Internet-Draft D. Shaw
Expires September 2003 Jabberwocky Tech
March 2003
The OpenPGP HTTP Keyserver Protocol (HKP)
draft-shaw-openpgp-hkp-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document specifies a series of conventions to implement an
OpenPGP keyserver using the Hypertext Transfer Protocol (HTTP). As
this document is a codification and extension of a protocol that is
already in wide use, strict attention is paid to backward
compatibility with these existing implementations.
Table of Contents
Status of this Memo ...................................... 1
Abstract ................................................. 1
Table of Contents ........................................ 1
1. Introduction ............................................. 2
1.1 Terms ................................................ 2
2. HKP and HTTP ............................................. 2
3. Requesting Data From A Keyserver ......................... 3
3.1 Basic Variables ...................................... 3
3.1.1 The "search" variable .......................... 3
3.1.1.1 Key ID and V4 Fingerprint Searches ..... 3
3.1.1.2 V3 Fingerprint Searches ................ 3
3.1.1.3 Text Searches .......................... 4
3.1.2 The "op" (operation) Variable .................. 4
3.1.2.1 The "get" operation .................... 4
3.1.2.2 The "index" operation .................. 4
3.1.2.3 The "vindex" (verbose index) operation.. 4
3.1.2.4 Other operations ....................... 5
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3.2 Modifier Variables ................................... 5
3.2.1 The "options" variable ......................... 5
3.2.2 The "fingerprint" variable ..................... 5
3.2.3 The "exact" variable ........................... 6
3.2.3 Other variables ................................ 6
3.4 Request Examples ..................................... 6
4. Submitting Keys To A Keyserver ........................... 6
5. Output Formats ........................................... 6
5.1 Machine Readable Output .............................. 7
5.2 Machine Readable Indexes ............................. 7
6. Extended Status Codes .................................... 8
7. Locating a HKP Keyserver ................................. 9
8. Security Considerations .................................. 9
9. IANA Considerations ...................................... 9
10. Normative References ..................................... 10
11. Author's Address ......................................... 10
1. Introduction
For ease of use, public key cryptography requires a key distribution
system. For many years, the most commonly used system has been a key
server - a server that stores public keys and can be searched for a
given key. The HTTP Keyserver Protocol is a OpenPGP keyserver
implemented using HTTP.
1.1. Terms
This document uses the terms "MUST", "SHOULD" and "MAY" as defined in
RFC-2119 [1], along with the negated forms of those terms.
2. HKP and HTTP
As HKP is implemented over HTTP, everything in RFC-1945 [2] applies
to HKP as well, and HKP error codes are the same as the ones used in
HTTP. In order to give as much information to the client about error
conditions, is good practice to return the most specific error code
possible: for example, returning 404 ("Not Found") rather than 400
("Bad Request") when a key is not found.
This document gives suggested HTTP error codes for several common
situations. Note that these are only suggestions, and
implementations may have good reasons (such as not revealing the
reason why a request failed) for using other error codes.
Due the very large deployment of HKP clients based on HTTP version
1.0, HKP keyservers MUST support HTTP 1.0. HKP keyservers MAY
additionally support other HTTP versions.
[ dshaw : I expect this to be controversial, but we've got tons of
deployed code that only works with 1.0. I'd be willing to discuss
removing this MUST or make it a SHOULD and add a "implementation
notes" section pointing out the problem instead. ]
By convention and history, HKP uses HTTP on TCP port 11371. It has
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been suggested by some that for reasons of maximum compatibility
with firewalls and filtering HTTP proxies, it is better to use the
standard HTTP port (TCP port 80). See section 7, Locating a HKP
Keyserver for an automated way for clients to discover the correct
port.
3. Requesting Data From A Keyserver
Keyserver requests are done via a HTTP GET URL that encodes the
request within it. Specifically, the abs_path (see [2], section
3.2) is built up of the base request "/pks/lookup", followed by any
variables. Arguments are passed through the usual means as
specified in [3], section 8.2.2. The variables may be given in any
order. Keyservers MUST ignore any unknown variables.
3.1. Basic Variables
All HKP requests contain the "op" (operation) and "search"
variables. The "op" variable determines what operation the
keyserver will take, and the "search" variable determines that keys
are operated on.
3.1.1. The "search" Variable
The search variable contains arbitrary text encoded as usual for a
HTTP URL. This text may represent the key ID of the key being
sought or some text from a user ID on the key being sought.
If any particular type of searching is not supported, the keyserver
should return an appropriate HTTP error code such as 501 ("Not
Implemented"). The server MUST NOT return an error code (such as
404 ("Not Found")) that could be mistaken by the client for a valid
response.
3.1.1.1. Key ID and V4 Fingerprint Searches
If a key is being sought by its key ID, the key ID string is
prefixed with an "0x" to indicate a hexadecimal number. Key ID
strings may be 8 digits (32-bit key ID), 16 digits (64-bit key ID),
32 digits (version 3 fingerprint), or 40 digits (version 4
fingerprint). The hexadecimal digits are not case sensitive.
A keyserver that allows searching by keyid MUST accept the 160-bit
version 4 fingerprint, 64-bit key IDs, and 32-bit key IDs in the
"search" variable. Note this does not mean that the keyserver will
actually use the full length of the request in the search, as it
may internally create a 32-bit or 64-bit key ID from a version 4
fingerprint (by taking the low-order 32 or 64 bits respectively),
or a 32-bit key ID from a 64-bit key ID (by taking low-order 32
bits). That said, a keyserver SHOULD use at least 64 bits of the
key ID if available.
3.1.1.2. V3 Fingerprint Searches
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The 128-bit version 3 fingerprint is represented by a leading "0x",
followed by 32 case-insensitive hexadecimal digits. Note that v3
fingerprints are treated differently and not grouped with keyid or
v4 fingerprint searches as it is not possible to calculate a 64-bit
or 32-bit keyid from a v3 fingerprint.
3.1.1.3. Text Searches
How a keyserver handles a textual search is implementation defined.
See also the definition of the "exact" variable for a method to
give additional instructions to the server on how the search is to
be executed.
3.1.2. The "op" (operation) Variable
The op variable specifies the operation to be performed on the
keyserver. The op variable is generally used with the "search"
variable to specify the keys that should be operated on.
3.1.2.1. The "get" operation
The "get" operation requests keys from the keyserver. A string that
specifies which key(s) to return is provided in the "search"
variable.
The response to a successful "get" request is a HTTP document
containing a keyring as specified in RFC-2440 [4], section 11.1, and
ASCII armored as specified in section 6.2.
The response may be wrapped in any HTML or other text desired, except
that the actual key data consisting of an initial line break, the
"-----BEGIN PGP PUBLIC KEY BLOCK-----" header, the armored key data
itself, the "-----END PGP PUBLIC KEY BLOCK-----" header, and a final
line break MUST NOT be modified from the form specified in [4].
If no keys match the request, the keyserver should return an
appropriate HTTP error code such as 404 ("Not Found").
3.1.2.2. The "index" Operation
The "index" operation requests a list of keys on the keyserver that
match the text or key ID in the "search" variable. Historically, the
"index" operation returned a human readable HTML document containing
links for each found key, but this is not required.
If the "index" operation is not supported, the keyserver should
return an appropriate HTTP error code such as 501 ("Not
Implemented").
3.1.2.3. The "vindex" (verbose index) Operation
The "vindex" operation is similar to "index" in that it provides a
list of keys on the keyserver that match the text of key ID in the
"search" variable. Historically, a "vindex" response was the same as
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"index" with the addition of showing the signatures on each key, but
this is not required.
If the "vindex" operation is not supported, the keyserver should
return an appropriate HTTP error code such as 501 ("Not
Implemented").
3.1.2.4. Other Operations
Other site-specific or nonstandard operations can be indicated by
prefixing the operation name with the string "x-".
3.2. Modifier Variables
These variables are used to modify basic requests.
3.2.1. The "options" Variable
This variable takes one or more arguments, separated by commas.
These are used to modify the behavior of the keyserver on a
per-request basis.
3.2.1.1. The "mr" (Machine Readable) Option
The machine readable option instructs the server that a program
(rather than a person) is making the request, so the output may be
customized for that use. See Section 5, Output Formats for the
specific details of machine readable output.
3.2.1.2. The "nm" (No Modification) Option
As keyservers may modify submitted keys to suit a particular
policy, this option is used to inform the keyserver that the
submitter would rather have the submission fail completely then
have the submitted key(s) modified. An example of this would be a
keyserver that does not accept user IDs with an email address
outside of the local domain. If such a key was submitted, the
keyserver could trim any noncompliant user IDs before accepting the
key. If this option was set, then the key submission would fail.
3.2.1.3. Other Options
Other site-specific or nonstandard options can be indicated by
prefixing the option name with the string "x-".
3.2.2. The "fingerprint" Variable
This variable takes one argument: "on" or "off". If present and
on, it instructs the server to provide the key fingerprint for each
key in an "index" or "vindex" operation. This variable has no
effect on any other operation. The exact format of the displayed
fingerprint, like the "index" and "vindex" operations themselves,
is implementation defined.
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3.2.3. The "exact" Variable
This variable takes one argument: "on" or "off". If present and
on, it instructs the server to search for an exact match for the
contents of the "search" variable. The exact meaning of "exact
match" is implementation defined.
3.2.3. Other Variables
Other site-specific or nonstandard variables can be indicated by
prefixing the variable name with the string "x-".
3.4. Request Examples
Search for all keys containing the string "dshaw":
http://keys.example.com:11371/pks/lookup?search=dshaw&op=index
Get key 0x99242560 (32-bit key ID):
http://keys.example.com:11371/pks/lookup?op=get&search=0x99242560
4. Submitting Keys To A Keyserver
Keyserver submissions are done via a HTTP POST URL. Specifically,
the abs_path (see [2], section 3.2) is set to "/pks/add", and the key
data is provided via HTTP POST as specified in [2], section 8.3, and
[3], section 8.2.3.
The body of the POST message contains a "keytext" variable which is
set to an ASCII armored keyring as specified in [4], sections 6.2
and 11.1. The ASCII armored keyring should also be urlencoded as
specified in [3], section 8.2.1. Note that more than one key may
be submitted in a single transaction.
There may also be an "options" variable, as specified in section
3.2.1 above.
If a keyserver does not support adding keys via HTTP, then requests
to do so should return an appropriate HTTP error code, such as 403
("Forbidden") if key submission has been disallowed, or 501 ("Not
Implemented") if the server does not support HTTP key submission.
The keyserver MUST NOT return an error code (such as 404 ("Not
Found")) that could be mistaken by the client for a valid response.
5. Output Formats
HKP is intended for both human and programmatic use. The "machine
readable" option is used to tailor the output for a given use. In
general, the "human readable" output is implementation specific.
For interoperability, the "machine readable" output MUST carefully
follow the guidelines given here.
Note that there is an installed base of programs that do in fact
attempt to parse the human readable "index" format used in the pksd
keyserver. Care should be taken with the choice of an "index" format
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if compatibility with these programs is desired.
5.1. Machine Readable Output
When machine readable output is requested, several changes are made
to output:
- Key retrievals (op=get) do not contain any text aside from the
ASCII armored keyring. The document is also sent to the user
using Content-Type: application/pgp-keys as specified in RFC-3156
[6].
- Key indexes (op=index) use the format specified in Section 5.2,
Machine Readable Indexes.
5.2. Machine Readable Indexes
The machine readable index format is a list of records that can be
easily parsed by a machine. The document is 7-bit clean, and as such
is sent with no encoding and Content-Type: text/plain.
The machine readable response begins with an optional information
line:
info:<version>:<count>
<version> = this is the version of this output format.
Currently, this is the number 1.
<count> = the number of keys returned in this response. Note
this is the number of keys, and not the number of
lines returned. That is, it should match the number
of "pub:" lines returned.
If this optional line is not included, or the version information is
not supplied, the version number is assumed to be 1.
The key listings themselves are made up of several lines per key.
The first line specifies the primary key:
pub:<keyid>:<algo>:<keylen>:<creationdate>:<expirationdate>:<flags>
<keyid> = this is either the fingerprint or the key ID of the
key. Either the 16-digit or 8-digit key IDs are
acceptable, but obviously the fingerprint is best. A
keyserver should use the most specific of the key IDs
that it has available. Since it is not possible to
calculate the key ID from a V3 key fingerprint, for V3
keys this should be either the 16-digit or 8-digit
key ID only.
<algo> = the algorithm number from [4]. (i.e. 1==RSA, 17==DSA,
etc).
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<keylen> = the key length (i.e. 1024, 2048, 4096, etc.)
<creationdate> = creation date of the key in standard RFC-2440
[4] form (i.e. number of seconds since 1/1/1970
UTC time)
<expirationdate> = expiration date of the key in standard
RFC-2440 [4] form (i.e. number of seconds
since 1/1/1970 UTC time)
<flags> = letter codes to indicate details of the key, if any.
Flags may be in any order. The meaning of "disabled"
is implementation-specific. Note that individual
flags may be unimplemented, so the absence of a given
flag does not necessarily mean the absence of the
detail.
r == revoked
d == disabled
e == expired
Following the "pub" line are one or more "uid" lines to indicate user
IDs on the key:
uid:<escaped uid string>:<creationdate>:<expirationdate>:<flags>
<escaped uid string> = the user ID string, with HTTP %-escaping
for anything that isn't 7-bit safe as
well as for the ":" character. Any other
characters may be escaped, as desired.
creationdate, expirationdate, and flags mean the same here as in the
"pub" line. The information is taken from the self-signature, if
any, and applies to the user ID in question, and not to the key as a
whole.
Note that empty fields are allowed. For example, a key with no
expiration date would have the <expirationdate> field empty. Also,
a keyserver that does not track a particular piece of information
may leave that field empty as well. Colons for empty fields on the
end of each line may be left off, if desired.
6. Extended Status Codes
As HKP is implemented over HTTP, when a status or error code needs
to be returned, the most appropriate HTTP code should be used.
Occasionally there is a need to express a condition that cannot be
expressed via the HTTP 1.0 status code list. In these cases, an
additional HTTP header may be added to the response. This
additional header is of the form "X-HKP-Status:" and is followed by
one of the following status codes:
xxx - Submitted key was altered to match keyserver policy.
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xxx - Submitted key was rejected as per keyserver policy.
xxx - The search resulted in more responses then the keyserver
was willing to return.
7. Locating a HKP Keyserver
Clients are usually manually configured with the address of a HKP
keyserver. Client implementors should be aware that it is
reasonably common practice to use a single name in DNS that
resolves to multiple address records. When receiving a DNS
response with multiple addresses, clients SHOULD try each address
until a server is reached. The order to try these addresses in is
implementation defined.
A far more flexible scheme for listing multiple HKP keyservers in
DNS is the use of DNS SRV records as specified in RFC-2782 [5].
DNS SRV allows for different priorities and weights to be applied
to each HKP keyserver in the list, which allows an administrator
much more control over how clients will contact the servers. The
SRV symbolic service name for HKP keyservers is "hkp". For
example, the SRV record for HKP keyservers in domain "example.com"
would be "_hkp._tcp.example.com".
SRV records contain the port that the target server runs on, so SRV
can also be used to automatically discover the proper port for
contacting a HKP keyserver.
An additional use of SRV records is when a client needs to locate a
specified key by email address. For example, a client trying to
locate a key for isabella@silvie.example.com could consult
"_hkp._tcp.silvie.example.com".
HKP clients SHOULD support SRV records.
8. Security Considerations
As described here, a keyserver is a searchable database of public
keys accessed over the network. While there may be security
considerations arising from distributing keys in this manner, this
does not impact the security of OpenPGP itself.
Without some sort of trust relationship between the client and
server, information returned from a keyserver in search results
cannot be trusted by the client until the OpenPGP client actually
retrieves and checks the key for itself. This is important and
must be stressed: without a specific reason to treat information
otherwise, all search results must be regarded as untrustworthy and
informational only.
9. IANA Considerations
This document assigns the DNS SRV symbolic name "hkp".
10. Normative References
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[1] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[2] T. Berners-Lee, "Hypertext Transfer Protocol 1.0",
RFC 1945, May 1996.
[3] T. Berners-Lee, "Hypertext Markup Language - 2.0",
RFC 1866, November 1995.
[4] J. Callas, L. Donnerhacke, H. Finney and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[5] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC
2782, February 2000.
[6] M. Elkins, D. Del Torto, R. Levien, and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.
11. Author's Address
David Shaw
16 Farina Road
Newton, MA 02459
Email: dshaw@jabberwocky.com
Tel: +1 (617) 332-8443
This document is a formalization and extension of the HKP
originally implemented in the PKS keyserver by Marc Horowitz, which
in turn was based on earlier work by Brian LaMacchia and Michael
Graff. Without their grounding, this document would not exist.
The author would also like to thank Peter Gutmann for his work on
the Certstore protocol, some of which was applicable here, and the
members of the pgp-keyserver-folk mailing list who contributed
valuable comments and suggestions.
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