Set8 - CMSC 417 Computer Networks Prof. Ashok K Agrawala...

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CMSC 417 Computer Networks Prof. Ashok K Agrawala © 2011 Ashok Agrawala Set 8 December 11 CMSC417 Set 8 1
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Network Security CMSC417 Set 8 Cryptography Symmetric-Key Algorithms Public-Key Algorithms Digital Signatures Management of Public Keys Communication Security Authentication Protocols Email Security Web Security Social Issues Revised: August 2011
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Network Security CMSC417 Set 8 Security concerns a variety of threats and defenses across all layers
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Network Security (1) CMSC417 Set 8 Some different adversaries and security threats Different threats require different defenses
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Cryptography CMSC417 Set 8 Cryptography is a fundamental building block for security mechanisms. Introduction » Substitution ciphers » Transposition ciphers » One-time pads » Fundamental cryptographic principles »
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Introduction CMSC417 Set 8 The encryption model (for a symmetric-key cipher) Kerckhoff’s principle: Algorithms (E, D) are public; only the keys (K) are secret Alice Bob Trudy
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Substitution Ciphers CMSC417 Set 8 Substitution ciphers replace each group of letters in the message with another group of letters to disguise it Simple single-letter substitution cipher
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Transposition Ciphers CMSC417 Set 8 Transposition ciphers reorder letters to disguise them Simple column transposition cipher Key gives column order Column 5 6 7 8
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One-Time Pads (1) CMSC417 Set 8 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
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One-Time Pads (2) CMSC417 Set 8 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
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Fundamental Cryptographic Principles CMSC417 Set 8 1. Messages must contain some redundancy 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
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Symmetric-Key Algorithms CMSC417 Set 8 Encryption in which the parties share a secret key DES – Data Encryption Standard » AES – Advanced Encryption Standard » Cipher modes » Other ciphers » Cryptanalysis »
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Symmetric-Key Algorithms (1) CMSC417 Set 8 Use the same secret key to encrypt and decrypt; block ciphers operate a block at a time Product cipher combines transpositions/substitutions Permutation (transposition) box Substitution box Product with multiple P- and S-boxes
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Data Encryption Standard (1) CMSC417 Set 8 DES encryption was widely used (but no longer secure) DES steps A single iteration Contains transpositions
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This note was uploaded on 01/13/2012 for the course CMSC 417 taught by Professor Staff during the Fall '08 term at Maryland.

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Set8 - CMSC 417 Computer Networks Prof. Ashok K Agrawala...

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