add support for WKS

[azet]

If we opt to implement MTA-STS as is, we need to serve HTTPS, as a further enhancement we could add support for WKS (OpenPGP Web Key Service) which would allow users to automagically look up OpenPGP keys of a given e-mail address local to the MTA.

Implementation should be rather straight-forward. WKS key discovery roughly works like that:

For example the URI to lookup the key for [email protected] is:

  https://example.org/.well-known/openpgpkey/
  hu/iy9q119eutrkn8s1mk4r39qejnbu3n5q

(line has been wrapped for rendering purposes)

which is served simply via a /.well-known URL, where the account (e-mail address) is encoded as follows: z-base-32(SHA1(${address-local_part})).

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• z-base-32: "human-oriented base-32 encoding"
• RFC5785: "Defining Well-Known Uniform Resource Identifiers (URIs)"