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Comparing and contrasting micro-payment models for E-commerce systems
Abstract
Current macro-payment systems used by most E-commerce sites are not suitable for high-volume, low-cost produce or
service purpose, such as charging per-page for web site browsing. These payment technologies suffer from use of heavy-
weight encryption technologies and reliance on always on-line authorisation servers. Micro-payment systems offer an
alternative strategy of pay-as-you-go charging, even for very low cost, very high-volume charging. However, several
different micro-payment schemes exist, not all suitable for all E-commerce uses. We compare and contrast several micro-
payment models and outline a new micro-payment technology we have been developing.
Keywords: electronic commerce, micropayment, off-line.
Introduction
Macro-payment systems are used by most E-commerce systems today. These typically use credit card debiting, digital cash or
real-time bank transfers, where a customer pays for products or services before or at the time of delivery. Such systems typically use
complex encryption technologies and require communications with an authorisation server to request and confirm payment. This
model suits low-to-medium volume transactions of medium-to-high value e.g. books, food, office stationary, home appliances, toys
and so on.
There is a trend towards charging for site content on the Internet [2] in order for companies to make direct profits from
information they provide, rather than relying on fickle or insufficient on-line advertising revenue [9, 10]. For example, many sites
have become subscription-only access e.g. on-line newspapers, academic and trade periodicals, help and advice columns, and so on.
Subscription has the disadvantage of locking customers to one site (they need to subscribe to every site they want access to) and a
“one size fits all” scenario where even if the customer wants a few items from the site, they have to pay for them all.
An alternative model is where a customer pays as they go from a previously acquired (by macro-payment) E-wallet with E-
coins i.e. is charged per-page or per-group or per-download for material, often very low cost per item [7, 9, 10]. Ideally they can
move to other sites and use the same E-money. This is the micro-payment model of on-line information, product and service
purchase.
In the following sections we introduce an example scenario – an on-line newspaper – that wants to charge on an article usage
basis. We review the typical macro-payment model’s interactions between customer, vendor (E-newspaper site) and authoriser
(bank or credit-card company). We then compare several micro-payment models and discuss their various advantages and
disadvantages for supporting this pay-as-you-go purchasing model.
Motivation
Assume a reader wants to read an on-line newspaper. Using subscription-based payment, they would first have to
subscribe to the newspaper by supplying payment details (credit card etc) and the newspaper system would make an
electronic debit to pay for their subscription, by communicating with an authorisation server. The user would then
normally go to the newspaper’s site where they login with an assigned user name and password. The newspaper looks up
their details and provides them access to the current edition if their subscription is still current. If the user’s subscription
has run out, they must renew this by authorising a payment from their credit card. Figure 1 (a) outlines the key interaction use
cases for this scenario. Problems with this approach are that there is no anonymity for the user (the newspaper system knows
exactly who they are and when and what they read), they can not browse other newspapers without first subscribing to them too,
and they must pay for the whole newspaper, even if they want just one or two sections or articles.
An alternative approach is a micro-payment model. The user first goes to a broker and purchases “E-coins” using a single
macro-payment. These are stored in an E-wallet on the user’s machine. The user can then visit any newspaper site they wish, their
wallet giving the site an E-coin. Each time they view an article (or section or page, depending on the item charged for) their E-coin
is debited. The vendor redeems debits with the broker (for “real” money”) periodically e.g. each night/week. The user can move to
another site and unspent money associated with their E-coin is transferred from the first vendor to the second. If coins run out, theuser communicates with the broker and authorises another macro-payment debit. Figure 1 (b) outlines the key interaction use cases
for this scenario.
Customer
Read Article
Subscribe Make Macropayment
Authorisation
System
Debit Coin
Get E-coins
Customer
Read Article
Redeem Debits
Broker
(a) Typical macro-payment interaction model. (b) Possible micro-payment interaction model.
Figure 1. Two on-line newspaper interaction scenarios.
The standard macropayment methods cannot be effectively or efficiently applied for buying inexpensive information
goods, like single articles of an on-line newspaper, because transaction costs are too high. Encryption mechanisms used
are slow and each transaction typically “costs” a few cents. Macro-payment suits spending small numbers of large
amounts. An Internet micropayment system would allow spending large numbers of small amounts of money at web sites
in exchange for various content or services, as in the E-newspaper scenario above. The design of micro-payment systems
are usually quite different from existing macro-payment systems, since micropayment systems must be very simple,
secure, and efficient, with a very low cost per transaction. This must also be taken into consideration for transaction
security: high security leads to high costs and computation time. For micropayments low security can be applied.
There are a number of payment systems in various stages of development from proposals in an academic literature to
systems currently in commercial trials. Payment protocols that are exclusively designed for electronic payments in a
normal customer to vendor transaction can be categorized as either online or offline protocols. On-line payment systems
include iKP, Netbill, and CyberCash [1,4,15]. In on-line systems, every payment needs to be authorized by the central
payment authority that issued the coin in order to prevent double spending. This is called an on-line payment scheme
since the issuing bank is involved in every transaction. Unfortunately the central organization very quickly becomes a
potential bottle-neck and point-of-failure.
Protocols that do not rely on a third party (broker) to guard against double spending are called off-line micropayment
protocols, such as PayWord, MiniPay and NetPay [13,8,6]. These protocols are typically credit based. In some there is no
protection mechanism to prevent a customer from double spending, and spending more than the balance in their account
(overspending). Double spending is detected at the time of the clearing process, when the vendor turns in the received
coins to their respective banks. Once double spending is detected, the malicious customer are usually penalized and
expelled. Though off-line protocols have received a lot of attention from researchers and cryptographers, no off-line
payment systems yet exist in the general public use.
Macro-payment Model
Within macropayment systems there are three distinct payment methods that are credit card based, digital cash, and
electronic check (account based). Credit based payment systems, such as SET and CyberCash [14,15], are both online and
post paid payment by credit card. There are many payment systems based on e-cash payment, such as DigiCash which is
online and prepaid payment system, and CAFÉ which is offline and prepaid payment system [5,3]. NetCheque employs
an account server to transfer and authenticate electronic checks [12]. Figure 2 represents a model how credit cards can be
used in a secure way across the Internet. The protocol is called CyberCash that provides customer software, vendor
software and a gateway to support the secure communication of credit card transaction over the Internet.
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