Re: [dnsext] Clarifying the mandatory algorithm rules

[Ondřej Surý <[e-mail]>]

Wouter, Jelte,

On 18.11.2010 15:33, Jelte Jansen wrote:
> IMHO, we should either do downgrade protection for all cases

I suggest we properly define what a "downgrade protection" is and how 
does it protect the zone.  I've fallen for this trap too and it helped 
me a lot to write it down.

Let's define (I skip KSK and ZSK for sake of sanity and I skip RFC5011 
for the same reason):

Algorithm A is supported.
Algorithm B is also supported.

Zone X has following properties:
   1) DS record for algorithm A
   2) DNSKEY in apex for algorithm A - DNSKEY(A)
   3) Signatures with DNSKEY(A)

Zone Y has following properties:
   1) DS record for algorithm A
   2) DNSKEY in apex for algorithm A - DNSKEY(A)
   3) DNSKEY in apex for algorithm B - DNSKEY(B)
   4) Signatures with DNSKEY(A) and DNSKEY(B)

What you are saying is that Zone Y is more secure than Zone X.  And I 
don't think so, because there is a step when you rely only on algorithm A.

Now let's add:

Zone Z has following properties
   1) DS record for algorithm A
   2) DS record for algorithm B
   3) DNSKEY in apex for algorithm A - DNSKEY(A)
   4) DNSKEY in apex for algorithm B - DNSKEY(B)
   5) Signatures with DNSKEY(A) and DNSKEY(B)

I think that only Zone Z is more secure than Zone X and Zone Y.  And 
because you don't have to keep both DS records for algorithm A and 
algorithm B in the parent zone (ie. you can just exchange the DS) then 
the key algorithm rollover can be less painful.

I would even suggest that the condition of "having the chain of trust" 
from TA to the key with a specific algorithm also applies for Sam's case 
for introducing the new "unknown" algorithm.  But I haven't given it 
much thought and I rather thing it through more before coming to any 
conclusion.

Ondrej
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  Ondřej Surý
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