Ac as de las XV Jo nadas
de Ingenie ía Telemá ica
(JITEL 2021),
A Co uña (España),
27-29 de oc ub e de 2021.
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A p o ocol o da a exchange wi h ee samples
using sma con ac s
Ra ael Gen´
es-Du ´
an, Juan He n´
andez-Se ano, Osca Espa za, Miquel So iano, Jos´
e Luis Mu˜
noz-Tapia
Depa amen o de Ingenie ´
ıa Telem´
a ica,
Uni e si a Poli `
ecnica de Ca alunya (UPC)
( a ael.genes,j.he nandez,osca .espa za,miquel.so iano,jose.luis.munoz)@upc.edu
Ma a Bell´
es-Mu˜
noz
Uni e si a Pompeu Fab a (UPF)
[email p o ec ed]
Dis us be ween da a p o ide s and da a con-
sume s is one o he main obs acles hampe ing digi al-
da a comme ce o ake o . Da a p o ide s wan o
ge paid o wha hey o e , while da a consume s
wan o know exac ly wha a e hey paying o be o e
ac ually paying o i . In his pape , we summa ize
a p o ocol ha o e comes his obs acle by building
us based on wo main ideas. Fi s , a p obabilis ic
e i ica ion p o ocol, whe e some andom samples o
he eal da ase a e shown o buye s in o de o allow
hem o make an assessmen be o e commi ing any
paymen ; and second a gua an eed, p o ec ed paymen
p ocess, en o ced wi h sma con ac s on a public
blockchain, ha gua an ees he paymen o he da a i
and only i he da a p o ided mee s he ag eed e ms,
and ha e unds hones playe s o he wise.
Palab as Cla e—da a exchange, sma con ac ,
blockchain, DLT, paymen s
I. INTRODUCCI ´
ON
The use o da a has inc easingly become a c ucial ac o
in he success o businesses. Businesses no only collec
and analyse he da a hey gene a e, bu inc easingly ely on
hi d pa y da a o enhance i s business alue. In gene al,
making p ope da a ag eemen s is no easy, specially he
ask o aluing da a and con incing cus ome s o hei
alue wi hou gi ing hem away [1]. The c ea ion o
ma ke places add esses many o hese p oblems. Allowing
p o ide s and consume s o deal wi h common in e es s
in a pla o m whe e bo h pa ies can mee each o he
and ade in o ma ion sol es he in eg a ion p oblem o
connec ing consume s and p o ide s.
This a icle ocuses on he p oblem o con incing con-
sume s o da a alue, which can be seen as a o m o lack
o us owa ds da a p o ide s. T adi ionally, his p oblem
canno be sol ed wi hou p e iously es ablishing con i-
dence be ween pa ies. This ep esen s an en y ba ie o
new p o ide s in he ma ke , hu ing compe ence and hus,
educing u ili y o consume s. To exchange alue sa ely,
i is essen ial o ensu e ha consume s ge he p oduc
hey a e paying o and ha p o ide s ge paid. These wo
hings a e o en ca ied ou wi hou any s ic p o ocols and
gua an eed jus by exis ing us . Typically, coun e pa ies
ha know each o he om p e ious expe ience o ha a e
aligned wi h u u e in e es s, a e con iden ha no in en
o scam will be made by he o he pa y, since con idence
is o en mo e bene icial han gains om aud.
Bu , when s onge assu ance han ha is needed, i
is a common p ac ice o use a us ed hi d-pa y (TTP)
o whom all pa ies us o gua an ee ha he p ocess
is ca ied ou co ec ly by all indi iduals in ol ed. TTPs
en ail an ex a cos o all pa ies, and gene a e a single
poin o ailu e ha could p oduce c i ic delays and denial
o se ices. Dis ibu ed Ledge Technologies (DLTs) can
be seen as a pa adigm shi when i comes o he need o
TTPs. Using DLTs, all pa icipan s in he ne wo k can
main ain a se o synch onized da a (who owns wha )
wi hou he need o a cen al au ho i y (TTP) gua an eeing
in eg i y, ai ness and da a a ailabili y.
In his pape we summa ize DEFS (Da a Exchange wi h
F ee Sample P o ocol), a p o ocol ha add esses he lack-
o - us be ween p o ide s and consume s in a da a ade.
DEFS p ese es he secu i y, p i acy and ai ness s an-
da ds ha ma ke places should gua an ee, and i includes
he capabili y o checking some sample po ions o he
da ase be o e commi ing o pu chase.
II. BACKGROUND
A. Me kle Hash T ees
A Me kle hash ee (MHT) is an au hen ica ed da a
s uc u e whe e e e y lea node o he ee con ains he
204
c yp og aphic hash o a da a block and e e y non lea
node con ains he conca ena ed hashes o i s child nodes
[2]. MHTs allow o link a se o da a o a unique hash
alue, he Me kle hash ee oo (MHR), allowing e icien
and secu e e i ica ion o consis ency and con en o la ge
se s o da a.
Figu e 1 con ains an example o a MHT wi h 8 lea es.
To show ha a ce ain alue is s o ed in a lea o he MHT,
one can c ea e a Me kle p oo (MP), which consis s o a
lis o he addi ional nodes equi ed o compu e he oo
o he ee. Fo ins ance, a Me kle p oo showing ha h3
is s o ed in he MHT om Figu e 2 would consis o he
nodes h2,h01 h4567,h01234567. No e ha wi h h3and he
i s h ee nodes o his lis anyone can compu e he oo
o he ee. I he oo ma ches h01234567, hen he p oo
is alid p oo o membe ship o h3in he ee.
Fig. 1. MHT o 8 lea s.
Fig. 2. Me kle p oo o h3. MP(h3)=h01234567,h4567,h01,h2
III. STATE OF THE ART
Decen alized ma ke places ha e a isen as a solu ion
o enhance secu i y, so e eign y and us in da a ex-
changes [3], [4], [5].
One in e es ing ini ia i e is GAIA-X [6], which is an
Eu opean p ojec o de elop he ounda ions o a ed-
e a ed open da a in as uc u e connec ing bo h classical
a chi ec u es wi h decen alized in as uc u es in o de o
build a anspa en ecosys em o he end use s aking
ad an age o he decen alized bene i s.
One o he main echnologies ha is os e ing da a
ma ke places is he In e ne o Things (IoT ) wi h huge
amoun s o da a being gene a ed om senso s and de ices.
The inc easing necessi y o mone izing hese da a is also
pushing esea ch. In he li e a u e, we can ind se e al
wo ks ha p opose decen alized ma ke places o IoT
using dis ibu ed ledge echnologies o enhance he da a
exchanges wi h anspa ency, us and in eg i y [7], [8],
[9]. Among o he s, decen alized ma ke places a e being
implemen ed in new dis up i e scena ios such as a i icial
in elligence [10], sma ci ies [11], [12], and he connec ed
ca [13]. In ac , he alue o he da a is becoming mo e
and mo e impo an o he business in e ac ions which is
e lec ed in he new echnologies and hei necessi y o
gene a e his new e a o decen alized ma ke places.
An example o a decen alized da a ading solu ion is
p esen ed in [14]. As in ou p o ocol, he da a on sale
a e no s o ed on he blockchain bu in some ex e nal
(and possibly dis ibu ed) s o age pla o m. Simila o ou
p o ocol, he p oposed solu ion symme ically enc yp s
da a on sale and uses a Me kle ee o c yp og ams o
egis e he associa ed ades on he blockchain. Howe e ,
he solu ion p oposed no only equi es o gene a e sym-
me ic c yp og ams bu also each o hese c yp og ams
needs o be asymme ically signed. Addi ionally, au ho s
p opose o use Plain ex Checkable Enc yp ion (PCE) [15]
o check on-chain ha he c yp og ams ha e been co ec ly
enc yp ed. In DEFS, we a oid using asymme ic enc yp-
ion, which is much slowe han symme ic enc yp ion.
Ano he ema kable implemen a ion o a decen alized
da a ading solu ion is p esen ed in [16], whe e au ho s
p esen SDTE, a secu e blockchain-based da a ading
ecosys em. As ou p o ocol, SDTE ies o mi iga e he
exis ence o dishones pa ies in da a exchanges. Howe e ,
SDTE ocuses on an scena io in which he buye does no
need o ha e access o a comple e da ase bu i only needs
he indings om he da a analysis. Fo his case, SDTE
p oposes a da a p ocessing-as-a-se ice, whe e he buye
is paying o he analysis o he selle ’s da ase . SDTE
is build using an In el’s SGX-based secu e execu ion
en i onmen o p o ec he da a p ocessing, he sou ce da a
and he analysis esul s. As we will show in he ollowing
sec ion, DEFS is no designed as a da a p ocessing-as-a-
se ice bu as a da a exchange-as-a-se ice. In he la e ,
he selle wan s o buy he comple e da ase no compu ed
da a. Fo his scena io, DEFS p o ides a p obabilis ic
e i ica ion p o ocol and a con lic esolu ion p o ocol ha
is gua an eed and suppo ed by a sma con ac .
IV. DATA EXCHANGE PROTOCOL
In his sec ion we summa ize DEFS, a p o ocol ha
add esses he p oblem o da a ading be ween p o ide
and consume s using a sma con ac deployed in he
blockchain as a b oke . As we explained be o e, he use o
DLTs can eplace he ole o TTPs in paymen p ocesses.
When using DLTs, pa icipan s in he ne wo k can main-
ain synch onized da a and sha e paymen in o ma ion
wi hou he need o a cen al au ho i y, gua an eeing his
way he in eg i y, ai ness and a ailabili y o he da a. In
his manne , DEFS makes use o a sma con ac o p e-
se e he secu i y and p i acy s anda ds ha ma ke places
should gua an ee.
Ano he gap o co e in his da a ading scena io is gen-
e a ing us be ween da a consume s and da a p o ide s.
He e i comes he no el y o DEFS: ou p oposed da a
exchange p o ocol is designed wi h he capabili y o
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205
checking andom samples om he da ase , so ha con-
sume s a e able o in e i he comple e da ase is wo h o
be paid o , enhancing he us o he consume ’s side. On
he o he side, he sma con ac ac s as a b oke du ing
he paymen p ocedu e, ensu ing p o ide s ha hey will
ecei e he paymen o he da a hey exchanged.
A. P o ocol Explana ion
We assume ha be o e s a ing he p o ocol, a da a
p o ide ad e ises he da a o he public using o -
blockchain means, such as a da a ma ke place. Then, a
consume in e es ed in a pa icula da ase con ac s he
p o ide , who s a s he DEFS p o ocol o pe o m he
da a exchange and paymen . To p e en po en ial ex ensi e
leaks o he da a, i is impo an ha he e is one DEFS
p o ocol pe each indi idual consume . DEFS consis s o
h ee di e en phases:
1) P o ocol p epa a ion: in his ini ial phase, he
p o ide p epa es no only he da a o be exchanged,
bu also all he pa ame e s and c yp og aphic ma e-
ial necessa y o demons a e ha he da a exchange
is secu e and p i a e. Mo e speci ically, he p o ide :
•Di ides he comple e da ase in po ions. These
po ions a e chosen andomly om he da ase
(no consecu i ely).
•Gene a es a seed o gene a e symme ic c yp o-
g aphic keys.
•Uses hese keys o c ea e a MHT, whose oo
can be used o check he co ec ness o his
c yp og aphic ma e ial.
•Enc yp s a andom pe mu a ion o he da a
po ions wi h he keys, ob aining an enc yp ed
and andomized e sion o he whole da ase .
•C ea es ano he MHT using he hashes o hese
c yp og ams as lea es, whose oo can be used
o e i y he co ec ness o he c yp og ams
gene a ed.
•Deploys a sma con ac in he blockchain
con aining ce ain public pa ame e s and ha
sma con ac ac s as a b oke du ing he es
o he p o ocol.
I he consume has in e es in ob aining he da ase ,
he p o ocol con inues as ollows:
•The consume ecei es he whole da ase en-
c yp ed bu i canno be dec yp ed a ha e y
momen .
•The consume que ies he sma con ac o
ob ain he oo o he ee o c yp og ams and
e i ies ha all he c yp og ams belong o his
ee.
A his poin , all en i ies (consume , p o ide and
sma con ac ) a e eady o s a he p o ocol execu-
ion phase, in which he consume will ha e access
o he comple e da ase and pe o m he paymen .
2) P o ocol execu ion: in his phase, he consume will
be able o ge some samples o he da ase ( o ee)
o e alua e i i is wo h o pay, and i so, i will
ob ain he da ase and he p o ide will be paid:
•The consume will choose a andom some
sample po ions o be e ealed.
•The p o ide will disclose he keys o hose
samples, so he consume can e alua e he qual-
i y o he da ase .
•I he consume is no con inced, he p o ocol
ends he e. Howe e , i i decides ha i is wo h
paying he da ase , i will commi he paymen
o he sma con ac .
•The p o ide is asked o publish he seed ( ha
will disclose all he enc yp ion keys) in he
sma con ac .
•I he consume is able o p ope ly dec yp he
da ase , a e a imeou , he p o ide is paid and
he p o ocol ends.
•I he consume is able o p o e ha he e we e
p oblems wi h he p e ious p ocedu e, i s a s
he con lic esolu ion phase o ob ain a e und.
The ollowing phase will only be needed in case
he consume conside s ha is chea ed on.
3) Con lic esolu ion*: his phase is op ional, i only
akes place i he consume de ec s a p o ide mis-
beha iou . The ollowing a e he cases ha can
end wi h a e und i he is able o demons a e his
misbeha iou :
•Keys a e no p ope ly gene a ed.
•C yp og ams do no ha e he p ope o ma .
B. P o ocol P ope ies
The main p ope ies p o ided by ou p o ocol a e he
ollowing:
1) Da a samples e alua ion. The consume ge s a
ee se o ai samples o he da a being aded
be o e paying. The p o ocol ensu es ha nei he he
consume no he p o ide a e able o manipula e
he chosen da a o selec speci ic samples.
2) Paymen gua an ees. The p o ide ge s paid i and
only i he consume has access o he whole se
o da a. Tha is, he consume can no ge he da a
wi hou paying o i and he p o ide does no ge
paid wi hou disclosing he da a.
3) The solu ion is cos -e icien . Due o high ees on
public ledge s, DEFS minimizes he amoun o da a
s o ed on he ne wo k, which is also independen
o he quan i y o da a aded. This way, bo h he
amoun o da a s o ed and he numbe o in e ac ions
wi h he dis ibu ed ledge is cons an .
4) Non- epudia ion. The DEFS p o ocol ensu es ha
any pa y in ol ed in he exchange is no able
o cancel and/o deny he da a exchange once an
ag eemen is made. Since he hash unc ion used
o gene a e he MHT is assumed o be collision-
esis an , he MRC and MRK logged in he sma
con ac c ea ion will p e en o he da a o be aked
as bough o sold his way. Mo eo e , he use o a
public blockchain enhances he in eg i y o he ac o
ac ions.
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206
5) Li eness. The di e en imeou s gua an ee ha he
p o ocol eaches a inal s a e, e en when one o he
pa ies qui s in ad ance. The p o ide can cancel
he sma con ac i no consume eaches him ou
and he imeou s se a e paymen s ensu e ha
any coun e pa y can inalize he execu ion o he
p o ocol a ou ably o i i he o he pa y does no
ac on ime.
C. S a e Diag am
The p o ocol ope a ion and he in e ac ions be ween he
di e en s akeholde s and he sma con ac a e de ailed
in Figu e 3.
imeOu 2
elapsed
x Consume SC:→
con lic K(·)
con lic D(·)
x P o ide SC:→
eleaseSeed(·)
x P o ide SC:→
cons uc o (·)
con ac C ea ed
consume
HasPaid
seedReleased
consume
Re undAllowed
p o ide
Wi hd awalAllowed
x Consume SC:→
pay(·)
p o ide
chea ed?
ue alse
x P o ide SC:→
cancelCon ac (·)
imeOu 1
elapsed
x Consume SC:→
e und(·) x P o ide SC:→
wi hd aw(·)
Fig. 3. S a e diag am o he sma con ac .
V. CONCLUSIONS
Dis us is one o he main obs acles o implemen
exchanges be ween da a p o ide s and da a consume s in a
decen alized way. In his a icle, we summa ize a p o ocol
ha allows a consume o p obabilis ically ob ain and
check a subse o a da ase on sale om a p o ide be o e
commi ing he paymen . The p o ocol is execu ed using
a sma con ac deployed in a public dis ibu ed ledge .
Once he consume accep s o buy he da ase , he paymen
p ocess, he ag eed e ms, and he possible e unds a e
managed and en o ced by he sma con ac . To expose
he da ase , ou p o ocol spli s he da a in po ions and
enc yp s and s o es each po ion o -chain. Then, we c ea e
a MHT o he c yp og ams and ano he MHT o he
enc yp ion keys. The enc yp ion keys a e ela ed o each
o he using a c yp og aphic hash unc ion in a way ha
allows us o implemen a cos -e icien con lic esolu ion
mechanism. The secu i y analysis o ou p o ocol shows
ha consume s and p o ide s a e economically p o ec ed
and ha he p o ide can educe he isks o iden i y-
eplica ion a acks by adjus ing he amoun o ee samples
disclosed o he consume .
AGRADECIMIENTOS
This esea ch has been unded by i3Ma ke (H2020-
ICT-2019-2 g an numbe 871754). This wo k is also sup-
po ed by he TCO-RISEBLOCK (PID2019-110224RB-
I00), ARPASAT (TEC2015-70197-R), P ojec RTI2018-
102112-B-I00 (AEI/FEDER,UE) and by he Gene ali a
de Ca alunya g an 2014-SGR-1504.
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