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Versions: 00 01                                                         
ANIMA WG                                                     M. Pritikin
Internet-Draft                                             P. Kampanakis
Intended status: Informational                             Cisco Systems
Expires: January 9, 2017                                    July 8, 2016


                            BRSKI over CoAP
                    draft-pritikin-coap-bootstrap-00

Abstract

   This document provides an initial discussion of Bootstrapping of
   Remote Secure key infrastructures (BRSKI) when the device being
   bootstrapped speaks CoAP.  The HTTPS REST methods leveraged by BRSKI
   are mapped to CoAP methods.  Fragmentation management of large
   messages during EST certificate enrollment is addressed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Scope of solution . . . . . . . . . . . . . . . . . . . . . .   2
   4.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Message Bindings  . . . . . . . . . . . . . . . . . . . . . .   3
   6.  Data Fragmentation  . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Example fragmented response . . . . . . . . . . . . . . .   7
   7.  Proxying  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  CoAP Parameters . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Many IoT and other devices are expected to use CoAP over UDP
   extensively.  Bootstrapping these devices without requiring a full
   TCP stack is an often raised requirement for
   [I-D.ietf-anima-bootstrapping-keyinfra].  CoAP provides REST methods
   over DTLS and is substantially functional with the folling necessary
   additions:

   DTLS:  Because CoAP use of DTLS includes support for large handshake
      messages there is little to describe here.  BRSKI and EST
      [RFC7030] are expanded to include DTLS.

   REST:  The mapping of BRSKI and EST messages to CoAP REST calls is
      described.

   Fragmentation:  Use of block chaining to support fragmentation of
      large BRSKI and EST messages is described.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

3.  Scope of solution

   The definition of BRSKI over DTLS and CoAP is not intended to expand
   the scope of BRSKI to highly constrained devices. (ref: [RFC7228]).
   Instead it is intended to ensure that bootstrapping works for less
   constained devices that choose to limit their communications stack to
   UDP/CoAP.



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   The BRSKI document details extensions to EST as well as making
   section 5.7 requirements on EST flows.  This document's references to
   BRSKI are intended to include all BRSKI extensions and all existing
   EST messages.  This document could replace BRSKI -03 section 5.7.5.
   [[EDNOTE: making this section 5.8 might make the most sense.]]

   Support for Observe CoAP options ( https://tools.ietf.org/html/
   rfc7641 ) in Blocks with BRSKI is not supported in the current BRSKI/
   EST message flows and is thus out-of-scope of this discussion.
   Observe options could be used by the server to notify clients about a
   change in the cacerts or csr attributes (resources) and might be an
   area of future work.

4.  DTLS

   During the DTLS handshake, if fragmentation is needed, "DTLS provides
   a mechanism for fragmenting a handshake message over a number of
   records, each of which can be transmitted separately, thus avoiding
   IP fragmentation" [RFC6347].

   Within BRSKI and EST when "TLS" is referred to it is understood that
   CoAP security is provided using DTLS instead.  No other changes are
   necessary (all provisional modes etc are the same as for TLS).

   In a constrained CoAP environment, endpoints can't afford to
   establish a DTLS connection for every EST transaction.
   Authenticating and negotiating DTLS keys requires resources on low-
   end endpoints and consumes valuable bandwidth.  The DTLS connection
   SHOULD remain open for persistent EST connections.  For example a
   cacerts request that is followed by an enrollment request can
   leverage the same DTLS connection.  Given that after a successfull
   enrollment, it is more likely that a new EST transaction will take
   place after a significant amount of time, the DTLS connections can
   only be kept alive for EST messages that are relatively close to each
   other.

5.  Message Bindings

   This section describes BRSKI to CoAP message mappings.

   CoAP defines confirmed (CON), acknowledgements (ACK), reset (RST) and
   non-corfirmed (NON) message types.  For confirmable messages, the
   responses are CoAP ACKs or RSTs.  All /cacerts, /simpleenroll,
   /simplereenroll, /csrattrs, /fullcmc and /serverkeygen EST messages
   expect a response, so they are all CON messages.

   A CoAP message has the following fields (from [I-D.ietf-core-block]
   Figure 7):



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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Ver| T |  TKL  |      Code     |          Message ID           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Token (if any, TKL bytes) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Options (if any) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 1 1 1 1 1 1 1|    Payload (if any) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Then Ver, TKL, Token, Message ID are not affected in BRSKI.  Their
   use is the same as in CoAP.  The options that can be used in a CoAP
   header have the following format (from [I-D.ietf-core-block]
   Figure 8):

        0   1   2   3   4   5   6   7
      +---------------+---------------+
      |  Option Delta | Option Length |   1 byte
      +---------------+---------------+
      /         Option Delta          /   0-2 bytes
      \          (extended)           \
      +-------------------------------+
      /         Option Length         /   0-2 bytes
      \          (extended)           \
      +-------------------------------+
      \                               \
      /         Option Value          /   0 or more bytes
      \                               \
      +-------------------------------+

   Options are used to convey Uri-Host, Uri-Path, Uri-Port, Content-
   Format and more in BRSKI which will be used to communicate the HTTP
   fields used in BRSKI messages.  As for the HTTP response messages in
   BRSKI, they are translated to the Response Codes explained in
   [RFC7252] section 5.3.1

   BRSKI URLs are https based (https:// ), in CoAPs these will be
   assumed to be transformed to coaps (coaps://)

   Some examples of how an BRSKI message would be translated in CoAP
   follow.  [[EDNOTE: This section to be expanded to ensure it covers
   all BRSKI edge conditions.]]

   First let's see how a get cacerts message in EST would be in CoAP:





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     GET /.well-known/est/cacerts HTTP/1.1
        User-Agent: curl/7.22.0 (i686-pc-linux-gnu) libcurl/7.22.0
                    OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
        Host: 192.0.2.1:8085
        Accept: */*

   The corresponding CoAP fields would be:

     Ver = 1
     T = 0 (means CON)
     Code = 0x01 (0.01 is GET)
     Options
      Option1 (Uri-Host)
        Option Delta = 0x3
        Option Length = 0x9
        Option Value = 192.0.2.1
      Option2 (Uri-Port)
        Option Delta = 0xA
        Option Length = 0x4
        Option Value = 8085
      Option3 (Uri-Path)
        Option Delta = 0xD
        Option Length = 0xD
        Extended Option Delta = 0x08
        Extended Option Length = 0x14
        Option Value = /.well-known/est/cacerts HTTP/1.1
     Payload = [Empty]

   Now let's say we have a 200 OK response with a cert in EST:

      HTTP/1.1 200 OK
      Status: 200 OK
      Content-Type: application/pkcs7-mime
      Content-Transfer-Encoding: base64 (TODO: Verify if we need a new
                                         option registry for Encoding?)
      Content-Length: 4246 (TODO: this example overflows and would
                            need fragmentation. Choose a better example.
                            Regardless we might need an CoAP option for
                            the content-length ie the CoAP payload?)

      MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYCAQExADALBgkqhkiG9w0BBwGgggwMMIIC
      +zCCAeOgAwIBAgIJAJpY3nUZO3qcMA0GCSqGSIb3DQEBBQUAMBsxGTAXBgNVBAMT
      ...

   The corresponding CoAP fields would be:






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     Ver = 1
     T = 2 (means ACK)
     Code = 0x21 (TODO: Maybe we need to create a 0x200 respond code.)
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime; smime-type=certs-only,
                   application/csrattrs, application/pkcs10,
                   application/pkcs8,
                   application/pkcs12 )
     Payload = MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYCAQExA \
                 DALBgkqhkiG9w0BBwGgggwMMIIC...

   [[EDNOTE: We might need a new Option for the Retry-After response
   message.  We might need a new Option for the WWW-Authenticate
   response.]]

   [[EDNOTE: username/password authentication can be described but is
   not a primary focus for BRSKI.  It is important for generic EST
   exchanges but would an endpoint device with sufficient user interface
   to allow username/password input from an end user be required to use
   CoAP instead of a full HTTPS exchange?]]

6.  Data Fragmentation

   After the DTLS connection is established fragmentation will be needed
   for the CoAP messages which involve certificate enrollment and
   management.

   Certificates can vary greatly in size based on signature algorithms,
   key sizes, and the fields used but even with ECC certs BRSKI CoAP
   messages can still exceed sizes in MTU of 1280 for IPv6 or 60-80
   bytes for 6LoWPAN [RFC4919]) (see section 2 of
   [I-D.ietf-core-block]).  For 256-bit curve, common ECDSA cert sizes
   are 500-1000bytes which could fluctuate based on the algorithms, SANs
   and cert fields.  For 384-bit curves, ECDSA certs increase in size
   and can sometimes reach 1.5KB.

   There are times when the EST cacert response from the server can
   include many certs that exceed maximum packet size.  Or any one cert
   can be more than the MTU.  CoAP RFC section 4.6 describes the
   possible payload sizes: "if nothing is known about the size of the
   headers, good upper bounds are 1152 bytes for the message size and
   1024 bytes for the payload size".  Also "If IPv4 support on unusual



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   networks is a consideration, implementations may want to limit
   themselves to more conservative IPv4 datagram sizes such as 576
   bytes; per [RFC0791], the absolute minimum value of the IP MTU for
   IPv4 is as low as 68 bytes, which would leave only 40 bytes minus
   security overhead for a UDP payload".  A fragmentation solution for
   BRSKI and EST CoAP message is required.

   The [I-D.ietf-core-block] document describes how fragmentation can be
   done by using a pair of Block options added to the CoAP flow.  Block1
   options are used by the client PUT and POST requests.  A Block1 in a
   client request needs a Block1 option in the responses.  A Block2
   comes from a server response that will also need Block2 from the
   client to acknowledge the block and get the rest of blocks from the
   server.  So, Block1 is used when a request (POST for example) is done
   in BRSKI over CoAP with a payload that needs fragmentation.  Then the
   server responds with Block1 option to acknowledge the fragment-
   blocks.  Block2 is used when a BRSKI server response is big and needs
   fragmentation.  The Block2 acknowledgements are requests with the
   same options as the initial request and a Block2 option.  "To
   influence the block size used in a response, the requester MAY also
   use the Block2 Option on the initial request, giving the desired
   size, a block number of zero and an M bit of zero".

   In a scenario with a big BRSKI POST we might have Block1 options from
   client to server and Block2 from server to client.  In this case the
   Block1 blocks get completed and then the Block2 comes the other
   direction.  The BLOCK draft also defines Size1 and Size2 options.
   These are used to convey the size of the resources in the requests or
   response.  The Size1 response should be parseable by the client and
   server that should be able to follow and send BLOCK options
   afterwards if need be.  Size1 sent in the request could also give the
   server an idea about the total size of the Block1 options.
   Similarly, Size2 option defined in BLOCK should be parseable by the
   server that is asked to provide the size estimate and by the client
   that is getting a response with Size2 size estimates of the total
   Block2 options.

6.1.  Example fragmented response

   An example of a server cacerts response that exceeds the MTU is:











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   An example of a server cacerts response that exceeds the MTU is
   HTTP/1.1 200 OK
      Status: 200 OK
      Content-Type: application/pkcs7-mime; smime-type=certs-only
      Content-Transfer-Encoding: base64
      Content-Length: 1122

      MIIDOAYJKoZIhvcNAQcCoIIDKTCCAyUCAQExADALBgkqhkiG9w0BBwGgggMLMIID
      BzCCAe+gAwIBAgIBFTANBgkqhkiG9w0BAQUFADAbMRkwFwYDVQQDExBlc3RFeGFt
      cGxlQ0EgTndOMB4XDTEzMDUwOTIzMTU1M1oXDTE0MDUwOTIzMTU1M1owHzEdMBsG
      A1UEAxMUZGVtb3N0ZXA0IDEzNjgxNDEzNTIwggEiMA0GCSqGSIb3DQEBAQUAA4IB
      DwAwggEKAoIBAQClNp+kdz+Nj8XpEp9kaumWxDZ3eFYJpQKz9ddD5e5OzUeCm103
      ZIXQIxc0eVtMCatnRr3dnZRCAxGjwbqoB3eKt29/XSQffVv+odbyw0WdkQOIbntC
      Qry8YdcBZ+8LjI/N7M2krmjmoSLmLwU2V4aNKf0YMLR5Krmah3Ik31jmYCSvwTnv
      6mx6pr2pTJ82JavhTEIIt/fAYq1RYhkM1CXoBL+yhEoDanN7TzC94skfS3VV+f53
      J9SkUxTYcy1Rw0k3VXfxWwy+cSKEPREl7I6k0YeKtDEVAgBIEYM/L1S69RXTLuji
      rwnqSRjOquzkAkD31BE961KZCxeYGrhxaR4PAgMBAAGjUjBQMA4GA1UdDwEB/wQE
      AwIEsDAdBgNVHQ4EFgQU/qDdB6ii6icQ8wGMXvy1jfE4xtUwHwYDVR0jBBgwFoAU
      scRp5lujBKfYl6OLO7+5arIyQjwwDQYJKoZIhvcNAQEFBQADggEBACmxg1hvL6+7
      a+lFTARoxainBx5gxdZ9omSb0L+qL+4PDvg/+KHzKsDnMCrcU6M4YP5n0EDKmGa6
      4lY8fbET4tt7juJg6ixb95/760Th0vuctwkGr6+D6ETTfqyHnrbhX3lAhnB+0Ja7
      o1gv4CWxh1I8aRaTXdpOHORvN0SMXdcrlCys2vrtOl+LjR2a3kajJO6eQ5leOdzF
      QlZfOPhaLWen0e2BLNJI0vsC2Fa+2LMCnfC38XfGALa5A8e7fNHXWZBjXZLBCza3
      rEs9Mlh2CjA/ocSC/WxmMvd+Eqnt/FpggRy+F8IZSRvBaRUCtGE1lgDmu6AFUxce
      R4POrT2xz8ChADEA

   Block options in CoAP messages can contain fields, SZX, M and NUM
   which are not affected by BRSKI.

   Let's assume that the cacerts message will need to be broken up to 3
   messages.  The first block2 will be:




















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     Ver = 1
     T = 2 (means ACK)
     Code = 0x21 (means 2.01 success message.
            TODO: Do we need to create a 0x200 respond code.)
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                         smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x0D
     Payload = MIIMOQYJKoZIhvcNAQcCoIIMKjCCDCYC \
               AQExADALBgkqhkiG9w0BBwGgggwMMIIC... (512 bytes)

   The second block2:

     Ver = 1
     T = 2 (means ACK)
     Code = 0x21
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                        smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x1D
     Payload = ... (512 bytes)

   The third and final block2:




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     Ver = 1
     T = 2 (means ACK)
     Code = 0x21
     Options
       Option1 (Content-Format)
         Option Delta = 0xC
         Option Length = 0xD
         Extended Option Length = 0x09
         Option Value = <number for application/pkcs7-mime;
                         smime-type=certs-only>
                   (TODO: We need a new CoAP IANA registered value
                   application/pkcs7-mime, application/csrattrs,
                   application/pkcs10, application/pkcs8,
                   application/pkcs12 )
       Option2 (Block2)
         Option Delta = 0xD
         Option Length = 0x1
         Extended Option Delta = 0x16
         Option Value = 0x25
     Payload = ...

7.  Proxying

   [[EDNOTE: This section to be populated.  It will address how proxying
   can take place by an entity that resides at the edge of the CoAP
   network, such as the Registrar, and can reach the BRSKI server
   residing in a traditional "TCP setting".  ]]

8.  CoAP Parameters

   [[EDNOTE: This section to be populated.  It will address transmission
   parameters for BRSKI described in sections 4.7 and 4.8 of the CoAP
   draft.  BRSKI does not impose any unique parameters that affect the
   CoAP parameters in Table 2 and 3 in the CoAP draft but the ones in
   CoAP could be affecting BRSKI.  For example the processing delay of
   CAs could be less then 2s, but in this case they should send an CoAP
   ACK every 2s while processing. ]]

9.  Security Considerations

   [[EDNOTE: This section to be populated.  This document describes an
   existing protocol moved to CoAP and there should not be additional
   security concerns added beyond the protocol's or CoAP's specifics
   security considerations.]]







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10.  Normative References

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., and S.
              Bjarnason, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-03 (work in progress), June 2016.

   [I-D.ietf-core-block]
              Bormann, D. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-20 (work in progress), April 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <http://www.rfc-editor.org/info/rfc7030>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

Authors' Addresses

   Max Pritikin
   Cisco Systems

   Email: pritikin@cisco.com


   Panos Kampanakis
   Cisco Systems

   Email: pkampana@cisco.com







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