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y(In)c(this)h(approach,)e(the)i(tick)o(et)f(could)g(be)g(used)h(to) f(establish)h(a)g(se-)1974 282 y(cure)f(channel)f(\(needed)f(for)i(the) g(secure)g(distrib)n(ution)f(of)h(decryp-)1974 382 y(tion)31 b(k)o(e)o(ys\))g(between)g(the)g(user)h(and)f(the)g(service)h(pro)o (vider)d(and)1974 482 y(to)21 b(initialize)h(the)f(micropayment)e (session.)29 b(In)21 b(both)f(approaches,)1974 581 y(the)f(encrypted)e (stream)h(itself)i(could)e(be)h(distrib)n(uted)f(via)h(satellite,)1974 681 y(D)m(VD,)31 b(or)g(the)h(Internet.)57 b(The)31 b(brok)o(er)f(w)o (ould)h(al)o(w)o(ays)h(be)f(con-)1974 780 y(tacted)20 b(via)g(the)g(Internet.)2073 886 y(In)d(this)g(paper)m(,)f(we)h(will)g (focus)f(on)g(the)h(latter)g(approach:)k(we)c(as-)1974 986 y(sume)k(that)h(the)g(k)o(e)o(y)f(distrib)n(ution)f(serv)o(er)h (and)g(the)g(content)g(serv)o(er)1974 1086 y(are)j(serv)o(ers)f(on)g (the)h(Internet.)35 b(Ho)n(we)n(v)o(er)m(,)22 b(our)h(design)g (presented)1974 1185 y(belo)n(w)18 b(can)g(easily)g(be)g(adopted)f(to)i (the)f(former)f(approach)f(as)j(well.)1974 1437 y Fo(3)99 b(The)26 b(micr)n(opayment)g(pr)n(otocol)2073 1672 y Fm(W)-7 b(e)17 b(be)o(gin)e(the)g(detailed)g(description)f(of)i(our)f (payment)f(scheme)1974 1772 y(by)j(introducing)e(the)i(micropayment)e (protocol)g(that)j(serv)o(es)f(as)h(the)1974 1871 y(core)d(of)g(our)f (system.)24 b(Because)15 b(of)g(space)g(limitations,)h(we)g(present) 1974 1971 y(the)22 b(protocol)f(brie\003y)-5 b(.)30 b(A)23 b(detailed)e(description)g(can)h(be)g(found)f(in)1974 2071 y([2)o(].)2073 2177 y(Lik)o(e)34 b(man)o(y)f(other)g(micropayment) e(protocols)h(proposed)g(in)1974 2276 y(the)25 b(literature)g([1)o(,)h (4,)g(6)o(,)g(7],)g(ours)g(as)g(well)g(is)g(based)g(on)f(the)g(suc-) 1974 2376 y(cessi)n(v)o(e)19 b(release)f(of)h(elements)f(in)h(a)g (chain)g(of)f(cryptographic)d(hash)1974 2475 y(v)n(alues.)24 b(When)18 b(the)g(user)g Ff(U)28 b Fm(w)o(ants)18 b(to)h(b)n(uy)e(some) h(services)h(from)e(a)1974 2575 y(service)k(pro)o(vider)e Ff(S)5 b Fm(,)22 b(she)g(generates)f(a)h(fresh)f(chain)g(of)g(hash)h(v) n(al-)1974 2675 y(ues)27 b Ff(w)2173 2645 y Fe(0)2171 2695 y Fd(0)2209 2675 y Ff(;)14 b(w)2305 2687 y Fd(1)2342 2675 y Ff(;)g(w)2440 2645 y Fe(0)2438 2695 y Fd(1)2476 2675 y Ff(;)g(w)2572 2687 y Fd(2)2610 2675 y Ff(;)g(w)2708 2645 y Fe(0)2706 2695 y Fd(2)2743 2675 y Ff(;)g(:)g(:)g(:)g(;)g(w)2987 2687 y Fc(n)3032 2675 y Ff(;)g(w)3130 2645 y Fe(0)3128 2695 y Fc(n)3202 2675 y Fm(by)26 b(picking)g Ff(w)3650 2645 y Fe(0)3648 2695 y Fc(n)3721 2675 y Fm(at)i(ran-)1974 2774 y(dom)d(and)h(then)f(computing)f Ff(w)2905 2786 y Fc(i)2967 2774 y Fb(=)34 b Ff(h)p Fb(\()p Ff(w)3207 2744 y Fe(0)3205 2796 y Fc(i)3233 2774 y Fb(\))27 b Fm(and)e Ff(w)3499 2744 y Fe(0)3497 2796 y Fc(i)p Fe(\000)p Fd(1)3645 2774 y Fb(=)33 b Ff(h)p Fb(\()p Ff(w)3882 2786 y Fc(i)3910 2774 y Fb(\))1974 2874 y Fm(for)20 b Ff(i)j Fb(=)h Ff(n;)14 b(n)k Fq(\000)g Fb(1)p Ff(;)c(:)g(:)g(:)f(;)h Fb(1)p Fm(.)26 b(Ev)o(ery)20 b(tw)o(o)g(consecuti)n(v)o(e)f(hash)i(v)n(alues) 1974 2974 y Ff(w)2033 2986 y Fc(i)2061 2974 y Ff(;)14 b(w)2159 2943 y Fe(0)2157 2995 y Fc(i)2211 2974 y Fm(form)25 b(a)h(micropayment)c(tok)o(en.)41 b(Therefore,)24 b(intuiti)n(v)o(ely) -5 b(,)1974 3073 y(each)29 b(hash)g(v)n(alue)f(can)h(be)h(thought)d(of) i(as)h(a)g(half-tok)o(en.)50 b Ff(w)3822 3043 y Fe(0)3820 3094 y Fd(0)3887 3073 y Fm(is)1974 3173 y(called)26 b(the)h(root)f(of)h (the)g(hash)f(chain)g(and)g(it)i(is)g(not)e(a)h(half-tok)o(en)1974 3272 y(itself.)41 b(When)25 b(the)h(hash)f(chain)g(is)h(generated,)f Ff(U)35 b Fm(sends)25 b Ff(w)3753 3242 y Fe(0)3751 3293 y Fd(0)3815 3272 y Fm(to)h(a)1974 3372 y(brok)o(er)e Ff(B)29 b Fm(together)24 b(with)i(some)f(payment)f(information)e (\(e.g.,)k(a)1974 3472 y(credit)g(card)f(number\).)41 b Ff(B)31 b Fm(generates)26 b(a)g(tick)o(et,)i(which)e(includes)1974 3571 y Ff(w)2035 3541 y Fe(0)2033 3592 y Fd(0)2096 3571 y Fm(and)f(the)g(identi\002er)f(of)h Ff(B)t Fm(,)h(and)f(it)g(is)h (digitally)f(signed)f(by)h Ff(B)t Fm(.)1974 3671 y Ff(U)31 b Fm(starts)22 b(the)f(micropayment)e(session)i(by)g(sending)g(this)g (tick)o(et)h(to)1974 3771 y Ff(S)5 b Fm(.)2073 3877 y(In)35 b(principle,)i(the)e Ff(i)p Fm(-th)g(micropayment)d(from)i Ff(U)44 b Fm(to)36 b Ff(S)k Fm(has)1974 3976 y(three)18 b(steps.)25 b(First)19 b Ff(U)28 b Fm(sends)18 b(the)h(pair)f Fb(\()p Ff(w)3190 3988 y Fc(i)3218 3976 y Ff(;)c Fb(2)p Ff(i)e Fq(\000)g Fb(1\))18 b Fm(to)h Ff(S)k Fm(\(the)18 b(\002rst)1974 4076 y(half-tok)o(en\),)i(then)i Ff(S)27 b Fm(pro)o(vides)21 b(the)h Ff(i)p Fm(-th)g(piece)g(of)g(the)g(service) g(to)1974 4175 y Ff(U)9 b Fm(,)31 b(and)e(\002nally)g Ff(U)39 b Fm(sends)29 b(the)h(pair)f Fb(\()p Ff(w)3178 4145 y Fe(0)3176 4197 y Fc(i)3204 4175 y Ff(;)14 b Fb(2)p Ff(i)p Fb(\))29 b Fm(to)h Ff(S)k Fm(\(the)29 b(second)1974 4275 y(half-tok)o(en\).)22 b(Each)e(half-tok)o(en)d(can)j(be)g(check)o (ed)e(by)i Ff(S)25 b Fm(using)19 b(the)1974 4375 y(pre)n(viously)k (recei)n(v)o(ed)g(half-tok)o(en.)36 b(Ho)n(we)n(v)o(er)m(,)24 b(a)h(typical)f(service)1974 4474 y(session)h(consists)h(of)f(a)h (series)g(of)f(consecuti)n(v)o(e)e(micropayments,)1974 4574 y(and)i Ff(U)36 b Fm(can)25 b(send)h(the)g(second)f(half-tok)o(en) f(of)i(the)g Ff(i)p Fm(-th)g(payment)1974 4674 y Fb(\()p Ff(w)2067 4643 y Fe(0)2065 4695 y Fc(i)2093 4674 y Ff(;)14 b Fb(2)p Ff(i)p Fb(\))28 b Fm(and)g(the)h(\002rst)g(half-tok)o(en)d(of) i(the)g Fb(\()p Ff(i)d Fb(+)f(1\))p Fm(-st)29 b(payment)1974 4773 y Fb(\()p Ff(w)2065 4785 y Fc(i)p Fd(+1)2177 4773 y Ff(;)14 b Fb(2)p Ff(i)20 b Fb(+)h(1\))j Fm(in)g(a)g(single)g (message.)35 b(Furthermore,)22 b(since)i Ff(w)3917 4743 y Fe(0)3915 4795 y Fc(i)1974 4873 y Fm(can)g(al)o(w)o(ays)g(be)g (computed)e(from)h Ff(w)3067 4885 y Fc(i)p Fd(+1)3204 4873 y Fm(by)h Ff(S)5 b Fm(,)25 b(only)e(the)h(second)1974 4972 y(pair)18 b Fb(\()p Ff(w)2213 4984 y Fc(i)p Fd(+1)2325 4972 y Ff(;)c Fb(2)p Ff(i)e Fb(+)g(1\))18 b Fm(has)g(to)h(be)f(sent.)25 b(A)19 b(typical)f(service)g(session,)1974 5072 y(thus,)i(looks)f(lik)o (e)i(this:)1974 5225 y Ff(U)32 b Fq(!)23 b Ff(S)k Fb(:)65 b Ff(w)2394 5237 y Fd(1)2432 5225 y Ff(;)14 b Fb(1)1974 5325 y Ff(U)32 b Fq( )23 b Ff(S)k Fb(:)42 b Fn(\002r)o(st)21 b(piece)f(of)g(the)h(service)1859 5574 y Fm(3)p eop %%Page: 4 4 4 3 bop -182 83 a Ff(U)31 b Fq(!)23 b Ff(S)28 b Fb(:)65 b Ff(w)238 95 y Fd(2)276 83 y Ff(;)14 b Fb(3)-182 183 y Ff(U)31 b Fq( )23 b Ff(S)28 b Fb(:)42 b Fn(second)19 b(piece)h(of)g(the)h(service)-182 282 y Ff(:)14 b(:)g(:)-182 382 y(U)31 b Fq(!)23 b Ff(S)28 b Fb(:)65 b Ff(w)238 394 y Fc(l)264 382 y Ff(;)14 b Fb(2)p Ff(l)19 b Fq(\000)f Fb(1)-182 482 y Ff(U)31 b Fq( )23 b Ff(S)28 b Fb(:)42 b Fn(last)21 b(piece)e(of)i(the)f(service)-182 581 y Ff(U)31 b Fq(!)23 b Ff(S)28 b Fb(:)65 b Ff(w)240 551 y Fe(0)238 605 y Fc(l)264 581 y Ff(;)14 b Fb(2)p Ff(l)-83 728 y Fm(If)k(e)n(v)o(erything)e(goes)i(well,)g(then)g Ff(S)24 b Fm(can)17 b(present)h(the)g(tick)o(et)g(and)-182 828 y(the)e(pair)h Fb(\()p Ff(w)176 798 y Fe(0)174 851 y Fc(l)200 828 y Ff(;)d Fb(2)p Ff(l)r Fb(\))i Fm(to)h Ff(B)t Fm(,)h(which)e(pays)h Ff(S)22 b Fm(the)16 b(amount)g (correspond-)-182 927 y(ing)30 b(to)h Ff(l)i Fm(tok)o(ens)e(and)f(char) o(ges)g Ff(U)40 b Fm(with)31 b(the)g(same)g(amount.)56 b(If)-182 1027 y(something)28 b(goes)i(wrong)f(and)h(the)h(last)g (half-tok)o(en)d(is)j(missing,)-182 1127 y(then)22 b Ff(S)27 b Fm(can)c(only)f(present)f(the)i(tick)o(et)g(and)f(the)g(pair) h Fb(\()p Ff(w)1477 1139 y Fc(l)1503 1127 y Ff(;)14 b Fb(2)p Ff(l)21 b Fq(\000)f Fb(1\))-182 1226 y Fm(to)25 b Ff(B)t Fm(.)40 b(In)25 b(this)h(case,)g Ff(B)k Fm(pays)25 b Ff(S)30 b Fm(the)25 b(amount)f(corresponding)e(to)-182 1326 y Ff(l)k Fq(\000)f Fb(1)k Fm(tok)o(ens)g(and)f(char)o(ges)g Ff(U)39 b Fm(with)29 b(the)g(amount)f(correspond-)-182 1425 y(ing)f(to)h Ff(l)h Fm(tok)o(ens)e(\(which)g(lea)n(v)o(es)h(the)f (brok)o(er)f(with)i(a)g(surplus)f(of)-182 1525 y(the)f(v)n(alue)f(of)h (one)g(tok)o(en\).)41 b(The)26 b(surplus)f(of)h(the)g(brok)o(er)f(is)i (han-)-182 1625 y(dled)21 b(according)f(to)i(some)g(polic)o(y)f (\(e.g.,)g(it)i(can)f(be)g(distrib)n(uted)f(to)-182 1724 y(charity)e(purposes\).)-83 1824 y(In)26 b(our)g(micropayment)d (protocol,)j(none)f(of)h(the)h(parties)f(gain)-182 1924 y(an)o(y)21 b(\002nancial)h(adv)n(antage)f(by)h(cheating.)31 b(The)22 b(reason)g(is)h(that)g(the)-182 2023 y(service)h(pro)o(vider)e (can)i(ne)n(v)o(er)f(earn)h(more)g(than)g(the)h(v)n(alue)f(of)g(the) -182 2123 y(services)e(it)h(pro)o(vided,)d(since)j(the)f(user)g (authorizes)g(the)g(brok)o(er)f(to)-182 2222 y(pay)26 b(the)g(service)h(pro)o(vider)d(after)i(she)h(has)g(recei)n(v)o(ed)e (the)h(service)-182 2322 y(\(by)c(sending)g(the)h(second)f(half-tok)o (en\).)31 b(Similarly)-5 b(,)22 b(the)h(user)g(can)-182 2422 y(ne)n(v)o(er)h(recei)n(v)o(e)g(services)i(that)f(are)h(w)o(orth)f (more)g(than)g(that)g(she)h(is)-182 2521 y(char)o(ged)e(for)m(,)k (because)e(she)h(lets)h(the)f(brok)o(er)e(char)o(ge)h(her)g(before)-182 2621 y(recei)n(ving)g(the)i(service)f(\(when)g(releasing)g(the)h (\002rst)h(half-tok)o(en\).)-182 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b(pro)o(vider)e(speci\002ed)i(in)g(the)i(P)t Fj(R)q(O)q(V)t(I)t(D)t(E)t(R)t Fm(I)t Fj(N)t(F)t(O)i Fm(\002eld\))-16 4371 y(identi\002er)19 b(of)h(the)g(tick)o(et.)-99 4523 y Fq(\017)43 b Fm(E)t Fj(X)t(P)t Fm(T)t Fj(I)t(M)t(E)25 b Fm(speci\002es)e(an)f(e)o(xpiration)f(time)i(after)f(which)g(the)-16 4623 y(tick)o(et)31 b(should)f(not)h(be)h(used)f(by)g(the)g(user)g(and) g(cannot)f(be)-16 4723 y(redeemed)18 b(by)i(the)g(service)g(pro)o (vider)-5 b(.)-99 4874 y Fq(\017)43 b Fm(T)s Fj(O)t(K)t(E)t(N)t Fm(V)-6 b Fj(A)t(L)t(U)t(E)17 b Fm(is)f(the)g(v)n(alue)f(of)h(one)f (micropayment)e(tok)o(en)-16 4974 y(in)20 b(the)g(tick)o(et.)-99 5126 y Fq(\017)43 b Fm(B)t Fj(R)q(O)t(K)t(E)t(R)t Fm(I)t Fj(N)t(F)t(O)e Fm(contains)36 b(at)h(least)h(the)f(identi\002er)f(of)h (the)-16 5225 y(brok)o(er)23 b(that)i(issued)g(this)h(tick)o(et.)39 b(In)25 b(addition,)f(it)i(may)f(also)-16 5325 y(contain)19 b(the)h(public)f(k)o(e)o(y)h(certi\002cate)g(of)g(the)g(brok)o(er)-5 b(.)2057 83 y Fq(\017)43 b Fm(P)t Fj(R)q(O)q(V)t(I)t(D)t(E)t(R)t Fm(I)t Fj(N)t(F)t(O)31 b Fm(contains)c(at)h(least)g(the)g(identi\002er) f(of)g(the)2140 183 y(service)d(pro)o(vider)d(for)j(which)f(this)i (tick)o(et)f(is)h(v)n(alid,)g(and)e(the)2140 282 y(IP)17 b(addresses)g(and)f(port)h(numbers)e(of)i(its)h(content)e(serv)o(er)g (and)2140 382 y(its)21 b(k)o(e)o(y)f(distrib)n(ution)f(serv)o(er)-5 b(.)2057 541 y Fq(\017)43 b Fm(S)t Fj(E)t(R)o(V)t(I)t(C)t(E)t Fm(I)t Fj(N)t(F)t(O)26 b Fm(contains)d(information)e(about)h(the)i (stream,)2140 641 y(for)19 b(which)h(this)h(tick)o(et)f(is)h(v)n(alid.) 2057 800 y Fq(\017)43 b Fm(T)t Fj(E)t(C)t(H)t Fm(P)m Fj(A)t(R)t(A)t(M)t(S)26 b Fm(contains)21 b(parameters)g(that)h(are)g (necessary)2140 900 y(to)e(initialize)h(the)f(micropayment)d(scheme)j (\(i.e.,)f Ff(w)3660 869 y Fe(0)3658 920 y Fd(0)3696 900 y Fm(\),)h(and)g(a)2140 999 y(symmetric)d(session)h(k)o(e)o(y)f Ff(k)k Fm(encrypted)16 b(with)i(the)g(public)f(k)o(e)o(y)2140 1099 y Ff(K)2211 1111 y Fc(S)2284 1099 y Fm(\(i.e.,)26 b Fq(f)p Ff(k)s Fq(g)2591 1111 y Fc(K)2647 1119 y Fa(S)2691 1099 y Fm(\))g(of)f(the)g(k)o(e)o(y)f(distrib)n(ution)g(serv)o(er)g (speci-)2140 1198 y(\002ed)e(in)g(the)i(P)t Fj(R)q(O)q(V)t(I)t(D)t(E)t (R)t Fm(I)t Fj(N)t(F)t(O)h Fm(\002eld.)31 b(The)21 b(session)i(k)o(e)o (y)e Ff(k)k Fm(is)2140 1298 y(used)20 b(to)h(pro)o(vide)d(a)j(secure)g (channel)e(between)h(the)g(user)h(and)2140 1398 y(the)f(k)o(e)o(y)g (distrib)n(ution)f(serv)o(er)-5 b(.)2057 1557 y Fq(\017)43 b Fm(B)t Fj(R)q(O)t(K)t(E)t(R)t Fm(S)t Fj(I)t(G)25 b Fm(is)d(a)f(digital)g(signature)e(of)i(the)g(brok)o(er)e(on)h(the)2140 1656 y(pre)n(vious)26 b(\002elds,)j(which)e(pre)n(v)o(ents)f(ille)o (gitimate)h(modi\002ca-)2140 1756 y(tion)20 b(and)f(for)o(gery)f(of)i (the)g(tick)o(et.)1974 1957 y Fh(4.2)90 b(T)n(ick)o(et)23 b(acquisition)2036 2158 y Fm(1.)41 b(The)22 b(user)g Ff(U)31 b Fm(selects)23 b(a)g(service)f(pro)o(vider)e Ff(S)5 b Fm(,)22 b(and)g(a)g(service)2140 2257 y(\(stream\))i(of)g Ff(S)31 b Fm(that)24 b(she)h(w)o(ants)h(to)f(access.)39 b Ff(S)30 b Fm(may)24 b(adv)o(er)n(-)2140 2357 y(tise)c(its)g(streams)f (on)g(a)g(W)-7 b(eb)20 b(site,)g(so)f(the)g(user)g(can)g(mak)o(e)g(her) 2140 2456 y(selection)i(using)g(a)g(W)-7 b(eb)22 b(bro)n(wser)-5 b(.)28 b(Other)21 b(necessary)f(infor)n(-)2140 2556 y(mation,)g(such)h (as)h(the)f(IP)h(addresses)e(and)h(the)g(port)g(numbers)2140 2656 y(of)g(the)g(content)f(serv)o(er)g(and)h(the)g(k)o(e)o(y)f (distrib)n(ution)g(serv)o(er)g(of)2140 2755 y Ff(S)5 b Fm(,)25 b(and)e(the)h(price)g(of)f(one)h(encrypted)e(block)h(of)g (the)h(stream)2140 2855 y(can)c(also)g(be)h(do)n(wnloaded)c(from)i (this)i(W)-7 b(eb)21 b(site.)2036 3014 y(2.)41 b Ff(U)36 b Fm(generates)26 b(a)h(hash)g(chain)f(with)i(appropriate)c(length)i (ac-)2140 3114 y(cording)18 b(to)i(the)g(micropayment)d(protocol)h (presented)g(in)i(the)2140 3213 y(pre)n(vious)i(section,)i(and)f(a)h (symmetric)f(session)h(k)o(e)o(y)f Ff(k)s Fm(.)36 b(She)2140 3313 y(encrypts)20 b Ff(k)k Fm(with)d(the)g(public)f(k)o(e)o(y)g Ff(K)3241 3325 y Fc(S)3310 3313 y Fm(of)h(the)g(k)o(e)o(y)f(distrib)n (u-)2140 3413 y(tion)j(serv)o(er)f(of)g Ff(S)5 b Fm(,)24 b(which)e(is)i(also)f(obtained)f(from)g(the)g(W)-7 b(eb)2140 3512 y(site)29 b(of)f Ff(S)5 b Fm(.)50 b Ff(U)38 b Fm(generates)27 b(a)i(lar)o(ge)e(random)g(number)g(to)h(be)2140 3612 y(used)16 b(as)h(the)f(tick)o(et)g(identi\002er)m(,)g(chooses)f(an)h(e) o(xpiration)e(time,)2140 3711 y(and)21 b(sets)i(the)e(v)n(alue)g(of)h (one)f(micropayment)e(tok)o(en)i(accord-)2140 3811 y(ing)j(to)h(the)g (price)f(of)h(one)f(encrypted)f(block)h(of)g(the)h(desired)2140 3911 y(stream.)2036 4070 y(3.)41 b(Then)21 b Ff(U)31 b Fm(submits)22 b(the)g(data)g(required)e(to)i(construct)f(a)h(tick)o (et)2140 4170 y(\(partly)g(obtained)g(from)h(the)g(W)-7 b(eb)24 b(site)h(of)e Ff(S)5 b Fm(,)24 b(partly)f(gener)n(-)2140 4269 y(ated)29 b(by)g Ff(U)38 b Fm(herself)28 b(in)i(the)f(pre)n(vious) e(step\))i(to)h(the)f(brok)o(er)2140 4369 y Ff(B)i Fm(together)24 b(with)j(some)f(payment)e(information)g(\(e.g.,)j(her)2140 4468 y(credit)20 b(card)f(number\).)2036 4628 y(4.)41 b Ff(B)c Fm(checks)32 b(the)g(recei)n(v)o(ed)f(payment)g(information)f (and)i(the)2140 4727 y(solv)o(enc)o(y)19 b(of)i(the)g(user)-5 b(.)29 b(If)21 b(it)h(is)g(not)e(satis\002ed,)i(then)f(the)g(pro-)2140 4827 y(tocol)16 b(is)i(aborted.)k(Otherwise,)17 b Ff(B)k Fm(constructs)16 b(a)h(tick)o(et)g(using)2140 4926 y(the)j(recei)n(v)o (ed)f(information,)e(digitally)j(signs)h(it,)f(and)g(returns)2140 5026 y(it)29 b(to)g Ff(U)9 b Fm(.)50 b(It)29 b(also)g(stores)g(the)g (tick)o(et)g(and)f(the)g(payment)f(in-)2140 5126 y(formation)e(in)i (its)g(internal)f(database)h(for)f(future)f(use.)46 b(The)2140 5225 y(communication)27 b(between)i Ff(U)39 b Fm(and)29 b Ff(B)35 b Fm(is)30 b(protected)f(using)2140 5325 y(standard)19 b(security)h(protocols)e(\(e.g.,)h(SSL\).)1859 5574 y(4)p eop %%Page: 5 5 5 4 bop -120 83 a Fm(5.)41 b Ff(U)25 b Fm(v)o(eri\002es)16 b(the)g(recei)n(v)o(ed)f(tick)o(et)i(and)e(stores)i(it)g(together)e (with)-16 183 y(the)20 b(corresponding)d(hash)j(chain)f(and)h(session)g (k)o(e)o(y)-5 b(.)-182 409 y Fh(4.3)90 b(T)n(ick)o(et)23 b(usage)-120 635 y Fm(1.)41 b(The)26 b(user)g Ff(U)36 b Fm(connects)25 b(to)i(the)f(k)o(e)o(y)g(distrib)n(ution)g(serv)o(er)f (of)-16 735 y(the)19 b(service)f(pro)o(vider)f Ff(S)24 b Fm(\(the)18 b(necessary)g(addressing)g(infor)n(-)-16 834 y(mation)c(is)j(obtained)d(from)h(the)g(tick)o(et\),)h(and)f(sends) h(the)g(tick)o(et)-16 934 y(to)k(the)g(serv)o(er)-5 b(.)-120 1125 y(2.)41 b(The)19 b(k)o(e)o(y)h(distrib)n(ution)f(serv)o(er)g(v)o (eri\002es:)83 1315 y Fq(\017)41 b Fm(that)24 b(the)f(tick)o(et)h (contains)e(the)i(identi\002er)f(of)g Ff(S)5 b Fm(,)24 b(and)f(if)166 1415 y(this)e(is)g(not)f(the)g(case,)h(it)g(aborts)e (the)h(protocol;)83 1560 y Fq(\017)41 b Fm(the)22 b(e)o(xpiration)f (time)h(of)g(the)g(tick)o(et,)h(and)e(if)i(the)f(tick)o(et)166 1660 y(is)f(e)o(xpired,)e(it)i(aborts)e(the)h(protocol;)83 1805 y Fq(\017)41 b Fm(that)24 b(the)g(v)n(alue)f(of)g(one)g (micropayment)e(tok)o(en)i(of)g(the)166 1905 y(tick)o(et)33 b(corresponds)c(to)j(the)g(price)g(of)g(one)f(block)g(of)166 2004 y(the)19 b(requested)f(stream,)h(and)f(if)h(this)h(is)f(not)g(the) g(case,)g(it)166 2104 y(aborts)h(the)g(protocol;)83 2249 y Fq(\017)41 b Fm(the)18 b(signature)e(of)h(the)h(brok)o(er)m(,)e(and)h (if)g(it)i(is)f(not)f(correct,)166 2349 y(then)j(it)h(aborts)f(the)g (protocol;)83 2494 y Fq(\017)41 b Fm(that)16 b(the)f(tick)o(et)h(is)g (not)f(stored)f(in)i(its)g(internal)f(database,)166 2594 y(where)23 b(it)i(stores)f(used)f(tick)o(ets)h(until)g(their)f(e)o (xpiration)166 2693 y(time;)e(if)f(it)h(\002nds)f(this)h(tick)o(et,)f (it)h(aborts)f(the)g(protocol.)-120 2884 y(3.)41 b(If)22 b(all)g(the)g(v)o(eri\002cations)f(are)h(successful,)g(then)f(the)h(k)o (e)o(y)g(dis-)-16 2984 y(trib)n(ution)15 b(serv)o(er)g(stores)i(the)f 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Fm(results)25 b(in)2323 1918 y(the)20 b(root)g(of)g(the)g(hash)g(chain)g Ff(w)3247 1888 y Fe(0)3245 1939 y Fd(0)3304 1918 y Fm(stored)g(in)g(the)h(tick)o (et;)2323 2018 y(from)h(this)i(computation,)d(it)j(kno)n(ws)e(the)i (number)d Ff(l)k Fm(of)2323 2117 y(tok)o(ens)19 b(used;)2240 2245 y Fq(\017)41 b Fm(that)17 b(this)h(tick)o(et)f(has)h(not)e(been)h (redeemed)f(before,)g(and)2323 2344 y(if)k(this)h(is)g(not)f(the)g (case,)g(it)h(aborts)f(the)g(protocol.)2036 2505 y(4.)41 b(If)33 b(all)i(the)e(v)o(eri\002cations)g(are)g(successful,)k(then)c Ff(B)39 b Fm(marks)2140 2604 y(the)24 b(tick)o(et)g(as)h(redeemed,)e (pays)h(the)g(service)g(pro)o(vider)m(,)e(and)2140 2704 y(char)o(ges)e(the)h(user)g(according)e(to)i(the)g(rules)g(speci\002ed) g(in)g(the)2140 2804 y(micropayment)c(protocol.)1974 3031 y Fo(5)99 b(Analysis)2073 3241 y Fm(In)29 b(this)g(section,)h(we)f (brie\003y)f(discuss)h(ho)n(w)f(our)f(scheme)i(ad-)1974 3341 y(dresses)20 b(the)h(problems)d(listed)j(in)f(Section)g(1.)2057 3510 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b(operations)h(of)i(the)f (micropayment)e(protocol,)47 b(we)-182 5225 y(used)16 b(the)h(AB)m(A)g(\(www)-5 b(.aba.net.au\))13 b(implementation)i(of)h (the)h(Ja)n(v)n(a)-182 5325 y(Cryptography)f(Extension)j(interf)o(ace)g (\(www)-5 b(.ja)n(v)n(asoft.com\).)2073 83 y(W)e(e)37 b(fully)e(implemented)e(only)h(the)i(tick)o(et)f(usage)g(protocol.)1974 183 y(T)m(ick)o(et)24 b(acquisition)f(is)i(simulated)e(by)h(a)g(simple) g(utility)g(program)1974 282 y(that)g(generates)g(tick)o(ets)h(from)e (data)h(stored)g(in)h(an)f(input)g(\002le.)38 b(W)-7 b(e)1974 382 y(did)20 b(not)g(implement)e(the)j(tick)o(et)f(redemption) e(protocol.)1974 599 y Fo(7)99 b(Conclusion)2073 800 y Fm(W)-7 b(e)28 b(presented)d(an)h(electronic)g(payment)e(scheme)i (for)g(broad-)1974 899 y(cast)c(multimedia)e(streams.)29 b(Our)21 b(scheme)f(respects)i(the)f(pay-per)n(-)1974 999 y(use)16 b(principle,)f(mak)o(es)h(cheating)e(uninteresting)g(for)h (both)g(the)h(user)1974 1099 y(and)21 b(the)g(service)g(pro)o(vider)m (,)d(resists)23 b(for)o(gery)18 b(and)j(o)o(v)o(er)n(-spending,)1974 1198 y(protects)36 b(sensiti)n(v)o(e)g(payment)g(information)e(and)i (user)h(pri)n(v)n(ac)o(y)-5 b(,)1974 1298 y(and)24 b(allo)n(ws)h(the)g (identi\002cation)f(of)g(misbeha)n(ving)f(users.)39 b(It)26 b(uses)1974 1398 y(a)f(trusted)f(party)-5 b(,)24 b(called)g(the)h(brok) o(er)m(,)e(that)i(issues)g(a)g(tick)o(et)g(to)f(the)1974 1497 y(user)m(,)19 b(which)f(allo)n(ws)i(her)e(to)i(access)g(the)f (service)g(pro)o(vided)d(by)j(the)1974 1597 y(service)h(pro)o(vider)d (in)j(a)h(semi-anon)o(ymous)16 b(w)o(ay)-5 b(.)25 b(In)20 b(addition,)e(the)1974 1696 y(tick)o(et)k(guarantees)f(to)h(the)h (service)f(pro)o(vider)d(that)k(it)g(will)f(be)h(paid)1974 1796 y(for)17 b(the)h(service)f(pro)o(vision.)22 b(W)-7 b(e)19 b(described)e(the)g(protocols)g(run)g(by)1974 1896 y(the)k(user)m(,)h(the)f(service)g(pro)o(vider)m(,)e(and)i(the)h (brok)o(er)e(in)i(details.)29 b(W)-7 b(e)1974 1995 y(brie\003y)19 b(analyzed)f(our)h(scheme)g(and)g(reported)f(about)g(a)i(prototype)1974 2095 y(implementation.)1974 2312 y Fo(Refer)n(ences)1974 2513 y Fm([1])40 b(R.)34 b(Anderson,)h(C.)f(Manif)o(a)n(v)n(as,)i(and)d (C.)h(Sutherland.)d(Net-)2112 2612 y(Card)i(\226)g(A)g(practical)g (electronic)e(cash)i(system.)g(T)-6 b(echnical)2112 2712 y(Report,)34 b(Cambridge)d(Uni)n(v)o(ersity)-5 b(,)33 b(Computer)d(Laboratory)-5 b(,)2112 2812 y(1995.)1974 2962 y([2])40 b(L.)29 b(Butty)5 b(\264)-33 b(an.)27 b(Remo)o(ving)g (the)h(\002nancial)g(incenti)n(v)o(e)f(to)h(cheat)2112 3062 y(in)46 b(micropayment)c(schemes.)j Fn(IEE)h(Electr)l(onics)f (Letter)o(s)p Fm(,)2112 3162 y(36\(2\):132\226133,)15 b(January)k(2000.)1974 3313 y([3])40 b(A.)59 b(Freier)m(,)68 b(P)-9 b(.)59 b(Karlton,)68 b(and)58 b(P)-9 b(.)59 b(K)m(ocher)-5 b(.)58 b(The)h(SSL)2112 3412 y(protocol.)50 b(V)-9 b(ersion)52 b(3.0.)f(Internet)g(Draft.)h(\(a)n(v)n(ailable)g(at)2112 3512 y(http://home.netscape.com/eng/ssl3/\))1974 3663 y([4])40 b(R.)57 b(Hauser)m(,)64 b(M.)56 b(Steiner)m(,)64 b(and)56 b(M.)g(W)-7 b(aidner)i(.)56 b(Micro-)2112 3762 y(payments)34 b(based)h(on)g(iKP.)g(T)-6 b(echnical)34 b(Report)h(RZ)h(2791,)2112 3862 y(IBM)21 b(Research,)f(February)e (1996.)1974 4013 y([5])40 b(G.)29 b(La)o(wton.)f(V)-5 b(ideo)28 b(streams)i(into)e(the)h(mainstream.)f Fn(IEEE)2112 4112 y(Computer)p Fm(,)20 b(33\(7\):12\22617,)c(July)k(2000.)1974 4263 y([6])40 b(T)-6 b(.)44 b(Pedersen.)f(Electronic)f(payment)h(of)g (small)h(amounts.)2112 4363 y(T)-6 b(echnical)31 b(Report)h(D)m (AIMI-PB-495,)g(Aarhus)f(Uni)n(v)o(ersity)-5 b(,)2112 4462 y(Computer)19 b(Science)h(Department,)e(1995.)1974 4613 y([7])40 b(R.)g(Ri)n(v)o(est)f(and)f(A.)h(Shamir)-5 b(.)39 b(P)o(ayW)-7 b(ord)38 b(and)g(MicroMint:)2112 4713 y(T)-7 b(w)o(o)28 b(simple)g(micropayment)d(schemes.)j(T)-6 b(echnical)27 b(report,)2112 4812 y(MIT)20 b(Laboratory)e(for)h (Computer)g(Science,)h(1996.)1974 4963 y([8])40 b(E.)21 b(v)n(on)g(F)o(aber,)g(R.)g(Hammelrath,)f(and)h(F-P)-9 b(.)21 b(Heider)-5 b(.)21 b(The)g(se-)2112 5063 y(cure)k(distrib)n (ution)f(of)i(digital)f(contents.)f(In)i Fn(Pr)l(oceedings)e(of)2112 5162 y(the)h(13th)f(Annual)f(Computer)h(Security)h(Applications)e(Con-) 2112 5262 y(fer)m(ence)p Fm(,)d(San)g(Die)o(go,)f(California,)h (December)e(1997.)1859 5574 y(6)p eop %%Trailer end userdict /end-hook known{end-hook}if %%EOF