Re: [Mathmesh] [Cfrg] A different approach to key escrow
Nico Williams <nico@cryptonector.com> Mon, 09 September 2019 16:38 UTC
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Date: Mon, 09 Sep 2019 11:38:14 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Phillip Hallam-Baker <phill@hallambaker.com>
Cc: Michael Richardson <mcr+ietf@sandelman.ca>, mathmesh@ietf.org, cfrg@irtf.org
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Subject: Re: [Mathmesh] [Cfrg] A different approach to key escrow
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On Thu, Sep 05, 2019 at 10:26:22PM -0400, Phillip Hallam-Baker wrote: > On Wed, Sep 4, 2019 at 2:46 AM Michael Richardson <mcr+ietf@sandelman.ca> > wrote: > > Nico Williams <nico@cryptonector.com> wrote: > > > I don't see why in the first case you'd need a separate > > > service to store the private keys encrypted in the master > > > secret. All the entities that hold secret shares can also > > > hold copies of the encrypted record along with the secret > > > shares. > > > > This allows the secrets to split and sent to appropriate entities > > once at the beginning of the process, and they never need to be > > updated again. Alice can generate as many keys (and as frequently) > > as she wishes, interacting only with the escrow service. Assuming > > that I understand correctly. If you use a master secret to to generate all the others, indeed. The first scheme, however, requires storing updated encrypted secrets payloads. That's the "service" that we'd asked about, but I assumed the services that store secret shares would also store those payloads. > > > However, you might find it difficult to generate key pairs > > > from a master secret, especially if you ever need to generate > > > keys on a hardware token. If you won't have that requirement, > > > and if you're willing to have that as an anti-requirement, > > > then the second scheme is simpler, indeed, and the entities > > > that store key shares will need to store smaller records. > > > > You are saying: If you use a hardware token, then you need the escrow > > service. > > Isn't it the case that some hardware tokens have no mechanism to get the > > private key out? I guess they are SOL regardless of scheme. Yes, though I expect wrapped key export to be generally available, still: > I agree. If someone is using a hardware token, the hardware token needs to > manage the escrow mechanism itself (e.g. like the way Luna cards do). But > it will in any case need a way to import a private master key generated > elsewhere. not supporting such tokens is a reasonable thing to do. > > Couldn't we generate per-key escrow keys from the master key using > > the same KDF mechanism? Doesn't that get us the best of both worlds, > > with the confusion as to how the different That enlarges the amount of secret share storage needed, which gets us back to Phillip's desire to avoid that. A nice feature of this scheme is that there's a fixed number of fixed-sized keyshares generated early on and this need never change unless you want to re-key everything. Speaking of which, how does master secret change work? > Hmm... that might be the way to do it. The only problem is that then you > need access to the offline master to be able to generate application key > sets and the idea of the offline master is that you should only need to > ever use it when changing your administration device. Nico --
- [Mathmesh] A different approach to key escrow Phillip Hallam-Baker
- Re: [Mathmesh] A different approach to key escrow Michael Richardson
- Re: [Mathmesh] A different approach to key escrow Phillip Hallam-Baker
- Re: [Mathmesh] [Cfrg] A different approach to key… Nico Williams
- Re: [Mathmesh] A different approach to key escrow Michael Richardson
- Re: [Mathmesh] A different approach to key escrow Phillip Hallam-Baker
- Re: [Mathmesh] A different approach to key escrow Michael Richardson
- Re: [Mathmesh] A different approach to key escrow Phillip Hallam-Baker
- Re: [Mathmesh] [Cfrg] A different approach to key… Nico Williams
- Re: [Mathmesh] [Cfrg] A different approach to key… Phillip Hallam-Baker