Authentication in Distributed Systems: Theory, Concepts, and Tools

1. Authentication in Dist Systems Presented in cs294-4 P2P Systems by Sailesh Krishnamurthy Oct 1 2003

2. Access Control Authentication • Model elements: • Objects, resources (files,processes etc) • Requests to perform operations on objects • Principals - request sources, use Channels • Guardian - request validator • Easy in centralized systems • OS implements all channels, “knows” all principals • Hard in distributed systems • Long path from request originator • Different kinds of channels • Parts can break, get inaccessible

3. Paper contributions • Authentication theory for distributed systems (why theory ?) • Assumptions about authority and trust • Formally represent these assumptions • Carefully examine the assumptions • Describes a practical system based on the theory • Uses the theory to explain several mechanisms

4. Concepts • Principals • Simple • People: Lampson, Abadi • Machines: coeus, mammoth • Roles: manager, secretary • Named sets of principals: Services, Groups • Channels (principals that “say” things) • Wires, IO ports, N/W addr, encryption keys • Compound principals • Roles: Abadi as manager • Delegations: Mike for Burrows • Conjunctions: Lampson ^ Wobber

5. Statements • Statements - made by principals • Simple: request for file foobar.tex • Compound: Bob as secretary requests file bar.tex • Trusted Computing Base - keep it small

6. Statements • Primitive statements (e.g. “read file foo”) • s^s’ (s and s’) • s  s’ (s implies s’) • s  s’ (s is equivalent to s’) • If ‘A’ is a principal and ‘s’ is a statement, • A says s is a statement • If ‘A’ and ‘B’ are principals • AB (A speaks for B) is a statement • A | B ( A quotes B) is a statement

7. More on statements • Needham-Schroeder auth ticket • {Kab,A}Ks can be written:Kbs says Kab  A • If ‘s’ is an axiom it is represented as s. • Some axioms: • (A says s ^ A says (s  s’))  A says s’ • If s then A says s for every principal A • A says (s ^ s’)  (A says s) ^ (A says s’)

8. Principals • Let ‘A’ and ‘B’ be principals, ‘C’ be a channel. • (A^B) says s  (A says s) ^ (B says s) • (A|B) says s  (A says B says s) • What if A lied - B did not say s ? • (AB)  ((A says s)  (B says s))

9. Tools: handoffs, joint authorities • (A says (B  A))  (B  A) • If you see A says s, simply conclude ‘s’ if it is of the form B  A. • Simply states that A allows B to speak for itself. • Third Parties! • ((A’  A) ^ A’ says (BA))  (B  A) • Joint Authorities • ((A’^B  B) ^ (B  A’))  (B  A)

10. Joint authorities • Useful for certificate revocation • Refreshing requires source availability • Hard to make a source that is both: • Secure • Highly available • Solution: use 2 sources • One is highly secure with a long lifetime • Other is highly available, uses a short lifetime • However, both must agree to validate certificate

11. Channels and Encryption • Encryption channel: • Dec(K,Enc(K-1,x)) = x for a message x • Encryption enforces: • Security: If you know Enc(K-1,x) but not K, then should not be able to compute x • Integrity: If you know x but not K-1 should not compute a y such that Dec(K,y) = x • 2 forms: public keys, shared keys • Public keys can be simulated with stateless symmetric key

12. Named principals • Pull vs Push to get credentials • Pull: receiver looks up named principal to get credential • Push: sender provides credentials • CA: external certificate authority • Paths: able to trace “up” and “down” an authority tree • Groups:Members “speak for” groups • Certificates: P1 => G, P2 => G etc. • Or, for each member Pi, store Enc(Kp,Kg-1) in G’s database

13. Roles and Programs • Can be run with different priorities • Use digests (MD5) of prog text to confirm identity • Similar to booting a machine • Different OS’s on a given machine

14. Other stuff • Delegation • Login (session keys) • Authenticating IPC • Use authenticating agents • Access Control • Standard ACL plus theory

15. Relevance to P2P systems ? • How can peers authenticate themselves to each other ? • RIAA trace lookups for file sharing systems ? • Authenticate steps in multiple-hop DHTs (like Chord/Viceroy) in a manner similar to the paths ?