Re: [OAUTH-WG] why are we signing?

[Brian Eaton <[e-mail]>]

On Mon, Nov 9, 2009 at 6:28 AM, John Kemp <[e-mail]> wrote:
> If we are only interested in i) [authenticating the entity] then signing any piece of the message might
> be sufficient. If we are interested in ii) [binding the signature to the message] (or some other security property)
> then we will need to identify which pieces of the message we want to provide
> that, or other, security properties for.

OK, let me try to summarize what I've heard on this thread about the
different use-cases for message signing:

- sign the HTTP request
  Used to prevent MITM from replaying token to a different URL.  Also
limits the replay attack window to minutes instead of hours.

- sign various other parts of the message
   DKIM: signs various message headers
   SIP: unspecified, just says "relevant parts of SIP request"
   XMPP: signs from, to, and purpose of message (roughly)
   OpenSocial: signs identity parameters
   Simple Web Tokens: signs identity parameters

XMPP and OpenSocial both needed to invent their own signature base
string logic in order to reuse OAuth, and I strongly suspect that the
SIP folks will need to do the same.

Several of those applications involve legitimate proxies that
legitimately munge certain parts of the message, and are supposed to
leave other parts untouched.

Cheers,
Brian