Michael Peirce
Donal O'Mahony
At the time of writing, it has been estimated that there may be over 30 million users of the Internet spread across 96 different countries[12] using over 6.6 million host computers[15], and these figures are rising very rapidly. This means that an effective electronic payment system must be highly scaleable. In practice, the system must support large numbers of buyers and sellers affiliated to many different banks. The problem of detection of double spending is particularly acute, and solutions must be found that allow for large numbers of payments to take place without requiring unreasonably large databases to be maintained. In the following section, we discuss related work on two systems for electronic payment and go on to propose a new set of protocols that surmounts some of their inherent problems.
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The first, Ecash, is a fully anonymous electronic cash system, using
numbered bank accounts and blind signatures. The second, NetCash, uses
identified electronic cash giving a more scaleable but less anonymous
system.
Electronic cash is the electronic equivalent of real paper cash, and can be implemented using public-key cryptography, digital signatures, and blind signatures. In an electronic cash system there is usually a bank, responsible for issuing currency, customers who have accounts at the bank and can withdraw and deposit currency, and merchants who will accept currency in exchange for goods or a service. Every customer, merchant, and bank has its own public/private key pair. The keys are used to encrypt, for security, and to digitally sign, for authentication, blocks of data that represent coins. A bank digitally signs coins using its private key. Customers and merchants verify the coins using the bank's widely-available public key. Customers sign bank deposits and withdrawals with their private key, and the bank uses the customer's public key to verify the signature.
The Ecash system consists of three main entities:
Ecash is implemented using RSA public-key cryptography. Every user in the system has their own public/private key pair. Special client and merchant software is required to use the Ecash system. The client software is called a "cyberwallet" and is responsible for withdrawing and depositing coins from a bank, and paying or receiving coins from a merchant.
When the bank receives the message, it checks the signature. The withdrawal amount can then be debited from the signature owner's account. The bank signs the coins with a private key.
After signing the blind coins, the bank returns them to the user, encrypted with the user's public key. The user can then decrypt the message, and unblind the coins by dividing out the blinding factor. Since the bank couldn't see the serial numbers on the coins it was signing there is no way to now trace these coins back to the user who withdrew them. In this way the cash is fully anonymous.
{Coins}K[public,Merchant]
If they disagree or do not have the exact denominations necessary to make a correct payment, the merchant is sent a payment refusal message.
{{Coins}K[private,Merchant]}K[public,Bank]
Ecash client and merchant software is available for many platforms. Currently no real money is used in the system, but an Ecash trial[11] with 10,000 participants, each being given 100 "cyberbucks" for free has been running since late 1994. There are many sample Web shops at which to spend cyberbucks.
The strengths of Ecash are its full anonymity and security. The electronic cash used is untraceable, due to the blind signatures used when generating coins.
By employing secure protocols using RSA public key cryptography, the Ecash system is safe from eavesdropping, and message tampering. Coins cannot be stolen while they are in transit. However, the protection of coins on the local machine could be strengthened by password protection and encryption.
The main problem with Ecash may be the size of the database of spent coins. If a large number of people start using the system, the size of this database could become very large and unmanageable. Keeping a database of the serial number of every coin ever spent in the system is not a scalable solution. Digicash plans to use multiple banks each minting and managing their own currency with interbank clearing to handle the problems of scalability. It seems likely that the bank host machine has an internal scalable structure so that it can be set up not only for a 10,000 user bank, but also for a 1,000,000 user bank. Under the circumstances, the task of maintaining and querying a database of spent coins is probably beyond today's state-of-the-art database systems.
The NetCash system consists of buyers, merchants, and currency
servers. An organisation wishing to set up and manage a currency
server obtains insurance for the new currency from a central
certification authority. The currency server generates a
public/private key pair. The public key is then certified by being
signed by the central authority. This certificate contains a
certificate ID, name of the currency server, currency server's public
key, issue date and an expiry date, all signed by the central
authority:
An electronic cheque can be exchanged with a currency server for electronic coins. The currency server is trusted not to record to whom the coins are issued. To further aid anonymity a holder of coins can go to any currency server and exchange valid coins for new ones. The currency server does not know who is exchanging coins, only the network address of where they are coming from. By performing the exchange and by choosing any currency server to do this with, it becomes difficult to track the path of the coins. If a currency server receives coins that were not minted by it, it will contact the minting currency server to validate those coins.
Figure 2 shows how a buyer uses NetCash coins to purchase an item from a merchant. In this transaction the buyer remains anonymous since the merchant will only know the network address of where the buyer is coming from. NetCash assumes that the buyer has or can obtain the public key of the merchant, and that the merchant has the public key of the currency server.
Implementation details of how the NetCash protocols might be linked with applications such as the Web are not available, but it could be done in a similar fashion to Ecash using an out-of-band communications channel. The transaction consists of the following four steps, starting from when the buyer attempts to pay the merchant:
The buyer can then use the transaction identifier and the public session key to obtain the service purchased.
This is the basic purchase protocol used in NetCash. While it prevents double spending it does not protect the buyer from fraud. There is nothing to stop the merchant spending the buyer's coins without providing a receipt.
Extensions to the protocol are detailed in [14]. These are more complex and give protection against fraud for both the merchant and buyer. There are also mechanisms to allow the merchant to be fully anonymous to the buyer. Partially off-line protocols where the bank does not need to be contacted during a purchase are also described. These however rely on the buyer contacting the currency server beforehand, and knowing who the merchant is at that time. They use a time window in which the coins are only valid for certain short lengths of time. Full technical details are given in [14].
The advantages of NetCash are that it is scalable and secure. It is scalable since multiple currency servers are present and security is provided by the cryptographic protocols used. Possible disadvantages of the system are that it uses many session keys and in particular public key session keys. To generate a public key of suitable length to be secure takes a very large amount of time compared with that involved in generating a symmetric session key. This could compromise the performance of the system as a whole.
NetCash is not fully anonymous, unlike Ecash. It is difficult but not impossible for a currency server to keep records of who it issues coins to and who it receives them back from. The ability to exchange coins and use any or multiple currency servers increases the anonymity of the system.
A NetCash system is currently being implemented, but no details are given as to how it will be linked with applications such as the Web. NetCheque will be used to provide cheques which can be used to buy coins or which can be issued when coins are traded in.
The two payment systems outlined each have their strengths and weaknesses. Ecash is a fully secure system that provides for very strong anonymity. The use of banks within the system reflects current practice in non-electronic payment systems. Successful operation of the Ecash system depends on the maintenance of a central database of all coins ever issued within the system. If it were to become accepted as a global payment system, this would quickly become a major problem.
NetCash uses identified coins with multiple currency servers, and thus, while anonymity is maintained, there is only a requirement to keep track of all currency currently in circulation. This makes for a much more scaleable solution to the payment problem. NetCash is also fully secure, and achieves this using protocols that are quite complex in nature.
The PayMe system and protocol set are now presented. Many of the design ideas are based on a close examination of systems such as NetCash, Ecash and other related systems such as Magic Money[7] and Netbill[20,21]. In this way PayMe is a collection of the successful parts from existing systems, minus the failings of those systems.
PayMe is an on-line electronic cash system. The entities involved are banks and users. Users can be either buyers or merchants but each has the same functionality. They can make payments, accept payments, or deal with the bank. Each bank mints its own identified electronic cash with serial numbers. Double spending of coins is prevented by the bank maintaining a database of coins in circulation. This scales better than the blind signature electronic cash approach. Any user in the PayMe system can accept payments and make payments. Merchants can receive payments for selling Web goods but they can also make payments to the buyers. This can be used for making refunds or in pay-out services.
A simple model showing the basic functionality of the PayMe system can be seen in Figure 3.
Both symmetric and public-key cryptography are used. Each entity has its own public/private key pair. It is a stand-alone system which has been tailored for use with the Web.
The PayMe system uses its own secure communications protocol, the PayMe Transfer Protocol(PMTP), to communicate between entities. This provides security and a means of communicating out-of-band, that is, outside the Web's HTTP protocol. This approach was adopted to allow a full prototype to be developed that could eventually be used with any emerging Web security standard.
When these five fields are put together and signed with the bank's private key, a valid coin is created. An example coin is of the form:
A bank within the PayMe system mints coins, maintains a database of the serial numbers of coins in current circulation to prevent double spending, and manages the accounts of merchants and buyers.
PMTP is the set of secure messages designed to provide the communications necessary in the PayMe system. It uses both symmetric and public-key cryptography. PMTP consists of six request-response message types.(1) For each of the six message types there are three different possible message identifiers. There is one request message identifier and two different response message identifiers. These have been called request, response and refusal respectively. A request is where the receiver is being asked to perform an action. A response message identifier indicates that the action has been performed and the message body contains the results of that action. A refusal is where the receiver refused to perform the action, and the message body may contain a reason for this refusal.
The first three messages are used by a bank account owner to withdraw or deposit coins, or obtain a bank statement from the bank for that account.
Either a buyer or merchant can use this mechanism to help hide their identity. When a user withdraws coins from a bank the bank could record the numbers on the coins and who it gave them to. Then when a merchant later deposits the coins the bank could check to whom it issued the coins. In this way the spending habits of a user could be recorded.
However, if during a purchase a merchant exchanges the coins rather than depositing them, then the bank does not know who has performed the exchange. Either the merchant or buyer, or even another trusted third party could perform this exchange to "launder" the money, making it more difficult to trace spending habits.
Table 1 : Ask_payment Messages
Table 2 : Pay_coins Messages.(2)
Having performed the check the bank then removes the serial numbers from the database, thereby invalidating the coins. This must be done because otherwise the same coins could be presented many times and they would always be valid. The merchant is given new coins in replacement, or the amount can be credited to his bank account.
Coin backups and log files are maintained to increase the fault tolerance of the system. In this way the chance of losing coins, and hence monetary value, is kept to a minimum if any of the entities crash.
PayMe could be used for schemes other than just monetary payment. A coin within the system could be used to represent a unit of CPU time, or connection time to a limited resource, in order to provide resource sharing in an institution. Jobs which require units of CPU time could be submitted or initiated through the Web where the merchant would be the CPU host requesting the PayMe coins representing time on that CPU.
For applications where anonymity is important the exchange coins mechanism can be used to anonymously exchange the coins with a bank preventing the bank knowing who now holds the new coins. In an environment where anonymity is not necessary or desirable the banks involved can be configured to refuse any requests to exchange certain coins, such as those representing CPU time. In this way the bank can record to who it issues the coins and who then deposits them, knowing for certain that no anonymous exchange has taken place. Thus the configuration of the bank can control the anonymity available to its users.
The final implemented system provides a secure and scalable means of paying for all types of Web services. It would seem to be more scaleable than the fully anonymous Ecash system, and more efficient than the complicated protocols and use of both symmetric and asymmetric session keys of NetCash.
[2] D.Chaum, A.Fiat, and M.Naor. Untraceable Electronic Cash. In Advances in Cryptology - Crypto '88 Proceedings, Springer-Verlag, Berlin, 1990, pages 319-327.
[3] D.Chaum. Blind Signatures for Untraceable Payments. In Advances in Cryptology - Crypto '82 Proceedings, Plenum Press, 1983, pages 199-203.
[4] D.Chaum. Security without Identification: Transaction Systems to Make Big Brother Obsolete. Communications of the ACM v.28, n.10, October 1985, pages 1030-1044.
[5] D.Chaum. Online Cash Checks. In Advances in Cryptology, EuroCrypt '89 Proceedings, Springer-Verlag, Berlin, 1989, pages 288-293.
[6] D. Chaum, A. Fiat, and M. Naor. Untraceable Electronic Cash. In Advances in Cryptology - Crypto '88 Proceedings, Springer-Verlag, Berlin, 1990, pages 319-327.
[7] Product Cypher. Magic Money Digital Cash System, ftp://ftp.csn.org, 1994.
[8] Product Cypher. The PGPTools Security Toolkit, ftp://ftp.csn.org, 1994.
[9] DigiCash. Ecash, http://www.digicash.com/, 1994.
[10] DigiCash Press Release. World's First Electronic Cash Payment over Computer Networks, May 27th 1994.
[11] DigiCash Press Release, Ecash Trial is Now Worldwide, January 6 1995.
[12] Larry Landweber. International Connectivity. Version 14 - June 15, 1995. ftp://ftp.cs.wisc.edu/connectivity_table/Connectivity_table.ps Computer Sciences Dept. University of Wisconsin - Madison, 1210 W. Dayton St., Madison, WI 53706, U.S.A.
[13] Gennady Medvinsky and B.Clifford Neuman. Electronic Currency for the Internet. Electronic Markets Vol 3. No. 9/10, October 1993, pages 23-24.
[14] Gennady Medvinsky and B. Clifford Neuman. NetCash: A design for practical electronic currency on the Internet. In Proceedings of the First ACM Conference on Computer and Communications Security, November 1993.
[15] Network Wizards, Menlo Park, California, http://www.nw.com/. Internet Domain Survey, July 1995.
[16] B. Clifford Neuman and Gennady Medvinsky. Requirements of Network Payment: The NetCheque Perspective. In Proceedings of IEEE Compcon'95, San Francisco, U.S.A., March 1995.
[17] M. Peirce, PayMe: Secure Payment for World Wide Web Services, B.A. (Mod) Project Report, Computer Science Department, Trinity College Dublin, Dublin 2, Ireland. May 1995.
[18] Jeffrey Rothfeder, Privacy for Sale, Simon & Schuster, 1992.
[19] Tony Sanders, Ari Luotonen, George Philips, John Franks, and Rob McCool. The CGI Specification. http://hoohoo.ncsa.uiuc.edu/cgi/interface.html
[20] Marvin Sirbu and J. Douglas Tygar. Netbill: An Electronic Commerce System Optimized for Network Delivered Information and Services. In Proceedings of IEEE Compcon '95, March 1995.
[21] J. D. Tygar. NetBill: An Internet Commerce System Optimized for Network Delivered Services. Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, 1995.
[22] University of Southern California Chronicle. The Check is in the E-mail, usc-chronicle-941107, November 1994.
[23] Phil Zimmermann. PGP User's Guide, Volume I: Essential Topics. Phil's Pretty Good Software, ftp://ftp.pegasus.esprit.ec.org/pub/arne/pgpdoc1.ps.gz. October 1994.
[24] Phil Zimmermann. PGP User's Guide, Volume II: Special Topics. Phil's Pretty Good Software, ftp://ftp.pegasus.esprit.ec.org/pub/arne/pgpdoc2.ps.gz. October 1994.
(2) A Money_bag is an implementation structure which holds coins.
Michael Peirce [http://www.cs.tcd.ie/www/mepeirce/mepeirce.html]
Computer Science Department
Trinity College
Dublin 2
Ireland
Email: Michael.Peirce@cs.tcd.ie
Michael Peirce graduated with a B.A.(Mod) in Computer Science from Trinity College Dublin, Ireland in June 1995. His final year dissertation concerned the design of a new scalable anonymous electronic payment mechanism for the purchase of goods and services on the WWW. He has acted as the maintainer of a highly popular WWW page on Electronic Payment since December '94. He was senior technical reviewer for the popular "Internet: The Complete Reference", and chief researcher for "The Internet Yellow Pages" (1st and 2nd editions), both published by Osborne/McGraw-Hill. He has completed a summer internship with Hitachi Research Laboratory in Dublin. Currently he is pursuing a Masters degree, working in the area of mobility, with the Networks and Telecommunications Research Group (NTRG) at Trinity College, Dublin.
Donal O'Mahony [http://www.cs.tcd.ie/www/omahony/omahony.html]
Computer Science Department
Trinity College
Dublin 2
Ireland
Email: Donal.OMahony@cs.tcd.ie
Donal O'Mahony received B.A., B.A.I. and Ph.D. degrees from Trinity College Dublin, Ireland. After a brief career in industry at SORD Computer Systems in Tokyo and IBM in Dublin, he joined Trinity College as a lecturer in Computer Science in 1984. He is author of many papers and articles on networking and security and coauthor of "Local Area Networks and their Applications" published by Prentice-Hall. At Trinity, he coordinates a research group working in the areas of Networks and Telecommunications. Within this group, projects are ongoing in X.500, Electronic Data Interchange (EDI), Networked multi-media data streams and Network Security. Dr. O'Mahony has acted as consultant to government and private industry organizations across Europe on a wide variety of projects involving strategic networking issues.