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Data Communications and Computer Networks Chapter 6 CS 3830 Lecture 29

This lecture introduces the fundamental concepts of symmetric and public key cryptography within the realm of network security. It explains how Bob and Alice share a symmetric key to secure their communications and discusses algorithms like DES and AES, highlighting their structures and security levels. In addition, the lecture covers public key cryptography, including the RSA algorithm, its operational principles, and key generation process. Providing essential knowledge for future cybersecurity professionals, the lecture emphasizes the importance of encryption in safeguarding information across networks.

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Data Communications and Computer Networks Chapter 6 CS 3830 Lecture 29

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  1. Data Communications and Computer NetworksChapter 6CS 3830 Lecture 29 Omar Meqdadi Department of Computer Science and Software Engineering University of Wisconsin-Platteville Introduction

  2. K K A-B A-B K (m) m = K ( ) A-B A-B Symmetric key cryptography symmetric key crypto: Bob and Alice share know same (symmetric) key: K • e.g., key is knowing substitution pattern in mono alphabetic substitution cipher • Q: how do Bob and Alice agree on key value? encryption algorithm decryption algorithm ciphertext plaintext plaintext message, m K (m) A-B A-B 8: Network Security

  3. Symmetric key crypto: DES DES: Data Encryption Standard • US encryption standard [NIST 1993] • 56-bit symmetric key, 64-bit plaintext input • Implemented in hardware—fast! • How secure is DES? • DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than 1 day (about 10 years ago) • no known “backdoor” decryption approach 8: Network Security

  4. AES: Advanced Encryption Standard • new (Nov. 2001) symmetric-key NIST standard, replacing DES • processes data in 128 bit blocks • 128, 192, or 256 bit keys • brute force decryption (try each key) taking 1 sec on DES, takes 149 trillion years for AES 8: Network Security

  5. T7 T1 T6 T4 T3 T2 T8 T5 Block Cipher 64-bit input 8bits 8bits 8bits 8bits 8bits 8bits 8bits 8bits loop for n rounds 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 8 bits 64-bit scrambler 64-bit output • block ciphers: DES, AES 8: Network Security

  6. + Cipher Block Chaining m(1) = “HTTP/1.1” c(1) = “k329aM02” block cipher t=1 • cipher block: if input block repeated, will produce same cipher text: … m(17) = “HTTP/1.1” c(17) = “k329aM02” block cipher t=17 • cipher block chaining: XOR ith input block, m(i), with previous block of cipher text, c(i-1) • c(0) transmitted to receiver in clear • what happens in “HTTP/1.1” scenario from above? m(i) c(i-1) block cipher c(i) 8: Network Security

  7. Public key cryptography symmetric key crypto • requires sender, receiver know shared secret key • Q: how to agree on key in first place (particularly if never “met”)? public key cryptography • radically different approach [Diffie-Hellman76, RSA78] • sender, receiver do not share secret key • public encryption key known to all • private decryption key known only to receiver 8: Network Security

  8. + K (m) B - + m = K (K (m)) B B Public key cryptography + Bob’s public key K B - Bob’s private key K B encryption algorithm decryption algorithm plaintext message plaintext message, m ciphertext 8: Network Security

  9. K (K (m)) = m B B - + 2 1 Public key encryption algorithms Requirements: need K ( ) and K ( ) such that . . + - B B + given public key K , it should be impossible to compute private key K B - B RSA: Rivest, Shamir, Adleman algorithm 8: Network Security

  10. + - K K B B RSA: Choosing keys 1. Choose two large prime numbers p, q. (e.g., 1024 bits each) 2. Compute n = pq, z = (p-1)(q-1) 3. Choose e (with e<n) that has no common factors with z. (e, z are “relatively prime”). 4. Choose d such that ed-1 is exactly divisible by z. (in other words: ed mod z = 1 ). 5.Public key is (n,e).Private key is (n,d). 8: Network Security

  11. 1. To encrypt bit pattern, m, compute d e m = c mod n c = m mod n e (i.e., remainder when m is divided by n) d e m = (m mod n) mod n RSA: Encryption, decryption 0. Given (n,e) and (n,d) as computed above 2. To decrypt received bit pattern, c, compute d (i.e., remainder when c is divided by n) Magic happens! c 8: Network Security

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