Re: [Trans] AD Review: draft-ietf-trans-rfc6962-bis-26.txt

[Rob Stradling <[e-mail]>]

I've just opened 2 PRs, which we (the authors) will deal with and then 
publish version -29.  I believe this will resolve all of the concerns 
raised in EKR's previous review.

On 20/04/18 00:46, Eric Rescorla wrote:
> Hi Paul,
> 
> I still have to review this. Will try to do so in the next week or so
> 
> -Ekr
> 
> 
> On Thu, Apr 19, 2018 at 6:30 AM, Paul Wouters <[e-mail] 
> <mailto:[e-mail]>> wrote:
> 
>     On Thu, 7 Sep 2017, Eric Rescorla wrote:
> 
>     Eric,
> 
>     Are all your concerns raised in the AD review met by version -28 ?
> 
>     The diff since your last review:
> 
>     https://tools.ietf.org/rfcdiff?url2=draft-ietf-trans-rfc6962-bis-28.txt&url1=draft-ietf-trans-rfc6962-bis-26.txt
>     <https://tools.ietf.org/rfcdiff?url2=draft-ietf-trans-rfc6962-bis-28.txt&url1=draft-ietf-trans-rfc6962-bis-26.txt>
> 
>     Paul
> 
>         Date: Thu, 7 Sep 2017 10:41:45
>         From: Eric Rescorla <[e-mail] <mailto:[e-mail]>>
>         Cc: "[e-mail] <mailto:[e-mail]>" <[e-mail]
>         <mailto:[e-mail]>>
>         To: Ben Laurie <[e-mail] <mailto:[e-mail]>>
>         Subject: Re: [Trans] AD Review: draft-ietf-trans-rfc6962-bis-26.txt
> 
> 
> 
> 
>         On Thu, Sep 7, 2017 at 3:24 AM, Ben Laurie <[e-mail]
>         <mailto:[e-mail]>> wrote:
> 
> 
>                On 5 September 2017 at 18:46, Eric Rescorla <[e-mail]
>         <mailto:[e-mail]>> wrote:
> 
> 
>                      On Tue, Sep 5, 2017 at 9:04 AM, Ben Laurie
>         <[e-mail] <mailto:[e-mail]>> wrote:
> 
> 
>                            On 4 September 2017 at 00:17, Eric Rescorla
>         <[e-mail] <mailto:[e-mail]>> wrote:
>                                  Hi folks,
> 
>         Please find enclosed the first cut of my AD review of this draft.
> 
>         Note: the original of this review is on Phabricator at:
> 
>         https://mozphab-ietf.devsvcdev.mozaws.net/D13
>         <https://mozphab-ietf.devsvcdev.mozaws.net/D13>
> 
>         If you want to see comments in context -- which is a lot easier
>         -- you
>         can go there. Also, you can create an account and respond inline if
>         you like.  If you elect to, let me know if you run into problems.
> 
>         -Ekr
> 
> 
>         Note: I have not yet reviewed the algorithms in S 2.1. I plan to do
>         that separately, but figured it would be useful to provide the
>         rest of
>         my review on the assumption that the changes to that section will be
>         modest if any.
> 
> 
>         High-Level:
> 
>         1. This document makes a variety of claims about the assurances that
>         clients get that only obtain if some as-yet-to-be-specified
>         third-party verifiability mechanism is implemented. For instance, in
>         the intro:
> 
>            Certificate Transparency aims to mitigate the problem of
>         misissued
>            certificates by providing append-only logs of issued
>         certificates.
>            The logs do not need to be trusted because they are publicly
>            auditable.
> 
> 
>         As the extensive discussion following Richard Barnes's and my
>         previous
>         comments should make clear, this is only a property of CT if you
>         also
>         have some mechanism for third-party verifiability of STHs, and this
>         document does not supply that. In the actually deployed -- we can
>         debate deployable separately the deployability of some of the
>         proposals for how to get this-- versions of CT, what clients get is
>         SCTs, which are effectively countersignatures and in fact do require
>         trusting the logs. This is implicitly acknowledged by proposals that
>         RPs only accept certificates with >1 SCT.
> 
> 
>         The purpose of multiple SCTs is to avoid the death of a single
>         log causing the death of a large number of certificates. It is
>         not about trust.
> 
> 
>         That seems like the reason for the server to offer it, but not
>         for the client to require it.
> 
>                      Line 763
>             Maximum Chain Length:  The longest chain submission the log is
>                willing to accept, if the log chose to limit it.
>         Nit: chooses
> 
> 
>         Past tense seems correct?
> 
> 
>         I'm willing to dert this to the RFC Editor.
> 
> 
> 
> 
>         Line 785
>             accepted trust anchor, using only the chain of intermediate CA
>             certificates provided by the submitter.
>         Why is this a 2119 MUST? It seems wise, but not necessarily a
>         conformance requirement
> 
> 
>         "To avoid being overloaded by invalid submissions"
> 
> 
>         There seem like one way to prevent this, but not the only one.
> 
> 
>                Line 816
>             anchor used to verify the chain (even if it was omitted from the
>             submission).  The log MUST present this chain for auditing upon
>             request (see Section 5.6).  This prevents the CA from
>         avoiding blame
>         What happens in cases of multiple chains. For instance, say that
>         the submitter provides superfluous certificates?
> 
> 
>         Not allowed.
> 
> 
>         Hmm...  So your theory is that the submitter does path construction
> 
> 
>         Yep - and that's borne out by practice.
> 
> 
>         OK, well, then it would help if the document were clearer on
>         this point.
> 
> 
> 
> 
> 
>         Line 837
>                 opaque LogID<2..127>;
>         This seems to be the first use of the TLS specification
>         language, but I don't see a cite. Please provide one,
> 
> 
>         See s1.2.
> 
> 
>         OK. Thanks.
> 
> 
> 
>         Line 926
>                 opaque TBSCertificate<1..2^24-1>;
>         NIT: there's no actual way a TBSCertificate can be 1 byte.
> 
> 
>         Line 948
>             "tbs_certificate".  The length of the "issuer_key_hash" MUST
>         match
>             HASH_SIZE.
>         Is this true? What happens if we have two CAs that share a key?
> 
> 
>         Eh?
> 
> 
>         Say that a CA spins up two subordinates that happen to share the
>         same key. I agree it's foolish.
>           return an empty
> 
> 
> 
>         Line 1522
>             permissible.  These entries SHALL be sequential beginning
>         with the
>             entry specified by "start".
>         How does the client know which of the above two cases has occurred?
> 
> 
>         The response includes an STH, which says how big the tree is.
>         Probably should be the latest one known to that server.
> 
> 
> 
>         Line 1552
>             TLS servers MUST use at least one of the three mechanisms listed
>             below to present one or more SCTs from one or more logs to
>         each TLS
>         This needs to somehow be clear that it only applies to TLS
>         servers that are compliant with this specification, as it's not
>         a new requirement on all
>         TLS servers.
> 
> 
>         Surely its the other way round: i.e. new requirements on all TLS
>         servers have to be made clear?
> 
> 
>         I'm not sure what you mean. This document does not get to
>         require that all TLS servers do CT. Was that your intent?
> 
> 
>         What I meant is that if every time you mention a thing you have
>         to say "but only if you are conforming to this RFC" then you a)
>         make the whole thing a lot more
>         cumbersome, b) are stating the obvious (i.e. that you only have
>         to conform to the RFC if you have decided to conform to the RFC).
> 
> 
>         I don't think it's merely stating the obvious in that we have
>         *other* RFCs which actually do attempt to retrospectively impose
>         requirements on all uses of TLS. See, for
>         instance: https://tools.ietf.org/html/rfc7465
>         <https://tools.ietf.org/html/rfc7465>. And this is a different
>         story, in that we are not (I assume) going to go around saying
>         that TLS stacks that don't do CT are not
>         TLS conformant.
> 
>         I think the text I would use here would be "CT-using TLS servers
>         MUST...."
> 
> 
>                Surely if you are making a new requirement for TLS
>         compliance then you have to explicitly say so?
> 
> 
>         Well, my point is that the way that this document is written in
>         fact does so, and so you have to look at "Updates" to find out
>         that that's not the case.
> 
>         -Ekr
> 
> 
> 
> 
> 
>         -EKr
> 
> 
> 
>         Line 1595
>             been struck off for misbehavior, has had a key compromise,
>         or is not
>             known to the TLS client).  For example:
>         Maybe replace "For example:" with "Some ways this can happen are..."
> 
> 
>         Line 1599
>                misissuance from clients.  Including SCTs from different logs
>                makes it more difficult to mount this attack.
>         Assuming that the server is malicious, why would it include
>         multiple SCTs? It seems like requiring multiple SCTs does in
>         fact provide this defense,
>         but that's not an argument for servers to provide multiples.
> 
> 
>         Line 1627
>                       SerializedTransItem trans_item_list<1..2^16-1>;
>                   } TransItemList;
>         Structurally, it's kind of a mess to have this be the place that
>         you make TransItems self-contained (by having a defined length
>         field). What about
>         other places I might want to concatenate TransItems. Why don't
>         you instead make TransItem self-contained, like so:
> 
>         struct {
>                    VersionedTransType versioned_type;
>                    uint16 length;   // NEW
>                    select (versioned_type) {
>                        case x509_entry_v2:
>         TimestampedCertificateEntryDataV2;
>                        case precert_entry_v2:
>         TimestampedCertificateEntryDataV2;
>                        case x509_sct_v2: SignedCertificateTimestampDataV2;
>                        case precert_sct_v2:
>         SignedCertificateTimestampDataV2;
>                        case signed_tree_head_v2: SignedTreeHeadDataV2;
>                        case consistency_proof_v2: ConsistencyProofDataV2;
>                        case inclusion_proof_v2: InclusionProofDataV2;
>                    } data;
>                } TransItem;
>         This is pretty much the universal TLS convention.
> 
> 
>         Line 1649
>         6.4.  transparency_info TLS Extension
>         This extension appears not to have any explicit support for CT
>         entries for intermediate certs. Am I just supposed to glue
>         together all the TransItems?
> 
> 
>         Line 1651
>             Provided that a TLS client includes the "transparency_info"
>         extension
>             type in the ClientHello, the TLS server SHOULD include the
>         You need to provide an actual definition of what the client
>         includes, and having the server ignore the contents is bad mojo.
>         TLS convention is for the
>         client to include an empty extension and the server to validate
>         that it is in fact empty.
> 
> 
>         Line 1654
>             "transparency_info" extension in the ServerHello with
>             "extension_data" set to a "TransItemList".  The TLS server
>         SHOULD
>             ignore any "extension_data" sent by the TLS client. 
>         Additionally,
>         IMPORTANT: The normative language here is kind of confusing. It
>         SHOULD include the extension but if it's included, it MUST
>         consist of TransItemList,
>         no? And surely only SHOU
>         Also, I'm not sure this is the right logic. If the server knows
>         that it has the SCT information in the certificate or in OCSP,
>         why SHOULD It send this
>         extension. I would think, rather that servers should aim to send
>         information at most once, so that it should only send the
>         extension if it contains
>         information that's not in the cert/OCSP. as it pretty much has
>         to send those anyway. Otherwise, don't we just end up in a world
>         where if this info is
>         in OCSP and certs, it's always sent twice, because the client
>         doesn't know where the info is, and so has to always offer the
>         extension.
> 
> 
>         Line 1658
>             session is resumed, since session resumption uses the original
>             session information.
>         Does this mean the client MUST abort the handshake if the server
>         includes it?
> 
> 
>         Line 1668
>             o  The TLS client includes the "transparency_info" extension
>         type in
>                the ClientHello.
>         This condition is non-sensical, because if the client *doesn't*
>         include the extension, the server cannot send the
>         transparency_info extension at all.
> 
> 
>         Line 1722
>         8.  Clients
>         Given the imminent standardization of TLS 1.3, you need to
>         somehow provide a mapping for client-side CT for that, I think
> 
> 
>         Line 1739
>             view.  The exact mechanisms will be in separate documents,
>         but it is
>             expected there will be a variety.
>         Given the somewhat science fictional status of Gossip, this
>         entire paragraph should be stricken
> 
> 
>         Line 1747
>             MUST implement all of the three mechanisms by which TLS
>         servers may
>             present SCTs (see Section 6).  TLS clients MAY also accept
>         SCTs via
>             the "status_request_v2" extension ([RFC6961]).  TLS clients that
>         IMPORTANT: This also needs to be rewritten so it makes clear
>         it's not a general levy on TLS clients.
> 
>         Line 1770
>             In addition to normal validation of the server certificate
>         and its
>             chain, TLS clients SHOULD validate each received SCT for
>         which they
>             have the corresponding log's parameters.  To validate an
>         SCT, a TLS
>         IMPORTANT: Why is this a SHOULD and not a MUST? If you support
>         CT at all, why would you not do this?
> 
>         Line 1791
>             TLS clients MUST NOT consider valid any SCT whose timestamp
>         is in the
>             future.
>         What's the reason for this? If your clock is slightly wrong,
>         this is going to cause new certs to fail even if they otherwise
>         would have succeeded
>         (because the notBefore and notAfter are more conservative).
> 
> 
>         Line 1800
>             will disclose to the log which TLS server the client has been
>             communicating with.
>         IMPORTANT: This "Note" just mentions in passing a huge privacy
>         issue. You need to be a lot clearer about this.
> 
>         Line 1823
>             "transparency_info" and "status_request" TLS extensions in the
>             ClientHello.
>         IMPORTANT: This is not consistent with the requirements on the
>         server.
>         Trying to reconstruct the reasoning here, the client can only
>         decide that the server is noncompliant if it has given the
>         server a chance to send the
>         SCTs by every mechanism., otherwise the server might just want
>         to send the SCT some other way. However, if servers can
>         optionally ignore
>         transparency_info (it's a SHOULD above), then you can have two
>         compliant implementations with the server having a CT-compliant
>         cert and yet the client
>         declares it noncompliant. To fix this, you need to require the
>         server to respond to "transparency_info"
> 
> 
>         Line 1831
>             "CachedObject" of type "ct_compliant" in the "cached_info"
>         extension.
>             The "hash_value" field MUST be 1 byte long with the value 0.
>         You should explain why this is one byte long (that the PDU is
>         defined as having a minimum length of 1). Also the server should
>         be required to check
>         it.
> 
> 
>         Line 1842
>             watches.  It may also want to keep copies of entire logs. 
>         In order
>             to do this, it should follow these steps for each log:
>         Why is this not a 2119 SHOULD?
> 
>         Also, what does "in order to do this" refer to? Clearly not how
>         to keep copies.... Presumably, how to poll the log.
> 
> 
>         Line 1864
>             8.  Either:
>         IMPORTANT: You seem to be missing there part where you actually
>         look at the entries to verify that they don't contain bogus data
>         (e.g., certificates
>         for your domain). I get that it's implicit here, but given that
>         you provide an algorithm, that should be an explicit stage.
>         This is a pretty odd algorithm. If I understand it correctly,
>         1-4 are setup steps and then 5-9 is supposed to be repeated, but
>         I could just do this
>         once and stop at 4.
> 
> 
>         Line 1912
>             STHs it receives, ensure that each entry can be fetched and
>         that the
>             STH is indeed the result of making a tree from all fetched
>         entries.
>         IMPORTANT: How do you verify MMD?
> 
>         Line 1944
>             If it should become necessary to deprecate an algorithm used
>         by a
>             live log, then the log should be frozen as specified in
>         Section 4.13
>             and a new log should be started.  Certificates in the frozen
>         log that
>         RFC 2119 SHOULD? Isn't this a MUST, though?
> 
> 
>         Line 1958
>             "transparency_info" TLS extension.  IANA should update this
>         extension
>             type to point at this document.
>         IMPORTANT: You'll need to fill in the new field specified in
>         https://tlswg.github.io/draft-ietf-tls-iana-registry-updates/#rfc.section.6
>         <https://tlswg.github.io/draft-ietf-tls-iana-registry-updates/#rfc.section.6>
> 
>         Line 2009
>             |                                | ECDSA (NIST P-256) |    
>                  |
>             |                                | with HMAC-SHA256   |    
>                  |
>             |                                |                    |    
>                  |
>         Why are you defining both algorithms?
> 
> 
>         Line 2150
>             (with the intention of actually running a CT log that will be
>             identified by the allocated Log ID).
>         This seems like it's not a great thing to be asking an expert to
>         do, as it seems to require business arrangements. Is it really
>         that valuable to save
>         a few bytes here?
> 
> 
>         Line 2163
>             that the log has misbehaved, which will be discovered when
>         the SCT is
>             audited.  A signed timestamp is not a guarantee that the
>         certificate
>             is not misissued, since appropriate monitors might not have
>         checked
>         IMPORTANT: This is not correct, because the client does not know
>         that the monitors are verifying the data that it is. See my
>         general comments on
>         public verifiability above.
> 
>         Line 2182
>             operating correctly.  As a log is allowed to serve an STH
>         that's up
>             to MMD old, the maximum period of time during which a misissued
>             certificate can be used without being available for audit is
>         twice
>         Nit: up to the MMD old
> 
> 
>         Line 2211
>             compute the proofs from the log) or communicate with the log via
>             proxies.
>         This also seems quite handwavy in light of the facts on the ground.
> 
> 
>         Line 2237
>                and STHs can be stored by the log and served to other
>         clients that
>                submit the same certificate or request the same STH.
>         This needs to be expanded. Who is this risk against? The log or
>         someone else? If the log, what's the logs incentive?
> 
> 
>         Line 2243
>             reduce the effectiveness of an attack where a CA and a log
>         collude
>             (see Section 6.1).
>         See my comments in 6.1 about this.
> 
> 
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> 
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> 
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-- 
Rob Stradling
Senior Research & Development Scientist
Email: [e-mail]
Bradford, UK
Office: [phone]
ComodoCA.com

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