On 30 September 2014 19:10, Mike Jones
<Michael(_dot_)Jones(_at_)microsoft(_dot_)com> wrote:
- In Section 4.1.5, why is TLS required to fetch digitally signed
X.509 certificates?
This question was explicitly discussed by the working group at IETF 87.
The discussion is recorded in *the minutes
<http://www.ietf.org/proceedings/87/minutes/minutes-87-jose>* as follows:
If there is an x5u pointing to a certification issued by a major CA, is
TLS required for the HTTP query used to retrieve this certificate? TLS
shouldn't be needed since the certificate is a signed object. Therefore,
the "MUST" use TLS for cert retrieval should be changed to "SHOULD". This
is an application decision. Mike Jones doesn't want removal of TLS in the
case where there's no external means to verify the retrieved key. Matt
Miller: agrees with the jku case, but argues that for x5u, there is a class
of applications where it isn't known if the retrieved object is
self-protecting (like a certificate) until after it is retrieved. Even if
the object appears to be self-protecting, if the retriever doesn't have a
trusted root for that object, it might not be able to verify the protection
anyhow. So it use of TLS might still be preferable instead of having to
potentially retrieve an object twice, once over HTTP and then over HTTPS.
Joe Hildebrand wanted to know what the upside of this proposal is. Richard
says it saves on TLS handshakes; Hildebrand envisions a world where TLS is
ubiquitous. Paul Hoffman said that a similar issue in DANE ended up being
dropped after a couple of months of discussion. Richard agreed to drop the
TLS MUST to SHOULD proposal.
Russ is certainly right that for certs that chain back to a known root of
trust, integrity protection isn’t necessary. It’s also the case that this
is not true of all certificates that might be used – especially self-signed
certificates intended only to carry a key value. One possibility is to say
that TLS MUST be used unless the certificate chains back to a known trust
root accepted by the application. Of course, just requiring TLS is
simpler. Do people in the working group want to re-open this issue or are
people content with the current requirements?
I thought the requirement was confusing as well, however I thought the
rationale might be entirely different:
JOSE exists, in no small part, because of the difficulty in handling X.509
within JavaScript. Therefore even where a certificate [chain] is entirely
valid and properly signed, a JOSE client dereferencing the URI almost
certainly cannot validate the certificate - yet can validate those used in
the TLS handshake because of the difference in the API calls possible.
Indeed, it also cannot validate that the certificate's public key matches
the private key in the JSON object; making use of a trusted source more
important still.
So I do support making the x5u require a dereference over TLS, however I
also think the rationale[s] should be carefully documented in the
specification otherwise the requirement is likely to be ignored.
(More concretely, I think the specification ought to make TLS a SHOULD
here, and explain the rationale, and in particular say that it might be
relaxed only if a consuming application can perform X.509/PKIX path
validation).
Dave.