Distributing OpenPGP Keys with Signed Keylist Subscriptions
draft-mccain-keylist-02

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Network Working Group                                          M. McCain
Internet-Draft                                                       FLM
Intended status: Experimental                                     M. Lee
Expires: April 16, 2019                                               TI
                                                                N. Welch
                                                                     FLM
                                                        October 13, 2018

      Distributing OpenPGP Keys with Signed Keylist Subscriptions
                        draft-mccain-keylist-02

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
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   This Internet-Draft will expire on April 16, 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
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   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 . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Note to Readers . . . . . . . . . . . . . . . . . . . . .   3
   2.  Functions and Procedures  . . . . . . . . . . . . . . . . . .   3
     2.1.  Subscribing to Keylists . . . . . . . . . . . . . . . . .   4
     2.2.  Periodic Updates  . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Cryptographic Verification of Keylists  . . . . . . . . .   5
   3.  Data Element Formats  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Keylist . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Signature . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  In Practice . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  Security Benefits . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Security Drawbacks  . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   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 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, which is included in the keylist data format specified in
       Section 3.

   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.

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   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
   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.

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   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"
       }
     ]
   }

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 both the authority key and the
   keylist distribution system were to 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.

6.  IANA Considerations

   This document has no actions for IANA.

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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

   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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