Abstract Specification of Crypto Protocols and their Attack Models in MSR


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1 Abstract Specification of Crypto Protocols and their Attack Models in MSR Iliano Cervesato ITT Industries, NRL Washington DC Software Engineering Institute  CMU August 6 th, 2001
2 Outline I. DolevYao specifications II. Specifying Protocols in MSR III.Access Control IV. Specifying Attacker Models V. Inferring the DolevYao Attacker Specifying CryptoProtocols and their Intruder Models in MSR 2
3 Part I DolevYao Specification of Security Protocols Specifying CryptoProtocols and their Intruder Models in MSR 3
4 Why is Protocol Analysis Difficult? Subtle cryptographic primitives DolevYao abstraction Distributed hostile environment Prudent engineering practice Inadequate specification languages the devil is in details Specifying CryptoProtocols and their Intruder Models in MSR 4
5 DolevYao Abstraction Symbolic data No bitstrings Perfect cryptography No guessing of keys Public knowledge soup Magic access to data Specifying CryptoProtocols and their Intruder Models in MSR 5
6 pictorially s a k b k a Specifying CryptoProtocols and their Intruder Models in MSR 6
7 Languages to Specify What? Message flow Message constituents Operating environment Protocol goals Specifying CryptoProtocols and their Intruder Models in MSR 7
8 Desirable Properties Unambiguous Simple Flexible Applies to a wide class of protocols Insightful Gives insight about protocols Specifying CryptoProtocols and their Intruder Models in MSR 8
9 Part II Specifying Protocols in MSR Specifying CryptoProtocols and their Intruder Models in MSR 9
10 What s in MSR 2.0? Multiset rewriting with existentials Dependent types w/ subsorting Memory predicates New New Constraints New Specifying CryptoProtocols and their Intruder Models in MSR 10
11 Terms Atomic terms Principal names Keys Nonces Term constructors ( ) {_} _ [_] _ {{_}} _ A k n D e f i n a b l e Specifying CryptoProtocols and their Intruder Models in MSR 11
12 Rules x 1 : τ 1. x n : τ n. y 1 : τ 1. lhs rhs y n : τ n. N(t) Network L(t,, t) Local state M A (t,, t) Memory χ Constraints N(t) Network L(t,, t) Local state M A (t,, t) Memory Specifying CryptoProtocols and their Intruder Models in MSR 12
13 Types of Terms A: princ n: nonce k: shk A B k: pubk A Types can depend on term Captures relations between objects k : privk k (definable) Specifying CryptoProtocols and their Intruder Models in MSR 13
14 Subtyping τ :: msg Allows atomic terms in messages Definable Nontransmittable terms Subhierarchies Specifying CryptoProtocols and their Intruder Models in MSR 14
15 Role state predicates Hold data local to a role instance Lifespan = role L l (A,t,, t) Invoke next rule L l = control (A,t,, t) = data Specifying CryptoProtocols and their Intruder Models in MSR 15
16 Memory Predicates New M A (t,, t) Hold private info. across role exec. Support for subprotocols Communicate data Pass control Interface to outside system Implements intruder Specifying CryptoProtocols and their Intruder Models in MSR 16
17 Constraints New χ Guards over interpreted domain Abstract Modular Invoke constraint handler E.g.: timestamps (T E = T N + T d ) (T N < T E ) Specifying CryptoProtocols and their Intruder Models in MSR 17
18 Type of predicates Dependent sums Σx: τ. τ τ (x) x τ Forces associations among arguments x E.g.: princ (A) x pubk A (k A) x privk k A Specifying CryptoProtocols and their Intruder Models in MSR 18
19 Roles Role state pred. var. declarations Generic roles Role owner Anchored roles L: τ (x 1 ) 1 x x τ n (xn) x:τ. lhs y:τ. rhs x:τ. lhs y:τ. rhs L: τ (x1) 1 x x τ n (xn) x:τ. lhs y:τ. rhs x:τ. lhs y:τ. rhs A A Specifying CryptoProtocols and their Intruder Models in MSR 19
20 NeedhamSchroeder PublicKey Protocol (fragment) A B: {n A, A} kb B A: {n A, n B } ka A B: {n B } kb ( Usual Notation ) Specifying CryptoProtocols and their Intruder Models in MSR 20
21 MSR 2.0 NS Initiator A L: princ x princ (B) x pubk B x nonce. B: princ k B : pubk B n A :nonce. L(A,B,k B,n A ) N({n A,A} kb ) k A : pubk A k A : privk k A n A,n B : nonce L(A,B,k B,n A ) N({n A,n B } ka ) N({n B } kb ) Specifying CryptoProtocols and their Intruder Models in MSR 21
22 MSR 2.0 NS Responder L: princ (B) x princ (A) x pubk B (kb) x privk k B x nonce x pubk A x nonce. B k B : pubk B k B : privk k B A: princ n A : nonce k A : pubk A N({n A,A} kb ) n B :nonce. L( ) N({n A,n B } ka ) n B : nonce L(B,k B,k B,A,n A,k A,n B ) N({n B } kb ) Specifying CryptoProtocols and their Intruder Models in MSR 22
23 Type Checking New Σ P t has type τ in Γ Catches: Γ t : τ Encryption with a nonce P is welltyped in Σ Transmission of a long term key Circular key hierarchies, Static and dynamic uses Decidable Specifying CryptoProtocols and their Intruder Models in MSR 23
24 Execution Model 1step firing P C C Activate roles Generates new role state pred. names Instantiate variables Apply rules Skips rules Specifying CryptoProtocols and their Intruder Models in MSR 24
25 Snapshots Active role set C = [S] R Σ State N(t) L l (t,, t) M A (t,, t) Signature a : τ L l : τ M _ : τ Specifying CryptoProtocols and their Intruder Models in MSR 25
26 Rule application Constraint check F, χ n:τ. G(n) Σ = χ (constraint handler) Firing [S 1 ] R Σ [S 2 ] R Σ, c:τ c not in S 1 S, F S, G(c) Specifying CryptoProtocols and their Intruder Models in MSR 26
27 Properties Admissibility of parallel firing Type preservation Access control preservation Completeness of DolevYao intruder New Specifying CryptoProtocols and their Intruder Models in MSR 27
28 Part III Access Control Specifying CryptoProtocols and their Intruder Models in MSR 28
29 Access Control r is ACvalid for A in Γ Catches Γ A r New P is ACvalid in Σ Σ P A signing/encrypting with B s key A accessing B s private data, Static Decidable Gives meaning to DolevYao intruder Specifying CryptoProtocols and their Intruder Models in MSR 29
30 Overview Interpret incoming information Collect received data Access unknown data Construct outgoing information Generate data Use known data Access new data Verify access to data Specifying CryptoProtocols and their Intruder Models in MSR 30
31 Processing a Rule Γ A lhs >> Δ Γ; Δ A rhs Γ A lhs rhs Specifying CryptoProtocols and their Intruder Models in MSR 31
32 Processing Predicates on the LHS Network messages Γ ; Δ A t >> Δ Γ; Δ A N(t) >> Δ Memory predicates Γ ; Δ A t 1,,t n >> Δ Γ; Δ A M A (t 1,,t n ) >> Δ Specifying CryptoProtocols and their Intruder Models in MSR 32
33 Interpreting Data on the LHS Pairs Γ; Δ A t 1, t 2 >> Δ Γ; Δ A (t 1, t 2 ) >> Δ Encrypted terms Γ; Δ A k >> Δ Γ; Δ A t >> Δ Γ; Δ A {t} k >> Δ Elementary terms Γ; (Δ,x) A x >> (Δ,x) (Γ,x:τ); Δ A x >> (Δ,x) Specifying CryptoProtocols and their Intruder Models in MSR 33
34 Accessing Data on the LHS Shared keys Γ; (Δ,k) A k >> (Δ,k) (Γ,x:shK A B); Δ A x >> (Δ,x) Public keys (Γ,k:pubK A,k :privk k); (Δ,k ) A k >> (Δ,k ) (Γ,k:pubK A,k :privk k); Δ A k >> (Δ,k ) Specifying CryptoProtocols and their Intruder Models in MSR 34
35 Generating Data on the RHS Nonces (Γ, x:nonce); (Δ, x) A rhs Γ; Δ A x:nonce. rhs Specifying CryptoProtocols and their Intruder Models in MSR 35
36 Constructing Terms on the RHS Pairs Γ; Δ A t 1 Γ; Δ A t 2 Γ; Δ A (t 1, t 2 ) Sharedkey encryptions Γ; Δ A t Γ; Δ A k Γ; Δ A {t} k Specifying CryptoProtocols and their Intruder Models in MSR 36
37 Accessing Data on the RHS Principal Γ, B:princ A B Shared key Γ, B:princ, k:shk A B A k Public key Γ, B:princ, k:pubk B A k Private key Γ, k:pubk A, k :privk k A k Specifying CryptoProtocols and their Intruder Models in MSR 37
38 Part IV Specifying Attacker Models Specifying CryptoProtocols and their Intruder Models in MSR 38
39 Execution with an Attacker P, P I C C Selected principal(s): Generic capabilities: Welltyped ACvalid I P I Modeled completely within MSR Specifying CryptoProtocols and their Intruder Models in MSR 39
40 The DolevYao Intruder Specific protocol suite P DY Underlies every protocol analysis tool Completeness still unproved!!! Specifying CryptoProtocols and their Intruder Models in MSR 40
41 Capabilities of the DY Intruder Intercept / emit messages Split / form pairs Decrypt / encrypt with known key Look up public information Generate fresh data Specifying CryptoProtocols and their Intruder Models in MSR 41
42 DY Intruder Data access M I (t) : Intruder knowledge A: princ. M I (A) I A: princ k: shk I A M I (k) I + dual A: princ k: pubk A M I (k) I k: pubk I k : privk k M I (k ) I No nonces, no other keys, Specifying CryptoProtocols and their Intruder Models in MSR 42
43 DY Intruder Data Generation Safe data n:nonce. M I (n) I m:msg. M I (m) I Anything else? A,B:princ. k:shk A B. M I (k) I??? It depends on the protocol!!! Automated generation Specifying CryptoProtocols and their Intruder Models in MSR 43
44 DY Intruder vs. AC ACvalid Welltyped DolevYao intruder New/ DY Intruder inferred from AC rules AC rules inferred from prot. spec. (with help) Specifying CryptoProtocols and their Intruder Models in MSR 44
45 Completeness of DY Intruder If P [S] R Σ [S ] R Σ with all welltyped and ACvalid Then P, P DY [S] R Σ [S ] R Σ Specifying CryptoProtocols and their Intruder Models in MSR 45
46 Consequences Justifies design of current tools Support optimizations DY intr. often too general/inefficient Generic optimizations Per protocol optimizations Restrictive environments Caps multiintruder situations Specifying CryptoProtocols and their Intruder Models in MSR 46
47 Part V New Inferring the DolevYao Intruder Specifying CryptoProtocols and their Intruder Models in MSR 47
48 The DolevYao Intruder Model Interpret incoming information Collect received data Access unknown data Construct outgoing information Generate data Use known data Access new data Same operations as AC! Specifying CryptoProtocols and their Intruder Models in MSR 48
49 Accessing Principal Names Γ, B:princ A B I B:princ. M I (B) I Specifying CryptoProtocols and their Intruder Models in MSR 49
50 What did we do? Instantiate acting principal to I Accessed data Intruder knowledge Metavariables Rule variables Ignore context Γ Specifying CryptoProtocols and their Intruder Models in MSR 50
51 Checking it out: Shared Keys Γ, A:princ, B:princ, k:shk A B A k I I I B: princ k: shk I B M I (k) I + dual Specifying CryptoProtocols and their Intruder Models in MSR 51
52 Getting Confident: Pub./Priv. Keys Γ, B:princ, k:pubk B A k I B: princ k: pubk B M I (k) I Γ, A:princ, k:pubk A, k :privk k A k I k: pubk I k : privk k M I (k ) Specifying CryptoProtocols and their Intruder Models in MSR 52 I I I
53 Constructing Messages: Pairs Γ; Δ A t 1 Γ; Δ A t I I 2 Γ; Δ A (t 1, t 2 ) I t 1,t 2 :msg. M I (t 1 ), M I (t 2 ) M I ((t 1,t 2 )) I Specifying CryptoProtocols and their Intruder Models in MSR 53
54 Now, what did we do? Instantiate acting principal to I Accessed data Intruder knowledge Metavariables Rule variables Ignore Γ and knowledge context Δ Premises antecedent Conclusion consequent Auxiliary typing derivation gives types Specifying CryptoProtocols and their Intruder Models in MSR 54
55 Carrying on: SharedKey Encrypt. Γ; Δ A t Γ; Δ A k I Γ; Δ A {t} I k I A,B: princ k: shk A B t: msg M I (t), M I (k) M I ({t} k ) I Similar for publickey encryption Specifying CryptoProtocols and their Intruder Models in MSR 55
56 Generating Data: Nonces (Γ, x:nonce); (Δ, x) A rhs Γ; Δ A x:nonce. rhs I I x:nonce. M I (x) I Similarly for other generated data Specifying CryptoProtocols and their Intruder Models in MSR 56
57 Now, what did we do? Instantiate acting principal to I Accessed data Intruder knowledge Metavariables Rule variables Ignore Γ and knowledge context Δ Premises antecedent Conclusion consequent Auxiliary typing derivation gives types One intruder rule for each AC rule Save generated object Specifying CryptoProtocols and their Intruder Models in MSR 57
58 Interpreting SharedKey Encrypt. Γ; Δ A k >> Δ Γ; Δ A t >> Δ I Γ; Δ A {t} k >> Δ I I A,B: princ k: shk A B t: msg M I ({t} k ), M I (k) M I (t) I Similar for publickey encryption pairing Specifying CryptoProtocols and their Intruder Models in MSR 58
59 Now, what did we do? Instantiate acting principal to I Accessed data Intruder knowledge Metavariables Rule variables Ignore Γ and knowledge context Δ Premises antecedent Conclusion consequent Auxiliary typing derivation gives types One intruder rule for each AC rule Save generated object Premises consequent Conclusion antecedant Specifying CryptoProtocols and their Intruder Models in MSR 59
60 Network Rules Γ ; Δ A t >> Δ Γ; Δ A N(t) >> Δ t:msg. N(t) M I (t) I Γ ; Δ A t Γ; Δ A N(t) t:msg. M I (t) N(t) I Specifying CryptoProtocols and their Intruder Models in MSR 60
61 Other Rules? Either redundant, or t:msg. N(t) N(t) I or, innocuous (but sensible) t 1,,t n :msg. M I (t 1,,t n ) M I (t 1 ),,M I (t n ) I Specifying CryptoProtocols and their Intruder Models in MSR 61
62 Dissecting AC 5 activities: Interpret message components on LHS Constructors atoms Pattern matching Trivial Access data (keys) on LHS Generate data on RHS Trivial Construct messages on RHS Access data on RHS Trivial Specifying CryptoProtocols and their Intruder Models in MSR 62
63 Accessing data Annotate the type of freely accessible data princ: + type Make it conditional for dep. types pubk: * princ > + type privk: ΠA: + princ. + pubk A > + type Specifying CryptoProtocols and their Intruder Models in MSR 63
64 Generating data Again, annotate types nonce:! type shk: + princ > + princ >! type shk: + princ > + princ >! type Specifying CryptoProtocols and their Intruder Models in MSR 64
65 Interpreting constructors Mark arguments as input or output _,_: msg > msg > msg {_} _ :  msg > ΠA: + princ. ΠB: + princ. + shk A B > msg {{_}} _ :  msg > ΠA: + princ. Πk: + pubk A. + privk k > msg hash: + msg > msg [_] _ : + msg > ΠA: * princ. Πk: * sigk A. * verk k > msg Specifying CryptoProtocols and their Intruder Models in MSR 65
66 Annotating declarations Integrates semantics of types and constructors Trimmed down version of AC Allows constructing AC rules Allows constructing the DolevYao intruder Specifying CryptoProtocols and their Intruder Models in MSR 66
67 alternatively Compute AC rules from protocol There are finitely many annotations Check protocol against each of them Keep the most restrictive ones that validate the protocol Exponential! More efficient algorithms? Specifying CryptoProtocols and their Intruder Models in MSR 67
68 Wrapup Specifying CryptoProtocols and their Intruder Models in MSR 68
69 CaseStudies Full NeedhamSchroeder publickey OtwayRees NeumanStubblebine repeated auth. OFT group key management Specifying CryptoProtocols and their Intruder Models in MSR 69
70 Conclusions Framework for specifying protocols Precise Flexible Powerful Provides Type /AC checking Sequential / parallel execution model Insights about DolevYao intruder Specifying CryptoProtocols and their Intruder Models in MSR 70
71 Future work Experimentation ClarkJacob library Fairexchange protocols More multicast Pragmatics Typereconstruction Operational execution model(s) Implementation Automated specification techniques Specifying CryptoProtocols and their Intruder Models in MSR 71
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