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Internet Draft                                       Editor: Peter Gutmann
draft-ietf-pkix-certstore-http-03.txt                University of Auckland
December 2002
Expires June 2003

                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.
Note that other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may
be updated, replaced, or obsoleted by other documents at any time.  It is
inappropriate to use Internet-Drafts as reference material or to cite them
other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

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/HTTPS) as an interface mechanism 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 Hypertext
Transfer Protocol (HTTP) or optionally HTTPS (throughout the remainder of this
document the generic term HTTP will be used to cover either option) 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 that specifies one or more certificates to be returned from the
query.  The query URI may be specified in a certificate SubjectInfoaccess or
AuthorityInfoAccess extension or configured at the client (see section 3).

Permitted attribute types and associated values are described below.
Arbitrary-length binary values (indicated in the table below) are converted
into a search key by the process described in section 2.1.  Note that the
values are checked for an exact match, and are therefore case-sensitive.

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

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

iHash        Y     Search key derived from the issuer DN as it
                   appears in the certificate, CRL, or other object.

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

name         N     Subject CommonName contained in the certificate.

sHash        Y     Search key derived from the subject DN as it
                   appears in the certificate or other object.

sKIDHash     Y     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 and/or server (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/mixed response.

Responses to unsuccessful queries (for example to indicate a non-match or an
error conditions) are handled in the standard manner as per [RFC2068].
Clients should in particular be aware that in some instances servers may
return HTTP type 3xx redirection requests to explicitly redirect queries to
another server.  Obviously, implicit DNS-based redirection is also possible.

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

2.1 Convering Binary Blobs into Search Keys

The fields marked as binary data in the table in section 2 are of arbitrary
length and contain non-textual data.  Both of these properties make them
unsuited for direct use in HTTP queries.  In order to make them usable, they
are first hashed down to a fixed-length 160-bit value and then base64-encoded:

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

  Step 2: Encode the hash value using base64-encoding to produce a
          27-byte text-only value

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

2.2 Implementation Notes

Although clients will always submit a fixed 160-bit value, servers are free to
utilise as many bits of this value as they require, for example a server may
choose to use only the first 40 or 64 or 80 or 128 bits for efficiency in
searching and maintaining indices.

The base64-encoded form of the identifier should 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.

Various network efficiency considerations need to be taken into account when
implementing this certificate distribution mechanism.  For example, a
simplistic implementation that performs two writes (the HTTP header and the
certificate written seperately) followed by a read will interact badly with
TCP delayed-ACK and slow-start.  The reason for this is that the TCP MSS is
typically 1460 bytes on a LAN (Ethernet) or 512/536 bytes on a WAN, while HTTP
headers are ~200-300 bytes, far less than the MSS.  When an HTTP message is
first sent, the TCP congestion window begins at one segment, with the TCP
slow-start then doubling its size for each ACK.  Sending the headers
separately will send one short segment and a second MSS-size segment,
whereupon the TCP stack will wait for the responder's ACK before continuing.
The responder gets both segments, then delays its ACK for 200ms in the hopes
of piggybacking it on responder data, which is never sent since it's still
waiting for the rest of the HTTP body from the initiator.  As a result, this
results in a 200ms (+ assorted RTT) delay in each message sent.

There are various other considerations that need to be taken into account in
order to provide maximum efficiency.  These are covered in depth elsewhere
[Spero] [Heidemann] [Nielsen].  A rule of thumb for optimal performance is to
combine the HTTP header and data payload into a single write (any reasonable
HTTP implementation will do this anyway, thanks to the considerable body of
experience that exists for HTTP server performance tuning), and to keep the
HTTP headers to a minimum to try and fit data within a TCP MSS.  Since this
protocol doesn't involve a web browser, there's no need to include the usual
headers covering browser versions and languages and so on; a minimal set of
content-type/encoding and host and session control information will suffice.

2.3 Examples

To convert the subject DN C=NZ, O=... CN=Fred Dagg 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 this to obtain:

    lkxwxB7JCOXKRSUQ1sgoOhrB3+I

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/1.1

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].  Remaining
required headers such as the "Host" header required by HTTP 1.1 have been
omitted for the sake of clarity.

To fetch the CA certificate that issued the email certificate:

  <Convert the issuer DN to a search key>
  GET /search-cgi?iHash=<search key> HTTP/1.1

Alternatively, if chaining is by key identifier:

  <Extract the keyIdentifier from the authorityKeyIdentifier>
  GET /search-cgi?sKID=<search key> HTTP/1.1

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/1.1

To fetch the CRL for the certificate:

  <Convert the issuer DN to a search key>
  GET /search-cgi?iHash=<search key> HTTP/1.1

Note that since the differentiator is the URI base, the above two queries
appear identical (since the URI base isn't shown) but are in fact distinct.

2.4 Rationale

The identifiers are taken from PKCS #15 [PKCS15], a standard that 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 that are required for typical certificate lookup operations are
analysed in some detail in [Gutmann].

Another possible identifier that 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 that 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 query types have been specifically chosen to be not just an HTTP interface
to LDAP but as a general-purpose retrieval mechanism that 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.

Multiple response are returned as multipart/mixed rather than an ASN.1
SEQUENCE OF Certificate or PKCS #7/CMS certificate chain because this is more
straightforward to implement with standard web-enabled tools.  An additional
advantage is that it doesn't restrict this access mechanism to DER-based data,
allowing it to be extended to other certificate types such as XML, PGP, and
SPKI.

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 that 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.  HKP 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 MAY use of the SubjectInfoAccess (SIA) and AuthorityInfoAccess
(AIA) extension [RFC3280] 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 ::= { id-ad 7 }

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 purposes.  Note
that it doesn't mean that the provision of certificate store access services
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.

A second case occurs when the certificate access service is being provided by
web-enabled embedded devices such as Universal Plug and Play devices [UPNP].
These devices have 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

If certificate access as described in this document is implemented by the
device then it SHOULD use these URIs in preference to other alternatives (see
the rationale for more on this requirement).

3.2.1 Examples

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 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 that 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 Implementation Notes

The well-known location option can frequently be automatically derived by user
software from currently-known parameters.  For example if the recipient's
email address is @hotmail.com, the user software would go to
certificates.hotmail.com and request the certificate.  If the recipient worked
for a government department, the certificate would be requested at
certificates.departmentname.gov.  In addition user software may maintain a
list of known certificate sources in the way that known CA lists are
maintained by web browsers.  The specific mention of support for redirection
in section 2 emphasises the fact that many sites will outsource the
certificate-storage task.  At worst all that will be required is the addition
of a single static web page pointing to the real server.  Alternatives such as
DNS CNAME RRs are obviously also possible, but aren't quite as easy to set up
as HTTP redirects and won't work well across domains.

Implementations that require the use of nonstandard locations or ports or
HTTPS rather than HTTP in combination with well-known locations should use an
HTTP redirect at the well-known location to point to the nonstandard location.
For example if the print spooler in section 3.2 used an SSL-protected server
named printspooler-server with an abs_path portion of cert_access, it would
use an HTTP 302 redirect to https://printspooler-server/cert_access.  This
combines the plug-and-play capability of well-known locations with the ability
to use nonstandard locations and ports.

A single server can be used to handle both CRLDP and AIA/SIA queries provided
the CRLDP form uses an HTTP URI.  Since CRLDP points to a single static
location for a CRL, a query can be pre-constructed and stored in the CRLDP
extension.  Software that uses the CRLDP will retrieve the single CRL that
applies to the certificate from the server, and software that uses the
AIA/SIA can retrieve any CRL from the server.  Similar pre-constructed URIs
may also be useful in other circumstances, for example for links on web pages,
to place in appropriate locations like the issuerAltName, or even for tech
support staff to email to users who can't find the certificate themselves.

3.5 Rationale

The SIA and AIA extensions are 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 SIA/AIA extension indicates "how to access CA information and
services for the subject/issuer of the certificate in which the extension
appears", in this case the CRL store interface that provides CRLs for any
certificates issued by the CA.  In addition CRLDP associates other attribute
information with a query that 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 that 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.

Protocols such as UPnP have their own means of disseminating device and
protocol information.  For example, UPnP uses SOAP, which provides a
GetPublicKeys action for pulling device keys and a PresentKeys action for
pushing control point keys.  The text in section 3.2 is not meant to imply
that this document overrides the existing UPnP mechanism, but merely that if a
device implements the mechanism describe here, it should use the naming scheme
in section 3.2 rather than using arbitrary names.

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 that can be used
to override cacheing ("Pragma: no-cache" for HTTP 1.0), 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.

4. IANA Considerations

The AIA/SIA accessMethod types are identified by object identifiers (OIDs).
OIDs were assigned from an arc contributed to the PKIX Working Group by RSA
Security.  Should additional accessMethods be introduced (for example for
attribute certificates or non-X.509 certificate types), the advocates for such
accessMethods are expected to assign the necessary OIDs from their own arcs.
No action by the IANA is necessary for this document or any anticipated
updates.

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.

  [Heidemann] "Performance Interactions Between P-HTTP and TCP
              Implementations", J.Heidemann, ACM Computer Communications
              Review, April 1997.

  [Nielsen] "Network Performance Effects of HTTP/1.1, CSS1, and PNG",
            H.Nielsen, J.Gettys, A.Baird-Smith, E.Prud'hommeaux, H.Wium Lie,
            and C.Lilley, 24 June 1997,
            http://www.w3.org/Protocols/HTTP/1.0/Performance/Pipeline.html.

  [PKCS11] PKCS #11 Cryptographic Token Interface Standard, RSA Laboratories,
           December 1999.

  [PKCS15] PKCS #15 Cryptographic Token Information Syntax Standard, RSA
           Laboratories, June 2000.

  [RFC1866] "Hypertext Markup Language - 2.0", RFC 1866, T. Berners-Lee and D.
            Connolly, November 1995.

  [RFC2068] "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, J. Gettys, J.
            Mogul, H. Frystyk, and T. Berners-Lee, January 1997.

  [RFC2119] "Key Words for Use in RFCs to Indicate Requirement Levels",
            RFC 2119, S.Bradner, March 1997.

  [RFC3280] "Internet X.509 Public Key Infrastructure: Certificate and CRL
            Profile", RFC 3280, R. Housley, W. Ford, W. Polk, and D. Solo,
            April 2002.

  [RFC2585] "Internet X.509 Public Key Infrastructure: Operational Protocols:
            FTP and HTTP", RFC 2585, R. Housley and P. Hoffman, May 1999

  [Spero] "Analysis of HTTP Performance Problems", S.Spero, July 1994,
          http://www.w3.org/Protocols/HTTP/1.0/HTTPPerformance.html.

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

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