ACE                                                      P. van der Stok
Internet-Draft                                                Consultant
Intended status: Standards Track                           P. Kampanakis
Expires: July 26, 2018                                     Cisco Systems
                                                                S. Kumar
                                               Philips Lighting Research
                                                           M. Richardson
                                                              M. Furuhed
                                                             Nexus Group
                                                                 S. Raza
                                                               RISE SICS
                                                        January 22, 2018

                    EST over secure CoAP (EST-coaps)


   Enrollment over Secure Transport (EST) [RFC7030] is used as a
   certificate management protocol over HTTPS.

   Low-resource devices often use the lightweight Constrained
   Application Protocol (CoAP) [RFC7252] for message exchanges.  This
   document defines how to transport EST payloads over secure CoAP (EST-
   coaps).  This allows low-resource constrained devices to re-use
   existing EST functionality.  Example low-resource use cases for EST
   are: secure bootstrapping and certificate enrollment.

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-
   Drafts is at

   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 July 26, 2018.

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Copyright Notice

   Copyright (c) 2018 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  EST operational differences . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conformance to RFC7925 profiles . . . . . . . . . . . . . . .   4
   3.  Protocol Design and Layering  . . . . . . . . . . . . . . . .   5
     3.1.  Payload format  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Message Bindings  . . . . . . . . . . . . . . . . . . . .   6
     3.3.  CoAP response codes . . . . . . . . . . . . . . . . . . .   6
     3.4.  Message fragmentation . . . . . . . . . . . . . . . . . .   7
     3.5.  Deployment limits . . . . . . . . . . . . . . . . . . . .   8
   4.  Discovery and URI . . . . . . . . . . . . . . . . . . . . . .   8
   5.  DTLS Transport Protocol . . . . . . . . . . . . . . . . . . .  10
   6.  Proxying  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Content-Format registry . . . . . . . . . . . . . . . . .  12
     8.2.  Resource Type registry  . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     9.1.  proxy considerations  . . . . . . . . . . . . . . . . . .  15
     9.2.  EST server considerations . . . . . . . . . . . . . . . .  15
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   11. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  EST messages to EST-coaps  . . . . . . . . . . . . .  20
     A.1.  cacerts . . . . . . . . . . . . . . . . . . . . . . . . .  20
     A.2.  csrattrs  . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.3.  enroll / reenroll . . . . . . . . . . . . . . . . . . . .  23
     A.4.  serverkeygen  . . . . . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Encoding for server side key generation  . . . . . .  27

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   Appendix C.  EST-coaps Block message examples . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   Enrollment over Secure Transport (EST) [RFC7030] is used for
   authenticated/authorized endpoint certificate enrollment (and
   optionally key provisioning) through a Certificate Authority (CA) or
   Registration Authority (RA).  This functionality is also needed for
   low resource devices.

   "Classical" EST uses HTTPS and this specification defines a new
   transport for EST using CoAP.  It also profiles the use of EST to a
   smaller subset.

   IPv6 over Low-power Wireless Personal Area Networks (6LoWPANs)
   [RFC4944] on IEEE 802.15.4 [ieee802.15.4] wireless networks are
   becoming common in many industry application domains such as lighting
   controls.  Although IEEE 802.15.4 defines how security can be enabled
   between nodes within a single mesh network, it does not specify the
   provisioning and management of the keys.  Therefore, securing a
   6LoWPAN network with devices from multiple manufacturers with
   different provisioning techniques is often tedious and time
   consuming.  An example use case is the application of Bootstrapping
   of Remote Secure Infrastructures (BRSKI)
   [I-D.ietf-anima-bootstrapping-keyinfra].  The low resource aspects
   are detailed for 6tisch in [I-D.ietf-6tisch-minimal-security] and

   Constrained networks use DTLS [RFC6347], CoAP [RFC7252], and UDP
   instead of TLS [RFC5246], HTTP [RFC7230] and TCP.  EST-coaps replaces
   the invocations of TLS and HTTP by DTLS and CoAP invocations thus
   enabling EST for CoAP-based low-resource devices.

   Because the relatively large EST messages cannot be readily
   transported over constrained (6LoWPAN, LLN) wireless networks, this
   document specifies the use of CoAP Block-Wise Transfer ("Block")
   [RFC7959] to fragment EST messages at the application layer.

1.1.  EST operational differences

   Only the differences to EST with respect to operational scenarios are
   described in this section.  EST-coaps server differs from EST server
   as follows:

   o  Replacement of TLS by DTLS and HTTP by CoAP, resulting in:

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      *  DTLS-secured CoAP sessions between EST-coaps client and EST-
         coaps server.

   o  Only certificate-based client authentication is supported, which
      results in:

      *  The EST-coaps client does not support HTTP Basic authentication
         (as described in Section 3.2.3 of [RFC7030]).

      *  The EST-coaps client does not support authentication at the
         application layer (as described in Section 3.2.3 of [RFC7030]).

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Many of the concepts in this document are taken over from [RFC7030].
   Consequently, much text is directly traceable to [RFC7030].  The same
   document structure is followed to point out the differences and
   commonalities between EST and EST-coaps.

2.  Conformance to RFC7925 profiles

   This section shows how EST-coaps fits into the profiles of low-
   resource devices as described in [RFC7925].

   EST-coaps can transport certificates and private keys.  Private keys
   can be transported as response to a request to a server-side key
   generation as described in section 4.4 of [RFC7030].

   The mandatory cipher suite for DTLS is
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 defined in [RFC7251] which is the
   mandatory-to-implement cipher suite in CoAP.  Additionally, the curve
   secp256r1 MUST be supported [RFC4492]; this curve is equivalent to
   the NIST P-256 curve.  The hash algorithm is SHA-256.  DTLS
   implementations MUST use the Supported Elliptic Curves and Supported
   Point Formats Extensions [RFC4492]; the uncompressed point format
   MUST be supported; [RFC6090] can be used as an implementation method.

   The EST-coaps client MUST be configured with an explicit TA database
   or at least an implicit TA database from its manufacturer.  The
   authentication of the EST-coaps server by the EST-coaps client is
   based on Certificate authentication in the DTLS handshake.

   The authentication of the EST-coaps client is based on client
   certificate in the DTLS handshake.  This can either be

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   o  DTLS with a previously issued client certificate (e.g., an
      existing certificate issued by the EST CA); this could be a common
      case for simple re-enrollment of clients;

   o  DTLS with a previously installed certificate (e.g., manufacturer-
      installed certificate or a certificate issued by some other

3.  Protocol Design and Layering

   EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise
   Transfer [RFC7959] to transport CoAP messages in blocks thus avoiding
   (excessive) 6LoWPAN fragmentation of UDP datagrams.  The use of
   "Block" for the transfer of larger EST messages is specified in
   Section 3.4.  The Figure 1 below shows the layered EST-coaps

   |    EST request/response messages               |
   |    CoAP for message transfer and signaling     |
   |    DTLS for transport security                 |
   |    UDP for transport                           |

                    Figure 1: EST-coaps protocol layers

   The EST-coaps protocol design follows closely the EST design.  The
   parts supported by EST-coaps are identified by their message types:

   o  Simple enroll and reenroll, for CA to sign public client-identity

   o  CA certificate retrieval, needed to receive the complete set of CA

   o  CSR Attributes request messages, informs the client of the fields
      to include in generated CSR.

   o  Server-side key generation messages, to provide a private client-
      identity key when the client is too restricted or because of lack
      of an entropy source.  [EDNOTE: Encrypting these keys is
      important.  RFC7030 specifies how the private key can be encrypted
      with CMS using symmetric or asymmetric keys.  Mention how
      symmetric key can be derived for EST server side key generation
      from the TLS KEM draft.]

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3.1.  Payload format

   The content-format (media type equivalent) of the CoAP message
   determines which EST message is transported in the CoAP payload.  The
   media types specified in the HTTP Content-Type header (see section
   3.2.2 of [RFC7030]) are in EST-coaps specified by the Content-Format
   Option (12) of CoAP.  The combination of URI path-suffix and content-
   format used for CoAP MUST map to an allowed combination of path-
   suffix and media type as defined for EST.  The required content-
   formats for these request and response messages are defined in
   Section 8.  The CoAP response codes are defined in Section 3.3.

   EST-coaps is designed for use between low-resource devices using CoAP
   and hence does not need to send base64-encoded data.  Simple binary
   is more efficient (30% less payload compared to base64) and well
   supported by CoAP.  Therefore, the content formats specification in
   Section 8 requires the use of binary for all EST-coaps Content-

3.2.  Message Bindings

   This section describes the general EST CoAP message characteristics.

   It is RECOMMENDED to use CoAP CON messages.  This recommendation does
   not influence the communication efficiency because all EST-coaps
   messages expect a response.

   The Ver, TKL, Token, and Message ID values of the CoAP header are not
   influenced by EST.

   CoAP options are used to convey Uri-Host, Uri-Path, Uri-Port,
   Content-Format and more in CoAP.  The CoAP Options are used to
   communicate the HTTP fields specified in the EST REST messages.

   EST URLs are HTTPS based (https://), in CoAP these will be assumed to
   be transformed to coaps (coaps://)

   Appendix A includes some practical examples of EST messages
   translated to CoAP.

3.3.  CoAP response codes

   Section 5.9 of [RFC7252] specifies the mapping of HTTP response codes
   to CoAP response codes.  Every time the HTTP response code 200 is
   specified in [RFC7030] in response to a GET (POST) request, in EST-
   coaps the equivalent CoAP response code 2.05 (2.01) MUST be used.
   Response code HTTP 202 in EST is mapped to CoAP _.__.  In
   [I-D.hartke-core-pending] it is specified how multiple concurrently

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   open requests may be handled.  All other HTTP 2xx response codes are
   not used by EST.  For the following HTTP 4xx error codes that may
   occur: 400, 401, 403, 404, 405, 406, 412, 413, 415; the equivalent
   CoAP response code for EST-coaps is 4.xx.  For the HTTP 5xx error
   codes: 500, 501, 502, 503, 504 the equivalent CoAP response code is

3.4.  Message fragmentation

   DTLS defines fragmentation only for the handshake part and not for
   secure data exchange (DTLS records).  [RFC6347] states that to avoid
   using IP fragmentation, which involves error-prone datagram
   reconstitution, invokers of the DTLS record layer SHOULD size DTLS
   records so that they fit within any Path MTU estimates obtained from
   the record layer.  In addition, invokers residing on a 6LoWPAN over
   IEEE 802.15.4 network SHOULD attempt to size CoAP messages such that
   each DTLS record will fit within one or two IEEE 802.15.4 frames.

   That is not always possible.  Even though ECC certificates are small
   in size, they can vary greatly based on signature algorithms, key
   sizes, and OID fields used.  For 256-bit curves, common ECDSA cert
   sizes are 500-1000 bytes which could fluctuate further based on the
   algorithms, OIDs, SANs and cert fields.  For 384-bit curves, ECDSA
   certs increase in size and can sometimes reach 1.5KB.  Additionally,
   there are times when the EST cacerts response from the server can
   include multiple certs that amount to large payloads.  Section 4.6 of
   CoAP [RFC7252] 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".
   Section 4.6 of [RFC7252] also suggests that IPv4 implementations may
   want to limit themselves to more conservative IPv4 datagram sizes
   such as 576 bytes.  From [RFC0791] follows that 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.  Thus, even
   with ECC certs, EST-coaps messages can still exceed sizes in MTU of
   1280 for IPv6 or 60-80 bytes for 6LoWPAN [RFC4919] as explained in
   section 2 of [RFC7959].  EST-coaps needs to be able to fragment EST
   messages into multiple DTLS datagrams.  Fine-grained fragmentation of
   EST messages is essential.

   To perform fragmentation in CoAP, [RFC7959] specifies the "Block1"
   option for fragmentation of the request payload and the "Block2"
   option for fragmentation of the return payload of a CoAP flow.

   The BLOCK draft defines SZX in the Block1 and Block2 option fields.
   These are used to convey the size of the blocks in the requests or

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   The CoAP client MAY specify the Block1 size and MAY also specify the
   Block2 size.  The CoAP server MAY specify the Block2 size, but not
   the Block1 size.  As explained in Section 1 of [RFC7959]), blockwise
   transfers SHOULD be used in Confirmable CoAP messages to avoid the
   exacerbation of lost blocks.

   The Size1 response MAY be parsed by the client as a size indication
   of the Block2 resource in the server response or by the server as a
   request for a size estimate by the client.  Similarly, Size2 option
   defined in BLOCK should be parsed by the server as an indication of
   the size of the resource carried in Block1 options and by the client
   as a maximum size expected in the 4.13 (Request Entity Too Large)
   response to a request.

   Examples of fragmented messages are shown in Appendix C.

3.5.  Deployment limits

   Although EST-coaps paves the way for the utilization of EST for
   constrained devices on constrained networks, some devices will not
   have enough resources to handle the large payloads that come with
   EST-coaps.  The specification of EST-coaps is intended to ensure that
   EST works for networks of constrained devices that choose to limit
   their communications stack to UDP/CoAP.  It is up to the network
   designer to decide which devices execute the EST protocol and which

4.  Discovery and URI

   EST-coaps is targeted to low-resource networks with small packets.
   Saving header space is important and an additional EST-coaps URI is
   specified that is shorter than the EST URI.

   In the context of CoAP, the presence and location of (path to) the
   management data are discovered by sending a GET request to "/.well-
   known/core" including a resource type (RT) parameter with the value
   "ace.est" [RFC6690].  Upon success, the return payload will contain
   the root resource of the EST resources.  It is up to the
   implementation to choose its root resource; throughout this document
   the example root resource /est is used.  The example below shows the
   discovery of the presence and location of management data.

     REQ: GET /.well-known/core?rt=ace.est

     RES: 2.05 Content
   </est>; rt="ace.est"

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   The additional EST-coaps server URIs differ from the EST URI by
   replacing the scheme https by coaps and by specifying a shorter
   resource path names:


   The CoAP short URI exists next to the URI defined in [RFC7030].


   Figure 5 in section 3.2.2 of [RFC7030] enumerates the operations and
   corresponding paths which are supported by EST.  Table 1 provides the
   mapping from the EST URI path to the shorter EST-coaps URI path.

                     | EST              | EST-coaps |
                     | /cacerts         | /crts     |
                     | /simpleenroll    | /sen      |
                     | /simplereenroll  | /sren     |
                     | /csrattrs        | /att      |
                     | /serverkeygen    | /skg      |

                                  Table 1

   When discovering the root path for the EST resources, the server MAY
   return the full resource paths and the used content types.  This is
   useful when multiple content types are specified for EST-coaps
   server.  For example, the following more complete response is

     REQ: GET /.well-known/core?rt=ace.est

     RES: 2.05 Content
   </est>; rt="ace.est"
   </est/crts>; rt="ace.est";ct=TBD1
   </est/sen>; rt="ace.est";ct=TBD1 TBD4
   </est/sren>; rt="ace.est";ct=TBD1 TBD4
   </est/att>; rt="ace.est";ct=TBD4
   </est/skg>; rt="ace.est";ct=TBD1 TBD4 TBD2

   The return of the content-types allows the client to choose the most
   appropriate one from multiple content types.

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5.  DTLS Transport Protocol

   EST-coaps depends on a secure transport mechanism over UDP that can
   secure (confidentiality, authenticity) the CoAP messages exchanged.

   DTLS is one such secure protocol.  When "TLS" is referred to in the
   context of EST, it is understood that in EST-coaps, security is
   provided using DTLS instead.  No other changes are necessary (all
   provisional modes etc. are the same as for TLS).

   CoAP was designed to avoid fragmentation.  DTLS is used to secure
   CoAP messages.  However, fragmentation is still possible at the DTLS
   layer during the DTLS handshake when using ECC ciphersuites.  If
   fragmentation is necessary, "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"

   CoAP and DTLS can provide proof of identity for EST-coaps clients and
   server with simple PKI messages conformant to section 3.1 of
   [RFC5272].  EST-coaps supports the certificate types and Trust
   Anchors (TA) that are specified for EST in section 3 of [RFC7030].

   Channel-binding information for linking proof-of-identity with
   connection-based proof-of-possession is optional for EST-coaps.  When
   proof-of-possession is desired, a set of actions are required
   regarding the use of tls-unique, described in section 3.5 in
   [RFC7030].  The tls-unique information translates to the contents of
   the first "Finished" message in the TLS handshake between server and
   client [RFC5929].  The client is then supposed to add this "Finished"
   message as a ChallengePassword in the attributes section of the
   PKCS#10 Request Info to prove that the client is indeed in control of
   the private key at the time of the TLS session when performing a
   /simpleenroll, for example.  In the case of EST-coaps, the same
   operations can be performed during the DTLS handshake.  In the event
   of handshake message fragmentation, the Hash of the handshake
   messages used in the MAC calculation of the Finished message

   PRF(master_secret, finished_label, Hash(handshake_messages))

   MUST be computed as if each handshake message had been sent as a
   single fragment [RFC6347].

   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

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   SHOULD remain open for persistent EST connections.  For example, an
   EST cacerts request that is followed by a simpleenroll request can
   use the same authenticated DTLS connection.  Given that after a
   successful enrollment, it is more likely that a new EST transaction
   will take place after a significant amount of time, the DTLS
   connections SHOULD only be kept alive for EST messages that are
   relatively close to each other.

   Support for Observe CoAP options [RFC7641] is out-of-scope for this
   document.  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.

6.  Proxying

   In real-world deployments, the EST server will not always reside
   within the CoAP boundary.  The EST-server can exist outside the
   constrained network in a non-constrained network that supports TLS/
   HTTP.  In such environments EST-coaps is used by the client within
   the CoAP boundary and TLS is used to transport the EST messages
   outside the CoAP boundary.  A proxy entity at the edge is required to
   operate between the CoAP environment and the external HTTP network.
   The ESTcoaps-to-HTTPS proxy SHOULD terminate EST-coaps downstream and
   initiate EST connections over TLS upstream.

   One possible use-case, shown in one figure below, is expected to be
   deployed in practice:

   o  A proxy between any EST-client and EST-server

                                        Constrained Network
                     .---------.    .----------------------------.
                     |   RA    |    |.--------------------------.|
                     '---------'    ||                          ||
                           |        ||                          ||
   .------.  HTTP   .-----------------.  CoAPS   .-----------.  ||
   | EST  |<------->|ESTcoaps-to-HTTPS|<-------->| EST Client|  ||
   |Server|over TLS |      Proxy      |          '-----------'  ||
   '------'         '-----------------'                         ||
                                    ||                          ||

               ESTcoaps-to-HTTPS proxy at the CoAP boundary.

   Table 1 contains the URI mapping between the EST-coaps and EST the
   proxy SHOULD adhere to.  Section 7 of [RFC8075] and Section 3.3
   define the mapping between EST-coaps and HTTP response codes, that

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   determines how a proxy translates CoAP response codes from/to HTTP
   status codes.  The mapping from Content-Type to media type is defined
   in Section 8.  The conversion from binary to BSD64 needs to be done
   in the proxy.  Conversion is possible because a TLS link exists
   between EST-coaps-to-HTTP proxy and EST server and a corresponding
   DTLS linked exists between EST-coaps-to-HTTP proxy and EST client.

   Due to fragmentation of large messages into blocks, an EST-coaps-to-
   HTTP proxy SHOULD reassemble the BLOCKs before translating the binary
   content to BSD64, and consecutively relay the message upstream into
   the HTTP environment.

   For the discovery of the EST server by the EST client in the coap
   environment, the EST-coaps-to-HTTP proxy MUST announce itself
   according to the rules of Section 4.  The available functions of the
   proxies MUST be announced with as many resource paths.  The discovery
   of EST server in the http environment follow the rules specified in

   [ EDNOTE: PoP will be addressed here. ]

   A proxy SHOULD authenticate the client downstream and it should be
   authenticated by the EST server or CA upstream.  The Registration
   Authority (RA) is necessary to (re-)create the secure connection from
   DTLS to TLS and vice versa.  A trust relationship needs to be pre-
   established between the proxy and the EST servers to be able to proxy
   these connections on behalf of various clients.

   [EDNOTE: To add more details about trust relations in this section. ]

7.  Parameters

   [EDNOTE: This section to be populated.  It will address transmission
   parameters described in sections 4.7 and 4.8 of the CoAP draft.  EST
   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 EST.  For example, the processing delay of CAs could be
   less then 2s, but in this case they should send a CoAP ACK every 2s
   while processing.]

8.  IANA Considerations

8.1.  Content-Format registry

   Additions to the sub-registry "CoAP Content-Formats", within the
   "CoRE Parameters" registry are needed for the below media types.
   These can be registered either in the Expert Review range (0-255) or
   IETF Review range (256-9999).

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       *  application/pkcs7-mime

       *  Type name: application

       *  Subtype name: pkcs7-mime

       *  ID: TBD1

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5751]

       *  Applications that use this media type: EST


       *  application/pkcs8

       *  Type name: application

       *  Subtype name: pkcs8

       *  ID: TBD2

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5958]

       *  Applications that use this media type: EST


       *  application/csrattrs

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       *  Type name: application

       *  Subtype name: csrattrs

       *  ID: TBD3

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC7030]

       *  Applications that use this media type: EST


       *  application/pkcs10

       *  Type name: application

       *  Subtype name: pkcs10

       *  ID: TBD4

       *  Required parameters: None

       *  Optional parameters: None

       *  Encoding considerations: binary

       *  Security considerations: As defined in this specification

       *  Published specification: [RFC5967]

       *  Applications that use this media type: EST

8.2.  Resource Type registry

   Additions to the sub-registry "CoAP Resource Type", within the "CoRE
   Parameters" registry are needed for a new resource type.

   o  rt="ace.est" needs registration with IANA.

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9.  Security Considerations

9.1.  proxy considerations

   The proxy proposed in Section 6 must be deployed with great care, and
   only when the recommended connections are impossible.

   [EDNOTE: To add more details about trust relations through proxies in
   this section. ]

9.2.  EST server considerations

   The security considerations of section 6 of [RFC7030] are only
   partially valid for the purposes of this document.  As HTTP Basic
   Authentication is not supported, the considerations expressed for
   using passwords do not apply.

   Given that the client has only limited resources and may not be able
   to generate sufficiently random keys to encrypt its identity, it is
   possible that the client uses server generated private/public keys to
   encrypt its certificate.  The transport of these keys is inherently
   risky.  A full probability analysis MUST be done to establish whether
   server side key generation enhances or decreases the probability of
   identity stealing.

   When a client uses the Implicit TA database for certificate
   validation, the client cannot verify that the implicit data base can
   act as an RA.  It is RECOMMENDED that such clients include "Linking
   Identity and POP Information" Section 5 in requests (to prevent such
   requests from being forwarded to a real EST server by a man in the
   middle).  It is RECOMMENDED that the Implicit Trust Anchor database
   used for EST server authentication be carefully managed to reduce the
   chance of a third-party CA with poor certification practices from
   being trusted.  Disabling the Implicit Trust Anchor database after
   successfully receiving the Distribution of CA certificates response
   (Section 4.1.3 of [RFC7030]) limits any vulnerability to the first
   DTLS exchange.

   In accordance with [RFC7030], TLS cipher suites that include
   "_EXPORT_" and "_DES_" in their names MUST NOT be used.  More
   information about recommendations of TLS and DTLS are included in

   As described in CMC, Section 6.7 of [RFC5272], "For keys that can be
   used as signature keys, signing the certification request with the
   private key serves as a POP on that key pair".  The inclusion of tls-
   unique in the certification request links the proof-of-possession to

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   the TLS proof-of-identity.  This implies but does not prove that the
   authenticated client currently has access to the private key.

   Regarding the CSR attributes that the CA may list for inclusion in an
   enrollment request, an adversary could exclude attributes that a
   server may want, include attributes that a server may not want, and
   render meaningless other attributes that a server may want.  The CA
   is expected to be able to enforce policies to recover from improper
   CSR requests.

   Interpreters of ASN.1 structures should be aware of the use of
   invalid ASN.1 length fields and should take appropriate measures to
   guard against buffer overflows, stack overruns in particular, and
   malicious content in general.

10.  Acknowledgements

   The authors are very grateful to Klaus Hartke for his detailed
   explanations on the use of Block with DTLS.  The authors would like
   to thank Esko Dijk and Michael Verschoor for the valuable discussions
   that helped in shaping the solution.  They would also like to thank
   Peter Panburana from Cisco for his feedback on technical details of
   the solution.  Constructive comments were received from Eliot Lear,
   Jim Schaad, Hannes Tschofenig, and Julien Vermillard.

11.  Change Log


      removed all motivation to and dependence on BRKI

      Supports full EST, except password support

      discovery limited to EST functions

      /.well-known/est is alternative path to short coap path

      proxy discussion is simplified to one case


      binary instead of CBOR binary in mime types.

      supported content types are discoverable.

      DTLS POP text improved.

      First version of Security considerations section written.

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      First version of Proxying section written.

      Various text improvements.


      Merging of draft-vanderstok-ace-coap-est-00 and draft-pritikin-

      URI and discovery are modified

      More text about 6tisch bootstrap including EDHOC and OSCoAP

      mapping to DICE IoT profiles

      adapted to BRSKI progress

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <>.

   [RFC5967]  Turner, S., "The application/pkcs10 Media Type", RFC 5967,
              DOI 10.17487/RFC5967, August 2010,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

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   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,

   [RFC8075]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for Mapping Implementations: HTTP to
              the Constrained Application Protocol (CoAP)", RFC 8075,
              DOI 10.17487/RFC8075, February 2017,

12.2.  Informative References

              Stok, P. and K. Hartke, "The 'Pending' Response Code for
              the Constrained Application Protocol (CoAP)", draft-
              hartke-core-pending-01 (work in progress), August 2017.

              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-01 (work in progress),
              February 2017.

              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-04 (work in progress), October

              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-09 (work in progress), October 2017.

              Institute of Electrical and Electronics Engineers, "IEEE
              Standard 802.15.4-2006", 2006.

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   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,

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   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,

Appendix A.  EST messages to EST-coaps

   This section takes all examples from Appendix A of [RFC7030], changes
   the payload from Base64 to binary and replaces the http headers by
   their CoAP equivalents.

   The corresponding CoAP headers are only shown in Appendix A.1.
   Creating CoAP headers are assumed to be generally known.

   [EDNOTE: The payloads of the examples need to be re-generated with
   appropriate tools and example certificates.]

A.1.  cacerts

   In EST-coaps, a coaps cacerts IPv4 message can be:

   GET coaps://[]/est/crts

   The corresponding CoAP header fields are shown below.  The use of
   block and DTLS are worked out in Appendix C.

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     Ver = 1
     T = 0 (CON)
     Code = 0x01 (0.01 is GET)
      Option1 (Uri-Host)
        Option Delta = 0x3  (option nr = 3)
        Option Length = 0x9
        Option Value =
      Option2 (Uri-Port)
        Option Delta = 0x4  (option nr = 4+3=7)
        Option Length = 0x4
        Option Value = 8085
      Option3 (Uri-Path)
        Option Delta = 0x4   (option nr = 7+4= 11)
        Option Length = 0x9
        Option Value = /est/crts
     Payload = [Empty]

   A 2.05 Content response with a cert in EST-coaps will then be:

   2.05 Content (Content-Format: application/pkcs7-mime)

   with CoAP fields

     Ver = 1
     T = 2 (ACK)
     Code = 0x45 (2.05 Content)
       Option1 (Content-Format)
         Option Delta = 0xC  (option nr = 12)
         Option Length = 0x2
         Option Value = TBD1 (defined in this document)

     Payload =

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A.2.  csrattrs

   In the following valid /csrattrs exchange, the EST-coaps client
   authenticates itself with a certificate issued by the connected CA.

   The initial DTLS handshake is identical to the enrollment example.
   The IPv6 CoAP GET request looks like:

   GET coaps://[2001:db8::2:1]:61616/est/att

   A 2.05 Content response contains attributes which are relevant for
   the authenticated client.  In this example, the EST-coaps server two
   attributes that the client can ignore when they are unknown to him.:

A.3.  enroll / reenroll

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

   During the Enroll/Reenroll exchange, the EST-coaps client uses a CSR
   (PKCS#10) request in the POST request payload.

   After verification of the certificate by the server, a 2.05 Content
   response with the issued certificate will be returned.

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   POST [2001:db8::2:1]:61616/est/sen
   (Content-Format: application/pkcs10)

   2.05 Content (Content-Format: application/pkcs7-mime)

   [EDNOTE: If POP is used, make sure tls-unique in the CSR is a valid
   HMAC output. ]

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A.4.  serverkeygen

   During this valid /serverkeygen exchange, the EST-coaps client
   authenticates itself using the certificate provided by the connected

   [EDNOTE: the client incudes a CSR with a public key that the server
   should ignore, so we need a content-format here. ]

   [EDNote: If POP is used, make sure tls-unique in the CSR is a valid
   HMAC output. ]

   The initial DTLS handshake is identical to the enrollment example.
   The CoAP GET request looks like:

   POST coaps://[]/est/skg

   2.05 Content (Content-Format: application/pkcs8)

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   Without the DecryptKeyIdentifier attribute, the response has no
   additional encryption beyond DTLS.  [EDNOTE: Add comment about
   deriving symmetric keys by using the TLS KEM draft. ]

   The response contains first a preamble that can be ignored.  The EST-
   coaps server can use the preamble to include additional explanations,
   like ownership or support information

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Appendix B.  Encoding for server side key generation

   Sever side key generation for CoAP can be implemented efficiently
   using multipart encoding

   [EDNOTE: text to be written.]

Appendix C.  EST-coaps Block message examples

   This section provides a detailed example of the messages using DTLS
   and BLOCK option Block2.  The minimum PMTU is 1280 bytes, which is
   the example value assumed for the DTLS datagram size.  The example
   block length is taken as 64 which gives an SZX value of 2.

   The following is an example of a valid /cacerts exchange over DTLS.
   The content length of the cacerts response in appendix A.1 of
   [RFC7030] is 4246 bytes using base64.  This leads to a length of 2509
   bytes in binary.  The CoAP message adds around 10 bytes, the DTLS
   record 29 bytes.  To avoid IP fragmentation, the CoAP block option is
   used and an MTU of 127 is assumed to stay within one IEEE 802.15.4
   packet.  To stay below the MTU of 127, the payload is split in 39
   packets with a payload of 64 bytes each, followed by a packet of 13
   bytes.  The client sends an IPv6 packet containing the UDP datagram
   with the DTLS record that encapsulates the CoAP Request 40 times.
   The server returns an IPv6 packet containing the UDP datagram with
   the DTLS record that encapsulates the CoAP response.  The CoAP
   request-response exchange with block option is shown below.  Block
   option is shown in a decomposed way indicating the kind of Block
   option (2 in this case because used in the response) followed by a
   colon, and then the block number (NUM), the more bit (M = 0 means
   last block), and block size exponent (2**(SZX+4)) separated by
   slashes.  The Length 64 is used with SZX= 2 to avoid IP
   fragmentation.  The CoAP Request is sent with confirmable (CON)
   option and the content format of the Response is /application/

   GET []/est/crts     -->
                 <--   (2:0/1/39) 2.05 Content
       GET URI (2:1/1/39)                           -->
                 <--   (2:1/1/39) 2.05 Content
        GET URI (2:65/1/39)                         -->
                 <--   (2:65/0/39) 2.05 Content

   For further detailing the CoAP headers of the first two blocks are
   written out.

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   The header of the first GET looks like:

     Ver = 1
     T = 0 (CON)
     Code = 0x01 (0.1 GET)
      Option1 (Uri-Host)
        Option Delta = 0x3  (option nr = 3)
        Option Length = 0x9
        Option Value =
      Option2 (Uri-Port)
        Option Delta = 0x4   (option nr = 3+4=7)
        Option Length = 0x4
        Option Value = 8085
      Option3 (Uri-Path)
        Option Delta = 0x4    (option nr = 7+4=11)
        Option Length = 0x9
        Option Value = /est/crts
     Payload = [Empty]

   The header of the first response looks like:

     Ver = 1
     T = 2 (ACK)
     Code = 0x45 (2.05 Content.)
       Option1 (Content-Format)
         Option Delta = 0xC  (option 12)
         Option Length = 0x2
         Option Value = TBD1
       Option2 (Block2)
         Option Delta = 0xB  (option 23 = 12 + 11)
         Option Length = 0x1
         Option Value = 0x0A (block number = 0, M=1, SZX=2)
     Payload =

   The second Block2:

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     Ver = 1
     T = 2 (means ACK)
     Code = 0x45 (2.05 Content.)
       Option1 (Content-Format)
         Option Delta = 0xC   (option 12)
         Option Length = 0x2
         Option Value = TBD1
       Option2 (Block2)
         Option Delta = 0xB  (option 23 = 12 + 11)
         Option Length = 0x1
         Option Value = 0x1A (block number = 1, M=1, SZX=2)
     Payload =

   The 40th and final Block2:

     Ver = 1
     T = 2 (means ACK)
     Code = 0x21
       Option1 (Content-Format)
         Option Delta = 0xC   (option 12)
         Option Length = 0x2
         Option Value = TBD1
       Option2 (Block2)
         Option Delta = 0xB  (option 23 = 12 + 11)
         Option Length = 0x2
         Option Value = 0x272 (block number = 39, M=0, SZX=2)
     Payload = 73a30d0c006343116f58403100

Authors' Addresses

   Peter van der Stok


   Panos Kampanakis
   Cisco Systems


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   Sandeep S. Kumar
   Philips Lighting Research
   High Tech Campus 7
   Eindhoven  5656 AE


   Michael C. Richardson
   Sandelman Software Works


   Martin Furuhed
   Nexus Group


   Shahid Raza
   Isafjordsgatan 22
   Kista, Stockholm  16440


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