Pubkey Distribution Concept

Feedback appreciated (use gnupg-devel@ or see EasyGpg2016).

Solution overview

To find the pubkey of a given email-address an email client should:

  1. Check its local cache
  2. Ask the mail service provider via HTTPS, as specified in (Diff between v01 and v02)
  3. Ask the mail service provider via DNS(SEC), as specified in
  4. Consult a classic pubkey server (to see if there is a single match)
  5. Resort to a fallback method using an initial email exchange.

Ask the mail service provider (MSP)

WKD draft spec 02 specifies the main features of how to ask the MSP. This part is called the web key directory (WKD).

2018-05-04: Webclients only using Javascript will not be able to do the DNS SRV record request as mentioned in draft 05 by themself. For maximum compatibility Bernhard's recommendation is to implement WKD serving without DNS SRV record for now. See gnupg-devel@ for the (ongoing) discussion.

2016-09-15: Smaller changes and additions are still being discussed, but the main features are very likely to stay, so that implementations for clients and the MSP side have started.

(attachment:web-key-service-discovery.svg: see metadata in the SVG source-code for license of compilation and sub-graphics.)

The diagram shows the most important use-case:

The request URL looks like

With XXXX being a 32-char long string constructed of the mapped local part of the email, SHA-1 hashed and z-Base-32 encoded.

The reason for using this encoding instead of a standard hex encoding is to visually distinguish such an item from a fingerprint. Furthermore, in contrast to Base-64 and other Base-32 encodings, z-Base-32 has been optimized for easier human use.

Case-insensitive local part

Almost all email servers do not decide between different casing in the email local part, though it is in the standards. Because it is so rare, it not supported. The local part is always lower-cased before the encoding.

A common example for case-insensitivity are visiting cards which capitalize the canonical lowercase mail address for easier reading.

Just one uid -- optional no comments policy

It is one email-address (via one uid) on the pubkey that gets served. If email aliases are in use, they will require a client to manage several pubkey (variants), one per email address. This design allows a simpler check (is this the email address the user submitted for?) and prevents other info to be inserted. Less desirable info would be an addition uid with an email address that the user does not control.

The optional mailbox-only policy allows the MSP to reject further contents coming with the pubkey, thus revealing less about the email address when serving the pubkey. Think about the real-name or insults in the pubkeys' UID.

Proxy to your mailprovider from your webserver

If your MX record and A records are served by different providers the recommendation is to proxy/redirect only the .well-known/openpgpkey URL to the webserver of the MSP. Publishing recipes for common web-servers configurations may ease this. (Some have pointed out that this may be considered an added layer of control for the domain owner.)

Manage pubkey by email

The client will send an email to a submission email-address, may get a challenge email back to prove that it is in possession of the private key. Handling is done automatically without user interaction. This is called the web key service (WKS).

The draft spec 02 also shows how the pubkey publication shall be managed between email client and provider.

2016-10-07: Some changes - mainly additions - are in discussion, like:

Fallback method

image:web-key-service-fallback.svg attachment:web-key-service-fallback.svg, see metadata in the SVG source-code for license of compilation and sub-graphics.

The diagram shows how encryption will be enabled in case the MSP does not offer the "web key service":

Design goals and constraints

According to user story T1 in EasyGpg2016/VisionAndStories we want to find a single pubkey for a given email address even if there has not been a previous email exchange. It has to work without user interaction (to the extend possible). In T2 and T3 we think about what happens if the communication partner is not ready for this communication.

Assuming both partners have already been set up, we have to solve the following problems reliably without user interaction:

  1. How can I get the pubkey for the email address? ("Discovery", "pubkey distribution")
  2. How can I gain some basic level of trust that this is the current pubkey of the owner with the email address he wants me to use ("building trust" or "validation").

Within the EasyGpg-contract we are attempting a solution that is comparatively easy to implement to raise chances that it will actually widen the user base. It shall also be designed in a way that it can be introduced step by step together with existing and future solutions ("lean" introduction). The main design idea is to use the already existing relation of an email address owner to his or her mail service provider (MSP). It is an intrinsic common interest of both parties to actually provide a verified relationship so that only the owners can connect to their email storage.

The drawback of this design idea is that an man-in-the-middle attack by the MSP is a lot easier compared to the web of trust approach where several external parties are involved and users must explicitly argument about trust relations. The idea is that users with a higher security interest (like Annika or Bob) will build additional trust using advanced methods, like a mutual check of the fingerprints of their public keys. The tracked communication history also allows to detect indications of an MSP trying to attack its users.

Main design idea

How to use the trust relation to the MSP technically?

An email account owner has already trusted her email service provider to accept, deliver and possibly store her communication. Technically the connection today is made via TLS secured protocol variants of something like IMAP (receiving, access to storage), SMTP (outbound sending) or HTTP in case of a web application. In each case the user will have to authenticate to use the service.

The design idea is to use this existing trust relation and find

  1. a protocol to ask the MSP for the pubkey of a specific email address
  2. a way for email applications to give, update, and (maybe) delete their pubkey at the MSP.

Query via HTTPS instead of DNS

The first design idea in the STEED paper from 2011 was to use DNS records and later profit from DNSSEC. There is a DANE OpenPGPkey Draft.

Drawbacks of DNS are:

An advantage of DNS is that there already exist implementations (GnuPG 2.1) and some real world deployments. Some MSPs already offer DANE OPENPGPKEY.

In 2015 the design idea was revisited to use a well-known URL via an HTTPS request.

Advantages for HTTPS are:

Less attractive alternatives/documenting the design process

Using URL encoding in the request

Instead of the SHA-1 hashing and z-Base-32, would be possible to let the HTTP URL standards do the encoding for the transport and leave the responsibility for hashing at the MSP server.

Advantages are:

Disregarded because:

Only using email exchanges (and no lookup)

Possible advantages are


Considered less attractive because:

Attaching a pubkey to all emails

The idea is to always attach one pubkey to all emails the user sends out. This could help with

Considered less attractive because:

Managing by HTTPS

It would be possible to manage the uploading of pubkeys to the mailserver-provider via an authenticated HTTPS connection.

This is considered less attractive than handling by email because it is estimated to be harder to implement overall.

The advantages of authenticated HTTPS would have been:

Resorting to a central fallback server

In case if the MSP does not support a pubkey service, it was proposed to use a central fallback server.

The main advantage:

The main drawbacks:

Overall an email exchange as fallback method is preferred because it is close to a natural way users build up trust in first email exchanges and comes without the drawbacks of the central fallback server.

See more discussion at /FallbackServer).

Using classic (pub)"key" servers only

Our unattended use for sending the email would not work in many cases. It would be by pure chance that only one public user id will be found on the public servers ("discovery") and that it carries a signature from a pubkey that I already "trust" to sign others ("validation"). Anyone can upload a pubkey with a specific email address. Instead a user could use the fallback method of exchanging signed emails before communicating more sensitive contents or using a more complex validation method, for example the web of trust.

However asking keys servers is a good idea to sometimes find revocation certificates or new signatures for pubkey I already know. This is important if an MSP service drops out completly. So a key server check is part of the proposed steps.

Future other services?

It is possible that other pubkey distribution concepts will be proposed and implemented that solve the same problems in a better way. There already are some concepts of how a decentralized directory can be kept honest in public.

If an email service provider offers a different service for a very large fraction of users, GnuPG may for practical reasons adopt this method for this particular MSP.

There is a list of some known other approaches below that may become more relevant in the future. The goal of the EasyGpg2016 contract is to implement and deploy something simple in 2016 as proposed in this concept. The other approaches seems to adhere to an improved pubkey validation via the public and seem to require more players to join to be more useful than the model proposed here.

TODO , e.g.

EasyGpg2016/PubkeyDistributionConcept (last edited 2022-12-09 10:21:07 by Christoph Klassen)