On 01/15/2016 01:37 PM, Andrew Gallagher wrote:
On 15/01/16 21:02, Doug Barton wrote:
On 01/15/2016 12:21 PM, Andrew Gallagher wrote:
| I've
| worked on several projects for more than one financial institution,
| and airgaps like this are considered barely sufficient for some
| important keys. (Of course in such projects the idea of a
| certification subkey not on the airgapped machine would be
| completely unacceptable...)
That's interesting, and you have made me curious ... what's the threat
model? And what is that key certifying?
Most relevant example, a system where users can register their
authorisation keys against a semi-automated authority which signs them
for trust by a third system. The root key that certifies the automated
authority keys is offline. Essentially a private root CA.
Now, this example is using x509 rather than pgp,
Right, that's what I suspected. I have set up similar systems myself,
and I'm very familiar with security requirements there.
X.509 is very different from PGP, although I do understand that in some
ways the semantics are the same. Most particularly X.509 is used
primarily to establish trust relationships between systems, not people.
So the ability for a system to identify itself to another system,
without human review being involved, is something much more precious
that deserves a higher degree of protection.
OTOH, PGP is designed primarily to establish trust relationships between
people, with human review of the results an integral part of the process.
I read your example, and there are numerous flaws with your theoretical
threat model. Let's assume your premise, that someone could root a
laptop, and by so doing gain access to use all of the PGP keys on that
laptop (Note, I disagree with this premise, but let's grant it for
argument's sake). There is no need to deal with the certification key at
all in order to do the kind of damage you proposed. All you need to do
is sign a message that authorizes your nefarious deeds. Said attacker
would also have the ability to decrypt all manner of messages and/or
data, all of which are likely to be vastly more interesting than what
you propose.
In fact, I assert with a great deal of confidence that *for PGP*, the
certification key is the least interesting key of the bunch, and yet
it's the one that people have created this intricate protection
mechanism for.
Further I don't see signing as all that interesting either. As has been
discussed several times on this list the primary area of reliability for
signing is to make sure that the message that arrived was the one that
was sent. But it provides no guarantee about who was in control of the
key when the message was signed, whether the signer was coerced, etc. We
can infer things about these topics from our knowledge/beliefs about the
sender, but I can't think of any rational person would go along with a
request to "Pay Joe $10,000" just because the message was PGP signed.
Forget the validity of the key, that kind of request would require
serious OOB authentication.
Glossing over authentication (because there's no real use case for those
keys yet), that leaves us with encryption, and that's where it's at, my
friends. But unless you really enjoy making your life harder than it has
to be, you can't routinely use encryption with an air-gapped key, so I
remain unconvinced that there is a use case for air-gapping PGP keys.
But I'm still willing to listen. :)
Doug
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