This is a summary and outcome of two session of the 3rd Open PGP Email Summit discussing how to deal with Key/Email Validations.

The topic is a follow-up from OpenPGPEmailSummit201512/EmailValidation.

Some notes about the Web Key Service in GnuPG added by aheinecke which could not attend the meeting.

Terminology

  • Key Server: A server that manages keys
  • Validating Server: A server that validates keys
  • Validated-Keys Server: A server that only holds validated keys
  • Servers can have multiple roles together
  • Validation Signature: A signature that signals a successful validation

Categorizing Key and Validation Servers

Current examples:

  pure Key Server both Key- and Validating Server pure Validating Server
doesn’t add PGP signatures for signing SKS Mailvelope, Google, WKS  
adds PGP signatures for signing   GMX TNG

Better Table?:

  Key Server only holds keys validated by Validating Server adds PGP signature for validation
SKS yes      
Google yes itself    
GMX yes itself yes yes
Mailvelope yes GMX and ??? yes  
TNG     yes yes
WKS yes itself yes  

Standard Validation Signatures

The discussion went around the topic: In case servers validate email addresses and sign this validation in the key, can we establish an improved (backward compatible) signature format?

What informations should standard validation signatures hold?

So, let’s first list what we want to signal with a validation signature (and see what can we do already with standard OpenPGP signatures):

  Currently with OpenPGP Goal How?
What was validated? the (person behind a) UID the email address in a UID new field for the exact email address
How was validated? only signature/certification levels (“0: no statement”, “1: didn’t validate”, “2: casual validation”, “3: extensive validation”) open lis of keywords signaling how was validated (e.g. “enc-email-click” for “click on URL after getting an encrypted email”) new field with predefined possible values
When did the validation happen? Currently there is only the timestamp of the signature and an optional expiry date. This can be a problem if the validation happened earlier than adding the signature (e.g. when signing later another key for the same email address). A clear statement when exactly the validation happened. An expiry date still makes sense to helkp to filter out expires signatures. recommendation to always set the expiry date to e.g. 1 year and a new field for the validation date
Who validated? defined by the signing key no change here (we still want that trusting a key that represents the validation gives trust to the validated keys)
Details of the validation policy Policy URL no change here (it makes sense to give the ability to add an URL that explains the validation (policy) in details
??? ??? signed certificate timestamp

What general requirements should apply for a standard validation signatures?

So, then let’s talk about the general requirements for a new format:

  • Backward Compatibilty: The new format should be backward compatible. That is, we use the existing OpenPGP standard for signature and just add some supplementary details that would simply be ignored by existing clients.
  • Easy to identify: There should be one common aspect signaling clearly to client that this standard extension is used.
  • Easy to process: It should be as easy as possible to process the additional informations.
  • Easy to extend: Whatever we define in the first version, it should be easy to extend the concept without breaking backward compatibility.
  • KISS: Let’s start just with a fist approach, useful for itself and avoid to make it too complex by trying to be perfect.

Initial Proposed Format

  • The proposal is to standardize for email validation signatures that they
    • always expire after 1 year (for validation once each year the expiry date should be 13 month after validation)
      • So, setting this expiry date is strongly recommended
    • always have a certification check level of 2 (“casual validation”)
    • use a signature notation to provide all missing details

A signature notation is a standard existing (but not so well known) OpenPGP feature consisting of a key and a value. As an example, see among the signatures in https://hkps.pool.sks-keyservers.net/pks/lookup?op=vindex&search=0x0B7F8B60E3EDFAE3:

sig  exp2  08AB4849 2014-02-08 2015-02-08 __________ Niels Lauken
    Policy URL: http://niels.dest-unreach.be/pgp-key-signing-policy.txt
    Notation data: occasion@niels.dest-unreach.be "Zimmermann–Sassaman based key signing party at FOSDEM2014 on 2014-02-02"

Here the last row is from a signature notation, having

  • the key “occasion@niels.dest-unreach.be” and
  • the value “Zimmermann–Sassaman based key signing party at FOSDEM2014 on 2014-02-02”

Has standard signature notation for validated email addresses has:

  • the standard key validation@openpgp-email.org
    • this is what clients would look for to see whether this standard is supported
  • as value a Base64 encoded JSON structure with all necessary informations

    • For example:

      {"validation":
 {"validations": [
   { "date": "2014-12-31",
     "approach": "enc-email-click",
     "email": "nico@josuttis.de" },
  ]
 }
}

That is we would standardize the following fields:

date the date of the validation mandatory
email the email address that was validated mandatory
approach approach used for the validation (see below) mandatory

For the approach, the following values are defined:

enc-email-click sent encrypted email with URL to click on
enc-email-reply sent encrypted email, which had to be sent back after decryption with new encryption
server-owns-key server owns the email address and the private key (for servers that manage the keys for their users such as company wide key servers for their employees)
server-has-key server owns email and knows the key because the use uploaded the private key

Open: Something (in combination) with DKIM as part of the validation?

This is a draft, all is still open for discussion.

Feedback

Please send comments and feedback to Nico Josuttis, nico(at)enigmail.net (Fingerprint: CFEA 3B9F 9D8E B52D BD3F 7AF6 1C16 A70A F92D 28F5)

Updated: