Re: [TLS] draft-rescorla-tls-subcerts

[Andrei Popov <[e-mail]>]

Naïve question: why not simply get a constrained CA certificate and issue short-validity end entity certs? Unless I’m missing something, this would work with existing TLS implementations, no extensions required.

Short-lived credential approach seems more viable than draft-mglt-lurk-tls-requirements-00 (which requires an additional round-trip between the Edge Server and Content Provider).

Cheers,

Andrei

From: TLS [mailto:[e-mail]] On Behalf Of Eric Rescorla
Sent: Thursday, July 7, 2016 12:29 PM
To: [e-mail]
Subject: [TLS] draft-rescorla-tls-subcerts

We've talked several times about designing some sort of TLS delegation
mechanism. A few of us got together and put together some initial thoughts
about the options at:
https://www.ietf.org/id/draft-rescorla-tls-subcerts-00.txt

The general idea here is to have some mechanism for allowing what
are effectively end-entities to issue short-lived credentials that allow other
entities to act on their behalf (e.g., for CDN use cases).
Comments welcome.

In terms of the security analysis, it's obviously very important that this mechanism
not present a risk to existing TLS servers. The mechanism designed here is
intended to be future safe in that sense, though perhaps we've missed something.

I also wanted to clarify a couple points about attacks where the certificate that signs the delegated credential is also used for ordinary TLS operation (which generally is a practice that's pretty scary). As noted above it's important that existing certs not be usable this way, but maybe future certs would be.

1. It's important to construct the delegated credential in such a way that you can't use a TLS server as a signing oracle. If you choose "option 2" where you define a new structure, then it's probably sufficient to use the TLS 1.3 "context-including" digitally-signed production proposed by AGL. If you you choose "option 1" where the delegated credential is an X.509 cert, then you'd need to make some rules about fixing portions of the cert that the TLS client can't control.

2. If you're concerned about attacks like those of Jager et al. which exploit RSA decryption, what's important is that the attacker not be able to get the server to do TLS 1.2-style static RSA with the key. Playing with the usage bits definitely makes it harder to configure the server this way (because it's likely to cause bustage) but may not be enough, because sufficiently busted clients and server might be willing to use them that way anyway.

In the next rev, we'll update the draft to make these points more clearly.

-Ekr