Theory and Design of Low-latency Anonymity Systems (Lecture 1) Paul Syverson U.S. Naval Research Laboratory syverson@itd.nrl.navy.mil http://www.syverson.org 1
Course Outline Lecture 1: • Usage examples, basic notions of anonymity, types of anonymous comms systems • Crowds: Probabilistic anonymity, predecessor attacks Lecture 2: • Onion routing basics: simple demo of using Tor, network discovery, circuit construction, crypto, node types and exit policies • Economics, incentives, usability, network effects 2
Course Outline Lecture 3: • Formalization and analysis, possibilistic and probabilistic definitions of anonymity • Hidden services: responder anonymity, predecessor attacks revisited, guard nodes Lecture 4: • Link attacks • Trust 3
Preliminaries Lots of collaborators in what I am presenting. Some of the main ones, alphabetically: George Danezis, Roger Dingledine, Matt Edman, Joan Feigenbaum, Aaron Johnson, Nick Mathewson, Lasse Øverlier I try to remember to cite work of others as I go. Full citations should be in.... 4
Preliminaries Book forthcoming in 2007. Full draft in 1-3 months. We would be happy to give a draft to any attendee of these lectures. Especially we would like to get your comments. Contact George or me if you want a copy. 5
Preliminaries Please interrupt if you have questions, want clarification, etc. 6
Preliminaries Please interrupt if you have questions, want clarification, etc. In bocca al lupo. 7
Anonymous communications Technical Governmental/Social 1. What is it? 2. Why does it matter? 3. How do we build it? 8
1. What is anonymity anyway? 9
Informally: anonymity means you can't tell who did what “Who wrote this blog post?” “Who's been viewing my webpages?” “Who's been emailing patent attorneys?” 10
Formally: anonymity means indistinguishability within an “anonymity set” Alice1 Alice2 Attacker can't distinguish Alice3 which Alice is talking to Bob Alice4 Bob Alice5 Alice6 Alice7 Alice8 .... 11
Formally: anonymity means indistinguishability within an “anonymity set” Alice1 Alice2 Attacker can't distinguish Alice3 which Alice is talking to Bob Alice4 Bob Alice5 Alice6 Alice7 Alice8 .... Can't distinguish? Basic anonymity set size Probability distribution within anonymity set .... 12
We have to make some assumptions about what the attacker can do. Alice Anonymity network Bob watch Alice! watch (or be!) Bob! Control part of the network! Etc, etc. 13
Anonymity isn't confidentiality: Encryption just protects contents. “Hi, Bob!” “Hi, Bob!” <gibberish> Alice attacker Bob 14
Anonymity isn't steganography: Attacker can tell that Alice is talking; just not to whom. Bob1 Alice1 Anonymity network Alice2 Bob2 ... AliceN 15
Anonymity isn't steganography: Attacker can tell that Alice is talking; just not to whom. Bob1 Alice1 Anonymity network Alice2 Bob2 ... AliceN Wrinkle: Alice may be trying to hide that she is talking to the anonymity network. 16
Anonymity isn't just wishful thinking “You can't prove it was me!” “Promise you won't look!” “Promise you won't remember!” “Promise you won't tell!” “I didn't write my name on it!” “Isn't the Internet already anonymous?” 17
Anonymity isn't just wishful thinking “You can't prove it was me!” Often statistical likelyhood matters more than legal proof . “Promise you won't look!” Will others have incentives & ability to keep promises? “Promise you won't remember!” Our goal is technical “Promise you won't tell!” protections without reliance on policy promises. Not what we're talking “I didn't write my name on it!” about. “Isn't the Internet already anonymous?” No! 18
2. Why does anonymity matter? 19
Anonymity serves different interests for different user groups. Governments Businesses “It's traffic-analysis “It's network security!” resistance!” Anonymity “It's reachability and “It's privacy!” censorship circumvention!” Private citizens Human rights advocates 20
Regular citizens don't want to be watched and tracked. “I sell the logs.” Blogger Hostile Bob Alice “Oops, I lost the logs.” 8-year-old Incompetent Bob Alice “Hey, they aren't Indifferent Bob Sick my secrets.” Alice .... Name, address, (the network can track too) age, friends, Consumer interests Alice (medical, financial, etc), unpopular opinions, Union illegal opinions.... member 21 Alice
Many people don't get to see the internet that you can see... 22
23
24
25
26
27
and they can't speak on the internet either... 28
It's not only about dissidents in faraway lands 29
Regular citizens don't want to be watched and tracked. “I look for you to Crime Stalker Bob do you harm.” Target Alice Human Censor/Blocker “I control your Rights Bob worldview and who Worker you talk to.” Alice “I imprison you for .... Name, address, seeing/saying the age, friends, wrong things.” Oppressed interests Alice (medical, financial, etc), unpopular opinions, illegal opinions.... 30
Law enforcement needs anonymity to get the job done. Investigated “Why is alice.fbi.gov reading my website?” suspect “Why no, alice.localpolice.gov! Officer Sting I would never sell counterfeits on ebay!” Alice target “Is my family safe if I Organized go after these guys?” Crime Witness/informer “Are they really going to ensure Anonymous Alice my anonymity?” tips 31
Businesses need to protect trade secrets... and their customers. “Oh, your employees are reading Competitor our patents/jobs page/product sheets?” “Hey, it's Alice! Give her the 'Alice' version!” Competitor “Wanna buy a list of Alice's suppliers? AliceCorp What about her customers? Compromised What about her engineering network department's favorite search terms?” Compromised/ “We attack Alice's customers with malicious malware, and watch for when she notices us.” hosts 32
Governments need anonymity for their security “What will you bid for a list of Baghdad IP addresses that get email from .gov?” Untrusted ISP “What bid for the hotel room from which Agent someone just logged in to foo.navy.mil?” Alice Compromised “What does the CIA Google for?” service 33
Aside: other benefits of an anonymity system Besides protecting affiliation, etc. can provide “poor man’s VPN”. Access to the internet despite • Network port policy disconnects • DNS failure 34
Governments need anonymity for their security “Do I really want to reveal my internal network topology?” Shared network “Do I want all my partners to know extent/pattern of my comms with Coalition other partners?” member Hostile Alice “How can I establish network communication with locals without a trusted network?” Semitrusted “How can I avoid selective blocking of network my communications?” 35
You can't be anonymous by yourself: private solutions are ineffective... Alice's small Citizen “One of the 25 ... anonymity net Alice users on AliceNet.” Officer Municipal Investigated Alice “Looks like a cop.” anonymity net suspect AliceCorp AliceCorp Competitor/ “It's somebody at anonymity net malware host AliceCorp!” 36
... so, anonymity loves company! Citizen “???” ... Alice Officer Investigated Alice Shared “???” suspect anonymity net AliceCorp Competitor “???” 37
Don't bad people use anonymity? 38
Current situation: Bad people on internet are doing fine Trojans Viruses Exploits Botnets Zombies Espionage Phishing DDoS Spam Extortion 39
Giving good people a fighting chance -Resist DDoS -Reduce malware Anonymity Network -Resist -Encourage - Protect operations Identity Theft informants -Protect free speech and cyberstalking and -Freedom of access -Protect kids online analysts/operatives 40
3. How does anonymity work ? 41
Dining Cryptographers 42
Dining Cryptographers 43
Dining Cryptographers H T T 44
Dining Cryptographers B: Different H T T A: Different C: Same 45
Dining Cryptographers B: Same (Lie) H T T A: Different (True) C: Same (True) Number of "Different"s odd: Signal 1 Number of "Different"s even: No Signal 0 46
Dining Cryptographers (DC Nets) Invented by Chaum, 1988 Strong provable properties Versions without collision or abuse problems have high communication and computation overhead Don't scale very well 47
Mixes 48
49
50
51
52
53
Mixes Invented by Chaum 1981 (not counting ancient Athens) As long as one mix is honest, network hides anonymity up to capacity of the mix Sort of Flooding - Trickling - Many variants Timed - Pool - ... 54 -
Recommend
More recommend
