Network Working Group
Request for Comments: 2585
Category: Standards Track

R. Housley
SPYRUS
P. Hoffman
IMC
May 1999

Internet X.509 Public Key Infrastructure

Operational Protocols: FTP and HTTP

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Abstract

The protocol conventions described in this document satisfy some of the operational requirements of the Internet Public Key Infrastructure (PKI). This document specifies the conventions for using the File Transfer Protocol (FTP) and the Hypertext Transfer Protocol (HTTP) to obtain certificates and certificate revocation lists (CRLs) from PKI repositories. Additional mechanisms addressing PKIX operational requirements are specified in separate documents.

1 Introduction

This specification is part of a multi-part standard for the Internet Public Key Infrastructure (PKI) using X.509 certificates and certificate revocation lists (CRLs). This document specifies the conventions for using the File Transfer Protocol (FTP) and the Hypertext Transfer Protocol (HTTP) to obtain certificates and CRLs from PKI repositories. Additional mechanisms addressing PKI repository access are specified in separate documents.

1.1. Model

The following is a simplified view of the architectural model assumed by the Internet PKI specifications.

      +---+
      | C |                       +------------+
      | e | <-------------------->| End entity |
      | r |       Operational     +------------+
      | t |       transactions          ^
      |   |      and management         |  Management
      | / |       transactions          |  transactions
      |   |                             |                PKI users
      | C |                             v
      | R |       -------------------+--+-----------+-----------------
      | L |                          ^              ^
      |   |                          |              |   PKI management
      |   |                          v              |       entities
      | R |                       +------+          |
      | e | <---------------------| RA   | <---+    |
      | p |  Publish certificate  +------+     |    |
      | o |                                    |    |
      | s |                                    |    |
      | I |                                    v    v
      | t |                                +------------+
      | o | <------------------------------|     CA     |
      | r |   Publish certificate          +------------+
      | y |   Publish CRL                         ^
      |   |                                       |
      +---+                        Management     |
      
                                   transactions   |
                                                  v
                                              +------+
                                              |  CA  |
                                              +------+

The components in this model are:

   End Entity:  user of PKI certificates and/or end user system that is
   
                the subject of a certificate;
   
   CA:          certification authority;
   
   RA:          registration authority, i.e., an optional system to
                which a CA delegates certain management functions;
   
   Repository:  a system or collection of distributed systems that store
                certificates and CRLs and serves as a means of
                distributing these certificates and CRLs to end
                entities.

1.2. Certificate and CRL Repository

Some CAs mandate the use of on-line validation services, while others distribute CRLs to allow certificate users to perform certificate validation themselves. In general, CAs make CRLs available to certificate users by publishing them in the Directory. The Directory is also the normal distribution mechanism for certificates. However, Directory Services are not available in many parts of the Internet today. The File Transfer Protocol (FTP) defined in RFC 959 and the Hypertext Transfer Protocol (HTTP) defined in RFC 2068 offer alternate methods for certificate and CRL distribution.

End entities and CAs may retrieve certificates and CRLs from the repository using FTP or HTTP. End entities may publish their own certificate in the repository using FTP or HTTP, and RAs and CAs may publish certificates and CRLs in the repository using FTP or HTTP.

2 FTP Conventions

Within certificate extensions and CRL extensions, the URI form of GeneralName is used to specify the location where issuer certificates and CRLs may be obtained. For instance, a URI identifying the subject of a certificate may be carried in subjectAltName certificate extension. An IA5String describes the use of anonymous FTP to fetch certificate or CRL information. For example:

      ftp://ftp.netcom.com/sp/spyrus/housley.cer
      ftp://ftp.your.org/pki/id48.cer
      ftp://ftp.your.org/pki/id48.no42.crl

Internet users may publish the URI reference to a file that contains their certificate on their business card. This practice is useful when there is no Directory entry for that user. FTP is widely deployed, and anonymous FTP are accommodated by many firewalls. Thus, FTP is an attractive alternative to Directory access protocols for certificate and CRL distribution. While this service satisfies the requirement to retrieve information related to a certificate which is already identified by a URI, it is not intended to satisfy the more general problem of finding a certificate for a user about whom some other information, such as their electronic mail address or corporate affiliation, is known.

For convenience, the names of files that contain certificates should have a suffix of ".cer". Each ".cer" file contains exactly one certificate, encoded in DER format. Likewise, the names of files that contain CRLs should have a suffix of ".crl". Each ".crl" file contains exactly one CRL, encoded in DER format.

3 HTTP Conventions

Within certificate extensions and CRL extensions, the URI form of GeneralName is used to specify the location where issuer certificates and CRLs may be obtained. For instance, a URI identifying the subject of a certificate may be carried in subjectAltName certificate extension. An IA5String describes the use of HTTP to fetch certificate or CRL information. For example:

      http://www.netcom.com/sp/spyrus/housley.cer
      http://www.your.org/pki/id48.cer
      http://www.your.org/pki/id48.no42.crl

Internet users may publish the URI reference to a file that contains their certificate on their business card. This practice is useful when there is no Directory entry for that user. HTTP is widely deployed, and HTTP is accommodated by many firewalls. Thus, HTTP is an attractive alternative to Directory access protocols for certificate and CRL distribution. While this service satisfies the requirement to retrieve information related to a certificate which is already identified by a URI, it is not intended to satisfy the more general problem of finding a certificate for a user about whom some other information, such as their electronic mail address or corporate affiliation, is known.

4 MIME registrations

Two MIME types are defined to support the transfer of certificates and CRLs. They are:

      application/pkix-cert
      application/pkix-crl

4.1. application/pkix-cert

   To: ietf-types@iana.org
   Subject: Registration of MIME media type application/pkix-cert

MIME media type name: application

MIME subtype name: pkix-cert

   Required parameters: None

Optional parameters: version (default value is "1")

Encoding considerations: will be none for 8-bit transports and most likely Base64 for SMTP or other 7-bit transports

Security considerations: Carries a cryptographic certificate

   Interoperability considerations: None
   
   Published specification: draft-ietf-pkix-ipki-part1

Applications which use this media type: Any MIME-complaint transport

Additional information:

     Magic number(s): None
     File extension(s): .CER
     Macintosh File Type Code(s): none

Person & email address to contact for further information: Russ Housley <housley@spyrus.com>

   Intended usage: COMMON
   
   Author/Change controller:
   Russ Housley <housley@spyrus.com>

4.2. application/pkix-crl

   To: ietf-types@iana.org
   Subject: Registration of MIME media type application/pkix-crl

MIME media type name: application

MIME subtype name: pkix-crl

   Required parameters: None

Optional parameters: version (default value is "1")

Encoding considerations: will be none for 8-bit transports and most likely Base64 for SMTP or other 7-bit transports

Security considerations: Carries a cryptographic certificate revocation list

   Interoperability considerations: None
   
   Published specification: draft-ietf-pkix-ipki-part1

Applications which use this media type: Any MIME-complaint transport

Additional information:

     Magic number(s): None
     File extension(s): .CRL
     Macintosh File Type Code(s): none

Person & email address to contact for further information: Russ Housley <housley@spyrus.com>

   Intended usage: COMMON
   
   Author/Change controller:
   Russ Housley <housley@spyrus.com>

References

   [RFC 959]   Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)",
               STD 5, RFC 959, October 1985.
   
   [RFC 1738]  Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
               Resource Locators (URL)", RFC 1738, December 1994.
   
   [RFC 2068]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and
               T. Berners-Lee; "Hypertext Transfer Protocol -- HTTP/1.1",
               RFC 2068, January 1997.

Security Considerations

Since certificates and CRLs are digitally signed, no additional integrity service is necessary. Neither certificates nor CRLs need be kept secret, and anonymous access to certificates and CRLs is generally acceptable. Thus, no privacy service is necessary.

HTTP caching proxies are common on the Internet, and some proxies do not check for the latest version of an object correctly. If an HTTP request for a certificate or CRL goes through a misconfigured or otherwise broken proxy, the proxy may return an out-of-date response.

Operators of FTP sites and World Wide Web servers should authenticate end entities who publish certificates as well as CAs and RAs who publish certificates and CRLs. However, authentication is not necessary to retrieve certificates and CRLs.

Authors' Addresses

Russell Housley
SPYRUS
381 Elden Street, Suite 1120
Herndon, VA 20170 USA

EMail:

          housley@spyrus.com

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA 95060 USA

EMail:

          phoffman@imc.org

Full Copyright Statement

This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.