Example of a product cipher Repeated substitution and transposition Feistel

Example of a product cipher repeated substitution and

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Example of a product cipher Repeated substitution and transposition Feistel Cipher LN4: Symmetric Key Cryptography 10 Classical Feistel Network For each round i ( i =1,2,..., n ), compute L i = R i- 1 R i = L i- 1 f(R i- 1 , K i ) where f is the round function and K i is the sub- key decryption is accomplished via R i- 1 = L i L i- 1 = R i f(R i- 1 , K i ) LN4: Symmetric Key Cryptography 11 Cipher Parameters Fiestel Cipher depends on : Block size: larger block sizes mean greater security Key Size: larger key size means greater security Number of rounds: multiple rounds offer increasing security, typically 16 rounds Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis. Round function: greater complexity means greater resistance to cryptanalysis Feistel Cipher Parameters LN4: Symmetric Key Cryptography 12 Symmetric Encryption Algorithms (DES) Data Encryption Standard (DES) The most widely used encryption scheme The algorithm is referred to as the Data Encryption Algorithm (DEA) DES is a block cipher processed in 64-bit blocks 56-bits key 8 parity bits are stripped off from the full 64-bit key (8 characters) 16 subkeys are generated for the 16 rounds Symmetric Encryption Algorithms
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4 LN4: Symmetric Key Cryptography 13 DES Subkey Generation - round # 1 drops 8 parity bits- effective key size becomes 56 bits permutes the bits and divides into two 28-bits rotates the bits left by single bit permutes and extracts 48 bits as a subkey LN4: Symmetric Key Cryptography 14 DES Subkey Generation One bit shift – round 1,2,9 and 16 Two bit shift for the remaining rounds LN4: Symmetric Key Cryptography 15 DES Round Each of the sixteen rounds takes a 64-bit block as input and produces a 64-bit block as output The output from the initial permutation is the input to round one Round one’s output is the input to round two Round two’s output is the input to round three The output from round sixteen is the 64-bit block of ciphertext LN4: Symmetric Key Cryptography 16 Single DES Round Similar to Fiestel Cipher structure 64-bit plaintext is divided into two 32-bit blocks (L &R) L i is the unchanged R i-1 (previous round) R i-1 goes through F function E table-expanded to 48bits and permuted 48 bits XORed with subkey K i Substitution produces 32-bit > 8 S-boxes > each takes 6 bits and produces 4 bits > transformation is defined by substitution tables > different substitution table for each S-box Permutes the output of S-box R i is L i-1 XORed with permuted output K i
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5 LN4: Symmetric Key Cryptography 17 DES Initial Permutation Table LN4: Symmetric Key Cryptography 18 Brute Force Attack: Exhaustive Key Search LN4: Symmetric Key Cryptography 19 Encryption Algorithms (Triple DES) Apply DES algorithm three times Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) C = ciphertext : E K [X] = encryption of X using key K P = Plaintext : D K [Y] = decryption of Y using key K Key length of 168 bits = 3 * 56 But, due to Meet-in-the-middle attack (see following slides), the
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  • Three '18
  • Cryptography, Advanced Encryption Standard, Block cipher, Symmetric Key Cryptography, Block cipher modes of operation

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