02. Usable Encryption. Blase Ur and Mainack Mondal March 28 th, 2018 CMSC / 33210
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1 02. Usable Encryption Blase Ur and Mainack Mondal March 28 th, 2018 CMSC /
2 This World of Ours 2
3 Encryption: A crash course 3
4 Encryption basics Putting information in code so that unauthorized people can t read it What might you want to encrypt? or text message Individual file Hard drive, USB stick Communication with a website Everything 4
5 One-way functions Easy to compute in one direction, but hard to compute in the other Hash function Small change in input big change in output md5( blase ) = 12b872adb2588c668d706d847fc1da7e Used for storing passwords Current research: make hashing slow (Older and less good) bcrypt: iterated hashes scrypt and Argon2: memory-hard 5
6 Trapdoor functions Easy to compute in one direction, but hard to compute in the other unless you have some extra information Encryption: reversible (if you know secret) this is a test Xe0yUqyOnY8JskyCQ2cYIg== Xe0yUqyOnY8JskyCQ2cYIg== and chicago (secret) this is a test 6
7 Two main encryption approaches Symmetric encryption: Same key used for encryption and decryption Requires key exchange (out of band) Prominent examples: AES, DES, etc. Asymmetric encryption: Different keys used for encryption and decryption Keypair: public key and private key Prominent examples: RSA, ElGamal, elliptic curve crypto 7
8 Properties of encryption Secrecy: Is Blase the only person who can decrypt my message? Integrity: Has someone tampered with Blase s message? Authenticity: Did this message really come from Blase? 8
9 Encryption historically Caesar shift Substitution cipher 9
10 Encryption historically Vigenère cipher Enigma machine 10
11 Block ciphers (symmetric) Old (deprecated): DES 3DES AES (Advanced Encryption Standard) Rijndael Cipher (chosen in 2001) 128-bit blocks 128-, 192-, or 256-bit keys RTmV0 No known* feasible* attacks on AES Timing side-channel attacks possible 11
12 Public key (asymmetric) crypto 1970s Present 12
13 Diffie-Hellman key exchange 1976: Diffie, Hellman, Merkle (Multiplicative group of integers modulus p) Generator g Prime number p Secrets (integers) x and y 13
14 Diffie-Hellman key exchange 14
15 RSA Generate a key: Choose primes p, q Calculate n = p q Private key: λ(n) = lcm(p-1, q-1) Choose integer e so 1 < e < λ(n) and e, λ(n) coprime Solve for d such that d e 1 (mod λ(n)) Release e and n as the public key, but keep d private To encrypt: c m e (mod n) To decrypt: c d Equivalent to m ed (mod n), which is just m 15
16 Usable encryption 16
17 Why do user studies? Purpose Assess needs Evaluate Understand tradeoffs Decide what to build Determine whether system meets requirements and what needs to be improved Decide which features/approaches/systems best fit particular needs Find root causes Determine where redesigns or new approaches are needed 17
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19 User study steps Identify research questions, metrics, and use cases Decide on type of study and design study protocol Develop detailed scripts, surveys, scenarios, incentives, instrumentation, prototypes, recruiting materials, etc. Obtain ethics approval Pilot and iterate on study design Collect data Analyze results Repeat some or all of these steps as needed 19
20 Usable security study challenges Keeping it real (ecological validity) Create realistic sense of risk (but not real risk) Provide realistic incentives Don t bias participants Measuring the right thing Design the right protocol Control the variables Instrument Observing infrequent events and small differences Legal, ethical, and practical issues 20
21 Why Johnny can t encrypt Why can t Johnny encrypt? Why was it so hard for participants to complete the tasks? How did the experimenters motivate the tasks and get participants to care about security? What role did attackers play in this user study? 21
22 Why Johnny can t encrypt Classic paper in usable security (1999) Interfaces are bad Metaphors are wrong (and confusing) Opaque process Key management is difficult 22
23 Why Johnny can t encrypt Security principles Unmotivated user Abstraction property Lack of feedback Barn door property Weakest link property Cognitive walkthrough vs. user test Bad metaphors 23
24 Why Glenn couldn t encrypt 24
25 Why Glenn couldn t encrypt Snowden image public domain from Laura Poitras. Document image CC by GNOME icon authors Greenwald image CC by David dos Dantos - mynewsdesk, NSA logo CC by the Electronic Frontier Foundation 25
26 Why Glenn couldn t encrypt Snowden image public domain from Laura Poitras. Document image CC by GNOME icon authors Greenwald image CC by David dos Dantos - mynewsdesk, NSA logo CC by the Electronic Frontier Foundation 26
27 Why Glenn couldn t encrypt Snowden image public domain from Laura Poitras. Document image CC by GNOME icon authors Greenwald image CC by David dos Dantos - mynewsdesk, NSA logo CC by the Electronic Frontier Foundation 27
28 Why Glenn couldn t encrypt 1:50 3:37, 4:10 4:58, 11:15 11:43 And yet, Greenwald still didn't bother learning security protocols. The more he sent me, the more difficult it seemed, he says. I mean, now I had to watch a f***ing video...? Snowden ended up reaching out to Laura Poitras instead
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34 Complexity of asymmetric encryption User creates a keypair Public key should be widely distributed Private key should never be distributed Private key protected with a password Two very different functions: Encrypting (secrecy) Signing (authenticity) Need person s key to communicate 34
35 (Just some) usability problems Encryption is rarely configured by default Public/private key encryption How to get someone s public key? How do I make it work on my phone? You often need a good password and you can t lose it or forget it Configuring multiple devices Only paranoid people use encryption 35
36 Open vs. closed ecosystem Open standards Don t require specific ecosystem or software Messaging apps (some security-focused) (Sometimes) can minimize configuration More centralized trust 36
37 Whatsapp 37
38 Important distinctions End-to-end encryption: messages encrypted between sender and recipient In theory, providers can t access Perfect forward secrecy: compromise of key does not compromise previous session keys As opposed to messages being encrypted between you and the company (e.g., WhatsApp), and then between the company and recipient 38
39 Do you have the right key? Man-in-the-middle attack Ways of trusting a person key binding: Public-key infrastructure (certifying authorities) Web of trust (someone you trust vouches) Exchange keys out of band Platform provider verifies Key servers: 39
40 Verifying you have the right key 40
41 Locally verifying key Out-of-band channel Interaction Bump, button press, shake devices Location-limited channel Bluetooth, sounds, wired connection Short string comparisons 41
42 Remotely verifying key GnuPG 3A70 F9A0 4ECD B5D7 8A89 D32C EDA0 A352 66E2 C53D OpenSSH ef:6d:bb:4c:25:3a:6d:f8:79:d3:a7:90:db:c9:b4:25 bubblebabble xucef-masiv-zihyl-bicyr-zalot-cevyt-lusobnegul-biros-zuhal-cixex OTR 4206EA15 1E C8BA9366 B972A136 C WhatsApp
43 Remotely verifying key 43
44 Current Research in Cryptography 44
45 Fully Homomorphic Encryption Homomorphism: transformation between sets that preserves relations between elements Allows for computation on encrypted data Partially homomorphic systems: Fully homomorphic systems (2010 now) 45
46 Searchable Encryption Can we have things (e.g., documents) that are encrypted yet still searchable? Conceptualize security roughly as leaking nothing but the results of the search queries Can we have a database that is encrypted yet can still be queried? Proxy that rewrites queries and returns decrypted results 46
47 Oblivious RAM Algorithms can give away information based on their access patterns Can we automatically rewrite algorithms so that their patterns of memory access don t leak information? 47
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