ACE Working Group                                            G. Selander
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                                 S. Raza
Expires: 6 May 2021                                                 RISE
                                                              M. Furuhed
                                                                   Nexus
                                                              M. Vucinic
                                                               T. Claeys
                                                                   INRIA
                                                         2 November 2020


                  Protecting EST Payloads with OSCORE
                 draft-selander-ace-coap-est-oscore-04

Abstract

   This document specifies public-key certificate enrollment procedures
   protected with lightweight application-layer security protocols
   suitable for Internet of Things (IoT) deployments.  The protocols
   leverage payload formats defined in Enrollment over Secure Transport
   (EST) and existing IoT standards including the Constrained
   Application Protocol (CoAP), Concise Binary Object Representation
   (CBOR) and the CBOR Object Signing and Encryption (COSE) format.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 6 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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 publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Operational Differences with EST-coaps  . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  EDHOC . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Certificate-based Authentication  . . . . . . . . . . . .   6
     3.3.  Channel Binding . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Optimizations . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  RPK-based Trust Anchors . . . . . . . . . . . . . . . . .   7
   4.  Protocol Design and Layering  . . . . . . . . . . . . . . . .   8
     4.1.  Discovery and URI . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Distribution of RPKs  . . . . . . . . . . . . . . . . . .   8
     4.3.  Mandatory/optional EST Functions  . . . . . . . . . . . .   9
     4.4.  Payload formats . . . . . . . . . . . . . . . . . . . . .   9
     4.5.  Message Bindings  . . . . . . . . . . . . . . . . . . . .  11
     4.6.  CoAP response codes . . . . . . . . . . . . . . . . . . .  11
     4.7.  Message fragmentation . . . . . . . . . . . . . . . . . .  11
     4.8.  Delayed Responses . . . . . . . . . . . . . . . . . . . .  11
   5.  HTTP-CoAP Proxy . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Other Authentication Methods . . . . . . . . . . . .  16
     A.1.  TTP Assisted Authentication . . . . . . . . . . . . . . .  16
     A.2.  PSK Based Authentication  . . . . . . . . . . . . . . . .  18
   Appendix B.  CBOR Encoding of EST Payloads  . . . . . . . . . . .  18
     B.1.  Distribution of CA Certificates (/crts) . . . . . . . . .  18
     B.2.  Enrollment/Re-enrollment of Clients (/sen, /sren) . . . .  19
       B.2.1.  CBOR Certificate Request Examples . . . . . . . . . .  20
       B.2.2.  ASN.1 Certificate Request Examples  . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21





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1.  Introduction

   One of the challenges with deploying a Public Key Infrastructure
   (PKI) for the Internet of Things (IoT) is certificate enrollment,
   because existing enrollment protocols are not optimized for
   constrained environments [RFC7228].

   One optimization of certificate enrollment targeting IoT deployments
   is specified in EST-coaps ([I-D.ietf-ace-coap-est]), which defines a
   version of Enrollment over Secure Transport [RFC7030] for
   transporting EST payloads over CoAP [RFC7252] and DTLS [RFC6347],
   instead of secured HTTP.

   This document describes a method for protecting EST payloads over
   CoAP or HTTP with OSCORE [RFC8613].  OSCORE specifies an extension to
   CoAP which protects the application layer message and can be applied
   independently of how CoAP messages are transported.  OSCORE can also
   be applied to CoAP-mappable HTTP which enables end-to-end security
   for mixed CoAP and HTTP transfer of application layer data.  Hence
   EST payloads can be protected end-to-end independent of underlying
   transport and through proxies translating between between CoAP and
   HTTP.

   OSCORE is designed for constrained environments, building on IoT
   standards such as CoAP, CBOR [RFC7049] and COSE [RFC8152], and has in
   particular gained traction in settings where message sizes and the
   number of exchanged messages needs to be kept at a minimum, such as
   6TiSCH [I-D.ietf-6tisch-minimal-security], or for securing multicast
   CoAP messages [I-D.ietf-core-oscore-groupcomm].  Where OSCORE is
   implemented and used for communication security, the reuse of OSCORE
   for other purposes, such as enrollment, reduces the code footprint.

   In order to protect certificate enrollment with OSCORE, the necessary
   keying material (notably, the OSCORE Master Secret, see [RFC8613])
   needs to be established between EST-oscore client and EST-oscore
   server.  For this purpose we assume the use of the lightweight
   authenticated key exchange protocol EDHOC [I-D.ietf-lake-edhoc].
   Other methods for key establishment are described in Appendix A.

   Other ways to optimize the performance of certificate enrollment and
   certificate based authentication described in this draft include the
   use of:

   *  Compact representations of X.509 certificates (see
      [I-D.mattsson-cose-cbor-cert-compress])

   *  Certificates by reference (see [I-D.ietf-cose-x509])




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   *  Compact representations of EST payloads (see Appendix B)

1.1.  Operational Differences with EST-coaps

   The protection of EST payloads defined in this document builds on
   EST-coaps [I-D.ietf-ace-coap-est] but transport layer security is
   replaced, or complemented, by protection of the transfer- and
   application layer data (i.e., CoAP message fields and payload).  This
   specification deviates from EST-coaps in the following respects:

   *  The DTLS record layer is replaced, or complemented, with OSCORE.

   *  The DTLS handshake is replaced, or complemented, with the
      lightweight authenticated key exchange protocol EDHOC
      [I-D.ietf-lake-edhoc], and makes use of the following features:

      -  Authentication based on certificates is complemented with
         authentication based on raw public keys.

      -  Authentication based on signature keys is complemented with
         authentication based on static Diffie-Hellman keys, for
         certificates/raw public keys.

      -  Authentication based on certificate by value is complemented
         with authentication based on certificate/raw public keys by
         reference.

   *  One new EST function, /rpks, is defined for installation of
      compact explicit TAs in the EST client.

   *  The EST payloads protected by OSCORE can be proxied between
      constrained networks supporting CoAP/CoAPs and non-constrained
      networks supporting HTTP/HTTPs with a CoAP-HTTP proxy protection
      without any security processing in the proxy (see Section 5).  The
      concept "Registrar" and its required trust relation with EST
      server as described in Section 6 of [I-D.ietf-ace-coap-est] is
      therefore redundant.

   So, while the same authentication scheme (Diffie-Hellman key exchange
   authenticated with transported certificates) and the same EST
   payloads as EST-coaps also apply to EST-oscore, the latter specifies
   other authentication schemes and a new matching EST function.  The
   reason for these deviations is that a significant overhead can be
   removed in terms of message sizes and round trips by using a
   different handshake, public key type or transported credential, and
   those are independent of the actual enrollment procedure.





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   Appendix A discusses yet other authentication and secure
   communication methods.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  These
   words may also appear in this document in lowercase, absent their
   normative meanings.

   This document uses terminology from [I-D.ietf-ace-coap-est] which in
   turn is based on [RFC7030] and, in turn, on [RFC5272].

   The term "Trust Anchor" follows the terminology of [RFC6024]: "A
   trust anchor represents an authoritative entity via a public key and
   associated data.  The public key is used to verify digital
   signatures, and the associated data is used to constrain the types of
   information for which the trust anchor is authoritative."  One
   example of specifying more compact alternatives to X.509 certificates
   for exchanging trust anchor information is provided by the
   TrustAnchorInfo structure of [RFC5914], the mandatory parts of which
   essentially is the SubjectPublicKeyInfo structure [RFC5280], i.e., an
   algorithm identifier followed by a public key.

3.  Authentication

   This specification replaces the DTLS handshake in EST-coaps with the
   lightweight authenticated key exchange protocol EDHOC
   [I-D.ietf-lake-edhoc].  During initial enrollment the EST-oscore
   client and server run EDHOC [I-D.ietf-lake-edhoc] to authenticate and
   establish the OSCORE security context with which the EST payloads are
   protected.

   EST-oscore clients and servers MUST perform mutual authentication.
   The EST server and EST client are responsible for ensuring that an
   acceptable cipher suite is negotiated.  The client MUST authenticate
   the server before accepting any server response.  The server MUST
   authenticate the client and provide relevant information to the CA
   for decision about issuing a certificate.











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3.1.  EDHOC

   EDHOC supports authentication with certificates/raw public keys
   (referred to as "credentials"), and the credentials may either be
   transported in the protocol, or referenced.  This is determined by
   the identifier of the credential of the endpoint, ID_CRED_x for x=
   Initiator/Responder, which is transported in an EDHOC message.  This
   identifier may be the credential itself (in which case the credential
   is transported), or a pointer such as a URI to the credential (e.g.,
   x5t, see [I-D.ietf-cose-x509]) or some other identifier which enables
   the receiving endpoint to retrieve the credential.

3.2.  Certificate-based Authentication

   EST-oscore, like EST-coaps, supports certificate-based authentication
   between EST client and server.  In this case the client MUST be
   configured with an Implicit or Explicit Trust Anchor (TA) [RFC7030]
   database, enabling the client to authenticate the server.  During the
   initial enrollment the client SHOULD populate its Explicit TA
   database and use it for subsequent authentications.

   The EST client certificate SHOULD conform to [RFC7925].  The EST
   client and/or EST server certificate MAY be a (natively signed) CBOR
   certificate [I-D.mattsson-cose-cbor-cert-compress].

3.3.  Channel Binding

   The [RFC5272] specification describes proof-of-possession as the
   ability of a client to prove its possession of a private key which is
   linked to a certified public key.  In case of signature key, a proof-
   of-possession is generated by the client when it signs the PKCS#10
   Request during the enrollment phase.  Connection-based proof-of-
   possession is OPTIONAL for EST-oscore clients and servers.

   When desired the client can use the EDHOC-Exporter API to extract
   channel-binding information and provide a connection-based proof-of
   possession.  Channel-binding information is obtained as follows

   edhoc-unique = EDHOC-Exporter("EDHOC Unique", length),

   where length equals the desired length of the edhoc-unique byte
   string.  The client then adds the edhoc-unique byte string as a
   challengePassword (see Section 5.4.1 of [RFC2985]) in the attributes
   section of the PKCS#10 Request to prove to the server that the
   authenticated EDHOC client is in possession of the private key
   associated with the certification request, and signed the
   certification request after the EDHOC session was established.




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3.4.  Optimizations

   *  The last message of the EDHOC protocol, message_3, MAY be combined
      with an OSCORE request, enabling authenticated Diffie-Hellman key
      exchange and a protected CoAP request/response (which may contain
      an enrolment request and response) in two round trips
      [I-D.palombini-core-oscore-edhoc].

   *  The certificates MAY be compressed, e.g. using the CBOR encoding
      defined in [I-D.mattsson-cose-cbor-cert-compress].

   *  The certificate MAY be referenced instead of transported
      [I-D.ietf-cose-x509].  The EST-oscore server MAY use information
      in the credential identifier field of the EDHOC message
      (ID_CRED_x) to access the EST-oscore client certificate, e.g., in
      a directory or database provided by the issuer.  In this case the
      certificate may not need to be transported over a constrained link
      between EST client and server.

   *  Conversely, the response to the PKCS#10 request MAY be a reference
      to the enrolled certificate rather than the certificate itself.
      The EST-oscore server MAY in the enrolment response to the EST-
      oscore client include a pointer to a directory or database where
      the certificate can be retrieved.

3.5.  RPK-based Trust Anchors

   A trust anchor is commonly a self-signed certificate of the CA public
   key.  In order to reduce transport overhead, the trust anchor could
   be just the CA public key and associated data (see Section 2), e.g.,
   the SubjectPublicKeyInfo, or a public key certificate without the
   signature.  In either case they can be compactly encoded, e.g. using
   CBOR encoding [I-D.mattsson-cose-cbor-cert-compress].  A client MAY
   request an unsigned trust anchors using the /rpks function (see
   Section 4.2).

   Client authentication can be performed with long-lived RPKs installed
   by the manufacturer.  Re-enrollment requests can be authenticated
   through a valid certificate issued previously by the EST-oscore
   server or by using the key material available in the Implicit TA
   database.

   TODO: Sanity check this.  Review the use of Implicit TA vs. Explicit
   TA.







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4.  Protocol Design and Layering

   EST-oscore uses CoAP [RFC7252] and Block-Wise [RFC7959] to transfer
   EST messages in the same way as [I-D.ietf-ace-coap-est].  Instead of
   DTLS record layer, OSCORE [RFC8613] is used to protect the EST
   payloads.  Figure 1 below shows the layered EST-oscore architecture.

            +------------------------------------------------+
            |          EST request/response messages         |
            +------------------------------------------------+
            |   CoAP with OSCORE   |   HTTP with OSCORE      |
            +------------------------------------------------+
            |   UDP  |  DTLS/UDP   |   TCP   |   TLS/TCP     |
            +------------------------------------------------+

                    Figure 1: EST protected with OSCORE.

   EST-oscore follows much of the EST-coaps and EST design.

4.1.  Discovery and URI

   The discovery of EST resources and the definition of the short EST-
   coaps URI paths specified in Section 5.1 of [I-D.ietf-ace-coap-est],
   as well as the new Resource Type defined in Section 9.1 of
   [I-D.ietf-ace-coap-est] apply to EST-oscore.  Support for OSCORE is
   indicated by the "osc" attribute defined in Section 9 of [RFC8613],
   for example:

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

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

4.2.  Distribution of RPKs

   The EST client can request a copy of the current CA public keys.

   TODO: Map relevant parts of section 4.1 of RFC 7030 and other EST
   function related content from RFC7030 and EST-coaps.

   RATIONALE: EST-coaps provides the /crts operation.  A successful
   request from the client to this resource will be answered with a bag
   of certificates which is subsequently installed in the Explicit TA.
   Motivated by the specification of more compact trust anchors (see
   Section 2) we define here the new EST function /rpks which returns a
   set of RPKs to be installed in the Explicit TA database.





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4.3.  Mandatory/optional EST Functions

   The EST-oscore specification has the same set of required-to-
   implement functions as EST-coaps.  The content of Table 1 is adapted
   from Section 5.2 in [I-D.ietf-ace-coap-est] and uses the updated URI
   paths (see Section 4.1).

               +===============+===========================+
               | EST functions | EST-oscore implementation |
               +===============+===========================+
               | /crts         | MUST                      |
               +---------------+---------------------------+
               | /sen          | MUST                      |
               +---------------+---------------------------+
               | /sren         | MUST                      |
               +---------------+---------------------------+
               | /skg          | OPTIONAL                  |
               +---------------+---------------------------+
               | /skc          | OPTIONAL                  |
               +---------------+---------------------------+
               | /att          | OPTIONAL                  |
               +---------------+---------------------------+

                    Table 1: Mandatory and optional EST-
                              oscore functions

   TODO: Add /rpks OPTIONAL

4.4.  Payload formats

   Similar to EST-coaps, EST-oscore allows transport of the ASN.1
   structure of a given Media-Type in binary format.  In addition, EST-
   oscore uses the same CoAP Content-Format Options to transport EST
   requests and responses . Table 2 summarizes the information from
   Section 5.3 in [I-D.ietf-ace-coap-est].
















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           +=======+===================================+=======+
           | URI   | Content-Format                    | #IANA |
           +=======+===================================+=======+
           | /crts | N/A (req)                         | -     |
           +-------+-----------------------------------+-------+
           |       | application/pkix-cert (res)       | 287   |
           +-------+-----------------------------------+-------+
           |       | application/pkcs-7-mime;smime-    | 281   |
           |       | type=certs-only (res)             |       |
           +-------+-----------------------------------+-------+
           | /sen  | application/pkcs10 (req)          | 286   |
           +-------+-----------------------------------+-------+
           |       | application/pkix-cert (res)       | 287   |
           +-------+-----------------------------------+-------+
           |       | application/pkcs-7-mime;smime-    | 281   |
           |       | type=certs-only (res)             |       |
           +-------+-----------------------------------+-------+
           | /sren | application/pkcs10 (req)          | 286   |
           +-------+-----------------------------------+-------+
           |       | application/pkix-cert (res)       | 287   |
           +-------+-----------------------------------+-------+
           |       | application/pkcs-7-mime;smime-    | 281   |
           |       | type=certs-only (res)             |       |
           +-------+-----------------------------------+-------+
           | /skg  | application/pkcs10 (req)          | 286   |
           +-------+-----------------------------------+-------+
           |       | application/multipart-core (res)  | 62    |
           +-------+-----------------------------------+-------+
           | /skc  | application/pkcs10 (req)          | 286   |
           +-------+-----------------------------------+-------+
           |       | application/multipart-core (res)  | 62    |
           +-------+-----------------------------------+-------+
           | /att  | N/A (req)                         | -     |
           +-------+-----------------------------------+-------+
           |       | application/csrattrs (res)        | 285   |
           +-------+-----------------------------------+-------+

                Table 2: EST functions and there associated
                        Media-Type and IANA numbers

   NOTE: CBOR is becoming a de facto encoding scheme in IoT settings.
   There is already work in progress on CBOR encoding of X.509
   certificates [I-D.mattsson-cose-cbor-cert-compress], and this can be
   extended to other EST messages, see Appendix B.







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4.5.  Message Bindings

   The EST-oscore message characteristics are identical to those
   specified in Section 5.4 of [I-D.ietf-ace-coap-est].  It is
   RECOMMENDED that

   *  The EST-oscore endpoints support delayed responses

   *  The endpoints supports the following CoAP options: OSCORE, Uri-
      Host, Uri-Path, Uri-Port, Content-Format, Block1, Block2, and
      Accept.

   *  The EST URLs based on https:// are translated to coap://, but with
      mandatory use of the CoAP OSCORE option.

4.6.  CoAP response codes

   See Section 5.5 in [I-D.ietf-ace-coap-est].

4.7.  Message fragmentation

   The EDHOC key exchange is optimized for message overhead, in
   particular the use of static DH keys instead of signature keys for
   authentication (e.g., method 3 of [I-D.ietf-lake-edhoc]).  Together
   with various measures listed in this document such as CBOR payloads
   (Appendix B), CBOR certificates
   [I-D.mattsson-cose-cbor-cert-compress], certificates by reference
   (Section 3.4), and trust anchors without signature (Section 3.5), a
   significant reduction of message sizes can be achieved.

   Nevertheless, depending on application, the protocol messages may
   become larger than available frame size resulting in fragmentation
   and, in resource constrained networks such as IEEE 802.15.4 where
   throughput is limited, fragment loss can trigger costly
   retransmissions.

   It is RECOMMENDED to prevent IP fragmentation, since it involves an
   error-prone datagram reconstitution.  To limit the size of the CoAP
   payload, this specification mandates the implementation of CoAP
   option Block1 and Block2 fragmentation mechanism [RFC7959] as
   described in Section 5.6 of [I-D.ietf-ace-coap-est].

4.8.  Delayed Responses

   See Section 5.7 in [I-D.ietf-ace-coap-est].






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5.  HTTP-CoAP Proxy

   As noted in Section 6 of [I-D.ietf-ace-coap-est], in real-world
   deployments, the EST server will not always reside within the CoAP
   boundary.  The EST-server can exist outside the constrained network
   in a non-constrained network that supports HTTP but not CoAP, thus
   requiring an intermediary CoAP-to-HTTP proxy.

   Since OSCORE is applicable to CoAP-mappable HTTP (see Section 11 of
   [RFC8613]) the EST payloads can be protected end-to-end between EST
   client and EST server independent of transport protocol or potential
   transport layer security which may need to be terminated in the
   proxy, see Figure 2.  Therefore the concept "Registrar" and its
   required trust relation with EST server as described in Section 6 of
   [I-D.ietf-ace-coap-est] is redundant.

   The mappings between CoAP and HTTP referred to in Section 9.1 of
   [I-D.ietf-ace-coap-est] apply, and additional mappings resulting from
   the use of OSCORE are specified in Section 11 of [RFC8613].

   OSCORE provides end-to-end security between EST Server and EST
   Client.  The use of TLS and DTLS is optional.

                                           Constrained-Node Network
      .---------.                       .----------------------------.
      |   CA    |                       |.--------------------------.|
      '---------'                       ||                          ||
           |                            ||                          ||
       .------.  HTTP   .-----------------.   CoAP   .-----------.  ||
       | EST  |<------->|  CoAP-to-HTTP   |<-------->| EST Client|  ||
       |Server|  (TLS)  |      Proxy      |  (DTLS)  '-----------'  ||
       '------'         '-----------------'                         ||
                                        ||                          ||
           <------------------------------------------------>       ||
                            OSCORE      ||                          ||
                                        |'--------------------------'|
                                        '----------------------------'

             Figure 2: CoAP-to-HTTP proxy at the CoAP boundary.

6.  Security Considerations

   TBD

7.  Privacy Considerations

   TBD




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8.  IANA Considerations

9.  Acknowledgments

10.  References

10.1.  Normative References

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

   [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>.

   [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>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", Work in Progress,
              Internet-Draft, draft-ietf-lake-edhoc-01, 2 August 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-lake-
              edhoc-01.txt>.



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   [I-D.ietf-ace-coap-est]
              Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
              "EST over secure CoAP (EST-coaps)", Work in Progress,
              Internet-Draft, draft-ietf-ace-coap-est-18, 6 January
              2020, <http://www.ietf.org/internet-drafts/draft-ietf-ace-
              coap-est-18.txt>.

10.2.  Informative References

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <https://www.rfc-editor.org/info/rfc2985>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5914]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Format", RFC 5914, DOI 10.17487/RFC5914, June 2010,
              <https://www.rfc-editor.org/info/rfc5914>.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <https://www.rfc-editor.org/info/rfc6024>.

   [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>.

   [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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   [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>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, 10 December 2019, <http://www.ietf.org/
              internet-drafts/draft-ietf-6tisch-minimal-security-
              15.txt>.

   [I-D.ietf-ace-oscore-profile]
              Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "OSCORE Profile of the Authentication and Authorization
              for Constrained Environments Framework", Work in Progress,
              Internet-Draft, draft-ietf-ace-oscore-profile-13, 27
              October 2020, <http://www.ietf.org/internet-drafts/draft-
              ietf-ace-oscore-profile-13.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-35, 24 June 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-ace-oauth-
              authz-35.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP", Work
              in Progress, Internet-Draft, draft-ietf-core-oscore-
              groupcomm-09, 23 June 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-core-oscore-groupcomm-09.txt>.

   [I-D.ietf-cose-x509]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header parameters for carrying and referencing X.509
              certificates", Work in Progress, Internet-Draft, draft-
              ietf-cose-x509-07, 17 September 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-cose-
              x509-07.txt>.



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   [I-D.mattsson-cose-cbor-cert-compress]
              Raza, S., Hoglund, J., Selander, G., Mattsson, J., and M.
              Furuhed, "CBOR Profile of X.509 Certificates", Work in
              Progress, Internet-Draft, draft-mattsson-cose-cbor-cert-
              compress-01, 13 July 2020, <http://www.ietf.org/internet-
              drafts/draft-mattsson-cose-cbor-cert-compress-01.txt>.

   [I-D.palombini-core-oscore-edhoc]
              Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
              and G. Selander, "Combining EDHOC and OSCORE", Work in
              Progress, Internet-Draft, draft-palombini-core-oscore-
              edhoc-00, 13 July 2020, <http://www.ietf.org/internet-
              drafts/draft-palombini-core-oscore-edhoc-00.txt>.

Appendix A.  Other Authentication Methods

   In order to protect certificate enrollment with OSCORE, the necessary
   keying material (notably, the OSCORE Master Secret, see [RFC8613])
   needs to be established between EST-oscore client and EST-oscore
   server.  In this appendix we analyse alternatives to EDHOC, which was
   assumed in the body of this specification.

A.1.  TTP Assisted Authentication

   Trusted third party (TTP) based provisioning, such as the OSCORE
   profile of ACE [I-D.ietf-ace-oscore-profile] assumes existing
   security associations between the client and the TTP, and between the
   server and the TTP.  This setup allows for reduced message overhead
   and round trips compared to the full-fledged EDHOC key exchange.
   Following the ACE terminology the TTP plays the role of the
   Authorization Server (AS), the EST-oscore client corresponds to the
   ACE client and the EST-oscore server is the ACE Resource Server (RS).



















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        +------------+                               +------------+
        |            |                               |            |
        |            | ---(A)- Token Request ------> |  Trusted   |
        |            |                               |   Third    |
        |            | <--(B)- Access Token -------  | Party (AS) |
        |            |                               |            |
        |            |                               +------------+
        | EST-oscore |                                  |     ^
        |   Client   |                                 (F)   (E)
        |(ACE Client)|                                  V     |
        |            |                               +------------+
        |            |                               |            |
        |            | -(C)- Token + EST Request --> | EST-oscore |
        |            |                               | server (RS)|
        |            | <--(D)--- EST response ------ |            |
        |            |                               |            |
        +------------+                               +------------+

     Figure 3: Accessing EST services using a TTP for authenticated key
                      establishment and authorization.

   During initial enrollment the EST-oscore client uses its existing
   security association with the TTP, which replaces the Implicit TA
   database, to establish an authenticated secure channel.  The
   [I-D.ietf-ace-oscore-profile] ACE profile RECOMMENDS the use of
   OSCORE between client and TTP (AS), but TLS or DTLS MAY be used
   additionally or instead.  The client requests an access token at the
   TTP corresponding the EST service it wants to access.  If the client
   request was invalid, or not authorized according to the local EST
   policy, the AS returns an error response as described in
   Section 5.6.3 of [I-D.ietf-ace-oauth-authz].  In its responses the
   TTP (AS) SHOULD signal that the use of OSCORE is REQUIRED for a
   specific access token as indicated in section 4.3 of
   [I-D.ietf-ace-oscore-profile].  This means that the EST-oscore client
   MUST use OSCORE towards all EST-oscore servers (RS) for which this
   access token is valid, and follow Section 4.3 in
   [I-D.ietf-ace-oscore-profile] to derive the security context to run
   OSCORE.  The ACE OSCORE profile RECOMMENDS the use of CBOR web token
   (CWT) as specified in [RFC8392].  The TTP (AS) MUST also provision an
   OSCORE security context to the EST-oscore client and EST-oscore
   server (RS), which is then used to secure the subsequent messages
   between the client and the server.  The details on how to transfer
   the OSCORE contexts are described in section 3.2 of
   [I-D.ietf-ace-oscore-profile].

   Once the client has retrieved the access token it follows the steps
   in [I-D.ietf-ace-oscore-profile] to install the OSCORE security
   context and presents the token to the EST-oscore server.  The EST-



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   oscore server installs the corresponding OSCORE context and can
   either verify the validity of the token locally or request a token
   introspection at the TTP.  In either case EST policy decisions, e.g.,
   which client can request enrollment or reenrollment, can be
   implemented at the TTP.  Finally the EST-oscore client receives a
   response from the EST-oscore server.

A.2.  PSK Based Authentication

   Another method to bootstrap EST services requires a pre-shared OSCORE
   security context between the EST-oscore client and EST-oscore server.
   Authentication using the Implicit TA is no longer required since the
   shared security context authenticates both parties.  The EST-oscore
   client and EST-oscore server need access to the same OSCORE Master
   Secret as well as the OSCORE identifiers (Sender ID and Recipient ID)
   from which an OSCORE security context can be derived, see [RFC8613].
   Some optional parameters may be provisioned if different from the
   default value:

   *  an ID context distinguishing between different OSCORE security
      contexts to use,

   *  an AEAD algorithm,

   *  an HKDF algorithm,

   *  a master salt, and

   *  a replay window size.

Appendix B.  CBOR Encoding of EST Payloads

   Current EST based specifications transport messages using the ASN.1
   data type declaration.  It would be favorable to use a more compact
   representation better suitable for constrained device
   implementations.  In this appendix we list CBOR encodings of requests
   and responses of the mandatory EST functions (see Section 4.3).

B.1.  Distribution of CA Certificates (/crts)

   The EST client can request a copy of the current CA certificates.  In
   EST-coaps and EST-oscore this is done using a GET request to /crts
   (with empty payload).  The response contains a chain of certificates
   used to establish an Explicit Trust Anchor database for subsequent
   authentication of the EST server.






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   CBOR encoding of X.509 certificates is specified in
   [I-D.mattsson-cose-cbor-cert-compress].  CBOR encoding of certificate
   chains is specified below.  This allows for certificates encoded
   using the CBOR certificate format, or as binary X.509 data wrapped as
   a CBOR byte string.

   CDDL:

   certificate chain = (
      + certificate : bstr
   )
   certificate = x509_certificate / cbor_certificate

B.2.  Enrollment/Re-enrollment of Clients (/sen, /sren)

   Existing EST standards specify the enrollment request to be a PKCS#10
   formated message [RFC2986].  The essential information fields for the
   CA to verify are the following:

   *  Information about the subject, here condensed to the subject
      common name,

   *  subject public key, and

   *  signature made by the subject private key.

   CDDL:

   certificate request = (
      subject_common_name : bstr,
      public_key : bstr
      signature : bstr,
      ? ( signature_alg : int, public_key_info : int )
   )

   The response to the enrollment request is the subject certificate,
   for which CBOR encoding is specified in
   [I-D.mattsson-cose-cbor-cert-compress].

   The same message content in request and response applies to re-
   enrollment.

   TODO: PKCS#10 allows inclusion of attributes, which can be used to
   specify extension requests, see Section 5.4.2 of [RFC2985].  CBOR
   encoding of the challengePassword attribute needs to be defined (see
   Section 3.3).  What other attributes are relevant?





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B.2.1.  CBOR Certificate Request Examples

   Here is an example of CBOR encoding of certificate request as defined
   in the previous section.

   114 bytes:

   ( h'0123456789ABCDF0',
   h'61eb80d2abf7d7e4139c86b87e42466f1b4220d3f7ff9d6a1ae298fb9adbb464',
   h'30440220064348b9e52ee0da9f9884d8dd41248c49804ab923330e208a168172dca
   e1 27a02206a06c05957f1db8c4e207437b9ab7739cb857aa6dd9486627b8961606a2
   b68ae' )

   In the example above the signature is generated on an ASN.1 data
   structure.  To validate this, the receiver needs to reconstruct the
   original data structure.  Alternatively, in native mode, the
   signature is generated on the profiled data structure, in which case
   the overall overhead is further reduced.

B.2.2.  ASN.1 Certificate Request Examples

   A corresponding certificate request of the previous section using
   ASN.1 is shown in Figure 4.

       SEQUENCE {
          SEQUENCE {
            INTEGER 0
            SEQUENCE {
              SET {
                SEQUENCE {
                  OBJECT IDENTIFIER commonName (2 5 4 3)
                  UTF8String '01-23-45-67-89-AB-CD-F0'
                  }
                }
              }
            SEQUENCE {
              SEQUENCE {
                OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
                OBJECT IDENTIFIER prime256v1 (1 2 840 10045 3 1 7)
                }
              BIT STRING
                (65 byte public key)
              }
          SEQUENCE {
            OBJECT IDENTIFIER ecdsaWithSHA256 (1 2 840 10045 4 3 2)
            }
          BIT STRING
            (64 byte signature)



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                         Figure 4: ASN.1 Structure.

   In Base64, 375 bytes:

   -----BEGIN CERTIFICATE REQUEST-----
   MIHcMIGEAgEAMCIxIDAeBgNVBAMMFzAxLTIzLTQ1LTY3LTg5LUFCLUNELUYwMFkw
   EwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEYeuA0qv31+QTnIa4fkJGbxtCINP3/51q
   GuKY+5rbtGSeZn3l8rVbU0jVEBWvKhAd98JeqgsuauGHRNWt2FqJ1aAAMAoGCCqG
   SM49BAMCA0cAMEQCIAZDSLnlLuDan5iE2N1BJIxJgEq5IzMOIIoWgXLcrhJ6AiBq
   BsBZV/HbjE4gdDe5q3c5y4V6pt2UhmJ7iWFgaitorg==
   -----END CERTIFICATE REQUEST-----

   In hex, 221 bytes:

   3081dc30818402010030223120301e06035504030c1730312d32332d34352d36
   372d38392d41422d43442d46303059301306072a8648ce3d020106082a8648ce
   3d0301070342000461eb80d2abf7d7e4139c86b87e42466f1b4220d3f7ff9d6a
   1ae298fb9adbb4649e667de5f2b55b5348d51015af2a101df7c25eaa0b2e6ae1
   8744d5add85a89d5a000300a06082a8648ce3d04030203470030440220064348
   b9e52ee0da9f9884d8dd41248c49804ab923330e208a168172dcae127a02206a
   06c05957f1db8c4e207437b9ab7739cb857aa6dd9486627b8961606a2b68ae

Authors' Addresses

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com


   Shahid Raza
   RISE

   Email: shahid.raza@ri.se


   Martin Furuhed
   Nexus

   Email: martin.furuhed@nexusgroup.com


   Malisa Vucinic
   INRIA

   Email: malisa.vucinic@inria.fr





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   Timothy Claeys
   INRIA

   Email: timothy.claeys@inria.fr















































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