rfc4387
Network Working Group P. Gutmann, Ed.
Request for Comments: 4387 University of Auckland
Category: Standards Track February 2006
Internet X.509 Public Key Infrastructure
Operational Protocols: Certificate Store Access via 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
Copyright (C) The Internet Society (2006).
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.
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Table of Contents
1. Introduction ....................................................2
2. HTTP Certificate Store Interface ................................3
2.1. Converting Binary Blobs into Search Keys ...................4
2.2. Attribute Types: X.509 .....................................5
2.3. Attribute Types: PGP .......................................6
2.4. Attribute Types: XML .......................................6
2.5. Implementation Notes and Rationale .........................6
2.5.1. Identification ......................................7
2.5.2. Checking of Input Values ............................9
2.5.3. URI Notes ..........................................10
2.5.4. Responses ..........................................11
2.5.5. Performance Issues .................................12
2.5.6. Miscellaneous ......................................13
2.6. Examples ..................................................14
3. Locating HTTP Certificate Stores ...............................15
3.1. Information in the Certificate ............................15
3.2. Use of DNS SRV ............................................16
3.2.1. Example ............................................16
3.3. Use of a "well-known" Location ............................16
3.3.1. Examples ...........................................17
3.4. Manual Configuration of the Client Software ...............18
3.5. Implementation Notes and Rationale ........................18
3.5.1. DNS SRV ............................................18
3.5.2. "well-known" Locations .............................19
3.5.3. Information in the Certificate .....................19
3.5.4. Miscellaneous ......................................20
4. Security Considerations ........................................20
5. IANA Considerations ............................................22
6. Acknowledgements ...............................................22
7. References .....................................................22
7.1. Normative References ......................................22
7.2. Informative References ....................................23
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 as an interface mechanism to obtain certificates or
public keys, and certificate revocation lists (CRLs), from PKI
repositories. Throughout the remainder of this document the generic
term HTTP will be used to cover either option.
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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
allow HTTP to be used as a general-purpose, transparent interface to
any type of certificate or key store including flat files, standard
databases such as Berkeley DB and relational databases, and
traditional X.500/LDAP directories. Typical applications would
include use with web-enabled relational databases (which most
databases are) or simple {key,value} 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].
2. HTTP Certificate Store Interface
The GET method is used in combination with an HTTP query URI
[RFC2616] to retrieve certificates from the underlying certificate
store:
http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
The parameters for the query portion of the URI are a certificate or
key identifier consisting of an attribute type and a value that
specifies one or more certificates or public keys to be returned from
the query:
query = attribute '=' value
Certificates and public keys are retrieved from one URI (the
certificate URI) and CRLs from another URI (the revocation URI).
These may or may not correspond to the same certificate store and/or
server (the exact interpretation is a local configuration issue).
The query value MUST be encoded using the form-urlencoded media type
[RFC2854]. Further details of URI construction, size limits, and
other factors are given in [RFC2616].
Responses to unsuccessful queries (for example, to indicate a non-
match or an error condition) are handled in the standard manner as
per [RFC2616]. 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.
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If more than one certificate matches a query, it MUST be returned as
a multipart/mixed response. The returned data MUST be returned
verbatim; it MUST NOT use any additional content- or transfer-
encoding at the HTTP level (for example, it can't be compressed or
encoded as base64 or quoted-printable text). Implementations SHOULD
NOT use chunked encoding in responses.
The query component of the URI MAY optionally contain additional
attribute/value pairs separated by the standard ampersand delimiter
'&' that specify further actions to be taken by the certificate
store. Certificate stores SHOULD ignore any additional unrecognised
attribute/value pairs present in the URI.
Other information, such as naming conventions and MIME types, is
specified in [RFC2585] (with additional MIME types for non-X.509
content in [RFC3156] and [RFC3275]).
2.1. Converting Binary Blobs into Search Keys
Some fields (indicated by the "Process" column in the tables below)
are of arbitrary length and/or contain non-textual data. Both of
these properties make them unsuited for direct use in HTTP queries.
In order to make them usable, fields for which the processing option
is "Hash" are first hashed down to a fixed-length 160-bit value.
Fields for which the processing option is "Hash" or "Base64" are
base64-encoded to transform the binary data into textual forms:
Processing Processing step
option
"Hash" Hash the key value using SHA-1 [FIPS180] to produce a
160-bit value, then continue with the base64 encoding
step that follows.
"Hash" Encode the binary value using base64 encoding to produce
"Base64" a 27-byte text-only value. Base64 encoding of the 20
byte value will produce 28 bytes, and the last byte will
always be a '=' padding character. The 27-byte value is
created by dropping the trailing '=' character.
For cases where the binary value is smaller or larger than the 20-
byte SHA-1 output (for example, with 64-bit/8 byte PGP key IDs), the
final value is created by removing any trailing '=' padding from the
encoding of the binary value (this is a generalisation of the above
case).
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Implementations MUST verify that the base64-encoded values submitted
in requests contain only characters in the ranges 'a'-'z', 'A'-'Z',
'0'-'9', '+', and '/'. Queries containing any other character MUST
be rejected. (See the implementation notes in Section 2.5 and the
security considerations in Section 4 for more details on this
requirement.)
2.2. Attribute Types: X.509
Permitted attribute types and associated values for use with X.509
certificates and CRLs are described below. Arbitrary-length binary
values (as 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 (after decoding of any form-urlencoded
[RFC2854] portions if this is necessary) and are therefore case
sensitive.
Attribute Process Value
--------- ------- -----
certHash Hash Search key derived from the SHA-1 hash of the
certificate (sometimes called the certificate
fingerprint or thumbprint).
uri None Subject URI associated with the certificate,
without the (optional) scheme specifier. The URI
type depends on the certificate. For S/MIME
certificates, it would be an email address; for
SSL/TLS certificates, it would be the server's DNS
name (this is usually also specified as the
CommonName); for IPsec certificates, it would be
the DNS name/IP address; and so on.
iHash Hash Search key derived from the DER-encoded issuer DN
as it appears in the certificate, CRL, or other
object.
iAndSHash Hash Search key derived from the certificate's
DER-encoded issuerAndSerialNumber [RFC3852].
name None Subject CommonName contained in the certificate.
sHash Hash Search key derived from the DER-encoded subject
DN as it appears in the certificate or other
object.
sKIDHash Hash Search key derived from the certificate's
subjectKeyIdentifier (specifically the contents
octets of the KeyIdentifier OCTET STRING).
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Certificate URIs MUST support retrieval by all the above attribute
types.
CRL URIs MUST support retrieval by the iHash and sKIDHash attribute
types, which identify the issuer of the CRL. In addition, CRL URIs
MAY support retrieval by certHash and iAndSHash attribute types, for
cases where this is required by the use of the
issuingDistributionPoint extension. A CRL query MUST return the
matching CRL with the greatest thisUpdate value (in other words, the
most recent CRL).
2.3. Attribute Types: PGP
Permitted attribute types and associated values for use with PGP
public keys and key revocation information are described below.
Binary values (as indicated in the table below) are converted into a
search key by the process described in Section 2.1.
Attribute Process Value
--------- ------- -----
email None email address associated with the key.
fingerprint Base64 160-bit PGP key fingerprint [RFC2440].
keyID Base64 64-bit PGP key ID [RFC2440].
name None User name associated with the key.
Key URIs MUST support retrieval by all of the above attribute types.
Revocation URIs MUST support retrieval by the fingerprint and keyID
attribute types, which identify the issuer of the key revocation.
2.4. Attribute Types: XML
Permitted attribute types and associated values for use with XML are
as specified in sections 2.2 and 2.3. Since XML allows arbitrary
attributes to be associated with the <RetrievalMethod> child element
of <KeyInfo> [RFC3275], there are no additional special requirements
for use with XML.
2.5. Implementation Notes and Rationale
This informative section documents the rationale behind the design in
Section 2 and provides guidance for implementors.
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2.5.1. Identification
The identifiers are taken from PKCS #15 [PKCS15], a standard that
covers (among other things) a transparent interface to a
certificate/public key store. These identifiers have been field
proven, as they have 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].
The URI identifier type specifies the identifier associated with the
certificate's intended usage with a given Internet security protocol.
For example, an SSL/TLS server certificate would contain the server's
DNS name (this is traditionally also specified as the CommonName or
CN) an S/MIME certificate would contain the subject's email address;
an IPsec certificate would contain a DNS name or IP address; and a
SIP certificate would contain a SIP URI. A modicum of common sense
is assumed when deciding upon an appropriate URI field value.
For historical reasons going back to its primary use as a means of
looking up users' S/MIME email certificates, some clients may specify
the URI attribute name as "email" rather than "uri". Although not
required by this specification, servers may choose to allow the use
of "email" as an alias for "uri".
In addition, it is common practice to use the Internet identifier
associated with the certificate's intended field of application as
the CN for the certificate when this is the most sensible name for
the certificate subject. For example, an SSL/TLS server certificate
will contain the server's DNS name in the CN field. In web-enabled
devices, this may indeed be the only name that exists for the device.
It is therefore quite possible that the URI will duplicate the CN,
and that it may be the only identifier present (that is, there's no
full DN but only a single CN field).
By long-standing convention, URIs in certificates are given without a
scheme specifier. For example, an SSL/TLS server certificate would
contain www.example.com rather than https://www.example.com, and an
S/MIME certificate would contain user@example.com rather than
mailto:user@example.com. This convention is extended to other URI
types as well, so that a certificate containing the (effective) URIs
im:user@example.com and xmpp:user@example.com would be queried using
the single URI user@example.com. The certificate store would then
return all certificates containing this URI, leaving it to the client
to determine which one is most appropriate for its use. This
approach is taken both because for the most common URI types there's
no schema specifier (see the paragraphs above) and no easy way to
determine what the intended use is (an SSL/TLS server certificate is
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simply one presented by an SSL/TLS server), and because the relying
party/client is in a better position to judge the certificate's most
appropriate use than the certificate store server.
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 name and email address are an exact copy of what is present in
the certificate, without any canonicalisation or rewriting (other
than the transport encoding required by HTTP). This follows standard
implementation practice, which transfers an exact copy of these data
items in order to avoid problems due to character set translation,
handling of whitespace, and other issues.
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 that searching for
structured name data isn't a supported feature, since this is a
simple interface to a {key,value} 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.
Although clients will always submit a fixed 160-bit value, servers
are free to use as many bits of this value as they require. For
example, a server may choose to use only the first 40, 64, 80, or 128
bits for efficiency in searching and maintaining indices.
PGP has traditionally encoded IDs using a C-style 0xABCDEF notation
based on the display format used for IDs in PGP 2.0. Unfortunately,
strings in this format are also valid strings in the base64 format,
complicated further by the fact that near-misses such as 0xABCDRF
could be either a mistyped attempt at a hex ID or a valid base64 ID.
For this reason, and to ensure consistency, base64 IDs are used
throughout this specification. The search keys used internally will
be binary values, so whether these are converted from ASCII-hex or
base64 is immaterial in the long run.
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
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rfc822Name, rfc822Mailbox, emailAddress, mail, or email) where
multiple name forms exist.
In some cases, users may require additional, application-specific
attribute types. For example, a healthcare application that uses a
healthcare ID as the primary key for its databases may require the
ability to perform certificate lookups based on this healthcare ID.
The formatting and use of such application-specific identifiers is
beyond the scope of this document. However, they should begin with
'x-' to ensure that they don't conflict with identifiers that may be
defined in future versions of this specification.
2.5.2. Checking of Input Values
The attribute value portion of the identifier should be carefully
checked for invalid characters since allowing raw data presents a
security risk. Consider, for example, a certificate/public key 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. Even a read-only query can
be problematic; for example, setting <search key> to "UNION SELECT
password FROM master.sysxlogins" will list all passwords in an SQL
Server database (in an easily-decrypted format) if the user is
running under the sa (DBA) account. For this reason, only valid
base64 encodings should be allowed. The same checking applies to
queries by name or email address.
Straightforward sanitisation of queries may not be sufficient to
prevent all attacks; for example, a filter that removes the SQL query
string "DELETE" can be bypassed by submitting the string embedded in
another instance of the string. Removing "DELETE" from
"DELDELETEETE" leaves the outer "DELETE" in place. Abusing the
truncation of over-long strings by filters can also be used as a
means of attack, with the attacker ensuring that the truncation
occurs in the middle of an escape sequence, bypassing the filtering.
Although in theory recursive filtering may help here, the use of
parameterised queries (often called placeholders) that aren't
vulnerable to SQL injection should be used to avoid these attacks.
More information on securing database back-ends may be found in
[Birkholz], and more comments on sanitisation and safety concerns may
be found in the security considerations section.
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2.5.3. URI Notes
Pre-constructed URIs that fetch a certificate/public key matching a
fixed search criterion may be useful for items such as web pages or
business cards, or even for technical support/helpdesk staff who want
to mail to users but can't find the certificate themselves. These
URIs may also be used to enforce privacy measures when distributing
certificates by perturbing the search key in a manner known only to
the certificate/public key store, or to the certificate store and
users (in other words, by converting the URI into a capability). For
example, a user with a newly-issued certificate could be instructed
to fetch it with a key of "x-encrCertHash=...", which is decrypted by
the certificate store to fetch the appropriate certificate, ensuring
that only the certificate owner can fetch their certificate
immediately after issue. Similarly, an organisation that doesn't
want to make its certificates available for public query might
require a MAC on search keys (e.g., "x-macCertHash=...") to ensure
that only authorised users can search for certificates (although a
more logical place for access control, if a true web server is being
used to access the store, would obviously be at the HTTP level).
The query types have been specifically chosen to be not just an HTTP
interface to LDAP but a general-purpose retrieval mechanism that
allows arbitrary certificate/public key storage mechanisms (with a
bias towards simple {key,value} stores, which are deployed almost
universally, whether as ISAM, Berkeley DB, or an RDBMS) to be
employed as back-ends. This specification has been deliberately
written to be technology neutral, allowing any {key,value} lookup
mechanism to be used. It doesn't matter if you choose to have
trained chimpanzees look up certificates in books of tables, as long
as your method can provide the correct response with reasonable
efficiency.
Certificate/public key 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 independent URIs, it's possible to implement the
two stores using mechanisms tailored to the data they contain.
PGP combines key and revocation information into a single data object
so that it's possible to return both public keys and revocation
information from the same URI. If distinct key and revocation
servers are available, these can provide a slight performance gain
since fetching revocation information doesn't require fetching the
key that it applies to. If no separate servers are available, a
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single server can be used to satisfy both types of queries with a
slight performance loss, since fetching revocation information will
also fetch the public key data associated with the revocation data.
2.5.4. Responses
The disallowance of exotic encoding forms reflects the fact that most
clients (and many servers, particularly for embedded devices) are not
general-purpose web browsers or servers capable of handling an
arbitrary range of encoding forms and types, but simply basic HTTP
engines attached to key management applications. In other words, the
HTTP interface is a rudimentary add-on to a key management
application, rather than key-management being an add-on to a
general-purpose web client or server. Eliminating unnecessary
choices simplifies the implementation task and reduces code size and
complexity, with an accompanying decrease in the probability of
security issues arising from the added complexity.
The use of an "Accept-encoding: identity" header would achieve the
same effect as disallowing any additional encodings and may indeed be
useful since section 14.3 of [RFC2616] indicates that the absence of
this header may be taken to mean that any encoding is permitted.
However, this unnecessarily bloats the HTTP header in a potentially
performance-affecting manner (see Section 2.5.5), whereas
establishing a requirement that the response be returned without any
additional decoration avoids the need to specify this in each
request. Implementations should therefore omit the Accept-encoding
header entirely or if it has to be included, include "identity" or
the wildcard "*" as an accepted content-encoding type.
Use of chunked encoding is given as a SHOULD NOT rather than a MUST
NOT because support for it is required by [RFC2616]. Nevertheless,
this form of encoding is strongly discouraged, as the data quantities
being transferred (1-2kB) make it entirely unnecessary, and support
for this encoding form is vulnerable to various implementation bugs,
some of which may affect security. However, implementors should be
aware that many versions of the Apache web server will unnecessarily
use chunked encoding when returning responses. Although it would be
better to make this a MUST NOT, this would render clients that
rejected it incompatible with the world's most widely used web
server. For this reason, support for chunked encoding is strongly
discouraged but is nevertheless permitted. Clients that choose not
to support it should be aware that they may run into problems when
communicating with Apache-based HTTP certificate stores.
Multiple responses are returned as multipart/mixed rather than an
ASN.1 SEQUENCE OF Certificate or PKCS #7/CMS certificate chain
(degenerate signed data containing only certificates) because this is
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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.
2.5.5. Performance Issues
Where high throughput/performance under load is a critical issue, a
main-memory database that acts as a form of content cache may be
interposed between the on-disk database and the HTTP interface
[Garcia-Molina]. A main-memory database provides the same
functionality as an on-disk database and is fully transparent to the
HTTP front-end, but offers buffer management and retrieval facilities
optimised for memory-resident data. Where further scalability is
required, the content-caching system could be implemented as a
cluster of main-memory databases [Ji].
Various network efficiency considerations need to be taken into
account when implementing this certificate/public key distribution
mechanism. For example, a simplistic implementation that performs
two writes (the HTTP header and the certificate, written separately)
followed by a read will interact badly with TCP delayed-ACK and
slow-start. This occurs because 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, there is a 200ms (+assorted RTT)
delay in each message sent.
There are various other considerations that need to be taken into
account to provide maximum efficiency. These are covered in depth
elsewhere [Spero] [Heidemann] [Nielsen]. In addition, modifications
to TCP's behaviour, such as the use of 4K initial windows [RFC3390]
(designed to reduce small HTTP transfer times to a single RTT),
should also ameliorate some of these issues.
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 to fit data within the TCP
MSS. For example, since this protocol doesn't involve a web browser,
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there's no need to include various common browser-related headers
such as ones detailing software versions or acceptable languages.
2.5.6. Miscellaneous
The interface specified in this document is a basic read-only type
that will be used by the majority of clients. The handling of
updates (both insertion and deletion) is a complex issue involving
both technological issues (a variety of fields used for indexing and
information retrieval need to be specified in a technology-neutral
manner, or the certificate store needs to perform its own parsing of
the item being added, moving it from a near-universal key=value
lookup mechanism to a full public-key/certificate processing system)
and political ones (who can perform updates to the certificate store,
and under what conditions?). Because of this complexity, the details
of any potential update mechanism are left as a local configuration
issue, although they may at some point be covered in a future
document if there is sufficient demand.
Concerns have been raised over the use of HTTP as a substrate
[RFC3205]. The mechanism described here, which implements a
straightforward request/response protocol with the same semantics as
traditional HTTP requests, is unaffected by these issues.
Specifically, it does not implement any form of complex RPC
mechanism, does not require HTTP security measures, is not affected
by firewalls (since it uses only a basic HTTP GET rather than
layering a new protocol on top of HTTP), and has well-defined MIME
media types specified in standards documents. As such, the concerns
expressed in [RFC3205] do not apply here. In addition, although a
number of servers still don't fully support some of the more advanced
features of HTTP 1.1 [Krishnamurthy], the minimal subset used here is
well supported by the majority of servers and HTTP implementations.
This access mechanism is similar to the PGP HKP protocol [HKP];
however, the latter is almost entirely undocumented and requires that
implementors reverse-engineer other implementations. Because of this
lack of standardisation, no attempt has been made to ensure
interoperability or compatibility with HKP-based servers, although
PGP developers provided much valuable input for this document. One
benefit that HKP does bring is extensive implementation experience,
which indicates that this is a very workable solution to the problem
of a simple certificate/public key retrieval mechanism. HKP servers
have been implemented using flat files, Berkeley DB, and various
databases, such as Postgres and MySQL.
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2.6. 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
(Note the absence of trailing '=' padding.) This is the search key
to use in the query URI.
To fetch all certificates useful for sending encrypted email to
foo@example.com:
GET /search.cgi?email=foo%40example.com HTTP/1.1
(For simplicity, the additional Host: header required by [RFC2616] is
omitted here and in the following examples.) 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 [RFC2854].
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?sHash=<search key> HTTP/1.1
Alternatively, if chaining is by key identifier:
<Extract the keyIdentifier from the authorityKeyIdentifier>
GET /search.cgi?sKIDHash=<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
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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.
To retrieve a key using XML methods, the <KeyName> (which contains
the string identifier for the key), used with the subject DN hash
above, would be:
<KeyName KeyID="sHash">lkxwxB7JCOXKRSUQ1sgoOhrB3+I</KeyName>.
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 DNS SRV
- 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 a well-known point of information access to
relying parties, CAs SHOULD use 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 }
where:
id-ad OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5)
pkix(7) 48 }
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The corresponding accessLocation is the query URI. The use of this
facility provides a CA with a convenient, standard location to
indicate where further certificates may be found, for example, for
certification 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 DNS SRV
DNS SRV is a facility for specifying the location of the server(s)
for a specific protocol and domain [RFC2782]. For the certificate
store interface, the DNS SRV symbolic name for the certificate store
interface SHALL be "certificates". The name for the CRL store
interface SHALL be "crls". The name for the PGP public key store
SHALL be "pgpkeys". The name for the PGP revocation store SHALL be
"pgprevocations". Handling of additional DNS SRV facilities, such as
the priority and weight fields, is as per [RFC2782].
3.2.1. Example
If a CA with the domain example.com were to make its certificates
available via an HTTP certificate store interface, the server details
could be obtained by a lookup on:
_certificates._tcp.example.com
and
_crls._tcp.example.com
This would return the server(s) and port(s) for the service as
specified in [RFC2782].
3.3. 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
("certificates.", "crls.", "pgpkeys.", or "pgprevocations.") to the
domain name to obtain the net_loc portion of the URI, and by
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
pgpkeys.<domain_name>/search.cgi
pgprevocations.<domain_name>/search.cgi
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Certificate store service providers SHOULD use these URIs in
preference to other alternatives. Note that the use of "search.cgi"
does not imply the use of CGI scripts [RFC3875]. This would be the
exception rather than the rule, since it would lead to a rather
inefficient implementation; it merely provides one possible (and
relatively simple to set up) implementation alternative (see the
rationale for more on this).
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 make 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,
"crls/search.cgi" for CRLs, "pgpkeys/search.cgi" for PGP public keys,
and "pgprevocations/search.cgi" for PGP revocation information to the
net_loc. The URI form of the "well-known" location is therefore:
<net_loc>/certificates/search.cgi
<net_loc>/crls/search.cgi
<net_loc>/pgpkeys/search.cgi
<net_loc>/pgprevocations/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).
3.3.1. Examples
If a CA with the domain example.com were to make its certificates
available via an HTTP certificate store interface, the "well-known"
query URIs for certificates and CRLs would be:
http://certificates.example.com/search.cgi
http://crls.example.com/search.cgi
A home automation controller with the IP address 192.0.2.1 (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:
http://192.0.2.1/certificates/search.cgi
http://192.0.2.1/crls/search.cgi
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A print server with DNS name "printspooler" would make certificates
for web-enabled printers that it communicates with available as:
http://printspooler/certificates/search.cgi
http://printspooler/crls/search.cgi
3.4. Manual Configuration of the Client Software
The accessLocation for the HTTP certificate/public key/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.
3.5. Implementation Notes and Rationale
This informative section documents the rationale behind the design in
Section 3 and provides guidance for implementors.
3.5.1. DNS SRV
The optimal solution for the problem of service location would be DNS
SRV. Unfortunately, the operating system used by the user group most
desperately in need of this type of handholding has no support for
anything beyond the most basic DNS address lookups, making it
impossible to use DNS SRV with anything but very recent Win2K and XP
systems. To make things even more entertaining, several of the
function names and some of the function parameters changed at various
times during the Win2K phase of development, and the behaviour of
portions of the Windows sockets API changed in undocumented ways to
match. This leads to an unfortunate situation in which a Unix
sysadmin can make use of DNS SRV to avoid having to deal with
technical configuration issues, but a Windows'95 user can't. Because
of these problems, an alternative to DNS SRV is provided for
situations where it's not possible to use this.
The SRV or "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
@example.com, the user software would query
_certificates._tcp.example.com or go to certificates.example.com and
request the certificate. 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 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 also possible but may not be
as easy to set up as HTTP redirects (corporate policies tend to be
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RFC 4387 Certificate Store Access via HTTP February 2006
more flexible in regard to web page contents than modifying DNS
configurations would be).
3.5.2. "well-known" Locations
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 that it be handled by the provider's main web
server, while using a distinct server URI allows for it be 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 3.3 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.
3.5.3. Information in the Certificate
Implementations that require the use of nonstandard locations, 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 3.3 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.
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.
A single server can be used to handle both CRLDP and AIA/SIA queries
provided that 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
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circumstances (for example, for links on web pages) to place in
appropriate locations like the issuerAltName, or even for technical
support/helpdesk staff to email to users who can't find the
certificate themselves, as described in Section 2.5. The resulting
certstore URL, when clicked on by the user, will directly access the
certificate when used in conjunction with any certificate-aware
application, such as a browser or mail program.
3.5.4. Miscellaneous
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, SSDP) to be
queried for certificates without requiring further user
configuration. Note that in practice no embedded device would ever
use the address given in the example (the de facto standard address
for web-enabled embedded devices is 192.168.1.x and not 192.0.2.x);
however, IETF policy requires the use of this non-address for
examples.
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 3.3 is not meant to imply that this document overrides the
existing UPnP mechanism, but merely that, if a device implements the
mechanism described here, it should use the naming scheme in Section
3.3 rather than use 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. [RFC2616]
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 caching ("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 back-end used to retrieve certificates. Allowing
attackers to submit arbitrary queries may allow them to manipulate
the certificate store in unexpected ways if the back-end tries to
interpret the query contents. For example, if a certificate store is
implemented using an RDBMS for which the calling application
assembles a complete SQL string to perform the query, and the SQL
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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. Even a read-only query
can be problematic; for example, setting <search key> to "UNION
SELECT password FROM master.sysxlogins" will list all passwords in an
SQL Server database (in an easily decrypted format) if the user is
running under the sa (DBA) account. Straightforward sanitisation of
queries may not be sufficient to prevent all attacks; for example, a
filter that removes the SQL query string "DELETE" can be bypassed by
submitting the string embedded in another instance of the string.
Removing "DELETE" from "DELDELETEETE" leaves the outer "DELETE" in
place. Abusing the truncation of over-long strings by filters can
also be used as a means of attack, in which the attacker ensures that
the truncation occurs in the middle of an escape sequence, bypassing
the filtering. The use of parameterised queries (often called
placeholders) that aren't vulnerable to SQL injection should be used
to avoid these attacks.
In addition, since some query data may be encoded/decoded before
being sent to the back-end, applications should check both the
encoded and decoded form for valid data. A simple means of avoiding
these problems is to use parameterised commands rather than hand-
assembling SQL strings for use in queries (this is also more
efficient for most database interfaces). The use of parameterised
commands means that the query value is never present in any position
where it could be interpreted as a portion of the query command.
Alongside filtering of queries, the back-end 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. Server-specific
security measures may also be employed; for example, the SQL Server
provides the built-in db_datareader account that only allows read
access to tables (but see the note above about what can be done even
with read-only access) and the ability to run the server under a
dedicated low-privilege account (a standard feature of Unix systems).
The mechanism described in this document is not intended to function
as a trusted directory/database. In particular, users should not
assume that just because they fetched a public key or certificate
from an entity claiming to be X, that X has made any statement about
the veracity of the public key or certificate. The use of a signed
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representation of the items stored removes the need to depend on the
certificate store for any security service other than availability.
Although it's possible to implement a trusted directory/database
using HTTPS or some other form of secured/trusted link, this is a
local policy/configuration issue, and in the absence of such
additional security measures users should apply appropriate levels of
verification to any keys or certificates fetched before they take
them into use.
5. IANA Considerations
No action by IANA is needed. The AIA/SIA accessMethod types are
identified by object identifiers (OIDs) from an arc managed by the
PKIX working group. 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.
6. Acknowledgements
Anders Rundgren, Blake Ramsdell, Jeff Jacoby, David Shaw, and members
of the ietf-pkix working group provided useful input and feedback on
this document.
7. References
7.1. Normative References
[FIPS180] Federal Information Processing Standards Publication
(FIPS PUB) 180-1, Secure Hash Standard, 17 April
1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2440] Callas, J., Donnerhacke, L., Finney, H., and R.
Thayer, "OpenPGP Message Format", RFC 2440, November
1998.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public
Key Infrastructure Operational Protocols: FTP and
HTTP", RFC 2585, May 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
for specifying the location of services (DNS SRV)",
RFC 2782, February 2000.
[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media
Type", RFC 2854, June 2000.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T.
Roessler, "MIME Security with OpenPGP", RFC 3156,
August 2001.
[RFC3275] Eastlake 3rd, D., Reagle, J., and D. Solo,
"(Extensible Markup Language) XML-Signature Syntax
and Processing", RFC 3275, March 2002.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo,
"Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC
3280, April 2002.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
7.2. Informative References
[Birkholz] "Special Ops: Host and Network Security for
Microsoft, Unix, and Oracle", Erik Birkholz et al,
Syngress Publishing, November 2002.
[Garcia-Molina] "Main Memory Database Systems", Hector Garcia-Molina
and Kenneth Salem, IEEE Transactions on Knowledge and
Data Engineering, Vol.4, No.6 (December 1992), p.509.
[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.
[HKP] "A PGP Public Key Server", Marc Horowitz, 2000,
http://www.mit.edu/afs/net.mit.edu/project/pks/
thesis/paper/thesis.html. A more complete and up-
to-date overview of HKP may be obtained from the
source code of an open-source OpenPGP implementation
such as GPG.
Gutmann Standards Track [Page 23]
RFC 4387 Certificate Store Access via HTTP February 2006
[Ji] "Affinity-based Management of Main Memory Database
Clusters", Minwen Ji, ACM Transactions on Internet
Technology, Vol.2, No.4 (November 2002), p.307.
[Krishnamurthy] "PRO-COW: Protocol Compliance on the Web - A
Longitudinal Survey", Balachander Krishnamurthy and
Martin Arlitt, Proceedings of the 3rd Usenix
Symposium on Internet Technologies and Systems
(USITS'01), March 2001, p.109.
[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/
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.
[RFC3205] Moore, K., "On the use of HTTP as a Substrate", BCP
56, RFC 3205, February 2002.
[RFC3390] Allman, M., Floyd, S., and C. Partridge, "Increasing
TCP's Initial Window", RFC 3390, October 2002.
[RFC3875] Robinson, D. and K. Coar, "The Common Gateway
Interface (CGI) Version 1.1", RFC 3875, October 2004.
[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.
Author's Address
Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
EMail: pgut001@cs.auckland.ac.nz
Gutmann Standards Track [Page 24]
RFC 4387 Certificate Store Access via HTTP February 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Gutmann Standards Track [Page 25]
ERRATA
