Subject: [Sip] RFC3261 issue: no integrity protection for ACK

From: "Jeroen van Bemmel" <[email-masked]>
Date: Wed, 27 Jun 2007 08:57:44 +0200

RFC3261 section 22.1 states:

Under an authentication scheme that uses responses to carry values
   used to compute nonces (such as Digest), some problems come up for
   any requests that take no response, including ACK.  For this reason,
   any credentials in the INVITE that were accepted by a server MUST be
   accepted by that server for the ACK.  UACs creating an ACK message
   will duplicate all of the Authorization and Proxy-Authorization
   header field values that appeared in the INVITE to which the ACK
   corresponds.  Servers MUST NOT attempt to challenge an ACK.

This means that integrity cannot be verified for the original request URI or 
any body (e.g. in case of offerless INVITE scenario) for ACKs. It also means 
that proxies and UAS need to maintain state in order to authenticate ACKs. 
Even though ACK cannot be challenged, it would still be possible for the UAC 
to calculate a digest as usual, using the same username/password as INVITE 
and the same nonce.

Is this an omission, is there any reason why this is done the way it is?

Regards,
Jeroen 

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