[trill] TRILL IPsec encapsulation

Yaron Sheffer <yaronf.ietf@gmail.com> Wed, 22 July 2015 17:48 UTC

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Date: Wed, 22 Jul 2015 19:48:34 +0200
From: Yaron Sheffer <yaronf.ietf@gmail.com>
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Cc: kathleen.moriarty.ietf@gmail.com, Stephen Farrell <stephen.farrell@cs.tcd.ie>
Subject: [trill] TRILL IPsec encapsulation
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I have read the TRILL-over-IP draft (draft-ietf-trill-over-ip-03) as a kind of early SecDir review, focusing on its use of IPsec. Thanks to Donald Eastlake who graciously provided me with a crash course on TRILL. He's not responsible for any stupidity on my part, of course.

So here are some comments:

- The draft currently uses IPsec but not IKE or any kind of key management. The end result is that data is being encrypted by very long-lived keys, not enjoying the benefit of forward secrecy etc. Please use IKEv2 and do NOT use IPsec directly. RFC 4107 explains why.
- There is in fact IPsec-with-multicast, but it's not widely deployed and is based on the obsolete IKEv1. Instead, I suggest to use unicast encapsulation with IKEv2. I suppose this means that you'd want to only encapsulate data but not IS-IS frames.
- The draft currently derives encryption keys from IS-IS keys. This is problematic at several levels:
  * The IS-IS key is common to a large group of devices (a.k.a. "a group key") and so is likely to be compromised.
  * The key is used directly for encryption, as noted.
  * The key is derived using HMAC, which is specified incorrectly in the draft (one parameter instead of two).
  * The derived key is identical for all routers/links.
- I would suggest to use a derived key for authentication only, and to derive it differently for each link - although I realize that this does not raise the security level significantly. Something like: link-psk = HMAC(IS-IS-key, 6-byte-system-id-1 | 6-byte-system-id-2).
- Note that IKE generates a different encryption key for each link even if everybody is using the same authentication key (pre-shared secret). But it's still a bad practice for all principals to have the same key...
- Longer term it would improve security hugely if each router had an authenticated identity of its own. In other words, its own certificate and private key.
- Please don't define your own MTI algorithms. Just use RFC 7321.