larger key size means greater security Number of rounds multiple rounds offer

# Larger key size means greater security number of

• Monash University
• FIT 3031
• Notes
• VarshineeS
• 64
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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 Feistel Cipher Parameters
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 22 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 character) 16 subkeys in 16 rounds Concern : Proved insecure in today’s fast processing power Symmetric Encryption Algorithms
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 23 Substitution & Permutation Substitution a binary word is replaced by some other binary word also known as S-box impractical to build 64-bit blocks multiple S-boxes of smaller blocks are used Example: for an input ’011001’ to an S-box, the output may be ’1001’ Permutation : A binary word has its bits reordered (permute) Also known as P-box Example: 1 st bit may become 7 th bit, 2 nd bit 12 th bit and so on Substitution & Permutation
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 24 DES Subkey Generation - round # 1 drop 8 parity bits effective key size 56 bits permute the bits and divide into two 28-bits rotate the bits left by single bit permute and extract 48 bits as a subkey
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 25 DES Subkey Generation One bit shift – round 1,2,9 and 16 Two bit shift for the remaining rounds
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 26 DES Round Each of the sixteen rounds takes a 64-bit block as inpu t 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
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 27 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
LN2:Symmetric Key Encryption : FIT3031 Information and Network Security 28 Encryption Algorithms (DES …) Encryption Algorithms (DES…):Time to break a

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• Fall '14
• Cryptography, Advanced Encryption Standard, Block cipher, Symmetric Key Encryption