Message-Authenticator for CoA/DM vs Access Request/Accounting/Status
peter at crypt.co.nz
Mon Jun 9 13:52:39 CEST 2014
I've been reading through RFC 2869 section 5.14
And RFC 5176 Section 3.4
And from how I understand it and how this is implemented by FreeRadius in
3.0.3 and 2.2.5.
Access-Request or Accounting or Status Messages:
The Message-Authenticator is calculated after the packet Authenticator has
been generated using a HMAC-MD5 with the Message-Authenticator being a zero
16 byte string and then the HMAC-MD5 calculated using the shared secret as
the key then replaced over the 16 zero byte field with the calculated value.
So a packet would look like this if the User-Name was "bob" with password
"hello" and the shared secret of "testing123"
Request to HMAC-MD5 calculate:
Whereas because of this line in RFC5176 with my emphasis on the word before:
"The Message-Authenticator Attribute is calculated and inserted in the packet
*before* the Request Authenticator is calculated."
So this means that the Message-Authenticator HMAC value should be
calculated on the assumption the Packet Authenticator is all zero bytes so
it would look something like this:
And then the Packet Authenticator and the Message-Authenticator gets added
in and you end up with a packet like this:
Is this correct? As that is how it seems to be working for me. And I just
wanted to make sure I was approaching this correctly. As it seems a little
strange that the CoA/DM messages would prefer to have a null Authenticator
message when calculating a Message-Authenticator. But it seems to be the
way it is.
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