Analyzing an Anonymous Fair Exchange Ecommerce Protocol

1
Analyzing an Anonymous Fair Exchange E-commerce
Protocol
CS 259

• Adam Barth
• (joint work with Andrew Tappert)

2
Protocol Overview

• Protocol proposed in Ray and Ray 2001
• Protocol presented in pseudocode
• Five roles
• Customer and customers bank
• Merchant and merchants bank
• Trusted third party
• Anonymous fair exchange of money for a digital
  good
• Wanted to look at non-trace-based properties
• Employed MOCHA, an ATL model checker
• Customer assured of obtaining correct product by
  cross validation (not modeled)
• Had enough to look at without this

3
Protocol Overview (no TP)
1
2
5
C
M
6
3
8
4
7
CB
MB

• Preamble (on a private channel) M gt TP m K1
  Mipub
• Preamble (on a private channel) TP gt C m, K1
  Mipub
• C gt M PO CC(PO), Ciprv Cipub, Mipub
• M gt C CC(PO), Miprv m.r, K1xK2 CC(m.r,
  K1xK2), Miprv r, K1
  CC(r, K1), Miprv Macct, MBpub CC(Macct,
  MBpub), Miprv
• C gt CB MTI, Cprv, CBpub 4)
  CB gt C P, Bcprv, Cpub
• C gt M P, Bcprv, Mipub
• M gt MB P, Bcprv, MBpub 7)
  MB gt M ack, MBprv
• M gt C K2inv, Cipub CC(K2inv), Miprv rinv,
  Cipub CC(rinv), Miprv

4
Formalizing Protocol Specification

• Protocol has many messages
• Eight, not including the trusted party
• Many terms in each message
• MOCHA bug limited total number of variables
• Too complex to keep track of every term directly
• Modeled messages as Boolean variables
• Set to true when sent
• Dishonest parties can forge messages
• Based on the messages in their possession

5
Design of Our MOCHA Model
hc
n
hm
hmb
hcb

• Honest principals interact with network
• Dishonest principals folded into network
• Network records messages seen by dishonest
  parties
• Dishonest can forge messages with enough
  knowledge
• Each corrupt principal adds more initial knowledge

6
Honest Customer Module (1)

• module hc -- honest customer
• external o2, o2a, o4, o4a, o8, oB bool
• interface i1, i3, i5, i5a, iA,
• cprod / customer has received
  product /,
• dc bool
• atom
• controls i1, i3, i5, i5a, iA, cprod, dc
• reads o2, o2a, o4, o4a, o8, oB,
• i1, i3, i5, i5a, iA, cprod, dc
• init
• true
• -gt i1' false
• i3' false
• i5' false
• i5a' false
• iA' false
• cprod' false

Vars for messages
Customer dishonesty flag
Initially has no messages
7
Honest Customer Module (2)
Rules for updating state

• update
• i1
• -gt i1' true
• i1 o2 o2a i3
• -gt i3' true
• i1 o2 o2a i3 o4 o4a i5
  i5a
• -gt i5' true
• i1 o2 o2a i3 o4 o4a i5
  i5a
• -gt i5a' true
• i1 o2 o2a i3 o4 o4a i5
  i5a o8 iA
• -gt
• i1 o2 o2a i3 o4 o4a i5
  i5a o8 iA
• -gt iA' true
• (o8 oB) cprod
• -gt cprod' true
• endatom
• endmodule

Gets product from message 8 or B (part of TP
resolution)
8
Network Module (1)

• Able to record messages for dishonest roles
• i1 (dm dnet) m1 -gt m1'
  true
• i2 (dc dnet) m2 -gt m2'
  true
• i2a (dc dnet) m2a -gt m2a'
  true
• i3 (dcb dnet) m3 -gt m3'
  true
• i4 (dc dnet) m4 -gt m4'
  true
• i4a (dc dnet) m4a -gt m4a'
  true
• i5 (dm dnet) m5 -gt m5'
  true
• i5a (dm dnet) m5a -gt m5a'
  true
• i6 (dmb dnet) m6 -gt m6'
  true
• i7 (dm dnet) m7 -gt m7'
  true
• i8 (dc dnet) m8 -gt m8' true

Dishonest client or network can record message 4
Knowledge vars
9
Network Module (2)

• Forge messages
• (dc ii mm) m1 -gt
  m1' true
• m1 dm m2 -gt
  m2' true
• dm m2a -gt
  m2a' true
• dc m3 -gt
  m3' true
• (dcb (dc dmb)) m4 -gt
  m4' true
• (dcb dc) m4a -gt
  m4a' true
• ((m4 dc) dcb dmb) m5 -gt
  m5' true
• (dc ii mm) m5a -gt
  m5a' true
• ((m5 dm) dmb (dm dcb)) m6 -gt
  m6' true
• dmb m7 -gt
  m7' true
• m1 dm m8 -gt
  m8' true
• m1 m2 m5 oA -gt oA' true iitp'
  true

Dishonest client can forge message 3 at will
10
What Did We Do With the Model?

• MOCHA allowed us to run model by hand
• Useful to debug the model
• Tested some invariants (trace-based properties)
• Intruder can't get product unless he's acting as
  merchant or customer
• inv "inv1" (nprod dm dc)
• Customer only gets prod when merchant is paid
• inv "inv2" (cprod mpay)
• inv "inv3" (cprod mpay)

11
More Complex ATL Properties

• Honest customer eventually gets product
• atl "atl1" (ltlt hc gtgt F (cprod))
• When payment is sent, honest customer eventually
  gets the product
• atl "atl2" (i5 ltlt hc gtgt F (cprod))
• Exchange can be successfully completed by honest
  parties
• atl "atl3" (ltlt hc, hm, hcb gtgt F (cprod mpay))

cb needed to make payment token
12
Fairness

• Dishonest merchant can't get paid without honest
  customer having a strategy to get product (DM
  model)
• atl "cfair" ((ltlt n gtgt F (npay (ltlt hc gtgt F
  (cprod)))))
• Dishonest customer can't get product without
  honest merchant having a strategy to get paid (DC
  model)
• atl "mfair" ((ltlt n, hcb gtgt F (nprod
• 
  (ltlt hm gtgt F (mpay)))))

Dishonest parties folded into network
Dishonest customer still needs help from honest
bank
13
Balance

• Dishonest customer cant get to a point where
• (1) Customer can force receiving product
• (2) Merchant cant force getting paid
• atl "cbal" ((ltlt n, hcb gtgt F
• Dishonest merchant cant get to a point where
• (1) Merchant can force getting paid
• (2) Customer cant force receiving product
• atl "mbal" ((ltlt n gtgt F ((ltlt n gtgt F npay)
• (ltlt hc,
  hcb gtgt F cprod))))

((ltlt n gtgt F nprod)
(ltlt hm gtgt F mpay))))
14
Four Attacks on the Protocol

• Analysis reveals four attacks
• Malicious banks can steal product
• Banks share a signing key (should use group sigs)
• Man-in-the-middle can steal product
• Ephemeral keys can be replaced (need another sig)
• Dishonest merchant can get paid without giving
  prod
• Customer and TP stuck in a loop (need TP state)
• Unbalanced in favor of customer
• Customer can force outcome with payment token

15
How the Attacks Were Found

• All found by hand while constructing model
• Did not see them before building the model
• MOCHA found traced-based attacks 1 and 2
• MOCHA should have found attack 4
• Ran for 150 hours with no answer

16
Conclusions

• Think carefully about your models!
• Process of creating formal model uncovers bugs
• Large impact on model checkers efficiency
• MOCHA limitations frustrating
• Usually used for simpler models?
• Checking invariants successful
• Checking ATL properties time consuming
• MOCHA didnt answer in a reasonable time