Modeling security 1

Models - encryption r Alice and Bob have the same key k r Alice and Bob exchange encrypted messages r Eve wants to get the plaintext. What can she do? r What can Eve do? r The model we studied: eavesdropping, known plaintext, chosen plaintext/ciphertext r 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 and more than 2 80 chosen ciphertexts. 2

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

4 Administrative Details r Grade – 100% test r Office hours :Wednesday 13:00-14:00 r – r Tel r Bibliography:  Security engineering / R. Anderson  Foundations of Cryptography, I and II/ O. Goldreich

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

Know the system r 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 r Any of these elements can be vulnerable r The model considers just the communication link and input to the encryption algorithm 6

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

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

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

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

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