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Internet Draft                                      Editor: Peter Gutmann
draft-ietf-pkix-certstore-http-01.txt               University of Auckland
December 4, 2001
Expires June 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
the Internet Engineering Task Force (IETF), its areas, and its working groups.
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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 described below.
Arbitrary-length binary values are converted into a search key by the process
described in section 2.1.

Attribute       Value
---------       -----
certHash        Search key derived from the SHA-1 hash of the certificate
                (sometimes called the certificate fingerprint).

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

iHash           Search key derived from the certificate's issuer DN as it
                appears in the certificate.

iAndSHash       Search key derived from the certificate's issuerAndSerialNumber
                [RFC2630].

name            CommonName contained in the certificate.

sHash           Search key derived from the certificate's subject DN as it
                appears in the certificate.

sKID            Search key derived from the certificate's subjectKeyIdentifier.

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>

The query value MUST be encoded using the form-urlencoded media type [RFC1866].

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 Converting Binary Blobs into Search Keys

Some of the fields which are used in queries are of arbitrary length and
contain binary data.  Both of these properties make them unsuited for direct
used in HTTP queries.  In order to make them usable, they are first hashed down
to a fixed-length 128-bit value and then base64-encoded:

  Step 1: Hash the key data using SHA-1 to produce a 160-bit value

  Step 2: Encode the first 128 bits of the hash value using base64-encoding to
          produce a 22-byte text-only value

The one exception to this process is the subjectKeyIdentifier, which is already
a hashed value.  In this case it isn't hashed again, but only base64-encoded as
per step 2.

Certificate stores should verify that the base64-encoded values submitted in
requests contain only characters in the range 'a'-'z', 'A'-'Z', '0'-'9', '/',
or '.'.  Queries containing any other character MUST be rejected (see the
rationale in section 2.2 and security considerations in section 4 for more
details on this requirement).

2.2 Examples

To convert the subject DN C=NZ, O=... CN=John Smith into a search key:

  Hash the DN, in the DER-encoded form it appears in the certificate, to
  obtain:

    96 4C 70 C4 1E C9 08 E5 CA 45 25 10 D6 C8 28 3A 1A C1 DF E2

  base-64 encode the first 128 bits:

    96 4C 70 C4 1E C9 08 E5 CA 45 25 10 D6 C8 28 3A

  to obtain:

    lkxwxB7JCOXKRSUQ1sgoOh

This is the search key to use in the query URI.

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

  GET /search-cgi?email=foo%40bar.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.  Note the encoding of the '@' symbol as per [RFC1866].

To fetch the CA certificate which issued the email certificate:

  <Convert the issuer DN to a search key>
  GET /search-cgi?iHash=<search key> HTTP/1.0

Alternatively, if chaining is by key identifier:

  <Extract the keyIdentifier from the authorityKeyIdentifier>
  GET /search-cgi?sKID=<search key> HTTP/1.0

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

  <Convert the subject DN to a search key>
  GET /search-cgi?sHash=<search key> HTTP/1.0

To fetch the CRL for the certificate:

  <Convert the issuer DN to a search key>
  GET /search-cgi?iHash=<search key> 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].

Another possible identifier which has been suggested is an IP address or DNS
name, which will be required for web-enabled embedded devices.  This is
necessary to allow for example a home automation controller to be queried for
certificates for the devices which it controls.  Since this value is regarded
as the CN for the device, common practice is to use this value for the CN in
the same way that web server certificates set the CN to the server's DNS name,
so this option is already covered in a widely-accepted manner.

The binary search key sizes are limited to 128 bits to save space in search
indexes, and because there's no advantage to be gained from using the full 160
bits.  The base64-encoded form of the identifier must be carefully checked for
invalid characters, since allowing raw data through presents a security risk.
Consider for example a certificate store implemented using an RDBMS in which
the SQL query is built up as "SELECT certificate FROM certificates WHERE iHash
= " + <search key>.  If <search key> is set to "ABCD;DELETE FROM certificates"
the results of the query will be quite different from what was expected by the
certificate store administrators.  For this reason only valid base64 encodings
should be allowed.  The same checking applies to queries by name or email
address.

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
HTTP LDAP gateways 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 locate servers from which certificates may be retrieved, relying
parties can employ one or more of the following strategies:

  Information contained in the certificate
  Use of a "well-known" location
  Manual configuration of the client software

The intent of the various options provided here is to make the certificate
store access as transparent as possible, only requiring manual user
configuration as a last resort.

3.1 Information in the Certificate

In order to convey to relying parties a well-known point of information access,
CAs SHALL provide the capability to include the AuthorityInfoAccess (AIA)
extension [RFC2459] in certificates.  The OID value for the accessMethod is one
of:

  id-ad-http-certs     OBJECT IDENTIFIER ::= { id-ad 6 }
  id-ad-http-crls      OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 3029 5 1 }

and the corresponding accessLocation is the query URI.

  [It may be preferable to provide a third accessMethod for attribute
   certificates.  The id-ad-http-certs implicitly included these (that is, it
   didn't try to exclude them and the access mechanism is identical to the one
   for certificates), however it has been pointed out that it would be useful
   to separate identity from attribute certificates]

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.

3.2 Use of a "well-known" Location

If no other location information is available, the certificate store interface
may be located at a "well-known" location constructed from the service
provider's domain name.  In the usual case the URI is constructed by prepending
the type of information to be retrieved, either "certificates." or "crls.", to
the domain name to obtain the net_loc portion of the URI and appending a fixed
abs_path portion "search.cgi".  The URI form of the "well-known" location is
therefore:

  certificates.<domain_name>/search.cgi
  crls.<domain_name>/search.cgi

Service providers SHOULD use these URIs in preference to other alternatives.

For example if a CA with the domain kiwisign.com were to make its certificates
available via an HTTP certificate store interface, the "well-known" query URIs
for certificates and CRLs would be:

  certificates.kiwisign.com/search.cgi
  crls.kiwisign.com/search.cgi

A second case occurs when the service is being provided by web-enabled embedded
devices such as Universal Plug and Play devices [UPNP].  In this case the
device has a single, fixed net_loc (either an IP address or a DNS name) and
makes services available via an HTTP interface.  In this case the URI is
constructed by appending a fixed abs_path portion "certificates/search.cgi" for
certificates and "crls/search.cgi" for CRLs to the net_loc.  The URI form of
the "well-known" location is therefore:

  <net_loc>/certificates/search.cgi
  <net_loc>/crls/search.cgi

Web-enabled devices SHOULD use these URIs in preference to other alternatives.

For example a home automation controller with IP address 192.168.1.1 (a control
point in UPNP terminology) would make certificates for devices such as HVAC
controllers, lighting and appliance controllers, and fire and physical
intrusion detection devices available as:

  192.168.1.1/certificates/search.cgi
  192.168.1.1/crls/search.cgi

A print server with DNS name "printspooler" would make certificates for web-
enabled printers which it communicates with available as:

  printspooler/certificates/search.cgi
  printspooler/crls/search.cgi

3.3 Manual Configuration of the Client Software

The accessLocation for the HTTP certificate/CRL store MAY be configured locally
at the client.  This can be used if no other information is available, or if it
is necessary to override other information.

3.4 Rationale

An AIA extension is used to indicate the location for the CRL store interface
rather than the CRLDistributionPoint (CRLDP) extension since the two perform
entirely different functions.  A CRLDP contains "a pointer to the current CRL",
a fixed location containing a CRL for the current certificate, while the AIA
extension indicates "how to access CA information and services for the issuer
of the certificate in which the extension appears", in this case the CRL store
interface which provides CRLs for any certificates issued by the CA.  In
addition CRLDP associates other attribute information with a query which is
incompatible with the simple query mechanisms presented in this document.

The well-known location URI is designed to make hosting options as flexible as
possible.  Locating the service at www.<domain name> would generally require it
to be handled by the provider's main web server, while using a distinct server
URI allows it to handled as desired by the provider.  Although there will no
doubt be servers which implement the interface using Apache and Perl scripts, a
more logical implementation would consist of a simple network interface to a
key-and-value lookup mechanism such as Berkeley DB.  The URI form presented in
section 3.2 allows for maximum flexibility, since it will work with both web
servers/CGI scripts and non-web-server-based network front-ends for certificate
stores.

Web-enabled (or more strictly HTTP-enabled) devices are intended to be plug-
and-play, with minimal (or no) user configuration necessary.  The "well-known"
URI allows any known device (for example one discovered via UPNP's Simple
Service Discovery Protocol) to be queried for certificates without requiring
further user configuration.

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.

Care should be taken to ensure that only valid queries are fed through to the
backend used to retrieve certificates.  Allowing an attacker to submit
arbitrary queries may allow them to manipulate the certificate store in
unexpected ways if the backend tries to interpret the query contents.  For
example if a certificate store is implemented using an RDBMS in which the SQL
query is built up as "SELECT certificate FROM certificates WHERE iHash = " +
<search key> and <search key> is set to "X;DELETE FROM certificates" the
results of the query will be quite different from what was expected by the
certificate store administrator.  The same applies to queries by name and email
address.

Alongside filtering of queries, the backend should be configured to disable any
form of update access via the web interface.  For Berkeley DB this restriction
can be imposed by opening the certificate store in read-only mode from the web
interface.  For relational databases, it can be imposed through the SQL
GRANT/REVOKE mechanism, for example "REVOKE ALL ON certificates FROM webuser;
GRANT SELECT ON certificates TO webuser" will allow read-only access of the
appropriate kind for the web interface.

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.

  RFC1866 Hypertext Markup Language - 2.0, T. Berners-Lee and D. Connolly,
        November 1995.

  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

  UPNP Universal Plug and Play Device Architecture, Version 1.0, UPnP Forum, 8
        June 2000.

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