Distributing OpenPGP Keys with Signed Keylist Subscriptions
draft-mccain-keylist-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
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|---|---|---|---|
| Authors | R. Miles McCain , Micah Lee , Nat Welch | ||
| Last updated | 2018-08-06 | ||
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draft-mccain-keylist-00
Network Working Group M. McCain
Internet-Draft FLM
Intended status: Experimental M. Lee
Expires: February 7, 2019 TI
N. Welch
FLM
August 6, 2018
Distributing OpenPGP Keys with Signed Keylist Subscriptions
draft-mccain-keylist-00
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 standard
format for the keylist file as well as requirements for clients who
wish to implement keylist subscription functionality.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 7, 2019.
Copyright Notice
Copyright (c) 2018 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
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 2
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Functions and Procedures . . . . . . . . . . . . . . . . . . 3
2.1. Subscribing to Keylists . . . . . . . . . . . . . . . . . 3
2.2. Periodic Updates . . . . . . . . . . . . . . . . . . . . 4
2.3. Cryptographic Verification of Keylists . . . . . . . . 4
3. Data Element Formats . . . . . . . . . . . . . . . . . . . . 5
3.1. Keylist . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Signature . . . . . . . . . . . . . . . . . . . . . . . . 5
4. In Practice . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5.1. Security Benefits . . . . . . . . . . . . . . . . . . . . 6
5.2. Security Drawbacks . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 standard 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.
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] .
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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 keys
identified by their 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.
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.
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 (defined in [RFC3986] ), the keylist's signature URI, and the
fingerprint of the authority key used to sign the keylist. The
protocol used to retrieve thbe keylist and its signature SHOULD be
HTTPS (see [RFC2818] ), however other implementation are possible. A
client implementing keylist functionality MUST support the retrieval
of keylists and signatures over HTTPS. All other protocols are
OPTIONAL.
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A client MUST NOT employ a trust-on-first-use 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. Periodic Updates
The primary purpose of keylists is to enable periodic updates of
OpenPGP clients' internal keystores. 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.
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 a keyserver. If it is already present
and not revoked, refresh it from a keyserver. If it is present
and revoked, ignore it. The method by which keys are retrieved
and updated is out of scope.
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,
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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 definitions of the data types we will be creating
to support this new feature set.
3.1. Keylist
The keylist MUST be encoded in UTF-8 [RFC3629] . Each line MUST begin
either with a comment, a public key fingerprint, or whitespace. A
comment is defined as a string of characters between a hash symbol
(#, U+0023) and a newline or an end of file (EOF). The keylist
SHOULD end with a newline. The fingerprint MUST be the full
40-character hexadecimal public key fingerprint, as defined in
OpenPGP Message Format [RFC4880] . Space characters (' ', U+0020) MAY
be included anywhere in the fingerprint. Lines SHOULD NOT exceed 128
characters in length.
It is RECOMMENDED that keylist maintainers describe each key using a
comment, for example:
1326 CB16 ... DDBF 52A1 # Miles' Key
To extract the public key fingerprints from a keylist, a client
SHOULD perform the following steps, in order:
1. Strip the keylist of all comments, as defined above, including
the preceding hash symbol but excluding the trailing newline.
2. Strip the keylist of all non-breaking whitespace.
Performing these steps will result in one public key fingerprint per
line.
3.2. Signature
The signature file MUST be an ASCII-armored 'detached signature' of
the keylist file, as defined in OpenPGP Message Format [RFC4880] .
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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 authority key and the keylist
distribution system were to both be compromised, it would be possible
for an attacker to distribute false keys. We believe, however, that
the security benefits of this system strongly outweigh the drawbacks.
If the client does not perform an update regularly, there is the
possibility that keys will be just as outdated as they would be
without a keylist subscription.
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6. IANA Considerations
This document has no actions for IANA.
7. 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>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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>.
Authors' Addresses
R. Miles McCain
First Look Media
Email: ietf@sendmiles.email
URI: https://rmrm.io
Micah Lee
The Intercept
Email: micah.lee@theintercept.com
URI: https://micahflee.com/
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Nat Welch
First Look Media
Email: nat.welch@firstlook.media
URI: https://natwelch.com
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