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Re: [resend] Use of DNS to distribute keys
> From: atkinson@itd.nrl.navy.mil (Ran Atkinson)
> To: ipsec@ans.net, namedroppers@nic.ddn.mil
> > For several years now I've been thinking that the DNS is
> >probably a really good way to distribute keys (or key certificates).
> >For example, if each host had a public key accessible via the DNS, one
> >could more easily setup a secure session key between oneself and the
> >remote host that one wished to communicate with. Also, one might be
> >able to encrypt UDP packets using asymmetric encryption for the odd
> >case where one only wanted to send one or two packets and thereby
> >avoid the overhead of setting up a session key for extremely brief
> >sessions.
>
> This is a great idea I have also had myself. Key certificates are
> generally too big and clunky to be in DNS but public keys would work
> fine. There is no reason for the keys stored in DNS to be embedded in
> a certificate because you can use secure communication with the DNS
> server based on the key from the next highest level in the DNS
> hierarchy. Caching these keys is kind of like caching IP address
> info.
This, obviously, is the way to go. So I have surprised to have received
private mails saying that we don't need secure DNS because we have key
certificate mechanism.
Some people does not understand that key certificate mechanism does not
scale unless a tree of servers are formed.
> All you need to complete the picture is to magicly know (or get
> via an e-mailed certificate or something) the public keys of the root
> DNS servers.
And, as we need public keys to construct the DNS tree, we don't need
any key certificates of servers.
> A 1024 bit RSA key, which most people consider secure, is only 128
> bytes. An appropriate RSA digital signature is going to be about the
> same size. I guess I should do the detailed arithmetic but it seems
> to me like a public key containing DNS response should fit into the
> DNS 512 bytes UDP limit.
I have found a exception. A reply packet for NS query will contain, as
glue information, addresses AND public keys of multiple name servers.
Thus the 512 bytes limit does matter if there is three name servers with
glue information (quite common).
It should be noted that the NS reply for the root name servers has
once exceeded the UDP limit even without any public keys.
So, if we must extend UDP size limit or must use TCP.
Masataka Ohta
References: