Re: [core] Last Call: <draft-ietf-core-block-18.txt> (Block-wise transfers in CoAP) to Proposed Standard

[Göran Selander <[e-mail]>]

Jim,

On 2016-02-01 23:52, "Jim Schaad" <[e-mail]> wrote:

>Göran,
>
>It I always more helpful to be on the same page.

Yes, I hope we get there soon :-) Clearly we have different solutions in
mind and value the drawbacks differently.

More inline.


>
>> -----Original Message-----
>> From: Göran Selander [mailto:[e-mail]]
>> Sent: Monday, February 01, 2016 1:55 PM
>> To: Jim Schaad <[e-mail]>; [e-mail]
>> Cc: [e-mail]; [e-mail]; [e-mail];
>> [e-mail]
>> Subject: Re: [core] Last Call: <draft-ietf-core-block-18.txt>
>>(Block-wise transfers
>> in CoAP) to Proposed Standard
>> 
>> Jim,
>> 
>> Yes, apparently we do talk past each other. Inline.
>> 
>> On 2016-02-01 21:28, "Jim Schaad" <[e-mail]> wrote:
>> 
>> >Göran,
>> >
>> >You apparently have missed the main point of what I was saying in my
>> >message.
>> >
>> >I see both of these cases as having a single message to be protected:
>> >1.  I am sending a small message that does not get fragmented 2.  I am
>> >sending a large message that needs to be fragmented into 5 messages
>> >using the Block-wise transfer options.
>> >
>> >In both cases I am doing an integrity protection/encryption operation
>> >on a single message that might later be fragmented into multiple
>>pieces.
>> 
>> I don’t understand how your solution protects CoAP metadata like Code,
>> Content-Format and Uri-Path. I may be wrong but it seems you are only
>> protecting CoAP payload, which is sufficient for certain use cases, but
>>for other
>> use cases we also need to protect metadata. The blockwise draft applies
>>to both
>> kind of use cases.
>
>Remember that I am looking at dealing with this as a single message.
>There is going to be a solution for using COSE with CoAP that is
>protecting the headers in a single message.  I am assuming that this is
>the same solution that needs to be used for dealing with the fragmented
>message as well.  The set of options from the fragmented message is built
>into an external object which is then authenticated as part of the
>process of authenticating the base block.
>
>This is where the current BLOCK draft falls apart on its own merits.  It
>does not discuss what should happen if the set of options is not the same
>between all of the blocks. This is a problem if one is using security or
>not.  The correct set of headers is the set that needs to be protected,
>but there is no way to determine what the set of headers is supposed to
>be.
>
>> 
>> 
>> > I do not want to integrity protect each of the 5 block transfer
>> >messages independently because of a number of problems which you have
>> >pointed out about messages getting re-fragmented in transit.
>> 
>> My point of view is that messages should not be re-fragmented in
>>transit.
>
>Why do you believe that this is a problem?  I cannot see any problems
>with it myself.


I find it problematic with a Block option which whenever used has as
effect that the receiver cannot verify the integrity of the received CoAP
message; that requires the message to be cached; and that only after
having received an unspecified number of subsequent messages it is maybe
possible to verify the integrity of.



>
>> 
>> > Additionally, I do not want to deal with each messages separately
>> >because I do not want to be in a situation where a block could be
>> >changed for a different block from a different stream.
>> 
>> This is not an issue if you integrity protect the appropriate Block
>>option, which
>> contains the number of the block and if it is the last.
>> Re-fragmentation prevents Block options from being possible to integrity
>> protect between client and server.
>
>That is insufficient to provide the protection.  In addition you would
>need to be able to protect some value which says that all of these are
>from a single fragmented message as well.  This means that one would need
>to ensure that this value is not reused under some set of constraints.
>What happens if I received a block 4 of 5 from server A and a block 4 of
>5 from server B.  The block values are going to be correct, but they are
>from different streams.  So protecting what stream they are from is
>needed as well as making sure that there is some value which to protect
>from different streams from the same server.  (Ok - I just found another
>thing that I need to make sure that the BLOCK draft supports correctly.)

As mentioned, we assume a security association per server so we can tell
apart blocks from different servers. For OSCOAP we assume a freshness
parameter which could potentially be used to distinguish different
fragments from the same server, unless there is parallel transmission of
different streams. And as you noted, in that case there must be some other
way to distinguish the fragments, and that information is potentially
already integrity protected (e.g. by being part of an option that is
integrity protected) or can become integrity protected.


>
>> 
>> >
>> >This would be the case of sending A1 A2 A3 A4 and receiving A1 A2 B3 A4
>> >where each of the fragments individually verifies, but the entire
>> >stream has been changed.  Doing a single integrity operation over the
>> >entire stream of (A1 A2 A3 A4) means that receiving (A1 A2 B3 A4) would
>> >fail to validate because the entire stream of bytes would fail
>>validation.
>> 
>> This is not an issue if you integrity protect the appropriate Block
>>option.
>
>Not that the block option  is the same for both A3 and B3.
>NUM = 3, M = no, SZX=1K

Well I meant you integrity protect also the Block option, in addition to
the other options, payload and freshness parameter.

>
>> 
>> 
>> >
>> >This means that I treat security identically if the block options are
>> >there or if the block options are not there.
>> 
>> Yes, since you focus on an over-the-top solution you can do that. But
>>the only
>> over-the-top solution I know of that protects metadata is the one
>>mentioned
>> below which moves REST out of CoAP.
>> 
>> >
>> >There are other discussions that need to be dealt with about options
>> >and how they interact with security, but the report of the group
>> >working on this has not yet come up for review. I am just worried about
>> >getting this draft finished.
>> 
>> And I’m worried that this draft voids the candidate solutions for
>>protecting
>> payload and metadata.
>
>There I cannot help as I have not seen anything yet.

Sorry, I thought it was clear that I was referring to constructions like
draft-selander-ace-object-security or draft-bergmann-ace-dcaf-cose.

Göran


>
>Jim
>
>> 
>> Göran
>> 
>> 
>> >
>> >jim
>> >
>> >> -----Original Message-----
>> >> From: Göran Selander [mailto:[e-mail]]
>> >> Sent: Monday, February 01, 2016 12:20 AM
>> >> To: Jim Schaad <[e-mail]>; [e-mail]
>> >> Cc: [e-mail]; [e-mail];
>> >>[e-mail];  [e-mail]
>> >> Subject: Re: [core] Last Call: <draft-ietf-core-block-18.txt>
>> >>(Block-wise transfers  in CoAP) to Proposed Standard
>> >>
>> >> Hi Jim
>> >>
>> >> Thanks for picking up the thread.
>> >>
>> >> We spent some time in Prague thinking about this. You are right that
>> >>there are  different options with different characteristics. Here are
>> >>the cases we have  considered.
>> >>
>> >> Just to recap the objective, this is about protecting communication
>> >>between  CoAP client and CoAP server while allowing legitimate
>> >>operations of one or  more intermediary CoAP proxies. Client and
>> >>server are assumed to have a  security association.
>> >>
>> >> On a high level the candidate solutions are either built entirely on
>> >>top of CoAP or  add new CoAP options.
>> >>
>> >> Looking at the former first, one solution is to wrap just the CoAP
>> >>payload/content, e.g. using COSE [1]. (We called this object security
>> >>of content,  OSCON). This is useful if the payload includes certain
>> >>meta data, like in the case  of CWT, or if certain information, such
>> >>as resource identifier etc., is implicit  from the security
>> >>association.
>> >>
>> >> This solution however does not protect the metadata sent in the CoAP
>> >>message,  such as e.g. Code (GET/DELETE/etc), Uri-Path or Content
>> >>Format.
>> >> Even if such information would be integrity protected, e.g. using
>> >>External-AAD in  [1], it neither protects messages which do not have
>> >>payload, like e.g.
>> >>GET
>> >> requests, nor does it address confidentiality desirable for a subset
>> >>of such  metadata for privacy reasons.
>> >>
>> >> An alternative solution on top of CoAP is to move the RESTful
>> >>protocol out of  CoAP and only use POST with some dummy Uri-Path,
>> >>Content Format etc. In this  way all messages could carry a protected
>> >>object as in [1] and the nature of the  interaction and content is
>> >>contained in this object. This is probably violating the  purpose of
>> >>CoAP too much to be of any interest.
>> >>
>> >> Those are the only solutions we have considered on top of CoAP. I'm
>> >>not sure if  the solution you propose is related to one of these?
>> >>
>> >> All other solutions I'm aware of which address the general problem
>> >>space, e.g.
>> >> OSCOAP, are built "within" CoAP, using CoAP options to carry
>> >>protected objects  (such as [1]) which include integrity protection of
>> >>the payload and meta-data.
>> >> Now, what happens if the payload is large? If the originating
>> >>endpoint does the  fragmentation then the destination endpoint can
>> >>verify the integrity.
>> >>If a proxy
>> >> using the blockwise draft (re-)fragments the payload (which also
>> >>changes a  Block option) such that it is different when reaching the
>> >>destination endpoint,  then integrity verification will fail. The
>> >>destination endpoint cannot distinguish a  legitimate
>> >>introduction/change of payload and Block option from any other  change
>> >>of the message, hence it cannot verify integrity. This can be used to
>> >>disable integrity protection at CoAP layer also for shorter messages,
>> >>since the  destination endpoint must treat the existence of a Block
>> >>option as a generic  "security off" button.
>> >>
>> >> This is quite different from the CoAP options standardized so far
>> >>where you can  protect individual messages between client and server
>> >>at the same time as  allowing legitimate proxy operation, even
>> >>verifying that intermediate nodes has  performed the correct CoAP
>> >>option manipulations such as e.g. when forward  proxies change the
>> >>Uri-* options making the message reach the correct  destination URI.
>> >>
>> >> IMHO CoAP should not allow an intermediate device to legitimately
>> >>turn off  integrity protecting between client and server. CoAP should
>> >>definitely support  integrity protection of short messages between
>> >>client and server through  proxies. This is where CoAP shines
>> >>brightly. Given that, it is straightforward to  integrity protect
>> >>messages fragmented at the endpoints.
>> >> And with the Block options integrity protected the entire message
>> >>built up of the  fragments is automatically protected as well.
>> >>
>> >> Göran
>> >>
>> >> [1] https://tools.ietf.org/html/draft-ietf-cose-msg
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> On 2016-01-29 00:18, "Jim Schaad" <[e-mail]> wrote:
>> >>
>> >> >Göran,
>> >> >
>> >> >I finally got caught up on reading the CORE mailing list (lots of
>> >> >boredom on issues I don’t think I care about) and I did not find any
>> >> >responses to your mail on this issue.  I would like to propose a
>> >> >different solution to the problem which I think you will find both
>> >> >workable and potential not requiring any updates to the current
>>draft.
>> >> >
>> >> >When I read this draft the first time, I read it as a network
>> >> >fragmentation draft rather than as a messaging draft.  As such I did
>> >> >not have the same concerns about object security as you seem to
>>have.
>> >> >I made the decision that I would apply the security to the entirety
>> >> >of the message being sent, and then fragment it into blocks
>>afterwards.
>> >> >Such an approach allows for a number of things that you are having
>> >> >problems with to be ignored.
>> >> >
>> >> >How the fragmentation is done, is change or is removed become
>> >> >immaterial as the end recipient would need to have all of the
>> >> >fragments delivered and in the correct order in order to process the
>> >> >message and do validation.
>> >> >
>> >> >Overhead is smaller because the overhead of encrypting/signing at
>> >> >the object security level is done once rather than once per
>> >> >fragment.  This allows for fewer bytes to be sent across the wire.
>> >> >
>> >> >The headers of the first message in the fragment are the ones that
>> >> >the object security system would be using both for security
>> >> >calculation purposes and for the receiver to process the validated
>>message.
>> >> >
>> >> >There are still some question that potentially need to be dealt
>>with:
>> >> >
>> >> >1) Are the block option headers authenticated?  The probable answer
>> >> >should be no as they are designed to be changed by intermediaries.
>> >> >This can be deferred until the general discussion about the rest of
>> >> >the current headers.
>> >> >
>> >> >2) What options are required to be copied forward into subsequent
>> >> >messages and which can be omitted?  I was unable to find any
>> >> >guidance on this issue from reading the document and thus would
>> >> >naively make the assumption that all options not specified by this
>> >> >document are copied forward and should be checked to make sure that
>> >> >they are unchanged in future messages.  However I doubt that is the
>>desire
>> of the authors.
>> >> >This however is not a security specific issue and needs to be
>> >> >addressed in this document.
>> >> >
>> >> >3) Do we want to apply per message security as well - that is an
>> >> >issue that can and should be punted to a future object security
>>draft.
>> >> >However, I don't see the point except to protect the ACK/NACK or
>> >> >lack of on each individual hop.  But this is point-to-point not
>>end-to-end.
>> >> >
>> >> >Jim
>> >> >
>> >> >
>> >> >
>> >> >> -----Original Message-----
>> >> >> From: core [mailto:[e-mail]] On Behalf Of Göran
>> >> >>Selander
>> >> >> Sent: Wednesday, November 25, 2015 11:07 PM
>> >> >> To: [e-mail]
>> >> >> Cc: [e-mail]; [e-mail];
>> >> >>[e-mail];  [e-mail]
>> >> >> Subject: Re: [core] Last Call: <draft-ietf-core-block-18.txt>
>> >> >>(Block-wise transfers  in CoAP) to Proposed Standard
>> >> >>
>> >> >>
>> >> >>
>> >> >> There was a thread on the CoRE WG mailing list a couple of months
>> >> >>ago on the  topic of blockwise and object security. The starting
>> >> >>point was a question if CoAP  proxies can (re-)partition messages
>> >> >>into blocks as defined in this draft, and the  implications on
>> >> >>end-to-end security between client and server through such a
>> >> >>proxy. The conclusions of that discussion has an impact on this
>> >> >>draft, but there  are no considerations of this kind made in
>> >> >>version -18. More details are given  below, including some
>> >> >>alternative proposals for how to address this.
>> >> >>Apologies
>> >> >> for the long e-mail.
>> >> >>
>> >> >>
>> >> >> Background:
>> >> >>
>> >> >> There is an ongoing discussion in CoRE and ACE WGs since a year on
>> >> >>the
>> >> >>end-to-
>> >> >> end security properties of CoAP, i.e. protecting the communication
>> >> >>between a  client and a server through proxies. RFC 7252 and other
>> >> >>specifications in the  CoAP suite define a set of legitimate proxy
>> >> >>operations on CoAP messages which  requires DTLS to be terminated
>> >> >>at proxies. This implies that the proxy has access  not only with
>> >> >>the data required for perform the intended proxy operation but is
>> >> >>also able to eavesdrop or manipulate any part of the CoAP payload
>> >> >>and metadata in transit between client and server without being
>> >> >>protected or  detected by DTLS.
>> >> >>
>> >> >>
>> >> >> One way to mitigate this threat is to complement or replace DTLS
>> >> >>with application layer protection of CoAP payload and metadata
>> >> >>between client and  server for the use cases where the proxy should
>> >> >>not be fully trusted.
>> >> >> This has been discussed in the CoRE WG meetings during the three
>> >> >>last IETF F2F  meetings and there are draft solutions using the
>> >> >>message format being  developed in the COSE WG.
>> >> >>
>> >> >>
>> >> >> With the COAP proxy operations standardized so far it has been
>> >> >>possible to  protect the CoAP messages adequately with security on
>> >> >>transport layer,  application layer or a combination thereof. In
>> >> >>the case where the legitimate  proxy operation is predictable by
>> >> >>client and server, application layer security can  be defined to
>> >> >>both verify that no illegitimate changes has been performed as
>> >> >>well as verifying the legitimate changes. In the case where proxy
>> >> >>operations are  not predictable — even if the data the proxy is
>> >> >>operating on cannot be protected  — it has so far been possible to
>> >> >>use other information elements to provide the  required end-to-end
>> security properities.
>> >> >>For example, the CoAP header field  Token may be changed by a
>> >> >>proxy, but instead a transaction identifier can be  introduced in
>> >> >>the application security wrapper (COSE
>> >> >> header) to define a message (exchange) identifier common to client
>> >> >>and server.
>> >> >>
>> >> >>
>> >> >> Blockwise:
>> >> >>
>> >> >> With the definition of blockwise transfer as specified in this
>> >> >>draft a proxy may  partition or re-partitioning a message into
>> >> >>blocks where the size of the blocks  are decided by the proxy. As a
>> >> >>consequence, it is not possible to integrity protect  individual
>> >> >>blocks end-to-end between client and server: DTLS does not protect
>> >> >>the message data within the proxy, and application layer integrity
>> >> >>protection of individual blocks cannot be performed unless the
>> >> >>partitioning into blocks as  received by one endpoint is identical
>> >> >>to that sent by the other endpoint. Hence,  when CoAP Block options
>> >> >>are used as defined in this draft, end-to-end security  of the
>> >> >>individual CoAP request and response breaks down. For example: a
>> >> >>proxy  may addBlock options, send any number of blocks with any
>> >> >>payload to an  endpoint without being possible to detect or protect
>> >> >>against. In contrast to the  existing standards in the CoAP suite,
>> >> >>in this case it is not possible to bypass the  construction and
>> >> >>define a secure end-to-end block partitioning with less than
>> >> >>disabling block partitioning as specified in this draft.
>> >> >>
>> >> >>
>> >> >> One solution to this is to disallow proxies to re-partition a
>> >> >>message, thus  redefine the Block options such that they are
>> >> >>possible to integrity protect end-  to-end.  Integrity protecting
>> >> >>each block and corresponding Block options as  defined in the
>> >> >>current draft has additional benefits: If any block in the sequence
>> >> >>fails verification, it can be individually requested to be resent.
>> >> >>When all blocks  has been verified the entire message has been
>> >> >>verified.  A receiving node may  even perform certain actions based
>> >> >>on received verified blocks before the entire  message has been
>>received.
>> >> >>
>> >> >>
>> >> >> Instead of delegating to proxies to partition into blocks, the
>> >>sending
>> >> >>endpoint
>> >> >> would need to anticipate or get information about the relevant
>> >> >>block size, e.g.
>> >> >> using a size indication in the link-format description [RFC6990].
>> >> >>Additional
>> >> >> methods for blocksize discovery may also be defined.
>> >> >> While this may not be as simple as leaving it entirely to the
>> >> >>proxy
>> >>to
>> >> >>decide,
>> >> >> considering the additional security benefits I believe this is the
>> >> >>right trade off to  make.
>> >> >>
>> >> >>
>> >> >> An alternative solution is to prevent proxies from re-partitioning
>> >> >>a message only  in the case where end-to-end security of CoAP
>> >> >>message is applied,
>> >>which
>> >> >>in
>> >> >> current solution proposals is indicated with the presence of a
>> >>certain
>> >> >>CoAP
>> >> >> option X (which e.g. contains the COSE object).
>> >> >> This would have the same benefits as the previous solution, but
>> >> >>requires the  code in the proxy implementing this draft to be aware
>> >> >>of option X,
>> >>and
>> >> >>hence
>> >> >> that dependency needs to be specified in this draft. And option X
>> >> >>is
>> >>not
>> >> >> standardized yet, so would require introducing a placeholder.
>> >> >>
>> >> >>
>> >> >> There are other alternatives as well but this e-mail is already
>> >> >>too long.
>> >> >> The main point I wanted to make is that given that we now have a
>> >>better
>> >> >> understanding of how to achieve security between client and server
>> >> >>through  proxies compared to when RFC7252 was written, my opinon is
>> >> >>that we should  not ignore these security issues in new standards.
>> >> >>
>> >> >>
>> >> >>
>> >> >> Göran
>> >> >>
>> >> >>
>> >> >>
>> >> >> On 2015-11-20 22:32, "The IESG" <[e-mail]> wrote:
>> >> >>
>> >> >> >
>> >> >> >The IESG has received a request from the Constrained RESTful
>> >> >> >Environments WG (core) to consider the following document:
>> >> >> >- 'Block-wise transfers in CoAP'
>> >> >> >  <draft-ietf-core-block-18.txt> as Proposed Standard
>> >> >> >
>> >> >> >The IESG plans to make a decision in the next few weeks, and
>> >>solicits
>> >> >> >final comments on this action. Please send substantive comments
>> >> >> >to
>> >>the
>> >> >> >[e-mail] mailing lists by 2015-12-04. Exceptionally,
>> >> >> >comments
>> >>may
>> >> >> >be sent to [e-mail] instead. In either case, please retain
>> >> >> >the beginning of the Subject line to allow automated sorting.
>> >> >> >
>> >> >> >Abstract
>> >> >> >
>> >> >> >
>> >> >> >   CoAP is a RESTful transfer protocol for constrained nodes and
>> >> >> >   networks.  Basic CoAP messages work well for the small
>> >> >> > payloads
>> >>we
>> >> >> >   expect from temperature sensors, light switches, and similar
>> >> >> >   building-automation devices.  Occasionally, however,
>>applications
>> >> >> >   will need to transfer larger payloads -- for instance, for
>> >>firmware
>> >> >> >   updates.  With HTTP, TCP does the grunt work of slicing large
>> >> >> >   payloads up into multiple packets and ensuring that they all
>> >>arrive
>> >> >> >   and are handled in the right order.
>> >> >> >
>> >> >> >   CoAP is based on datagram transports such as UDP or DTLS,
>>which
>> >> >> >   limits the maximum size of resource representations that can
>>be
>> >> >> >   transferred without too much fragmentation.  Although UDP
>> >>supports
>> >> >> >   larger payloads through IP fragmentation, it is limited to 64
>>KiB
>> >> >> >   and, more importantly, doesn't really work well for
>>constrained
>> >> >> >   applications and networks.
>> >> >> >
>> >> >> >   Instead of relying on IP fragmentation, this specification
>> >>extends
>> >> >> >   basic CoAP with a pair of "Block" options, for transferring
>> >>multiple
>> >> >> >   blocks of information from a resource representation in
>>multiple
>> >> >> >   request-response pairs.  In many important cases, the Block
>> >>options
>> >> >> >   enable a server to be truly stateless: the server can handle
>>each
>> >> >> >   block transfer separately, with no need for a connection
>>setup or
>> >> >> >   other server-side memory of previous block transfers.
>> >> >> >
>> >> >> >   In summary, the Block options provide a minimal way to
>>transfer
>> >> >> >   larger representations in a block-wise fashion.
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> >The file can be obtained via
>> >> >> >https://datatracker.ietf.org/doc/draft-ietf-core-block/
>> >> >> >
>> >> >> >IESG discussion can be tracked via
>> >> >> >https://datatracker.ietf.org/doc/draft-ietf-core-block/ballot/
>> >> >> >
>> >> >> >
>> >> >> >No IPR declarations have been submitted directly on this I-D.
>> >> >> >
>> >> >> >
>> >> >>
>> >> >> _______________________________________________
>> >> >> core mailing list
>> >> >> [e-mail]
>> >> >> https://www.ietf.org/mailman/listinfo/core
>> >> >
>> >
>> >
>