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Versions: (draft-vanderstok-ace-coap-est) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

ACE                                                      P. van der Stok
Internet-Draft                                                Consultant
Intended status: Standards Track                           P. Kampanakis
Expires: August 31, 2019                                   Cisco Systems
                                                           M. Richardson
                                                                     SSW
                                                                 S. Raza
                                                               RISE SICS
                                                       February 27, 2019


                    EST over secure CoAP (EST-coaps)
                       draft-ietf-ace-coap-est-09

Abstract

   Enrollment over Secure Transport (EST) is used as a certificate
   provisioning protocol over HTTPS.  Low-resource devices often use the
   lightweight Constrained Application Protocol (CoAP) for message
   exchanges.  This document defines how to transport EST payloads over
   secure CoAP (EST-coaps), which allows constrained devices to use
   existing EST functionality for provisioning certificates.

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 https://datatracker.ietf.org/drafts/current/.

   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 August 31, 2019.

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  DTLS and conformance to RFC7925 profiles  . . . . . . . . . .   6
   5.  Protocol Design . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Discovery and URIs  . . . . . . . . . . . . . . . . . . .   9
     5.2.  Mandatory/optional EST Functions  . . . . . . . . . . . .  11
     5.3.  Payload formats . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Message Bindings  . . . . . . . . . . . . . . . . . . . .  13
     5.5.  CoAP response codes . . . . . . . . . . . . . . . . . . .  14
     5.6.  Message fragmentation . . . . . . . . . . . . . . . . . .  14
     5.7.  Delayed Responses . . . . . . . . . . . . . . . . . . . .  15
     5.8.  Server-side Key Generation  . . . . . . . . . . . . . . .  17
   6.  HTTPS-CoAPS Registrar . . . . . . . . . . . . . . . . . . . .  19
   7.  Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  Deployment limitations  . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Content-Format Registry . . . . . . . . . . . . . . . . .  21
     9.2.  Resource Type registry  . . . . . . . . . . . . . . . . .  22
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  23
     10.1.  EST server considerations  . . . . . . . . . . . . . . .  23
     10.2.  HTTPS-CoAPS Registrar considerations . . . . . . . . . .  24
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     13.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  EST messages to EST-coaps  . . . . . . . . . . . . .  30
     A.1.  cacerts . . . . . . . . . . . . . . . . . . . . . . . . .  30
     A.2.  enroll / reenroll . . . . . . . . . . . . . . . . . . . .  32
     A.3.  serverkeygen  . . . . . . . . . . . . . . . . . . . . . .  34
     A.4.  csrattrs  . . . . . . . . . . . . . . . . . . . . . . . .  36
   Appendix B.  EST-coaps Block message examples . . . . . . . . . .  37
     B.1.  cacerts . . . . . . . . . . . . . . . . . . . . . . . . .  37
     B.2.  enroll / reenroll . . . . . . . . . . . . . . . . . . . .  41
   Appendix C.  Message content breakdown  . . . . . . . . . . . . .  42
     C.1.  cacerts . . . . . . . . . . . . . . . . . . . . . . . . .  42
     C.2.  enroll / reenroll . . . . . . . . . . . . . . . . . . . .  44
     C.3.  serverkeygen  . . . . . . . . . . . . . . . . . . . . . .  45



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Change Log

   EDNOTE: Remove this section before publication

   -09

      WGLC comments taken into account

      consensus about discovery of content-format

      added additional path for content-format selection

      merged DTLS sections

   -08

      added application/pkix-cert Content-Format TBD287.

      discovery text clarified

      Removed text on ct negotiation in connection to multipart-core

      removed text that duplicates or contradicts RFC7252 (thanks Klaus)

      Stated that well-known/est is compulsory

      Use of response codes clarified.

      removed bugs: Max-Age and Content-Format Options in Request

      Accept Option explained for est/skg and added in enroll example

      Added second URI /skc for server-side key gen and a simple cert
      (not PKCS#7)

      Persistence of DTLS connection clarified.

      Minor text fixes.

   -07:

      redone examples from scratch with openssl

      Updated authors.

      Added CoAP RST as a MAY for an equivalent to an HTTP 204 message.



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      Added serialization example of the /skg CBOR response.

      Added text regarding expired IDevIDs and persistent DTLS
      connection that will start using the Explicit TA Database in the
      new DTLS connection.

      Nits and fixes

      Removed CBOR envelop for binary data

      Replaced TBD8 with 62.

      Added RFC8174 reference and text.

      Clarified MTI for server-side key generation and Content-Formats.
      Defined the /skg MTI (PKCS#8) and the cases where CMS encryption
      will be used.

      Moved Fragmentation section up because it was referenced in
      sections above it.

   -06:

      clarified discovery section, by specifying that no discovery may
      be needed for /.well-known/est URI.

      added resource type values for IANA

      added list of compulsory to implement and optional functions.

      Fixed issues pointed out by the idnits tool.

      Updated CoAP response codes section with more mappings between EST
      HTTP codes and EST-coaps CoAP codes.

      Minor updates to the MTI EST Functions section.

      Moved Change Log section higher.

   -05:

      repaired again

      TBD8 = 62 removed from C-F registration, to be done in CT draft.

   -04:





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      Updated Delayed response section to reflect short and long delay
      options.

   -03:

      Removed observe and simplified long waits

      Repaired Content-Format specification

   -02:

      Added parameter discussion in section 8

      Concluded Content-Format specification using multipart-ct draft

      examples updated

   -01:

      Editorials done.

      Redefinition of proxy to Registrar in Section 6.  Explained better
      the role of https-coaps Registrar, instead of "proxy"

      Provide "observe" Option examples

      extended block message example.

      inserted new server key generation text in Section 5.8 and
      motivated server key generation.

      Broke down details for DTLS 1.3

      New Media-Type uses CBOR array for multiple Content-Format
      payloads

      provided new Content-Format tables

      new media format for IANA

   -00

      copied from vanderstok-ace-coap-04








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2.  Introduction

   "Classical" 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).  EST transports messages over HTTPS.

   This document defines a new transport for EST based on the
   Constrained Application Protocol (CoAP) since some Internet of Things
   (IoT) devices use CoAP instead of HTTP.  Therefore, this
   specification utilizes DTLS [RFC6347] and CoAP [RFC7252] instead of
   TLS [RFC8446] and HTTP [RFC7230].

   EST responses can be relatively large and for this reason this
   specification also uses CoAP Block-Wise Transfer [RFC7959] to offer a
   fragmentation mechanism of EST messages at the CoAP layer.

   This document also profiles the use of EST to only support
   certificate-based client authentication.  HTTP Basic or Digest
   authentication (as described in Section 3.2.3 of [RFC7030]) are not
   supported.

3.  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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Many of the concepts in this document are taken from [RFC7030].
   Consequently, much text is directly traceable to [RFC7030].

4.  DTLS and conformance to RFC7925 profiles

   This section describes how EST-coaps fits into the profiles of low-
   resource devices described in [RFC7925].  EST-coaps can transport
   certificates and private keys.  Certificates are responses to
   (re-)enrollment requests or requests for a trusted certificate list.
   Private keys can be transported as responses to a server-side key
   generation request as described in Section 4.4 of [RFC7030] and
   discussed in Section 5.8 of this document.

   EST-coaps depends on a secure transport mechanism that secures the
   exchanged CoAP messages.  DTLS is one such secure protocol.  No other
   changes are necessary regarding the secure transport of EST messages.





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   +------------------------------------------------+
   |    EST request/response messages               |
   +------------------------------------------------+
   |    CoAP for message transfer and signaling     |
   +------------------------------------------------+
   |    Secure Transport                            |
   +------------------------------------------------+

                    Figure 1: EST-coaps protocol layers

   As per sections 3.3 and 4.4 of [RFC7925], the mandatory cipher suite
   for DTLS in EST-coaps is TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
   [RFC7251].  Curve secp256r1 MUST be supported [RFC8422]; this curve
   is equivalent to the NIST P-256 curve.  Additionally, crypto agility
   is important, and the recommendations in Section 4.4 of [RFC7925] and
   any updates to it concerning Curve25519 and other curves also apply.

   DTLS 1.2 implementations must use the Supported Elliptic Curves and
   Supported Point Formats Extensions in [RFC8422].  Uncompressed point
   format must also be supported.  DTLS 1.3 [I-D.ietf-tls-dtls13]
   implementations differ from DTLS 1.2 because they do not support
   point format negotiation in favor of a single point format for each
   curve.  Thus, support for DTLS 1.3 does not mandate point format
   extensions and negotiation.

   CoAP was designed to avoid IP 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 several records, each of which
   can be transmitted separately, thus avoiding IP fragmentation"
   [RFC6347].

   The authentication of the EST-coaps server by the EST-coaps client is
   based on certificate authentication in the DTLS handshake.  The EST-
   coaps client MUST be configured with at least an Implicit TA database
   which will enable the authentication of the server the first time
   before updating its trust anchor (Explicit TA) [RFC7030].

   The authentication of the EST-coaps client MUST be with a client
   certificate in the DTLS handshake.  This can either be

   o  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  a previously installed certificate (e.g., manufacturer IDevID
      [ieee802.1ar] or a certificate issued by some other party); the



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      server is expected to trust that certificate.  IDevID's are
      expected to have a very long life, as long as the device, but
      under some conditions could expire.  In that case, the server MAY
      want to authenticate a client certificate against its trust store
      although the certificate is expired (Section 10).

   EST-coaps supports the certificate types and Trust Anchors (TA) that
   are specified for EST in Section 3 of [RFC7030].

   CoAP and DTLS can provide proof-of-identity for EST-coaps clients and
   servers with simple PKI messages as described in Section 3.1 of
   [RFC5272].  Moreover, 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 consists of the contents of
   the first "Finished" message in the (D)TLS handshake between server
   and client [RFC5929].  The client adds the "Finished" message as a
   ChallengePassword in the attributes section of the PKCS#10 Request
   [RFC5967] to prove that the client is indeed in control of the
   private key at the time of the (D)TLS session establishment.

   In the case of EST-coaps, the same operations can be performed during
   the DTLS handshake.  For DTLS 1.2, in the event of handshake message
   fragmentation, the Hash of the handshake messages used in the MAC
   calculation of the Finished message must be computed as if each
   handshake message had been sent as a single fragment (Section 4.2.6
   of [RFC6347]).  The Finished message is calculated as shown in
   Section 7.4.9 of [RFC5246].  Similarly, for DTLS 1.3, the Finished
   message must be computed as if each handshake message had been sent
   as a single fragment (Section 5.8 of [I-D.ietf-tls-dtls13]) following
   the algorithm described in 4.4.4 of [RFC8446].

   In a constrained CoAP environment, endpoints can't always 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.  To alleviate this
   situation, an EST-coaps DTLS connection MAY remain open for
   sequential EST transactions.  For example, an EST csrattrs request
   that is followed by a simpleenroll request can use the same
   authenticated DTLS connection.  However, when a cacerts request is
   included in the set of sequential EST transactions, some additional
   security considerations apply regarding the use of the Implicit and
   Explicit TA database as explained in Section 10.1.

   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



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   that are relatively close to each other.  In some cases, like NAT
   rebinding, keeping the state of a connection is not possible when
   devices sleep for extended periods of time.  In such occasions,
   [I-D.ietf-tls-dtls-connection-id] negotiates a connection ID that can
   eliminate the need for new handshake and its additional cost.

5.  Protocol Design

   EST-coaps uses CoAP to transfer EST messages, aided by Block-Wise
   Transfer [RFC7959] to avoid IP fragmentation.  The use of Blocks for
   the transfer of larger EST messages is specified in Section 5.6.
   Figure 1 shows the layered EST-coaps architecture.

   The EST-coaps protocol design follows closely the EST design.  The
   supported message types in EST-coaps are:

   o  CA certificate retrieval needed to receive the complete set of CA
      certificates.

   o  Simple enroll and re-enroll for a CA to sign public client
      identity key.

   o  Certificate Signing Request (CSR) attribute messages that inform
      the client of the fields to include in a CSR.

   o  Server-side key generation messages to provide a private client
      identity key when the client choses so.

5.1.  Discovery and URIs

   EST-coaps is targeted for low-resource networks with small packets.
   Saving header space is important and short EST-coaps URIs are
   specified in this document.  These URIs are shorter than the ones in
   [RFC7030].  Two example EST-coaps resource path names are:

   coaps://est-coaps.example.org:<port>/.well-known/est/<short-est>
   coaps://est-coaps.example.org:<port>/.well-known/est/
                                              ArbitraryLabel/<short-est>

   The short-est strings are defined in Table 1.  The ArbitraryLabel
   path-segment, if used, SHOULD be of the shortest length possible
   (Sections 3.1 and 3.2.2 of [RFC7030].  Arbitrary Labels are usually
   defined and used by EST CAs in order to route client requests to the
   appropriate certificate profile.

   The EST-coaps server URIs, obtained through discovery of the EST-
   coaps root resource(s) as shown below, are of the form:




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   coaps://est-coaps.example.org:<port>/<root-resource>/<short-est>
   coaps://est-coaps.example.org:<port>/<root-resource>/
                                              ArbitraryLabel/<short-est>

   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 (PKCS#7)                 |
           | /serverkeygen    | /skc (application/pkix-cert)  |
           +------------------+-------------------------------+

                     Table 1: Short EST-coaps URI path

   The /skg message is the EST /serverkeygen equivalent where the client
   requests for a certificate in PKCS#7 format and a private key.  If
   the client prefers a single application/pkix-cert certificate instead
   of PKCS#7, he will make an /skc request.

   Clients and servers MUST support the short resource URIs.

   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.  The example below shows the
   discovery of the presence and location of EST-coaps resources.
   Linefeeds are included only for readability.

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

     RES: 2.05 Content
   </est/crts>;rt="ace.est.crts";ct="281 TBD287",
   </est/sen>;rt="ace.est.sen";ct="281 TBD287",
   </est/sren>;rt="ace.est.sren";ct="281 TBD287",
   </est/att>;rt="ace.est.att";ct=285,
   </est/skg>;rt="ace.est.skg";ct=62,
   </est/skc>;rt="ace.est.skc";ct=62

   The first three lines of the discovery response above MUST be
   returned if the server supports resource discovery.  The last three



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   lines are only included if the corresponding EST functions are
   implemented.  The Content-Formats in the response allow the client to
   request one that is supported by the server.

   Discoverable port numbers can be returned in the response payload.
   An example response payload for non-default CoAPS server port 61617
   follows below.  Linefeeds were included only for readability.

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

     RES: 2.05 Content
   <coaps://[2001:db8:3::123]:61617/est/crts>;rt="ace.est.crts";
                 ct="281 TBD287",
   <coaps://[2001:db8:3::123]:61617/est/sen>;rt="ace.est.sen";
                 ct="281 TBD287",
   <coaps://[2001:db8:3::123]:61617/est/sren>;rt="ace.est.sren";
                 ct="281 TBD287",
   <coaps://[2001:db8:3::123]:61617/est/att>;rt="ace.est.att";
                 ct=285,
   <coaps://[2001:db8:3::123]:61617/est/skg>;rt="ace.est.skg";
                 ct=62,
   <coaps://[2001:db8:3::123]:61617/est/skc>;rt="ace.est.skc";
                 ct=62

   The server MUST support the default /.well-known/est root resource.
   The server SHOULD support resource discovery when he supports non-
   default URIs (like /est or /est/ArbitraryLabel) or ports.  The client
   SHOULD use resource discovery when /.well-known/est fails or when the
   client is unaware of the available EST-coaps resources.

   It is up to the implementation to choose its root resource;
   throughout this document the example root resource /est is used.

5.2.  Mandatory/optional EST Functions

   This specification contains a set of required-to-implement functions,
   optional functions, and not specified functions.  The latter ones are
   deemed too expensive for low-resource devices in payload and
   calculation times.

   Table 2 specifies the mandatory-to-implement or optional
   implementation of the EST-coaps functions.  Discovery of the
   existence of optional functions is described in Section 5.1.








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              +------------------+--------------------------+
              | EST Functions    | EST-coaps implementation |
              +------------------+--------------------------+
              | /cacerts         | MUST                     |
              | /simpleenroll    | MUST                     |
              | /simplereenroll  | MUST                     |
              | /csrattrs        | OPTIONAL                 |
              | /serverkeygen    | OPTIONAL                 |
              | /fullcmc         | Not specified            |
              +------------------+--------------------------+

                   Table 2: List of EST-coaps functions

   While [RFC7030] permits a number of these functions to be used
   without authentication, this specification requires that the client
   MUST be authenticated for all functions.

5.3.  Payload formats

   EST-coaps is designed for low-resource devices and hence does not
   need to send Base64-encoded data.  Simple binary is more efficient
   (30% smaller payload) and well supported by CoAP.  Thus, the payload
   for a given Media-Type follows the ASN.1 structure of the Media-Type
   and is transported in binary format.

   The Content-Format (HTTP 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 (Section 3.2.2
   of [RFC7030]) are specified by the Content-Format Option (12) of
   CoAP.  The combination of URI-Path and Content-Format in EST-coaps
   MUST map to an allowed combination of URI and Media-Type in EST.  The
   required Content-Formats for these requests and response messages are
   defined in Section 9.1.  The CoAP response codes are defined in
   Section 5.5.

   Content-Format TBD287 can be used in place of 281 to carry a single
   certificate instead of a PKCS#7 container in a /crts, /sen, /sren or
   /skg response.  Content-Format 281 MUST be supported by EST-coaps
   servers.  Servers MAY also support Content-Format TBD287.  It is up
   to the client to support only Content-Format 281, TBD287 or both.
   The client will use a COAP Accept Option in the request to express
   the preferred response Content-Format.  If an Accept Option is not
   included in the request, the client is not expressing any preference
   and the server SHOULD choose format 281.

   Content-Format 286 is used in /sen, /sren and /skg requests and 285
   in /att responses.




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   A representation with Content-Format identifier 62 contains a
   collection of representations along with their respective Content-
   Format.  The Content-Format identifies the Media-Type application/
   multipart-core specified in [I-D.ietf-core-multipart-ct].  For
   example, a collection, containing two representations in response to
   a EST-coaps server-side key generation /skg request, could include a
   private key in PKCS#8 [RFC5958] with Content-Format identifier 284
   (0x011C) and a single certificate in a PKCS#7 container with Content-
   Format identifier 281 (0x0119).  Such a collection would look like
   [284,h'0123456789abcdef', 281,h'fedcba9876543210'] in diagnostic CBOR
   notation.  The serialization of such CBOR content would be


      84                  # array(4)
      19 011C             # unsigned(284)
      48                  # bytes(8)
         0123456789ABCDEF # "\x01#Eg\x89\xAB\xCD\xEF"
      19 0119             # unsigned(281)
      48                  # bytes(8)
         FEDCBA9876543210 # "\xFE\xDC\xBA\x98vT2\x10"

                   Multipart /skg response serialization

   When the client makes an /skc request the certificate returned with
   the private key is a single X.509 certificate (not a PKCS#7
   container) with Content-Format identifier TBD287 (0x011F) instead of
   281.  In cases where the private key is encrypted with CMS (as
   explained in Section 5.8) the Content-Format identifier is 280
   (0x0118) instead of 284.  The key and certificate representations are
   ASN.1 encoded in binary format.  An example is shown in Appendix A.3.

5.4.  Message Bindings

   The general EST-coaps message characteristics are:

   o  All EST-coaps request messages expect an acknowledgement (with a
      response payload); EST-coaps requests are confirmable CON CoAP
      messages.

   o  The CoAP Options used are Uri-Host, Uri-Path, Uri-Port, Content-
      Format, Block, Accept and Location-Path.  These CoAP Options are
      used to communicate the HTTP fields specified in the EST REST
      messages.  The URI-host and Uri-Port Options can be omitted from
      the COAP message sent on the wire.  When omitted, they are
      logically assumed to be the transport protocol destination address
      and port respectively.  Explicit Uri-Host and Uri-Port Options are
      typically used when an endpoint hosts multiple virtual servers and




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      uses the Options to route the requests accordingly.  Other COAP
      Options should be handled in accordance with [RFC7252].

   o  EST URLs are HTTPS based (https://), in CoAP these are assumed to
      be translated to CoAPS (coaps://)

   Table 1 provides the mapping from the EST URI path to the EST-coaps
   URI path.  Appendix A includes some practical examples of EST
   messages translated to CoAP.

5.5.  CoAP response codes

   Section 5.9 of [RFC7252] and Section 7 of [RFC8075] specify 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 request (/cacerts, /csrattrs), the equivalent CoAP response code
   2.05 or 2.03 MUST be used in EST-coaps.  Similarly, 2.01, 2.02 or
   2.04 MUST be used in response to EST POST requests (/simpleenroll,
   /simplereenroll, /serverkeygen).

   HTTP response code 202 with a Retry-After header in [RFC7030] has no
   equivalent in CoAP.  Retry-After is used in EST for delayed server
   responses.  Section 5.7 specifies how EST-coaps handles delayed
   messages.

   EST makes use of HTTP 204 and 404 responses when a resource is not
   available for the client.  The equivalent CoAP codes to use in an
   EST-coaps responses are 2.04 and 4.04.  Additionally, EST's HTTP 401
   error translates to 4.01 in EST-coaps.  Other EST HTTP error messages
   are 400, 423 and 503.  Their equivalent CoAP errors are 4.00, 4.03
   and 5.03 respectively.  In case a CoAP Option is unrecognized and
   critical, the server is expected to return a 4.02 (Bad Option).

5.6.  Message fragmentation

   DTLS defines fragmentation only for the handshake 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 [ieee802.15.4] 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 in EST-coaps.  Even though ECC
   certificates are small in size, they can vary greatly based on
   signature algorithms, key sizes, and Object Identifier (OID) fields



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   used.  For 256-bit curves, common ECDSA cert sizes are 500-1000 bytes
   which could fluctuate further based on the algorithms, OIDs, Subject
   Alternative Names (SAN) and cert fields.  For 384-bit curves, ECDSA
   certificates increase in size and can sometimes reach 1.5KB.
   Additionally, there are times when the EST cacerts response from the
   server can include multiple certificates 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.  Even with ECC,
   EST-coaps messages can still exceed MTU sizes on the Internet or
   6LoWPAN [RFC4919] (Section 2 of [RFC7959]).  EST-coaps needs to be
   able to fragment messages into multiple DTLS datagrams.

   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.  As explained
   in Section 1 of [RFC7959], block-wise transfers should be used in
   Confirmable CoAP messages to avoid the exacerbation of lost blocks.
   The EST-coaps client and server MUST support Block2.  Block1 MUST be
   supported for EST-coaps enrollment requests that exceed the Path MTU.

   [RFC7959] also defines Size1 and Size2 Options to provide size
   information about the resource representation in a request and
   response.  EST-client and server MAY support Size1 and Size2 Options.

   Examples of fragmented EST-coaps messages are shown in Appendix B.

5.7.  Delayed Responses

   Server responses can sometimes be delayed.  According to
   Section 5.2.2 of [RFC7252], a slow server can acknowledge the request
   and respond later with the requested resource representation.  In
   particular, a slow server can respond to an EST-coaps enrollment
   request with an empty ACK with code 0.00, before sending the
   certificate to the client after a short delay.  If the certificate
   response is large, the server will need more than one Block2 blocks
   to transfer it.

   This situation is shown in Figure 2.  The client sends an enrollment
   request that uses N1+1 Block1 blocks.  The server uses an empty 0.00
   ACK to announce the delayed response which is provided later with
   2.04 messages containing N2+1 Block2 Options.  The first 2.04 is a
   confirmable message that is acknowledged by the client.  Onwards,
   having received the first 256 bytes in the first Block2 block, the
   client asks for a block reduction to 128 bytes in a confirmable



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   enrollment request and acknowledges the Block2 blocks sent up to that
   point.


   POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
          <-- (ACK) (1:0/1/256) (2.31 Continue)
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
          <-- (ACK) (1:1/1/256) (2.31 Continue)
                         .
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
          <-- (0.00 empty ACK)
                         |
          ...... short delay before certificate is ready ......
                         |
         <-- (CON) (1:N1/0/256)(2:0/1/256)(2.04 Changed) {Cert resp}
                                      (ACK)                     -->
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/128)          -->
          <-- (ACK) (2:1/1/128) (2.04 Changed) {Cert resp}
                         .
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/128)          -->
          <-- (ACK) (2:N2/0/128) (2.04 Changed) {Cert resp}

               Figure 2: EST-COAP enrollment with short wait

   If the server is very slow (i.e. minutes) in providing the response
   (i.e. when a manual intervention is needed), he SHOULD respond with
   an ACK containing response code 5.03 (Service unavailable) and a Max-
   Age Option to indicate the time the client SHOULD wait to request the
   content later.  After a delay of Max-Age, the client SHOULD resend
   the identical CSR to the server.  As long as the server responds with
   response code 5.03 (Service Unavailable) with a Max-Age Option, the
   client SHOULD keep resending the enrollment request until the server
   responds with the certificate or the client abandons for other
   reasons.

   To demonstrate this scenario, Figure 3 shows a client sending an
   enrollment request that uses N1+1 Block1 blocks to send the CSR to
   the server.  The server needs N2+1 Block2 blocks to respond, but also
   needs to take a long delay (minutes) to provide the response.
   Consequently, the server uses a 5.03 ACK response with a Max-Age
   Option.  The client waits for a period of Max-Age as many times as he
   receives the same 5.03 response and retransmits the enrollment
   request until he receives a certificate in a fragmented 2.01




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   response.  Note that the server asks for a decrease in the block size
   when acknowledging the first Block2.



   POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
          <-- (ACK) (1:0/1/256) (2.31 Continue)
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
          <-- (ACK) (1:1/1/256) (2.31 Continue)
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
        <-- (ACK) (1:N1/0/256) (2:0/0/128)(5.03 Service Unavailable)
                                                           (Max-Age)
                         |
                         |
   Client tries one or more times after Max-Age with identical payload
                         |
                         |
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
        <-- (ACK) (1:N1/0/256) (2:0/1/128) (2.01 Created){Cert resp}
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/128)           -->
        <-- (ACK) (2:1/1/128) (2.01 Created) {Cert resp}
                         .
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/128)          -->
          <-- (ACK) (2:N2/0/128) (2.01 Created) {Cert resp}


               Figure 3: EST-COAP enrollment with long wait

5.8.  Server-side Key Generation

   Constrained devices sometimes do not have the necessary hardware to
   generate statistically random numbers for private keys and DTLS
   ephemeral keys.  Past experience has also shown that low-resource
   endpoints sometimes generate numbers which could allow someone to
   decrypt the communication or guess the private key and impersonate as
   the device [PsQs] [RSAorig].  Additionally, random number key
   generation is costly, thus energy draining.  Even though the random
   numbers that constitute the identity/cert do not get generated often,
   an endpoint may not want to spend time and energy generating
   keypairs, and just ask for one from the server.

   In these scenarios, server-side key generation can be used.  The
   client asks for the server or proxy to generate the private key and
   the certificate which are transferred back to the client in the



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   server-side key generation response.  In all respects, the server
   SHOULD treat the CSR as it would treat any enroll or re-enroll CSR;
   the only distinction here is that the server MUST ignore the public
   key values and signature in the CSR.  These are included in the
   request only to allow re-use of existing codebases for generating and
   parsing such requests.

   The client /skg request is for a certificate in a PKCS#7 container
   and private key in two application/multipart-core elements.
   Respectively, an /skc request is for a single application/pkix-cert
   certificate and a private key.  The private key Content-Format
   requested by the client is depicted in the PKCS#10 CSR request.  If
   the request contains SMIMECapabilities and DecryptKeyIdentifier or
   AsymmetricDecryptKeyIdentifier the client is expecting Content-Format
   280 for the private key.  Then the private key is encrypted
   symmetrically or asymmetrically as per [RFC7030].  The symmetric key
   or the asymmetric keypair establishment method is out of scope of the
   specification.  A /skg or /skc request with a CSR without
   SMIMECapabilities expects an application/multipart-core with an
   unencrypted PKCS#8 private key with Content-Format 284.

   The EST-coaps server-side key generation response is returned with
   Content-Format application/multipart-core
   [I-D.ietf-core-multipart-ct] containing a CBOR array with four items
   (Section 5.3) .  The two representations (each consisting of two CBOR
   array items) do not have to be in a particular order since each
   representation is preceded by its Content-Format ID.  The private key
   can be in unprotected PKCS#8 [RFC5958] format (Content-Format 284) or
   protected inside of CMS SignedData (Content-Format 280).  The
   SignedData is signed by the party that generated the private key,
   which may be the EST server or the EST CA.  The SignedData is further
   protected by placing it inside of a CMS EnvelopedData as explained in
   Section 4.4.2 of [RFC7030].  In summary, the symmetrically encrypted
   key is included in the encryptedKey attribute in a KEKRecipientInfo
   structure.  In the case where the asymmetric encryption key is
   suitable for transport key operations the generated private key is
   encrypted with a symmetric key which is encrypted by the client
   defined (in the CSR) asymmetric public key and is carried in an
   encryptedKey attribute in a KeyTransRecipientInfo structure.
   Finally, if the asymmetric encryption key is suitable for key
   agreement, the generated private key is encrypted with a symmetric
   key which is encrypted by the client defined (in the CSR) asymmetric
   public key and is carried in an recipientEncryptedKeys attribute in a
   KeyAgreeRecipientInfo.

   [RFC7030] recommends the use of additional encryption of the returned
   private key.  For the context of this specification, clients and
   servers that choose to support server-side key generation MUST



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   support unprotected (PKCS#8) private keys (Content-Format 284).
   Symmetric or asymmetric encryption of the private key (CMS
   EnvelopedData, Content-Format 280) SHOULD be supported for
   deployments where end-to-end encryption needs to be provided between
   the client and a server.  Such cases could include architectures
   where an entity between the client and the CA terminates the DTLS
   connection (Registrar in Figure 4).

6.  HTTPS-CoAPS Registrar

   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 which case it will support TLS/HTTP instead of
   CoAPS.  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 Registrar at the edge is required to
   operate between the CoAP environment and the external HTTP network as
   shown in Figure 4.

                                        Constrained Network
   .------.                         .----------------------------.
   |  CA  |                         |.--------------------------.|
   '------'                         ||                          ||
      |                             ||                          ||
   .------.  HTTP   .-----------------.   CoAPS  .-----------.  ||
   | EST  |<------->|EST-coaps-to-HTTPS|<------->| EST Client|  ||
   |Server|over TLS |   Registrar     |          '-----------'  ||
   '------'         '-----------------'                         ||
                                    ||                          ||
                                    |'--------------------------'|
                                    '----------------------------'

       Figure 4: EST-coaps-to-HTTPS Registrar at the CoAP boundary.

   The EST-coaps-to-HTTPS Registrar MUST terminate EST-coaps downstream
   and initiate EST connections over TLS upstream.  The Registrar MUST
   authenticate and OPTIONALLY authorize the clients and it MUST be
   authenticated by the EST server or CA.  The trust relationship
   between the Registrar and the EST server SHOULD be pre-established
   for the Registrar to proxy these connections on behalf of various
   clients.

   When enforcing Proof-of-Possession (POP) linking, the DTLS tls-unique
   value of the (D)TLS session is used to prove that the private key
   corresponding to the public key is in the possession of the client
   and was used to establish the connection as explained in Section 4.
   The POP linking information is lost between the EST-coaps client and
   the EST server when a Registrar is present.  The EST server becomes



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   aware of the presence of a Registrar from its TLS client certificate
   that includes id-kp-cmcRA [RFC6402] extended key usage extension
   (EKU).  As explained in Section 3.7 of [RFC7030], the EST server
   SHOULD apply an authorization policy consistent with a Registrar
   client.  For example, it could be configured to accept POP linking
   information that does not match the current TLS session because the
   authenticated EST client Registrar has verified this information when
   acting as an EST server.

   For some use cases, clients that leverage server-side key generation
   might prefer for the enrolled keys to be generated by the Registrar
   if the CA does not support server-side key generation.  Such
   Registrar is responsible for generating a new CSR signed by a new key
   which will be returned to the client along with the certificate from
   the CA.  In these cases, the Registrar MUST support random number
   generation using proper entropy.

   Table 1 contains the URI mappings between EST-coaps and EST that the
   Registrar MUST adhere to.  Section 5.5 of this specification and
   Section 7 of [RFC8075] define the mappings between EST-coaps and HTTP
   response codes, that determine how the Registrar MUST translate CoAP
   response codes from/to HTTP status codes.  The mapping from CoAP
   Content-Format to HTTP Media-Type is defined in Section 9.1.
   Additionally, a conversion from CBOR major type 2 to Base64 encoding
   MUST take place at the Registrar when server-side key generation is
   supported.  If CMS end-to-end encryption is employed for the private
   key, the encrypted CMS EnvelopedData blob MUST be converted to binary
   in CBOR type 2 downstream to the client.

   Due to fragmentation of large messages into blocks, an EST-coaps-to-
   HTTP Registrar MUST reassemble the BLOCKs before translating the
   binary content to Base64, and consecutively relay the message
   upstream.

   If necessary, the EST-coaps-to-HTTP Registrar will support resouce
   discovery according to the rules in Section 5.1.

7.  Parameters

   This section addresses transmission parameters described in sections
   4.7 and 4.8 of [RFC7252].  EST does not impose any unique values on
   the CoAP parameters in [RFC7252], but the EST parameter values need
   to be tuned to the CoAP parameter values.

   It is RECOMMENDED, based on experiments, to follow the default CoAP
   configuration parameters ([RFC7252]).  However, depending on the
   implementation scenario, retransmissions and timeouts can also occur
   on other networking layers, governed by other configuration



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   parameters.  A change in a server parameter MUST ensure the adjusted
   value is also available to all the endpoints with which these
   adjusted values are to be used to communicate.

   Some further comments about some specific parameters, mainly from
   Table 2 in [RFC7252]:

   o  NSTART: A parameter that controls the number of simultaneous
      outstanding interactions that a client maintains to a given
      server.  An EST-coaps client is not expected to interact with more
      than one servers at the same time, which is the default NSTART
      value defined in [RFC7252].

   o  DEFAULT_LEISURE: This setting is only relevant in multicast
      scenarios, outside the scope of EST-coaps.

   o  PROBING_RATE: A parameter which specifies the rate of re-sending
      non-confirmable messages.  The EST messages are defined to be sent
      as CoAP confirmable messages, hence this setting is not
      applicable.

   Finally, the Table 3 parameters in [RFC7252] are mainly derived from
   Table 2.  Directly changing parameters on one table would affect
   parameters on the other.

8.  Deployment limitations

   Although EST-coaps paves the way for the utilization of EST by
   constrained devices in constrained networks, some classes of devices
   [RFC7228] will not have enough resources to handle the 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 DTLS/CoAP.  It is up to the
   network designer to decide which devices execute the EST protocol and
   which do not.

9.  IANA Considerations

9.1.  Content-Format Registry

   Additions to the sub-registry "CoAP Content-Formats", within the
   "CoRE Parameters" registry [COREparams] are specified in Table 3.
   These have been registered provisionally in the Expert Review range
   (0-255).







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   +------------------------------+-------+----------------------------+
   | HTTP Media-Type              |    ID | Reference                  |
   +------------------------------+-------+----------------------------+
   | application/pkcs7-mime;      |   280 | [RFC7030] [I-D.ietf-lamps- |
   | smime-type=server-generated- |       | rfc5751-bis]               |
   | key                          |       |                            |
   | application/pkcs7-mime;      |   281 | [I-D.ietf-lamps-rfc5751-bi |
   | smime-type=certs-only        |       | s]                         |
   | application/pkcs8            |   284 | [RFC5958] [I-D.ietf-lamps- |
   |                              |       | rfc5751-bis]               |
   | application/csrattrs         |   285 | [RFC7030] [RFC7231]        |
   | application/pkcs10           |   286 | [RFC5967] [I-D.ietf-lamps- |
   |                              |       | rfc5751-bis]               |
   | application/pkix-cert        | TBD28 | [RFC2585]                  |
   |                              |     7 |                            |
   +------------------------------+-------+----------------------------+

                     Table 3: New CoAP Content-Formats

   It is suggested that 287 is allocated to TBD287.

9.2.  Resource Type registry

   This memo registers new Resource Type (rt=) Link Target Attributes in
   the "Resource Type (rt=) Link Target Attribute Values" subregistry
   under the "Constrained RESTful Environments (CoRE) Parameters"
   registry.

   o  rt="ace.est.crts".  This resource depicts the support of EST get
      cacerts.

   o  rt="ace.est.sen".  This resource depicts the support of EST simple
      enroll.

   o  rt="ace.est.sren".  This resource depicts the support of EST
      simple reenroll.

   o  rt="ace.est.att".  This resource depicts the support of EST CSR
      attributes.

   o  rt="ace.est.skg".  This resource depicts the support of EST
      server-side key generation with the returned certificate in a
      PKCS#7 container.

   o  rt="ace.est.skc".  This resource depicts the support of EST
      server-side key generation with the returned certificate in
      application/pkix-cert format.




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10.  Security Considerations

10.1.  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.
   The transport of these keys is inherently risky.  Analysis SHOULD be
   done to establish whether server-side key generation enhances or
   decreases the probability of identity stealing.

   It is also RECOMMENDED that the Implicit Trust Anchor database used
   for EST server authentication is carefully managed to reduce the
   chance of a third-party CA with poor certification practices
   jeopardizing authentication.  Disabling the Implicit Trust Anchor
   database after successfully receiving the Distribution of CA
   certificates response (Section 4.1.3 of [RFC7030]) limits any risk to
   the first DTLS exchange.  Alternatively, in a case where a /sen
   request immediately follows a /crt, a client MAY choose to keep the
   connection authenticated by the Implicit TA open for efficiency
   reasons (Section 4).  A client that pipelines EST-coaps /crt request
   with other requests in the same DTLS connection SHOULD revalidate the
   server certificate chain against the updated Explicit TA from the
   /crt response before proceeding with the subsequent requests.  If the
   server certificate chain does not authenticate against the database,
   the client SHOULD close the connection without completing the rest of
   the requests.  The updated Explicit TA MUST continue to be used in
   new DTLS connections.

   In cases where the IDevID used to authenticate the client is expired
   the server MAY still authenticate the client because IDevIDs are
   expected to live as long as the device itself (Section 4).  In such
   occasions, checking the certificate revocation status or authorizing
   the client using another method is important for the server to ensure
   that the client is to be trusted.

   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
   [RFC7525].

   As described in CMC, Section 6.7 of [RFC5272], "For keys that can be
   used as signature keys, signing the certification request with the



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   private key serves as a POP on that key pair".  The inclusion of tls-
   unique in the certificate request links the proof-of-possession to
   the TLS proof-of-identity.  This implies but does not prove that only
   the authenticated client currently has access to the private key.

   What's more, POP linking uses tls-unique as it is defined in
   [RFC5929].  The 3SHAKE attack [tripleshake] poses a risk by allowing
   a man-in-the-middle to leverage session resumption and renegotiation
   to inject himself between a client and server even when channel
   binding is in use.  The attack was possible because of certain (D)TLS
   implementation imperfections.  In the context of this specification,
   an attacker could invalidate the purpose of the POP linking
   ChallengePassword in the client request by resuming an EST-coaps
   connection.  Even though the practical risk of such an attack to EST-
   coaps is not devastating, we would rather use a more secure channel
   binding mechanism.  Such a mechanism could include an updated tls-
   unique value generation like the tls-unique-prf defined in
   [I-D.josefsson-sasl-tls-cb] by using a TLS exporter [RFC5705] in TLS
   1.2 or TLS 1.3's updated exporter (Section 7.5 of [RFC8446]).  Such
   mechanism has not been standardized yet.  Adopting a channel binding
   value generated from an exporter would break backwards compatibility.
   Thus, in this specification we still depend in the tls-unique
   mechanism defined in [RFC5929], especially since the practicality of
   such an attack would not expose any messages exchanged with EST-
   coaps.

   Regarding the Certificate Signing Request (CSR), a 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.

10.2.  HTTPS-CoAPS Registrar considerations

   The Registrar proposed in Section 6 must be deployed with care, and
   only when the recommended connections are impossible.  When POP
   linking is used the Registrar terminating the TLS connection
   establishes a new one with the upstream CA.  Thus, it is impossible
   for POP linking to be enforced end-to-end for the EST transaction.
   The EST server could be configured to accept POP linking information
   that does not match the current TLS session because the authenticated
   EST Registrar client has verified this information when acting as an
   EST server.

   The introduction of an EST-coaps-to-HTTP Registrar assumes the client
   can trust the registrar using its implicit or explicit TA database.



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   It also assumes the Registrar has a trust relationship with the
   upstream EST server in order to act on behalf of the clients.  When a
   client uses the Implicit TA database for certificate validation, he
   SHOULD confirm if the server is acting as an RA by the presence of
   the id-kp-cmcRA EKU [RFC6402] in the server certificate.

   In a server-side key generation case, if no end-to-end encryption is
   used, the Registrar may be able see the private key as it acts as a
   man-in-the-middle.  Thus, the client puts its trust on the Registrar
   not exposing the private key.

   Clients that leverage server-side key generation without end-to-end
   encryption of the private key (Section 5.8) have no knowledge if the
   Registrar will be generating the private key and enrolling the
   certificates with the CA or if the CA will be responsible for
   generating the key.  In such cases, the existence of a Registrar
   requires the client to put its trust on the registrar doing the right
   thing if it is generating the private key.

11.  Contributors

   Martin Furuhed contributed to the EST-coaps specification by
   providing feedback based on the Nexus EST over CoAPS server
   implementation that started in 2015.  Sandeep Kumar kick-started this
   specification and was instrumental in drawing attention to the
   importance of the subject.

12.  Acknowledgements

   The authors are very grateful to Klaus Hartke for his detailed
   explanations on the use of Block with DTLS and his support for the
   Content-Format specification.  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 for his feedback on technical details of the solution.
   Constructive comments were received from Benjamin Kaduk, Eliot Lear,
   Jim Schaad, Hannes Tschofenig, Julien Vermillard, John Manuel, Oliver
   Pfaff, Pete Beal and Carsten Bormann.

   Interop tests were done by Oliver Pfaff, Thomas Werner, Oskar
   Camezind, Bjorn Elmers and Joel Hoglund.

   Robert Moskowitz provided code to create the examples.








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13.  References

13.1.  Normative References

   [I-D.ietf-core-multipart-ct]
              Fossati, T., Hartke, K., and C. Bormann, "Multipart
              Content-Format for CoAP", draft-ietf-core-multipart-ct-02
              (work in progress), August 2018.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-30 (work in progress),
              November 2018.

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

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,
              <https://www.rfc-editor.org/info/rfc2585>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5967]  Turner, S., "The application/pkcs10 Media Type", RFC 5967,
              DOI 10.17487/RFC5967, August 2010,
              <https://www.rfc-editor.org/info/rfc5967>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

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





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

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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,
              <https://www.rfc-editor.org/info/rfc8075>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

13.2.  Informative References

   [COREparams]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters", <https://www.iana.org/assignments/core-
              parameters/core-parameters.xhtml>.

   [I-D.ietf-lamps-rfc5751-bis]
              Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", draft-ietf-lamps-rfc5751-bis-12
              (work in progress), September 2018.

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
              "Connection Identifiers for DTLS 1.2", draft-ietf-tls-
              dtls-connection-id-02 (work in progress), October 2018.

   [I-D.josefsson-sasl-tls-cb]
              Josefsson, S., "Channel Bindings for TLS based on the
              PRF", draft-josefsson-sasl-tls-cb-03 (work in progress),
              March 2015.





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   [I-D.moskowitz-ecdsa-pki]
              Moskowitz, R., Birkholz, H., Xia, L., and M. Richardson,
              "Guide for building an ECC pki", draft-moskowitz-ecdsa-
              pki-04 (work in progress), September 2018.

   [ieee802.15.4]
              Institute of Electrical and Electronics Engineers, "IEEE
              Standard 802.15.4-2006", 2006.

   [ieee802.1ar]
              Institute of Electrical and Electronics Engineers, "IEEE
              802.1AR Secure Device Identifier", December 2009.

   [PsQs]     Nadia Heninger, Zakir Durumeric, Eric Wustrow, J. Alex
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", USENIX Security Symposium
              2012 ISBN 978-931971-95-9, August 2012.

   [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,
              <https://www.rfc-editor.org/info/rfc4919>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <https://www.rfc-editor.org/info/rfc5272>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <https://www.rfc-editor.org/info/rfc5929>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <https://www.rfc-editor.org/info/rfc5958>.

   [RFC6402]  Schaad, J., "Certificate Management over CMS (CMC)
              Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
              <https://www.rfc-editor.org/info/rfc6402>.

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



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   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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,
              <https://www.rfc-editor.org/info/rfc7251>.

   [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, <https://www.rfc-editor.org/info/rfc7525>.

   [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,
              <https://www.rfc-editor.org/info/rfc7925>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,
              <https://www.rfc-editor.org/info/rfc8422>.

   [RSAorig]  Petr Svenda, Matus Nemec, Peter Sekan, Rudolf Kvasnovsky,
              David Formanek, David Komarek, Vashek Matyas, "The
              Million-Key Question - Investigating the Origins of RSA
              Public Keys", USENIX Security Symposium 2016 ISBN
              978-1-931971-32-4, August 2016.

   [tripleshake]
              Karthikeyan Bhargavan, Antoine Delignat-Lavaud, Cedric
              Fournet, Alfredo Pironti, Pierre-Yves Strub, "Triple
              Handshakes and Cookie Cutters: Breaking and Fixing
              Authentication over TLS", IEEE Security and Privacy ISBN
              978-1-4799-4686-0, May 2014.






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Appendix A.  EST messages to EST-coaps

   This section shows similar examples to the ones presented in
   Appendix A of [RFC7030].  The payloads in the examples are the hex
   encoded binary, generated with 'xxd -p', of the PKI certificates
   created following [I-D.moskowitz-ecdsa-pki].  Hex is used for
   visualization purposes because a binary representation cannot be
   rendered well in text.  The hexadecimal representations would not be
   transported in hex, but in binary.  The payloads are shown
   unencrypted.  In practice the message content would be transferred
   over an encrypted DTLS tunnel.

   The certificate responses included in the examples contain Content-
   Format 281 (application/pkcs7).  If the client had requested Content-
   Format TBD287 (application/pkix-cert) by querying /est/skc, the
   server would respond with a single DER binary certificate.

   These examples assume a short root resource path of "/est".

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

   The message content breakdown is presented in Appendix C.

A.1.  cacerts

   In EST-coaps, a cacerts message can be:

   GET coaps://est-coaps.example.org:9085/est/crts
   (Accept:  281)

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


















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     Ver = 1
     T = 0 (CON)
     Code = 0x01 (0.01 is GET)
     Token = 0x9a (client generated)
     Options
     Option (Uri-Host)
        Option Delta = 0x3  (option# 3)
        Option Length = 0x9
        Option Value = est-coaps.example.org
     Option (Uri-Port)
        Option Delta = 0x4  (option# 3+4=7)
        Option Length = 0x4
        Option Value = 9085
      Option (Uri-Path)
        Option Delta = 0x4   (option# 7+4=11)
        Option Length = 0x5
        Option Value = "est"
      Option (Uri-Path)
        Option Delta = 0x0   (option# 11+0=11)
        Option Length = 0x6
        Option Value = "crts"
      Option (Accept)
        Option Delta = 0x6   (option# 11+6=17)
        Option Length = 0x2
        Option Value = 281
     Payload = [Empty]

   The Uri-Host and Uri-Port Options can be omitted if they coincide
   with the transport protocol destination address and port
   respectively.  Explicit Uri-Host and Uri-Port Options are typically
   used when an endpoint hosts multiple virtual servers and uses the
   Options to route the requests accordingly.

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

   2.05 Content (Content-Format: 281)
      {payload with certificate in binary format}

   with CoAP fields












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     Ver = 1
     T = 2 (ACK)
     Code = 0x45 (2.05 Content)
     Token = 0x9a   (copied from request by server)
     Options
       Option (Content-Format)
         Option Delta = 0xC  (option# 12)
         Option Length = 0x2
         Option Value = 281

     [ The hexadecimal representation below would NOT be transported
     in hex, but in binary. Hex is used because a binary representation
     cannot be rendered well in text. ]

     Payload =
   3082027b06092a864886f70d010702a082026c308202680201013100300b
   06092a864886f70d010701a082024e3082024a308201f0a0030201020209
   009189bcdf9c99244b300a06082a8648ce3d0403023067310b3009060355
   040613025553310b300906035504080c024341310b300906035504070c02
   4c4131143012060355040a0c0b4578616d706c6520496e63311630140603
   55040b0c0d63657274696669636174696f6e3110300e06035504030c0752
   6f6f74204341301e170d3139303130373130343034315a170d3339303130
   323130343034315a3067310b3009060355040613025553310b3009060355
   04080c024341310b300906035504070c024c4131143012060355040a0c0b
   4578616d706c6520496e6331163014060355040b0c0d6365727469666963
   6174696f6e3110300e06035504030c07526f6f742043413059301306072a
   8648ce3d020106082a8648ce3d03010703420004814994082b6e8185f3df
   53f5e0bee698973335200023ddf78cd17a443ffd8ddd40908769c55652ac
   2ccb75c4a50a7c7ddb7c22dae6c85cca538209fdbbf104c9a38184308181
   301d0603551d0e041604142495e816ef6ffcaaf356ce4adffe33cf492abb
   a8301f0603551d230418301680142495e816ef6ffcaaf356ce4adffe33cf
   492abba8300f0603551d130101ff040530030101ff300e0603551d0f0101
   ff040403020106301e0603551d1104173015811363657274696679406578
   616d706c652e636f6d300a06082a8648ce3d0403020348003045022100da
   e37c96f154c32ec0b4af52d46f3b7ecc9687ddf267bcec368f7b7f135327
   2f022047a28ae5c7306163b3c3834bab3c103f743070594c089aaa0ac870
   cd13b902caa1003100

   The breakdown of the payload is shown in Appendix C.1.

A.2.  enroll / reenroll

   During the (re-)enroll exchange the EST-coaps client uses a CSR
   (Content-Format 286) request in the POST request payload.  The Accept
   option tells the server that the client is expecting Content-Format
   281 (PKCS#7) in the response.  As shown in Appendix C.2, the CSR
   contains a ChallengePassword which is used for POP linking
   (Section 4).



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   POST [2001:db8::2:1]:61616/est/sen
   (Token: 0x45)
   (Accept: 281)
   (Content-Format: 286)

   [ The hexadecimal representation below would NOT be transported
   in hex, but in binary. Hex is used because a binary representation
   cannot be rendered well in text. ]

   3082018b30820131020100305c310b3009060355040613025553310b3009
   06035504080c024341310b300906035504070c024c413114301206035504
   0a0c0b6578616d706c6520496e63310c300a060355040b0c03496f54310f
   300d060355040513065774313233343059301306072a8648ce3d02010608
   2a8648ce3d03010703420004c8b421f11c25e47e3ac57123bf2d9fdc494f
   028bc351cc80c03f150bf50cff958d75419d81a6a245dffae790be95cf75
   f602f9152618f816a2b23b5638e59fd9a073303406092a864886f70d0109
   0731270c2576437630292a264a4b4a3bc3a2c280c2992f3e3c2e2c3d6b6e
   7634332323403d204e787e60303b06092a864886f70d01090e312e302c30
   2a0603551d1104233021a01f06082b06010505070804a013301106092b06
   010401b43b0a01040401020304300a06082a8648ce3d0403020348003045
   02210092563a546463bd9ecff170d0fd1f2ef0d3d012160e5ee90cffedab
   ec9b9a38920220179f10a3436109051abad17590a09bc87c4dce5453a6fc
   1135a1e84eed754377

   After verification of the CSR by the server, a 2.01 Content response
   with the issued certificate will be returned to the client.

























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   2.01 Created
   (Token: 0x45)
   (Content-Format: 281)

   [ The hexadecimal representation below would NOT be transported
   in hex, but in binary. Hex is used because a binary representation
   cannot be rendered well in text. ]

   3082026e06092a864886f70d010702a082025f3082025b0201013100300b
   06092a864886f70d010701a08202413082023d308201e2a0030201020208
   7e7661d7b54e4632300a06082a8648ce3d040302305d310b300906035504
   0613025553310b300906035504080c02434131143012060355040a0c0b45
   78616d706c6520496e6331163014060355040b0c0d636572746966696361
   74696f6e3113301106035504030c0a3830322e3141522043413020170d31
   39303133313131323931365a180f39393939313233313233353935395a30
   5c310b3009060355040613025553310b300906035504080c024341310b30
   0906035504070c024c4131143012060355040a0c0b6578616d706c652049
   6e63310c300a060355040b0c03496f54310f300d06035504051306577431
   3233343059301306072a8648ce3d020106082a8648ce3d03010703420004
   c8b421f11c25e47e3ac57123bf2d9fdc494f028bc351cc80c03f150bf50c
   ff958d75419d81a6a245dffae790be95cf75f602f9152618f816a2b23b56
   38e59fd9a3818a30818730090603551d1304023000301d0603551d0e0416
   041496600d8716bf7fd0e752d0ac760777ad665d02a0301f0603551d2304
   183016801468d16551f951bfc82a431d0d9f08bc2d205b1160300e060355
   1d0f0101ff0404030205a0302a0603551d1104233021a01f06082b060105
   05070804a013301106092b06010401b43b0a01040401020304300a06082a
   8648ce3d0403020349003046022100c0d81996d2507d693f3c48eaa5ee94
   91bda6db214099d98117c63b361374cd86022100a774989f4c321a5cf25d
   832a4d336a08ad67df20f1506421188a0ade6d349236a1003100

   The breakdown of the request and response is shown in Appendix C.2.

A.3.  serverkeygen

   In a serverkeygen exchange the CoAP POST request looks like
















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   POST coaps://192.0.2.1:8085/est/skg
   (Token: 0xa5)
   (Accept: 62)
   (Content-Format: 286)

   [ The hexadecimal representation below would NOT be transported
   in hex, but in binary. Hex is used because a binary representation
   cannot be rendered well in text. ]

   3081cf3078020100301631143012060355040a0c0b736b67206578616d70
   6c653059301306072a8648ce3d020106082a8648ce3d030107034200041b
   b8c1117896f98e4506c03d70efbe820d8e38ea97e9d65d52c8460c5852c5
   1dd89a61370a2843760fc859799d78cd33f3c1846e304f1717f8123f1a28
   4cc99fa000300a06082a8648ce3d04030203470030440220387cd4e9cf62
   8d4af77f92ebed4890d9d141dca86cd2757dd14cbd59cdf6961802202f24
   5e828c77754378b66660a4977f113cacdaa0cc7bad7d1474a7fd155d090d

   The response would follow [I-D.ietf-core-multipart-ct] and could look
   like
































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   2.01 Content
   (Token: 0xa5)
   (Content-Format: 62)

   [ The hexadecimal representations below would NOT be transported
   in hex, but in binary. Hex is used because a binary representation
   cannot be rendered well in text. ]

   84                                   # array(4)
   19 011C                              # unsigned(284)
   58 8A                                # bytes(138)
   308187020100301306072a8648ce3d020106082a8648ce3d030107046d30
   6b02010104200b9a67785b65e07360b6d28cfc1d3f3925c0755799deeca7
   45372b01697bd8a6a144034200041bb8c1117896f98e4506c03d70efbe82
   0d8e38ea97e9d65d52c8460c5852c51dd89a61370a2843760fc859799d78
   cd33f3c1846e304f1717f8123f1a284cc99f
   19 0119                              # unsigned(281)
   59 01D3                              # bytes(467)
   308201cf06092a864886f70d010702a08201c0308201bc0201013100300b
   06092a864886f70d010701a08201a23082019e30820143a0030201020208
   126de8571518524b300a06082a8648ce3d04030230163114301206035504
   0a0c0b736b67206578616d706c65301e170d313930313039303835373038
   5a170d3339303130343038353730385a301631143012060355040a0c0b73
   6b67206578616d706c653059301306072a8648ce3d020106082a8648ce3d
   030107034200041bb8c1117896f98e4506c03d70efbe820d8e38ea97e9d6
   5d52c8460c5852c51dd89a61370a2843760fc859799d78cd33f3c1846e30
   4f1717f8123f1a284cc99fa37b307930090603551d1304023000302c0609
   6086480186f842010d041f161d4f70656e53534c2047656e657261746564
   204365727469666963617465301d0603551d0e04160414494be598dc8dbc
   0dbc071c486b777460e5cce621301f0603551d23041830168014494be598
   dc8dbc0dbc071c486b777460e5cce621300a06082a8648ce3d0403020349
   003046022100a4b167d0f9add9202810e6bf6a290b8cfdfc9b9c9fea2cc1
   c8fc3a464f79f2c202210081d31ba142751a7b4a34fd1a01fcfb08716b9e
   b53bdaadc9ae60b08f52429c0fa1003100

   The private key in the response above is without CMS EnvelopedData
   and has no additional encryption beyond DTLS (Section 5.8).

   The breakdown of the request and response is shown in Appendix C.3

A.4.  csrattrs

   Below is a csrattrs exchange








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

   RES:
   2.05 Content
   (Content-Format: 285)

   [ The hexadecimal representation below would NOT be transported
   in hex, but in binary. Hex is used because a binary representation
   cannot be rendered well in text. ]

   307c06072b06010101011630220603883701311b131950617273652053455
   420617320322e3939392e31206461746106092a864886f70d010907302c06
   0388370231250603883703060388370413195061727365205345542061732
   0322e3939392e32206461746106092b240303020801010b06096086480165
   03040202

   A 2.05 Content response should contain attributes which are relevant
   for the authenticated client.  This example is copied from
   Section A.2 in [RFC7030], where the base64 representation is replaced
   with a hexadecimal representation of the equivalent binary format.
   The EST-coaps server returns attributes that the client can ignore if
   they are unknown to him.

Appendix B.  EST-coaps Block message examples

   Two examples are presented in this section:

   1.  a cacerts exchange shows the use of Block2 and the block headers

   2.  an enroll exchange shows the Block1 and Block2 size negotiation
       for request and response payloads.

   The payloads are shown unencrypted.  In practice the message contents
   would be binary formatted and transferred over an encrypted DTLS
   tunnel.  The corresponding CoAP headers are only shown in
   Appendix B.1.  Creating CoAP headers is assumed to be generally
   known.

B.1.  cacerts

   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 cacerts exchange over DTLS.  The
   content length of the cacerts response in appendix A.1 of [RFC7030]



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   contains 639 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 9 packets with a payload of 64 bytes each, followed by a last
   tenth packet of 63 bytes.  The client sends an IPv6 packet containing
   the UDP datagram with the DTLS record that encapsulates the CoAP
   request 10 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 (block-
   option:NUM/M/size) indicating the kind of Block Option (2 in this
   case) followed by a colon, and then the block number (NUM), the more
   bit (M = 0 in Block2 response means it is last block), and block size
   with exponent (2**(SZX+4)) separated by slashes.  The Length 64 is
   used with SZX=2 to avoid IP fragmentation.  The CoAP Request is sent
   confirmable (CON) and the Content-Format of the response, even though
   not shown, is 281 (application/pkcs7-mime; smime-type=certs-only).
   The transfer of the 10 blocks with partially filled block NUM=9 is
   shown below

      GET coaps://est-coaps.example.org:9085/est/crts (2:0/0/64)  -->
                    <--   (2:0/1/64) 2.05 Content
      GET coaps://est-coaps.example.org:9085/est/crts (2:1/0/64)  -->
                    <--   (2:1/1/64) 2.05 Content
                                  |
                                  |
                                  |
      GET coaps://est-coaps.example.org:9085/est/crts (2:9/0/64) -->
                    <--   (2:9/0/64) 2.05 Content

   The header of the GET request looks like



















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     Ver = 1
     T = 0 (CON)
     Code = 0x01 (0.1 GET)
     Token = 0x9a    (client generated)
     Options
      Option (Uri-Host)
        Option Delta = 0x3  (option# 3)
        Option Length = 0x9
        Option Value = est-coaps.example.org
      Option (Uri-Port)
        Option Delta = 0x4   (option# 3+4=7)
        Option Length = 0x4
        Option Value = 9085
      Option (Uri-Path)
        Option Delta = 0x4    (option# 7+4=11)
        Option Length = 0x5
        Option Value = "est"
      Option (Uri-Path)Uri-Path)
        Option Delta = 0x0    (option# 11+0=11)
        Option Length = 0x6
        Option Value = "crts"
      Option (Accept)
        Option Delta = 0x6   (option# 11+6=17)
        Option Length = 0x2
        Option Value = 281
     Payload = [Empty]

   The Uri-Host and Uri-Port Options can be omitted if they coincide
   with the transport protocol destination address and port
   respectively.  Explicit Uri-Host and Uri-Port Options are typically
   used when an endpoint hosts multiple virtual servers and uses the
   Options to route the requests accordingly.

   For further detailing the CoAP headers, the first two and the last
   blocks are written out below.  The header of the first Block2
   response looks like















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     Ver = 1
     T = 2 (ACK)
     Code = 0x45 (2.05 Content)
     Token = 0x9a     (copied from request by server)
     Options
       Option
         Option Delta = 0xC  (option# 12 Content-Format)
         Option Length = 0x2
         Option Value = 281
       Option
         Option Delta = 0xB  (option# 12+11=23 Block2)
         Option Length = 0x1
         Option Value = 0x0A (block#=0, M=1, SZX=2)

     [ The hexadecimal representation below would NOT be transported
     in hex, but in binary. Hex is used because a binary representation
     cannot be rendered well in text. ]

     Payload =
   3082027b06092a864886f70d010702a082026c308202680201013100300b
   06092a864886f70d010701a082024e3082024a308201f0a0030201020209
   009189bc

   The second Block2:

     Ver = 1
     T = 2 (means ACK)
     Code = 0x45 (2.05 Content)
     Token = 0x9a     (copied from request by server)
     Options
       Option
         Option Delta = 0xC  (option# 12 Content-Format)
         Option Length = 0x2
         Option Value = 281
       Option
         Option Delta = 0xB  (option 12+11=23 Block2)
         Option Length = 0x1
         Option Value = 0x1A (block#=1, M=1, SZX=2)

     [ The hexadecimal representation below would NOT be transported
     in hex, but in binary. Hex is used because a binary representation
     cannot be rendered well in text. ]

     Payload =
   df9c99244b300a06082a8648ce3d0403023067310b300906035504061302
   5553310b300906035504080c024341310b300906035504070c024c413114
   30120603




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   The 10th and final Block2:

     Ver = 1
     T = 2 (means ACK)
     Code = 0x45      (2.05 Content)
     Token = 0x9a     (copied from request by server)
     Options
       Option
         Option Delta = 0xC  (option# 12 Content-Format)
         Option Length = 0x2
         Option Value = 281
       Option
         Option Delta = 0xB  (option# 12+11=23 Block2 )
         Option Length = 0x2
         Option Value = 0x92 (block#=9, M=0, SZX=2)

     [ The hexadecimal representation below would NOT be transported
     in hex, but in binary. Hex is used because a binary representation
     cannot be rendered well in text. ]

     Payload =
   2ec0b4af52d46f3b7ecc9687ddf267bcec368f7b7f1353272f022047a28a
   e5c7306163b3c3834bab3c103f743070594c089aaa0ac870cd13b902caa1
   003100

B.2.  enroll / reenroll

   In this example, the requested Block2 size of 256 bytes, required by
   the client, is transferred to the server in the very first request
   message.  The block size 256=(2**(SZX+4)) which gives SZX=4.  The
   notation for block numbering is the same as in Appendix B.1.  The
   header fields and the payload are omitted for brevity.



















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   POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) {CSR req} -->
          <-- (ACK) (1:0/1/256) (2.31 Continue)
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) {CSR req} -->
          <-- (ACK) (1:1/1/256) (2.31 Continue)
                         .
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256){CSR req} -->
          <-- (ACK) (1:N1/0/256)(2:0/1/256)(2.04 Changed){Cert resp}
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256)           -->
          <-- (ACK) (2:1/1/256)(2.04 Changed) {Cert resp}
                         .
                         .
                         .
   POST [2001:db8::2:1]:61616/est/sen (CON)(2:N2/0/256)          -->
          <-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp}


            Figure 5: EST-COAP enrollment with multiple blocks

   N1+1 blocks have been transferred from client to the server and N2+1
   blocks have been transferred from server to client.

Appendix C.  Message content breakdown

   This appendix presents the breakdown of the hexadecimal dumps of the
   binary payloads shown in Appendix A.

C.1.  cacerts

   The breakdown of cacerts response containing one root CA certificate
   is



















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   Certificate:
       Data:
           Version: 3 (0x2)
           Serial Number:
               91:89:bc:df:9c:99:24:4b
       Signature Algorithm: ecdsa-with-SHA256
           Issuer: C=US, ST=CA, L=LA, O=Example Inc,
                   OU=certification, CN=Root CA
           Validity
               Not Before: Jan  7 10:40:41 2019 GMT
               Not After : Jan  2 10:40:41 2039 GMT
           Subject: C=US, ST=CA, L=LA, O=Example Inc,
                    OU=certification, CN=Root CA
           Subject Public Key Info:
               Public Key Algorithm: id-ecPublicKey
                   Public-Key: (256 bit)
                   pub:
                       04:81:49:94:08:2b:6e:81:85:f3:df:53:f5:e0:be:
                       e6:98:97:33:35:20:00:23:dd:f7:8c:d1:7a:44:3f:
                       fd:8d:dd:40:90:87:69:c5:56:52:ac:2c:cb:75:c4:
                       a5:0a:7c:7d:db:7c:22:da:e6:c8:5c:ca:53:82:09:
                       fd:bb:f1:04:c9
                   ASN1 OID: prime256v1
                   NIST CURVE: P-256
           X509v3 extensions:
               X509v3 Subject Key Identifier:
   24:95:E8:16:EF:6F:FC:AA:F3:56:CE:4A:DF:FE:33:CF:49:2A:BB:A8
               X509v3 Authority Key Identifier:
                   keyid:
   24:95:E8:16:EF:6F:FC:AA:F3:56:CE:4A:DF:FE:33:CF:49:2A:BB:A8

               X509v3 Basic Constraints: critical
                   CA:TRUE
               X509v3 Key Usage: critical
                   Certificate Sign, CRL Sign
               X509v3 Subject Alternative Name:
                   email:certify@example.com
       Signature Algorithm: ecdsa-with-SHA256
            30:45:02:21:00:da:e3:7c:96:f1:54:c3:2e:c0:b4:af:52:d4:
            6f:3b:7e:cc:96:87:dd:f2:67:bc:ec:36:8f:7b:7f:13:53:27:
            2f:02:20:47:a2:8a:e5:c7:30:61:63:b3:c3:83:4b:ab:3c:10:
            3f:74:30:70:59:4c:08:9a:aa:0a:c8:70:cd:13:b9:02:ca









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C.2.  enroll / reenroll

   The breakdown of the enrollment request is

   Certificate Request:
       Data:
           Version: 0 (0x0)
           Subject: C=US, ST=CA, L=LA, O=example Inc,
                                   OU=IoT/serialNumber=Wt1234
           Subject Public Key Info:
               Public Key Algorithm: id-ecPublicKey
                   Public-Key: (256 bit)
                   pub:
                       04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                       9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                       0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                       be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                       56:38:e5:9f:d9
                   ASN1 OID: prime256v1
                   NIST CURVE: P-256
           Attributes:
               challengePassword        : <256-bit POP linking value>
           Requested Extensions:
               X509v3 Subject Alternative Name:
                   othername:<unsupported>
       Signature Algorithm: ecdsa-with-SHA256
            30:45:02:21:00:92:56:3a:54:64:63:bd:9e:cf:f1:70:d0:fd:
            1f:2e:f0:d3:d0:12:16:0e:5e:e9:0c:ff:ed:ab:ec:9b:9a:38:
            92:02:20:17:9f:10:a3:43:61:09:05:1a:ba:d1:75:90:a0:9b:
            c8:7c:4d:ce:54:53:a6:fc:11:35:a1:e8:4e:ed:75:43:77

   The CSR contained a ChallengePassword which is used for POP linking
   (Section 4).

   The breakdown of the issued certificate is
















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   Certificate:
       Data:
           Version: 3 (0x2)
           Serial Number: 9112578475118446130 (0x7e7661d7b54e4632)
       Signature Algorithm: ecdsa-with-SHA256
           Issuer: C=US, ST=CA, O=Example Inc, OU=certification,
                              CN=802.1AR CA
           Validity
               Not Before: Jan 31 11:29:16 2019 GMT
               Not After : Dec 31 23:59:59 9999 GMT
           Subject: C=US, ST=CA, L=LA, O=example Inc,
                                OU=IoT/serialNumber=Wt1234
           Subject Public Key Info:
               Public Key Algorithm: id-ecPublicKey
                   Public-Key: (256 bit)
                   pub:
                       04:c8:b4:21:f1:1c:25:e4:7e:3a:c5:71:23:bf:2d:
                       9f:dc:49:4f:02:8b:c3:51:cc:80:c0:3f:15:0b:f5:
                       0c:ff:95:8d:75:41:9d:81:a6:a2:45:df:fa:e7:90:
                       be:95:cf:75:f6:02:f9:15:26:18:f8:16:a2:b2:3b:
                       56:38:e5:9f:d9
                   ASN1 OID: prime256v1
                   NIST CURVE: P-256
           X509v3 extensions:
               X509v3 Basic Constraints:
                   CA:FALSE
               X509v3 Subject Key Identifier:
   96:60:0D:87:16:BF:7F:D0:E7:52:D0:AC:76:07:77:AD:66:5D:02:A0

               X509v3 Authority Key Identifier:
                   keyid:
   68:D1:65:51:F9:51:BF:C8:2A:43:1D:0D:9F:08:BC:2D:20:5B:11:60

               X509v3 Key Usage: critical
                   Digital Signature, Key Encipherment
               X509v3 Subject Alternative Name:
                   othername:<unsupported>
       Signature Algorithm: ecdsa-with-SHA256
            30:46:02:21:00:c0:d8:19:96:d2:50:7d:69:3f:3c:48:ea:a5:
            ee:94:91:bd:a6:db:21:40:99:d9:81:17:c6:3b:36:13:74:cd:
            86:02:21:00:a7:74:98:9f:4c:32:1a:5c:f2:5d:83:2a:4d:33:
            6a:08:ad:67:df:20:f1:50:64:21:18:8a:0a:de:6d:34:92:36

C.3.  serverkeygen

   The following is the breakdown of the server-side key generation
   request.




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   Certificate Request:
       Data:
           Version: 0 (0x0)
           Subject: O=skg example
           Subject Public Key Info:
               Public Key Algorithm: id-ecPublicKey
                   Public-Key: (256 bit)
                   pub:
                       04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
                       be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
                       52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
                       9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
                       1a:28:4c:c9:9f
                   ASN1 OID: prime256v1
                   NIST CURVE: P-256
           Attributes:
               a0:00
       Signature Algorithm: ecdsa-with-SHA256
            30:44:02:20:38:7c:d4:e9:cf:62:8d:4a:f7:7f:92:eb:ed:48:
            90:d9:d1:41:dc:a8:6c:d2:75:7d:d1:4c:bd:59:cd:f6:96:18:
            02:20:2f:24:5e:82:8c:77:75:43:78:b6:66:60:a4:97:7f:11:
            3c:ac:da:a0:cc:7b:ad:7d:14:74:a7:fd:15:5d:09:0d

   Following is the breakdown of the private key content of the server-
   side key generation response.

   Private-Key: (256 bit)
   priv:
       0b:9a:67:78:5b:65:e0:73:60:b6:d2:8c:fc:1d:3f:
       39:25:c0:75:57:99:de:ec:a7:45:37:2b:01:69:7b:
       d8:a6
   pub:
       04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
       be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
       52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
       9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
       1a:28:4c:c9:9f
   ASN1 OID: prime256v1
   NIST CURVE: P-256

   The following is the breakdown of the certificate in the server-side
   key generation response payload.









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   Certificate:
       Data:
           Version: 3 (0x2)
           Serial Number: 1327972925857878603 (0x126de8571518524b)
       Signature Algorithm: ecdsa-with-SHA256
           Issuer: O=skg example
           Validity
               Not Before: Jan  9 08:57:08 2019 GMT
               Not After : Jan  4 08:57:08 2039 GMT
           Subject: O=skg example
           Subject Public Key Info:
               Public Key Algorithm: id-ecPublicKey
                   Public-Key: (256 bit)
                   pub:
                       04:1b:b8:c1:11:78:96:f9:8e:45:06:c0:3d:70:ef:
                       be:82:0d:8e:38:ea:97:e9:d6:5d:52:c8:46:0c:58:
                       52:c5:1d:d8:9a:61:37:0a:28:43:76:0f:c8:59:79:
                       9d:78:cd:33:f3:c1:84:6e:30:4f:17:17:f8:12:3f:
                       1a:28:4c:c9:9f
                   ASN1 OID: prime256v1
                   NIST CURVE: P-256
           X509v3 extensions:
               X509v3 Basic Constraints:
                   CA:FALSE
               Netscape Comment:
                   OpenSSL Generated Certificate
               X509v3 Subject Key Identifier:
   49:4B:E5:98:DC:8D:BC:0D:BC:07:1C:48:6B:77:74:60:E5:CC:E6:21
               X509v3 Authority Key Identifier:
                   keyid:
   49:4B:E5:98:DC:8D:BC:0D:BC:07:1C:48:6B:77:74:60:E5:CC:E6:21

       Signature Algorithm: ecdsa-with-SHA256
            30:46:02:21:00:a4:b1:67:d0:f9:ad:d9:20:28:10:e6:bf:6a:
            29:0b:8c:fd:fc:9b:9c:9f:ea:2c:c1:c8:fc:3a:46:4f:79:f2:
            c2:02:21:00:81:d3:1b:a1:42:75:1a:7b:4a:34:fd:1a:01:fc:
            fb:08:71:6b:9e:b5:3b:da:ad:c9:ae:60:b0:8f:52:42:9c:0f

Authors' Addresses

   Peter van der Stok
   Consultant

   Email: consultancy@vanderstok.org







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   Panos Kampanakis
   Cisco Systems

   Email: pkampana@cisco.com


   Michael C. Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/


   Shahid Raza
   RISE SICS
   Isafjordsgatan 22
   Kista, Stockholm  16440
   SE

   Email: shahid@sics.se































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