Distributing OpenPGP Key Fingerprints with Signed Keylist Subscriptions
First Look Media
ietf@sendmiles.email
https://rmrm.io
The Intercept
micah.lee@theintercept.com
https://micahflee.com/
Google
nat@natwelch.com
https://natwelch.com
Security
OpenPGP
GPGSync
GPG
Keylist
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.
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.
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.
This document has no actions for IANA.