Distributing OpenPGP Key Fingerprints with Signed Keylist Subscriptions

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 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.

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 .

This document uses the terms "OpenPGP", "public key", "private key", "signature", and "fingerprint" as defined by OpenPGP Message Format (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 . 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.

RFC Editor: please remove this section prior to publication. Development of this Internet draft takes place on GitHub at firstlookmedia/Keylist-RFC. We are still considering whether this Draft is better for the Experimental or Informational track. All feedback is appreciated.

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.

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 ), 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 ), 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 ).

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. 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. Obtain a current copy of the keylist's signature data from its URI, which is included in the keylist data format specified in . 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. 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. Validate the format of the keylist according to . 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. 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 ) 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 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.

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.

The following are format specifications for the keylist file and its signature file.

The keylist MUST be a valid JavaScript Object Notation (JSON) Data Interchange Format 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. 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 . 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" } ] }

The signature file MUST be an ASCII-armored 'detached signature' of the keylist file, as defined in OpenPGP Message Format .

Keylists SHOULD NOT be well-known resources . 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.

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>

The keylist subscription functionality defined in this document provides a number of security benefits, including: The ability for new keys to be quickly distributed across an organization. Removing 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 without centralized public key hosting.

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.

A keylist MAY reference public keys corresponding to email addresses across several different domains. Because managing OPENPGPKEY DNS Records 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.

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.

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