RetrievalMethod

RetrievalMethod is supposed to specify a way to retrieve
the sender's public key. However, it's unspecified what
will be retrieved by dereferencing the URI in question.

Now, if the URI is HTTP, then the data will of course be typed
However, there is no standard list of MIME types for
keying material.

If the URI isn't HTTP (e.g. FTP) then the data may be 
untyped. 

I don't see that this feature is very useful unless the
core spec provides some way of specifiying what the implementation
should expect to retrieve.

-Ekr