Subject: [Ietf-krb-wg] errata for RFC 4120 5.2.6.2?

From: Ken Raeburn <[email-masked]>
Date: Thu, 2 Apr 2009 00:59:35 -0400

I don't think this has come up yet, and Jeff couldn't find it in his  
archives....

In 5.2.6.2:

The field-by-field description says the ad-checksum field uses the  
session key.

The description below that says:

    For KDC-issued elements, this is prevented because the elements are
    signed by the KDC by including a checksum encrypted using the
    server's key (the same key used to encrypt the ticket or a key
    derived from that key).  Elements encapsulated with in the KDC- 
issued
    element MUST be ignored by the application server if this  
"signature"
    is not present.

First, the checksum isn't encrypted.  (Well, for some cryptosystems it  
might be implemented that way, but not necessarily always.  And if it  
were a checksum computed and then encrypted, it would be an  
EncryptedData.)

Second, the key used is different in the two descriptions.  I think  
the server's key is the right one to use, not the session key.

Third, if the "signature" (checksum) isn't present, the message is  
simply an invalid encoding; it's not optional.  We should specify that  
the checksum has to be verified or the encapsulated elements ignored.

Fourth, "with in" should be "within".

I suggest submitting an erratum that changes the ad-checksum  
description to refer to "the server's key" in the field-by-field  
description (both times it's mentioned), and in the text above  
replaces "encrypted" with "computed", "with in" with "within", and  
"not present" with "not valid"...

Ken
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From: Sam Hartman <[email-masked]>
Date: Fri, 03 Apr 2009 15:23:44 -0400

I think the session key is used to encrypt the encapsulated elements
and the checksum is keyed with the ticket key.