Re: Gen-ART review of draft-ietf-pkix-rfc2560bis-15

I have given this a go by expanding the note as follows:

NOTE: The "revoked" state for known non-issued certificate serial
         numbers is allowed in order to reduce the risk of relying
         parties using CRLs as a fall back mechanism, which would be
         considerably higher if an "unknown" response was returned. The
         "revoked" status is still optional in this context in order to
         maintain backwards compatibility with deployments of RFC 2560.
         For example, the responder may not have any knowledge about
         whether a requested serial number has been assigned to any
         issued certificate, or the responder may provide pre produced
         responses in accordance with RFC 5019 and, for that reason, is
         not capable of providing a signed response for all non-issued
         certificate serial numbers.


Does this solve you issue.
I think this is as far as dare to go without risking a heated debate.

/Stefan


On 3/27/13 5:08 PM, "Black, David" <david(_dot_)black(_at_)emc(_dot_)com> wrote:

Stefan,

Is this important enough to do that?


IMHO, yes - the "running code" aspects of existing responder
behavior/limitations
are definitely important enough for an RFC like this that revises a
protocol spec,
and the alternatives to "revoked" feel like an important complement to
those
aspects (discussion what to do instead when responder
behavior/limitations are
encountered).

I appreciate the level of work that may be involved in capturing this, as
I've had my share of contentious discussions in WGs that I chair - FWIW,
I'm currently chairing my 4th and 5th WGs.  OTOH, when a WG has put that
much
time/effort into reaching a (compromise) decision, it really is valuable
to record why the decision was reached to avoid recovering that ground
in the future and (specific to this situation) to give implementers some
more context/information on how the protocol is likely to work in
practice.

Thanks,
--David

-----Original Message-----
From: Stefan Santesson [mailto:stefan(_at_)aaa-sec(_dot_)com]
Sent: Wednesday, March 27, 2013 11:38 AM
To: Black, David; sts(_at_)aaa-sec(_dot_)com; mmyers(_at_)fastq(_dot_)com; 
ambarish(_at_)gmail(_dot_)com;
slava(_dot_)galperin(_at_)gmail(_dot_)com; 
cadams(_at_)eecs(_dot_)uottawa(_dot_)ca; gen-art(_at_)ietf(_dot_)org
Cc: pkix(_at_)ietf(_dot_)org; Sean Turner; ietf(_at_)ietf(_dot_)org
Subject: Re: Gen-ART review of draft-ietf-pkix-rfc2560bis-15

I could.

My worry is just that this is such a contentious subject and it took us
x
hundreds of emails to reach this state, that if I add more explanations,
people will start disagreeing with it and that we end up in a long
debate
on how to correctly express this.

Is this important enough to do that?

/Stefan

On 3/27/13 3:30 PM, "Black, David" <david(_dot_)black(_at_)emc(_dot_)com> 
wrote:

Hi Stefan,

Does this answer your question?


Yes, please add some of that explanation to the next version of the

draft

;-).
Coverage of existing responder behavior/limitations (important "running
code"
concerns, IMHO) and alternatives to using "revoked" ("have a number of
tools
to prevent the client from accepting a bad certificate") seem

particularly

relevant.

Thanks,
--David

-----Original Message-----
From: Stefan Santesson [mailto:stefan(_at_)aaa-sec(_dot_)com]
Sent: Wednesday, March 27, 2013 7:44 AM
To: Black, David; sts(_at_)aaa-sec(_dot_)com; mmyers(_at_)fastq(_dot_)com;

ambarish(_at_)gmail(_dot_)com;

slava(_dot_)galperin(_at_)gmail(_dot_)com; 
cadams(_at_)eecs(_dot_)uottawa(_dot_)ca; gen-art(_at_)ietf(_dot_)org
Cc: pkix(_at_)ietf(_dot_)org; Sean Turner; ietf(_at_)ietf(_dot_)org
Subject: Re: Gen-ART review of draft-ietf-pkix-rfc2560bis-15

Hi David,

Yes I missed to respond to that aspect.

This is a bit complicated, but we have a large legacy to take into
account
where some responders implements just RFC 2560, while some deliver
pre-generated responses according to RFC 5019 (Light-weight OCSP). LW
responders are not capable of producing a signed response at the

time of

responding and in case such responder finds a request for a

certificate

where no pre-produced response exists, it will reply with an unsigned
error response "unauthorized", which also is a legitimate way to
respond.
So the actual OCSP responder may actually know that the certificate

was

never issued, but since it delivers pre-produced responder through a
CDN,
it can not provide a revoked response in real time.

So the major aim with the current writing is to declare that the

revoked

response is a MAY because there are other valid alternatives.

We also want to avoid putting down a SHOULD respond revoked if a
certificate is known to be not-issued, because that would require us

to

define what "known to be non-issued" actually means. And that could

be

quite tricky as OCSP responders by no means are required to have this
knowledge.

The OCSP responder simply have a number of tools to prevent the

client

from accepting a bad certificate.
This update of OCSP simply allows responders to use the "revoked"
response
as a preventive measure, without mandating it.

This is also related to activities in the CA Browser Forum where they
put
down requirements on responders complying with CAB rules to not

respond

"good" to certificates that were never issued.
With this update in OCSP, they can now mandate in their policies both
the
fact that their responders MUST know if a certificate was never

issued

and
MUST respond "revoked".

So we allow other communities to raise the bar even if the base

standard

defines the response as optional.

In theory we could possibly say that responding revoked is optional,

but

if you choose between revoked and unknown then you SHOULD favour

revoked

over unknown. But such nested requirements just feels bad and

impossible

to test compliance against. I'd much rather just leave it optional. I
think the Note gives a clear recommendation on this and the rationale
without spelling it out as a requirement.

Does this answer your question?


On 3/27/13 12:51 AM, "Black, David" 
<david(_dot_)black(_at_)emc(_dot_)com> wrote:

Hi Stefan,

This looks good - thank you for the prompt response.

It looks like my speculation on item [1] was wrong, so could you

respond

to the question below, please?:

[1] Section 2.2:

      NOTE: The "revoked" state for known non-issued

certificate serial

              numbers is allowed in order to reduce the risk of

relying

              parties using CRLs as a fall back mechanism,

which would be

              considerably higher if an "unknown" response was

returned.


Given this explanation, I'm surprised that the use of "revoked"

instead of

"unknown" for a known non-issued certificate is a "MAY"

requirement

and

not a "SHOULD" requirement.  Why is that the case?


--------------

Beyond that, the proposed actions (or proposed non-actions) on items
[2]-[5]
are fine with me, Sean's taken care of the author permissions item

from

idnits, and I assume someone has or will check the ASN.1 .

Thanks,
--David

-----Original Message-----
From: Stefan Santesson [mailto:stefan(_at_)aaa-sec(_dot_)com]
Sent: Monday, March 25, 2013 10:21 PM
To: Black, David; sts(_at_)aaa-sec(_dot_)com; 
mmyers(_at_)fastq(_dot_)com;

ambarish(_at_)gmail(_dot_)com;

slava(_dot_)galperin(_at_)gmail(_dot_)com; 
cadams(_at_)eecs(_dot_)uottawa(_dot_)ca; gen-art(_at_)ietf(_dot_)org
Cc: pkix(_at_)ietf(_dot_)org; Sean Turner; ietf(_at_)ietf(_dot_)org
Subject: Re: Gen-ART review of draft-ietf-pkix-rfc2560bis-15

Hi David,

Thanks for the review.
My reply in line.

On 3/26/13 1:25 AM, "Black, David" 
<david(_dot_)black(_at_)emc(_dot_)com> wrote:

Authors,

I am the assigned Gen-ART reviewer for this draft. For

background on

Gen-ART, please
see the FAQ at

<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.


Please resolve these comments along with any other Last Call

comments

you

may receive.

Document: draft-ietf-pkix-rfc2560bis-15
Reviewer: David L. Black
Review Date: March 25, 2013
IETF LC End Date: March 27, 2013

Summary:
This draft is on the right track but has open issues, described

in

the

review.

This draft updates the OCSP protocol for obtaining certificate

status

with some minor extensions.

Because this is a "bis" draft, I reviewed the diffs against RFC

2560.


I did not check the ASN.1.  I also did not see a writeup for this

draft

in the data tracker, and so will rely on the document shepherd to
ensure that the ASN.1 has been checked when the writeup is

prepared.


I found five open issues, all of which are minor, plus one idnits

item

that is probably ok, but should be double-checked.

Minor issues:

[1] Section 2.2:

      NOTE: The "revoked" state for known non-issued

certificate serial

              numbers is allowed in order to reduce the risk of

relying

              parties using CRLs as a fall back mechanism,

which would be

              considerably higher if an "unknown" response was

returned.


Given this explanation, I'm surprised that the use of "revoked"

instead of

"unknown" for a known non-issued certificate is a "MAY"

requirement

and

not a "SHOULD" requirement.  Why is that the case?

It appears that the reason is that the use of "revoked" in this

situation

may be dangerous when serial numbers can be predicted for

certificates

that
will be issued in the future.  If that's what's going on, this

concern

is

already explained in the security considerations section, but it

should

also be mentioned here for completeness.


No, this is not the main reason. The main reason is the one stated

as a

Note: in this section:

NOTE: The "revoked" state for known non-issued certificate serial
numbers
is allowed in order to reduce the risk of relying parties using

CRLs

as

a
fall back mechanism, which would be considerably higher if an

"unknown"

response was returned.


[2] Section 4.2.2.2:

      The key that signs a certificate's status information

need not be

the

      same key that signed the certificate. It is necessary

however to

      ensure that the entity signing this information is

authorized to
do

      so.  Therefore, a certificate's issuer MAY either sign

the OCSP

      responses itself or it MAY explicitly designate this

authority to

      another entity.

The two instances of "MAY" in the above text were both "MUST" in

RFC

2560.


The RFC 2560 text construction of "MUST" or "MUST" is a bit odd,

but

the two

"MAY"s in this draft are even worse, as they allow "MAY do

something

else

entirely", despite being enclosed in an either-or construct.  I

strongly

suspect that the latter was not intended, so the following would

be

clearer:

      The key that signs a certificate's status information

need not be

the

      same key that signed the certificate. It is necessary

however to

      ensure that the entity signing this information is

authorized to
do

      so.  Therefore, a certificate's issuer MUST do one of the

following:

              - sign the OCSP responses itself, or
              - explicitly designate this authority to another

entity.



I Agree. I will adopt your text.


[3] Section 4.3:

Is the "SHOULD" requirement still appropriate for the DSA with

SHA-1

combo

(vs. a "MAY" requirement)?  This requirement was a "MUST" in RFC

2560,

but

I wonder about actual usage of DSA in practice.


The change in algorithm requirements was provided by RFC 6277, and
further
refined in this draft in accordance with requests from Sean

Turner.


[4] Section 5, last paragraph:

      Responding a "revoked" state to certificate that has

never been

      issued may enable someone to obtain a revocation response

for a

      certificate that is not yet issued, but soon will be

issued, if
the

      CA issues certificates using sequential certificate

serial number

      assignment.

The above text after starting with the "if" is too narrow - it

should

say:


      if the certificate serial number of the certificate that
      will be issued can be predicted or guessed by the

requester.

      Such prediction is easy for a CA that issues certificates
      using sequential certificate serial number assignment.

There's also a nit in original text - its first line should be:

      Responding with a "revoked" state for a certificate that

has never

been

Good suggestions. I will update accordingly.


[5] Section 5.1.1:

      In archival applications it is quite possible that an OCSP

responder

      might be asked to report the validity of a certificate on

a date
in

      the distant past. Such a certificate might employ a

signing method

      that is no longer considered acceptably secure. In such
      circumstances the responder MUST NOT generate a signature

using a

      signing mechanism that is not considered acceptably

secure.


This could use an additional warning that certificate archival

should

not rely solely on signatures in archived certificates for

ensuring

the

validity and integrity of the archived certificates because the

signature

algorithm(s) may transition to no longer being considered

acceptably

secure at some point after the certificates are archived.


This note if I remember correctly is imported from RFC 6277,

which is

incorporated into this document. The reason behind the text is

only

to

avoid usages of insecure algorithms.
Historical validation is a real can of worms that I really would

like to

keep a tight lid on. I really want to avoid doing recommendations

in

this
space as it may trigger a whole flood of things that could be

equally

important to say about this subject.


Nits:

idnits 2.12.15 said:

 -- The document seems to lack a disclaimer for pre-RFC5378

work,

but

may

    have content which was first submitted before 10 November

2008.

If

you
    have contacted all the original authors and they are all

willing

to

grant
    the BCP78 rights to the IETF Trust, then this is fine, and

you

can

ignore
    this comment.  If not, you may need to add the pre-RFC5378
disclaimer.
    (See the Legal Provisions document at
    http://trustee.ietf.org/license-info for more information.)

This looks like it's ok because all the authors of RFC 2560 are

also

authors of
this draft, but it should be double-checked.



I defer this one to Sean. I think he has this one under control.


Thanks again for the review.

/Stefan


Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
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