Subject: Re: [keyassure] Using trust anchor language

From: Stephen Kent <[email-masked]>
Date: Wed, 16 Feb 2011 19:23:06 -0500

Sorry for the very tardy reply.

>  > -----Original Message-----
>>  From: Stephen Kent [mailto:[email-masked]]
>>
>>  Sorry for being dense, but can you explain how the SNI extension and IP
>>  exhaustion relate to the issue of extracting a DNS name from a cert vs.
>>  inferring it from the DNS record?
>
>Ordinarily, in order to host multiple TLS-endpoints (web virtual 
>hosting) on a single-IP, you have two choices:
>
>1. A cert with multiple SANs
>2. use SNI so a client can indicate which end-endpoint (hostname) it 
>is trying to reach.
>
>#1 is extremely problematic when virtual-hosting services for 
>different entities, or lots of entities.  For example, CDNs will 
>often try to virtual a number of websites at the same IP, and will 
>have a certificate with SANs that span a multitude of 
>organizations/businesses.
>
>#2 is problematic because SNI isn't supported in Windows-XP.
>
>Currently, #1 or a single-IP per "website/hostname" is required.
>
>With TLSA if we don't have to embed the hostname in the certificate 
>offered during the TLS handshake, we don't care about SNI if a 
>client supports TLSA.  Now, what client will support TLSA and not 
>SNI, well, that's why this isn't really that big a deal I suppose..

Thanks for the clarification of the TLS problem generated by virtual 
hosting, used with older versions of WIndows.

In DANE there is no issuance cost for having a lot of SANs in the 
server cert, or for changing the cert to add or remove SANs, but I 
admit that it could get big if there are a lot of sites hosted on the 
same machine.

If there are no attacks that arise if we ignore the name in a cert, then
we could do so as a way of avoiding this problem, and so I understand 
your motivation.

However, if the goal of DANE is to provide a cert to be used as a TA, 
to verify an EE cert for a web site, this is a problem, as it would 
interfere with cert chaining from the TA to the EE cert. So we do 
have some tradeoffs to consider.

Steve