Network Working Group M. McCain
Internet-Draft FLM
Intended status: Standards Track M. Lee
Expires: August 16, 2019 TI
N. Welch
Google
February 12, 2019
Distributing OpenPGP Keys with Signed Keylist Subscriptions
draft-mccain-keylist-03
Abstract
This document specifies a system by which an OpenPGP client may
subscribe to an organization's keylist to keep its internal keystore
up-to-date. Ensuring that all members of an organization have their
colleagues' most recent PGP public keys is critical to maintaining
operational security. Without the most recent keys and a source of
trust for those keys (as this document specifies), users must
manually update and sign each others keys -- a system that is
untenable in larger organizations. This document proposes a
experimental format for the keylist file as well as requirements for
clients who wish to implement this experimental keylist subscription
functionality.
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/.
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 August 16, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Note to Readers . . . . . . . . . . . . . . . . . . . . . 3
2. Functions and Procedures . . . . . . . . . . . . . . . . . . 3
2.1. Subscribing to Keylists . . . . . . . . . . . . . . . . . 4
2.2. Automatic Updates . . . . . . . . . . . . . . . . . . . . 4
2.3. Cryptographic Verification of Keylists . . . . . . . . . 5
3. Data Element Formats . . . . . . . . . . . . . . . . . . . . 5
3.1. Keylist . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Signature . . . . . . . . . . . . . . . . . . . . . . . . 7
4. In Practice . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5.1. Security Benefits . . . . . . . . . . . . . . . . . . . . 7
5.2. Security Drawbacks . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document specifies a system by which clients may subscribe to
cryptographically signed keylists. This system allows for seamless
key rotation across entire organizations and enhances operational
security. To enable cross-client compatibility, this document
provides a experimental format for the keylist, its cryptographic
verification, and the method by which it is retreived by the client.
The user interface by which a client provides this functionality to
the user is out of scope, as is the process by which the client
retrieves public keys. Other non-security-related implementation
details are also out of scope.
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1.1. Requirements Notation
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] .
1.2. Terminology
This document uses the terms "OpenPGP", "public key", "private key",
"signature", and "fingerprint" as defined by OpenPGP Message Format
[RFC4880] .
The term "keylist" is defined as a list of OpenPGP public key
fingerprints and accessible via a URI. The exact format of this data
is specified in Section 3 .
An "authority key" is defined as the OpenPGP secret key used to sign
a particular keylist. Every keylist has a corresponding authority
key, and every authority key has at least one corresponding keylist.
A single authority key SHOULD NOT be used to sign multiple keylists.
To be "subscribed" to a keylist means that a program will retreive
that keylist on a regular interval. After retrieval, that program
will perform an update to an internal OpenPGP keystore.
A "client" is a program that allows the user to subscribe to
keylists. A client may be an OpenPGP client itself or a separate
program that interfaces with an OpenPGP client to update its
keystore.
1.3. Note to Readers
RFC Editor: please remove this section prior to publication.
Development of this Internet draft takes place on GitHub at Keylist-
RFC [1].
A mailing list is available for discussion at Keylists mailing list
[2].
2. Functions and Procedures
As new keys are created and other keys are revoked, it is critical
that all members of an organization have the most recent set of keys
available on their computers. Keylists enable organizations to
publish a directory of OpenPGP keys that clients can use to keep
their internal keystores up-to-date.
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2.1. Subscribing to Keylists
A single client may subscribe to any number of keylists. When a
client first subscribes to a keylist, it SHOULD update or import
every key present in the keylist into its local keystore. Keylist
subscriptions SHOULD be persistent -- that is, they should be
permanently stored by the client to enable future automatic updates.
To subscribe to a keylist, the client must be aware of the keylist
URI (see [RFC3986]), and the fingerprint of the authority key used to
sign the keylist. The protocol used to retrieve the keylist and its
signature SHOULD be HTTPS (see [RFC2818]), however other
implementation MAY be supported. A client implementing keylist
functionality MUST support the retrieval of keylists and signatures
over HTTPS. All other protocols are OPTIONAL.
A client MUST NOT employ a trust-on-first-use (TOFU) model for
determining the fingerprint of the authority key; it must be
explicitly provided by the user.
The process by which the client stores its keylist subscriptions is
out of scope, as is the means by which subscription functionality is
exposed to the end-user.
2.2. Automatic Updates
The primary purpose of keylists is to enable periodic updates of
OpenPGP clients' internal keystores. We RECOMMEND that clients
provide automatic 'background' update functionality; we also regonize
that automatic background updates are not possible in every
application (specifically cross-platform CLI tools).
When automatic background updates are provided, we RECOMMEND that
clients provide a default refresh interval of less than one day,
however we also RECOMMEND that clients allow the user to select this
interval. The exact time at which updates are performed is not
critical.
To perform an update, the client MUST perform the following steps on
each keylist to which it is subscribed. The steps SHOULD be
performed in the given order.
1. Obtain a current copy of the keylist from its URI.
2. Obtain a current copy of the keylist's signature data from its
URI, which is included in the keylist data format specified in
Section 3.
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3. Using the keylist and the keylist's signature, cryptographically
verify that the keylist was signed using the authority key. If
the signature does not verify, the client MUST abort the update
of this keylist and SHOULD alert the user. The client SHOULD NOT
abort the update of other keylists to which it is subscribed,
unless they too fail signature verification.
4. Validate the format of the keylist according to Section 3 . If
the keylist is in an invalid format, the client MUST abort the
update this keylist and SHOULD alert the user.
5. For each fingerprint listed in the keyfile, if a copy of the
associated public key is not present in the client's local
keystore, retrieve it from the keyserver specified by the keylist
(see Section 3) or, if the keylist specifies no keyserver, from
any keyserver. If the key is already present and not revoked,
refresh it from a keyserver. If it is present and revoked, do
nothing.
2.3. Cryptographic Verification of Keylists
To ensure authenticity of a keylist during an update, the client MUST
verify that the keylist's data matches its cryptographic signature,
and that the public key used to verify the signature matches the
authority key fingerprint given by the user.
For enhanced security, it is RECOMMENDED that keylist operators sign
each public key listed in their keylist with the authority private
key. This way, an organization can have an internal trust
relationship without requiring members of the organization to certify
each other's public keys.
3. Data Element Formats
The following are format specifications for the keylist file and its
signature file.
3.1. Keylist
The keylist MUST be a valid JavaScript Object Notation (JSON) Data
Interchange Format [RFC8259] object with specific keys and values, as
defined below. Note that unless otherwise specified, 'key' in this
section refers to JSON keys -- not OpenPGP keys.
To encode metadata, the keylist MUST have a "metadata" root key with
an object as the value ("metadata object"). The metadata object MUST
contain a "signature_uri" key whose value is the URI string of the
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keylist's signature file. All metadata keys apart from
"signature_uri" are OPTIONAL.
The metadata object MAY contain a "keyserver" key with the value of
the URI string of the keyserver from which the OpenPGP keys in the
keylist should be retrieved.
The metadata object MAY contain a "comment" key with the value of any
string. The metadata object MAY also contain other arbitrary key-
value pairs.
The keylist MUST have a "keys" key with an array as the value. This
array contains a list of OpenPGP key fingerprints and metadata about
them. Each item in the array MUST be an object. Each of these
objects MUST have a "fingerprint" key with the value of a string that
contains the full 40-character hexadecimal public key fingerprint, as
defined in OpenPGP Message Format [RFC4880] . Any number of space
characters (' ', U+0020) MAY be included at any location in the
fingerprint string. These objects MAY contain "name", "email", and
"comment" key-value pairs, as well as any other key-value pairs
relevant.
The following is an example of a valid keylist.
{
"metadata": {
"signature_uri": "https://www.example.com/keylist.json.asc",
"comment": "This is an example of a keylist file"
},
"keys": [
{
"fingerprint": "927F419D7EC82C2F149C1BD1403C2657CD994F73",
"name": "Micah Lee",
"email": "micah.lee@theintercept.com",
"comment": "Each key can have a comment"
},
{
"fingerprint": "1326CB162C6921BF085F8459F3C78280DDBF52A1",
"name": "R. Miles McCain",
"email": "0@rmrm.io"
},
{
"fingerprint": "E0BE0804CF04A65C1FC64CC4CAD802E066046C02",
"name": "Nat Welch",
"email": "nat.welch@firstlook.org"
}
]
}
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3.2. Signature
The signature file MUST be an ASCII-armored 'detached signature' of
the keylist file, as defined in OpenPGP Message Format [RFC4880] .
4. In Practice
GPG Sync, an open source program created by one of the authors,
implements this experimental standard. GPG Sync is used by First
Look Media and the Freedom of the Press Foundation to keep OpenPGP
keys in sync across their organizations, as well as to publish their
employee's OpenPGP keys to the world. These organizations
collectively employ more than 200 people and have used the system
described in this document successfully for multiple years.
GPG Sync's existing code can be found at
<https://github.com/firstlookmedia/gpgsync>
First Look Media's keylist file can be found at
<https://github.com/firstlookmedia/gpgsync-firstlook-fingerprints>
5. Security Considerations
5.1. Security Benefits
The keylist subscription functionality defined in this document
provide a number of security benefits, including:
o The ability for new keys to be quickly distributed across an
organization.
o It removes the complexity of key distribution from end users,
allowing them to focus on the content of their communications
rather than on key management.
o The ability for an organization to prevent the spread of falsely
attributed keys by centralizing the public key discovery process
within their organization.
5.2. Security Drawbacks
There is a situation in which keylist subscriptions could pose a
potential security threat. If the both the authority key and the
keylist distribution system were to be compromised, it would be
possible for an attacker to distribute any key of their choosing to
the subscribers of the keylist. The potential consequences of this
attack are limited, however, because the attacker cannot remove or
modify the keys already present on subscribers' systems.
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6. IANA Considerations
This document has no actions for IANA.
7. References
7.1. Normative References
[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/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[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/info/rfc8259>.
7.2. URIs
[1] https://github.com/firstlookmedia/keylist-rfc
[2] https://www.freelists.org/list/keylists
Authors' Addresses
R. Miles McCain
First Look Media
Email: ietf@sendmiles.email
URI: https://rmrm.io
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Micah Lee
The Intercept
Email: micah.lee@theintercept.com
URI: https://micahflee.com/
Nat Welch
Google
Email: nat@natwelch.com
URI: https://natwelch.com
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