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.

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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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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation
• 1.2. Terminology
• 1.3. Note to Readers
• 2. Functions and Procedures
• 2.1. Subscribing to Keylists
• 2.2. Automatic Updates
• 2.3. Cryptographic Verification of Keylists
• 3. Data Element Formats
• 3.1. Keylist
• 3.2. Signature
• 4. In Practice
• 5. Security Considerations
• 5.1. Security Benefits
• 5.2. Security Drawbacks
• 6. IANA Considerations
• 7. Normative References
• Authors' Addresses

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.

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.

A mailing list is available for discussion at Keylists mailing list.

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 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.
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 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"
    }
  ]
}
					

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:

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

6. IANA Considerations

This document has no actions for IANA.

7. Normative References

Authors' Addresses