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Modeling security

Modeling security. Models - encryption. Alice and Bob have the same key k Alice and Bob exchange encrypted messages Eve wants to get the plaintext. What can she do? What can Eve do? The model we studied: eavesdropping, known plaintext, chosen plaintext/ciphertext Bad news for the attacker

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Modeling security

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  1. Modeling security

  2. Models - encryption • Alice and Bob have the same key k • Alice and Bob exchange encrypted messages • Eve wants to get the plaintext. What can she do? • What can Eve do? • The model we studied: eavesdropping, known plaintext, chosen plaintext/ciphertext • Bad news for the attacker • Any reasonable algorithm (e.g. AES) beats all of these • The best current (2012) attack on AES in the model breaks it in time 2126.2 and more than 280 chosen ciphertexts.

  3. New security goals • We “know” how to do confidentiality, integrity and authentication. • Is anything else even needed • Hint: we need to work even with those we don’t completely trust • n-out-of n secret sharing • t-out-of n secret sharing

  4. Administrative Details • Grade – 100% test • Office hours :Wednesday 13:00-14:00 • E-mail – niv.gilboa@gmail.com • Tel. 054-6501047 • Bibliography: • Security engineering / R. Anderson • Foundations of Cryptography, I and II/ O. Goldreich

  5. Principles for the attacker • The model is a simplification • Complexity is your friend • Find the weakest link • Know the system • Model assumes that attacker has full knowledge. Real life is different. • Know the attacks

  6. Know the system • An encryption system includes • Users • Physical environment • Plaintext storage & plaintext input • Encryption software / hardware • (Almost always) – a larger hardware / software system such as a computer • A key • A communication channel • Any of these elements can be vulnerable • The model considers just the communication link and input to the encryption algorithm

  7. Know the Attacks • On users • Social attacks – bribe the user, threaten the user, swindle the user • Outside our scope • Physical environment • Input attacks: e.g. key loggers, a bug in the wall • Outside our scope

  8. More attacks • Device hardware • Side channel attacks – e.g. timing • Fault attacks – e.g. glitch • Optical attacks • Chemical attacks • Question: so many options, why isn’t this part of the model? • Answer: they all require physical proximity and that is considered impossible

  9. An economic approach • An attacker’s potential utility • Money, knowledge, ideology, vengeance • The risks • Cost of an attack • Probability of failure • Risk of getting caught • Example – table of attacks on encryption system

  10. What about the defense? • A breach / exploit / attack has costs • Defense has its own costs • Problems • All attack avenues must be adequately blocked • Attacker’s model: plans, capabilities, utility are imperfectly known • Principles • Know the attacks and the defenses • Layered defense • Redundancy is good • But, remember the utility!

  11. Shocking example Do you know what this is? Here’s a hint

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