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Fault Tree Representation of Security Requirements

This work explores representing security requirements using fault trees, making complex requirements intuitive and enabling better understanding and analysis. The approach leverages graphical presentation and Dependence Trees for medium-sized specifications, bridging the gap between formal and informal use. The method, exemplified with protocols like Kerberos 5 and GDOI, offers a fresh perspective on expressing, verifying, and managing security requirements efficiently and intuitively.

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Fault Tree Representation of Security Requirements

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  1. ? Fault Tree Representation of Security Requirements

  2. Couple Dozen Connectives Work in progress One Picture is Worth aThousand Words Iliano Cervesatoiliano@itd.nrl.navy.mil ITT Industries, inc @ NRL Washington, DC http://theory.stanford.edu/~iliano Joint work with Cathy Meadows UMBC meeting Baltimore, MD October 1-2, 2003

  3. How this work came about Analysis of GDOI group protocol • Requirements expressed in NPATRL • Novel group properties • Medium size specifications • Dozen operators • Lots of fine-tuning • Difficult to read and share specs. • Informal use of fault trees • Intuitive visualization medium • Became favored language • Formal relation with NPATRL Fault Tree Representation of Security Requirements

  4. Security Requirements Describe what a protocol should do • Verified by • Model checking • Mathematical proof • Pattern-matching (in some cases) • Expressed • Informally • Semi-formally • Formal language • Adequate for toy protocols BUT, do not scale to real protocols Fault Tree Representation of Security Requirements

  5. Example: Kerberos 5 [CSFW’02] • Semi-formal • But very precise • Bulky and unintuitive • Requires several readings to grasp Fault Tree Representation of Security Requirements

  6. Example: GDOI [CCS’01] • Formal • NPATRL protocol spec. language • Ok for a computer • Bulky and unintuitive for humans • About 20 operators Fault Tree Representation of Security Requirements

  7. Example: Authentication [Lowe, CSFW’97] • Informal • Made precise as CSP expressions • Simple, but … • … many very similar definitions Fault Tree Representation of Security Requirements

  8. The Problem • Desired properties are difficult to • Phrase & get right • Explain & understand • Modify & keep right • Examples • Endless back and forth on GDOI • Are specs. right now? • K5 properties read over and over Fault Tree Representation of Security Requirements

  9. Dealing with Textual Complexity • HCI response: graphical presentation • Our approach: Dependence Trees • Re-interpretation of fault trees • 2D representation of NPATRL • Intuitive for medium size specs. Fault Tree Representation of Security Requirements

  10. Example: Kerberos 5 • Excises the gist of the theorem • Highlights dependencies • Fairly intuitive • … in a minute … Fault Tree Representation of Security Requirements

  11. Example: GDOI • Isomorphic to NPATRL specifications • Much more intuitive • … in a minute … Fault Tree Representation of Security Requirements

  12. Example:Authentication • Formalize definitions • Easy to compare … • … and remember … Fault Tree Representation of Security Requirements

  13. Rest of this Talk • Logic for protocol specs • NPATRL Logic • NRL Protocol Analyzer fragment • Model checking • Precedence trees • Fault trees • NPATRL semantics • Analysis of an example • Future Work Fault Tree Representation of Security Requirements

  14. NPATRL • Formal language for protocol requirements • Simple temporal logic • Designed for NRL Protocol Analyzer • Simplify input of protocol specs • Sequences of events that should not occur • Applies beyond NPA • Used for many protocols • SET, GDOI, … Fault Tree Representation of Security Requirements

  15. NPATRL Logic • Events initiator_accept_key( A, (B,S), (KAB,nA), N) • Classical connectives: , , , … • “Previously”: #( ) initiator_accept_key(A, (B,S), (KAB,nA), N)  # server_sent_key(S, (A,B), (KAB), _) name round actuator other agents terms Fault Tree Representation of Security Requirements

  16. NPA Fragment NPA uses a small fragment of NPATRL R ::= a  F F ::= E | E | F1 F2 | F1 F2 E ::= #a | #(a  F) • Efficient model checking Fault Tree Representation of Security Requirements

  17. Fault Trees • Safety analysis of system design • Root is a failure situation • Extended to behavior descriptions • Inner nodes are conditions enabling fault • Events • Combinators (logical gates) • Example • A passenger needs aticketand a photo IDtoboard a plane, butshould not carry aweapon canBoard hasTicket hasID carriesWeapon Fault Tree Representation of Security Requirements

  18. R ::= a  F F ::= E | E | F1 F2 | F1 F2 E ::= #a | #(a  F) Precedence Trees • Fault tree representation of NPATRLNPA • Isomorphism a a R ::= E ::= a F F F ::= E E F1 F2 F1 F2 Fault Tree Representation of Security Requirements

  19. Conclusions • Explored tree representation of protocol reqs. • Promising initial results • Complex requirements now intuitive • Precedence trees • Draw from fault trees research • Specialized to NPATRL and NPA • NPATRL semantics • Better understanding of NPATRL • Papers • “A Fault-Tree Representation of NPATRL Security Requirements”, with Cathy Meadows • WITS’03 • TCS (long version, submitted) Fault Tree Representation of Security Requirements

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