Fault Tree Represent at ion of Securit y Requirement s 0
Work in progress Work in progress One Picture is Worth a Thousand Words Couple Dozen Connectives I liano Cer vesat o iliano@itd.nrl.navy.mil I TT I ndust r ies, inc @ NRL Washingt on, DC ht t p:/ / t heory.st anf ord.edu/ ~iliano Joint work with Cathy Meadows UMBC meeting Baltimore, MD October 1-2, 2003
How this work came about Analysis of GDOI group prot ocol ! Requir ement s expressed in NP ATRL " Novel gr oup pr oper t ies " Medium size specif icat ions – Dozen oper at ors " Lot s of f ine-t uning ! Dif f icult t o read and shar e specs. ! I nf ormal use of f ault t rees " I nt uit ive visualizat ion medium " Became f avor ed language ! For mal r elat ion wit h NPATRL Fault Tree Represent at ion of Securit y Requirement s 2
Security Requirements Describe what a prot ocol should do • Ver if ied by ! Model checking ! Mat hemat ical pr oof ! Pat t er n-mat ching (in some cases) • Expr essed ! I nf or mally ! Semi-f or mally ! For mal language • Adequat e f or t oy pr ot ocols BUT, do not scale t o r eal prot ocols Fault Tree Represent at ion of Securit y Requirement s 3
Example: Kerberos 5 [CSFW’02] • Semi-f ormal ! But ver y pr ecise • Bulky and unint uit ive ! Requir es sever al r eadings t o gr asp Fault Tree Represent at ion of Securit y Requirement s 4
Example: GDOI [CCS’01] • Formal ! NPATRL pr ot ocol spec. language • Ok f or a comput er • Bulky and unint uit ive f or humans ! About 20 oper at or s Fault Tree Represent at ion of Securit y Requirement s 5
Example: Authentication [Lowe, CSFW’97] • I nf ormal ! Made pr ecise as CSP expr essions • Simple, but … ! … many ver y similar def init ions Fault Tree Represent at ion of Securit y Requirement s 6
The Problem • Desired propert ies are dif f icult t o ! Phrase & get right ! Explain & underst and ! Modif y & keep right • Examples ! Endless back and f ort h on GDOI " Ar e specs. r ight now? ! K5 propert ies read over and over Fault Tree Represent at ion of Securit y Requirement s 7
Dealing with Textual Complexity • HCI response: graphical present at ion • Our approach: Dependence Trees ! Re-int erpret at ion of f ault t rees ! 2D represent at ion of NPATRL ! I nt uit ive f or medium size specs. Fault Tree Represent at ion of Securit y Requirement s 8
Example: Kerberos 5 • Excises t he gist of t he t heor em • Highlight s dependencies • Fair ly int uit ive ! … in a minut e … Fault Tree Represent at ion of Securit y Requirement s 9
Example: GDOI • I somor phic t o NP ATRL specif icat ions • Much mor e int uit ive ! … in a minut e … Fault Tree Represent at ion of Securit y Requirement s 10
Example: Authentication • Formalize def init ions • Easy t o compare … ! … and r emember … Fault Tree Represent at ion of Securit y Requirement s 11
Rest of this Talk • Logic f or prot ocol specs ! NPATRL Logic ! NRL Prot ocol Analyzer f ragment ! Model checking • Precedence t rees ! Fault t rees ! NPATRL semant ics • Analysis of an example • Fut ure Work Fault Tree Represent at ion of Securit y Requirement s 12
NPATRL • Formal language f or prot ocol requirement s ! Simple t empor al logic • Designed f or NRL Prot ocol Analyzer ! Simplif y input of pr ot ocol specs " Sequences of event s t hat should not occur ! Applies beyond NP A • Used f or many prot ocols ! SET, GDOI , … Fault Tree Represent at ion of Securit y Requirement s 13
NPATRL Logic • Event s init iat or _accept _key( A, (B,S), (K AB ,n A ), N) name round act uat or ot her agent s t erms • Classical connect ives: ∧ , ∨ , ¬ , … • “Previously”: # ( ) init iat or _accept _key(A, (B,S), (K AB ,n A ), N) ⇒ # server _sent _key(S, (A,B), (K AB ), _) Fault Tree Represent at ion of Securit y Requirement s 14
NPA Fragment NPA uses a small f ragment of NPATRL R ::= a ⇒ F F ::= E | ¬ E | F 1 ∧ F 2 | F 1 ∨ F 2 E ::= # a | # (a ∧ F) • Ef f icient model checking Fault Tree Represent at ion of Securit y Requirement s 15
Fault Trees • Saf et y analysis of syst em design ! Root is a f ailure sit uat ion " Ext ended t o behavior descr ipt ions ! I nner nodes ar e condit ions enabling f ault " Event s " Combinat or s (logical gat es) canBoard • Example ! A passenger needs a t icket and a phot o I D hasTicket hasI D t o boar d a plane, but should not carr y a weapon carriesWeapon Fault Tree Represent at ion of Securit y Requirement s 16
R ::= a ⇒ F F ::= E | ¬ E | F 1 ∧ F 2 | F 1 ∨ F 2 Precedence Trees E ::= # a | # (a ∧ F) • Fault t ree represent at ion of NPATRL NPA ! I somor phism a a R ::= E ::= a F F F ::= E E F 1 F 2 F 1 F 2 Fault Tree Represent at ion of Securit y Requirement s 17
“Recency Freshness” in GDOI if a member accept s a key f r om t he cont r oller in a pr ot ocol r un, no newer key should have been dist r ibut ed pr ior t o t he mem- ber ' s r equest member_accept _key(M,G,(K GM ,K old ),N) ⇒ # gcks_loseparwisekey(G,(),(M,K GM ),_) ∨ ¬ ( # ( member_request key(M,G,(),N) ∧ # gcks_creat ekey(G,(),K new ,K old ),_))) Fault Tree Represent at ion of Securit y Requirement s 18
“Sequential Freshness” in GDOI if a member accept s a key f r om t he gr oup cont r oller in a pr ot o- col r un, t hen it should not have pr eviously accept ed a lat er key member_accept _key(M,G,(K GM ,K old ),_) ⇒ # gcks_loseparwisekey(G,(),(M,K GM ),_) ∨ ¬ ( # (member_accept key(M,G,(K GM ,K new ),_) ∧ # ( gcks_creat ekey(G,(),K new ,K ’ ),_) ∧ # gcks_creat ekey(G,(),K old ,K ’’ ),_)))) Fault Tree Represent at ion of Securit y Requirement s 19
Conclusions • Explor ed t r ee r epr esent at ion of pr ot ocol reqs. ! Pr omising init ial r esult s ! Complex r equir ement s now int uit ive • Pr ecedence t r ees ! Dr aw f r om f ault t r ees r esear ch ! Specialized t o NPATRL and NPA ! NPATRL semant ics ! Bet t er under st anding of NPATRL • Paper s ! “A Fault -Tr ee Repr esent at ion of NPATRL Secur it y Requir ement s”, wit h Cat hy Meadows " WI TS’03 " TCS (long version, submit t ed) Fault Tree Represent at ion of Securit y Requirement s 20
Future Work – Theory • What propert ies can be expressed? ! All of saf et y? ! Liveness? • Graphical equivalence of requirement s? • Expressive power ! Recur sive t rees? ! Mor e complex quant if ier pat t er ns? • Graphical gist of t heorems ! Usef ul classes? ! Pr oof s? Fault Tree Represent at ion of Securit y Requirement s 21
Future Work – Practice • Gain f urt her experience ! Can t hey be used f or ot her requir ement s? • Scaling up ! When ar e t r ees so big t hey ar e non-int uit ive? " Exist ing r equir ement s? ! Modular it y • I nt eract ion wit h f ault t ree communit y ! Br oader applicat ions of dependence t rees? ! Tools we can use? " NPATRL < -> dependence t r ees Fault Tree Represent at ion of Securit y Requirement s 22
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