Lecture 12

Security (and privacy): 

Security (and privacy) Larry Rudolph With help from Srini Devedas, Dwaine Clark

Who cares about security?: 

Who cares about security? Especially important with wireless devices Are you allowed to use this device? authorization Are you who you say you are? authentication Is this device what you think it is? authentication

Where do we need security?: 

Where do we need security? Communicate thru insecure area Over internet Through air (bluetooth, 802.11, ..) USB keyboard, Monitor via cable Assumed to be secure Shared resources or devices

Public Key Cryptosystem: 

Public Key Cryptosystem Very quick overview Two keys: K1=Public Key, K2= Private Key Encrypt message, M: E(M,K1) Decrypt message M: D(M,K2) where M = D( E(M,K1), K2) and M = E( D(M,K2), K1) Given K1, M, E(M,K1) cannot easily compute K2.

Signing Messages: 

Signing Messages Alice andamp; Bob have keys A1, A2, B1, B2 When alice sends message M to bob E( M andamp; E(A, A2) , B1 ) Bob decrypts message and then uses Alice’s public key, A1 to decrypt the name Alice (A). Bob knows that Alice sent the msg ??

Certification Authority: 

Certification Authority No, it all depends on having correct public keys. How did Alice know that B1 is Bob’s public key? Use a certification authority: some trusted site that associates keys with names. Hierarchy of CA’s

Point to point communication: 

Point to point communication Public Key scheme: many people can send messages to Alice But basically a one-to-one protocol: With signing and with replies Not well suited: pervasive computing Environment filled with devices

Want group keys: 

Want group keys Want all students in SMA 5508 and 6.893 to access course web site Want all SMA students to access SMA’s main site, etc. A person belongs to many groups Grant access based on group Add/remove people from group

Slide9: 

Director’s Office TA TA Usage Scenario

Access Control: 

Access Control Security Model Useful mechanism in guarding access to resources Suitable for dynamic environments Each resource maintains a list referencing a set of valid keys Granting, delegating, revoking access user/application does not know accessibility of resource without explicitly attempting access Resource

SPKI/SDSI Introduction(Simple Public-Key Infrastructure/Simple Distributed Security Infrastructure): 

SPKI/SDSI Introduction (Simple Public-Key Infrastructure/Simple Distributed Security Infrastructure) A group key infrastructure Build secure, scalable distributed computing systems Fine-grained access control over an untrusted network

SPKI/SDSI Introduction(Simple Public-Key Infrastructure/Simple Distributed Security Infrastructure): 

SPKI/SDSI Introduction (Simple Public-Key Infrastructure/Simple Distributed Security Infrastructure) Designed by Ron Rivest, Butler Lampson and Carl Ellison Each public key is a CA Name certificate: defines a name in issuer’s name space Authorization certificate: grants a specific authorization from issuer to subject

SPKI/SDSI: Name Certificates: 

SPKI/SDSI: Name Certificates Local name spaces Groups

SPKI/SDSI: Name Certificate: 

SPKI/SDSI: Name Certificate (cert (issuer (name (public-key (rsa-pkcs1-md5 (e #23#) (n |AMMgMuKpqK13pHMhC8kuxaSeCo+yt8TadcgnG8bEo+erdrSBveY3C MBkkZqrM0St4KkmMuHMXhsp5FX71XBiVW1+JGCBLfI7hxWDZCxGTMg bR4Fk+ctyUxIv3CQ93uYVkg9ca6awCxtS0EI7sLuEB+HKuOLjzTsH+ +Txw9NAHq4r|))) friends)) (subject (public-key (rsa-pkcs1-md5 (e #23#) (n |AKg3tOzoJ5PGQ5q9jzxzwxE8o6bIZ6/cE8gEL+1xJa23viE3bz68ru hpD5muqJ+uyDCNxgAZ0JVXJazmX1QjiGudj9kEmuni8gJRLZRu0T5E3 K7OU2dodu0kdDg32kym7+ooZNe/F0zWGekfESeezyQ25kvNO3XQvMHX afWcYjRw|)))))

SPKI/SDSI: Authorization Model: 

SPKI/SDSI: Authorization Model Simple trust policy model Authorizations specified in flexible, user-defined tags Authorizations can be defined as specific or as general as desired Delegation (specific)

SPKI/SDSI: Authorization Certificate: 

SPKI/SDSI: Authorization Certificate (cert (issuer (public-key (rsa-pkcs1-md5 (e #23#) (n |AMMgMuKpqK13pHMhC8kuxaSeCo+yt8TadcgnG8bEo+erdrSBveY3C MBkkZqrM0St4KkmMuHMXhsp5FX71XBiVW1+JGCBLfI7hxWDZCxGTMg bR4Fk+ctyUxIv3CQ93uYVkg9ca6awCxtS0EI7sLuEB+HKuOLjzTsH+ +Txw9NAHq4r|)))) (subject (public-key (rsa-pkcs1-md5 (e #23#) (n |AKg3tOzoJ5PGQ5q9jzxzwxE8o6bIZ6/cE8gEL+1xJa23viE3bz68ru hpD5muqJ+uyDCNxgAZ0JVXJazmX1QjiGudj9kEmuni8gJRLZRu0T5E3 K7OU2dodu0kdDg32kym7+ooZNe/F0zWGekfESeezyQ25kvNO3XQvMHX afWcYjRw|)))) (tag (http (* set GET POST) (* prefix http://ostrich.lcs.mit.edu/demo/))) (propagate))

SPKI/SDSI: Tag: 

(tag (http (* set GET POST) (* prefix http://ostrich.lcs.mit.edu/demo/))) SPKI/SDSI: Tag Intuitively, a tag is a set of requests.

Proxy to Proxy: 

Proxy to Proxy Alice (Client Proxy) Bob (Server Proxy) Da (private key) Ea (public key) Alice’s client certs List of CA certs Db (private key) Eb (public key) ACL Server certs Set up SSL connection: Server auth Session key for privacy Freshness (nonce) Protection from MIM Initialization:

Proxy to ProxyCase 1: user’s key is directly on the ACL : 

Proxy to Proxy Case 1: user’s key is directly on the ACL Alice (Client) Bob (Server) Da (private key) Ea (public key) Alice’s client certs List of CA certs Db (private key) Eb (public key) ACL Server certs [tag]Da Response ACL: {Ec, Eb, Ea} Signed by alice

Proxy to Proxy: 

Proxy to Proxy Alice (Client Proxy) Bob (Server Proxy) Da (private key) Ea (public key) Alice’s client certs List of CA certs Db (private key) Eb (public key) ACL Server certs [tag]Da Rejected: [tag]Da, certs ACL Case 2: user’s key is 'indirectly' on the ACL Client performs certificate chain discovery. Server verifies certificate chain. ACL: {‘Eb friends’}

Certificate Chaining Example: 

Certificate Chaining Example Bob’s ACL says only MIT faculty are allowed to access his server. Alice’s first request is simply signed with Alice’s key, and Bob rejects this request. Alice’s second request contains a chain consisting of the following certificates: A certificate saying she is an LCS Professor A second certificate saying LCS Professors are MIT faculty

Certificate Chain Discovery(Client Proxy): 

Certificate Chain Discovery (Client Proxy) Derive certificate chains Input: device’s ACL, requestor’s public key, requestor’s set of signed certificates, tag Output: a chain of certificates leading from an entry on the ACL to the requestor’s public key. (The certificate chain consists of signed certificates. It proves that the requestor is authorized to perform the tag’s operations on the device.) * Recall, intuitively, a tag is a set of requests.

Certificate Chain Verification (Server Proxy): 

Certificate Chain Verification (Server Proxy) Verify certificate chains Input: device’s ACL, requestor’s public key, requestor’s certificate chain, tag Output: 1 if certificate chain proves that the public key is authorized to perform the tag’s operations on the device; 0 otherwise.

Proxy to Proxy: 

Proxy to Proxy Alice (Client Proxy) Bob (Server Proxy) Da (private key) Ea (public key) Alice’s client certs List of CA certs Db (private key) Eb (public key) ACL Server certs [tag]Da Rejected: [tag]Da, certs ACL Case 2 revisited user’s key is 'indirectly' on the ACL Signed request provides proof of authenticity of the request Certificate chain provides proof that the request is authorized

Example: Public resource: 

Example: Public resource Mary wants to turn on/off a public light switch. Light switch’s proxy may require requests to be signed for auditing purposes.

Example: user’s key directly on ACL: 

Example: user’s key directly on ACL Mary wants to log into an account on a dialup machine. ACL: {Ec, Ef, Em}

Example:user’s key is indirectly on ACL: 

Example:user’s key is indirectly on ACL Mary wants to play music on John’s speaker. ACL: {‘Ej friends’}

More “fun”: 

More 'fun' Integrating access control with name lookup services Trusting untrusting devices Using public terminals in Startbucks as a cache for handheld