Trade Working Group Ko Fujimura
INTERNET-DRAFT NTT
Expires: August 2001 February 2001
Requirements for Generic Voucher Trading
<draft-ietf-trade-drt-requirements-02.txt>
Status of this Memo
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Abstract
In purchase and other trading transactions, it is often required to
credit loyalty points, collect digital coupons or gift certificates,
and so on. These activities can be generalized using the concept of
a "voucher", which is a digital representation of the right to claim
goods or services. This document contains a voucher trading model
and the requirements for the following points:
- A voucher trading system to circulate vouchers securely
- A language to describe diverse types of vouchers
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Table of Contents
Status of this Memo ................................................1
Abstract ...........................................................1
1. Background ......................................................2
2. Terminology and Model ...........................................3
2.1 Voucher ......................................................3
2.2 Participants .................................................3
2.3 Voucher trading system (VTS) .................................4
3. VTS Requirements ................................................5
3.1 Capability to handle diversity ...............................5
3.2 Ensuring security ............................................5
3.3 Ensuring practicality ........................................6
4. GVL Requirements ................................................6
4.1 Semantics ....................................................6
4.2 Syntax .......................................................7
4.3 Security .....................................................7
4.4 Efficiency ...................................................8
4.5 Coordination .................................................8
5. VTP Requirements ................................................8
6. Application Examples ............................................8
7. Q & A ..........................................................10
8. Security Considerations ........................................11
9. Acknowledgments ................................................11
10. References .....................................................11
11. Author's Address ...............................................11
1. Background
It is often necessary to credit loyalty points, collect digital
coupons or gift certificates, etc, to complete purchases or other
trading transactions in the real world. The importance of these
activities is also being recognized in Internet Commerce. If a
different issuing or collecting system to handle such points or
coupons must be developed for each individual application, the
implementation cost will be too expensive, especially for small
businesses. Consumers may also be forced to install a number of
software modules to handle these points or coupons. A voucher
is a digital representation of the right to claim services or
goods. Using the concept of a voucher, a wide-range of
electronic-values including points or coupons can be handled in a
uniform manner with one trading software module.
This document presents the terminology, voucher trading model,
requirements on Voucher Trading System (VTS) that circulate vouchers
securely, and Generic Voucher Language (GVL), in which diverse
types of vouchers can be described.
Along with the Voucher Trading Protocol (VTP), VTS enables companies
or individuals to freely issue, transfer, and redeem various
types of vouchers via the Internet. This document does not
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include protocol-related requirements, which will be presented in a
separate document.
2. Terminology and Model
2.1 Voucher
A voucher is a digital representation of the right to claim goods or
services. To clarify the difference between vouchers and electronic
money/digital certificates, we introduce a formal definition of vouchers
in this document.
Let I be a voucher issuer, H be a voucher holder, P be the
issuer's promise to the voucher holder. A voucher is defined as
the 3-tuple of <I, P, H>.
Examples of P are as follows:
o Two loyalty points are added to the card. If you collect 50
points, you'll get one item free. (Loyalty points)
o Take 10% off your total purchase by presenting this card.
(Membership card)
o Take 50% off your total purchase with this coupon. (Coupon)
o The bearer can access "http://..." for one month free. (Free
ticket for sales promotion)
o The bearer can exchange the ordered clothes with this ticket.
(Exchange ticket or Delivery note)
o Seat number A-24 has been reserved for "a-concert" on April 2.
(Event ticket)
Note that P does not need to be described in terms of a natural
language as long as the contents of the vouchers are specified. For
example, a set of attribute name and value pairs described in XML can
be employed to define the contents.
2.2 Participants
There are four types of participants in the voucher trading model:
issuer, holder, collector, and VTS provider. Their roles are as
follows:
Issuer: Creates and issues a voucher. Guarantees contents of
the voucher.
Holder (or user): Owns the vouchers. Transfers and redeems
the voucher to other users or collector.
Collector (or examiner): Collects the voucher. In general,
compensated by goods or services rendered.
VTS Provider: Provides a VTS and guarantees that there are no
duplicate assignments or multiple use of vouchers.
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The IOTP model [IOTP] includes merchant, deliverer, consumer and other
participants. They take various roles in the settlement because a
merchant, for example, can be considered as an issuer, or holder
depending on whether the merchant creates the voucher
her/himself or purchases it from a wholesaler or manufacturer. A
merchant can also be a collector if the shop collects gift certificate
or coupons.
2.3 Voucher Trading System (VTS)
A voucher is generated by the issuer, and traded among users,
and finally is collected by the collector:
<I, P, H> <I, P, H'> <I, P, H'>
Issuer I --------> User H ---------> User H' ---------> Collector
Issue Transfer Redemption
Figure 1. Life cycle of vouchers
The VTS provider provides a VTS that enables vouchers to be
circulated among the participants securely.
A formal definition of VTS is as follows:
Definition: A voucher trading system (VTS) is a system that logically
manages a set of valid vouchers VVS, which is a subset of
{<I, P, H> | I in IS, P in PS, H in HS} where IS is the set of
issuers, PS is the set of promises, and HS is the set of holders;
VTS prevents them from being modified or reproduced except where
for the following three transactions: issue, transfer, and
redemption. The initial state of the VVS is an empty set.
Note that this does not imply that VVS is stored physically in a
centralized database. For example, one implementation may store them
in distributed smart cards carried by each holder [T00], or may store
them in multiple servers managed by each issuer or trusted third
parties. This is a trust policy and/or implementation issue [MF99].
Issue
An issue transaction is the action that creates the tuple of
<I, P, H> and adds it to the VVS with the issuer's intention.
Transfer
A transfer transaction is the action that rewrites the tuple of
<I, P, H> (in VVS) as <I, P, H'> (H<>H') to reflect the original
holder H's intention.
Redemption
There are two redemption transactions: presentation and
consumption.
A presentation transaction is the action that shows the tuple of
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<I, P, H> (in VVS) to reflect the holder H's intention. In this
case, the ownership of the voucher is retained when the voucher
is redeemed, e.g., redemption of licenses or passports.
A consumption transaction is the action that deletes the tuple of
<I, P, H> (in VVS) to reflect the holder H's intention. The
ownership of the voucher may be voided or the number of
times it is valid reduced when the voucher is redeemed, e.g.,
redemption of event tickets or telephone cards.
Note that one or more of these transactions can be executed as part of
the same IOTP purchase transaction. See details in Section 6.
3. VTS Requirements
A VTS must meet the following requirements
(1) It MUST handle diverse types of vouchers issued by different
issuers.
(2) It MUST prevent illegal acts such as alteration, forgery, and
reproduction, and ensure privacy.
(3) It MUST be practical in terms of implementation/operation cost and
efficiency.
Each of these requirements is discussed below in detail.
3.1 Capability of handling diversity
(a) Different issuers
Unlike a digital cash system that handles only the currency issued
by a specific issuer such as a central bank, the system MUST handle
the vouchers issued by different issuers.
(b) Various types of vouchers
Unlike a digital cash system that only handles a currency, the
system MUST handle various types of vouchers, such as gift
certificates, coupons, and loyalty points.
3.2 Ensuring security
(c) Preventing forgery
Voucher MUST NOT be counterfeited. Only the issuer can initiate an
issue transaction.
(d) Preventing alteration
Voucher MUST NOT be altered during circulation except for the
transfer transaction in which the voucher holder is rewritten.
Only the holder can initiate a transfer transaction.
(e) Preventing duplicate-redemption
Voucher MUST NOT be redeemable once it has been consumed (the
result of a redemption transaction). Only the holder can initiate a
redemption transaction.
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(f) Preventing reproduction
Voucher MUST NOT be reproduced while in circulation.
(g) Non-repudiation
It SHOULD NOT be possible to repudiate the issuance, transfer,
or redemption of a voucher when it is transferred or sold.
(h) Ensuring privacy
Current and previous ownership of voucher SHOULD be concealed.
(i) Trust manageability
If diverse types of vouchers are put into circulation, it would be
difficult for users to judge whether a voucher can be trusted or
not. To support such a judgment, a trust management system that
automatically verifies the trust of voucher SHOULD be supported.
3.3 Ensuring practicality
(j) Scalability
A centralized broker who sells all types of vouchers, or
centralized authority that authenticates all issuers or other
participants, SHOULD NOT be assumed. A system that relies on a
global, centralized organization is excessively frail; failure
in the organization causes complete system failure.
(k) Efficiency
It MUST be possible to implement VTS efficiently. Many applications
of vouchers, e.g., event ticket or transport passes, require
high performance, especially when the voucher is redeemed.
(l) Simplicity
It SHOULD be possible to implement VTS simply. Simplicity is
important to reduce the cost of implementation. It is also
important in in understanding the system, which is necessary for
people to trust the system.
4. GVL Requirements
To satisfy the diverse requirements placed on VTS (see above), we
believe that a Generic Voucher Language (GVL) that realizes various
voucher properties should be introduced. This approach ensures that
VTS is application independent.
In this section, we mainly discuss how Promise P of the voucher
<I, P, H> can be defined in GVL. Specifying I and H is an
implementation issue and can be achieved by using a public key, hash
of a public key, or other names with scope rule.
4.1 Semantics
(a) Validity control: The invalidation (punching) method that is
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executed when the voucher is redeemed depends on the
type of the voucher. For example, a loyalty point will
be invalidated if the point is redeemed but a membership card
can be used repeatedly regardless of the number of times
presented. The language MUST be able to define how validity is
modified. Additionally, the language MUST be able to define the
validity period, start date and end date.
(b) Transferability control: Some types of vouchers require
transferability. The language MUST be able to specify if an
voucher can be transferred.
(c) Circulation control: Depending on the type of the voucher,
various circulation requirements or restrictions must be
satisfied while in circulation [F99], for example, only
qualified shops can issue particular vouchers or only a
certain service provider can punch (invalidate) particular
vouchers. The language SHOULD be able to specify such
circulation requirements.
(d) Anonymity control: Different types of voucher will
require different levels of anonymity. The language SHOULD be
able to control the required level of anonymity.
(e) Understandability: The terms and description of a voucher
SHOULD be objectively understood by the participants,
because this will contribute to reducing the number of disputes
on the interpretation of the vouchers promised.
(f) State manageability: Some types of vouchers have
properties the values of which may change dynamically while in
circulation, e.g., payment status, reservation status, or
approval status. The language MAY support the definition of such
properties.
(g) Composability: Some types of vouchers consist of several
sub-vouchers, which may be issued separately from the
original vouchers typically because the vouchers are
issued by different organizations or issued at different times.
The language MAY support compound vouchers comprised of
multiple sub-vouchers.
4.2 Syntax
(a) To achieve consistency with other related standards shown below,
the syntax of the language MUST be based on XML [XML].
(b) The language syntax MUST enable any application-specific
property, e.g., seat number, flight number, etc to be defined. A
schema definition language that can be translated into
application-specific DTDs may be needed.
4.3 Security
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(a) The language MUST provide the parameters necessary to establish
security. Security requirements, however, mainly follow VTS
requirements described in Section 3 rather than GVL requirements.
4.4 Efficiency
(a) The vouchers may be stored in a smart card or PDA with
a restricted amount of memory. Large definitions may incur long
transfer and processing time, which may not be acceptable. The
language SHOULD enable the efficient definition of vouchers
4.5 Coordination
(a) The language specification SHOULD be consistent with the
following specifications:
(1) Internet Open Trading Protocol v1.0 [IOTP]
(2) XML-Signature [XMLDSIG]
(3) Extensible Markup Language (XML) Recommendation [XML]
(4) ECML Version 2 [ECML]
5. VTP Requirements
Requirements for the Voucher Trading Protocol (VTP) will be
discussed in a separate document or future version of this document.
6. Application Examples
This section describes, as a typical electronic commerce example
involving advertisement, payment, and delivery transactions, the use
of vouchers and VTS, and shows that vouchers can be used as an
effective way to coordinate autonomous services that have
not yet established trust among each other.
Figure 2 shows a typical electronic commerce example of a consumer
searching for goods or services and making a purchase:
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----------
------------------------------------------->| Ad |
| (1) Acquire a coupon | Agency |
| ----------
|
| (2) Send payment information ----------
| --------------------------------------->| Payment |
| | Acquire a gift certificate | Handler |
| | ----------
v v (3) Transfer the coupon &
---------- gift certificate ----------
| Consumer |<------------------------------------>| Merchant |
---------- Acquire an exchange ticket & ----------
^ loyalty points
|
| (4) Transfer the exchange ticket ----------
------------------------------------------->| Deliverer|
Supply goods or services | Handler |
----------
Figure 2. Application example of vouchers
(1) Use a search engine to find the desired goods or services and
acquire a coupon from an ad agency that represents the right to
purchase the goods or services at a discounted price.
(2) Acquire a gift certificate from a payment handler in exchange for
cash or payment information.
(3) Transfer the coupon and gift certificate to the merchant, and in
exchange acquire an exchange ticket and loyalty points.
(4) Transfer the exchange ticket to the deliverer handler and receive
the goods or services.
In this example, the coupon, gift certificate, and exchange ticket
each represent media coordinating the above four transactions.
Note that it is not necessary to trust the participants involved in
the transactions, but to trust the vouchers themselves. In
other words, there is no need to exchange contracts among the
participants beforehand if the vouchers themselves are trusted.
Take the exchange ticket as an example; even if the delivery handler
does not trust the consumer, the merchant that issued the exchange
ticket is trusted, and if the VTS guarantees that there is no
duplication in the trading process of the exchange ticket, there is no
problem in swapping the exchange ticket for the goods or services. In
the same way, even if the merchant does not trust the delivery handler,
the issuance of the exchange ticket can be verified, and if the VTS
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guarantees that there is no duplication in the trading process of the
exchange ticket, there is no problem in swapping the exchange ticket
for the goods or services (Fig. 3). In other words, if there is trust
in the issuer and the VTS, trust among the participants involved in
the transactions is not required.
Exchange Exchange
---------- ticket ---------- ticket ----------
| Consumer |-------->| Deliverer|-------->| Merchant |
| |<--------| Handler |<--------| |
---------- Goods or ---------- Goods or ----------
services services
Figure 3. Coordination of untrusted participants
using exchange ticket
In general, it is difficult to manage the trust of individuals rather
than companies, so this characteristic of VTS is especially effective.
Moreover, the transactions involving vouchers have desirable features
with respect to privacy protection. For example, in the above exchange
ticket scenario, the consumer can designate the delivery service by
himself, so the merchant does not even need to know any personal
information such as the delivery address. Furthermore, by designating
a convenience store etc. as the receiving point, the delivery service
does not need to know the address of the consumer.
7. Q & A
- Is it possible to implement a VTS using digital certificates?
If transferability is not required, a voucher can be easily
implemented as a digital certificate, i.e., Signed_I(I, P, H), where
the phrase "Signed_I" means that the entire block is signed by the
issuer's digital signature. If transferability is required, then H is
changed during the transfer, i.e., the signature is broken.
Additionally, online DB checking or tamper-resistant devices are
required to prevent duplicate-redemption.
- What is the difference from digital-cash?
VTS must handle various types of vouchers, such as gift certificates,
coupons, or loyalty points unlike a digital cash system which handles
only currency. Additionally, vouchers are issued by different
issuers.
- Is it possible to support "digital property rights?"
Digital property right can be represented as a voucher and it can
be traded using VTS. However, some protected rendering system would
be required to regenerate the digital contents securely in order to
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support digital property rights. These requirements are out of scope
of VTS.
8. Security Considerations
Security issues are discussed in Section 3.2 and 4.3.
9. Acknowledgments
I would like to thank Masayuki Terada, for his valuable comments.
I would also like to thank all the active member of IETF Trade WG,
especially Donald E. Eastlake 3rd, who is the chair of the WG, for
his help and suggestions.
10. References
[ECML] ECML Version 2, to appear.
[F99] K. Fujimura, H. Kuno, M. Terada, K. Matsuyama, Y. Mizuno, and J.
Sekine, "Digital-Ticket-Controlled Digital Ticket Circulation", 8th
USENIX Security Symposium, August 1999.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IOTP] D. Burdett, "The Internet Open Trading Protocol", RFC2801,
April 2000.
[MF99] K. Matsuyama and K. Fujimura, "Distributed Digital-Ticket
Management for Rights Trading System", 1st ACM Conferences on
Electronic Commerce, November 1999.
[T00] M. Terada, H. Kuno, M. Hanadate, and K. Fujimura, "Copy
Prevention Scheme for Rights Trading Infrastructure", 4th Smart Card
Research and Advanced Application Conference (CARDIS 2000), September
2000.
[XML] "Extensible Mark Up Language (XML) 1.0 (Second Edition)", A W3C
recommendation, <http://www.w3.org/TR/REC-xml>, October 2000.
[XMLDSIG] "XML-Signature Syntax and Processing", draft-ietf-xmldsig-
core-11.txt, in RFC Editor queue for publication as Proposed Standard.
Author's Address
Ko Fujimura
NTT Corporation
1-1 Hikari-no-oka, Yokosuka-shi, Kanagawa, 239-0847 JAPAN
Phone: +81-(0)468-59-3814
Fax: +81-(0)468-59-2241
Email: fujimura@isl.ntt.co.jp
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