The contents of this chapter include all of the following: discussion on block cipher and stream cipher, detail the symmetric key cryptography, we explored feistel cipher and its structure, the DES was also discussed in detail. | Network Security Lecture 15 Presented by: Dr. Munam Ali Shah Part 2 (c) Symmetric Key Cryptography Summary of the previous lecture We had a discussion on block cipher and stream cipher. We discussed in detail the symmetric key cryptography. We explored Feistel Cipher and its structure The DES was also discussed in detail Feistel Cipher The plaintext is divided into two halves The two halves pass through n rounds of processing then combine to produce the cipher block. Each round has as input and derived from the previous round as well as a sub-key derived from the overall All rounds have the same structure A substitution is performed on the left half of the data. This is done by applying a round function F to the right half of the data followed by the XOR of the output of that function and the left half of the data. 4 Design Features of Feistel Network Block Size: (larger block means greater security) 64 bits. Key Size:56-128 bits. Number of Rounds: a single round offers inadequate . | Network Security Lecture 15 Presented by: Dr. Munam Ali Shah Part 2 (c) Symmetric Key Cryptography Summary of the previous lecture We had a discussion on block cipher and stream cipher. We discussed in detail the symmetric key cryptography. We explored Feistel Cipher and its structure The DES was also discussed in detail Feistel Cipher The plaintext is divided into two halves The two halves pass through n rounds of processing then combine to produce the cipher block. Each round has as input and derived from the previous round as well as a sub-key derived from the overall All rounds have the same structure A substitution is performed on the left half of the data. This is done by applying a round function F to the right half of the data followed by the XOR of the output of that function and the left half of the data. 4 Design Features of Feistel Network Block Size: (larger block means greater security) 64 bits. Key Size:56-128 bits. Number of Rounds: a single round offers inadequate security, a typical size is 16 rounds. Sub-key Generation Algorithms: greater complexity should lead to a greater difficulty of cryptanalysis. Round function: Again, greater complexity generally means greater resistance to cryptanalysis. 5 A simple way to represent DES Classical Feistel Network 7 Outlines of today’s lecture Block Cipher Principles We will explore the limitations of DES Another symmetric key cryptography that is Advanced Encryption Standard (AES) will also be discussed. Objectives You would be able to present an understanding of Symmetric Key Cryptography. You would be able use understand the phases involved in DES. Block Cipher Principles Most symmetric block ciphers are based on a Feistel Cipher Structure Input: n bits plaintext block Output: n bits ciphertext block For 2 bits, there are 2n possible plaintext block Reversible mapping Plaintext Ciphertext 00 11 01 10 10 00 11 01 Irreversible mapping Plaintext Ciphertext 00 11 01 10 10 00 11 10 Most symmetric block .
