1	Accountability and Freedom Butler Lampson Microsoft September 26, 2005
2	Real-World Security It’s about risk, locks, and deterrence. Risk management: cost of security < expected loss Perfect security costs way too much Locks good enough that bad guys break in rarely Bad guys get caught and punished enough to be deterred, so police / courts must be good enough. Can recover from damage at an acceptable cost. Internet security similar, but little accountability Can’t identify the bad guys, so can’t deter them
3	How Much Security Security is costly—buy only what you need You pay mainly in inconvenience If there’s no punishment, you pay a lot People do behave this way We don’t tell them this—a big mistake The best is the enemy of the good Perfect security is the worst enemy of real security Feasible security Costs less than the value it protects Simple enough for users to manage Simple enough for vendors to implement
4	Causes of Security Problems Exploitable bugs Bad configuration TCB: Everything that security depends on Hardware, software, and configuration Does formal policy say what I mean? Can I understand it? Can I manage it? Why least privilege doesn’t work Too complicated, can’t manage it The unavoidable price of reliability is simplicity —Hoare
5	Defensive strategies Locks: Control the bad guys Coarse: Isolate—keep everybody out Medium: Exclude—keep the bad guys out Fine: Restrict—Keep them from doing damage Recover—Undo the damage Deterrence: Catch bad guys, punish them Auditing, police, courts or other penalties
6	The Access Control Model
7	Isolation Attacks on: Program Isolation Policy
8	Access Control Mechanisms: The Gold Standard Authenticate principals: Who made a request Mainly people, but also channels, servers, programs (encryption implements channels, so key is a principal) Authorize access: Who is trusted with a resource Group principals or resources, to simplify management Can define by a property, e.g. “type-safe” or “safe for scripting” Audit: Who did what when? Lock = Authenticate + Authorize Deter = Authenticate + Audit
9	Making Isolation Work Isolation is imperfect: Can’t get rid of bugs TCB = 10-50 M lines of code Customers want features more than correctness Instead, don’t tickle them. How? Reject bad inputs Code: don’t run or restrict severely Communication: reject or restrict severely Especially web sites Data: don’t send; don’t accept if complex
10	Accountability Can’t identify bad guys, so can’t deter them Fix? End nodes enforce accountability Refuse messages that aren’t accountable enough or strongly isolate those messages Senders are accountable if you can punish them All trust is local Need an ecosystem for Senders becoming accountable Receivers demanding accountability Third party intermediaries To stop DDOS attacks, ISPs must play
11	Enforcing Accountability Not being accountable enough means end nodes will reject inputs Application execution is restricted or prohibited Communication is restricted or prohibited Information is not shared or accepted Access to devices or networks is restricted or prohibited
12	For Accountability To Work Senders must be able to make themselves accountable This means pledging something of value Friendship Reputation Money … Receivers must be able to check accountability Specify what is accountable enough Verify sender’s evidence of accountability
13	Accountability vs. Access Control “In principle” there is no difference but Accountability is about punishment, not locks Hence audit is critical Accountability is very coarse-grained
14	The Accountability Ecosystem Identity, reputation, and indirection services Mechanisms to establish trust relationships Person to person and person to organization A flexible, simple user model for identity Stronger user authentication Smart card, cell phone, biometrics Application identity: signing, reputation
15	Accountable Internet Access Just enough to block DDoS attacks Need ISPs to play. Why should they? Servers demand it; clients don’t get locked out Regulation? A server asks its ISP to block some IP addresses ISPs propagate such requests to peers or clients Probably must be based on IP address Perhaps some signing scheme to traverse unreliable intermediaries? High priority packets can get through
16	"Partition world into two parts" Partition world into two parts: Green Safer/accountable Red Less safe/unaccountable Two aspects, mostly orthogonal User Experience Isolation mechanism Separate hardware with air gap VM Process isolation
17	Without R\|G: Today
18	With R\|G
19	Must Get Configuration Right
20	Why R\|G? Problems: Any OS will always be exploitable The richer the OS, the more bugs Need internet access to get work done, have fun The internet is full of bad guys Solution: Isolated work environments: Green: important assets, only talk to good guys Don’t tickle the bugs, by restricting inputs Red: less important assets, talk to anybody Blow away broken systems Good guys: more trustworthy / accountable Bad guys: less trustworthy or less accountable
21	Configuring Green Green = locked down = only whitelist inputs Requires professional management Few users can make these decisions Avoid “click OK to proceed” To escape, use Red Today almost all machines are Red
22	R\|G User Model Dilemma People don’t want complete isolation They want to: Cut/paste, drag/drop Share parts of the file system Share the screen Administer one machine, not multiple … But more integration can weaken isolation Add bugs Compromise security
23	Data Transfer Mediates data transfer between machines Drag / drop, Cut / paste, Shared folders Problems Red → Green : Malware entering Green → Red : Information leaking Possible policy Allowed transfers (configurable). Examples: No transfer of “.exe” from R to G Only transfer ASCII text from R to G Non-spoofable user intent; warning dialogs Auditing Synchronous virus checker; third party hooks, ...
24	Where Should Email/IM Run? As productivity applications, they must be well integrated in the work environment (green) Threats—A tunnel from the bad guys Executable attachments Exploits of complicated data formats Choices Run two copies, one in Green and one in Red Run in Green and mitigate threats Green platform does not execute arbitrary programs Green apps are conservative in the file formats they accept Route messages to appropriate machine
25	R\|G and Enterprise Networks
26	Summary Security is about risk management Cost of security < expected loss Security relies on deterrence more than locks Deterrence requires the threat of punishment This requires accountability Accountability needs an ecosystem Senders becoming accountable Receivers verifying accountability Accountability limits freedom Beat this by partitioning: red \| green Don’t tickle bugs in green, dispose of red