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Network Security Chapter 8 Network security problems can be divided roughly into 4 closely intertwined areas: secrecy (confidentiality), authentication, nonrepudiation, and integrity control. Question: what does non-repudiation mean? What does “integrity” mean? • Cryptography • Symmetric-Key Algorithms • Public-Key Algorithms • Digital Signatures • Management of Public Keys • Communication Security • Authentication Protocols • Email Security -- skip • Web Security • Social Issues -- skip Revised: August 2011 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Network Security Security concerns a variety of threats and defenses across all layers Application Authentication, Authorization, and non-repudiation Transport End-to-end encryption Network Firewalls, IP Security Link Packets can be encrypted on data link layer basis Physical Wireless transmissions can be encrypted CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Network Security (1) Some different adversaries and security threats • Different threats require different defenses CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cryptography Cryptography – 2 Greek words meaning “Secret Writing” Vocabulary: • Cipher – character-for-character or bit-by-bit transformation • Code – replaces one word with another word or symbol Cryptography is a fundamental building block for security mechanisms. • Introduction » • Substitution ciphers » • Transposition ciphers » • One-time pads » • Fundamental cryptographic principles » CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Introduction Most popular computer-based encryption alternatives: 1. Symmetric key – same key used to both encrypt and decrypt; popular for securing data comm protocols. 2. Public key – different keys used to encrypt than to decrypt (public key, private key); popular for encrypting keys (and some data) The encryption model (for a symmetric-key cipher) • Kerckhoff’s principle: Algorithms (E, D) are public; only the keys (K) are secret − Kerckhoff’s principal: algorithmsTrudy are public, keys are private Alice Bob Note: Names indicate roles. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Substitution Ciphers Substitution ciphers replace each group of letters in the message with another group of letters to disguise it Simple single-letter substitution cipher CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Transposition Ciphers Transposition ciphers reorder letters but not disguise them Key gives column order, starting with the lowest letter Column 5 (G) 6 (K) 7 (M) 8 (U) Simple column transposition cipher CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 One-Time Pads (1) Simple scheme for perfect secrecy: • XOR message with secret pad to encrypt, decrypt • Pad is as long as the message and can’t be reused! − It is a “one-time” pad to guarantee secrecy Different secret pad decrypts to the wrong plaintext As long as the pads are never re-used or revealed, this approach is unbreakable. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 One-Time Pads (2) – Quantum Cryptography Approach using lasers and photonic filters leveraging quantum mechanics (qubit polarization). See pages 773-776 in textbook. Approach cannot be eavesdropped upon Alice sending Bob a one-time pad with quantum crypto. • Bob’s guesses yield bits; Trudy misses some • Bob can detect Trudy since error rate increases CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Fundamental Cryptographic Principles 1. Messages must contain some redundancy redundancy = info not needed to understand the message so that active intruders cannot send random junk and have it be interpreted as a valid msg • All encrypted messages decrypt to something • Redundancy lets receiver recognize a valid message • But redundancy helps attackers break the design 2. Some method is needed to foil replay attacks • Without a way to check if messages are fresh then old messages can be copied and resent • For example, add a date stamp to messages CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Symmetric-Key Algorithms Symmetric key – the same Secret key is used to both encrypt and decrypt the message. Popular for communication protocols because encryption/decryption can be done rapidly and easily adaptable to hardware automation. Encryption in which the parties share a secret key » Approaches using block ciphers – takes a n-bit block of plaintext as an input and transforms it using the key into an n-bit block of cipher text (and vice versa). » Question: What happens if the secret key becomes revealed? • DES – Data Encryption Standard » • AES – Advanced Encryption Standard » • Cipher modes » • Other ciphers » • Cryptanalysis » CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Symmetric-Key Algorithms (1) Use the same secret key to encrypt and decrypt; block ciphers operate a block at a time • Same number of bits in as bits out • Product cipher combines transpositions/substitutions Basic elements of product ciphers (Pages 779-780) Permutation Substitution Product with multiple P- and S-boxes (transposition) box box Question: Who can explain what is happening in the Substitution Box? CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Data Encryption Standard (1) DES encryption was widely used (but no longer secure) • Developed by IBM in 1977 • Plaintext encrypted in blocks of 64-bits; 56-bit key; 19 stages of computation Contains transpositions & substitutions DES steps A single iteration CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Data Encryption Standard (2) Introduced 1979, popular through the 1990s. Uses 2 DES keys for encryption and 3 iterations (stages) of DES, using key 1 for 1st and 3rd iteration and key 2 for 2nd iteration. Triple encryption (“3DES”) with two 56-bit keys • Gives an adequate key strength of 112 bits − Instructor addition: 3DES not currently considered “adequate” today for most uses • Setting K1 = K2 allows for compatibility with DES Triple DES encryption Triple DES decryption Question: Why is the length of the encryption key important? CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Advanced Encryption Standard (1) AES replaces 3DES. Introduced in 2001 by US Government (NIST). Supports block size of 128-bits and encryption key lengths of 128, 192, or 256 bits. AES is the successor to DES: • Symmetric block cipher, key lengths up to 256 bits • Openly designed by public competition (1997-2000) • Available for use by everyone • Built as software (e.g., C) or hardware (e.g., x86) • Winner was Rijndael cipher • Now a widely used standard CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Advanced Encryption Standard (2) AES uses 10 rounds for 128-bit block and 128-bit key • Each round uses a key derived from 128-bit key • Each round has a mix of substitutions and rotations • All steps are reversible to allow for decryption Round keys are derived from 128-bit secret key CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cipher Modes (1) Cipher Modes prevent the same msg from being encrypted to the same bits each time. Question: Why is such variance desirable? Cipher modes set how long messages are encrypted • Instructor addition: 5 cipher modes for symmetric-key encryption • Each has strengths/weaknesses and preferential deployment usages 1. Encrypting each block independently, called ECB (Electronic Code Book) mode, is vulnerable to shifts With ECB mode, switching encrypted Leslie gets a blocks gives a different but valid message large bonus! CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cipher Modes (2) CBC protects against code books (i.e., same plaintext block will not result in the same ciphertext block); computed using a 64-bit block 2) CBC (Cipher Block Chaining) is a widely used mode • Chains blocks together with XOR to prevent shifts • Has a random IV (Initial Value) for different output CBC mode encryption CBC mode decryption CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cipher Modes (3) Cipher feedback mode enables a byte-by-byte encryption (no larger blocks) There are many other modes with pros / cons, e.g., 3) cipher feedback mode is similar to CBC mode but can operate a byte (rather than a whole block) at a time Encryption Decryption CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cipher Modes (4) 4) A stream cipher uses the key and IV to generate a stream that is a one-time pad; can’t reuse (key, IV) pair • Doesn’t amplify transmission errors like CBC mode Encryption Decryption Keystream is a sequence of output blocks that acts like a one-time pad. CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cipher Modes (5) Counter mode is often used for encrypting accessible stored data such as databases 5) Counter mode (encrypt a counter and XOR it with each message block) allows random access for decryption Encryption above; repeat the operation to decrypt CN5E by Tanenbaum & Wetherall, © Pearson
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