[ietf-dkim] A question about public key formats

[Steve Atkins]

The OpenSSL library gives you access to the public key of an RSA pair  
in several formats (PKCS#1, X509 and I think XML <RSAKeyValue>) and  
the perl wrapper, Crypt::OpenSSL::RSA provides access to two of those  
via functions:

get_public_key_string: Return the Base64/DER-encoded PKCS1  
representation of the public key.

get_public_key_x509_string: Return the Base64/DER-encoded  
representation of the "subject public key", suitable for use in X509  
certificates.

It's not entirely clear to me from RFC4871 which of those is the  
correct one to use - as it just points at RFC3447, which is not the  
clearest of documents, and doesn't seem to say anything relevant. If I  
had to guess, I'd say the former, as that's the form that's described  
as a PKCS#1 format.

The implementations I've seen, and the ("INFORMATIVE") examples in  
RFC4871 seem to use the latter.

I don't see how the normative section of 4871 suggests that the X509  
format is the right one to use. Can anyone point me at the line of  
reasoning there?

Cheers,
   Steve