Network Working Group T. Kause
Internet-Draft Tectia
Updates: 4210 (if approved) M. Peylo
Intended status: Standards Track NSN
Expires: November 8, 2012 May 07, 2012
Internet X.509 Public Key Infrastructure -- HTTP Transport for CMP
draft-ietf-pkix-cmp-transport-protocols-18.txt
Abstract
This document describes how to layer the Certificate Management
Protocol over HTTP. It is the "CMPtrans" document referenced in RFC
4210 and therefore updates the reference given therein.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. HTTP-Based Protocol . . . . . . . . . . . . . . . . . . . . . 6
3.1. HTTP Versions . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Persistent Connections . . . . . . . . . . . . . . . . . . 6
3.3. General Form . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Media Type . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Communication Workflow . . . . . . . . . . . . . . . . . . 7
3.6. HTTP Request-URI . . . . . . . . . . . . . . . . . . . . . 7
3.7. Pushing of Announcements . . . . . . . . . . . . . . . . . 7
3.8. HTTP Considerations . . . . . . . . . . . . . . . . . . . 8
4. Compatibility Issues with Legacy Implementations . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The Certificate Management Protocol (CMP) [RFC4210] requires a well
defined transport mechanism to enable End Entities (EEs),
Registration Authorities (RAs) and Certification Authorities (CAs) to
pass PKIMessage sequences between them. This document defines the
transport mechanism which was removed from the main CMP specification
with the second release and referred to be in a separate document.
The first version of the CMP specification [RFC2510] included a brief
description of a simple transport protocol layer on top of TCP. Its
features was simple transport level error-handling and a mechanism to
poll for outstanding PKI messages. Additionally it was mentioned
that PKI messages could also be conveyed using file-, E-mail- and
HTTP-based transport, but those were not specified in detail.
The current version of the CMP specification [RFC4210] incorporated
its own polling mechanism and thus the need for a transport protocol
providing this functionality vanished. The remaining features CMP
requires from its transport protocols are connection and error
handling.
During the long time it existed as draft, this RFC was undergoing
drastic changes. The "TCP-Based Management Protocol" was enhanced
and a TCP-Messages-over-HTTP transport specification appeared. As
both proved to be needless and cumbersome, implementers preferred to
use plain HTTP transport. This document now reflects that by
exclusively describing HTTP as transport protocol for CMP.
The usage of HTTP for transporting CMP messages exclusively uses POST
method for requests, effectively tunneling CMP over HTTP. While this
is generally considered as bad practice and should not be emulated,
there are good reasons to do so for transporting CMP. HTTP is used
as it is generally easy to implement and able to traverse network
borders utilizing ubiquitous proxies. Most importantly, HTTP is
already commonly used in existing CMP implementations. Other HTTP
request methods such as GET are not used as PKI management operations
can only be triggered using CMP's PKI messages which need to be
transported within a POST request.
With its status codes HTTP provides needed error reporting
capabilities. General problems on the server side as well as those
directly caused by the respective request can be reported to the
client.
As CMP implements a transaction ID, identifying transactions spanning
over more than just a single request/response pair, the statelessness
of HTTP is not blocking its usage as transport protocol for CMP
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messages.
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2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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3. HTTP-Based Protocol
For direct interaction between two entities, where a reliable
transport protocol like TCP is available, HTTP SHOULD be utilized for
conveying CMP messages.
3.1. HTTP Versions
Implementations MUST support HTTP/1.0 [RFC1945], and SHOULD support
HTTP/1.1 [RFC2616].
3.2. Persistent Connections
HTTP persistent connections [RFC2616] allow multiple interactions to
take place on the same HTTP connection. However, neither HTTP nor
this protocol are designed to correlate messages on the same
connection in any meaningful way; persistent connections are only a
performance optimization. In particular, intermediaries can do
things like mix connections from different clients into one
"upstream" connections, terminate persistent connections and forward
requests as non-persistent requests, etc. As such, implementations
MUST NOT infer that requests on the same connection come from the
same client (e.g., for correlating PKI messages with ongoing
transactions); every message is to be evaluated in isolation.
3.3. General Form
A DER-encoded PKIMessage is sent as the entity-body of an HTTP POST
request. If this HTTP request is successful, the server returns the
CMP response in the body of the HTTP response. The HTTP response
status code in this case MUST be 200; other "Successful 2xx" codes
MUST NOT be used for this purpose. HTTP responses to pushed CMP
Announcement messages (i.e., CA Certificate Announcement, Certificate
Announcement, Revocation Announcement, and CRL Announcement) utilize
the status codes 201 and 202 to identify whether the received
information was processed.
While "Redirection 3xx" status codes MAY be supported by
implementations, clients should only be enabled to automatically
follow them after careful consideration of possible security
implications.
All applicable "Client Error 4xx" or "Server Error 5xx" status codes
may be used to inform the client about errors.
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3.4. Media Type
The Internet Media Type "application/pkixcmp" MUST be set in the HTTP
header when conveying a PKIMessage.
3.5. Communication Workflow
In CMP most communication is initiated by the end entities where
every CMP request triggers a CMP response message from the CA or RA.
The CMP Announcement messages described in Section 3.7 are an
exception. Their creation may be triggered by certain events or done
on a regular basis by a CA. The recipient of the Announcement only
replies with an HTTP status code acknowledging the receipt or
indicating an error but not with a CMP response.
If the receipt of an HTTP request is not confirmed by receiving an
HTTP response, it MUST be assumed that the transported CMP message
was not successfully delivered to its destination.
3.6. HTTP Request-URI
The Request-URI is formed as specified in [RFC3986].
A server implementation MUST handle Request-URI paths with or without
a trailing slash as identical.
An example of a Request-Line and a Host header field in an HTTP/1.1
header, sending a CMP request to a server, located in the "/cmp" path
of the host "example.com", would be
POST /cmp HTTP/1.1
Host: example.com
or in the absoluteURI form
POST http://example.com/cmp/ HTTP/1.1
Host: example.com
3.7. Pushing of Announcements
A CMP server may create event-triggered announcements or generate
them on a regular basis. It MAY utilize HTTP transport to convey
them to a suitable recipient. As no request messages are specified
for those announcements they can only be pushed to the recipient.
If an EE wants to poll for a potential CA Key Update Announcement or
the current CRL, a PKI Information Request using a General Message as
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described in E.5 of [RFC4210] can be used.
When pushing Announcement messages, PKIMessage structures are sent as
the entity-body of an HTTP POST request.
Suitable recipients for CMP announcements might e.g. be repositories
storing the announced information such as directory services. Those
listen for incoming messages, utilizing the same HTTP Request-URI
scheme as defined in Section 3.6.
The following PKIMessages are announcements that may be pushed by a
CA. The prefixed numbers reflect ASN.1 numbering of the respective
element.
[15] CA Key Update Announcement
[16] Certificate Announcement
[17] Revocation Announcement
[18] CRL Announcement
CMP Announcement messages do not require any CMP response. However,
the recipient MUST acknowledge receipt with a HTTP response having an
appropriate status code and an empty body. When not receiving such
response it MUST be assumed that the delivery was not successful and
if applicable the sending side may retry sending the Announcement
after waiting for an appropriate time span.
If the announced issue was successfully stored in a database or was
already present, the answer MUST be an HTTP response with a "201
Created" status code and empty message body.
In case the announced information was only accepted for further
processing, the status code of the returned HTTP response MAY also be
"202 Accepted". After an appropriate delay, the sender may then try
to send the Announcement again and may repeat this until it receives
a confirmation that it had been successfully processed. The
appropriate duration of the delay and the option to increase it
between consecutive attempts should be carefully considered.
A receiver MUST answer with a suitable 4xx or 5xx HTTP error code
when a problem occurs.
3.8. HTTP Considerations
While implementations MAY make use of all defined features of the
HTTP protocol, they SHOULD keep the protocol utilization as simple as
possible. E.g. there is no benefit in using chunked Transfer-
Encoding as the length of an ASN.1 sequence is know when starting to
send it.
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There is no need for the clients to send an "Expect" request-header
field with the "100-continue" expectation and wait for a "100
Continue" status as described in chapter 8.2.3 of [RFC2616]. The CMP
payload sent by a client is relatively small, so having extra
messages exchanged is more inefficient as the server will anyway only
seldom reject a message without evaluating the body.
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4. Compatibility Issues with Legacy Implementations
As this document was subject of multiple changes during the long
period of time it was created in, implementations using a different
approach for HTTP transport may exist. While only those
implementations according to this specification are compliant,
implementers should to be aware that there might be existing ones
which behave differently.
Legacy implementations might also use an unregistered "application/
pkixcmp-poll" MIME type as it was specified in earlier drafts of this
document. Here, the entity-body of an HTTP POST request contains the
DER-encoded PKIMessage prefixed by an additional so-called TCP-
Message field. The "TCP-Based Management Protocol" specifying those
TCP-Messages has been described in draft versions of this document
but was removed.
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5. Security Considerations
The following aspects need to be considered by implementers and
users:
1. There is the risk for denial of service attacks through resource
consumption by opening many connections to an HTTP server.
Therefore idle connections should be terminated after an
appropriate timeout, maybe also depending on the available free
resources. After sending a CMP Error Message, the server should
close the connection even if the CMP transaction is not yet fully
completed.
2. Without being encapsulated in effective security protocols such
as TLS [RFC5246] there is no integrity protection at the HTTP
protocol level. Therefore information from the HTTP protocol
should not be used to change state of the transaction.
3. Client users should be aware that storing the target location of
a HTTP response with the "301 Moved Permanently" status code
could be exploited by a man-in-the-middle attacker to block them
permanently from contacting the correct server.
4. If no measures to authenticate and protect the HTTP responses to
pushed Announcement messages are in place their information
regarding the Announcement's processing state may not be trusted.
In that case the overall design of the PKI system must not depend
on the Announcements being reliably received and processed by
their destination.
5. CMP provides inbuilt integrity protection and authentication.
The information communicated unencrypted in CMP messages does not
contain sensitive information endangering the security of the PKI
when intercepted. However, it might be possible for an
eavesdropper to utilize the available information to gather
confidential technical or business critical information.
Therefore users of the HTTP transport for CMP might want to
consider using HTTP over TLS according to [RFC2818] or virtual
private networks created e.g. by utilizing Internet Protocol
Security according to [RFC4301].
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6. IANA Considerations
The IANA has already registered the MIME media type "application/
pkixcmp" for identifying CMP sequences due to an request made in
connection with [RFC2510].
No further action by the IANA is necessary for this document or any
anticipated updates.
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7. Acknowledgments
Until the fifth draft version of this document, released on November
24th 2000, the sole authors were Amit Kapoor and Ronald Tschlaer from
Certicom. Up to this point the now removed TCP-Based transport was
described in detail. They are not available for this working on this
document anymore at the time it is entering the "Authors Final Review
state AUTH48". As they therefore cannot approve this document as it
would be necessary, their names were moved to this section. Their
contact data as originally stated by them is as follows:
Amit Kapoor
Certicom
25801 Industrial Blvd
Hayward, CA
US
Email: amit@trustpoint.com
Ronald Tschalaer
Certicom
25801 Industrial Blvd
Hayward, CA
US
Email: ronald@trustpoint.com
The authors gratefully acknowledge the contributions of various
members of the IETF PKIX Working Group and the ICSA CA-talk mailing
list (a list solely devoted to discussing CMP interoperability
efforts).
By providing ideas, giving hints and doing invaluable review work,
the following alphabetically listed individuals have significantly
contributed to this document:
Tomas Gustavsson, Primekey
Peter Gutmann, University of Auckland
Wolf-Dietrich Moeller, Nokia Siemens Networks
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure Certificate Management Protocols",
RFC 2510, March 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
8.2. Informative References
[RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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Authors' Addresses
Tomi Kause
Tectia Corporation
Kumpulantie 3
Helsinki 00520
Finland
Email: toka@tectia.com
Martin Peylo
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Email: martin.peylo@nsn.com
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