[Acme] Short term certificates - two options

[Yaron Sheffer <[e-mail]>]

Hi,

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

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

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


Option 1: Certificate Pull

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

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

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

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

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


Option 2: Certificate Delegation

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

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

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

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


Comparison:

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

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

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

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


I would appreciate your input!

Thanks,

      Yaron


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