draft-ietf-pkix-certstore-http-03.txt

PKIX的RFC英文文档

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.
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Drafts.

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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/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 <[e-mail]> 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 [e-mail]:

  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