Subject: [dhcwg] 3315 - Reconfigure Key Protocol

From: "Bernie Volz \(volz\)" <[email-masked]>
Date: Tue, 26 Dec 2006 14:18:58 -0500

Ralph, et al:

I presume that the reconfigure key should be be part of the HMAC-MD5
calculation. However, in the text below it never says how to do that
calculation -- I would have thought that the key should be placed in the
Auth option's HMAC-MD5 field but it says below to zero that field? That
seems wrong to me?

21.5.2. Server considerations for Reconfigure Key protocol

   The server selects a Reconfigure Key for a client during the
   Request/Reply, Solicit/Reply or Information-request/Reply message
   exchange.  The server records the Reconfigure Key and transmits that
   key to the client in an Authentication option in the Reply message.

   The Reconfigure Key is 128 bits long, and MUST be a cryptographically
   strong random or pseudo-random number that cannot easily be
   predicted.

   To provide authentication for a Reconfigure message, the server
   selects a replay detection value according to the RDM selected by the
   server, and computes an HMAC-MD5 of the Reconfigure message using the
-> Reconfigure Key for the client.  The server computes the HMAC-MD5
-> over the entire DHCP Reconfigure message, including the
-> Authentication option; the HMAC-MD5 field in the Authentication
-> option is set to zero for the HMAC-MD5 computation.  The server
   includes the HMAC-MD5 in the authentication information field in an
   Authentication option included in the Reconfigure message sent to the
   client.

"The server computes the HMAC-MD5 over the entire DHCP Reconfigure
message, including the Authentication option with the Auth-Info Type of
2 and the Reconfigure Key in the HMAC-MD5 field. The server then places
the resulting computed HMAC-MD5 value in the HMAC-MD5 field."

I guess this is another errata for 3315bis?

---

The client would then copy the HMAC-MD5 value out of this area, place
the Reconfigure Key there, do the calculation and if the computed
HMAC-MD5 value matches the copied out value, the Reconfigure message
authenticates.

- Bernie

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