零售角度央行数字货币技术【完整版】

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　Raphael Auer raphael.auer@bis.org

　Rainer Böhme rainer.boehme@uibk.ac.at

　 The technology of retail central bank digital currency 1

　 Central bank digital currencies (CBDCs) promise to provide cash-like safety and convenience for peer-to-peer

　payments.

　To

　do

　so,

　they

　must

　be

　resilient

　and

　accessible.

　They

　should

　also safeguard the user’s privacy, while allowing for effective law enforcement. Different technical designs

　satisfy

　these

　attributes

　to

　varying

　degrees,

　depending

　on

　whether

　they

　feature intermediaries,

　a

　conventional

　or

　distributed

　infrastructure,

　account-

　or

　token-based

　access, and

　retail

　interlinkages

　across

　borders.

　We

　set

　out

　the

　underlying

　trade-offs

　and

　the

　related hierarchy of design choices. JEL

　classification:

　E42,

　E44,

　E51,

　E58,

　G21,

　G28.

　The question of whether central banks should issue digital currency to the general public has attracted increasing attention. This special feature sketches out some key technological

　design

　considerations

　for

　a

　retail

　CBDC,

　in

　the

　event

　that

　a

　central bank decided to issue one. We do not investigate the case for or against issuance, the systemic implications, or how these might be managed. 2

　We

　 structure

　 our

　 approach

　 around

　 consumer

　 needs

　 and

　 the

　 associated technical design choices. Current electronic retail money represents a claim on an intermediary, rather than functioning as the digital equivalent of cash. CBDCs could potentially

　provide

　a

　cash-like

　certainty

　for

　peer-to-peer

　payments.

　At

　the

　same time, they should offer convenience, resilience, accessibility, privacy and ease of use in cross-border payments. Different technical designs meet these criteria to varying degrees, with attendant technical trade-offs. We explore these issues. The aim is not to promote or highlight any particular approach, but to lay some groundwork for more systematic discussions.

　 1

　We

　 thank

　 Morten

　 Bech,

　 Codruta

　 Boar,

　 Claudio

　 Borio,

　 Stijn

　 Claessens,

　 Benoît

　 Cœuré,

　 Jon

　 Frost, Leonardo

　Gambacorta,

　Marc

　Hollanders,

　Henry

　Holden,

　Ross

　Leckow,

　Cyril

　Monet,

　Hyun

　Song

　Shin, Rastko

　 Vrbaski,

　 Amber

　 Wadsworth

　 and

　 Philip

　 Wooldridge

　 for

　 comments,

　 and

　 Haiwei

　 Cao,

　 Giulio Cornelli

　and

　Alan

　Villegas

　for

　exceptional

　research

　assistance.

　The

　views

　expressed

　in

　this

　article

　are those

　of

　the

　authors

　and

　do

　not

　necessarily

　reflect

　those

　of

　the

　Bank

　for

　International

　Settlements.

　2

　For

　the

　systemic

　implications,

　see

　the

　survey

　in

　CPMI-MC

　(2018).

　Andolfatto

　(2018),

　Kumhof

　and Noone

　(2018),

　and

　Bindseil

　(2020)

　examine

　how

　the

　impact

　on

　the

　central

　bank’s

　balance

　sheet

　can be

　managed,

　while

　Brunnermeier

　and

　Niepelt

　(2019)

　investigate

　how

　financial

　instability

　risks

　can be

　mitigated.

　 Our

　approach

　is

　graphically

　represented

　in

　the

　“CBDC

　pyramid”,

　which

　maps consumer

　needs

　onto

　the

　associated

　design

　choices

　for

　the

　central

　bank.

　This scheme forms a hierarchy in which the lower layers represent design decisions that feed into subsequent, higher-level decisions. We

　start

　by

　introducing

　the

　four

　main

　design

　choices,

　as

　represented

　in

　the four layers of the CBDC pyramid. We

　assess the legal

　structure of

　claims

　and the operational

　roles

　of

　the

　central

　bank

　and

　private

　institutions

　in

　different

　CBDC architectures.

　We

　discuss

　the

　choice

　between

　distributed

　ledger

　technology

　(DLT) and

　a

　centrally

　controlled

　infrastructure.

　We

　compare

　token-based

　systems

　and account-based

　systems.

　Before

　concluding,

　we

　assess

　how

　the

　development

　of CBDCs might reinforce current efforts to overhaul cross-border payments.

　 From

　consumer

　needs

　to

　design

　choices:

　the

　CBDC

　pyramid

　 The

　focus

　of

　our

　approach

　is

　the

　“retail”

　aspect

　of

　CBDC;

　we

　ask

　what

　consumer needs a CBDC could address. 3

　We thus sketch the development of a CBDC through an approach that proceeds from consumer needs to design choices. 4

　The left-hand side

　of

　the

　CBDC

　pyramid

　(Graph 1)

　sets

　out

　such

　consumer

　needs

　and

　six associated features that would make a CBDC useful. Starting with cash-like peer-to- peer

　 usability,

　 these

　 features

　 also

　 comprise

　 convenient

　 real-time

　 payments, payments

　security,

　privacy,

　wide

　accessibility

　and

　ease

　of

　use

　in

　cross-border payments. The pyramid’s right-hand side lays out the associated design choices. The consumer’s prime need is that the CBDC embodies a cash-like claim on the central

　bank,

　ideally

　transferable

　in

　peer-to-peer

　settings.

　Today,

　even

　consumers who

　normally

　prefer

　to

　pay

　electronically

　are

　confident

　that,

　if

　an

　episode

　of financial turmoil were to threaten, they could shift their electronic money holdings into cash. This flight to cash has been seen in many crisis episodes, including recent ones.

　The

　main

　concern

　is

　that

　if,

　in

　the

　future,

　cash

　were

　no

　longer

　generally

　 3

　All

　 private

　 sector

　 non-financial

　 users

　 are

　 referred

　 to

　 as

　 “consumers”

　 in

　 what

　 follows.

　 For

　 a discussion

　of

　“wholesale”

　CBDC

　for

　use

　in

　the

　financial

　industry,

　see

　CPMI-MC

　(2018).

　4

　The

　survey

　in

　Boar

　et

　al

　(2020)

　highlights

　that

　central

　banks

　have

　advanced

　other

　motivations

　for issuance,

　 including

　 monetary

　 policy

　 implementation

　 and

　 financial

　 stability

　 considerations.

　 These aspects

　are

　considered

　in

　the

　CBDC

　design

　frameworks

　of

　Fung

　and

　Halaburda

　(2016),

　Bjerg

　(2017), CPMI-MC

　 (2018),

　 Mancini-Griffoli

　 et

　 al

　 (2018),

　 Wadsworth

　 (2018),

　 Kahn

　 et

　 al

　 (2019)

　 and

　 Adrian (2019).

　 Although

　 it

　 takes

　 a

　 more

　 positive

　 stance

　 towards

　 CBDC,

　 our

　 focus

　 on

　 technical

　 design elements

　is

　related

　to

　Pichler

　et

　al’s

　(2020)

　analysis

　of

　the

　limits

　of

　CBDC

　when

　compared

　with

　cash.

　 86 BIS Quarterly Review, March 2020 Key takeaways

　 A trusted and widely usable retail CBDC must be secure and accessible, offer cash-like convenience and safeguard privacy.  Various technical designs satisfy these criteria to different degrees, and the associated trade-offs need to be identified.  The design of a retail CBDC needs to balance the credibility of direct claims on the central bank with the benefits of using payment intermediaries.

　accepted, a severe financial crisis might create further havoc by disrupting day-to- day business and retail transactions. 5

　At

　the

　same

　time,

　consumers

　are

　unlikely

　to

　adopt

　a

　CBDC

　if

　it

　is

　less convenient

　to

　use

　than

　today’s

　electronic

　payments.

　Banks

　and

　payment

　service providers

　run

　sophisticated

　infrastructures that

　can

　handle

　peak demand,

　such

　as on Singles Day in China or Black Friday in the United States. And intermediaries help to smooth the flow of payments by taking on risk, for example during connectivity breaks or offline payments. These

　two

　needs

　–

　cash-like

　safety

　and

　convenience

　of

　use

　–

　lead

　to

　the foundational

　design

　consideration

　for

　a

　CBDC

　(see

　lowest

　layer

　of

　pyramid

　in Graph 1):

　the

　choice

　of

　the

　operational

　architecture,

　and

　how

　it

　will

　balance

　the consumer’s demand for a cash-like claim on the central bank with the convenience that

　intermediaries

　confer

　on

　the

　payment

　system.

　The

　choice

　is

　shaped

　by

　two questions. Is the CBDC a direct claim on the central bank or is the claim indirect, via payment

　intermediaries?

　What

　is

　the

　operational

　role

　of

　the

　central

　bank

　and

　of private sector intermediaries in day-to-day payments? Further, the consumer’s need for cash-like payment safety means that a CBDC must be secure not only from the insolvency or technical glitches of intermediaries, but

　also

　from

　outages

　at

　the

　central

　bank.

　The

　choice

　is

　whether

　to

　base

　this infrastructure on a conventional centrally controlled database or instead on DLT – technologies

　that

　differ

　in

　their

　efficiency

　and

　degree

　of

　protection

　from

　single

　The

　CBDC

　pyramid

　Graph 1

　The CBDC pyramid maps consumer needs (left-hand side) onto the associated design choices for the central bank (right-hand side). The four layers of the right-hand side form a hierarchy in which the lower layers represent design choices that feed into subsequent, higher- level decisions. Source: Authors’ elaboration.

　 5

　In

　Sweden,

　where

　cash

　use

　has

　already

　declined

　substantially,

　considerations

　along

　these

　lines

　have led

　the

　central

　bank

　to

　propose

　a

　review

　of

　the

　concept

　of

　legal

　tender

　(Sveriges

　Riksbank

　(2019)).

　 BIS Quarterly Review, March 2020

　87

　points of failure. Importantly, this decision can only be made once the architecture has been decided upon, as DLT is only feasible for some operational setups. This is why the choice of infrastructure lies in the pyramid’s second layer. Two

　 further

　 consumer

　 needs

　 are

　 easy,

　 universal

　 access

　 and

　 privacy

　 by default. 6

　From

　a

　technical

　perspective,

　there

　is

　an

　underlying

　trade-off

　between privacy and ease of access on the one hand and ease of law enforcement on the other. The associated design choice – the pyramid’s third layer – is whether access to

　the

　CBDC

　is

　tied

　to

　an

　identity

　system

　(ie

　an

　account-based

　technology)

　or instead via

　cryptographic

　schemes that do

　not

　require

　identification

　(ie

　an

　access technology based on so-called digital tokens). The final consumer need we consider is that CBDCs should also enable cross- border payments. At a design level, this could be arranged via technical connections at

　the

　wholesale

　level

　that

　are

　built

　on

　today’s

　systems.

　Alternatively,

　novel interlinkages could be envisaged at the retail level, ie allowing consumers to hold foreign digital currencies directly. Importantly, the means of implementing the latter option would depend on whether the CBDC was account- or token-based. This is why this design choice belongs in the top layer of the pyramid.

　 Architecture:

　indirect

　or

　direct

　claims,

　and

　the

　operational role

　fo...

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