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y(the)c(pack)o(et,)g (decreased)f(its)i(stock)f(of)g(nuggets.)2020 2528 y(The)31 b(basic)g(problem)f(with)h(this)h(approach)d(is)j(that)f(it)h(might) 1937 2627 y(be)25 b(dif)n(\002cult)f(to)i(estimate)f(the)g(number)e(of) i(nuggets)f(that)h(are)g(re-)1937 2726 y(quired)17 b(to)i(reach)f(a)h (gi)n(v)o(en)e(destination.)23 b(If)c(the)f(originator)f(under)n(-)1937 2824 y(estimates)29 b(this)f(number)m(,)g(then)f(the)h(pack)o(et)f (will)i(be)f(discarded,)1937 2923 y(and)19 b(the)g(originator)f(loses)i (its)h(in)m(v)o(estment)c(in)j(this)g(pack)o(et.)k(If)c(the)1937 3022 y(originator)h(o)o(v)o(er)n(-estimates)h(the)i(number)d(\(lik)o(e) j(in)f(our)g(e)o(xample)1937 3121 y(abo)o(v)o(e\),)h(then)h(the)g(pack) o(et)g(will)h(arri)n(v)o(e,)f(b)n(ut)g(the)g(originator)e(still)1937 3220 y(loses)e(the)g(remaining)e(nuggets)g(in)i(the)f(pack)o(et)3309 3189 y Fk(3)3341 3220 y Fp(.)27 b(The)20 b(model)g(de-)1937 3318 y(scribed)g(in)g(the)g(ne)o(xt)g(subsection)f(o)o(v)o(ercomes)f (this)i(problem.)1937 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4082 y(mentioned)d(in)j([3].)-118 4181 y Fh(\017)41 b Fg(Slo)o(wly)31 b(changing)f(neighborhood.)54 b Fp(W)-7 b(e)32 b(assume)e(that)g(the)-118 4280 y(neighborhood)d(of)k (a)g(terminode)f(does)g(not)h(change)f(v)o(ery)g(f)o(ast.)-118 4379 y(This)e(mak)o(es)f(it)i(feasible)e(for)h(the)f(terminode)f(to)i (k)o(eep)f(track)h(of)-118 4477 y(its)d(neighbors)e(by)h(running)e(a)j (sort)g(of)f(\223hello)g(protocol\224)e(at)j(re)o(g-)-118 4576 y(ular)e(time)h(interv)n(als.)36 b(Besides)25 b(disco)o(v)o(ering) c(its)k(neighbors,)d(the)-118 4675 y(security)28 b(module)e(of)i(the)h (terminode)d(uses)j(the)f(hello)g(protocol)-118 4774 y(to)34 b(establish)g(shared)g(secrets)g(with)g(the)g(security)g (modules)f(of)-118 4873 y(its)21 b(neighbors)d(\(dif)n(ferent)g (secrets)j(with)f(dif)n(ferent)f(neighbors,)f(of)-118 4971 y(course\).)51 b(The)29 b(establishment)f(of)h(the)g(shared)f (secret)i(is)g(based)-118 5070 y(on)20 b(public)f(k)o(e)o(y)h (cryptography)c(and)k(relies)h(on)f(the)g(e)o(xisting)g(pub-)-118 5169 y(lic)k(k)o(e)o(y)g(infrastructure.)33 b(In)24 b(addition)e(to)i (the)g(shared)f(secret,)i(we)-118 5268 y(require)g(that)i(the)g (security)f(module)g(agrees)g(on)h(the)g(initial)g(v)n(al-)1937 1810 y(ues)20 b(of)g(tw)o(o)g(counters)f(with)h(each)g(of)g(its)h (neighbors.)i(The)c(shared)1937 1909 y(secret)g(and)f(the)g(tw)o(o)h (counters)e(are)h(used)h(to)f(protect)g(the)g(commu-)1937 2007 y(nication)e(between)f(neighboring)f(security)i(modules)f(and)h (will)i(be)1937 2106 y(discussed)i(further)f(in)h(Section)g(IV.)1937 2205 y Fh(\017)42 b Fg(Omnidir)o(ectional)33 b(antennae.)66 b Fp(W)-7 b(e)35 b(assume)f(that)g(the)g(ter)n(-)1937 2304 y(minodes)25 b(use)i(omnidirectional)d(antennae,)i(which)g(means)g (that)1937 2403 y(a)c(message)g(sent)g(by)g(a)g(terminode)e(can)i(be)g (heard)f(by)g(all)i(the)e(ter)n(-)1937 2501 y(minodes)32 b(within)h(the)h(communication)c(range)j(of)g(the)g(sender)-5 b(.)1937 2600 y(W)e(e)32 b(further)e(assume)h(that)g(such)g(a)g (message)g(can)g(not)f(only)h(be)1937 2699 y(heard,)19 b(b)n(ut)h(it)h(is)h(understood)17 b(by)j(all)h(of)f(the)g(neighbors.)k (By)c(this,)1937 2798 y(we)31 b(mean)g(that)g(all)h(the)f(neighbors)e (recei)n(v)o(e)h(the)h(message)g(and)1937 2897 y(can)g(determine)g(who) g(the)g(sender)g(and)g(the)h(intended)e(recei)n(v)o(er)1937 2995 y(are)19 b(and)f(what)h(the)g(content)f(of)g(the)h(message)g(is) 3328 2965 y Fk(6)3361 2995 y Fp(.)25 b(Depending)17 b(on)1937 3094 y(the)24 b(MA)m(C)f(layer)h(used,)f(this)i(may)e(require)f(that)i (the)f(terminodes)1937 3193 y(agree)16 b(on)g(further)g(parameters)f (with)i(their)g(neighbors)d(during)i(the)1937 3292 y(hello)24 b(protocol.)37 b(If,)25 b(for)f(instance,)h(access)g(to)g(the)g(shared) e(radio)1937 3391 y(resource)g(is)i(based)f(on)g(code)f(di)n(vision)h (\(CDMA\),)f(then)h(the)g(ter)n(-)1937 3489 y(minode)15 b(should)g(inform)g(its)j(neighbors)c(about)h(all)i(the)g(codes)e(that) 1937 3588 y(it)j(uses,)h(in)e(order)g(for)g(the)g(neighbors)f(to)h(be)h (able)f(to)h(recei)n(v)o(e)f(mes-)1937 3687 y(sages)k(sent)f(by)g(the)g (terminode.)1937 3786 y Fh(\017)42 b Fg(Symmetry)23 b(of)i(the)f (neighbor)h(r)o(elationship.)37 b Fp(F)o(or)24 b(the)g(sak)o(e)1937 3885 y(of)h(simplicity)-5 b(,)26 b(we)g(assume)g(that)g(the)f(neighbor) e(relationship)i(is)1937 3983 y(symmetric,)20 b(which)g(means)g(that)h (if)g(terminode)e(A)i(is)h(a)f(neighbor)1937 4082 y(of)g(terminode)f (B,)i(then)f(terminode)e(B)j(is)h(a)e(neighbor)f(of)h(termin-)1937 4181 y(ode)f(A)g(as)h(well.)1937 4280 y Fh(\017)42 b Fg(Reliable)20 b(communication)g(between)g(neighbors.)25 b Fp(Another)1937 4379 y(simplifying)i(assumption)g(we)i(made)f(is)h (that)g(the)f(communica-)1937 4477 y(tion)h(channel)f(between)g (neighboring)e(terminodes)h(is)j(reliable.)1937 4576 y(This)35 b(means)g(that)g(if)h(a)f(message)g(is)h(sent)f(successfully) g(\(e.g.,)1937 4675 y(without)24 b(an)o(y)f(collision\),)h(then)g(it)h (arri)n(v)o(es)f(to)g(the)h(intended)d(ne)o(xt)1937 4774 y(hop)k(correctly)-5 b(.)42 b(W)-7 b(e)28 b(will)g(address)e(the)h (problem)e(of)h(unreliable)1937 4873 y(communication)19 b(links)j(in)g(a)g(future)f(paper)-5 b(.)29 b(W)-7 b(e)24 b(note,)d(ho)n(we)n(v)o(er)m(,)1937 4971 y(that)k(this)h(assumption)f (does)g(not)g(imply)f(that)i(end-to-end)c(com-)1990 5181 y Fi(6)2025 5205 y Fm(More)27 b(precisely)l(,)j(each)e(neighbor)h(can)e (see)g(the)h(bits)f(of)f(the)i(message,)h(al-)1937 5278 y(though)e(not)e(necessarily)k(understanding)f(the)e(real)h(meaning)f (of)g(the)g(message)1937 5351 y(\(e.g.,)16 b(in)i(case)f(of)g (end-to-end)j(encrypted)f(messages\).)p eop %%Page: 5 5 5 4 bop -118 438 a Fp(munication)29 b(is)k(reliable.)58 b(Since)32 b(messages)f(can)g(be)h(modi\002ed)-118 537 y(and)22 b(intercepted)f(by)h(the)g(forw)o(arding)e(terminodes)h (themselv)o(es,)-118 635 y(successfully)28 b(sending)g(a)h(message)g (to)f(the)h(ne)o(xt)f(hop)g(does)h(not)-118 734 y(mean)17 b(that)h(the)g(message)f(will)i(correctly)d(arri)n(v)o(e)h(to)h(the)g (\002nal)g(des-)-118 833 y(tination.)-118 932 y Fh(\017)41 b Fg(Pricing)o(.)64 b Fp(In)34 b(the)f(P)o(ack)o(et)g(Purse)h(Model,)h (we)f(assume)f(that)-118 1031 y(there)15 b(e)o(xists)h(a)g(mechanism)e (to)i(estimate)f(the)h(number)d(of)j(nuggets)-118 1129 y(that)h(the)g(originator)f(of)h(a)g(pack)o(et)g(must)g(load)g(in)g (the)g(pack)o(et)g(purse)-118 1228 y(in)23 b(order)f(for)g(the)h(pack)o (et)f(to)h(be)g(deli)n(v)o(ered)e(to)i(the)g(\002nal)g(destina-)-118 1327 y(tion.)40 b(Furthermore,)24 b(we)h(also)h(assume)f(that)h(there)f (is)h(a)g(mecha-)-118 1426 y(nism)h(to)g(determine)e(the)h(number)f(of) i(nuggets)e(that)i(a)g(forw)o(ard-)-118 1525 y(ing)22 b(terminode)f(can)i(acquire)e(from)h(a)h(pack)o(et)f(purse.)31 b(Similarly)-5 b(,)-118 1623 y(in)26 b(the)g(P)o(ack)o(et)g(T)m(rade)f (Model,)i(we)f(assume)g(that)g(there)g(e)o(xists)g(a)-118 1722 y(mechanism)18 b(to)h(determine)f(the)i(number)d(of)i(nuggets,)f 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b(T)t(H)t(E)g(M)t(O)t(D)t(E)t(L)t(S)-35 4956 y Fp(W)-7 b(e)26 b(use)e(the)h(tamper)e(resistant)i(security)f (module)f(to)i(enforce)-118 5054 y(the)g(beha)n(vior)e(described)g(by)h (the)h(models.)37 b(In)25 b(this)g(section,)g(we)-118 5153 y(present)31 b(the)h(description)e(of)h(this)i(module)d(and)h(the) h(protocols)-118 5252 y(that)22 b(it)g(runs)f(with)h(its)h(en)m (vironment.)j(Our)c(leading)e(design)i(prin-)-118 5351 y(ciple)e(is)g(to)g(put)g(as)g(little)h(as)f(possible)g(in)g(the)g (security)f(module)f(in)1937 438 y(order)h(to)h(rely)g(on)g(as)h(fe)n (w)f(assumptions)f(as)i(possible.)1937 627 y Fl(A.)41 b(Long)19 b(and)h(medium)f(term)i(data)f(in)g(the)g(security)h(module) 2020 772 y Fp(The)16 b(security)f(module)g(stores)i(and)e(manipulates)g (data)h(that)h(are)1937 871 y(critical)28 b(for)g(the)g(correct)g(beha) n(vior)e(of)i(the)h(system.)49 b(Since)29 b(the)1937 970 y(security)f(module)f(is)i(tamper)e(resistant,)k(these)d(data)g (cannot)g(be)1937 1069 y(corrupted)18 b(by)i(the)g(user)g(of)g(the)g (terminode)e(or)i(other)g(attack)o(ers.)2020 1172 y(The)25 b(follo)n(wing)f(long)g(term)h(data)g(are)h(stored)e(in)i(the)f (security)1937 1271 y(module)19 b Ff(S)5 b(M)k Fp(:)1937 1380 y Fh(\017)42 b Fg(Unique)49 b(identi\002er)-8 b(.)109 b Fp(The)48 b(security)g(module)f(stores)i(its)1937 1478 y(system-wide)19 b(unique)g(identi\002er)m(,)g(which)h(we)g(denote)f (by)h Ff(id)3721 1490 y Fe(S)s(M)3839 1478 y Fp(.)1937 1577 y Fh(\017)42 b Fg(Pri)o(v)o(ate)23 b(k)o(ey)-6 b(.)36 b Fp(The)24 b(security)f(module)f(has)j(a)f(public)f(k)o(e)o(y)g(and) 1937 1676 y(a)d(corresponding)15 b(pri)n(v)n(ate)j(k)o(e)o(y)-5 b(.)24 b(The)19 b(pri)n(v)n(ate)f(k)o(e)o(y)g(is)i(e)o(xclusi)n(v)o (ely)1937 1775 y(kno)n(wn)26 b(to)i Ff(S)5 b(M)37 b Fp(and,)29 b(thus,)g(it)g(must)f(be)g(stored)f(by)g Ff(S)5 b(M)k Fp(.)48 b(The)1937 1874 y(public)18 b(k)o(e)o(y)g(does)h(not)g(need)f (to)h(be)g(k)o(ept)g(secret,)g(therefore,)e(it)j(can)1937 1972 y(be)29 b(stored)f(else)n(where.)51 b(It)29 b(is)h(important,)f (ho)n(we)n(v)o(er)m(,)f(that)h(other)1937 2071 y(security)i(modules)f (associate)i(the)f(right)g(public)f(k)o(e)o(y)h(\(i.e.,)j(the)1937 2170 y(public)19 b(k)o(e)o(y)h(of)g Ff(S)5 b(M)k Fp(\))19 b(with)i(the)f(unique)e(identi\002er)i(of)g Ff(S)5 b(M)k Fp(.)24 b(This)1937 2269 y(is)g(ensured)d(with)i(the)g(help)f(of)g(the) h(assumed)f(public)g(k)o(e)o(y)g(infras-)1937 2368 y(tructure.)1937 2466 y Fh(\017)42 b Fg(Number)23 b(of)f(nuggets.)32 b Fp(Nuggets)22 b(are)g(represented)f(by)h(coun-)1937 2565 y(ters)f(in)g(the)f(security)g(modules)g(of)g(the)h(terminodes.)j(The)d (wealth)1937 2664 y(of)40 b(each)f(terminode)f(is)j(equal)e(to)h(the)g (v)n(alue)f(of)g(the)h(nugget)1937 2763 y(counter)d(in)i(its)g (security)f(module.)79 b(W)-7 b(e)40 b(denote)d(the)i(nugget)1937 2862 y(counter)19 b(in)h(the)g(security)g(module)f(by)g Ff(n)3121 2874 y Fe(S)s(M)3239 2862 y Fp(.)2020 2970 y(In)27 b(addition,)f(the)h(security)f(module)g(k)o(eeps)g(a)i(list)f (of)g(current)1937 3069 y(neighbors)d(and)h(maintains)g(data)h (associated)f(to)h(each)f(of)h(these.)1937 3168 y Ff(S)5 b(M)25 b Fp(stores)17 b(the)f(follo)n(wing)f(medium)g(term)h(data)g (for)g(each)g(neigh-)1937 3266 y(boring)j(security)g(module)g Ff(S)5 b(M)2874 3236 y Fh(0)2897 3266 y Fp(:)1937 3375 y Fh(\017)42 b Fg(Unique)33 b(identi\002er)-8 b(.)61 b Fp(The)32 b(system-wide)f(unique)g(identi\002er)1937 3474 y Ff(id)2009 3486 y Fe(S)s(M)2122 3470 y Fd(0)2170 3474 y Fp(of)20 b(the)g(neighbor)-5 b(.)1937 3572 y Fh(\017)42 b Fg(Shar)o(ed)18 b(secr)o(et)h(k)o(ey)-6 b(.)25 b Fp(When)19 b Ff(S)5 b(M)27 b Fp(and)19 b Ff(S)5 b(M)3342 3542 y Fh(0)3384 3572 y Fp(become)18 b(neigh-)1937 3671 y(bors,)23 b(the)o(y)f(establish)g(a)h(shared)f(secret)h(k)o(e)o(y)f Ff(k)3304 3683 y Fe(S)s(M)s(;S)s(M)3547 3667 y Fd(0)3597 3671 y Fp(between)1937 3770 y(them)17 b(using)g(the)g(hello)g(protocol) f(and)h(public)g(k)o(e)o(y)f(cryptography)-5 b(.)1937 3869 y(This)23 b(shared)e(secret)i(is)g(e)o(xclusi)n(v)o(ely)e(kno)n (wn)g(to)h Ff(S)5 b(M)32 b Fp(and)22 b Ff(S)5 b(M)3831 3839 y Fh(0)3853 3869 y Fp(,)1937 3968 y(and)20 b(it)h(is)g(used)f(to)g (protect)f(the)i(communication)c(between)i(them.)1937 4066 y(This)26 b(protection,)g(in)g(turn,)g(is)h(based)f(on)f (symmetric)g(k)o(e)o(y)h(cryp-)1937 4165 y(tography)i(for)j(ef)n (\002cienc)o(y)f(reasons.)57 b(Protection)30 b(is)i(necessary)-5 b(,)1937 4264 y(because)16 b(the)i(security)e(modules)g(cannot)g (communicate)f(directly)1937 4363 y(b)n(ut)j(only)f(through)e(their)j (hosting)f(terminodes,)f(which)h(are)h(under)1937 4462 y(the)i(control)f(of)h(\(potentially)f(malicious\))g(users.)1937 4560 y Fh(\017)42 b Fg(Sending)30 b(and)h(r)o(ecei)o(ving)e(counters.) 55 b Ff(S)5 b(M)39 b Fp(stores)31 b(a)f(send-)1937 4659 y(ing)22 b(counter)e Ff(c)2373 4671 y Fe(S)s(M)6 b Fh(!)p Fe(S)s(M)2665 4655 y Fd(0)2715 4659 y Fp(and)22 b(a)g(recei)n(ving)f (counter)f Ff(c)3554 4671 y Fe(S)s(M)6 b Fh( )p Fe(S)s(M)3846 4655 y Fd(0)1937 4758 y Fp(associated)29 b(with)h Ff(S)5 b(M)2632 4728 y Fh(0)2654 4758 y Fp(.)53 b(These)30 b(counters)e(are)h (used)g(to)h(detect)1937 4857 y(message)38 b(replay)-5 b(,)40 b(which,)i(as)c(mentioned)e(in)i(Subsection)f(II-)1937 4956 y(C,)i(w)o(ould)34 b(fool)g(the)h(security)f(module)f(to)i (process)g(the)f(same)1937 5054 y(message)24 b(twice.)38 b Ff(S)5 b(M)2628 5024 y Fh(0)2676 5054 y Fp(has)25 b(similar)f (counters)f Ff(c)3409 5066 y Fe(S)s(M)3522 5050 y Fd(0)3545 5066 y Fh(!)p Fe(S)s(M)3754 5054 y Fp(and)1937 5153 y Ff(c)1973 5165 y Fe(S)s(M)2086 5149 y Fd(0)2109 5165 y Fh( )p Fe(S)s(M)2293 5153 y Fp(,)35 b(which)c(are)h(associated)f (with)h Ff(S)5 b(M)k Fp(.)60 b(When)32 b Ff(S)5 b(M)1937 5252 y Fp(and)31 b Ff(S)5 b(M)2235 5222 y Fh(0)2290 5252 y Fp(become)31 b(neighbors,)h(the)o(y)f(initialize)h(their)g(recei)n (v-)1937 5351 y(ing)j(counters)f(to)h(random)e(v)n(alues)i(and)f(use)h (the)g(hello)g(proto-)p eop %%Page: 6 6 6 5 bop -118 438 a Fp(col)31 b(to)h(set)g(their)f(sending)g(counters)f (such)h(that)h(the)f(follo)n(wing)-118 537 y(holds:)50 b Ff(c)172 549 y Fe(S)s(M)6 b Fh(!)p Fe(S)s(M)464 532 y Fd(0)538 537 y Fc(=)46 b Ff(c)685 549 y Fe(S)s(M)798 532 y Fd(0)820 549 y Fh( )p Fe(S)s(M)1032 537 y Fc(+)27 b(1)33 b Fp(and)f Ff(c)1388 549 y Fe(S)s(M)1501 532 y Fd(0)1524 549 y Fh(!)p Fe(S)s(M)1754 537 y Fc(=)-118 635 y Ff(c)-82 647 y Fe(S)s(M)6 b Fh( )p Fe(S)s(M)210 631 y Fd(0)273 635 y Fc(+)35 b(1)p Fp(.)95 b(Then,)48 b(each)43 b(time)g Ff(S)5 b(M)52 b Fp(sends)44 b(a)g(mes-)-118 734 y(sage)28 b(to)h Ff(S)5 b(M)298 704 y Fh(0)321 734 y Fp(,)31 b(it)e(includes)e(the)i(current)e(v)n(alue)h(of)g(its)i (sending)-118 833 y(counter)h Ff(c)200 845 y Fe(S)s(M)6 b Fh(!)p Fe(S)s(M)492 829 y Fd(0)553 833 y Fp(in)33 b(the)g(message,)j (and)c(then)h(increments)-118 932 y(the)f(counter)-5 b(.)59 b(When)32 b Ff(S)5 b(M)41 b Fp(recei)n(v)o(es)31 b(a)i(message)f(from)f Ff(S)5 b(M)1776 902 y Fh(0)1798 932 y Fp(,)-118 1031 y(it)29 b(v)o(eri\002es)g(if)g(the)g(message)g 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y(cryptographic)c(hash)j(v)n(alue)f(of)h(the)h(content)e(of)h(the) g(pack)o(et.)28 b(Re-)-118 5351 y(play)21 b(of)g(the)h(pack)o(et)f (purse)g(is)i(pre)n(v)o(ented)c(by)i(the)h(use)f(of)h(an)f(e)n(v)o(er) 1937 438 y(increasing)16 b(counter)g(that)i(is)g(placed)f(in)g(the)g (purse.)24 b(This)17 b(solution)1937 537 y(is)26 b(preferable)d(to)i (the)g(application)f(of)g(time-stamps,)i(because)e(it)1937 635 y(does)30 b(not)g(require)g(the)g(security)g(module)f(to)i(ha)n(v)o (e)f(an)g(internal)1937 734 y(clock)g(and)g(to)g(run)g(clock)g (synchronization)d(protocols,)k(which)1937 833 y(w)o(ould)19 b(need)h(to)g(be)g(secured)g(as)h(well.)2020 956 y(The)d(originator)e (of)h(a)i(pack)o(et)e(cannot)g(re-use)h(the)g(nuggets)e(that)1937 1055 y(it)32 b(has)f(already)e(loaded)h(in)h(the)g(pack)o(et,)h (because)e(the)h(security)1937 1153 y(module)18 b(decreases)g(the)h (nugget)f(counter)g(when)g(creating)g(a)i(PPH)1937 1252 y(for)27 b(a)g(ne)n(w)g(pack)o(et.)46 b(An)27 b(intermediary)e(cannot)h (tak)o(e)i(out)e(more)1937 1351 y(nuggets)17 b(from)h(the)g(pack)o(et)g (than)g(it)h(deserv)o(es)e(for)h(the)g(pack)o(et)g(for)n(-)1937 1450 y(w)o(arding,)32 b(because)f(its)h(nugget)e(counter)g(can)g(be)i (manipulated)1937 1549 y(e)o(xclusi)n(v)o(ely)k(by)h(its)h(security)f (module,)j(which)d(beha)n(v)o(es)f(cor)n(-)1937 1647 y(rectly)-5 b(.)23 b(Moreo)o(v)o(er)m(,)16 b(the)i(intermediary)e(is)i (stimulated)g(to)g(forw)o(ard)1937 1746 y(the)32 b(pack)o(et,)i (because)d(its)i(nugget)d(counter)g(will)j(be)f(increased)1937 1845 y(only)21 b(if)h(an)g(ackno)n(wledgment)d(arri)n(v)o(es)i(from)g (the)h(ne)o(xt)f(hop,)g(and)1937 1944 y(this)g(is)g(possible)f(only)f (if)i(the)f(pack)o(et)f(has)i(been)e(forw)o(arded.)2020 2067 y(Our)f(solution)g(requires)g(each)g(hop)g(to)h(send)g(an)f(ackno) n(wledge-)1937 2165 y(ment)g(for)g(the)h(pack)o(et)f(it)h(recei)n(v)o (ed.)k(T)-6 b(erminodes,)17 b(ho)n(we)n(v)o(er)m(,)f(may)1937 2264 y(be)28 b(reluctant)g(to)g(send)g(ackno)n(wledgements,)f(because)h (sending)1937 2363 y(consumes)h(ener)o(gy)f(and)i(it)g(does)g(not)g(ha) n(v)o(e)f(an)o(y)h(direct)f(adv)n(an-)1937 2462 y(tages.)j(This)23 b(problem)e(is)j(related)e(to)g(f)o(air)h(e)o(xchange)d([6],)j([7])f (\(in)1937 2561 y(our)e(case,)i(pack)o(ets)f(for)f(ackno)n (wledgements\),)e(and)i(it)i(is)g(usually)1937 2659 y(solv)o(ed)d(with) g(the)h(in)m(v)n(olv)o(ement)d(of)i(a)h(trusted)f(third)g(party)f (\(TTP\).)1937 2758 y(W)-7 b(e)24 b(cannot,)d(ho)n(we)n(v)o(er)m(,)g (assume)h(the)g(e)o(xistence)g(of)g(TTPs)h(in)f(ter)n(-)1937 2857 y(minode)d(netw)o(orks.)24 b(The)c(problem)e(of)i(f)o(air)h(e)o (xchange)d(without)h(a)1937 2956 y(TTP)25 b(is)g(analyzed)e(in)h([8],)h (where)e(it)j(is)f(called)f Fl(unenfor)m(ced)e(safe)1937 3054 y(e)n(xc)o(hang)o(e)p Fp(.)j(The)c(author)e(pro)o(v)o(es)h(that)h (isolated)f(unenforced)e(safe)1937 3153 y(e)o(xchange)d(is)j(not)f (possible)g(if)h(the)f(last)h(step)g(of)f(the)g(e)o(xchange)e(has)1937 3252 y(some)g(costs.)24 b(A)16 b(proposed)e(solution)g(is)j(that)e(one) g(should)f(not)h(con-)1937 3351 y(sider)24 b(only)f(a)i(single)e (isolated)h(e)o(xchange,)f(b)n(ut)h(one)f(should)g(also)1937 3450 y(tak)o(e)d(into)g(account)f(possible)h(future)f(e)o(xchanges,)f (where)i(the)g(be-)1937 3548 y(ha)n(vior)25 b(of)i(the)f(parties)g(in)h (the)f(future)g(e)o(xchanges)e(may)i(depend)1937 3647 y(on)k(the)g(result)g(of)g(the)g(current)f(e)o(xchange.)52 b(If)30 b(misbeha)n(vior)e(in)1937 3746 y(the)22 b(present)f(can)g(be)h (punished)e(in)i(the)g(future,)e(then)i(unenforced)1937 3845 y(safe)27 b(e)o(xchange)f(becomes)g(possible.)46 b(In)27 b(our)f(implementation,)1937 3944 y(we)i(used)f(these)g(ideas)h (in)f(tw)o(o)h(w)o(ays)f(to)h(stimulate)f(terminodes)1937 4042 y(to)j(send)f(ackno)n(wledgments.)49 b(First,)32 b(we)e(reduced)e(the)h(cost)h(of)1937 4141 y(sending)18 b(an)h(ackno)n(wledgement)d(by)j(piggy)f(backing)g(it)i(to)g(a)f(nor)n (-)1937 4240 y(mal)31 b(pack)o(et)g(that)g(the)g(terminode)e(sends)j (an)o(yw)o(ay)d(\(e)o(xcept)h(for)1937 4339 y(the)24 b(destination)e(of)i(a)g(pack)o(et\).)34 b(Second,)23 b(we)h(introduced)d(\002nes,)1937 4438 y(in)g(order)e(to)i(punish)f (misbeha)n(ving)f(terminodes.)25 b(Moreo)o(v)o(er)m(,)18 b(the)1937 4536 y(\002ne)27 b(is)g(sent)g(in)g(the)g(purse)f(together)g (with)g(the)h(nuggets,)g(which)1937 4635 y(enforces)g(the)i(terminode)d (who)i(w)o(ants)h(the)f(nuggets)f(to)i(upload)1937 4734 y(the)18 b(\002ne)f(as)i(well)f(to)g(the)f(security)g(module,)g(which)g (will)h(decrease)1937 4833 y(the)i(nugget)f(counter)g(according)f(to)i (the)g(recei)n(v)o(ed)f(\002ne.)2020 4956 y(W)-7 b(e)27 b(should)f(note)f(that)i(e)o(xchanges)d(without)i(TTP)g(can)g(ne)n(v)o (er)1937 5054 y(achie)n(v)o(e)40 b(the)i(same)g(le)n(v)o(el)f(of)g(f)o (airness)h(as)g(those)f(with)h(TTP)-9 b(.)1937 5153 y(The)31 b(e)o(xistence)g(of)g(dif)n(ferent)f(le)n(v)o(els)i(of)f(f)o(airness)g (is)i(discussed)1937 5252 y(in)41 b([9],)46 b(where)41 b(the)g(authors)f(relate)h(the)h(dif)n(ferent)d(le)n(v)o(els)i(to)1937 5351 y(dif)n(ferent)31 b(equilibrium)g(concepts)h(in)h(game)f(theory)-5 b(.)62 b(Accord-)p eop %%Page: 10 10 10 9 bop -118 438 a Fp(ing)34 b(to)g(these)g(results,)k(our)33 b(implementation)f(achie)n(v)o(es)h(Nash-)-118 537 y(equilibrium)25 b(f)o(airness,)k(which)e(essentially)g(means)g(that)h(a)f(mis-)-118 635 y(beha)n(ving)20 b(party)h(may)g(cause)h(some)f(damage)g(to)g(a)i (correctly)d(be-)-118 734 y(ha)n(ving)25 b(one,)j(b)n(ut)e(it)i(also)f (loses)g(something)e(or)h(at)h(least)h(cannot)-118 833 y(gain)e(an)o(ything)f(\(apart)h(from)f(malicious)i(jo)o(y\))f(with)h (the)g(misbe-)-118 932 y(ha)n(vior)-5 b(.)-118 1116 y Fl(C.)42 b(Ef)o(\002ciency)-35 1259 y Fp(At)24 b(\002rst)g(sight,)h (our)e(solution)f(may)h(seem)h(a)g(bit)g(hea)n(vy)f(to)h(im-)-118 1358 y(plement.)45 b(Ho)n(we)n(v)o(er)m(,)26 b(the)i(o)o(v)o(erhead)c (generated)h(by)i(it)h(is)g(small)-118 1457 y(when)19 b(compared)f(to)i(all)g(the)g(functions)e(that)i(are)g(required)e(to)i (ac-)-118 1555 y(complish)i(pack)o(et)h(forw)o(arding.)33 b(In)23 b(particular)m(,)g(the)h(calculation)-118 1654 y(and)h(v)o(eri\002cation)f(of)h(the)h(P)o(ack)o(et)f(Purse)g(and)g (the)h(P)o(ack)o(et)f(T)m(rade)-118 1753 y(Headers)d(require)f(only)h (cryptographic)d(hash)j(function)f(compu-)-118 1852 y(tations,)c(which) g(can)g(be)g(done)f(v)o(ery)g(ef)n(\002ciently)g([10].)23 b(Public)17 b(k)o(e)o(y)-118 1951 y(cryptographic)h(operations)i(are)h (used)g(only)g(rarely)f(\(in)h(the)h(hello)-118 2049 y(protocol\).)41 b(Moreo)o(v)o(er)m(,)24 b(most)i(of)g(the)g (processing)f(load)h(will)h(be)-118 2148 y(supported)16 b(by)i(the)g(security)f(module;)h(to)g(some)g(e)o(xtent,)g(it)h(can)f (be)-118 2247 y(accomplished)f(in)h(parallel)g(with)h(the)g(processing) e(performed)f(by)-118 2346 y(the)k(main)g(processor)f(of)h(the)g (terminode.)-35 2448 y(Another)j(issue)h(is)h(the)f(length)f(of)h(the)g 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b(and)g(all)h(the)f(related)g(mechanisms)g (only)g(in)g(a)h(small)g(frac-)-118 3637 y(tion)g(of)g(pack)o(ets.)23 b(Then)17 b(the)g(majority)f(of)h(the)g(pack)o(ets)g(w)o(ould)g(not) -118 3736 y(carry)g(an)h(additional)f(header)f(and)i(w)o(ould)f(be)h (processed)f(without)-118 3835 y(an)o(y)j(call)h(to)g(the)f(security)g (module.)25 b(This)c(means,)f(ho)n(we)n(v)o(er)m(,)e(that)-118 3933 y(the)25 b(terminodes)e(w)o(ould)h(not)h(be)f(re)n(w)o(arded)g (for)g(the)h(forw)o(arding)-118 4032 y(of)20 b(each)g(pack)o(et,)g(and) g(we)h(w)o(ould)f(ha)n(v)o(e)f(to)i(ensure)f(that)g(the)o(y)g(for)n(-) -118 4131 y(w)o(ard)h(those)f(pack)o(ets)h(as)h(well)f(from)f(which)g (the)o(y)h(cannot)e(e)o(xpect)-118 4230 y(an)o(y)g(nuggets.)24 b(This)c(issue)h(is)g(left)g(for)e(future)g(w)o(ork.)531 4414 y(V)t(I)t(.)45 b(C)t Fm(O)t(N)t(C)t(L)t(U)t(S)t(I)t(O)t(N)-35 4557 y Fp(In)18 b(this)h(paper)m(,)e(we)i(addressed)e(the)i(problem)d (of)j(service)f(a)n(v)n(ail-)-118 4656 y(ability)j(in)g(terminode)e 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(as)i(mobile)e(stations)1937 734 y(are)24 b(under)e(the)i(control)f(of) h(end)f(users,)i(there)e(is)i(a)f(strong)f(temp-)1937 833 y(tation)d(to)h(alter)g(their)f(beha)n(vior)f(in)i(one)f(w)o(ay)g (or)h(another)-5 b(.)25 b(There-)1937 932 y(fore,)16 b(all)h(f)o(acets)f(of)g(security)g(ha)n(v)o(e)f(to)h(be)h(carefully)d (analyzed)h(and)1937 1031 y(implemented.)24 b(W)-7 b(e)22 b(are)f(currently)e(w)o(orking)g(on)h(the)h(inte)o(gration)1937 1129 y(of)15 b(the)h(proposed)e(solution)h(with)h(other)f(security)g (functions,)g(such)1937 1228 y(as)28 b(con\002dentiality)e(and)g(inte)o (grity)g(protection)g(of)h(communica-)1937 1327 y(tions.)2020 1426 y(Finally)-5 b(,)17 b(we)h(belie)n(v)o(e)e(that)h(introducing)e(a) i(kind)g(of)f(virtual)h(cur)n(-)1937 1525 y(renc)o(y)29 b(can)h(serv)o(e)g(se)n(v)o(eral)f(other)g(purposes)g(in)i(mobile)e (ad-hoc)1937 1623 y(W)-10 b(ANs.)40 b(First,)26 b(it)g(can)e(be)h(used) f(to)h(remunerate)e(not)h(only)g(com-)1937 1722 y(munication)30 b(services,)k(as)e(described)f(in)h(this)g(paper)m(,)h(b)n(ut)e(also) 1937 1821 y(information)24 b(services.)45 b(Second,)27 b(it)g(can)f(be)h(used)f(as)i(a)f(w)o(ay)f(to)1937 1920 y(pay)h(for)g(the)h(usage)f(of)g(backbones)f(or)h(satellite)i(links,)g (when)e(a)1937 2019 y(terminode)h(has)h(to)h(communicate)d(with)j(a)f (v)o(ery)g(distant)g(party)-5 b(.)1937 2117 y(In)22 b(this)h(case,)g (the)f(virtual)f(currenc)o(y)f(will)j(ha)n(v)o(e)f(to)g(be)g(con)m(v)o (erted)1937 2216 y(in)e(some)g(w)o(ay)h(into)f(\223hard\224)f(currenc)o (y)-5 b(.)2543 2390 y(A)q Fm(C)t(K)t(N)t(O)r(W)t(L)t(E)t(D)t(G)t(M)t(E) t(N)t(T)2020 2526 y Fp(W)e(e)20 b(are)g(thankful)d(to)i(Re\002k)h(Molv) n(a)f(for)f(proposing)f(the)i(use)h(of)1937 2625 y(e)n(v)o(er)25 b(increasing)g(sending)g(and)g(recei)n(ving)f(counters)h(instead)h(of) 1937 2723 y(time-stamps)20 b(to)g(detect)g(replays.)2673 2897 y(R)t Fm(E)t(F)t(E)t(R)t(E)t(N)t(C)t(E)t(S)1937 3007 y([1])75 b(J.-P)-7 b(.)19 b(Hubaux,)i(J.-Y)-9 b(.)19 b(Le)h(Boudec,)i(S.)d(Giordano,)j(and)f(M.)e(Hamdi,)35 b(\223The)2089 3080 y(T)-5 b(erminode)20 b(Project:)27 b(to)n(w)o(ard)20 b(mobile)g(ad-hoc)g(W)-8 b(ANs,)j(\224)30 b(in)19 b Fo(Pr)m(oceedings)2089 3153 y(of)e(the)h(Mobile)h(Multimedia) h(Confer)n(ence)o(,)f(MOMUC)p Fm(,)e(San)g(Die)o(go,)h(1999.)1937 3226 y([2])82 b(T)-5 b(erminodes)19 b(web)e(site.)25 b Fa(http://www.terminodes.org/)p Fm(.)1937 3300 y([3])75 b(J.-P)-7 b(.)44 b(Hubaux,)54 b(J.-Y)-9 b(.)45 b(Le)h(Boudec,)54 b(M.)45 b(V)-9 b(ojno)o(vi)t(\264)-26 b(c,)55 b(S.)46 b(Giordano,)2089 3373 y(M.)23 b(Hamdi,)i(L.)d(Blaze)n(vi)t(\264)-26 b(c,)28 b(and)c(L.)e(Butty)t(\264)-26 b(an,)48 b(\223T)-5 b(o)n(w)o(ard)25 b(mobile)f(ad-hoc)2089 3446 y(W)-8 b(ANs:)25 b(T)-5 b(erminodes,)g(\224)32 b(T)-5 b(echnical)22 b(Report)e(No.)f (DSC/2000/006,)i(Swiss)2089 3519 y(Federal)e(Institute)g(of)e(T)-5 b(echnology)l(,)19 b(Lausanne,)e(January)i(2000.)1937 3592 y([4])75 b(F)-5 b(.)20 b(Stajano)k(and)e(R.)e(Anderson,)40 b(\223The)22 b(resurrecting)i(duckling:)33 b(security)2089 3665 y(issues)19 b(for)f(ad-hoc)i(wireless)f(netw)o(orks,)-5 b(\224)31 b(in)19 b Fo(Pr)m(oceedings)h(of)f(the)g(7th)g(In-)2089 3738 y(ternational)f(W)-6 b(orkshop)17 b(on)e(Security)j(Pr)m(otocols)p Fm(,)e(Cambridge,)h(UK,)e(April)2089 3811 y(1999.)1937 3884 y([5])75 b(L.)16 b(Zhou)h(and)h(Z.)e(Haas,)26 b(\223Securing)19 b(ad-hoc)g(netw)o(orks,)-5 b(\224)28 b Fo(IEEE)15 b(Network)p Fm(,)2089 3957 y(13\(6\):24\22630,)k(No)o(v)o(ember)o(-December)g (1999.)1937 4030 y([6])75 b(J.)23 b(Zhou)g(and)i(D.)d(Gollmann,)49 b(\223)-5 b(A)23 b(f)o(air)i(non-repudiation)j(protocol,)-5 b(\224)49 b(in)2089 4103 y Fo(Pr)m(oceedings)17 b(of)f(the)h(1996)f (IEEE)e(Symposium)j(on)e(Security)j(and)e(Privacy)p Fm(,)2089 4176 y(pages)i(55\22661,)f(May)h(1996.)1937 4249 y([7])75 b(M.)14 b(Franklin)j(and)e(M.)f(Reiter)m(,)22 b(\223F)o(air)16 b(e)o(xchange)h(with)f(a)f(semi-trusted)i(third)2089 4322 y(party)l(,)-5 b(\224)20 b(in)14 b Fo(Pr)m(oceedings)h(of)f(the)h (4th)f(A)n(CM)g(Confer)n(ence)i(on)e(Computer)h(and)2089 4395 y(Communications)20 b(Security)p Fm(,)f(pages)f(1\2266,)e(April)i (1997.)1937 4468 y([8])75 b(T)-5 b(.)17 b(Sandholm,)27 b(\223Unenforced)21 b(e-commerce)e(transactions,)-5 b(\224)31 b Fo(IEEE)16 b(Inter)o(-)2089 4542 y(net)i(Computing)p Fm(,)g(1\(6\):47\22654,)h(No)o(v)o(ember)o(-December)g(1997.)1937 4615 y([9])75 b(L.)29 b(Butty)t(\264)-26 b(an)32 b(and)f(J.-P)-7 b(.)28 b(Hubaux,)70 b(\223T)-5 b(o)n(w)o(ard)31 b(a)f(formal)h(model)g (of)e(f)o(air)2089 4688 y(e)o(xchange)k(\226)c(a)h(game)h(theoretic)i (approach,)-5 b(\224)71 b(T)-5 b(echnical)33 b(Report)e(No.)2089 4761 y(SSC/1999/039,)f(Swiss)c(Federal)h(Institute)h(of)e(T)-5 b(echnology)l(,)30 b(Lausanne,)2089 4834 y(December)19 b(1999.)1937 4907 y([10])42 b(A.)21 b(Menezes,)j(P)-7 b(.)20 b(v)n(an)j(Oorschot,)g(and)g(S.)d(V)-7 b(anstone,)41 b Fo(Handbook)23 b(of)f(Ap-)2089 4980 y(plied)d(Crypto)o(gr)o(aphy)p Fm(.)26 b(CRC)18 b(Press,)e(1997.)p eop %%Trailer end userdict 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