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Versions: 00 01 02 03 04 05 06 07 08                                    
Internet-Draft                          Tom Arnold (CyberSource)
Category: Application                   Jason Eaton (CyberSource)
October 5, 1999
Expires in six months


            Simple Commerce Messaging Protocol (SCMP)
                  Version 1 Message Specification
                    (draft-arnold-scmp-04.txt)


Status of this Memo

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and 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 and may be updated, replaced, or obsoleted by other
documents at any time.  It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

1. Introduction

The Simple Commerce Messaging Protocol (SCMP) is a general-purpose
electronic commerce message protocol for secure, real-time
communication of a set of data from a sending agent's application
to a receiving agent's server. Additionally the response by the
receiving agent's server to the sending agent is the reply from the
request represented by the set of data in the message's payload. The
intent of this protocol is to define a method where trading partners can
perform on-line business requests in an environment where the sending
partner is fully authenticated, and the message cannot be repudiated.

The taxonomy of the SCMP message payload is not in the scope of this
document. The SCMP protocol does not specify payload definitions or
how trading partners are expected to process the payload, beyond basic
server-level functions related to processing SCMP headers. This intent
is to permit trading partners the flexibility to implement either a
standard commerce message format as in ANSI-X12 Electronic Data
Interchange (EDI) or some other non-standard payload format. This
document does give an example implementation of a payload format
based on [XML].

The only requirement on the message payload is that it be prepared
as specified in [MIME].

In this manner, SCMP fundamentally differs from many emerging
commerce message protocols. Beyond specifying the method for
encryption, authentication and handling, these other protocols
specify the contents of the message and details how a server is to
process and respond to a given message payload.

1.1. Document Overview

This document describes SCMP from the standpoint of how trading partners
would implement a client/server request processing system via an
untrusted network connection.

In a on-line, electronic commerce environment, trading partners require
a scalable, message handling system that will meet these minimum
requirements:

1.1.1   Real-time Request and Response

A single message containing all credentials and payload data is
prepared by a sending agent and sent to a receiving agent. The
receiving agent, upon verification of sender's credentials, should
process the payload, format a reply, and respond to the sender as
the response to the request.

1.1.2   Message Privacy

Through use of cryptographic methods, the privacy of the sender's
message payload should be assured should a message payload be
intercepted.

1.1.3   Message Integrity

If a message arrives in an incomplete or tampered condition from a
sending agent, the receiving agent's server should detect the condition,
deny message payload processing, and respond with an appropriate
error message.

1.1.4   Authentication of Sending and Receiving Agents

Messages between trading partners, as represented by sending and
receiving agents, must contain attributes that assure a given request
could only be from a specific trading partner. Additionally SCMP
requests and SCMP replies MUST be authenticated.

Prior to performing any application functions on a SCMP request
payload, the receiving agent's server MUST verify that the request has
been made by an authorized sender.

1.1.5   Non-repudiation

When a receiving agent's server receives a request to process a payload,
the receiving agent's application should guarantee that the sender
cannot, at some later time, refute having sent the request.

Non-repudiation is defined as, the inability of either trading partner
(sender or receiver) to refute the sending of an SCMP request or SCMP
reply.

An Electronic Commerce provider will typically provide financial based
functionality such as authorization, settlement, and crediting, of
credit card accounts and/or merchant accounts. This functionality
requires that the Electronic Commerce Provider execute financial
transactions on behalf of the merchant, or trading partner. Therefore it
is desirable that the transaction directives which are given in an SCMP
message are non-refutable.

1.1.6   Payload Independence

The messaging system should perform consistently for all payload formats.

1.1.7   Standards Based

The messaging protocol should be based on proven, existing cryptography
and Internet standards.

1.1.8   Use of Standard Credentials

Standard credential formats should be used to maximize interoperability
of common Public Key Infrastructures.

1.1.9   Transport Independent

The message should be transportable over the most common Internet
transport protocols.

1.1.10 Service Level Guarantee

The receiving agent should guarantee a response within the time
designated by the sender, or reject the message with an appropriate
error message.

1.1.11 State Independence

State dependency by either a sender's or receiver's application should
be minimalized as to support multiple transport mechanisms.

1.2. Terminology

Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD NOT
are used in conformance to the definitions in [MUSTSHOULD].

1.3. Definitions

Several terms will be used when specifying SCMP.

Trading Partners     Two entities wishing to perform some on-line
                     request processing where authentication,
                     privacy, integrity and non-repudiation of the
                     requests are important. Trading partners have
                     established a trusted relationship between each
                     other.

Client               An application program that executes on a remote
                     system, used by a trading partner to request
                     services from a server via an untrusted or
                     publicly switched packet network, like the
                     Internet.

Server               An application program used to process SCMP
                     messages received from a client, and generate
                     appropriate replies which are sent back to the
                     client.

Sending Agent        An entity that operates or uses a Client for
                     requesting on-line services from a server.

Receiving Agent      An entity that operates a Server, receives and
                     processes requests from a plurality of Clients.

Request              An SCMP formatted message containing a set of
                     directives set in a textual form requesting a set
                     of directives be executed on behalf of the sending
                     agent.

Reply                An SCMP formatted message containing a set of
                     result data generated as a result of processing an
                     SCMP request.

Payload              The meaningful content provided by a client to a
                     server, encapsulated in an SCMP message. Similarly
                     the meaningful content provided by a server to a
                     client, encapsulated in an SCMP message.

Services             Groups of operations and/or algorithms implemented
                     by the server application which are executed as
                     designated by the payload. Each available group of
                     operations and/or algorithms is a service.

2. Payload Encapsulation

The payload of an SCMP message MUST be prepared as a standard MIME
entity as defined in the [MIME] specification. The [SMIME] document
describes how the resulting MIME entity SHOULD be cryptographically
enhanced according to [PKCS-7].

An SCMP compliant server SHOULD implement the three message types as
described in [SMIME], signed, enveloped, and signed/enveloped. An SCMP
compliant server MUST implement signed/envelope message type as
described in [SMIME].

For non-repudiation concerns, the trading partners MUST exchange
signed or signed/enveloped SCMP message types.

SCMP error messages MUST be of signed type and NOT encrypted.

All of the header fields defined in this document are subject to the
general syntactic rules for header fields specified in [RFC822].

In addition to the standard MIME headers, a compliant implementation
MUST define "SCMP-protocol-version" and "SCMP-sender-name". These
headers added to the outer MIME entity, as described in [MIME].

Use of any additional standard SMIME (outside MIME entity) headers are
assumed. These headers will most likely be ones that need to be
processed prior to payload decryption.

Both the sending agent and receiving agent MUST specify all SCMP
headers specified in this document.

2.1 SCMP Protocol Version

The SCMP-protocol-version header is used to designate the SCMP protocol
version. Server implementations MAY reject the request based upon
protocol version, before any message processing occurs.

An example SCMP-protocol-version header will be in this format:

  SCMP-protocol-version: 2.0

The value of the protocol-version header MUST be in the following
format, any number of digits, followed by a the special character ".",
followed by any number of digits. Where special character, and digits is
defined in [RFC822].

If a particular protocol version is not supported by the implimentation,
the receiving agent MUST reject the request with an appropriate SCMP
error message.

3. SCMP Payload-based Headers

This section describes the payload-based extensions that MUST be
implemented by both the client and server to ensure correct and proper
request processing.

Setting the SCMP service headers is the responsibility of the sending
agent's client application. Processing the SCMP payload headers is the
responsibility of the receiving agent's server application processing
the request.

The following headers are described for the payload of the SMIME
entity, and MUST be prepared as defined in [MIME]. Thus if the SMIME
message type is signed/enveloped ( which is recommended ), then the
SCMP headers will be encrypted with the sender's message payload.

Both the sending agent and receiving agent MUST specify all SCMP
headers specified in this document.

3.1. Request Time to Live

This describes the amount of actual processing time in seconds the
client expects the server to complete payload processing prior to
responding with an appropriate reply.

An SCMP server receiving a SCMP message MUST evaluate the request time
to live value and determine if it can execute the required service(s)
in the amount of time designated.  Assuming the server believes it can
complete the work within the allowed time, it will accept the request.
If not, the server MUST return an error to the client stating it
could not accept the request.

Once a server has accepted a request, it MUST process it until the time
to live value has been reached or until completion. If the time to live
value is reached during execution, the server MUST return an error to
the client stating that a time-out has occurred.

Application functions to ensure data consistency, integrity, or
rollback after the time to live value has been exceeded will be the
responsibility of the server application. A policy on what application
actions a server will take upon exceeding a time to live value SHOULD
be published by the receiving agent operating the server.

An example of a policy in this are would be one where a receiving
agent's server will continue processing the request after a request
time to live value has been exceeded. Given this policy, a client,
having received a time-out error message, would send a "request
status message" to the server, referencing the original
scmp-request-id (from the message that timed out) in the message
payload. The server's reply to this status message would be the reply
that would have been sent had the processing time not exceeded the
time to live metric.

The time to live header will be in this format:

    SCMP-request-time-to-live: 90

Where the value of the time-to-live header is a digit or digit(s) as
specified in [RFC822]. The value of the time-to-live is represented
as any number of digit(s) which will designate a number of seconds.

3.2. Message Type

This value specifies the type of payload that is contained in the SCMP
message. The intent of this header is to provide a meta-level
description of the message payload and allow a receiving server to
decide which services or associated algorithms to use in processing
the payload.

Message type is specified as follows:

    SCMP-message-type: [service-name]/[version-number]

Where service-name is text as specified in [RFC822] and version-number
is a digit or digit(s), followed by the special character ".", followed
by a digit or digit(s). Where digit and special character are defined in
[RFC822].

For instance, if a service was published called "CommerceService", the
SCMP-message-type would be represented as:

    SCMP-message-type: CommerceService/1

It is assumed that trading partners will agree on service names
before requests are processed.

If a particular message type is not supported by the implimentation,
the receiving agent MUST reject the request with an appropriate SCMP
error message.

3.3. Request ID

Request ID's MUST be generated by the client application, thus
assuring that the scmp-request-id is available in the event that the
request cannot be sent to the server due to errors.

The format of value of the request id header is 22 digits, where
digits is defined by [RFC822].

An example of a request scmp-request-id is:

        scmp-request-id: 0917293049096167904518

The scmp-request-id MUST be unique in the domain of a client
application and SHOULD NOT be easy to predict so as to prevent a
potential replay attack.

A client application, when preparing the scmp-request-id, SHOULD
perform a random number generation with sufficient degrees of
randomness, to ensure unpredictability, and generate a client side
time value, to ensure uniqueness of the result. These two data items
together SHOULD form the resulting scmp-request-id.

Servers MAY use a scmp-request-id as a reference and handle to the
original request during server message processing.

Servers MUST return the submitted request id back to the client via
the SCMP reply message in the SCMP-request-id header.

4. SCMP Data Block (Message Payload)

The payload or data block can be any arbitrary data type in the format
as specified by the SCMP-message-type. This payload forms the content
of the SMIME message as described in [SMIME].

6. Transport Implementations

SCMP can be implemented using any variety of transport methods as
defined by the service provider. Here are a few examples.

HTTP: This delivers a SCMP message to a server URL and should
      use a POST function. Used in this manner the SCMP reply
      would be the entity-body of the HTTP response. SCMP error
      messages would be the entity-body of the HTTP response.

SMTP: This will support a queued batch processing service. Used
      in this manner the SCMP messages would be the body of the SMTP
      message. SCMP error messages would be sent in the body of the
      SMTP message.

7. Receiving Server Functions

This section describes minimal server functions required to implement
SCMP.

7.1. General

A SCMP server receives a message from a client, processes the message
and generates a reply. If the message type is signed or signed/enveloped
the server initially validates the outer signature. If the outer
signature is not valid the server MUST NOT process the request further.

7.1.1. Message Timestamp

The time a request was received SHOULD be derived from the environment
which recieves the message. Clients and servers SHOULD be synchronized
using [NTP] or Secure NTP.

The message timestamp SHOULD be used, in combination with the
scmp-request-id, by the server to aid in detection of a potential
replay attack.

It is recommended that servers SHOULD run a client-visible NTP server
to allow SCMP client applications to synchronize clocks as required.

7.1.2 Support for Request Non-Repudiation

Support for non-repudiation MUST be included in any complete SCMP
implementation, as described in the following subsections.

Implemenations MAY support non-repudation of error message replies.
This document addresses the non-repudation concerns of the server or
receiving agent. The non-repudiation concerns of of the client or
sending agent MAY be fulfilled by the same means as the server or
receiving agent supports non-repudiation.

7.1.2.1 Client Message Signing

The client application must send signed or signed/enveloped message type
as specified in [SMIME].

7.1.2.2 Server Message Signing

The server application must send signed or signed/enveloped message type
as specified in [SMIME].

7.1.2.3 Server Processing

The receiving agent's server application evaluates the digital
signature, thereby guaranteeing that the message payload has not been
altered in transit, and that the message was, in fact, signed by a
specific trading partner (client) who possess the proper credentials.

7.1.2.4 Server Accounting

The receiving agent's server application MUST store the original signed/
encrypted message in an unprocessed state along with the timestamp for
identifying when the message was received.

7.1.2.5 Client Accounting

The sending agent's client application MAY store the original signed/
encrypted message in an unprocessed state along with the timestamp for
identifying when the message was received.

7.1.2.6 Revocation

All messages signed by a sending agent's client application in
accordance with [SMIME] and sent to a receiving agent's server SHALL be
considered non-repudiable.

To satisfy the non-repudiation requirements, the receiver of the message
MUST support revocation mechanisms for the certificates of the potential
senders of the SCMP messages that are accepted by the server application.

7.2. Application issues

The server MUST evaluate the signature of the message, if the message
is of signed or signed/enveloped type, prior to processing the message
payload. In performing this authentication process, the server MUST
validate the senders certificate and verify that the senders certificate
is not listed in any available revocation systems.

Assuming the SCMP message's signature is valid, the server will process
requests with the appropriate service designated by the SCMP-message-type value.

7.2.1. Request Serialization

A server SHOULD NOT guarantee serialized request processing. If requests
must be serialized, it is expected that all of the serialized
transactions will be received in a single message payload or that other
content specific serialization systems will be used.

7.2.2. Server Errors

A server may encounter several classes of error conditions. The
server MUST be capable of reporting an error as described in section 8
of this document. Error Detection may vary based on specific
implementation.

A server MUST be capable of detecting a duplicate scmp-request-id
and reply to the sending client application with an appropriate SCMP
error message. Duplicate request detection MUST be based on the
scmp-request-id and the distinguished name of the signer to prevent
denial of service attacks.

8. Protocol Level Error Messages

In general SCMP does not concern itself with application level errors.
Such errors MUST be returned in an SCMP reply with appropriate
application specific formatting.

8.1. Format

SCMP error messages MUST be signed SMIME messages. SCMP errors
MUST NOT be encrypted to permit clients to process encryption related
errors.

The format of SCMP errors is:

     SCMP <error number> <error message text>

Where the format of "error number" is a digit or digits as defined in
[RFC822] and "error message text" is text as defined in [RFC822].

8.2. Client Application Error Handling

Client action in the case of error return is error specific and not
defined. If the server fails to return any reply within the time to
live requested (due to unspecified server or network failure) the
client MAY re-send the request. Clients MUST NOT retry a request in
an interval which is less than the time to live value of the original
request.

9. Security Considerations

Security considerations are addressed throughout this document.

9.1 Encryption Strength

It is recommended that strong enough cryptographic methods be used to
ensure authenticity, integrity, non-repudiation, and privacy of the
payload.

9.2 Non-repudiation

Non-repudation implimentation is specified in section 7.1.2.

As addressed above, this document does not describe how a sending agent
may support non-repudiation. The intent of this document does describe
how a receiving agent can support non-repudiation.

If the receving agent accepts and processes a transaction after the
private key of the sending agent has been comprimised, that request is
refutable, or not non-refutable.

9.3 Public Certificate Considerations

9.3.1 Certificate Exchange

Every trading partner implementing SCMP SHOULD exchange certificates
that have been issued and signed by one or more mutually trusted
certificate authorities. Prior to establishing trading partner
relationships, the sender and receiver SHOULD acquire mutually
acceptable public root certificates from the agreed upon certificate
authority or authorities.

Sending and receiving agents MAY utilize certificate only messages to
exchange certificates as specified in [SMIME].

9.3.2 Certificate Authentication and Revocation

Trading partners, upon receiving or exchanging public key certificates
for the first time, SHOULD validate the certificate and certificate
chain before processing an SCMP request.

A server certificate revalidation policy, related to the frequency
certificates are revalidated against a certificate authority's
certificate revocation list, is not specified by SCMP. This matter is
left as a policy decision for the operator of the SCMP server.

The timestamp of a certificate revocation event SHOULD be the time the
private key was known to be comprimised, or the time that the revocation
event was made.

9.4 Private Key Considerations

9.4.1 Private Key Generation

Private key generation should be of a secure manner as not to jepordize
the integrety of the private key.

9.4.2 Private Key Storage

The sending agent, maintaining a SCMP client application, MUST
maintain the private key in a secure location.

9.4.3 Private Key Revocation

Should a sending agent loose control of their private key, they MUST
notify the agreed upon, trusted, certificate authority. This
notification mechanism is not defined in this document, and should
be done via an out of band mechanism.

9.5 Request Id

The request id MUST be unique as to prevent possible replay attack
senarios.

10. SCMP Message Example

[ OUTER MIME START ]
Content-Type: application/pkcs7-mime
Content-Transfer-Encoding: base64
Content-Length: 1024
SCMP-protocol-version: 2.0
SCMP-sender-name: Adobe

  [ INNER MIME START - enveloped entity ]
  SCMP-request-time-to-live: 90
  SCMP-message-type: Commerce/2.0
  SCMP-request-id: 0123456789012345678901
  Content-Type: application/pkcs7-mime
  Content-Transfer-Encoding: base64
  Content-Length: 512

    [PAYLOAD - signed entity ]

  [ INNER MIME END ]
[ OUTER MIME END ]

11. XML Payload Example

This section is intended to be an example implementation of a payload
and is NOT required for this protocol. The parties communicating could
agree on two "scmp-message-type" values. The first would be the exchange
of the DTD template, ( ie. scmp-message-type=widget xml definition ).
The second could be the actual data generated from that template, ( ie.
scmp-message-type=widget xml data ). The DTD would be transfered
defining the data format. The server could store the format for later
transferring of these types of messages. An example DTD follows.

<!ELEMENT request ( merchant_id, order+ )>
<!ATTLIST request number NUMBER #REQUIRED>
<!ELEMENT request type (#PCDATA)>
<!ELEMENT merchant_id ( #PCDATA )>
<!ELEMENT order ( product_sku )>
<!ATTLIST order_number NUMBER #REQUIRED>
<!ELEMENT product_sku (#PCDATA)>

This DTD would produce data as follows.

<request number="1">
<request type="widget xml data">
  <merchant_id>Widget Maker</merchant_id>
  <order_number="1">
    <product_sku>0349003</product_sku>
  </order>
</request>

This XML data would be the payload of the SCMP. When the agent recieves
this type of SCMP message they could validate the message format with
the previously received template.

12. Author's Address

Tom Arnold
CyberSource Corporation
550 S. Winchester Blvd., #301
San Jose, CA 95128
E-mail: toma@cybersource.com
Phone: 408-556-9100

Jason Eaton
CyberSource Corporation
550 S. Winchester Blvd., #301
San Jose, CA 95128
E-mail: jeaton@cybersource.com
Phone: 408-556-9100

13. Acknowledgements

The authors wish to recognize and thank several individuals
(listed in alphabetic order) who have and continue to
support the development of requirements and improvement of
this protocol.

Mike Agostino (Vulcan), Ron Bose (LitleNet), David Burdett (Mondex),
Leonard Cantor (IBM), Dan Corcoran (Equifax), Steve Crocker
(Crocker Assoc.), Tony Curwen (Ingram Micro), Donald Eastlake (IBM),
Richard Frank (Intertrust), James Gavin (Commercenet), Paul
Guthrie (VISA International), Lauren Hall (SIIA), Bengamin Hipp
(FUSA/Paymentech), Andy Jeffrey (Sonnet Financial), Helle Jespersen
(IBM), Sean Kiewiet (Hypercom.com), Connie Lindgreen (IBM),
Michael Myers (VeriSign), Allan Ottosen (PBS), John Pettitt
(Beyond.com), Jesse Rendleman (CyberSource), Don Sloan
(Tech Data), Carl Stucke (Equifax), Frank Tyksen (Portland
Software), Huy Vu (VISA USA), Sean Youssefi (CobWeb)

14. References

[SMIME]            S. Dusse, et. al, "S/MIME Version 2 Message
                   Specification", RFC 2311, IETF, March 1998.

[MIME]             "MIME Part1: Format of Internet Message Bodies", RFC
                   2045; "MIME Part2: Media Types", RFC 2046; "MIME Part
                   3: Message Header Extensions for Non-ASCII Text", RFC
                   2047; "MIME Part 4: Registration Procedures", RFC
                   2048; "MIME Part 5: Conformance Criteria and
                   Examples", RFC 2049, IETF.

[MUSTSHOULD]       "Key words for use in RFCs to Indicate Requirement
                   Levels", RFC 2119, IETF.

[NTP]              D. Mills. "Network Time Protocol", RFC 1119, IETF,
                   September 1989.

[PKCS-7]           B. Kaliski, "PKCS #7: Cryptograpic Message Syntax"
                   RFC 2315, IETF, March 1998.

[RFC822]           D. Crocker, "Standard for the format of arpa internet
                   text messages", RFC 822, IETF, August 1982.

[X.520]            "ITU-T Recommendation X.520: Information Technlogy -
                   Open Systems Interconnection - The Directory:
                   Selected Attributes Types, 1993.

[XML]              Extensible Mark Up Language. See http://www.w3.org/TR