Internet DRAFT - draft-gallagher-openpgp-hkp
draft-gallagher-openpgp-hkp
openpgp D. Shaw
Internet-Draft Jabberwocky Tech
Intended status: Informational A. Gallagher, Ed.
Expires: 5 September 2024 PGPKeys.EU
4 March 2024
OpenPGP HTTP Keyserver Protocol
draft-gallagher-openpgp-hkp-04
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.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://andrewgdotcom.gitlab.io/draft-gallagher-openpgp-hkp. Status
information for this document may be found at
https://datatracker.ietf.org/doc/draft-gallagher-openpgp-hkp/.
Discussion of this document takes place on the OpenPGP Working Group
mailing list (mailto:openpgp@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/openpgp/. Subscribe at
https://www.ietf.org/mailman/listinfo/openpgp/.
Source for this draft and an issue tracker can be found at
https://gitlab.com/andrewgdotcom/draft-gallagher-openpgp-hkp.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Shaw & Gallagher Expires 5 September 2024 [Page 1]
Internet-Draft HKP March 2024
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."
This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. HKP and HTTP . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Request Paths . . . . . . . . . . . . . . . . . . . . . . 4
3.2. HTTP Status Codes . . . . . . . . . . . . . . . . . . . . 4
4. Looking up Data from a Keyserver . . . . . . . . . . . . . . 5
4.1. The "op" (operation) Variable . . . . . . . . . . . . . . 6
4.1.1. The "get" operation . . . . . . . . . . . . . . . . . 6
4.1.2. The "index" operation . . . . . . . . . . . . . . . . 6
4.1.3. The "vindex" (verbose index) operation . . . . . . . 7
4.1.4. The "stats" (statistics/status) operation . . . . . . 7
4.1.5. The "hget" (hash get) operation . . . . . . . . . . . 7
4.1.6. Other operations . . . . . . . . . . . . . . . . . . 7
4.2. The "search" variable . . . . . . . . . . . . . . . . . . 7
4.2.1. Key ID and Fingerprint Searches . . . . . . . . . . . 8
4.2.2. Text Searches . . . . . . . . . . . . . . . . . . . . 8
4.3. Lookup Examples . . . . . . . . . . . . . . . . . . . . . 8
5. Submitting Keys To A Keyserver . . . . . . . . . . . . . . . 8
6. Modifier Variables . . . . . . . . . . . . . . . . . . . . . 9
6.1. The "options" Variable . . . . . . . . . . . . . . . . . 9
6.1.1. The "mr" (Machine Readable) Option . . . . . . . . . 9
6.1.2. The "nm" (No Modification) Option . . . . . . . . . . 9
6.1.3. Other Options . . . . . . . . . . . . . . . . . . . . 9
6.2. The "v" (Version) Variable . . . . . . . . . . . . . . . 10
6.3. The "fingerprint" Variable . . . . . . . . . . . . . . . 10
Shaw & Gallagher Expires 5 September 2024 [Page 2]
Internet-Draft HKP March 2024
6.4. The "hash" Variable . . . . . . . . . . . . . . . . . . . 10
6.5. The "exact" Variable . . . . . . . . . . . . . . . . . . 10
6.6. Other Variables . . . . . . . . . . . . . . . . . . . . . 10
7. Output Formats . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Machine Readable Output . . . . . . . . . . . . . . . . . 11
7.2. Machine Readable Indexes . . . . . . . . . . . . . . . . 11
8. Keyring Format . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Detached Revocations . . . . . . . . . . . . . . . . . . 14
9. Locating a HKP Keyserver . . . . . . . . . . . . . . . . . . 15
9.1. Key discovery . . . . . . . . . . . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 17
Appendix B. Document History . . . . . . . . . . . . . . . . . . 17
B.1. Changes Between -03 and -04 . . . . . . . . . . . . . . . 17
B.2. Changes Between -02 and -03 . . . . . . . . . . . . . . . 17
B.3. Changes Between -01 and -02 . . . . . . . . . . . . . . . 18
B.4. Changes Between -00 and -01 . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. HKP and HTTP
As HKP is implemented over HTTP, everything in [RFC1945] applies to
HKP as well, and HKP error codes are the same as the ones used in
HTTP.
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.
Shaw & Gallagher Expires 5 September 2024 [Page 3]
Internet-Draft HKP March 2024
(( 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. See issue #5. ))
When used over HTTPS, HKP is commonly referred to as "HKPS".
By convention and history, HKP uses HTTP on TCP port 11371, and HTTPS
on TCP port 443. These are often distinguished from generic use of
HTTP(S) by using the URI schemes "hkp" and "hkps". For reasons of
maximum compatibility with firewalls and filtering HTTP proxies, HKP
is also often served over the standard HTTP port (TCP port 80).
(( andrewg : We may wish to officially recommend HKPS. See issue #7.
))
See Section 9 for an automated way for clients to discover the
correct port.
3.1. Request Paths
HKP defines two paths, namely "/pks/lookup" for lookups (see
Section 4) and "/pks/add" for submission (see Section 5). A
keyserver MAY support requests to other paths, but these are
implementation dependent. These alternative paths have historically
been used to provide human-readable interfaces such as HTML forms,
and functionality extensions such as [SKS].
(( andrewg : SKS uses "/pks/hashquery" for bulk updates, and
hockeypuck also implements "/pks/delete" and "/pks/replace". See
issue #15. ))
3.2. HTTP Status Codes
When a status or error code needs to be returned by a keyserver, the
most appropriate HTTP code from [RFC9110] should be used. It 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.
Clients SHOULD understand the following codes:
Shaw & Gallagher Expires 5 September 2024 [Page 4]
Internet-Draft HKP March 2024
+===================+===================================+
| Status Code | Description |
+===================+===================================+
| 200 OK | Request succeeded |
+-------------------+-----------------------------------+
| 202 Accepted | Submitted key was altered to |
| | match keyserver policy |
+-------------------+-----------------------------------+
| 403 Forbidden | The requested operation is not |
| | permitted |
+-------------------+-----------------------------------+
| 404 Not found | The search returned no results, |
| | or path not found |
+-------------------+-----------------------------------+
| 410 Gone | Key has been permanently deleted, |
| | e.g. due to blacklisting |
+-------------------+-----------------------------------+
| 413 Content too | The search returned too many |
| large | responses |
+-------------------+-----------------------------------+
| 422 Unprocessable | Submitted key was rejected as per |
| content | keyserver policy |
+-------------------+-----------------------------------+
| 501 Not | The requested operation is not |
| implemented | supported |
+-------------------+-----------------------------------+
Table 1: Status Codes
In addition, a client SHOULD understand 3xx redirect codes.
(( andrewg : In draft-shaw-00 it was suggested that a novel header be
used for statuses that could not be represented by the HTTP response
codes of the time. This was only partially specified, and it is
unclear if any implementations of this header existed. In the
meantime many new HTTP response codes have been defined, so I am
using them instead - even if their semantics does not exactly match
that of [RFC9110]. NB therefore that codes 202, 410, 413, 422 may
not have been implemented anywhere yet. See issue #5. ))
4. Looking up Data from a Keyserver
Key lookups are done via an HTTP GET request. Specifically, the
abs_path (see [RFC1945], section 3.2) is built up of the base path
"/pks/lookup", followed by any variables. Variables are passed using
HTTP query strings as specified in [RFC1866], section 8.2.2.
Shaw & Gallagher Expires 5 September 2024 [Page 5]
Internet-Draft HKP March 2024
Most HKP lookups contain both the "op" (operation) and "search"
variables. The "op" variable determines what operation the keyserver
will take, and the "search" variable determines which keys are
operated on. There may also be modifier variables, as specified in
Section 6 below. The variables may be given in any order.
Keyservers MUST ignore any unknown variables.
4.1. 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.
If a particular operation is not supported, the keyserver should
return an appropriate HTTP error code such as 501 ("Not
Implemented").
4.1.1. The "get" operation
A keyserver SHOULD support the "get" operation.
The "get" operation requests keys from the keyserver by specifying
its key ID, fingerprint, or one of its user IDs. 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 an ASCII-armored keyring as specified in Section 8.
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
[crypto-refresh].
If no keys match the request, the keyserver SHOULD return an
appropriate HTTP error code such as 404 ("Not Found").
4.1.2. The "index" operation
A keyserver MAY support 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.
Shaw & Gallagher Expires 5 September 2024 [Page 6]
Internet-Draft HKP March 2024
4.1.3. The "vindex" (verbose index) operation
A keyserver MAY support the "vindex" 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
"index" with the addition of showing the signatures on each key, but
this is not required.
4.1.4. The "stats" (statistics/status) operation
A keyserver MAY support the "stats" operation.
The output of the "stats" operation is implementation-dependent, but
may include diagnostic output, configuration state, or other
metadata. The "search" variable is ignored when supplied with
"stats".
4.1.5. The "hget" (hash get) operation
A keyserver MAY support the "hget" operation.
"hget" requests a key from a keyserver by specifying its [SKS]
digest. The digest is provided in the "search" variable.
4.1.6. Other operations
Other site-specific or nonstandard operations can be indicated by
prefixing the operation name with the string "x-".
4.2. The "search" variable
The search variable contains arbitrary text encoded as usual for a
HTTP URL. This text may represent the key ID, or fingerprint, 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.
Shaw & Gallagher Expires 5 September 2024 [Page 7]
Internet-Draft HKP March 2024
4.2.1. Key ID and Fingerprint Searches
To search for a key by its key ID or fingerprint, a client SHOULD use
"op=get" and prefix the "search" string with an "0x" to indicate a
hexadecimal number. Key ID strings are 16 hexadecimal digits (64
bits). Fingerprint strings are either 32 (version 3), 40 (version
4), or 64 (version 6) hexadecimal digits. The hexadecimal digits are
not case sensitive.
A keyserver SHOULD accept fingerprints and MAY accept 64-bit key IDs
in the search string. A keyserver MUST NOT return results for 32-bit
"short key ID" searches, as these do not provide sufficient collision
resistance.
V3 keys are no longer considered secure, but MAY be distributed for
historical reference.
4.2.2. Text Searches
To search for a key by the text of a user ID, a client SHOULD use
"op=get" and SHOULD NOT prefix the "search" string with "0x".
Otherwise, how a keyserver handles textual search is implementation
defined. See also the definition of the "exact" variable
(Section 6.5) for a method to give additional instructions to the
server on how the search is to be executed.
4.3. Lookup Examples
Search for all keys containing the string "dshaw":
http://keys.example.com:11371/pks/lookup?search=dshaw&op=index
Get key 0xDEADBEEFDECAFBAD (64-bit key ID):
http://keys.example.com:11371/pks/lookup?op=get&search=0xDEADBEEFDECAFBAD
5. Submitting Keys To A Keyserver
A keyserver MAY accept submissions via an HTTP POST request.
Specifically, the abs_path (see [RFC1945], section 3.2) is set to
"/pks/add", and the key data is provided via HTTP POST as specified
in [RFC1945], section 8.3, and [RFC1866], section 8.2.3.
The body of the POST message contains a "keytext" variable which
contains an ASCII-armored keyring as specified in Section 8. The
ASCII armored keyring should also be urlencoded as specified in
[RFC1866], section 8.2.1. Note that more than one key may be
submitted in a single transaction.
Shaw & Gallagher Expires 5 September 2024 [Page 8]
Internet-Draft HKP March 2024
There may also be modifier variables, as specified in Section 6
below.
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.
6. Modifier Variables
These variables are used to modify basic requests.
6.1. The "options" Variable
This variable takes one or more option values, separated by commas.
These are used to modify the behavior of the keyserver on a per-
request basis. Each value indicates a boolean flag, where the
presence of the value indicates "true" and the absence "false".
6.1.1. 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 SHOULD be
in machine-readable format. See Section 7 for the specific details
of machine readable output.
6.1.2. 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 MAY trim any
noncompliant user IDs before accepting the key. If this option was
set, then such a key submission SHOULD fail with an appropriate error
code such as 422 (Unprocessable content).
"nm" is meaningful for submissions only.
6.1.3. Other Options
Other site-specific or nonstandard options can be indicated by
prefixing the option name with the string "x-". Non-standard options
MUST represent boolean values with a default value of "false".
Shaw & Gallagher Expires 5 September 2024 [Page 9]
Internet-Draft HKP March 2024
6.2. The "v" (Version) Variable
This variable identifies the version of machine readable output that
the client supports. Currently, only "v=1" is defined.
"v" is meaningful for machine readable output only.
6.3. 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. An implementation MAY decide to ignore this
variable or set the default behaviour to "on".
"fingerprint" is meaningful for lookups only.
6.4. The "hash" Variable
This variable takes one argument: "on" or "off". If present and on,
it instructs the server to provide the [SKS] digest of each key in an
"index" or "vindex" operation in the default human-readable form.
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. An implementation
MAY decide to ignore this variable or set the default behaviour to
"on".
"hash" is meaningful for lookups only.
6.5. 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. An implementation MAY decide to ignore this
variable or set the default behaviour to "on".
"exact" is meaningful for lookups only.
6.6. Other Variables
Other site-specific or nonstandard variables can be indicated by
prefixing the variable name with the string "x-".
Shaw & Gallagher Expires 5 September 2024 [Page 10]
Internet-Draft HKP March 2024
7. Output Formats
HKP is intended for both human and programmatic use. In general, the
default "human readable" output is implementation specific. The
"machine readable" option is used to tailor the output for automated
use. For interoperability, the "machine readable" output MUST
carefully follow the guidelines given here.
A client implementation SHOULD request machine readable output and
SHOULD NOT attempt to parse human-readable output.
7.1. Machine Readable Output
Clients requesting machine readable output:
* SHOULD supply "v=1" (Section 6.2) and "option=mr" (Section 6.1.1)
in the request.
* MUST silently ignore any content preceding or following a returned
armored key block.
* MUST silently ignore any keys with unknown versions or algorithms.
Keyservers returning machine readable output:
* MUST set the HTTP header "Access-Control-Allow-Origin: *", as
specified in [CORS].
* MUST set "Content-Type: application/pgp-keys" when returning keys
("op=get"), as specified in [RFC3156], section 7.
* MUST use the format specified in Section 7.2 when returning
indexes ("op=index").
* MAY return statistics in JSON format [RFC8259], the schema of
which is otherwise implementation-dependent.
7.2. Machine Readable Indexes
The machine readable index format is a list of newline-separated
records. The document is 7-bit clean, and as such is sent with no
encoding and Content-Type: text/plain.
The machine readable response MAY be prefixed by an information
record:
info:<version>:<count>
Shaw & Gallagher Expires 5 September 2024 [Page 11]
Internet-Draft HKP March 2024
+=========+===================================+
| Field | Description |
+=========+===================================+
| version | the version of this output format |
+---------+-----------------------------------+
| count | the number of keys returned |
+---------+-----------------------------------+
Table 2: Information Record Fields
If this line is not included, or the version information is not
supplied, the version number is assumed to be 1. Currently, only
version 1 is defined.
Note that "count" is the number of keys, and not the number of lines
returned. That is, it SHOULD match the number of "pub:" lines
returned.
The key listings themselves are made up of several records per key.
The first record specifies the primary key:
pub:<keyid>:<algorithm>:<keylen>:<creationdate>:<expirationdate>:<flags>:<version>
+================+=============================================+
| Field | Description |
+================+=============================================+
| keyid | the fingerprint or the key ID |
+----------------+---------------------------------------------+
| algorithm | the algorithm ID |
+----------------+---------------------------------------------+
| keylen | the key length in bits |
+----------------+---------------------------------------------+
| creationdate | creation date of the key |
+----------------+---------------------------------------------+
| expirationdate | expiration date of the key |
+----------------+---------------------------------------------+
| flags | letter codes to indicate details of the key |
+----------------+---------------------------------------------+
| version | the version of the key |
+----------------+---------------------------------------------+
Table 3: Public Key Record Fields
A keyserver MAY return a 16-digit key ID, but SHOULD return a
fingerprint if available. Since it is not possible to calculate the
key ID from a V3 key fingerprint, for V3 keys this SHOULD be the
16-digit key ID only.
Shaw & Gallagher Expires 5 September 2024 [Page 12]
Internet-Draft HKP March 2024
The algorithm ID is as specified in [crypto-refresh], section 9.1
i.e. 1==RSA, 17==DSA, etc.
Following the "pub" record are one or more "uid" records to indicate
user IDs on the key:
uid:<uidstring>:<creationdate>:<expirationdate>:<flags>
+================+=================================================+
| Field | Description |
+================+=================================================+
| uidstring | the user ID string |
+----------------+-------------------------------------------------+
| creationdate | creation date of the User ID |
+----------------+-------------------------------------------------+
| expirationdate | expiration date of the User ID |
+----------------+-------------------------------------------------+
| flags | letter codes to indicate details of the User ID |
+----------------+-------------------------------------------------+
Table 4: User ID Record Fields
The user ID string MUST use HTTP % encoding for anything that isn't
7-bit safe as well as for the ":" character. Any other characters
MAY be HTTP encoded, as desired.
The information for the "creationdate", "expirationdate", and "flags"
fields is taken from the User ID self-signature, if any, and applies
to the user ID in question, not to the key as a whole.
Primary key and User ID records may contain a "flags" field
containing a sequence of alphabetical characters, one per flag.
Flags MAY be given 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. Client implementations MUST ignore
unknown flags.
Shaw & Gallagher Expires 5 September 2024 [Page 13]
Internet-Draft HKP March 2024
+======+=============+
| Flag | Description |
+======+=============+
| r | revoked |
+------+-------------+
| d | disabled |
+------+-------------+
| e | expired |
+------+-------------+
Table 5: Record Flags
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. Client implementations
MUST ignore unknown trailing fields. All dates are given in the
number of seconds since midnight 1/1/1970 UTC.
8. Keyring Format
HKP uses an ASCII-armored keyring as its primary data representation
for both input and output.
A keyring is a sequence of one or more OpenPGP Transferable Public
Keys, concatenated directly, as specified in [crypto-refresh]
sections 10.1 and 3.6. An ASCII-armored keyring is a keyring that
has been encoded as a single armored block, as specified in
[crypto-refresh] section 6.2.
8.1. Detached Revocations
Traditionally, OpenPGP key revocation signatures have sometimes been
distributed as a detached "revocation certificate", as per
[crypto-refresh] section 10.1.3. An HKP implementation MAY accept an
ASCII-armored detached revocation as input, in place of an ASCII-
armored keyring.
(( andrewg : We may wish to specify multiple detached revocations,
and/or a mixture of detached revocations and TPKs. We may also wish
to specify them for both input and output. See issue #18. ))
Shaw & Gallagher Expires 5 September 2024 [Page 14]
Internet-Draft HKP March 2024
9. Locating a HKP Keyserver
Clients are usually manually configured with the address of a HKP
keyserver. Client implementations 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 [RFC2782]. 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" when used over plaintext
HTTP, or "hkps" when using HTTPS. 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. HKP clients SHOULD support SRV records.
9.1. Key discovery
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".
(( andrewg : key discovery is the subject of ongoing debate, and may
need to be left for another document. See issue #1. ))
10. 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.
Shaw & Gallagher Expires 5 September 2024 [Page 15]
Internet-Draft HKP March 2024
11. IANA Considerations
This document assigns the DNS SRV symbolic names "hkp" and "hkps",
the URI schemes "hkp" and "hkps", and the HKP port 11371.
(( andrewg : if we assign hkps, we may be required to specify a
dedicated port, even though nobody uses it. See issue #14. ))
12. References
12.1. Normative References
[CORS] "Cross Origin Resource Sharing", n.d.,
<https://fetch.spec.whatwg.org/#cors-protocol>.
[crypto-refresh]
Wouters, P., Huigens, D., Winter, J., and N. Yutaka,
"OpenPGP", October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-
crypto-refresh-13>.
[RFC1866] Berners-Lee, T. and D. Connolly, "Hypertext Markup
Language - 2.0", RFC 1866, DOI 10.17487/RFC1866, November
1995, <https://www.rfc-editor.org/rfc/rfc1866>.
[RFC1945] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945,
DOI 10.17487/RFC1945, May 1996,
<https://www.rfc-editor.org/rfc/rfc1945>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/rfc/rfc2782>.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156,
DOI 10.17487/RFC3156, August 2001,
<https://www.rfc-editor.org/rfc/rfc3156>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
Shaw & Gallagher Expires 5 September 2024 [Page 16]
Internet-Draft HKP March 2024
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
12.2. Informative References
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[SKS] "Synchronising Key Server Wiki", n.d.,
<https://github.com/sks-keyserver/sks-keyserver/wiki>.
Appendix A. Acknowledgments
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 authors 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.
Appendix B. Document History
Note to RFC Editor: this section should be removed before
publication.
B.1. Changes Between -03 and -04
* Reworded Section 4 for clarity.
* Separate section for keyring format.
* Specify detached revocations.
* Updated references.
B.2. Changes Between -02 and -03
* Clients SHOULD supply the v=1 api-versioning variable.
* Machine-readable output includes key version field.
Shaw & Gallagher Expires 5 September 2024 [Page 17]
Internet-Draft HKP March 2024
* Clients MUST silently ignore leading and trailing cruft, trailing
unknown fields, and unknown flags.
* Clients MUST silently ignore keys with unknown versions or
algorithms.
* All other m-r index specs (CORS, Content-Type etc.) are now MUST.
* Included the hash variable from SKS.
B.3. Changes Between -01 and -02
* Tightened up BCP-14 language.
* Included op=hget from SKS.
* Options now strictly boolean with default false, variables less
strict.
* More detail about HTTP status code usage.
B.4. Changes Between -00 and -01
* Improved text structure.
* Added references to HTTPS/HKPS, and hkp:/hkps: URL schemes.
* Forbade short IDs and deprecated V3 keys.
* Included op=stats from SKS.
* Mentioned CORS.
* Made use of terminology more consistent.
* Replaced custom status codes with standard HTTP status codes.
Authors' Addresses
Daphne Shaw
Jabberwocky Tech
Email: dshaw@jabberwocky.com
Andrew Gallagher (editor)
PGPKeys.EU
Email: andrewg@andrewg.com
Shaw & Gallagher Expires 5 September 2024 [Page 18]
