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Network Working Group                                          M. McCain
Internet-Draft                                                       FLM
Intended status: Standards Track                                  M. Lee
Expires: March 5, 2020                                                TI
                                                                N. Welch
                                                                  Google
                                                       September 2, 2019


Distributing OpenPGP Key Fingerprints with Signed Keylist Subscriptions
                        draft-mccain-keylist-05

Abstract

   This document specifies a system by which an OpenPGP client may
   subscribe to an organization's public keylist to keep its keystore
   up-to-date with correct keys from the correct keyserver(s), even in
   cases where the keys correspond to multiple (potentially
   uncontrolled) domains.  Ensuring that all members or followers of an
   organization have their colleagues' most recent PGP public keys is
   critical to maintaining operational security.  Without the most
   recent keys' fingerprints 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 March 5, 2020.







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Copyright Notice

   Copyright (c) 2019 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 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  . . . . . . . . . . . . . . . . . .   4
     2.1.  Subscribing to Keylists . . . . . . . . . . . . . . . . .   4
     2.2.  Automatic Updates . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Cryptographic Verification of Keylists  . . . . . . . . .   6
   3.  Data Element Formats  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Keylist . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Signature . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Well-Known URL  . . . . . . . . . . . . . . . . . . . . .   8
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Benefits . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Relation to Other Technologies  . . . . . . . . . . . . . . .   8
     6.1.  Web Key Directories . . . . . . . . . . . . . . . . . . .   9
     6.2.  OPENPGPKEY DNS Records  . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document specifies a system by which clients may subscribe to
   cryptographically signed 'keylists' of public key fingerprints.  The
   public keys do not necesssarily all correspond to a single domain.
   This system enhances operational security by allowing seamless key
   rotation across entire organizations without centralized public key



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   hosting.  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.

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 fingerprint type SHOULD be V4).

   The term "keylist" is defined as a list of OpenPGP public key
   fingerprints accessible via a URI in the format specified in
   Section 3.  Keylists SHOULD be treated as public documents, however a
   system administrator MAY choose, for example, to restrict access to a
   keylist to a specific subnet or private network.

   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
   firstlookmedia/Keylist-RFC [1].





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   We are still considering whether this Draft is better for the
   Experimental or Informational track.  All feedback is appreciated.

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 (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 public key; the
   authority public key fingerprint 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.

   The client MAY provide the option to perform all its network activity
   over a SOCKS5 proxy (see [RFC1928]).

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).





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   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.  If a current
       copy (i.e. not from local cache) cannot be obtained, the client
       SHOULD abort the update for this keylist and notify the user.
       The client SHOULD continue the update for other keylists to which
       it is subscribed, notwithstanding also failing the criteria
       described in this section.

   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.  If a current copy cannot be obtained, the client
       SHOULD abort the update and notify the user.  The client SHOULD
       continue the update for other keylists to which it is subscribed,
       notwithstanding also failing the criteria described in this
       section.

   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.  The client SHOULD
       continue the update for other keylists to which it is subscribed,
       notwithstanding also failing the criteria described in this
       section.

   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 either the
       key entry, the keylist (see Section 3) or, if the keylist
       specifies no keyserver, from the user's default keyserver.  If
       the public key cannot be found for a particular fingerprint, the
       client MUST NOT abort the entire update process; instead, it
       SHOULD notify the user that the key retrieval failed but
       otherwise merely skip updating the key and continue.  If the key



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       is already present and not revoked, refresh it from the keyserver
       determined in the same manner as above.  If it is present and
       revoked, do nothing for that particular key.

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 a HKP keyserver from which the OpenPGP keys in the
   keylist should be retrieved.  Each PGP key listed in the keylist MAY
   have a "keyserver" JSON key; if a PGP key in the keylist specifies a
   HKP keyserver that is different from the one described in the
   metadata object, the PGP key-specific keyserver should be used to
   retrieve that particular key (and not the key listed in the metadata
   object).

   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.




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   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" (the name of
   the PGP key's owner), "email" (an email of the PGP key's owner),
   "keyserver" (a HKP keyserver from which the key should be retrieved),
   and "comment" key-value pairs, as well as any other key-value pairs.

   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",
             "keyserver": "https://keys.openpgp.org/"
       },
       {
         "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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3.3.  Well-Known URL

   Keylists SHOULD NOT be well-known resources [RFC4880].  To subscribe
   to a keylist, the client must be aware not only of the keylist's
   location, but also of the fingerprint of the authority public key
   used to sign the keylist.  Furthermore, because keylists can
   reference public keys from several different domains, the expected
   host of the well-known location for a keylist may not always be self-
   evident.

4.  Implementation Status

   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 Benefits

   The keylist subscription functionality defined in this document
   provides a number of security benefits, including:

   o  The ability for new keys to be quickly distributed across an
      organization.

   o  Removing 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 without centralized public key hosting.

6.  Relation to Other Technologies








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6.1.  Web Key Directories

   Unlike Web Key Directories, keylists are not domain specific.  A
   keylist might contain public key fingerprints for email addresses
   across several different domains.  Moreover, keylists only provide
   references to public keys by way of fingerprints; Web Key Directories
   provide the public keys themselves.

6.2.  OPENPGPKEY DNS Records

   A keylist MAY reference public keys corresponding to email addresses
   across several different domains.  Because managing OPENPGPKEY DNS
   Records [RFC7929] for a particular domain requires control of that
   domain, the OPENPGPKEY DNS Record system is not suitable for cases in
   which keys are strewn about several different domains, including ones
   outside of the control of an organization's system adminitrators.

7.  Security Considerations

   There is a situation in which keylist subscriptions could pose a
   potential security threat.  If 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.

   Some organizations may wish to keep their keylists private.  While
   this may be achievable by serving keylists at URIs only accessible
   from specific subnets, keylists are designed to be public documents.
   As such, clients may leak the contents of keylists to keyservers --
   this specification ensures to the best of its ability the integrity
   of keylists, but not the privacy of keylists.

8.  IANA Considerations

   This document has no actions for IANA.

9.  References

9.1.  Normative References

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,
              <https://www.rfc-editor.org/info/rfc1928>.





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   [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>.

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/info/rfc7929>.

   [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>.

9.2.  URIs

   [1] https://github.com/firstlookmedia/keylist-rfc

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
   Google

   Email: nat@natwelch.com
   URI:   https://natwelch.com














































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