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Lecture notes on Computer and network security: Lecture 9 - Avinash Kak

Lecture 9, using block and stream ciphers for secure wired and wifi communications. The goals of this chapter are: To present 2DES and its vulnerability to the meet-in-the-middle attack, to present two-key 3DES and three-key 3DES, to present the five different modes in which a block cipher can be used in practical systems for secure communications,. | Lecture 9: Using Block and Stream Ciphers for Secure Wired and WiFi Communications Lecture Notes on “Computer and Network Security” by Avi Kak (kak@purdue.edu) February 11, 2016 9:19am c 2016 Avinash Kak, Purdue University Goals: • To present 2DES and its vulnerability to the meet-in-the-middle attack • To present two-key 3DES and three-key 3DES • To present the five different modes in which a block cipher can be used in practical systems for secure communications • To discuss stream ciphers and to review RC4 stream cipher algorithm • To review problems with the WEP protocol for home wireless networks • To review the Klein and PTW attacks on WEP • Using aircrack-ng to crack a WEP key CONTENTS Section Title Page 9.1 Multiple Encryptions with DES for a More Secure Cipher 3 9.2 Double DES 4 9.2.1 Can a Double-DES (2DES) Plaintext-to-Ciphertext Mapping be Equivalent to a Single-DES Mapping? 6 9.2.2 Vulnerability of Double DES to the Meet-in-the-Middle Attack 11 9.3 Triple DES with Two Keys 9.3.1 Possible Ways to Attack 3DES Based on Two Keys 16 18 9.4 Triple DES with Three Keys 22 9.5 Five Modes of Operation for Block Ciphers 24 9.5.1 The Electronic Codebook Mode (ECB) 28 9.5.2 The Cipher Block Chaining Mode (CBC) 38 9.5.3 The Cipher Feedback Mode (CFB) 40 9.5.4 The Output Feedback Mode (OFB) 43 9.5.5 The Counter Mode (CTR) 45 9.6 Stream Ciphers 48 9.7 The RC4 Stream Cipher Algorithm 52 9.8 WEP for WiFi 57 9.8.1 62 9.8.2 The PTW Attack for Figuring Out the WEP Root Key 71 9.8.3 9.9 The Klein Attack for Figuring Out the WEP Root Key Using the aircrack-ng Package to Break WEP in Under a Minute 73 Homework Problems 80 Computer and Network Security by Avi Kak Lecture 9 9.1: MULTIPLE ENCRYPTIONS WITH DES FOR A MORE SECURE CIPHER • As you already know, the DES cryptographic system was shown to not be very secure about 10 years ago. • We can obviously use AES cryptography that is designed to be extremely secure,