Re: [Acme] Short term certificates - two options

[Chris Drake <[e-mail]>]

Hi Yaron,

What is the use case for these?

Kind Regards,
Chris Drake


Wednesday, July 20, 2016, 7:51:57 PM, you wrote:

YS> Hi,

YS> At the LURK BoF this week there was some interest in having a solution
YS> where a domain owner can delegate to some other entity (which we will
YS> call "the TLS server") the authority to terminate TLS connections on its
YS> behalf, using short-term certificates. These certificates allow the
YS> domain owner to terminate the TLS server's authorization when necessary,
YS> without requiring certificate revocation - which we know doesn't work
YS> reliably. The certificates' validity is measured in days, e.g. 3 days.

YS> First, I would like to request the working group to adopt short-term
YS> certificates as a charter item.

YS> Second, I would like the group's advice in choosing between two very
YS> different approaches to this problem.


YS> Option 1: Certificate Pull

YS> This option is documented in the LURK draft [1], which will be modified
YS> to include feedback received this week, specifically to use more
YS> traditional certification request (CSR) flows. But the basic idea is
YS> very simple:

YS> 1. TLS server generates a CSR once every 3 days for www.example.com,
YS> sends it to the domain owner using an authenticated REST API.

YS> 2. Domain owner validates the CSR, forwards it to ACME server, gets back
YS> a short-term cert.

YS> 3. Domain owner returns the cert to the TLS server.

YS> If something bad happens, the domain owner simply stops forwarding
YS> requests from this particular TLS server.


YS> Option 2: Certificate Delegation

YS> This option moves more of the responsibility to the ACME server.

YS> 1. Domain owner contacts the ACME server and obtains a "delegation
YS> ticket" which is specific to the TLS server. The ticket is good for a
YS> long period, e.g. 1 year.

YS> 2. TLS server regularly contacts the ACME server, proves ownership of
YS> the delegation ticket, and receives a short-term certificate.

YS> If something bad happens, the domain owner contacts the ACME server and
YS> revokes the delegation ticket.


YS> Comparison:

YS> 1. Option 2 is clearly more complicated to specify and to implement.

YS> 2. Option 2 extends the ACME protocol. Many clients can ignore it, but
YS> servers will need to implement it.

YS> 3. Option 1 requires the domain owner to have a server available
YS> regularly, even if it is only a short REST interaction once every few
YS> days. Option 2 doesn't require any such server.

YS> 4. Option 1 looks to the ACME server as a normal cert request, and
YS> therefore will swamp the CT logs with lots of short-term certs. With
YS> Option 2, we can log to CT the issuance of the delegation ticket instead
YS> of the actual certificates.


YS> I would appreciate your input!

YS> Thanks,

YS>       Yaron


YS> [1] https://tools.ietf.org/html/draft-sheffer-lurk-cert-delegation-00