Donald E. Eastlake 3rd (dee@cybercash.com)
Wed, 5 Jul 95 23:31:28 EDT

You may find the following of interest. I have already sent it to
Internet-Drafts. I expect to be sending out at least one more draft
before the end of the week but this is the only one of the series
which potentially impacts HTML and HTTP.

Donald E. Eastlake 3rd +1 508-287-4877(tel) dee@cybercash.com
318 Acton Street +1 508-371-7148(fax) dee@world.std.com
Carlisle, MA 01741 USA +1 703-620-4200(main office, Reston, VA)

INTERNET-DRAFT Donald E. Eastlake 3rd
Updates RFC 821, 854, 959 CyberCash
Expires: 6 January 1996 7 July 1995

An Application Level Internet Payment Syntax
-- ----------- ----- -------- ------- ------

Status of This Document

This draft, file name draft-eastlake-internet-payment-00.txt, is
intended to be become one or more Proposed Standard RFCs.
Distribution of this document is unlimited. Comments should be sent
to the author <dee@cybercash.com>.

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''

To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu,
munnari.oz.au, or ftp.is.co.za.

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The Internet is becoming an increasingly commercial arena in which
information is being bought and sold and payments are rendered for
goods and services to be delivered outside of the Internet. Thus
far, the protocols and format used for such payments have been ad hoc
and proprietary.

This draft proposes a uniform application level syntax to support
such commerce. Specific specifications are given for how this syntax
fits into the World Wide Web, FTP, Telnet, and SMTP.


The contributions of the following persons to this draft are
gratefully acknowledged:

Brian Boesch <boesch@cybercash.com>
Phillip Hallam-Baker <hallam@w3.org>
David S. Raggett <dsg@w3.org>.

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Table of Contents

Status of This Document....................................1


Table of Contents..........................................3

1. Introductions...........................................4
1.1 Applications Level Applicability.......................4
1.2 Overview of this document..............................4

2. Price Tags..............................................6
2.1 Prices.................................................6
2.2 Payment System Strings.................................6
2.3 Price Tags.............................................7

3. Payments and Receipts...................................9

4. Use in the World Wide Web..............................11
4.1 Web Browser User Interface............................11
4.2 Anchor Embedded Costs.................................11
4.3 Page Header Price Tag.................................12
4.4 HTML Form Price Tags..................................13
4.5 Web Payments and Receipts.............................13
4.6 Payment Required Error................................14
4.7 Web Proxies...........................................15

5. Use in File Transfer Protocol..........................16

6. Use in Telnet..........................................17

7. Use in Simple Message Transfer Protocol................18

8. Protocols to Which Payment is not Applicable...........20
8.1 The Domain Name System................................20
8.2 The Finger Service....................................20
8.3 The Auth Service......................................20
8.4 The ECHO, DISCARD, and CHARGEN Services...............21

9. Security Considerations................................22

Author's Address..........................................23
Expiration and File Name..................................23

Appendix A: Initial Payment System Names..................24

Appendix B: Simplified BNF................................25

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1. Introductions

Applications level Internet commerce requires a means to (1) indicate
prices and acceptable methods of payment, (2) tender payment, and (3)
issue a receipt acknowledging payment or indicate if payment fails.

This document specifies a character string syntax for these three

1.1 Applications Level Applicability

Payment facilities could be applied at a number of levels. This
specification is concerned only with applications level data items
and services. It does not in any way concern itself with network
level packets or quality of service nor does it concern itself
directly with transport level connections or quantity or quality of
service except as these transport level measures impact application

This proposed syntax is concerned with such matters as access to a
web page page, storage of a file, initiation of a telnet session, or
conducting an extensive WAIS search. These are generally user
visible and meaningful data objects or tasks.

Within most legal systems, the owners of such data objects and/or the
owners of the facilities used to present such objects or perform such
tasks are frequently entitled to require some recompense if they
choose to do so. This document does not concern itself with the
morality of such laws or requirements but merely provides a syntax
whereby cooperating entities may speak at that level about prices and

There is no requirement that the "currencies" used with this syntax
be the usually recognized national or international currencies. For
example, some transactions could be denominated in frequent flyer
miles or other private artificial unit.

1.2 Overview of this document

Sections 2 and 3 below define a basic syntactic framework for price
tags, payments, and receipts.

Sections 4 through 7 specify a standard for inclusion of these items
in transactions for the World Wide Web (HTTP/HTML), File Transfer
Protocol (FTP), Telnet, and the Simple Message Transfer Protocol

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Section 8 lists some protocols to which application level payment
systems should not be applied.

Section 9 discusses security considerations.

Appendix A is an initial list of payment systems that are or are
planned to be usable via this syntax.

Appendix B gives a semi-formal BNF-like description of the syntax.

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2. Price Tags

A uniform price tag format is needed to indicate when payment is due,
how much, and what methods are acceptable by the seller. Such a
price tag must include the specification of one or more acceptable
payment systems (with a provision for payment system specific
information where needed) and will almost always include one or more

Sections 2.1 and 2.2 below describe prices and payment system strings
and Section 2.3 assembles these for complete price tags.

2.1 Prices

Prices are encoded as character strings consisting of a number
followed by a currency code.

These currency codes are the three letter ISO 4217 codes, Internet
Assigned Number Authority (IANA) registered four letter or longer
currency codes, or private use currency codes starting with "x-".
(ISO 4217 code normally consist of the two letter country code
followed by a letter mnemonic for the major unit of currency.)
Currency codes are case insensitive. One and two letter codes
appearing in the place of a country code are reserved for future use.

The number preceding the currency designation is the quantity of
major units of that currency. It may optionally have a decimal point
and additional decimal fraction digits for specifying minor unit or
fractions of units. (Some currencies, such as US Dollars or British
Pounds have minor units (cents and pennies). Others, such as Japanese
Yen and Italian Lira do not.) Fractional digits may continue
indefinitely after the decimal point but payment systems may define
how many digits they utilize.

Somes examples:

2.34gbp, 79ALL, 1.23456cad, 0.125usd

which signify 2 pounds and 34 pence sterling, seventy nine Albanian
Leks, one dollar and 23 and 456 thousandths cents Canadian, and one
eighth of a US dollar.

2.2 Payment System Strings

Payment system strings consist of the payment system name, a colon,
and any payment system specific information (such as what account

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within that payment system the payment should be made payable to).

Payment system names are case insensitive, four or more letters long,
and are indicated by a terminating colon. One to three letter codes
occurring in the place of payment system names are reserved for
future use.

Payment system specific information must be encoded so that it
contains no internal spaces or unusual characters as described in
Appendix B. It is up to the named payment system to encode and
decode any information it requires so as to fit within this syntax.
Use of the base64 encoding defined in RFC 1521 is recommended. The
payment system specific information, if any, appears immediately
after the payment system name and colon and is terminated by white
space or the end of the price tag character string.

A registry of payment system names is maintained by IANA. Initial
payment system names are listed in Appendix A. For experimental use
pursuant to bilateral agreement between the parties involved payment
system name starting with "x-" may be used. No names of this form
will be officially registered.

2.3 Price Tags

A complete price tag consists of a string of white space separated
prices and payment system strings. There must be at least one
payment system string present.

Normally there will also be at least one price. However, there are
circumstances under which the cost of a service in highly
unpredictable and the seller is, in effect, requesting a payment
system and account to which they can attempt to charge indefinite
amounts. Under these circumstances, it is recommended that a price
be listed which is a reasonable ceiling such that if costs exceed
that, the seller which have to present another price tag; however, it
is permitted to omit the price and list only a payment system in a
price tag.

Payment systems SHOULD provide a means for a limited amount of
arbitrary seller information to be included in the payment system
specific part of a price tag and be returned to the seller within a
payment message based on that price tag.

A price appearing after a payment system string applies only to that
system. Putting a price before the first payment system specific
information makes that price a default for every payment system
specified. The default can be overridden by specifying a different
amount for that currency after a particular payment system.

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For example:

33.45all foocash:xxxx 22eTb barsys:yyyy 9.999ghC

indicates that payment of twenty two Ethiopian Birrs via the foocash
payment system or 9.999 Ghanaian Cedis via the barsys system or 33.45
Albania Leks via either system is acceptable.

In cases where the cost of the service is not known in advance, the
price can be an estimate, deposit request, or the like, with any
overpayment refunded. Underpayment can be collected by requesting an
additional payment from the client. In the absence of trust between
the parties, frequent small payments may be required.

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3. Payments and Receipts

After encountering a price tag, either initially, during a session,
or in conjunction with a "payment required" error, an application
needs some method of tendering payment. This is done with a payment
system string with the same syntax as described in Section 2.2 above.
For example:


The payment system used in the payment is selected from among those
in the price tag as those are known to be supported by the seller.
Payment systems will normally include in the payment system specific
information some sort of serial or transaction number so that
retransmission of a message containing the string will not result in
duplicate payment.

Normally the seller will provide a receipt for the amount of money
actually collected or a message indicating payment failure or error.
This will be via a receipt character string which is also simply in
the form of a payment system string. For example:


Payment systems will normally include in the payment system specific
information of a receipt, in addition to an indication of how much
the receipt is for, some type of serial or transaction number, so
that retransmission of a message containing the receipt will not
result in confusion.

A null payment or receipt string is explicitly permitted in most
contexts as a way for an entity to indicate merely that it is payment
syntax aware.

One or more payment system names in isolation are permitted in a
payment or receipt context but only as a way to indicate that a
particular payment system is understood. Any actual payment or
receipt must have a colon and non-null payment specific information.
Only one such full payment system string can occur in a payment or

Depending of payment system details, a refund can be implemented in
two way. It can be a payment message from the seller to the buyer,
normally leading to a receipt from the buyer to the seller. Or the
seller may be able to directly refund to the buyer's account or the
like and simply send the buyer a receipt. In some payment system,
both refund techniques might be available. In others, refunding may
not be possible.

The content and/or encoding of the payment system specific

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information would normally differ between the price tag, payment, and
receipt contexts but this is a matter only of concern to the payment
system. Errors in formating or the like that are internal to a
payment or receipt should generally be handled by being logged and/or
reported by an error message encoded into a receipt. Errors in a
price tag may be reported in a payment or receipt. Great care should
be taken to be sure to avoid any situation that could result in an
endless loop of receipts.

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4. Use in the World Wide Web

The World Wide Web is a rapidly evolving system for information
interaction that is being increasingly used for commerce. It is
particularly well suited for the inclusion of payment systems,
especially any designed for efficient handling of very small payments
which might reasonably be incurred on a "per web page" basis or the

In the Web, a price is indicated by a COST parameter, as described
below, which can occur within an anchor, HTML document header, or
several places in a FORM. Payment can be included with any HTTP
request using the "ChargeTo:" header. A receipt can be included in
any HTTP reply using the "Receipt:" header.

4.1 Web Browser User Interface

It is important that small payments be closely integrated into the
browser user interface. An expected mode of operation will be one of
many small payments so the overhead associated with each must be
small. It is unacceptable for the user to necessarily interact with
a separate screen or window to approve each small payment although a
user who wishes to do so should have that option.

The user should be able to establish some threshold (default perhaps
around 0.1usd or equivalent) such that actions incurring that charge
or less are semi-automatic. That is, no special approval action is
required, although color coding or the like should be used to
distinguish toll links from free links, an optional sound could be
made when any money is sent, or other clues used to give the user a
feel for what is going on.

To avoid spending an unexpectedly large amount in small pieces,
possibly a bank graphic or the like should be displayed to show how
much cash is still available to the browser before the user will have
to take action. The act of refilling the bank would be a more heavy
weight operation requiring user interaction or, to get a default
amount, at least user approval.

4.2 Anchor Embedded Costs

A cost can appear in an anchor. This is a very strong hint that
payment of the indicated amount should accompany the GET operation
that occurs when following that link. Note, however, that it is
ultimately up to the server being hit to determine if payment is
adequate or to follow the course it chooses for different levels of

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The cost is given by a COST parameter in the anchor. For example:

<A COST="paymentsystem:xxxxxx 0.10usd 0.17cad"
Great stuff for one thin dime! </A>

It is recommended that toll links be shown in a different color or
type style from toll-free links. Browsers may wish to go further and
indicate different cost levels, particularly costs above or below any
"automatic approval" level the user has. When the user has their
pointer over the link, the browser may wish to display the payment
particulars in a similar way to that in which it displays the URL.
(Such a display could be filtered to the currency and/or payment
system(s) actually available to the user.)

Notice that the cost, if any, indicated by the anchor text ("Great
stuff..." above) could be different from the actual "COST=" parameter
which controls the payment sent with the request. In turn, the
"COST=" amount could be different from what the server really wants.
Or the server may provide different data or services for different
payment amount. Such variable payment schemes may be better handled
with a FORM as described below.

4.3 Page Header Price Tag

The cost for accessing an HTML page can be included in the header.
For example:

<head><title>Mating Habits of the Red Breasted Geek</title>
<cost> 0.75usd 0.99cad cybercash:A8jne8W2/sw== </cost></head>
<body> ... </body></HTML>

An attempt to get such a document without payment or with inadequate
payment should fail (see Payment Required Error section below). A
second attempt with payment will be required. This could be done in
a manner similar to an access restriction failure followed by a
second attempt with access authorization information.

Implementing page header cost requires that the HTML for a web page
be partly understood by the server, at least through the head, but
this is necessary to implement the "title:" and "link:" response
entity header fields anyway.

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4.4 HTML Form Price Tags

A cost can be associated with a form and with multiple choice items
within the form. For example:

<form COST="CyberCash:A8jne8W2/sw== 0.75usd 0.99cad" ACTION=POST>

Miscellaneous text, etc.

<input type="radio" name="extras" value="omit">plain vanilla
<input type="radio" name="extras" value="include"
COST="0.25usd 0.40cad">chocolate fudge

<p>Your quality of service: <select name="quality">
<option value="bronze"> Low<p>
<option value="silver" cost="0.10usd 0.17cad"> Medium<p>
<option value="gold" cost="0.20usd 0.34cad">
High<p> </select>

The COST associated with the form is a base price to which any
multiple choice item costs are added. The form level COST may be
omitted and COSTs can still appear with multiple choice items. The
COST associated with a "select" is a default which applies only if no
item is selected. When an item is selected, it over-rides the
selection level cost and become the price component added into the
total form price for that selection.

The normally required payment system string can be omitted from some
of the form COST parameters, in which case any prices add to the
amount for all payment systems. But one or more payment systems and
their payment system specific parameters must be determinable if any
payment is to be sent. The payment system specific information
associated with the last encountered instance of a payment system
field in processing the form is used. If no payment system field is
encountered, then no payment will be sent with the request even
though "COST=" parameters are present.

As with anchor costs, it is desirable to indicate the cost of
multiple choice items by color coding and the cost of activating the
form by color coding the submit button. Note that the submit button
could change from free to toll or the like as choices are made in the

4.5 Web Payments and Receipts

Any HTTP request can be accompanied with payment by including a
payment line in the message header. This consists of the "ChargeTo:"

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header label followed by a payment system string; however,
"ChargeTo:" can appear with one or more bare payment system names for
the purpose of indicating that the browser understands those systems
without conveying any actual payment. Examples:

ChargeTo: cybercash:A8jne8W2/sw==

ChargeTo: foocash barsys

The first example is in the form of a payment via cybercash. The
second example is an indication by the sender that it understands the
foocash and barsys payment systems.

The browser should keep track of such actual payments it has sent and
re-send the identical payment if the request needs to be retried with
access authorization information or due to a transient error, rather
than sending additional funds.

The collection of payment or the specifics of the failure of a
tendered payment are indicated back to the customer by a receipt line
in the response header. This consists of the "receipt:" header label
followed by a payment system string. For example:

Receipt: cybercash:A8jne8W2/sw==

There cases where a larger payment is collected initially and the
unused portion refunded or where adjustments are required after a
purchase. Because of this, ChargeTo and Receipt headers are both
allowed in both HTTP requests and responses.

4.6 Payment Required Error

If an HTTP request arrives without sufficient payment (or with none
at all) and payment is required by the server, the server can simply
provide a web page with limited or no actual information and possibly
one or more links with COST parameters embedded in them.
Alternatively, a "402 payment required" error is returned in which
case there must be a "www-cost:" response header field analogous to
the "www-authenticate:" header field for a "401 unauthorized""
response. The value of the www-cost field is the same as for the
COST parameter described above.

This is similar to an access restriction error in that the browser
can just try again with payment included the way they can try again
with access information. It may be possible to combine these by
returning a 402 error with the HTML accompanying the error having a
link with a COST parameter pointing to the originally sought item.
This would combine automatic charging for browsers that have 402

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error processing implemented with a convenient way for the user to
re-request with payment for browsers that understand anchor COST
parameters but do not automatically handle 402 errors.

4.7 Web Proxies

When information that has an owner and price is being cached and
served to multiple different users by a proxy, the payments should be
requested by the proxy. The safest thing for the proxy to do is to
send payment to the entity it retrieved the data from using an HTTP
request with a ChargeTo header and the PAYMENT method.

If the proxy understands the payment system well enough and there are
no firewall problems, the proxy may be able to collect the payment
and directly transfer funds to the information owner.

It is not expected that proxy payment collection will be perfect.
There will initially be many dumb proxies that don't understand
payment and there may be proxies that deliberately avoid collecting
and forwarding payment. But any large scale avoidance of payment
will be noticed. In any case, if the proxy can cache a copy, so
could the user, who could then give copies to all his friends. The
ease of automatically making small payments for information through
this syntax is hoped to produce a net reduction in unauthorized
information copying.

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5. Use in File Transfer Protocol

An FTP server may wish to charge for a file transfer (either way) or
for an FTP session.

It may do so by requesting that an ACCT command be sent via the 332
or 532 reply codes. 332 is used to indicate that a received command
is being held in abeyance pending receipt of an ACCT while 532
indicates that a received command has been abandoned due to lack of
payment and an ACCT command needs to be sent before attempting the
command again.

Price tags are indicated in the 332 or 532 text by a string at the
beginning of the form

<COST="foocash:xxxxx 0.05usd">

in the 332 or 532 text, i.e., a literal "<COST=" followed by a string
conforming to the definition herein of a price tag, followed by a
">". The word "cost" is case insensitive. Arbitrary additional text
may be included after the price tag.

A payment can be send by simply including a payment string, as
defined in section 3, after the ACCT command.

A successful receipt is rendered by returning a 233 reply with a
receipt payment system string as the beginning of its text. A
payment failure receipt is rendered by returning a 433 or 533 reply
depending on whether the failure is transient or permanent. In
either case, the receipt string can be terminated by white space and
additional text human readable text placed after the receipt string
in the reply.

(See RFC 959.)


Price Tags - in existing 332 and 532 replies.
Payments - in existing ACCT command.
Receipts - in new 233, 433, and 533, replies.

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6. Use in Telnet

A host may wish to charge for a Telnet session. Telnet option code
[TBD] is used to initially negotiate agreement of the two parties to
speak about payment. As with other Telnet options, either side can
sent IAC WILL xxx, in response to which an IAC DO xxx indicates
agreement and an IAC DON'T xxx indicate refusal. Or a party can send
IAC DO xxx to which IAC WILL xxx indicates agreement and an IAC WON'T
xxx indicates refusal.

After agreement to speak about payment has been reached, Telnet
subnegotiation strings can be exchanged, bracketed with IAC SB and
IAC SE. The initial subnegotiation byte indicates the type of
payment message following in the rest of the subnegotiation byte
string as follows:

Byte Meaning
---- -------

01 Price-tag
02 Payment
03 Receipt

If desired, arbitrary binary representations may be used for payment
system specific information after the colon terminating the system
name in payments and receipts (as long as bytes with value 255 are
doubled as per the Telnet standard). Termination can be unambiguously
determined by the IAC SE sequence. However, price tags must stick
with the ASCII syntax given herein as they must be parsed by systems
that may not understand any particular payment system.

(See RFCs 854, 855.)

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7. Use in Simple Message Transfer Protocol

A host or user may wish to charge for the receipt of mail. This is
accomplished via the new 332 reply code. This is an interim success
code that indicates that further information is required to complete
a pending command. Note that use of 332 after the SMTP RCPT command
would be a simple way to implement any particular user requiring
payment for mail to be delivered to them and its use after the MAIL
command would be a simple way to implement a system requiring payment
for mail from all or certain sources (although this information is
easy to forge).

Payment is indicated by the new ACCT command. This is followed by a
payment string as defined in section 3 above.

Charging for mail may cut a host or user off from the normal flow of
mail. It seems unlikely that most individuals or mailing lists would
be willing to pay to send mail to an address. However, it is easy to
envision cases where a service for which it would be reasonable to
charge is requested via email. Or there may be individuals who do
want to substantially cut themselves off from most mail or mail from
certain senders.

SMTP servers that speak ESMTP (see RFC 1651) may optionally give the
new EHLO keyword ACCT. However, ESMTP is designed for servers to
list features to be optionally invoked by clients. It is not really
appropriate as a means for servers to indicate features that they
will *require* of clients.

In any case, it is believed that no negotiation is necessary for an
SMTP server to use the new 332 reply code. RFC 821 is clear that the
receipt of any 3xx reply code after a MAIL, RCPT, etc. command is to
be considered an error. This is the appropriate understanding for an
SMTP client that does not understand payment when an SMTP server
requires payment.

The rules and state diagrams in RFC-821 are hereby amended and the
state diagram for MAIL, RCPT, SEND, SOML, and SAML is modified to the

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1 +---+ 1,3
FLOW FOR +----------->| E |<-----+
| 2 +---+ 2 |
+----------->| S |<-----+
| +---+ |
| |
| |
+---+ cmd +---+ 3 +---+ ACCT +---+
| B |--------->| W |----->| |------->| W |
+---+ +---+ +---+ +---+
| |
| 4,5 +---+ 4,5 |
+----------->| F |<-----+

A successful receipt is rendered by returning the new 233 reply with
a receipt payment system string as the beginning of its text. A
payment failure receipt is rendered by returning the new 433 or 533
replies depending on whether the failure is transient or permanent.
In either case, the receipt string can be terminated by white space
and additional text human readable text placed after the receipt
string in the reply.

The middle digit 3 in SMTP reply codes is reserved for accounting,
corresponding to its existing use in FTP.

(See RFCs 821, 1651.)


Price Tags - in new 332 reply.
Payments - in new ACCT command.
Receipts - in new 233, 433, and 533 replies.

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8. Protocols to Which Payment is not Applicable

Some protocols are sufficiently basic to the operation of the network
or provide sufficiently light-weight access to public information
that attempts to impose application level payment would be
inappropriate. Some of these protocols, listed below, SHOULD NOT
make use of this syntax or impose prices or payments.

8.1 The Domain Name System

Part of the philosophy of the Domain Name System (DNS) is that it
contains public information and generally gives the same answers to
all inquirers. It is used for such fundamental purposes as
translating domain names (such as tam.cybercash.com) into IP
addresses or specifying SMTP mail backup and routing servers. As
such, charges SHOULD NOT be imposed for DNS queries.

(See RFCs 1034, 1035.)

8.2 The Finger Service

Finger is an optional information service intended to permit remote
users to learn a limited amount of information about a user or users
on an Internet host. Information such as the time they last logged
in or contents of their ".plan" file. There are serious security
considerations involved in allowing finger access to a host and hosts
are free to decide how much such access, if any, they will provide.

In some cases, finger servers have been set up to act as information
retrieval or reporting mechanisms, but this was not the designed
purpose of finger and, in most cases, there are better mechanisms to
provide such access.

If finger access is provided because a site wishes to be open,
charges SHOULD NOT be imposed.

(See RFC 1288.)

8.3 The Auth Service

This service, when implemented, allows a remote host to determine the
user associated with a TCP connection. It is intended as a security
and auditing tool although it is weak in the face of anyone with
direct access to the TPC or IP level who was attempting to mislead

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it. Implementation is optional.

Those who chose to provide this service are doing so to cooperate in
such security or auditing at some sacrifice in the privacy of their
users. Charging for this service makes little sense in this context.

(See RFC 931.)

8.4 The ECHO, DISCARD, and CHARGEN Services

These are light weight services intended for network maintenance.
ECHO echoes the packet sent to it (see RFC 862), DISCARD throws away
packets sent to it but maintains the connection (see RFC 863), and
CHARGEN generates an infinite number of random characters and sends
them until the calling party disconnects (see RFC 864).

Hosts are free to decide which, if any, of these three services they
wish to provide (although ECHO is Recommended), but SHOULD NOT impose
any charges for them.

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9. Security Considerations

Getting authorization to construct payments may, depending on the
payment system, require the user to enter a passphrase. For example,
a passphrase might be required at the beginning of their session to
unlock a private key. Thus the user could be vulnerable to Trojan
horse web browsers, ftp clients, telnet clients, etc., as they are
to many other types of Trojan horse applications. Use of "secure"
application distribution with signed executables, checksums, virus
detection, etc., should be encouraged.

An adversary may be able to observe or modify traffic to and from an
application. Payment systems should be designed so that such
observation results in minimal loss of privacy and such observation
or modification can not result in hijacking a payment. Note that an
adversary that has complete control over application communications
can pretend to be a merchant just as it could by controlling an end
node. However, such impersonation from an end node may be easier to
trace and control than impersonation at an unknown point along the
communications path. Message (MOSS, SHTTP, etc.) and connection
(IPSEC, IPv6, SSL, etc.) security protocols are available to help
protect the communications path.

On receipt of an advance payment, a server is capable of charging the
user regardless of whether the server actually provides the data or
services being charged for. A server could even send back an error
message but keep and use the payment. Some means of automatically
logging payments that result in a software or human detectable
failure to deliver should be implemented so these can be examined for
patterns or cross checked with payment system statements of account.

A merchant can withhold and fail to send back to the user a receipt.
Applications should assume any payment sent will be collected
regardless of whether they get a receipt back.

With payment systems, a monetary cost can sometimes be associated
with downloaded data. Caching algorithms may wish to take this into
account and cache costly data in preference to free data. Servers
should accept the identical data request from the same net entity for
a reasonable amount of time even if the payment being presented
appears to be a duplicate. Transient errors may have prevented use of
the data previously downloaded for that request.

A bad client application could generate payments exceeding the funds
or authorization available to it. Servers should verify payments
promptly and be cautious of extending services or goods unless they
can confirms that payment is good. Applications and payment systems
should be designed to limit the amount of funds a rogue application
could transfer.

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[ISO 4217] - Codes for the representation of currencies and funds

[RFC 821] - J. Postel, "Simple Mail Transfer Protocol", 08/01/1982.

[RFC 854] - J. Postel, J. Reynolds, "Telnet option specifications",

[RFC 855] - J. Postel, J. Reynolds, "Telnet Protocol specification",

[RFC 959] - J. Postel, J. Reynolds, "File Transfer Protocol",

Author's Address

Donald E. Eastlake, 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA

Telephone: +1 508 287 4877
+1 703-620-4200 (main office, Reston, Virginia, USA)
email: dee@cybercash.com

Expiration and File Name

This draft expires 28 December 1995.

Its file name is draft-eastlake-internet-payment-00.txt.

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Appendix A: Initial Payment System Names

This is the initial alphabetic list of the initial registered payment
system names that intend to be usable via this syntax.

[send email to author, dee@cyercash.com, if you would like to be

Company Name Email Contact Home Page
------------ ------------- ---------
Payment System Name - (brief description)

CyberCash, Inc. info@cybercash.com http://www.cybercash.com
cybercash - (credit card)
cash - (cash)

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Appendix B: Simplified BNF

This is a BNF-like description of the Payment Protocol syntax syntax,
using the conventions of RFC822, except that "|" is used to designate
alternatives, and brackets [] are used around optional or repeated
elements. Briefly, literals are quoted with "", optional elements are
enclosed in [brackets], and elements may be preceded with <n>* to
designate n or more repetitions of the following element; n defaults
to 0.


isocurrency = alpha alpha alpha
ietfcurrency = 4*alpha
privatecurrency = "x-" 4*alpha
currency = isocurrency | ietfcurrency | privatecurrency
digits = 1*digit
decimalpoint = "." | ","
number = digits | digits decimalpoint *digit

cost = number currency

;payment system strings

ianapaysys = 4*alpha
privatepaysys = "x-" 4*alpha
paysysname = ianapaysys | privatepaysys

paysys = paysysname ":" *uchar

;price tag

pricetag = *sp paysys *[ 1*sp cost | 1*sp paysys ] *sp |
*sp *[ cost 1*sp | paysys 1*sp ] paysys *sp

;miscellaneous definitions

lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
"i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
"q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
"y" | "z"
hialpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
"I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
"Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
"Y" | "Z"
alpha = lowalpha | hialpha
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |

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"8" | "9"
other = "$" | "-" | "_" | "." | "+" | "/" | "=" | "@"
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
escape = "%" hex hex
sp = " "
uchar = alpha | digit | other | extra | escape

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