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
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