisSecret Key EncryptionSecret Key Encryption! Also known as symmetric encryption! Encrypted_Message = Encrypt(Key, Message)! Message = Decrypt(Key, Encrypted_Message)! Example: Encrypt = division! 433 = 48 R 1 (using divisor of 9)5-5©2007 Raj JainCSE571SWashington University in St. LouisSecret Key CryptographySecret Key Cryptography! Block Encryption[KPS Fig 3-1]5-6©2007 Raj JainCSE571SWashington University in St. LouisBlock Encryption (Cont)Block Encryption (Cont)! Short block length ⇒ tabular attack! 64-bit block! Transformations:" Substitution: replace k-bit input blocks with k-bit output blocks" Permutation: move input bits around. 1 → 13, 2 → 61, etc.! Round: Substitution round followed by permutation round and so on5-7©2007 Raj JainCSE571SWashington University in St. LouisData Encryption Standard (DES)Data Encryption Standard (DES)! Published by National Bureau of Standards in 1977! For commercial and unclassified government applications! 8 octet (64 bit) key. Each octet with 1 odd parity bit ⇒ 56-bit key! Efficient hardware implementation! Used in most financial transactions! Computing power goes up 1 bit every 2 years! 56-bit was secure in 1977 but is not secure today! Now we use DES three times ⇒ Triple DES = 3DES5-8©2007 Raj JainCSE571SWashington University in St. LouisDES StepsDES Steps! Total 18 steps: Initial permutation, 16 mangler rounds, Inverse of initial permutation[KPS Fig 3-2]5-9©2007 Raj JainCSE571SWashington University in St. LouisInitial and Final PermutationInitial and Final Permutation! Input bit 58 goes to output bit 1Input bit 50 goes to output bit 2, …5-10©2007 Raj JainCSE571SWashington University in St. LouisDES RoundDES Round[KPS Fig 3-6]5-11©2007 Raj JainCSE571SWashington University in St. LouisMangler FunctionMangler Function! 32-bit Rnand 48 bit Kn! 32 bit Rn= 8 × 4 bits ⇒ 8 × 6 bits! 48 bit key = 8 × 6 bits[KPS Fig 3-7]5-12©2007 Raj JainCSE571SWashington University in St. LouisDES Substitution BoxDES Substitution Box! Xor and S-Box5-13©2007 Raj JainCSE571SWashington University in St. LouisDES SDES S--Box (Cont)Box (Cont)! S-Box! 3. Permutation16thinput bit is the 1stoutput bit, …[KPS Fig 3-9][KPS Fig 3-17]5-14©2007 Raj JainCSE571SWashington University in St. LouisGeneration of PerGeneration of Per--Round KeysRound Keys! Divide in to 28-bit halves! Initial permutation: 57thbit of key becomes the 1stbit of output49thbit of key becomes the 2ndbit of output, …5-15©2007 Raj JainCSE571SWashington University in St. LouisGeneration of PerGeneration of Per--Round Keys (Cont)Round Keys (Cont)! Rotate left by 1 or 2 bits: In rounds 1, 2, 9, and 16 rotate 1-bit left, in other rounds rotate 2-bit left [KPS Fig 3-5]5-16©2007 Raj JainCSE571SWashington University in St. LouisGeneration of PerGeneration of Per--Round Keys (Cont)Round Keys (Cont)! Final permutation: 4 bits are discarded from each half ⇒ 24 bitsLeft-Half Right-Half14thinput bit becomes the 1stoutput bit, …! Bits 9, 18, 22, 25 of left half are discardedBits 35, 38, 53, and 54 of right half are discarded.5-17©2007 Raj JainCSE571SWashington University in St. LouisDES DecryptionDES Decryption! Identical to Encryption! Keys are used in reverse order5-18©2007 Raj JainCSE571SWashington University in St. LouisInternational Data Encryption AlgorithmInternational Data Encryption Algorithm! IDEA. Designed for software implementation! Encryption and Decryption are identical as in DES[KPS Fig 3-18]5-19©2007 Raj JainCSE571SWashington University in St. LouisInternational Data Encryption AlgorithmInternational Data Encryption Algorithm! 128-bit key is converted to 52 16-bit keys! Inverse of the encryption key is used for decryption in the reverse order! Has patent protection5-20©2007 Raj JainCSE571SWashington University in St. LouisAdvanced Encryption Standard (AES)Advanced Encryption Standard (AES)! Published by NIST in Nov 2001! Based on a competition won by Rijmen and Daemen (Rijndael)! Rijndael allows many block sizes and key sizes! AES restricts it to:" Block Size: 128 bits" Key sizes: 128, 192, 256 (AES-128, AES-192, AES-256)5-21©2007 Raj JainCSE571SWashington University in St. LouisBasic Structure of RijndaelBasic Structure of Rijndael! Number of Rounds Nr= 6 + max{Nb, Nk}! Nb= 32-bit words in the the block! Nk= 32-bit words in key! 4 rows × Nbcolumns (Nb = 4 for AES)[KPS Fig 3-23]5-22©2007 Raj JainCSE571SWashington University in St. LouisKey ExpansionKey Expansion! Key flows in octet by octet in 4-octet columns.! (Nr+1)Nbcolumns! Key expansion uses the same kind of primitive operations as the rounds! Rows, columns, round keys are numbered starting at 0, round numbers start at 15-23©2007 Raj JainCSE571SWashington University in St. LouisAES Primitive OperationsAES Primitive Operations! Xor! Substitution box! Rotation: column or row! MixColumn: Replace 32-bit word with another 32-bit word5-24©2007 Raj JainCSE571SWashington University in St. LouisRijndael SRijndael S--BoxBox[KPS Fig 3-27]5-25©2007 Raj JainCSE571SWashington University in St. LouisMixColumnMixColumn! 4 Input octets are used as an index to retrieve a column from the table[KPS Fig 3-26]5-26©2007 Raj JainCSE571SWashington University in St. LouisMixColumn (Cont)MixColumn (Cont)! Retrieved column is rotated vertically so that its top octet is in the same row as the input octet! Four rotated columns are xor'ed[KPS Fig 3-25]5-27©2007 Raj JainCSE571SWashington University in St.


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WUSTL CSE 571S - Secret Key Cryptography

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