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Advanced Operating Systems Lecture notes. Dr. Clifford Neuman Dr. Dongho Kim University of Southern California Information Sciences Institute. Focus on Authorization. Focusing on authorization and the management of policies used in the authorization decision.
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Advanced Operating Systems Lecture notes Dr. Clifford Neuman Dr. Dongho Kim University of Southern California Information Sciences Institute
Focus on Authorization • Focusing on authorization and the management of policies used in the authorization decision. • Not really new - this is a reference monitor. • Applications shouldn’t care about authentication or identity. • Separate policy from mechanism • Authorization may be easier to integrate with applications. • Hide the calls to the key management and authentication functions.
Generic Authorization and Access-control API • Allows applications to use the security infrastructure to implement security policies. gaa_get_object_eacl function called before other GAA API routines which require a handle to object EACL to identify EACLs on which to operate. Can interpret existing policy databases. gaa_check_authorizationfunction tells application whether requested operation is authorized, or if additional application specific checks are required GAA API SC,obj_id,op input gaa_get_ object_eacl gaa_check_ authorization Application output Yes,no,maybe
Credential transport (needed) • The GAA-API gets user & connection info from Security Context: • Evaluated and unevaluated credentials • Delegated authority • Cross-calls to transport to retrieve additional creds • The security context is provided as: • Output from GSS-API (requires many calls) • Credentials from transport or session protocols • SSL, ARDP • Other extensions are needed: • IPSec, pulled from Kernel, other extensions
Evaluation of credentials POLICY GAA API 5a EACL 5 gaa_get_object_eacl App 6 . . . 7 6b 1 4 gaa_check_authorization Transport Mechanism 6a 4a GAA API Security Context 2 3 GSS-API LIBRARY
Integrating security services • The GAA-API calls must be made by applications. • This is a major undertaking, but one which must be done no matter how one chooses to do authorization. • These calls are at the control points in the app • They occur at auditable events, and this is where records should be generated for ID systems • They occur at the places where one needs to consider dynamic network threat conditions. • Adaptive policies use such information from ID systems. • They occur at the right point for billable events.
Electronic commerce • Some authorization policies do not require user authentication at all - just that an item is paid for. • Policy specifies required payment. • Cross call to credential transport retrieves payment credentials and grants access. • If application used GAA-API, no change to the application is necessary, simply specify the payment policy instead of a more traditional identity based policy.
ID and Audit relation to GAA-API THREAT CONDITION UNDER ATTACK POLICY GAA API 5a EACL 5 gaa_get_object_eacl App 6 . . . 7 6b 1 4 gaa_check_authorization Transport Mechanism 6a 4a GAA API Security Context 2 3 GSS-API LIBRARY SECURITY AUDIT RECORDS
Application based ID • Without the GAA-API • Convince each application developer to add calls to audit functions in addition to all the other security calls they make (good luck). Of course it needs to do authentication too. • With the GAA-API • Get developers to use the GAA for authorization decisions instead of making multiple calls to implement their own authorization database. • Create module for GAA implementation that generates audit records according to policy. • Write policy (inc. adaptive or credential based) that says when to generate audit records.
CSci555: Advanced Operating SystemsLecture 7 - October 10 and 11, 2002Security Architecture Dr. Clifford Neuman University of Southern California Information Sciences Institute
Key distribution • Conventional cryptography • Single key shared by both parties • Public Key cryptography • Public key published to world • Private key known only by owner • Third party certifies or distributes keys • Certification infrastructure • Authentication
or Needham Schroeder KDC 1 2 S C ,4,5 3 Authentication w/ Conventional Crypto • Kerberos
DS 2 3 1 Authentication w/ PK Crypto • Based on public key certificates S C
KDC TGS 3. TgsReq 2. T+{Reply}Kc 1. Req 4. Ts+{Reply}Kt C S 5. Ts + {ts}Kcs Kerberos Third-party authentication service • Distributes session keys for authentication, confidentiality, and integrity
Public Key Cryptography (revisited) • Key Distribution • Confidentiality not needed for public key • Solves n2 problem • Performance • Slower than conventional cryptography • Implementations use for key distribution, then use conventional crypto for data encryption • Trusted third party still needed • To certify public key • To manage revocation • In some cases, third party may be off-line
Certificate-Based Authentication Certification authorities issue signed certificates • Banks, companies, & organizations like Verisign act as CA’s • Certificates bind a public key to the name of a user • Public key of CA certified by higher-level CA’s • Root CA public keys configured in browsers & other software • Certificates provide key distribution
Certificate-Based Authentication (2) Authentication steps • Verifier provides nonce, or a timestamp is used instead. • Principal selects session key and sends it to verifier with nonce, encrypted with principal’s private key and verifier’s public key, and possibly with principal’s certificate • Verifier checks signature on nonce, and validates certificate.
Attacker Secure Sockets Layer (and TLS) C S Hello Hello + CertS {PMKey}Ks [CertC + VerifyC ] VerifyS Encryption support provided between Browser and web server - below HTTP layer Client checks server certificate Works as long as client starts with the correct URL Key distribution supported through cert steps Authentication provided by verify steps
Trust models for certification • X.509 Hierarchical • Single root (original plan) • Multi-root (better accepted) • SET has banks as CA’s and common SET root • PGP Model • “Friends and Family approach” - S. Kent • Other representations for certifications • No certificates at all • Out of band key distribution • SSH
Global Authentication Service • Pair-wise trust in hierarchy • Name is derived from path followed • Shortcuts allowed, but changes name • Exposure of path is important for security • Compared to Kerberos • Transited field in Kerberos - doesn’t change name • Compared with X.509 • X.509 has single path from root • X.509 is for public key systems • Compared with PGP • PGP evaluates path at end, but may have name conflicts
Capability Based Systems - Amoeba “Authentication not an end in itself” • Theft of capabilities an issue • Claims about no direct access to network • Replay an issue • Modification of capabilities a problem • One way functions provide a good solution • Where to store capabilities for convenience • In the user-level naming system/directory • 3 columns • Where is authentication in Amoeba • To obtain initial capability
Security Architectures • DSSA • Delegation is the important issue • Workstation can act as user • Software can act as workstation - if given key • Software can act as developer - if checksum validated • Complete chain needed to assume authority • Roles provide limits on authority - new sub-principal • Proxies - Also based on delegation • Limits on authority explicitly embedded in proxy • Works well with access control lists
Distributed Authorization • It must be possible to maintain authorization information separate from the end servers • Less duplication of authorization database • Less need for specific prior arrangement • Simplified management • Based on restricted proxies which support • Authorization servers • Group Servers • Capabilities • Delegation
Proxies • A proxy allows a second principal to operate with the rights and privileges of the principal that issued the proxy • Existing authentication credentials • Too much privilege and too easily propagated • Restricted Proxies • By placing conditions on the use of proxies, they form the basis of a flexible authorization mechanism
PROXY CERTIFICATE Conditions: Use between 9AM and 5PM Grantee is user X, Netmask is 128.9.x.x, must be able to read this fine print, can you Proxy Proxy Grantor Restricted Proxies • Two Kinds of proxies • Proxy key needed to exercise bearer proxy • Restrictions limit use of a delegate proxy • Restrictions limit authorized operations • Individual objects • Additional conditions +
1 2 C S 3 Authorization and Group Services R 1. Authenticated authorization request (operation X) 2. [operation X only]R, {Kproxy} Ksession 3. [operation X only]R, authentication using Kproxy
Central Authorization • Authorization server usesextended ACLs • Conditions are not evaluated, but insteadattached to credentials • Groups implemented by auth server • Server grants right to assert group membership • Application servers configuredto use authorization server • Minimal local ACL • Can use multiple Authorization servers
Applied Security • Electronic commerce • SSL Applies authentication and encryption • NetCheque applies proxies • SET applies certification • End system security a major issue • What we have today • Firewalls • Web passwords, encryption, certificates • Windows 2000 uses Kerberos