Network Working Group W. Koch
Internet-Draft GnuPG Project
Intended status: Informational May 6, 2016
Expires: November 7, 2016
OpenPGP Web Key Service
draft-koch-openpgp-webkey-service-00
Abstract
This specification describes a service to locate OpenPGP keys by mail
address using a Web service and the HTTPS protocol. It also provides
a method for secure communication between the key owner and the mail
provider to publish and revoke the public key.
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
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This Internet-Draft will expire on November 7, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 2
3. Web Key Directory . . . . . . . . . . . . . . . . . . . . . . 2
3.1. Key Discovery . . . . . . . . . . . . . . . . . . . . . . 3
4. Web Key Directory Update Protocol . . . . . . . . . . . . . . 4
4.1. The Submission Address . . . . . . . . . . . . . . . . . 5
4.2. The Submission Mail . . . . . . . . . . . . . . . . . . . 6
4.3. The Confirmation Request . . . . . . . . . . . . . . . . 6
4.4. The Confirmation Response . . . . . . . . . . . . . . . . 7
4.5. Policy Flags . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Well-Known URI . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 9
A.1. Sample key . . . . . . . . . . . . . . . . . . . . . . . 10
A.2. Software Notes . . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 10
B.1. TODO . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This memo describes a method to associate OpenPGP keys with a mail
address and now to look them up using a web service with a well-known
URI. In addition a mail based protocol is given to allow a client to
setup such an association and to maintain it.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Web Key Directory
A major use case for OpenPGP is the encryption of mail. A common
difficulty of sending encrypted mails to a new communication partner
is to find the appropriate public key of the recipient. Unless an
off-channel key exchange has been done, there are no easy ways to
discover the required key. The common practice is to search the
network of public key servers for a key matching the recipient's mail
address. This practise bears the problem that the keyservers are not
able to give a positive confirmation that a key actually belongs to
the mail addresses given in the key. Further, there are often
several keys matching a mail address and thus one needs to pick a key
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on good luck. This is clearly not a secure way to setup an end-to-
end encryption. Even if the need for a trusted key for an initial
mail message is relinquished, a non-authenticated key may be a wrong
one and the actual recipient would receive a mail which she can't
decrypt, due to the use of a wrong key.
Methods to overcome this problem are
o sending an initial unencrypted messages with the public key
attached,
o using the OpenPGP DANE protocol to lookup the recipients key via
the DNS.
The first method has the obvious problems of not even trying to
encrypt the initial mail, an extra mail round-trip, and problems with
unattended key discovery.
The latter method works fine but requires that mail providers need to
set up a separate DNS resolver to provide the key. The
administration of a DNS zone is often not in the hands of small mail
installations. Thus an update of the DNS resource records needs to
be delegated to the ISP running the DNS service. Further, DNS
lookups are not encrypted and missing all confidentially. Even if
the participating MUAs are using STARTTLS to encrypt the mail
exchange, a DNS lookup for the key unnecessary identifies the local-
part of the recipients mail address to any passive eavesdroppers.
This memo specified a new method for key discovery using an encrypted
https connection.
3.1. Key Discovery
Although URIs are able to encode all kind of characters,
straightforward implementations of a key directory may want to store
the "local-part" of a mail address directly in the file system. This
forbids the use of certain characters in the "local-part". To allow
for such an implementation method the URI uses an encoded form of the
"local-part" which can be directly mapped to a file name.
OpenPGP defines its User IDs, and thus the mail address, as UTF-8
strings. To help with the common pattern of using capitalized names
(e.g. "Joe.Doe@example.org") for mail addresses, and under the
premise that almost all MTAs treat the "local-part" case-insensitive
and that the "domain-part" is required to be compared case-
insensitive anyway, all upper-case ASCII characters in a User ID are
mapped to lowercase. Non-ASCII characters are not changed.
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The so mapped "local-part" is hashed using the SHA-1 algorithm. The
resulting 160 bit digest is encoded using the Z-Base-32 method as
described in [RFC6189], section 5.1.6. The resulting string has a
fixed length of 32 octets. To form the URI, the scheme "https://" is
concatenated with the mapped "domain-part", the fixed string "./well-
known/openpgpkey/hu/", the "domain-part" again, and the above
constructed 32 octet string.
For example the URI to lookup the key for Joe.Doe@Example.ORG is:
https://example.org/.well-known/openpgpkey/
hu/example.org/iy9q119eutrkn8s1mk4r39qejnbu3n5q
(line has been wrapped for rendering purposes)
The HTTP GET method MUST return the binary representation of the
OpenPGP key for the given mail address. The key needs to carry a
User ID packet ([RFC4880]) with that mail address. Note that the key
may be revoked or expired - it is up to the client to handle such
conditions. The server MUST also accept a HEAD method so that a
client may only check for the existence of a key.
The server SHOULD return "application/pgp-key" as the content-type
for the data but clients MUST also accept "application/octet-string"
as content-type. The server MUST NOT return an ASCII armored version
of the key.
4. Web Key Directory Update Protocol
To put keys into the key directory a protocol to automate the task is
desirable. The protocol defined here is entirely based on mail and
the assumption that a mail provider can securely deliver mail to the
INBOX of a user (e.g. an IMAP folder). Note that the same protocol
may also be used for submitting keys for use with OpenPGP DANE.
We assume that the user already created a key for her mail account
alice@example.org. To install the key at her provider's Web Key
Directory, she performs the following steps:
1. She retrieves a file which contains one line with the mail
address used to submit the key to the mail provider. See below
for the syntax of that file. For a mail address at the domain
"example.org" the URI of the file is
https://example.org/.well-known/openpgpkey/submission-address
2. She sends her key using SMTP (or any other transport mechanism)
to the provider using the submission address. The content-type
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SHOULD be "application/pgp-key" and the key being a binary
attachment (which is then likely base64 encoded). Note that the
OpenPGP ASCII armor is not used.
3. The provider checks that the received key has a User ID which
* matches an account name of the provider,
* and that the from address matches that account.
4. The provider sends an encrypted message containing a nonce and
the fingerprint of the key to the mail account of the user. Note
that a similar scheme is used by the well known caff(1) tool to
help with key signing parties.
5. A legitimate user will be able to decrypt the message because she
created the key and is in charge of the private key. This step
verifies that the submitted key has actually been created by the
owner of the account.
6. The user sends the decrypted nonce back to the submission address
as a confirmation that the private key is owned by her and that
the provider may now publish the key. Also technically not
required, it is suggested that the mail to the provider is
encrypted. The public key for this is retrieved using the key
lookup protocol described above.
7. The provider receives the nonce, matches it with its database of
pending confirmations and then publishes the key. Finally the
provider sends a mail back to the user to notify her of the the
publication of her key.
The message data structures used for the above protocol are specified
in detail below. In the following sections the string "WELLKNOWN"
denotes the first part of an URI specific for a domain. In the
examples the domain "example.org" is assumed, thus
WELLKNOWN := https://example.org/.well-known/openpgpkey
The term "target key" denotes the to be published key, the term
"submission key" the key associated with the submission-address of
the mail provider.
4.1. The Submission Address
The address of the submission file is
WELLKNOWN/submission-address
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The file consists of exactly one line, terminated by a LF, or the
sequence of CR and LF, with the full mail address to be used for
submission of a key to the mail provider. For example the content of
the file may be
key-submission-example.org@directory.example.org
4.2. The Submission Mail
The mail used to submit a key to the mail provider MUST comply to the
PGP/MIME specification ([RFC3156], section 7), which states that the
Content-Type must be "application/pgp-keys", there are no required or
optional parameters, and the body part contains the ASCII-armored
transferable Public Key Packets as defined in [RFC4880], section
11.1.
If the mail provider has published an encryption key for the
submission-address in the Web Key Directory, the key to be published
MUST be submitted using a PGP/MIME encrypted message ([RFC3156],
section 4). The message MUST not be signed (because the authenticity
of the signing key has not yet been confirmed). After decryption of
the message at the mail provider a single "application/pgp-keys"
part, as specified above, is expected.
4.3. The Confirmation Request
The mail provider sends a confirmation mail in response to a received
key publication request. The message SHOULD be sent from the
submission-address of the mail provider to the mail address extracted
from the target key. The message needs to be encrypted to the target
key and MAY be signed by the submission key. PGP/MIME MUST be used
for encryption and signing; the Combined method ([RFC3156], section
6.2) MUST be used if the message is to be signed.
The Content-type used for the plaintext part MUST be "application/
vnd.gnupg.wkd". The body consists of name-value pairs with one name-
value pair per LF or CR+LF terminated line. Empty lines are allowed
and will be ignored by the receiver. A colon is used to terminate a
name.
In a confirmation request the following names MUST be send in the
specified order:
"type" The value must be "confirmation-request".
"from" This is the mailbox the user is expected to sent the
confirmation response to. The value must match the mailbox part
of the "From:" address of this request.
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"address" The value is the addr-spec part of the target key's mail
address. The value SHOULD match the addr-spec part of the
recipient's address. The value MUST be be UTF-8 encoded as
required for an OpenPGP User ID.
"fingerprint" The value is the fingerprint of the target key. The
fingerprint is given in uppercase hex encoding without any
interleaving spaces.
"nonce" The value is a string with a minimum length of 16 octets and
a maximum length of 64 octets. The string must entirely be made
up of random ASCII letters or digits. This nonce will be sent
back to the mail provider as proof that the recipient is the
legitimate owner of the target-key.
The receiver of the message decrypts the message, checks that the
"fingerprint" matches the target key, checks that the "address"
matches a User ID of the target key, and checks the other constrains
of the request format. If any constraint is not asserted, or the
fingerprint or User ID do not match the target key, or there is no
pending publication requests (i.e. a mail recently sent o the
submission address), the user MAY be notified about this fake
confirmation attempt.
In other cases the confirmation request is legitimate and the MUA
shall silently send a response as described in the next section.
4.4. The Confirmation Response
A response to a confirmation request MUST only be send in the
positive case; there is no negative confirmation response. A mail
service provider is expected to cancel a pending key submission after
a suitable time without a confirmation. The mail service provider
SHOULD not retry the sending of a confirmation request after the
first request has been send successfully.
The user MUST send the confirmation response from her target mail
address to the "from" address of the confirmation request. The
message MUST be signed and SHOULD be encrypted. The PGP/MIME
Combined format MUST be used for encryption and signing ([RFC3156],
section 6.2). The encryption key can be taken from the Web Key
Directory.
The Content-type used for the plaintext message MUST also be
"application/vnd.gnupg.wkd". The format is the same as described
above for the Confirmation Request. The body must contain three
name-value pairs in this order:
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"type" The value must be "confirmation-response".
"from" The value must match the mailbox part of the "From:" address
of this response.
"nonce" The value is the value of the "nonce" parameter from the
confirmation request.
4.5. Policy Flags
For key generation and submission it is sometimes useful to tell the
client about certain properties of the mail provider in advance.
This can be done with a file at the URL
WELLKNOWN/policy
The file contains keywords, one per line with each line terminated by
a LF or the sequence of CR and LF. Empty lines and lines starting
with a '#' character are considered comment lines. A keyword is made
up of lowercase letters, digits, hyphens, or dots. An underscore is
allowed as a name space delimiters; see below. The first character
must be a letter. Clients MUST use case-insensitive matching.
Currently defined keywords are:
"mailbox-only" The mail server provider does only accept keys with
only a mailbox in the User ID. In particular User IDs with a real
name in addition to the mailbox will be rejected as invalid.
More keywords will be defined in updates to this I-D. There is no
registry yet except for this document. For experimental use of new
features or for provider specific settings, keywords MUST be prefixed
with a domain name and an underscore.
5. Security Considerations
The use of SHA-1 for the mapping of the "local-part" to a fixed
string is not a security feature but merely used to map the local-
part to a fixed-sized string made from a well defined set of
characters. It is not intended to conceal information about a mail
address.
The domain name part of the mail address is not part of the hash to
avoid problems with internationalized domain names. Instead a
separate web service is required for each domain name.
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6. IANA Considerations
6.1. Well-Known URI
IANA is requested to assign a well-known URI in the "Well-Known URIs"
registry as defined by [RFC5785]:
URI suffix: openpgpkey
Change controller: IETF
Specification document: This
7. Normative References
[RFC0822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
August 1982, <http://www.rfc-editor.org/info/rfc822>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3156] Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
"MIME Security with OpenPGP", RFC 3156, August 2001.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, DOI
10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6189] Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
Media Path Key Agreement for Unicast Secure RTP", RFC
6189, DOI 10.17487/RFC6189, April 2011,
<http://www.rfc-editor.org/info/rfc6189>.
Appendix A. Test Vectors
For help implementing this specification a non-normative example is
given:
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A.1. Sample key
TODO
A.2. Software Notes
GnuPG supports the key discovery described in this document since
version 2.1.12. To use it, the new method "wkd" needs to be used
with the --auto-key-locate option.
Appendix B. Changes
o This is the initial draft.
B.1. TODO
o What about authenticated submission?
o Describe how to handle a key with several User IDs.
Author's Address
Werner Koch
GnuPG Project
Email: wk@gnupg.org
URI: https://gnupg.org
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