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

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This Internet-Draft will expire on February 7, 2019.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Notation
• 1.2. Terminology
• 2. Functions and Procedures
• 2.1. Subscribing to Keylists
• 2.2. Periodic 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 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] .

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.

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, 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] .

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 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.

6. IANA Considerations

This document has no actions for IANA.

7. Normative References

Authors' Addresses