[Freeipa-devel] PKI expert needed: [dane] "DANE-TA(2) ? Full(0)" certificate chain matching?

[Petr Spacek]

Hello list,

FYI  dane at ietf.org list hosts a discussion about PKI certificate handling in 
DNSSEC world. PKIX experts are needed here, please comment (not only) on 
following draft.

Glossary for the alphabet soup below:
http://tools.ietf.org/html/draft-ietf-dane-registry-acronyms-04
http://tools.ietf.org/html/rfc6698

Mailing list:
https://www.ietf.org/mailman/listinfo/dane

Have a nice day!

Petr^2 Spacek

-------- Original Message --------
Subject: [dane] Repost: "DANE-TA(2) ? Full(0)" certificate chain matching?
Date: Wed, 26 Mar 2014 22:25:11 +0000
From: Viktor Dukhovni <viktor1dane at dukhovni.org>
Reply-To: dane at ietf.org
To: dane at ietf.org


[
     Repost, no feedback received.  Question summary:

     * Are clients supposed to check "signed-by" via bare TA keys (2 1 0) that
       have no matching certificate in the server's chain (say root TA
       left out of chain per normal PKIX practice)?

     * Are clients supposed to be able to augment the server chain with any
       (2 0 0) DANE-TA certificate obtained from DNS, if that certificate is
       missing from the server's chain?
]

I believe we have reached consensus on DANE-EE(3) end-entity X.509
cert verification:

     - No name checks based on certificate content (TLSA base domain sufficient)
     - No expiration checks based on certificate content (DNSSEC sufficient)

It is time to consider the issues for DANE-TA(2).  The first issue
is:

With records of the form:

     _25.mx.example.com. TLSA DANE-TA(2) <selector> <some-digest-alg> {blob}

     where the matching type is really a digest, not Full(0), we've
     observed that the server operator had better include the matching
     certificate in his TLS handshake certificate (chain) message,
     because the verifying client can't match a digest against a
     certificate it does not have.

What do we want to say about:

     _25.mx.example.com. TLSA DANE-TA(2) SPKI(1) Full(0) {DER-encoded key}

In this case the client still typically has no corresponding
certificate in hand, and so, if the server does not provide a
matching certificate in the TLS handshake, the client cannot
"compare" the TLSA record with the server's chain.

However, if the server's chain starts with a certificate signed
with the trust anchor key in question (obtained from the TLSA
record), the client can in principle verify the chain.

The Postfix SMTP client in fact does exactly this, uses any "IN
TLSA 2 1 0" trust anchor keys to verify "signed-by" even when the
server does not present a corresponding TA issuing certificate in
his chain.

So the question is:

     - Are clients expected to do this?  In other words can servers
       expect to be able to elide TA certs from their chains when
       the TA key is in a "2 1 0" TLSA record?

     - If clients are not expected to do this, there is little point
       in "IN TLSA 2 1 0", with the key also inside a certificate in
       the server's chain, it makes a lot more sense to publish
       "IN TLSA 2 1 X" for some suitable set of digests X.

A related question is whether with "DANE-TA(2) Cert(0) Full(0)",
there is again a practical need to duplicate the certificate in
the server's chain "for matching", or whether this time the server
really can avoid duplication, because the client has the cert in
hand.

Note:  The OPS draft discourages "Full(0)" due to potential issues
with DNS payload bloat, but these are nevertheless valid, and it
would be good to settle on the semantics.

-- 
	Viktor.

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