Message-ID: <513899DF.60109@fifthhorseman.net>
Date: Thu, 07 Mar 2013 08:45:03 -0500
From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
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To: IETF OpenPGP <openpgp@ietf.org>
References: <5135BDE6.1070200@fifthhorseman.net>
 <6F1173CD-290C-4A38-BD80-152C5E553D1F@jabberwocky.com>
 <B18461E9-7F88-4B85-AAD7-83E31C79DBD4@callas.org>
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Subject: Re: [openpgp] marking subkeys as constrained for specific use -- new
 key usage flags?
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On 03/05/2013 11:10 AM, Jon Callas wrote:
> In this case, we have an authentication-only subkey that's intended to =
be used for OTR. If you mark it as authentication-only, it's not going to=
 be used for document signing, which is really what you want. It's possib=
le that some other authentication protocol could grab it, but is that rea=
lly a problem?

well, yes, this was my original concern.

i wrote:
>>  (e.g. it would be bad if someone who was able to compromise my
>> OTR client and steal my OTR key was able to use the secret key materia=
l
>> to impersonate me over SSH).

We already have systems in place (e.g. monkeysphere) that permit the use
of authentication-capable subkeys for ssh systems.  so if i was to mark
my OTR key as authentication-capable, and critical notations were not
widely respected, that wouldn't turn out very well.

> This brings us to the problem with criticality. It's supposed to keep s=
ome item from being used in an unknown way. But it can also fail in unexp=
ected ways. I've seen criticality flags cause all sorts of weird issues i=
n other systems, and the usual fix is not to make it critical.

If criticality is fraught with problems, doesn't that suggest extending
the usage flags is a more responsible way to go?

or should i create a subkey with all usage flags set to 0, and then
include a notation to indicate the use?  that way, the subkey wouldn't
be used by any existing system except the ones willing to parse and
interpret the notation, regardless of its criticality.

	--dkg


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