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Re: [ietf-smtp] DKIM encryption, was Request for discussion

2013-10-19 17:53:59
> multipart/encrypted, in other words. See RFC 1847.

Ahem, does let you encapsualte a message/rfc822 and encrypt it, doesn't
it?  It even works in ordinary MUAs.

No, the purpose of multipart/encrypted is to separate the encrypted content
from the encryption metadata. You can put anything, including a message/rfc822
part, inside, but you could do that with any random encypted container.

There are various advantages to having the separation. For example, you could
generate multiple types of metadata for the same encrypted payload, in which
case the metadata part could be a multipart/alternative. There are streaming
applications for bandwidth limited devices: Fetch the first part, decrypt the
interchange key, and then provide that key to a display that then uses URLFETCH
to pull the data, decrypt it, and display it.

That said, I don't any of the advantages of multipart/encrypted coming into
play in a scenario where the message is encrypted by the MSA and decrypted
by the MDA. It's really only useful in end-to-end cases.

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