Internet-Draft Tom Arnold (CyberSource)
Category: Security Jason Eaton (CyberSource)
August 31, 1998 Michael Jimenez (CyberSource)
Expires in six months
Simple Commerce Messaging Protocol (SCMP)
(draft-arnold-scmp-00.txt)
Version 1
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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1. Introduction
The Simple Commerce Messaging Protocol (SCMP) is a general-purpose
commerce transport protocol for securely communicating a set of data
from a sending agent's application to a receiving agent's server. And,
where the response by the receiving agent's sever to the sending agent
is, in fact, the reply from the transaction 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
transactions in an environment where the sending partner is fully
authenticated, and the message cannot be repudiated.
The SCMP message content, hereinafter referred to as message payload,
is not intended to be defined or specified. SCMP does not specify
payload contents 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 proprietary
transaction format.
The only requirement on the message payload is that it be identified to
the receiver utilizing MIME naming and formatting [MIME] and used to
identify the type of payload contained in the SCMP data area.
In this manner, SCMP fundamentally differs from many emerging
commerce message protocols. Beyond specifying the method for transport,
encryption, authentication and handling, these other protocols
specify the contents of the message and details how a server is to
process and respond to the message payload.
SCMP is intended as both an on-line and batch protocol. The exact
content of the message and the processing constraints are specified in
SCMP-headers.
1.1. Document Overview
This document describes SCMP from the standpoint of how trading partners
would implement a client/server transaction processing system where a
sending agent requests services via an untrusted network connection from
a server-based receiving agent.
In this environment, the typical requirements for authentication, non-
repudiation, message integrity, and privacy as discussed in [SMIME] and
assured by the proper use of the Secure/Multipurpose Internet Mail
Extensions [SMIME]. Beyond this, the trading partners require service-
based extensions to standard MIME and SMIME security services. These
service-based extensions are described within this document, while it is
assumed the trading partner will implement MIME and SMIME services as
described in [MIME] and [SMIME] respectively.
1.2. Terminology
Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD NOT
are used in conformances 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
transaction processing where authentication,
privacy, integrity and non-repudiation of the
transactions are important.
Client An application program that executes on a remote
system, used by a trading partner to request
services from a server via an un-trusted 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.
Transaction A discrete unit of service embodied in a single
SCMP message/reply pair.
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.
Request An SCMP message sent from a client to a server.
Reply An SCMP message sent from a server to a client.
Services Algorithms implemented by the server application
which are executed as designated by the payload.
Each available algorithm 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 [CMS], which is derived from PKCS #7, [PKCS-7].
An SCMP compliant server MUST implement three message types as described
in [SMIME], signed, enveloped, and signed/enveloped.
This document does not designate specific encryption algorithms or SMIME
message types. However, it is recommended, for non-repudiation concerns
that the trading partners SHOULD exchange signed or signed/enveloped
SCMP message types.
It is also recommended that strong enough cryptographic methods
be used to insure authenticity, integrity, non-repudiation, and privacy
of the payload. But, if the trading partners form a private agreement,
clear data or signed-only data MAY be exchanged. However, an SCMP
compliant server MUST support encryption even if encryption is not being
used.
3. SCMP Service-based Headers
This section describes the service-based extensions that MUST be
implemented by both the client and server to insure correct and proper
transaction processing. Processing the SCMP service headers is the
responsibility of the application processing the request.
3.1. Quality of Service:
This describes the amount of actual processing time in seconds the client
expects the server to fully complete payload processing prior to
responding with and appropriate error message, or reply.
An SCMP server receiving a SCMP message MUST evaluate the quality of service
value and determine if it can execute the required service(s) in the amount
of time designated by the quality of service header. Assuming the server
believes it can complete the work within the allowed time, it will accept
the job. If not, it MUST return an error to the client stating it could not
accept the transaction.
Once a server has accepted a job, it MUST process it until the quality of
service value has been reached or until completion. If the quality of
service value is reached during execution, the server MUST return an error
to the client stating that a timeout has occurred. Measures to ensure data
integrity after the quality of service value has been exceeded will be the
responsibility of the implementation.
The quality of service header will be in this format:
SCMP-quality-of-service: [batch | {0..n seconds}]
An SCMP-quality-of-service value of "batch" MUST cause the server to
respond with an acknowledgement reply to the client. The server SHOULD
then process the message according to an appropriate schedule and respond
to the client as appropriate after completing the actual processing.
Additionally, "batch" designates that there are no "real-time" quality of
service requirements for that SCMP message.
3.2 Return Path:
This describes the method used to respond to the client after completion
of a "batch" request. The client will not be notified upon "batch"
request completion if the return path is empty.
Return Path is specified as follows:
SCMP-return-path: "[protocol]:[destination]"
The list of protocols supported MUST be furnished via a private agreement
between trading partners.
Destination MUST be a valid value relative to the specified protocol.
For example:
SCMP-return-path: smtp/someone@host.domain
3.3. 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. The message type value SHOULD NOT be the sole
specifier of the services being requested by a client from a
server. The list of services to be performed on a specific payload
SHOULD be included in the message payload.
Message type is specified as follows:
SCMP-message-type: "[service-name]/[version-number]"
The assignment of service names MUST be provided by the server to a
client at the time a service is published. For instance, if a service
was published called "CommerceService", the SCMP-message-type might
be represented as:
"SCMP-message-type: CommerceService/1.0"
It is assumed that trading partners will agree on service names
before transactions are processed. Additionally servers MUST allow
service names to be configurable, reguardless of what the algorithm
which implements the service does.
3.4. Transaction ID:
A value in the format described in [822] for the Message-ID header
with the left part constrained to be a numeric value "message-number
@host.domain". Transaction ID's MUST be generated by the client
application. The Transaction-ID of the reply will be the inbound
Transaction-ID with a "Reply-" pre-appended.
An example of a request Transaction-ID is:
SCMP-transaction-id: 123456789@host.somewhere.com
A corresponding example reply Transaction ID would be:
SCMP-transaction-id: Reply-123456789@host.somewhere.com
The message-number portion of a Transaction ID's SHOULD (except as
required for error recovery) be unique and SHOULD NOT be easy to
predict to prevent a potential denial of service attack. A client
application when preparing the message-number portion should perform a
random number generation with sufficient degrees of randomness
so as to insure uniqueness and unpredictability of the result.
Servers MAY use a Transaction-ID as a reference and handle to the
original transaction.
3.5. Additional Header Information:
In addition to to the payload (inside MIME entity) headers stated in
sections 3.1-3.4, use of the remaining standard SMIME (outside MIME entity)
headers are assumed. This includes any additional implementation-specific
headers. These headers will most likely be ones that need to be processed
prior to payload decryption.
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].
5. Certificates
Every trading partner implementing SCMP MUST exchange certificates that
have been issued and signed by one or more mutually trusted certificate
authorities (CA). These certificates are used to guarantee the
authenticity of public keys. Prior to establishing trading relationships
on the basis of SCMP transactions, sender and receiver MUST have
acquired mutually acceptable trusted public root certificates in a trusted,
secure, out-of-band manner.
Trading partners, upon receiving or exchanging public key certificates
for the first time, SHOULD validate the certificate and certificate chain
before processing an SCMP transaction.
It is also recommended that the trading partners re-validate any
certificates and certificate chains on a scheduled basis.
Upon establishing a relationship between trading partners, the recipient
of a new certificate (the server in most cases) SHOULD validate the
certificate as soon as is practically possible. Certificate re-
validation policy, related to the frequency known 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.
6. Transport Implementations
SCMP can be implemented using any variety of transport methods as agreed
between trading partners. Here are a few examples.
http: This delivers a SCMP message to a server URL and should
use a PUT function.
electronic mail: This will support a queued batch processing service
diskette: SCMP message as a text file
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 transaction further.
7.1.1. Message Timestamp
The time a transaction was sent will be derived from the standard SMIME
date header. If a client is specifying a quality of service other than
"batch", the client SHOULD be synchronized using [NTP] or Secure NTP.
The sender of an SCMP message will place the time a message was
dispatched into the SMIME header in [MIME] format. The message timestamp
SHOULD be included in the SCMP payload if possible and used, in
combination with the transaction ID, by the server to prevent a replay
attack.
It is recommended that servers run a client-visible NTP server to
allow sending agents running SCMP client applications to synchronize
clocks as required.
7.2. Application issues
The server SHOULD evaluate the signature of the message, (if the message
is of signed or signed/enveloped type ), prior to processing the message
payload. Within this process the server SHOULD obtain the senders
certificate via. the distinguished name in the certificate as described
in [HOUSLEY].
Assuming the SCMP message's signature is valid, the server will process
transactions based on the SCMP-message-type value. Transactions may be
processed on-line or in batch mode, depending on the value of the
SCMP-quality-of-service header.
7.2.1. Quality of Service and Time
Assuming the SCMP-quality-of-service header is integer representing a
number of seconds (i.e., not "SCMP-quality-of-service: batch"), the
receiving server MUST determine if the quality of service is
attainable. In this function, the server will evaluate if sufficient
time has been allotted for the application functions to be completed.
Assuming the server determines the quality of service time allotted
is attainable, the server will begin processing the transaction.
Once a server has started processing a transaction, the server MUST
NOT terminate due to inability to meet the quality of service time
value.
In the event the server is unable to complete and reply within the
quality of service time value, the server MUST reply to the client
with a time-out error message. The server will finish processing the
transaction and continue processing the next SCMP message.
A client, having received a time-out error message, SHOULD send a
"request status message" to the server, referencing the original
SCMP-transaction-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
quality of service metric.
7.2.2. Transaction Serialization
A server may not guarantee serialized transaction processing. If
transactions 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.
7.2.3. 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. Detection may vary based on specific implementation.
A server MUST be capable of detecting a duplicate SCMP-transaction-id
and notify the sending client application of the duplicate transaction.
Duplicate transaction detection MUST be based on the SCMP-transaction-
id and the distinguished name of the signer to prevent denial of service
attacks. Servers MUST take steps to prevent error conditions in which
transaction retries overlap the original transaction processing. In this
case the server MUST NOT respond to the retry until the original result
is available.
In the event of a duplicate transaction being detected the server MUST:
1) lookup the prior transaction
2) verify the sender is the same; and
3) return an appropriate error message to the client.
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 are returned by a server as signed data. Errors MUST
NOT be encrypted to permit clients to process encryption related errors.
The format of SCMP errors is:
SCMP-Error: <error number> <error message text>
[To do - Need to define error numbers/error message text or domains
of error numbers -something like http? ]
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 twice the
quality of service requested (due so unspecified server or network
failure) the client SHOULD re-send the transaction. Upon receipt of the
duplicate transaction the server will respond as described in 7.1.3.
clients MUST NOT retry transactions in less than the quality of service
interval of the original transaction.
A server MAY want to provide the function to allow a client application
to send a status request referencing the original transaction-ID. In
this situation, a server SHOULD return the values of the reply from the
original transaction request (identified by the transaction-ID).
9. Author's Address
Tom Arnold
CyberSource Corporation
550 S. Winchester Blvd., #301
San Jose, CA 95128
E-mail: dptom@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
Michael Jimenez
CyberSource Corporation
550 S. Winchester Blvd., #301
San Jose, CA 95128
E-mail: mjimenez@cybersource.com
Phone: 408-556-9100
10. Acknowledgements
Several persons (in alphabetic order) have contributed, and continue to
contribute significantly through their participation in an industry-
based EMAP (Electronic products Messaging and Protocol) committee.
Through their participation and continued support SCMP will continue to
develop into a functioning, on-line Internet commerce messaging
protocol. Their contributions are greatly appreciated.
Ron Bose (LitleNet), Len Cantor (IBM), Hubert Chen (CyberSource), Tony
Curwen (Ingram Micro), Mike Myers (Verisign), John Pettitt (Beyond.com),
Jesse Rendleman (CyberSource), Don Sloan (Tech Data), Frank Tyksen
(Portland Software)
11. References
[822] D. Crocker, "Standard for the format of ARPA Internet
text messages." RFC 0822, IETF, Aug. 1982.
[SMIME] Blake Ramsdell, "S/MIME Message Specification",
work in progress, IETF Internet Draft, draft-ietf-
smime-msg-05.txt, Aug. 1998.
[CMS] R. Housley, "Cryptograpic Message Syntax", work
in progress, IETF Internet Draft,draft-ietf-
smime-cms-06.txt, June. 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: Cryptographic Message Syntax
Version 1-5", IETF, March 1998.