Anonymity - Background R. Newman

Topics Defining anonymity Need for anonymity Defining privacy Threats to anonymity and privacy Mechanisms to provide anonymity Applications of anonymity technology

Anonymity Mechanisms Cryptography Steganography Traffic Analysis Prevention (TAP) Mixes, crowds Data sanitization/scrubbing k-anonymity

Security Security is relative Must know what we want Relative to security goals Relative to threats (adversaries) Must know what we want Must know what we are up against Must understand what we have to work with

Adversaries Global vs. Restricted Passive vs. Active All links vs. some links All network nodes vs. some or no nodes Passive vs. Active Passive – listen only Active – remove, modify, replay, or inject new messages Cryptography Assumptions All unencrypted contents are observable All encrypted contents are not, without key

Symmetric Key Cryptography One key, Kab, associated with entities A and B Same key used for encryption and decryption: C=E(M,Kab), M=D(C,Kab)=D(E(M,Kab)Kab) For message M, ciphertext C = {M}K Anyone with Kab can form ciphertext Anyone with Kab can decrypt C

Symmetric Key Cryptography One key, Kab, associated with entities A and B For message M, MIC or MAC uses hash fcn If only A and B have Kab, then MAC (authentication, not non-repudation) If group key, then MIC (integrity check, but not authentication) Depending on E, may require crypto hash fcn

Public Key Cryptography Two keys, K and K-1, associated with entity A Hard to get K-1 from K and vice versa (unless...) K is public key, K-1 is private key Keys are inverses: {{M}K}K-1 = {{M}K-1}K = M For message M, ciphertext C = {M}K Anyone can send A ciphertext using K Only A has K-1 so only A can decrypt C For message M, signature S = {M}K-1 Anyone can verify M,S using K Only A can sign with K-1

Details we omit Limit on size of M, based on size of K Need to format M to avoid attacks on PKC Use confounder to foil guessed ptxt attacks Typical use of one-way hash H to distill large M to reasonable size for signing Typical use of PKC to distribute symmetric key for actual encryption/decryption of larger messages See http://www.rsa.com/rsalabs/ for standards

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