State and Protocols: The Envelope Example

Transcription

1 State and Protocols: The Envelope Example rigorous design for protocols using state Daniel J. Dougherty and Joshua D. Guttman Worcester Polytechnic Institute Thanks to: National Science Foundation (Grant CNS ). Mar 2013 DD & JDG ( WPI ) Envelope BiSS 1 / 23

2 Goal of this Talk Illustrate a: Logical approach to protocol refinement for protocols using state via Hardware Security Modules (specifically Trusted Platform Modules) using M. Ryan s envelope protocol DD & JDG ( WPI ) Envelope BiSS 2 / 23

3 Goal of this Talk Illustrate a: Diagrammatic approach to protocol refinement for protocols using state via Hardware Security Modules (specifically Trusted Platform Modules) using M. Ryan s envelope protocol DD & JDG ( WPI ) Envelope BiSS 2 / 23

4 Goal of this Talk Illustrate a: Diagrammatic approach to protocol refinement for protocols using state via Hardware Security Modules (specifically Trusted Platform Modules) using M. Ryan s envelope protocol DD & JDG ( WPI ) Envelope BiSS 2 / 23

5 Trusted Platform Modules Small cheap chip on motherboard of many PCs Offers: Cryptographic primitives Some protected storage Platform configuration registers that record certain event sequences State is a reliable record of those events Supports attestation Signed assertions about TPM state, reflecting system history DD & JDG ( WPI ) Envelope BiSS 3 / 23

6 The Envelope Protocol Alice deposits an encrypted secret { v } K s.t. Bob can obtain v Or Bob can certify v was never obtained, and never will be obtained But not both Bob may be adversarial, misbehaving to try to get both v and refuse({ v } K ) TPM used to achieve protocol goal DD & JDG ( WPI ) Envelope BiSS 4 / 23

7 The Envelope Protocol Alice deposits an encrypted secret { v } K s.t. Bob can obtain v Or Bob can certify v was never obtained, and never will be obtained But not both Bob may be adversarial, misbehaving to try to get both v and refuse({ v } K ) TPM used to achieve protocol goal DD & JDG ( WPI ) Envelope BiSS 4 / 23

8 The Envelope Protocol Alice deposits an encrypted secret { v } K s.t. Bob can obtain v Or Bob can certify v was never obtained, and never will be obtained But not both Bob may be adversarial, misbehaving to try to get both v and refuse({ v } K ) TPM used to achieve protocol goal DD & JDG ( WPI ) Envelope BiSS 4 / 23

9 The Envelope Protocol Alice deposits an encrypted secret { v } K s.t. Bob can obtain v Or Bob can certify v was never obtained, and never will be obtained But not both Bob may be adversarial, misbehaving to try to get both v and refuse({ v } K ) TPM used to achieve protocol goal DD & JDG ( WPI ) Envelope BiSS 4 / 23

10 Envelope Protocol: Security Goal This diagram never occurs within any execution: A { v } K v refuse({ v } K ) where v is fresh and unguessable DD & JDG ( WPI ) Envelope BiSS 5 / 23

11 Envelope Protocol: Security Goal This diagram never occurs within any execution: A { v } K v refuse({ v } K ) where v is fresh and unguessable DD & JDG ( WPI ) Envelope BiSS 5 / 23

12 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 Suppose: Just one TPM with just one PCR 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 6 / 23

13 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 Suppose: Just one TPM with just one PCR 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 6 / 23

14 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 Suppose: Just one TPM with just one PCR 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 6 / 23

15 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 Suppose: Just one TPM with just one PCR 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 6 / 23

16 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 Suppose: Just one TPM with just one PCR 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 6 / 23

17 Analyzing the Envelope Protocol Delaune-Kremer-Ryan-Steel: verify it via ProVerif Under some restrictions: Bounded number of reboots Anchored at initial, post-boot state Monolithic Our contributions: 1 Lift the restrictions above using explicit model of distributed system behavior 2 Proof method tailored to protocol refinement steps 3 Ensure disjoint encryption properties DD & JDG ( WPI ) Envelope BiSS 7 / 23

18 Analyzing the Envelope Protocol Delaune-Kremer-Ryan-Steel: verify it via ProVerif Under some restrictions: Bounded number of reboots Anchored at initial, post-boot state Monolithic Our contributions: 1 Lift the restrictions above using explicit model of distributed system behavior 2 Proof method tailored to protocol refinement steps 3 Ensure disjoint encryption properties DD & JDG ( WPI ) Envelope BiSS 7 / 23

19 Analyzing the Envelope Protocol Delaune-Kremer-Ryan-Steel: verify it via ProVerif Under some restrictions: Bounded number of reboots Anchored at initial, post-boot state Monolithic Our contributions: 1 Lift the restrictions above using explicit model of distributed system behavior 2 Proof method tailored to protocol refinement steps 3 Ensure disjoint encryption properties DD & JDG ( WPI ) Envelope BiSS 7 / 23

20 Strands with state synchronization A strand is a linear sequence of 1 transmission nodes t 2 reception nodes t 3 state synchronization nodes φ representing a single local session We view transmissions as receptions as state synchronizations as positive negative neutral DD & JDG ( WPI ) Envelope BiSS 8 / 23

21 Strands with state synchronization A strand is a linear sequence of 1 transmission nodes t 2 reception nodes t 3 state synchronization nodes φ representing a single local session We view transmissions as receptions as state synchronizations as positive negative neutral DD & JDG ( WPI ) Envelope BiSS 8 / 23

22 Bundles, executions Definition A bundle B is a finite directed acyclic graph of nodes closed backward on each strand every reception has a unique matching transmission t Definition An execution is a pair B, where B is a bundle and is a partial order extending ( ) such that linearly orders the neutral nodes certain state evolution rules are satisfied n 1 n 2 implies pcr after(n 1 ) = pcr before(n 2 ) or m. Neutral(m) n 1 m n 2 DD & JDG ( WPI ) Envelope BiSS 9 / 23

23 Bundles, executions Definition A bundle B is a finite directed acyclic graph of nodes closed backward on each strand every reception has a unique matching transmission t Definition An execution is a pair B, where B is a bundle and is a partial order extending ( ) such that linearly orders the neutral nodes certain state evolution rules are satisfied n 1 n 2 implies pcr after(n 1 ) = pcr before(n 2 ) or m. Neutral(m) n 1 m n 2 DD & JDG ( WPI ) Envelope BiSS 9 / 23

24 Bundles, executions Definition A bundle B is a finite directed acyclic graph of nodes closed backward on each strand every reception has a unique matching transmission t Definition An execution is a pair B, where B is a bundle and is a partial order extending ( ) such that linearly orders the neutral nodes certain state evolution rules are satisfied n 1 n 2 implies pcr after(n 1 ) = pcr before(n 2 ) or m. Neutral(m) n 1 m n 2 DD & JDG ( WPI ) Envelope BiSS 9 / 23

25 Bundles, executions Definition A bundle B is a finite directed acyclic graph of nodes closed backward on each strand every reception has a unique matching transmission t Definition An execution is a pair B, where B is a bundle and is a partial order extending ( ) such that linearly orders the neutral nodes certain state evolution rules are satisfied n 1 n 2 implies pcr after(n 1 ) = pcr before(n 2 ) or m. Neutral(m) n 1 m n 2 DD & JDG ( WPI ) Envelope BiSS 9 / 23

26 Bundles, executions Definition A bundle B is a finite directed acyclic graph of nodes closed backward on each strand every reception has a unique matching transmission t Definition An execution is a pair B, where B is a bundle and is a partial order extending ( ) such that linearly orders the neutral nodes certain state evolution rules are satisfied n 1 n 2 implies pcr after(n 1 ) = pcr before(n 2 ) or m. Neutral(m) n 1 m n 2 DD & JDG ( WPI ) Envelope BiSS 9 / 23

27 I. State Evolution Equations TPM Formalization Boot(n) implies Extend(n, x) implies pcr after(n) = 1 pcr after(n) = Hash(pcr before(n), x) Boot(n) or Extend(n, x) or pcr after(n) = pcr before(n) Will write pcr(n) when pcr after(n) = pcr before(n) DD & JDG ( WPI ) Envelope BiSS 10 / 23

28 I. State Evolution Equations TPM Formalization Boot(n) implies Extend(n, x) implies pcr after(n) = 1 pcr after(n) = Hash(pcr before(n), x) Boot(n) or Extend(n, x) or pcr after(n) = pcr before(n) Will write pcr(n) when pcr after(n) = pcr before(n) DD & JDG ( WPI ) Envelope BiSS 10 / 23

29 II. Prefix/Boot TPM Formalization Definition Prefix(x, y) iff, recursively, x = y or z, w. y = Hash(z, w) and Prefix(x, z) Lemma n 0 n 2 implies either Prefix(pcr before(n 0 ), pcr after(n 2 )) or n 1. n 0 n 1 n 2 and Boot(n 1 ) DD & JDG ( WPI ) Envelope BiSS 11 / 23

30 II. Prefix/Boot TPM Formalization Definition Prefix(x, y) iff, recursively, x = y or z, w. y = Hash(z, w) and Prefix(x, z) Lemma n 0 n 2 implies either Prefix(pcr before(n 0 ), pcr after(n 2 )) or n 1. n 0 n 1 n 2 and Boot(n 1 ) DD & JDG ( WPI ) Envelope BiSS 11 / 23

31 IIIa. Request/reply roles TPM Formalization power on extend x quote x Boot(n) Extend(n) Quote(n, x) up ext ok c c = [[ quote pcr(n), x ]] aik DD & JDG ( WPI ) Envelope BiSS 12 / 23

32 IIIb. Request/reply roles TPM Formalization bind x unbind { t } K r 2 Bind(n, x) Unbind(n, r 2) r 1 t r 1 = [[ bind K, x ]] aik r 2 = [[ bind K, pcr(n) ]] aik K fresh values K 1 uncompromised, in bind role DD & JDG ( WPI ) Envelope BiSS 13 / 23

33 Envelope Protocol: Refined Security Goal This diagram never occurs within any execution: A [[ bind K, Hash(u 1,obtain) ]] aik { v } K v refuse(u 1,{ v } K ) refuse(u 1, { v } K ) = [[ quote Hash(u 1, refuse), { v } K ]] aik DD & JDG ( WPI ) Envelope BiSS 14 / 23

34 Envelope Protocol: Refined Security Goal This diagram never occurs within any execution: A [[ bind K, Hash(u 1,obtain) ]] aik { v } K v refuse(u 1,{ v } K ) refuse(u 1, { v } K ) = [[ quote Hash(u 1, refuse), { v } K ]] aik DD & JDG ( WPI ) Envelope BiSS 14 / 23

35 Refusal Specification Must traverse Quote-able state { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Quote(n 2, { v } K ) pcr(n 2 ) = Hash(u 1, refuse) DD & JDG ( WPI ) Envelope BiSS 15 / 23

36 Refusal Specification Must traverse Quote-able state { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Quote(n 2, { v } K ) pcr(n 2 ) = Hash(u 1, refuse) DD & JDG ( WPI ) Envelope BiSS 15 / 23

37 Disclosure Specification Must traverse Unbindable state { v } K { v } K n 0 v v Unbind(n 0, [[ bind K, Hash(u 1, obtain) ]] aik ) so pcr(n 0 ) = Hash(u 1, obtain) DD & JDG ( WPI ) Envelope BiSS 16 / 23

38 Disclosure Specification Must traverse Unbindable state { v } K { v } K n 0 v v Unbind(n 0, [[ bind K, Hash(u 1, obtain) ]] aik ) so pcr(n 0 ) = Hash(u 1, obtain) DD & JDG ( WPI ) Envelope BiSS 16 / 23

39 Disclosure Specification Must traverse Unbindable state { v } K { v } K n 0 v v Unbind(n 0, [[ bind K, Hash(u 1, obtain) ]] aik ) so pcr(n 0 ) = Hash(u 1, obtain) DD & JDG ( WPI ) Envelope BiSS 16 / 23

40 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice DD & JDG ( WPI ) Envelope BiSS 17 / 23

41 Security Goal, and consequence This diagram never occurs within any execution: { v } K v refuse(u 1,{ v } K ) n 1. Boot(n 1 ) and n 0 n 1 n 2 or n 2 n 1 n 0 by the Prefix/Boot Lemma DD & JDG ( WPI ) Envelope BiSS 18 / 23

42 Security Goal, and consequence, 1 This diagram never occurs within any execution: { v } K n 0 n 1 n 2 v refuse(u 1,{ v } K ) n 1. Boot(n 1 ) and n 0 n 1 n 2 or n 2 n 1 n 0 by the Prefix/Boot Lemma DD & JDG ( WPI ) Envelope BiSS 18 / 23

43 Security Goal, and consequences, 2 This diagram never occurs within any execution: { v } K n 0 n 1 n 2 v refuse(u 1,{ v } K ) n 1. Boot(n 1 ) and n 0 n 1 n 2 or n 2 n 1 n 0 by the Prefix/Boot Lemma DD & JDG ( WPI ) Envelope BiSS 18 / 23

44 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice Produce u 1 = Hash(u 0, N) by extending some state u 0 with a fresh nonce N DD & JDG ( WPI ) Envelope BiSS 19 / 23

45 Implementation Idea Using a TPM 1 Use one TPM platform configuration register 2 Put it in a state u 1 3 Allow decryption if extended to Hash(u 1,obtain) 4 Generate refusal cert if extended to Hash(u 1,refuse) 5 Ensure u 1 can never again be value of PCR regardless of this choice Produce u 1 = Hash(u 0, N) by extending some state u 0 with a fresh nonce N DD & JDG ( WPI ) Envelope BiSS 19 / 23

46 Specification: PCR setup subprotocol, 1 This diagram never occurs within any execution: N freshly chosen at n 0 N must be sent encrypted A n 0...extend N... Extend(n 1, N) Boot(n 2) Extend(n 3, N) pcr after(n 1, Hash(u 0, N)) DD & JDG ( WPI ) Envelope BiSS 20 / 23

47 Specification: PCR setup subprotocol, 2 This diagram never occurs within any execution: N freshly chosen at n 0 N must be sent encrypted A n 0 {...extend N... } esk Extend(n 1, N) Boot(n 2) Extend(n 3, N) pcr after(n 1, Hash(u 0, N)) DD & JDG ( WPI ) Envelope BiSS 20 / 23

48 PCR Setup subprotocol Implementation strategies A delivers extend N to TPM in an encrypted session Then either A closes the session before transmitting { v } K or TPM tears down encrypted sessions before reboot DD & JDG ( WPI ) Envelope BiSS 21 / 23

49 Specification: PCR setup subprotocol, 2 This diagram never occurs within any execution: N freshly chosen at n 0 N must be sent encrypted A n 0 Extend(n 1, N) Boot(n 2) Extend(n 3, N) pcr after(n 1, Hash(u 0, N)) DD & JDG ( WPI ) Envelope BiSS 22 / 23

50 Diagrams and Logic { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Each box shows a structure (model) Positive existential formulas summarize structures Diagram says: Executions containing LHS contain RHS Homomorphisms preserve positive existential formulas DD & JDG ( WPI ) Envelope BiSS 23 / 23

51 Diagrams and Logic { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Each box shows a structure (model) Positive existential formulas summarize structures Diagram says: Executions containing LHS contain RHS Homomorphisms preserve positive existential formulas DD & JDG ( WPI ) Envelope BiSS 23 / 23

52 Diagrams and Logic { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Each box shows a structure (model) Positive existential formulas summarize structures Diagram says: Executions containing LHS contain RHS Homomorphisms preserve positive existential formulas DD & JDG ( WPI ) Envelope BiSS 23 / 23

53 Diagrams and Logic { v } K { v } K refuse(u 1,{ v } K ) n 2 refuse(u 1,{ v } K ) Each box shows a structure (model) Positive existential formulas summarize structures Diagram says: Executions containing LHS contain RHS Homomorphisms preserve positive existential formulas DD & JDG ( WPI ) Envelope BiSS 23 / 23