private algorithms and the DS record

[David Blacka <davidb@verisignlabs.com>]

I was thinking about the use of private algorithms in DNSSEC the other 
day, and it struck me that they might not, strictly speaking, work.

Note that records-11 has the following text in Appendex A.1.1:

   Algorithm number 253 is reserved for private use and will never be
   assigned to a specific algorithm.  The public key area in the DNSKEY
   RR and the signature area in the RRSIG RR begin with a wire encoded
   domain name, which MUST NOT be compressed.  The domain name indicates
   the private algorithm to use and the remainder of the public key area
   is determined by that algorithm.  Entities should only use domain
   names they control to designate their private algorithms.

Fair enough, but there are no instructions for the DS record.  So how 
would a resolver be able to distinguish between a known private 
algorithm from an unknown private algorithm from the delegation?

Note this text from protocol-09, section 5.2:

   If the validator does not support any of the algorithms listed in an
   authenticated DS RRset, then the resolver has no supported
   authentication path leading from the parent to the child.  The
   resolver should treat this case as it would the case of an
   authenticated NSEC RRset proving that no DS RRset exists, as
   described above.

I.e., it is expected that a DNSSEC validator would be able to make the 
determination that it does or does not support any of the algorithms 
directly from the DS set.  As soon as a DNSSEC validator knows about one 
private algorithm (per scheme), it suddenly cannot make this determination.

Or am I missing something?

--
David Blacka    <davidb@verisignlabs.com>
Sr. Engineer    VeriSign Applied Research

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