1. 1 Computer Security in the Real World Butler Lampson What people want from computer security is to be as secure with computers as they are in the real world. Real-world security is about value, locks, and police. When it works, you get good enough locks (not too many break-ins), good enough police (so break-ins aren't a paying business), and minimum interference with daily life. Computer security is hard because people don’t trust new things (especially when they don’t understand them), and computers are fast and complicated. The kind of computer break-ins most people care about are vandalism or sabotage that damages information or disrupts service, theft of money or information, and loss of privacy. Some people think that because computers are precise, perfect computer security should be possible. I'll explain why this is wrong, and talk about what kind of security is practical and how to get it.

2. 2 Computer Security in the Real World Butler Lampson Microsoft Outline What security is about Operating systems security Network security How they fit together

3. 3 Security: The Goal People believe that computers are as secure as real world systems, and it’s true. This is hard because: –People don’t trust new things. –Computers can do a lot of damage fast. –There are many places for things to go wrong. –Anonymous attacks are easy across a network.

4. 4 Real-World Security It’s about value, locks, and police.  Locks good enough that bad guys don’t break in very often.  Police and courts good enough that bad guys that do break in get caught and punished often enough.  Interferes with daily life less than value of loss.

5. 5 Dangers Vandalism or sabotage that –damages information –disrupts service Theft of money Theft of information Loss of privacy integrity availability integrity secrecy

6. 6 Vulnerabilities Bad (buggy or hostile) programs Bad (careless or hostile) people giving instructions to good programs Bad guy tapping or interfering with communications

7. 7 Defensive strategies Keep everybody out –Isolation Keep the bad guy out –Code signing, firewalls Let him in, but keep him from doing damage –Sandboxing, access control Catch him and prosecute him –Auditing, police

8. 8 The Access Control Model Guards control access to valued resources. Reference monitor Object Do operation Resource Principal GuardRequest Source

9. 9 Mechanisms—The Gold Standard Authenticating principals  Mainly people, but also machines, programs Authorizing access.  Usually for groups of principals Auditing Trusted computing base

10. 10 Levels of Security—Defense in Depth Network, with a firewall Operating system, with sandboxing –Basic OS (such as NT) –Higher-level OS (such as Java) Application that checks authorization directly All need authentication

11. 11 Why We Don’t Have “Real” Security People don’t buy it –Danger is small, so people buy features instead Secure systems do less because they’re older  Security is a pain »It has to be configured correctly »Users have to authenticate themselves Systems are complicated, so they have bugs. KISS – Keep It Simple, Stupid

12. 12 Operating System Security Assume secure channel from user Authenticate user by local password Map user to her SID + group SIDs –Local database for group memberships Access control by ACL on each resource –OS kernel is usually the reference monitor –Any RPC target can read SIDs of its caller ACLs are lists of SIDs –A program has SIDs of its logged in user

13. 13 NT Domain Security Just like OS except for authentication OS does RPC to domain for authentication –Secure channel to domain –Just do RPC(user, password) to get user’s SIDs Domain may do RPC to foreign domain –Pairwise trust and pairwise secure channels –SIDs include domain ID

14. 14 Distributed Systems Are Different Autonomous parts –In equipment –In management Fault tolerant –Partly broken but still working

15. 15 Web Security Today Server: Simplified from single OS –Establish secure channel with SSL –Authenticate user by local password (or certificate) –ACL on right to enter, or on user’s private state Browser: Basic authentication –Of server by DNS lookup, or by SSL + certificate –Of programs by supplier’s signature »Good programs run as user »Bad ones rejected or totally sandboxed

16. 16 Principals Authentication:Who sent a message? Authorization:Who is trusted? Principal — abstraction of “who”: –People Lampson, Gray –Machines SN12672948, Jumbo –Services microsoft.com, Exchange –Groups UW-CS, MS-Employees

17. 17 What Principals Do Principal says statement –Lampson says “read /MSR/Lampson/foo” –Microsoft-CA says “Lampson's key is #7438”

18. 18 Says things directlyC says s Has knownpossible receiverssecrecy possible sendersintegrity Examples –Within a node: operating system (pipes, etc.) –Between nodes: »Secure wiredifficult to implement »Network fantasy for most networks »Encryptionpractical Secure Channel

19. 19 Speaks For Principal A speaks for B: A  –Meaning: if A says something, B says it too. »Thus A is stronger than B. –Examples »Lampson  MSR group of people »Server-1  MSR-NFS group of servers »Key #7438  Lampson key for Lampson Handoff rule: If A says “B  A” then B  A –Reasonable if A is competent.

20. 20 Secure Channels via Encryption The channel is defined by the key: –Decrypt with K, encrypt with K –1 –K says s is a message which K can decrypt. If only A knows K –1, then K  A.

21. 21 Access control lists (ACLs) –An object O has an ACL that says: “Principal P may access O.” »Lampson may read and write doc »MSR may append to log ACLs must use names for principals –so that people can read them. For manageability –ACLs should be length 1—use groups of principals –ACLs should protect big groups of resources Authorization with ACLs P  r/w O Lampson  r/w doc MSR  r/w log

22. 22 Names: SDSI/SPKI A name is local to some name space A name space is defined by a key The key can bind names in its name space –K microsoft  K microsoft / Lampson –K microsoft says “K lampson  K microsoft / Lampson ” Path names can start from anywhere –K lampson / friends = K microsoft / Lampson/friends

23. 23 K lampson /DNS/MIT/Rivest =K MIT /Rivest =K lampson /Ron Names K Lampson K Rivest K Microsoft K DNS K MIT Microsoft DNS Rivest DNS MicrosoftMIT Lampson DNS Ron

24. 24 Authenticating a Channel Who can send on a channel? –C  P; C is the channel, P the sender. Just binding a name. In Microsoft’s name space –K microsoft says “K ws  K microsoft / WS ” –K microsoft says “K lampson  K microsoft / Lampson ” This is transitive, so –K lampson says “K DES  K lampson ” plus implies –K DES  K microsoft / Lampson

25. 25 Checking Access Givena requestQ says “ read O” an ACLP may r/w O = P  r/w O Check Q speaks for PQ  P rights are enough r/w  read Q  P  r/w O henceQ  r/w O

26. 26 Compatibility with Local OS? (1) Put network principals on OS ACLs (2) Let network principal speak for local one –Rivest@lcs.mit.edu  Redmond\rivest –Use network authentication »replacing local or domain authentication –Users and ACLs stay the same (3) Assign SIDs to network principals –Do this automatically –Use network authentication as before

27. 27 Groups and Group Credentials A group is a principal; its members speak for it –Lampson  MSR –Rashid  MSR –... Proving group membership: Use certificates. –K microsoft says “ Lampson  K microsoft / MSR ” which is just like –K microsoft says “K lampson  K microsoft / Lampson ” Must have groups of principals for manageability –ACLs should be of length 1

28. 28 Compound Principals (A andB) says s =A says s andB says s (A orB) says s =A says s orB says s (A asB) says s =A says “B says s” –Quoting or multiplexing (A forB) says s  (B and (A as B)) says s –Delegation. Examples: »Workstation-21 for Lampson »Payroll for Lampson

29. 29 Authenticating Programs A program can be authenticated by a digest: –K microsoft says “If image I has digest X then I is Word ” formally“X  K microsoft / Word ” This certificate makes node N willing to run I if Word is on the ACL for running on N. It also makes N assert that the running I is Word. –K N says “K N as port15  K microsoft / Word ” Applications –Virus control –Least privilege

30. 30 Authenticating Systems A machine N can store its own secret key A system can speak for another system: –K microsoft says “ N  K microsoft / Word ” This certificate lets N convince others that N is authorized to run Word –K N says “ K N as port15  K microsoft / Word ”

31. 31 Auditing Checking access: –Givena request Q says “ read O ” an ACL P may r/w O –Check Q speaks for PQ  P rights suffice r/w  read Auditing—Each step is justified by –a signed statement (certificate), or –a handoff rule

32. 32 Summary Gold standard –AuthenticationWho said it? –AuthorizationWho is trusted? –Auditing What happened? Theory –Principals: keys, names, compound –Speaks for: handoff, groups Trusted computing base –Keep it small and simple. –Validate each component carefully.

33. 33 References Why “real” security is hard –www.cl.cam.ac.uk/users/rja14 Distributed system security –Lampson et al. TOCS 10, 4 (Nov. 1992) –Wobber et al. TOCS 12, 1 (Feb. 1994) Simple Distributed Security Infrastructure (SDSI) –theory.lcs.mit.edu/~cis/sdsi.html Simple Public Key Infrastructure (SPKI) –ftp://ds.internic.net/internet-drafts/draft-ietf-spki- cert-structure-02.txt