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Internet Draft                                      Editor: Peter Gutmann
draft-ietf-pkix-certstore-http-00.txt               University of Auckland
November 10, 2001
Expires May 2002

                Internet X.509 Public Key Infrastructure
       Operational Protocols: Certificate Store Access via HTTP

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.  Internet-Drafts are working documents of
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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 Hypertext Transfer Protocol
(HTTP) as an interface mechanism to obtain certificates and certificate
revocation lists (CRLs) from PKI repositories (although RFC 2585 covers
fetching certificates via HTTP, this merely mentions that certificates may be
fetched from a static URL, which doesn't provide a general-purpose interface to
a certificate store).  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 Hypertext
Transfer Protocol (HTTP) as an interface mechanism to obtain certificates and
certificate revocation lists (CRLs) from PKI repositories.

Although RFC 2585 [RFC2585] covers fetching certificates via HTTP, this merely
mentions that certificates may be fetched from a static URL, which doesn't
provide any general-purpose interface capabilities to a certificate store.  The
conventions described in this document allows HTTP to be used as a general-
purpose, transparent interface to any type of certificate store ranging from
flat files through to standard databases such as Berkeley DB and relational
databases, as well as traditional X.500/LDAP directories.  Typical applications
would include use with web-enabled relational databases (which most current
databases are) or simple key/data lookup mechanisms such as Berkeley DB and its
various descendants.

Additional mechanisms addressing PKIX operational requirements are specified in
separate documents.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
described in [RFC2119].

This draft is being discussed on the "ietf-pkix" mailing list.  To join the
list, send a message to <ietf-pkix-request@imc.org> with the single word
"subscribe" in the body of the message.  Also, there is a Web site for the
mailing list at <http://www.imc.org/ietf-pkix>.

2. HTTP Certificate Store Interface

The GET method is used in combination with a query URI to retrieve certificates
from the underlying certificate store [RFC2068].  The parameters for the query
URI are a certificate identifier consisting of an attribute type and a value
which specifies one or more certificates to be returned from the query.  The
query URI may be specified in a certificate AuthorityInfoAccess extension or
configured at the client (see section 3).

Permitted attribute types and associated values are:

Attribute       Value
---------       -----
email           Email address contained in the certificate, typically as an
                rfc882Name attribute.

iHash           base64-encoded SHA-1 hash of the certificate's issuerName.

iAndSHash       base64-encoded SHA-1 hash of the certificate's
                issuerAndSerialNumber [RFC2630].

name            CommonName contained in the certificate.

sHash           base64-encoded SHA-1 hash of the certificate's subjectName.

sKID            base64-encoded certificate subjectKeyIdentifier.

  [Another possible identifier is the SHA-1 cert hash ("fingerprint") which
   isn't required directly to find a cert but may be required by some ancillary
   PKI protocols which identify certs in this manner]

The full URI is formed by concatenating the query URI and the attribute and
value.  Certificates are retrieved from one query URI (the certificate URI) and
CRLs from another query URI (the CRL URI).  These may or may not correspond to
the same certificate store (the exact interpretation is a local configuration
issue).  The form of the complete URI is therefore:

  <query URI> '?' <attribute> '=' <value>

Certificate URIs MUST support retrieval by all of the above attribute types.

CRL URIs MUST support retrival by the iHash and sKID attribute types, which
identify the issuer of the CRL.

If more than one certificate matches a query, it MUST be returned as a
multipart response.

  [Or a SEQUENCE OF Certificate?  This has the advantage that it takes a lot
   less code to parse, OTOH it may be harder to produce if what you're using is
   a web-enabled RDBMS, which is what most of them are]

In some instances servers may return HTTP type 3xx redirection requests to
redirect queries to another server.  Clients receiving this response SHOULD use
the returned URI to replace their existing one and resubmit the query to the
new server.

Other information such as naming conventions and MIME types are specified in
[RFC2585].

2.1 Examples

To fetch all certificates useful for sending encrypted email to foo@bar.com:

  GET /search-cgi?email=foo@bar.com HTTP/1.0

(in this case "/search-cgi" is the abs_path portion of the query URI, and the
request is submitted to the server located at the net_loc portion of the query
URI).

To fetch the CA certificate which issued the email certificate:

  <Hash the issuerName>
  GET /search-cgi?iHash=<base64-encoded hash> HTTP/1.0

Alternatively, if chaining is by key identifier:

  <Extract the keyIdentifier from the authorityKeyIdentifier>
  GET /search-cgi?sHash=<base64-encoded hash> HTTP/1.0

To fetch other certificates belonging to the same user as the email
certificate:

  <Hash the subjectName>
  GET /search-cgi?sHash=<base64-encoded hash> HTTP/1.0

To fetch the CRL for the certificate:

  <Hash the issuerName>
  GET /search-cgi?iHash=<base64-encoded hash> HTTP/1.0

2.2 Rationale

The identifiers are taken from PKCS #15 [PKCS15], a standard which covers
(among other things) a transparent interface to a certificate store.  These
identifiers have been field proven through having been in common use for a
number of years, typically via PKCS #11 [PKCS11].  Certificate stores and the
identifiers which are required for typical certificate lookup operations are
analysed in some detail in [Gutmann].

The query types have been specifically chosen to be not just an HTTP interface
to LDAP but as a general-purpose retrieval mechanism which allows arbitrary
certificate storage mechanisms (with a bias towards simple key/data stores,
which are deployed almost universally, whether as ISAM, Berkeley DB, or an
RDBMS) to be employed as back-ends.

Hashes are used for arbitrary-length fields such as ones containing DNs in
place of the full field to keep the length manageable.  In addition the use of
the hashed form emphasizes the fact that searching for structured name data
isn't a supported feature, since this is a simple interface to a key/data
certificate store rather than an HTTP interface to an X.500 directory.  Users
specifically requiring an HTTP interface to X.500 may use technology such as
web2ldap for this purpose.

The attributes are given shortened name forms (for example iAndSHash in place
of issuerAndSerialNumberHash) in order to keep the lengths reasonable, or
common name forms (for example email in place of rfc822Name, rfc822Mailbox,
emailAddress, mail, email, etc etc) where multiple name forms exist.

Certificate and CRL stores are allocated separate URIs because they may be
implemented using different mechanisms.  A certificate store typically contains
large numbers of small items while a CRL store contains a very small number of
potentially large items, by providing independant URIs it's possible to
implement the two stores using mechanisms tailored to the data they contain.

This access mechanism is similar to the PGP HKP protocol, however the latter is
almost entirely undocumented and requires implementors to reverse-engineer
other implementations.  Because of this lack of standardisation, no attempt has
been made to ensure interoperability or compatibility with HKP-based servers.
One benefit which HKP brings is extensive implementation experience, which
indicates that this is a very workable solution to the problem of a simple
key/certificate retrieval mechanism.  HKS servers have been implemented using
flat files, Berkeley DB, and various databases such as Postgres and MySQL.

3. Locating HTTP Certificate Stores

In order to convey to relying parties a well-known point of information access,
CAs SHALL provide the capability to include the AuthorityInfoAccess extension
[RFC2459] in certificates.  Alternatively, the accessLocation for the HTTP
certificate store MAY be configured locally at the client.  The OID value for
the accessMethod is one of:

  id-ad-http-certs     OBJECT IDENTIFIER ::= { id-ad 3 }
  id-ad-http-crls      OBJECT IDENTIFIER ::= { id-ad 4 }

  [NB: These are provisional values]

and the corresponding accessLocation is the query URI.

This provides a CA with a convenient place to indicate where further
certificates may be found, for example for path construction.  Note that it
doesn't mean that this service is limited to CAs only.

4. Security Considerations

HTTP caching proxies are common on the Internet, and some proxies may not check
for the latest version of an object correctly.  [RFC2068] specifies that
responses to query URLs should not be cached, and most proxies and servers
correctly implement the "Cache-Control: no-cache" mechanism which can be used
to override cacheing, however in the rare instance in which 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.

Author Address

Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
pgut001@cs.auckland.ac.nz

References

  Gutmann A Reliable, Scalable General-purpose Certificate Store, P.Gutmann,
        Proceedings of the 16th Annual Computer Security Applications
        Conference, December 2000.

  PKCS11 Cryptographic Token Interface Standard, RSA Laboratories, December
        1999.

  PKCS15 Cryptographic Token Information Syntax Standard, RSA Laboratories,
        June 2000.

  RFC2068 Hypertext Transfer Protocol -- HTTP/1.1, J. Gettys, J. Mogul, H.
        Frystyk, and T. Berners-Lee, January 1997.

  RFC2119 Key Words for Use in RFCs to Indicate Requirement Levels, S.Bradner,
        March 1997.

  RFC2459 Internet X.509 Public Key Infrastructure: Certificate and CRL
        Profile, R. Housley, W. Ford, W. Polk, and D. Solo, January 1999.

  RFC2585, Internet X.509 Public Key Infrastructure: Operational Protocols:
        FTP and HTTP, R. Housley and P. Hoffman, May 1999

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