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Versions: (draft-selander-ace-object-security) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 8613

CoRE Working Group                                           G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: October 1, 2018                                     Ericsson AB
                                                                L. Seitz
                                                               RISE SICS
                                                          March 30, 2018


     Object Security for Constrained RESTful Environments (OSCORE)
                   draft-ietf-core-object-security-12

Abstract

   This document defines Object Security for Constrained RESTful
   Environments (OSCORE), a method for application-layer protection of
   the Constrained Application Protocol (CoAP), using CBOR Object
   Signing and Encryption (COSE).  OSCORE provides end-to-end protection
   between endpoints communicating using CoAP or CoAP-mappable HTTP.
   OSCORE is designed for constrained nodes and networks supporting a
   range of proxy operations, including translation between different
   transport protocols.

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 October 1, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  The OSCORE Option . . . . . . . . . . . . . . . . . . . . . .   7
   3.  The Security Context  . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Security Context Definition . . . . . . . . . . . . . . .   7
     3.2.  Establishment of Security Context Parameters  . . . . . .  10
     3.3.  Requirements on the Security Context Parameters . . . . .  12
   4.  Protected Message Fields  . . . . . . . . . . . . . . . . . .  12
     4.1.  CoAP Options  . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  CoAP Header Fields and Payload  . . . . . . . . . . . . .  20
     4.3.  Signaling Messages  . . . . . . . . . . . . . . . . . . .  21
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  22
     5.1.  Kid Context . . . . . . . . . . . . . . . . . . . . . . .  23
     5.2.  Nonce . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     5.3.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  25
     5.4.  Additional Authenticated Data . . . . . . . . . . . . . .  26
   6.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  26
     6.1.  Encoding of the OSCORE Option Value . . . . . . . . . . .  27
     6.2.  Encoding of the OSCORE Payload  . . . . . . . . . . . . .  28
     6.3.  Examples of Compressed COSE Objects . . . . . . . . . . .  28
   7.  Message Binding, Sequence Numbers, Freshness and Replay
       Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Message Binding . . . . . . . . . . . . . . . . . . . . .  31
     7.2.  Sequence Numbers  . . . . . . . . . . . . . . . . . . . .  31
     7.3.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.4.  Replay Protection . . . . . . . . . . . . . . . . . . . .  32
     7.5.  Losing Part of the Context State  . . . . . . . . . . . .  32
   8.  Processing  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     8.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  34
     8.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  34
     8.3.  Protecting the Response . . . . . . . . . . . . . . . . .  36
     8.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  36
   9.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  38
   10. CoAP-to-CoAP Forwarding Proxy . . . . . . . . . . . . . . . .  38
   11. HTTP Operations . . . . . . . . . . . . . . . . . . . . . . .  39
     11.1.  The HTTP OSCORE Header Field . . . . . . . . . . . . . .  39
     11.2.  CoAP-to-HTTP Mapping . . . . . . . . . . . . . . . . . .  40
     11.3.  HTTP-to-CoAP Mapping . . . . . . . . . . . . . . . . . .  40
     11.4.  HTTP Endpoints . . . . . . . . . . . . . . . . . . . . .  41



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     11.5.  Example: HTTP Client and CoAP Server . . . . . . . . . .  41
     11.6.  Example: CoAP Client and HTTP Server . . . . . . . . . .  43
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  44
     12.1.  End-to-end Protection  . . . . . . . . . . . . . . . . .  44
     12.2.  Security Context Establishment . . . . . . . . . . . . .  45
     12.3.  Master Secret  . . . . . . . . . . . . . . . . . . . . .  45
     12.4.  Replay Protection  . . . . . . . . . . . . . . . . . . .  45
     12.5.  Client Aliveness . . . . . . . . . . . . . . . . . . . .  46
     12.6.  Cryptographic Considerations . . . . . . . . . . . . . .  46
     12.7.  Message Segmentation . . . . . . . . . . . . . . . . . .  46
     12.8.  Privacy Considerations . . . . . . . . . . . . . . . . .  47
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
     13.1.  COSE Header Parameters Registry  . . . . . . . . . . . .  47
     13.2.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  48
     13.3.  CoAP Signaling Option Numbers Registry . . . . . . . . .  48
     13.4.  Header Field Registrations . . . . . . . . . . . . . . .  48
     13.5.  Media Type Registrations . . . . . . . . . . . . . . . .  49
     13.6.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  51
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     14.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Appendix A.  Scenario Examples  . . . . . . . . . . . . . . . . .  55
     A.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  55
     A.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  56
   Appendix B.  Deployment Examples  . . . . . . . . . . . . . . . .  57
     B.1.  Master Secret Used Once . . . . . . . . . . . . . . . . .  57
     B.2.  Master Secret Used Multiple Times . . . . . . . . . . . .  58
   Appendix C.  Test Vectors . . . . . . . . . . . . . . . . . . . .  59
     C.1.  Test Vector 1: Key Derivation with Master Salt  . . . . .  59
     C.2.  Test Vector 2: Key Derivation without Master Salt . . . .  60
     C.3.  Test Vector 3: OSCORE Request, Client . . . . . . . . . .  61
     C.4.  Test Vector 4: OSCORE Request, Client . . . . . . . . . .  63
     C.5.  Test Vector 5: OSCORE Response, Server  . . . . . . . . .  64
     C.6.  Test Vector 6: OSCORE Response with Partial IV, Server  .  65
   Appendix D.  Overview of Security Properties  . . . . . . . . . .  66
     D.1.  Supporting Proxy Operations . . . . . . . . . . . . . . .  66
     D.2.  Protected Message Fields  . . . . . . . . . . . . . . . .  66
     D.3.  Uniqueness of (key, nonce)  . . . . . . . . . . . . . . .  67
     D.4.  Unprotected Message Fields  . . . . . . . . . . . . . . .  68
   Appendix E.  CDDL Summary . . . . . . . . . . . . . . . . . . . .  71
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  72
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  72

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   transfer protocol, designed for constrained nodes and networks
   [RFC7228], and may be mapped from HTTP [RFC8075].  CoAP specifies the



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   use of proxies for scalability and efficiency and references DTLS
   [RFC6347] for security.  CoAP-to-CoAP, HTTP-to-CoAP, and CoAP-to-HTTP
   proxies require DTLS or TLS [RFC5246] to be terminated at the proxy.
   The proxy therefore not only has access to the data required for
   performing the intended proxy functionality, but is also able to
   eavesdrop on, or manipulate any part of, the message payload and
   metadata in transit between the endpoints.  The proxy can also
   inject, delete, or reorder packets since they are no longer protected
   by (D)TLS.

   This document defines the Object Security for Constrained RESTful
   Environments (OSCORE) security protocol, protecting CoAP and CoAP-
   mappable HTTP requests and responses end-to-end across intermediary
   nodes such as CoAP forward proxies and cross-protocol translators
   including HTTP-to-CoAP proxies [RFC8075].  In addition to the core
   CoAP features defined in [RFC7252], OSCORE supports Observe
   [RFC7641], Block-wise [RFC7959], No-Response [RFC7967], and PATCH and
   FETCH [RFC8132].  An analysis of end-to-end security for CoAP
   messages through some types of intermediary nodes is performed in
   [I-D.hartke-core-e2e-security-reqs].  OSCORE essentially protects the
   RESTful interactions; the request method, the requested resource, the
   message payload, etc. (see Section 4).  OSCORE protects neither the
   CoAP Messaging Layer nor the CoAP Token which may change between the
   endpoints, and those are therefore processed as defined in [RFC7252].
   Additionally, since the message formats for CoAP over unreliable
   transport [RFC7252] and for CoAP over reliable transport [RFC8323]
   differ only in terms of CoAP Messaging Layer, OSCORE can be applied
   to both unreliable and reliable transports (see Figure 1).

               +-----------------------------------+
               |            Application            |
               +-----------------------------------+
               +-----------------------------------+  \
               |  Requests / Responses / Signaling |  |
               |-----------------------------------|  |
               |               OSCORE              |  | CoAP
               |-----------------------------------|  |
               | Messaging Layer / Message Framing |  |
               +-----------------------------------+  /
               +-----------------------------------+
               |          UDP / TCP / ...          |
               +-----------------------------------+

              Figure 1: Abstract Layering of CoAP with OSCORE

   OSCORE works in very constrained nodes and networks, thanks to its
   small message size and the restricted code and memory requirements in
   addition to what is required by CoAP.  Examples of the use of OSCORE



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   are given in Appendix A.  OSCORE does not depend on underlying
   layers, and can be used with non-IP transports (e.g.,
   [I-D.bormann-6lo-coap-802-15-ie]).  OSCORE may also be used in
   different ways with HTTP.  OSCORE messages may be transported in
   HTTP, and OSCORE may also be used to protect CoAP-mappable HTTP
   messages, as described below.

   OSCORE is designed to protect as much information as possible while
   still allowing CoAP proxy operations (Section 10).  It works with
   legacy CoAP-to-CoAP forward proxies [RFC7252], but an OSCORE-aware
   proxy will be more efficient.  HTTP-to-CoAP proxies [RFC8075] and
   CoAP-to-HTTP proxies can also be used with OSCORE, as specified in
   Section 11.  OSCORE may be used together with TLS or DTLS over one or
   more hops in the end-to-end path, e.g. transported with HTTPS in one
   hop and with plain CoAP in another hop.  The use of OSCORE does not
   affect the URI scheme and OSCORE can therefore be used with any URI
   scheme defined for CoAP or HTTP.  The application decides the
   conditions for which OSCORE is required.

   OSCORE uses pre-shared keys which may have been established out-of-
   band or with a key establishment protocol (see Section 3.2).  The
   technical solution builds on CBOR Object Signing and Encryption
   (COSE) [RFC8152], providing end-to-end encryption, integrity, replay
   protection, and binding of response to request.  A compressed version
   of COSE is used, as specified in Section 6.  The use of OSCORE is
   signaled in CoAP with a new option (Section 2), and in HTTP with a
   new header field (Section 11.1) and content type (Section 13.5).  The
   solution transforms a CoAP/HTTP message into an "OSCORE message"
   before sending, and vice versa after receiving.  The OSCORE message
   is a CoAP/HTTP message related to the original message in the
   following way: the original CoAP/HTTP message is translated to CoAP
   (if not already in CoAP) and protected in a COSE object.  The
   encrypted message fields of this COSE object are transported in the
   CoAP payload/HTTP body of the OSCORE message, and the OSCORE option/
   header field is included in the message.  A sketch of an exchange of
   OSCORE messages, in the case of the original message being CoAP, is
   provided in Figure 2.














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          Client                                          Server
             |      OSCORE request - POST example.com:      |
             |        Header, Token,                        |
             |        Options: {OSCORE, ...},               |
             |        Payload: COSE ciphertext              |
             +--------------------------------------------->|
             |                                              |
             |<---------------------------------------------+
             |      OSCORE response - 2.04 (Changed):       |
             |        Header, Token,                        |
             |        Options: {OSCORE, ...},               |
             |        Payload: COSE ciphertext              |
             |                                              |

                   Figure 2: Sketch of CoAP with OSCORE

   An implementation supporting this specification MAY implement only
   the client part, MAY implement only the server part, or MAY implement
   only one of the proxy parts.

1.1.  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.

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252], Observe [RFC7641], Block-wise [RFC7959],
   COSE [RFC8152], CBOR [RFC7049], CDDL [I-D.ietf-cbor-cddl] as
   summarized in Appendix E, and constrained environments [RFC7228].

   The term "hop" is used to denote a particular leg in the end-to-end
   path.  The concept "hop-by-hop" (as in "hop-by-hop encryption" or
   "hop-by-hop fragmentation") opposed to "end-to-end", is used in this
   document to indicate that the messages are processed accordingly in
   the intermediaries, rather than just forwarded to the next node.

   The term "stop processing" is used throughout the document to denote
   that the message is not passed up to the CoAP Request/Response layer
   (see Figure 1).

   The terms Common/Sender/Recipient Context, Master Secret/Salt, Sender
   ID/Key, Recipient ID/Key, and Common IV are defined in Section 3.1.






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2.  The OSCORE Option

   The OSCORE option (see Figure 3, which extends Table 4 of [RFC7252])
   indicates that the CoAP message is an OSCORE message and that it
   contains a compressed COSE object (see Section 5 and Section 6).  The
   OSCORE option is critical, safe to forward, part of the cache key,
   and not repeatable.

  +------+---+---+---+---+-----------------+--------+--------+---------+
  | No.  | C | U | N | R | Name            | Format | Length | Default |
  +------+---+---+---+---+-----------------+--------+--------+---------+
  | TBD1 | x |   |   |   | OSCORE          |  (*)   | 0-255  | (none)  |
  +------+---+---+---+---+-----------------+--------+--------+---------+
      C = Critical,   U = Unsafe,   N = NoCacheKey,   R = Repeatable
      (*) See below.

                        Figure 3: The OSCORE Option

   The OSCORE option includes the OSCORE flag bits (Section 6), the
   Sender Sequence Number and the Sender ID when present (Section 3).
   The detailed format and length is specified in Section 6.  If the
   OSCORE flag bits are all zero (0x00) the Option value SHALL be empty
   (Option Length = 0).  An endpoint receiving a CoAP message without
   payload, that also contains an OSCORE option SHALL treat it as
   malformed and reject it.

   A successful response to a request with the OSCORE option SHALL
   contain the OSCORE option.  Whether error responses contain the
   OSCORE option depends on the error type (see Section 8).

   For CoAP proxy operations, see Section 10.

3.  The Security Context

   OSCORE requires that client and server establish a shared security
   context used to process the COSE objects.  OSCORE uses COSE with an
   Authenticated Encryption with Additional Data (AEAD, [RFC5116])
   algorithm for protecting message data between a client and a server.
   In this section, we define the security context and how it is derived
   in client and server based on a shared secret and a key derivation
   function (KDF).

3.1.  Security Context Definition

   The security context is the set of information elements necessary to
   carry out the cryptographic operations in OSCORE.  For each endpoint,
   the security context is composed of a "Common Context", a "Sender
   Context", and a "Recipient Context".



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   The endpoints protect messages to send using the Sender Context and
   verify messages received using the Recipient Context, both contexts
   being derived from the Common Context and other data.  Clients and
   servers need to be able to retrieve the correct security context to
   use.

   An endpoint uses its Sender ID (SID) to derive its Sender Context,
   and the other endpoint uses the same ID, now called Recipient ID
   (RID), to derive its Recipient Context.  In communication between two
   endpoints, the Sender Context of one endpoint matches the Recipient
   Context of the other endpoint, and vice versa.  Thus, the two
   security contexts identified by the same IDs in the two endpoints are
   not the same, but they are partly mirrored.  Retrieval and use of the
   security context are shown in Figure 4.

                 .-------------.           .-------------.
                 |  Common,    |           |  Common,    |
                 |  Sender,    |           |  Recipient, |
                 |  Recipient  |           |  Sender     |
                 '-------------'           '-------------'
                      Client                   Server
                         |                       |
   Retrieve context for  | OSCORE request:       |
    target resource      |   Token = Token1,     |
   Protect request with  |   kid = SID, ...      |
     Sender Context      +---------------------->| Retrieve context with
                         |                       |  RID = kid
                         |                       | Verify request with
                         |                       |  Recipient Context
                         | OSCORE response:      | Protect response with
                         |   Token = Token1, ... |  Sender Context
   Retrieve context with |<----------------------+
    Token = Token1       |                       |
   Verify request with   |                       |
    Recipient Context    |                       |

            Figure 4: Retrieval and use of the Security Context

   The Common Context contains the following parameters:

   o  AEAD Algorithm.  The COSE AEAD algorithm to use for encryption.

   o  Key Derivation Function.  The HMAC based HKDF [RFC5869] used to
      derive Sender Key, Recipient Key, and Common IV.

   o  Master Secret.  Variable length, random byte string (see
      Section 12.3) containing the keying material used to derive
      traffic keys and IVs.



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   o  Master Salt.  Variable length byte string containing the salt used
      to derive traffic keys and IVs.

   o  Common IV.  Byte string derived from Master Secret and Master
      Salt.  Length is determined by the AEAD Algorithm.

   The Sender Context contains the following parameters:

   o  Sender ID.  Byte string used to identify the Sender Context and to
      assure unique AEAD nonces.  Maximum length is determined by the
      AEAD Algorithm.

   o  Sender Key. Byte string containing the symmetric key to protect
      messages to send.  Derived from Common Context and Sender ID.
      Length is determined by the AEAD Algorithm.

   o  Sender Sequence Number.  Non-negative integer used by the sender
      to protect requests and certain responses, e.g.  Observe
      notifications.  Used as 'Partial IV' [RFC8152] to generate unique
      nonces for the AEAD.  Maximum value is determined by the AEAD
      Algorithm.

   The Recipient Context contains the following parameters:

   o  Recipient ID.  Byte string used to identify the Recipient Context
      and to assure unique AEAD nonces.  Maximum length is determined by
      the AEAD Algorithm.

   o  Recipient Key. Byte string containing the symmetric key to verify
      messages received.  Derived from Common Context and Recipient ID.
      Length is determined by the AEAD Algorithm.

   o  Replay Window (Server only).  The replay window to verify requests
      received.

   All parameters except Sender Sequence Number and Replay Window are
   immutable once the security context is established.  An endpoint may
   free up memory by not storing the Common IV, Sender Key, and
   Recipient Key, deriving them from the Master Key and Master Salt when
   needed.  Alternatively, an endpoint may free up memory by not storing
   the Master Secret and Master Salt after the other parameters have
   been derived.

   Endpoints MAY operate as both client and server and use the same
   security context for those roles.  Independent of being client or
   server, the endpoint protects messages to send using its Sender
   Context, and verifies messages received using its Recipient Context.
   The endpoints MUST NOT change the Sender/Recipient ID when changing



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   roles.  In other words, changing the roles does not change the set of
   keys to be used.

3.2.  Establishment of Security Context Parameters

   The parameters in the security context are derived from a small set
   of input parameters.  The following input parameters SHALL be pre-
   established:

   o  Master Secret

   o  Sender ID

   o  Recipient ID

   The following input parameters MAY be pre-established.  In case any
   of these parameters is not pre-established, the default value
   indicated below is used:

   o  AEAD Algorithm

      *  Default is AES-CCM-16-64-128 (COSE algorithm encoding: 10)

   o  Master Salt

      *  Default is the empty string

   o  Key Derivation Function (KDF)

      *  Default is HKDF SHA-256

   o  Replay Window Type and Size

      *  Default is DTLS-type replay protection with a window size of 32
         [RFC6347]

   All input parameters need to be known to and agreed on by both
   endpoints, but the replay window may be different in the two
   endpoints.  The way the input parameters are pre-established, is
   application specific.  Considerations of security context
   establishment are given in Section 12.2 and examples of deploying
   OSCORE in Appendix B.

3.2.1.  Derivation of Sender Key, Recipient Key, and Common IV

   The KDF MUST be one of the HMAC based HKDF [RFC5869] algorithms
   defined in COSE.  HKDF SHA-256 is mandatory to implement.  The
   security context parameters Sender Key, Recipient Key, and Common IV



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   SHALL be derived from the input parameters using the HKDF, which
   consists of the composition of the HKDF-Extract and HKDF-Expand steps
   [RFC5869]:

      output parameter = HKDF(salt, IKM, info, L)

   where:

   o  salt is the Master Salt as defined above

   o  IKM is the Master Secret as defined above

   o  info is a CBOR array consisting of:

      info = [
          id : bstr,
          alg_aead : int / tstr,
          type : tstr,
          L : uint
      ]

   where:

   o  id is the Sender ID or Recipient ID when deriving keys and the
      empty string when deriving the Common IV.  The encoding is
      described in Section 5.

   o  alg_aead is the AEAD Algorithm, encoded as defined in [RFC8152].

   o  type is "Key" or "IV".  The label is an ASCII string, and does not
      include a trailing NUL byte.

   o  L is the size of the key/IV for the AEAD algorithm used, in bytes.

   For example, if the algorithm AES-CCM-16-64-128 (see Section 10.2 in
   [RFC8152]) is used, the integer value for alg_aead is 10, the value
   for L is 16 for keys and 13 for the Common IV.

3.2.2.  Initial Sequence Numbers and Replay Window

   The Sender Sequence Number is initialized to 0.  The supported types
   of replay protection and replay window length is application specific
   and depends on how OSCORE is transported, see Section 7.4.  The
   default is DTLS-type replay protection with a window size of 32
   initiated as described in Section 4.1.2.6 of [RFC6347].






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3.3.  Requirements on the Security Context Parameters

   As collisions may lead to the loss of both confidentiality and
   integrity, Sender ID SHALL be unique in the set of all security
   contexts using the same Master Secret and Master Salt.  To assign
   identifiers, a trusted third party (e.g., [I-D.ietf-ace-oauth-authz])
   or a protocol that allows the parties to negotiate locally unique
   identifiers can be used.  The Sender IDs can be very short.  The
   maximum length of Sender ID in bytes equals the length of AEAD nonce
   minus 6.  For AES-CCM-16-64-128 the maximum length of Sender ID is 7
   bytes.

   To simplify retrieval of the right Recipient Context, the Recipient
   ID SHOULD be unique in the sets of all Recipient Contexts used by an
   endpoint.  If an endpoint has the same Recipient ID with different
   Recipient Contexts, i.e. the Recipient Contexts are derived from
   different keying material, then the endpoint may need to try multiple
   times before finding the right security context associated to the
   Recipient ID.  The Client MAY provide a 'kid context' parameter
   (Section 5.1) to help the Server find the right context.

   While the triple (Master Secret, Master Salt, Sender ID) MUST be
   unique, the same Master Salt MAY be used with several Master Secrets
   and the same Master Secret MAY be used with several Master Salts.

4.  Protected Message Fields

   OSCORE transforms a CoAP message (which may have been generated from
   an HTTP message) into an OSCORE message, and vice versa.  OSCORE
   protects as much of the original message as possible while still
   allowing certain proxy operations (see Section 10 and Section 11).
   This section defines how OSCORE protects the message fields and
   transfers them end-to-end between client and server (in any
   direction).

   The remainder of this section and later sections focus on the
   behavior in terms of CoAP messages.  If HTTP is used for a particular
   hop in the end-to-end path, then this section applies to the
   conceptual CoAP message that is mappable to/from the original HTTP
   message as discussed in Section 11.  That is, an HTTP message is
   conceptually transformed to a CoAP message and then to an OSCORE
   message, and similarly in the reverse direction.  An actual
   implementation might translate directly from HTTP to OSCORE without
   the intervening CoAP representation.

   Protection of Signaling messages (Section 5 of [RFC8323]) is
   specified in Section 4.3.  The other parts of this section target
   Request/Response messages.



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   Message fields of the CoAP message may be protected end-to-end
   between CoAP client and CoAP server in different ways:

   o  Class E: encrypted and integrity protected,

   o  Class I: integrity protected only, or

   o  Class U: unprotected.

   The sending endpoint SHALL transfer Class E message fields in the
   ciphertext of the COSE object in the OSCORE message.  The sending
   endpoint SHALL include Class I message fields in the Additional
   Authenticated Data (AAD) of the AEAD algorithm, allowing the
   receiving endpoint to detect if the value has changed in transfer.
   Class U message fields SHALL NOT be protected in transfer.  Class I
   and Class U message field values are transferred in the header or
   options part of the OSCORE message, which is visible to proxies.

   Message fields not visible to proxies, i.e., transported in the
   ciphertext of the COSE object, are called "Inner" (Class E).  Message
   fields transferred in the header or options part of the OSCORE
   message, which is visible to proxies, are called "Outer" (Class I or
   U).  There are currently no Class I options defined.

   An OSCORE message may contain both an Inner and an Outer instance of
   a certain CoAP message field.  Inner message fields are intended for
   the receiving endpoint, whereas Outer message fields are used to
   enable proxy operations.  Inner and Outer message fields are
   processed independently.

4.1.  CoAP Options

   A summary of how options are protected is shown in Figure 5.  Note
   that some options may have both Inner and Outer message fields which
   are protected accordingly.  Certain options require special
   processing as is described in Section 4.1.3.















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                   +------+-----------------+---+---+
                   | No.  | Name            | E | U |
                   +------+-----------------+---+---+
                   |   1  | If-Match        | x |   |
                   |   3  | Uri-Host        |   | x |
                   |   4  | ETag            | x |   |
                   |   5  | If-None-Match   | x |   |
                   |   6  | Observe         |   | x |
                   |   7  | Uri-Port        |   | x |
                   |   8  | Location-Path   | x |   |
                   | TBD1 | OSCORE          |   | x |
                   |  11  | Uri-Path        | x |   |
                   |  12  | Content-Format  | x |   |
                   |  14  | Max-Age         | x | x |
                   |  15  | Uri-Query       | x |   |
                   |  17  | Accept          | x |   |
                   |  20  | Location-Query  | x |   |
                   |  23  | Block2          | x | x |
                   |  27  | Block1          | x | x |
                   |  28  | Size2           | x | x |
                   |  35  | Proxy-Uri       |   | x |
                   |  39  | Proxy-Scheme    |   | x |
                   |  60  | Size1           | x | x |
                   | 258  | No-Response     | x | x |
                   +------+-----------------+---+---+

                 E = Encrypt and Integrity Protect (Inner)
                 U = Unprotected (Outer)

                   Figure 5: Protection of CoAP Options

   Options that are unknown or for which OSCORE processing is not
   defined SHALL be processed as class E (and no special processing).
   Specifications of new CoAP options SHOULD define how they are
   processed with OSCORE.  A new COAP option SHOULD be of class E unless
   it requires proxy processing.

4.1.1.  Inner Options

   Inner option message fields (class E) are used to communicate
   directly with the other endpoint.

   The sending endpoint SHALL write the Inner option message fields
   present in the original CoAP message into the plaintext of the COSE
   object (Section 5.3), and then remove the Inner option message fields
   from the OSCORE message.





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   The processing of Inner option message fields by the receiving
   endpoint is specified in Section 8.2 and Section 8.4.

4.1.2.  Outer Options

   Outer option message fields (Class U or I) are used to support proxy
   operations, see Appendix D.1.

   The sending endpoint SHALL include the Outer option message field
   present in the original message in the options part of the OSCORE
   message.  All Outer option message fields, including the OSCORE
   option, SHALL be encoded as described in Section 3.1 of [RFC7252],
   where the delta is the difference to the previously included instance
   of Outer option message field.

   The processing of Outer options by the receiving endpoint is
   specified in Section 8.2 and Section 8.4.

   A procedure for integrity-protection-only of Class I option message
   fields is specified in Section 5.4.  Proxies MUST NOT change the
   order of option's occurrences, for options repeatable and of class I.

   Note: There are currently no Class I option message fields defined.

4.1.3.  Special Options

   Some options require special processing as specified in this section.

4.1.3.1.  Max-Age

   An Inner Max-Age message field is used to indicate the maximum time a
   response may be cached by the client (as defined in [RFC7252]), end-
   to-end from the server to the client, taking into account that the
   option is not accessible to proxies.  The Inner Max-Age SHALL be
   processed by OSCORE as a normal Inner option, specified in
   Section 4.1.1.

   An Outer Max-Age message field is used to avoid unnecessary caching
   of OSCORE error responses at OSCORE unaware intermediary nodes.  A
   server MAY set a Class U Max-Age message field with value zero to
   OSCORE error responses, which are described in Section 7.4,
   Section 8.2 and Section 8.4.  Such message field is then processed
   according to Section 4.1.2.

   Successful OSCORE responses do not need to include an Outer Max-Age
   option since the responses are non-cacheable by construction (see
   Section 4.2).




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4.1.3.2.  Proxy-Uri

   Proxy-Uri, when present, is split by OSCORE into class U options and
   class E options, which are processed accordingly.  When Proxy-Uri is
   used in the original CoAP message, Uri-* are not present [RFC7252].

   The sending endpoint SHALL first decompose the Proxy-Uri value of the
   original CoAP message into the Proxy-Scheme, Uri-Host, Uri-Port, Uri-
   Path, and Uri-Query options (if present) according to Section 6.4 of
   [RFC7252].

   Uri-Path and Uri-Query are class E options and SHALL be protected and
   processed as Inner options (Section 4.1.1).  Uri-Host being an Outer
   option SHOULD NOT contain privacy sensitive information.

   The Proxy-Uri option of the OSCORE message SHALL be set to the
   composition of Proxy-Scheme, Uri-Host, and Uri-Port options (if
   present) as specified in Section 6.5 of [RFC7252], and processed as
   an Outer option of Class U (Section 4.1.2).

   Note that replacing the Proxy-Uri value with the Proxy-Scheme and
   Uri-* options works by design for all CoAP URIs (see Section 6 of
   [RFC7252]).  OSCORE-aware HTTP servers should not use the userinfo
   component of the HTTP URI (as defined in Section 3.2.1 of [RFC3986]),
   so that this type of replacement is possible in the presence of CoAP-
   to-HTTP proxies.  In future documents specifying cross-protocol
   proxying behavior using different URI structures, it is expected that
   the authors will create Uri-* options that allow decomposing the
   Proxy-Uri, and specify in which OSCORE class they belong.

   An example of how Proxy-Uri is processed is given here.  Assume that
   the original CoAP message contains:

   o  Proxy-Uri = "coap://example.com/resource?q=1"

   During OSCORE processing, Proxy-Uri is split into:

   o  Proxy-Scheme = "coap"

   o  Uri-Host = "example.com"

   o  Uri-Port = "5683"

   o  Uri-Path = "resource"

   o  Uri-Query = "q=1"





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   Uri-Path and Uri-Query follow the processing defined in
   Section 4.1.1, and are thus encrypted and transported in the COSE
   object.  The remaining options are composed into the Proxy-Uri
   included in the options part of the OSCORE message, which has value:

   o  Proxy-Uri = "coap://example.com"

   See Sections 6.1 and 12.6 of [RFC7252] for more information.

4.1.3.3.  The Block Options

   Block-wise [RFC7959] is an optional feature.  An implementation MAY
   support [RFC7252] and the OSCORE option without supporting block-wise
   transfers.  The Block options (Block1, Block2, Size1, Size2), when
   Inner message fields, provide secure message segmentation such that
   each segment can be verified.  The Block options, when Outer message
   fields, enables hop-by-hop fragmentation of the OSCORE message.
   Inner and Outer block processing may have different performance
   properties depending on the underlying transport.  The end-to-end
   integrity of the message can be verified both in case of Inner and
   Outer Block-wise transfers provided all blocks are received.

4.1.3.3.1.  Inner Block Options

   The sending CoAP endpoint MAY fragment a CoAP message as defined in
   [RFC7959] before the message is processed by OSCORE.  In this case
   the Block options SHALL be processed by OSCORE as normal Inner
   options (Section 4.1.1).  The receiving CoAP endpoint SHALL process
   the OSCORE message before processing Block-wise as defined in
   [RFC7959].

4.1.3.3.2.  Outer Block Options

   Proxies MAY fragment an OSCORE message using [RFC7959], by
   introducing Block option message fields that are Outer
   (Section 4.1.2).  Note that the Outer Block options are neither
   encrypted nor integrity protected.  As a consequence, a proxy can
   maliciously inject block fragments indefinitely, since the receiving
   endpoint needs to receive the last block (see [RFC7959]) to be able
   to compose the OSCORE message and verify its integrity.  Therefore,
   applications supporting OSCORE and [RFC7959] MUST specify a security
   policy defining a maximum unfragmented message size
   (MAX_UNFRAGMENTED_SIZE) considering the maximum size of message which
   can be handled by the endpoints.  Messages exceeding this size SHOULD
   be fragmented by the sending endpoint using Inner Block options
   (Section 4.1.3.3.1).





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   An endpoint receiving an OSCORE message with an Outer Block option
   SHALL first process this option according to [RFC7959], until all
   blocks of the OSCORE message have been received, or the cumulated
   message size of the blocks exceeds MAX_UNFRAGMENTED_SIZE.  In the
   former case, the processing of the OSCORE message continues as
   defined in this document.  In the latter case the message SHALL be
   discarded.

   Because of encryption of Uri-Path and Uri-Query, messages to the same
   server may, from the point of view of a proxy, look like they also
   target the same resource.  A proxy SHOULD mitigate a potential mix-up
   of blocks from concurrent requests to the same server, for example
   using the Request-Tag processing specified in Section 3.3.2 of
   [I-D.ietf-core-echo-request-tag].

4.1.3.4.  Observe

   Observe [RFC7641] is an optional feature.  An implementation MAY
   support [RFC7252] and the OSCORE option without supporting [RFC7641].
   The Observe option as used here targets the requirements on
   forwarding of [I-D.hartke-core-e2e-security-reqs] (Section 2.2.1).

   An Observe intermediary MUST forward the OSCORE option unchanged.  In
   order for an OSCORE-unaware proxy to support forwarding of Observe
   messages [RFC7641], there SHALL be an Outer Observe option, i.e.,
   present in the options part of the OSCORE message.  With OSCORE,
   Observe intermediaries are forwarding messages without being able to
   re-send cached notifications to other clients.

   In order to support multiple concurrent Observe registrations in the
   same endpoint, Observe intermediaries are allowed to deviate from
   [RFC7641] and register multiple times to the same (root) resource,
   since the actual target resource is encrypted and not visible in the
   OSCORE message.  The processing of the CoAP Code for Observe messages
   is described in Section 4.2.

   The Observe option in the CoAP request may be legitimately removed by
   a proxy or ignored by the server.  In these cases, the server
   processes the request as a non-Observe request and produce a non-
   Observe response.  If the OSCORE client receives a response to an
   Observe request without an Outer Observe value, then it verifies the
   response as a non-Observe response, as specified in Section 8.4.  If
   the OSCORE client receives a response to a non-Observe request with
   an Outer Observe value, it stops processing the message, as specified
   in Section 8.4.

   It the server accepts the Observe registration, a Partial IV must be
   included in all notifications (both successful and error).  To secure



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   the order of notifications, the client SHALL maintain a Notification
   Number for each Observation it registers.  The Notification Number is
   a non-negative integer containing the largest Partial IV of the
   received notifications for the associated Observe registration (see
   Section 7.4).  The Notification Number is initialized to the Partial
   IV of the first successfully received notification response to the
   registration request.  In contrast to [RFC7641], the received Partial
   IV MUST always be compared with the Notification Number, which thus
   MUST NOT be forgotten after 128 seconds.  Further details of replay
   protection of notifications are specified in Section 7.4.  The client
   MAY ignore the Observe option value.

   Clients can re-register observations to ensure that the observation
   is still active and establish freshness again ([RFC7641]
   Section 3.3.1).  When an OSCORE observation is refreshed, not only
   the ETags, but also the partial IV (and thus the payload and OSCORE
   option) change.  The server uses the new request's Partial IV as the
   'request_piv' of new responses.

4.1.3.5.  No-Response

   No-Response [RFC7967] is an optional feature.  Clients using No-
   Response MUST set both an Inner (Class E) and an Outer (Class U) No-
   Response option, with the same value.

   The Inner No-Response option is used to communicate to the server the
   client's disinterest in certain classes of responses to a particular
   request.  The Inner No-Response SHALL be processed by OSCORE as
   specified in Section 4.1.1.

   The Outer No-Response option is used to support proxy functionality,
   specifically to avoid error transmissions from proxies to clients,
   and to avoid bandwidth reduction to servers by proxies applying
   congestion control when not receiving responses.  The Outer No-
   Response option is processed according to Section 4.1.2.

   Note the effect in step 8 of Section 8.4 when applied to No-Response.
   Applications should consider that a proxy may remove the Outer No-
   Response option from the request.  Applications using No-Response can
   specify policies to deal with cases where servers receive an Inner
   No-Response option only, which may be the result of the request
   having traversed a No-Response unaware proxy, and update the
   processing in Section 8.4 accordingly.  This avoids unnecessary error
   responses to clients and bandwidth reductions to servers, due to No-
   Response unaware proxies.






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4.1.3.6.  OSCORE

   The OSCORE option is only defined to be present in OSCORE messages,
   as an indication that OSCORE processing have been performed.  The
   content in the OSCORE option is neither encrypted nor integrity
   protected as a whole but some part of the content of this option is
   protected (see Section 5.4).  Nested use of OSCORE is not supported:
   If OSCORE processing detects an OSCORE option in the original CoAP
   message, then processing SHALL be stopped.

4.2.  CoAP Header Fields and Payload

   A summary of how the CoAP header fields and payload are protected is
   shown in Figure 6, including fields specific to CoAP over UDP and
   CoAP over TCP (marked accordingly in the table).

                       +------------------+---+---+
                       | Field            | E | U |
                       +------------------+---+---+
                       | Version (UDP)    |   | x |
                       | Type (UDP)       |   | x |
                       | Length (TCP)     |   | x |
                       | Token Length     |   | x |
                       | Code             | x |   |
                       | Message ID (UDP) |   | x |
                       | Token            |   | x |
                       | Payload          | x |   |
                       +------------------+---+---+

                 E = Encrypt and Integrity Protect (Inner)
                 U = Unprotected (Outer)

          Figure 6: Protection of CoAP Header Fields and Payload

   Most CoAP Header fields (i.e. the message fields in the fixed 4-byte
   header) are required to be read and/or changed by CoAP proxies and
   thus cannot in general be protected end-to-end between the endpoints.
   As mentioned in Section 1, OSCORE protects the CoAP Request/Response
   layer only, and not the Messaging Layer (Section 2 of [RFC7252]), so
   fields such as Type and Message ID are not protected with OSCORE.

   The CoAP Header field Code is protected by OSCORE.  Code SHALL be
   encrypted and integrity protected (Class E) to prevent an
   intermediary from eavesdropping on or manipulating the Code (e.g.,
   changing from GET to DELETE).

   The sending endpoint SHALL write the Code of the original CoAP
   message into the plaintext of the COSE object (see Section 5.3).



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   After that, the sending endpoint writes an Outer Code to the OSCORE
   message.  The Outer Code SHALL be set to 0.02 (POST) or 0.05 (FETCH)
   for requests.  For non-Observe requests the client SHALL set the
   Outer Code to 0.02 (POST).  For responses, the sending endpoint SHALL
   respond with Outer Code 2.04 (Changed) to 0.02 (POST) requests, and
   with Outer Code 2.05 (Content) to 0.05 (FETCH) requests.  Using FETCH
   with Observe allows OSCORE to be compliant with the Observe
   processing in OSCORE-unaware intermediaries.  The choice of POST and
   FETCH [RFC8132] allows all OSCORE messages to have payload.

   The receiving endpoint SHALL discard the Outer Code in the OSCORE
   message and write the Code of the COSE object plaintext (Section 5.3)
   into the decrypted CoAP message.

   The other currently defined CoAP Header fields are Unprotected (Class
   U).  The sending endpoint SHALL write all other header fields of the
   original message into the header of the OSCORE message.  The
   receiving endpoint SHALL write the header fields from the received
   OSCORE message into the header of the decrypted CoAP message.

   The CoAP Payload, if present in the original CoAP message, SHALL be
   encrypted and integrity protected and is thus an Inner message field.
   The sending endpoint writes the payload of the original CoAP message
   into the plaintext (Section 5.3) input to the COSE object.  The
   receiving endpoint verifies and decrypts the COSE object, and
   recreates the payload of the original CoAP message.

4.3.  Signaling Messages

   Signaling messages (CoAP Code 7.00-7.31) were introduced to exchange
   information related to an underlying transport connection in the
   specific case of CoAP over reliable transports [RFC8323].

   OSCORE MAY be used to protect Signaling if the endpoints for OSCORE
   coincide with the endpoints for the signaling message.  If OSCORE is
   used to protect Signaling then:

   o  To comply with [RFC8323], an initial empty CSM message SHALL be
      sent.  The subsequent signaling message SHALL be protected.

   o  Signaling messages SHALL be protected as CoAP Request messages,
      except in the case the Signaling message is a response to a
      previous Signaling message, in which case it SHALL be protected as
      a CoAP Response message.  For example, 7.02 (Ping) is protected as
      a CoAP Request and 7.03 (Pong) as a CoAP response.






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   o  The Outer Code for Signaling messages SHALL be set to 0.02 (POST),
      unless it is a response to a previous Signaling message, in which
      case it SHALL be set to 2.04 (Changed).

   o  All Signaling options, except the OSCORE option, SHALL be Inner
      (Class E).

   NOTE: Option numbers for Signaling messages are specific to the CoAP
   Code (see Section 5.2 of [RFC8323]).

   If OSCORE is not used to protect Signaling, Signaling messages SHALL
   be unaltered by OSCORE.

5.  The COSE Object

   This section defines how to use COSE [RFC8152] to wrap and protect
   data in the original message.  OSCORE uses the untagged COSE_Encrypt0
   structure with an Authenticated Encryption with Additional Data
   (AEAD) algorithm.  The key lengths, IV length, nonce length, and
   maximum Sender Sequence Number are algorithm dependent.

   The AEAD algorithm AES-CCM-16-64-128 defined in Section 10.2 of
   [RFC8152] is mandatory to implement.  For AES-CCM-16-64-128 the
   length of Sender Key and Recipient Key is 128 bits, the length of
   nonce and Common IV is 13 bytes.  The maximum Sender Sequence Number
   is specified in Section 12.

   As specified in [RFC5116], plaintext denotes the data that is to be
   encrypted and integrity protected, and Additional Authenticated Data
   (AAD) denotes the data that is to be integrity protected only.

   The COSE Object SHALL be a COSE_Encrypt0 object with fields defined
   as follows

   o  The 'protected' field is empty.

   o  The 'unprotected' field includes:

      *  The 'Partial IV' parameter.  The value is set to the Sender
         Sequence Number.  All leading zeroes SHALL be removed when
         encoding the Partial IV, except in the case of value 0 which is
         encoded to the byte string 0x00.  This parameter SHALL be
         present in requests.  In case of Observe notifications
         (Section 4.1.3.4) the Partial IV SHALL be present in responses,
         and otherwise the Partial IV will not typically be present in
         responses.





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      *  The 'kid' parameter.  The value is set to the Sender ID.  This
         parameter SHALL be present in requests and will not typically
         be present in responses.  An example where the Sender ID is
         included in a response is the extension of OSCORE to group
         communication [I-D.ietf-core-oscore-groupcomm].

      *  Optionally, a 'kid context' parameter as defined in
         Section 5.1.  This parameter MAY be present in requests and
         SHALL NOT be present in responses.

   o  The 'ciphertext' field is computed from the secret key (Sender Key
      or Recipient Key), AEAD nonce (see Section 5.2), plaintext (see
      Section 5.3), and the Additional Authenticated Data (AAD) (see
      Section 5.4) following Section 5.2 of [RFC8152].

   The encryption process is described in Section 5.3 of [RFC8152].

5.1.  Kid Context

   For certain use cases, e.g. deployments where the same kid is used
   with multiple contexts, it is necessary or favorable for the sender
   to provide an additional identifier of the security material to use,
   in order for the receiver to retrieve or establish the correct key.
   The kid context parameter is used to provide such additional input.
   The kid context and kid are used to determine the security context,
   or to establish the necessary input parameters to derive the security
   context (see Section 3.2).  The application defines how this is done.

   The kid context is implicitly integrity protected, as a manipulation
   that leads to the wrong key (or no key) being retrieved results in an
   error, as described in Section 8.2.

   A summary of the COSE header parameter kid context defined above can
   be found in Figure 7.

   Some examples of relevant uses of kid context are the following:

   o  If the client has an identifier in some other namespace which can
      be used by the server to retrieve or establish the security
      context, then that identifier can be used as kid context.  The kid
      context may be used as Master Salt (Section 3.1) for additional
      entropy of the security contexts (see for example Appendix B.2 or
      [I-D.ietf-6tisch-minimal-security]).

   o  In case of a group communication scenario
      [I-D.ietf-core-oscore-groupcomm], if the server belongs to
      multiple groups, then a group identifier can be used as kid
      context to enable the server to find the right security context.



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   +----------+--------+------------+----------------+-----------------+
   |   name   |  label | value type | value registry | description     |
   +----------+--------+------------+----------------+-----------------+
   |   kid    |  TBD2  | bstr       |                | Identifies the  |
   | context  |        |            |                | kid context     |
   +----------+--------+------------+----------------+-----------------+

     Figure 7: Additional common header parameter for the COSE object

5.2.  Nonce

   The AEAD nonce is constructed in the following way (see Figure 8):

   1.  left-padding the Partial IV (PIV) in network byte order with
       zeroes to exactly 5 bytes,

   2.  left-padding the Sender ID of the endpoint that generated the
       Partial IV (ID_PIV) in network byte order with zeroes to exactly
       nonce length minus 6 bytes,

   3.  concatenating the size of the ID_PIV (a single byte S) with the
       padded ID_PIV and the padded PIV,

   4.  and then XORing with the Common IV.

   Note that in this specification only algorithms that use nonces equal
   or greater than 7 bytes are supported.  The nonce construction with
   S, ID_PIV, and PIV together with endpoint unique IDs and encryption
   keys makes it easy to verify that the nonces used with a specific key
   will be unique, see Appendix D.3.

   If the Partial IV is not present in a response, the nonce from the
   request is used.  For responses that are not notifications (i.e. when
   there is a single response to a request), the request and the
   response should typically use the same nonce to reduce message
   overhead.  Both alternatives provide all the required security
   properties, see Appendix D.3 and Section 7.4.  The only non-Observe
   scenario where a Partial IV must be included in a response is when
   the server is unable to perform replay protection, see Section 7.5.2.
   For processing instructions see Section 8.











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              <- nonce length minus 6 B -> <-- 5 bytes -->
         +---+-------------------+--------+---------+-----+
         | S |      padding      | ID_PIV | padding | PIV |----+
         +---+-------------------+--------+---------+-----+    |
                                                               |
          <---------------- nonce length ---------------->     |
         +------------------------------------------------+    |
         |                   Common IV                    |->(XOR)
         +------------------------------------------------+    |
                                                               |
          <---------------- nonce length ---------------->     |
         +------------------------------------------------+    |
         |                     Nonce                      |<---+
         +------------------------------------------------+

                      Figure 8: AEAD Nonce Formation

5.3.  Plaintext

   The plaintext is formatted as a CoAP message without Header (see
   Figure 9) consisting of:

   o  the Code of the original CoAP message as defined in Section 3 of
      [RFC7252]; and

   o  all Inner option message fields (see Section 4.1.1) present in the
      original CoAP message (see Section 4.1).  The options are encoded
      as described in Section 3.1 of [RFC7252], where the delta is the
      difference to the previously included instance of Class E option;
      and

   o  the Payload of original CoAP message, if present, and in that case
      prefixed by the one-byte Payload Marker (0xFF).

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Code      |    Class E options (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1|    Payload (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      (only if there
        is payload)

                            Figure 9: Plaintext

   NOTE: The plaintext contains all CoAP data that needs to be encrypted
   end-to-end between the endpoints.



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5.4.  Additional Authenticated Data

   The external_aad SHALL be a CBOR array as defined below:

   external_aad = [
      oscore_version : uint,
      algorithms : [ alg_aead : int / tstr ],
      request_kid : bstr,
      request_piv : bstr,
      options : bstr
   ]

   where:

   o  oscore_version: contains the OSCORE version number.
      Implementations of this specification MUST set this field to 1.
      Other values are reserved for future versions.

   o  algorithms: contains (for extensibility) an array of algorithms,
      according to this specification only containing alg_aead.

   o  alg_aead: contains the AEAD Algorithm from the security context
      used for the exchange (see Section 3.1).

   o  request_kid: contains the value of the 'kid' in the COSE object of
      the request (see Section 5).

   o  request_piv: contains the value of the 'Partial IV' in the COSE
      object of the request (see Section 5).

   o  options: contains the Class I options (see Section 4.1.2) present
      in the original CoAP message encoded as described in Section 3.1
      of [RFC7252], where the delta is the difference to the previously
      included instance of class I option.

   The oscore_version and algorithms parameters are established out-of-
   band and are thus never transported in OSCORE, but the external_aad
   allows to verify that they are the same in both endpoints.

   NOTE: The format of the external_aad is for simplicity the same for
   requests and responses, although some parameters, e.g. request_kid,
   need not be integrity protected in the requests.

6.  OSCORE Header Compression

   The Concise Binary Object Representation (CBOR) [RFC7049] combines
   very small message sizes with extensibility.  The CBOR Object Signing
   and Encryption (COSE) [RFC8152] uses CBOR to create compact encoding



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   of signed and encrypted data.  COSE is however constructed to support
   a large number of different stateless use cases, and is not fully
   optimized for use as a stateful security protocol, leading to a
   larger than necessary message expansion.  In this section, we define
   a stateless header compression mechanism, simply removing redundant
   information from the COSE objects, which significantly reduces the
   per-packet overhead.  The result of applying this mechanism to a COSE
   object is called the "compressed COSE object".

   The COSE_Encrypt0 object used in OSCORE is transported in the OSCORE
   option and in the Payload.  The Payload contains the Ciphertext and
   the headers of the COSE object are compactly encoded as described in
   the next section.

6.1.  Encoding of the OSCORE Option Value

   The value of the OSCORE option SHALL contain the OSCORE flag bits,
   the Partial IV parameter, the kid context parameter (length and
   value), and the kid parameter as follows:

          0 1 2 3 4 5 6 7 <--------- n bytes ------------->
         +-+-+-+-+-+-+-+-+---------------------------------
         |0 0 0|h|k|  n  |      Partial IV (if any) ...
         +-+-+-+-+-+-+-+-+---------------------------------

          <- 1 byte -> <------ s bytes ----->
         +------------+----------------------+------------------+
         | s (if any) | kid context (if any) | kid (if any) ... |
         +------------+----------------------+------------------+

                    Figure 10: The OSCORE Option Value

   o  The first byte of flag bits encodes the following set of flags and
      the length of the Partial IV parameter:

      *  The three least significant bits encode the Partial IV length
         n.  If n = 0 then the Partial IV is not present in the
         compressed COSE object.  The values n = 6 and n = 7 are
         reserved.

      *  The fourth least significant bit is the kid flag, k: it is set
         to 1 if the kid is present in the compressed COSE object.

      *  The fifth least significant bit is the kid context flag, h: it
         is set to 1 if the compressed COSE object contains a kid
         context (see Section 5.1).





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      *  The sixth to eighth least significant bits are reserved for
         future use.  These bits SHALL be set to zero when not in use.
         According to this specification, if any of these bits are set
         to 1 the message is considered to be malformed and
         decompression fails as specified in item 3 of Section 8.2.

   o  The following n bytes encode the value of the Partial IV, if the
      Partial IV is present (n > 0).

   o  The following 1 byte encode the length of the kid context
      (Section 5.1) s, if the kid context flag is set (h = 1).

   o  The following s bytes encode the kid context, if the kid context
      flag is set (h = 1).

   o  The remaining bytes encode the value of the kid, if the kid is
      present (k = 1).

   Note that the kid MUST be the last field of the OSCORE option value,
   even in case reserved bits are used and additional fields are added
   to it.

   The length of the OSCORE option thus depends on the presence and
   length of Partial IV, kid context, kid, as specified in this section,
   and on the presence and length of the other parameters, as defined in
   the separate documents.

6.2.  Encoding of the OSCORE Payload

   The payload of the OSCORE message SHALL encode the ciphertext of the
   COSE object.

6.3.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples for
   requests and responses.  The examples assume the COSE_Encrypt0 object
   is set (which means the CoAP message and cryptographic material is
   known).  Note that the full CoAP unprotected message, as well as the
   full security context, is not reported in the examples, but only the
   input necessary to the compression mechanism, i.e. the COSE_Encrypt0
   object.  The output is the compressed COSE object as defined in
   Section 6, divided into two parts, since the object is transported in
   two CoAP fields: OSCORE option and payload.

   1.  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
       0x25, and Partial IV = 0x05





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       Before compression (24 bytes):

         [
         h'',
         { 4:h'25', 6:h'05' },
         h'aea0155667924dff8a24e4cb35b9'
         ]

       After compression (17 bytes):

         Flag byte: 0b00001001 = 0x09

         Option Value: 09 05 25 (3 bytes)

         Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)

   2.  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
       empty string, and Partial IV = 0x00

       Before compression (23 bytes):

         [
         h'',
         { 4:h'', 6:h'00' },
         h'aea0155667924dff8a24e4cb35b9'
         ]

       After compression (16 bytes):

         Flag byte: 0b00001001 = 0x09

         Option Value: 09 00 (2 bytes)

         Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)

   3.  Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid =
       empty string, Partial IV = 0x05, and kid context = 0x44616c656b

       Before compression (30 bytes):

         [
         h'',
         { 4:h'', 6:h'05', 8:h'44616c656b' },
         h'aea0155667924dff8a24e4cb35b9'
         ]






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       After compression (22  bytes):

         Flag byte: 0b00011001 = 0x19

         Option Value: 19 05 05 44 61 6c 65 6b (8 bytes)

         Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)

   4.  Response with ciphertext = 0xaea0155667924dff8a24e4cb35b9 and no
       Partial IV

       Before compression (18 bytes):

         [
         h'',
         {},
         h'aea0155667924dff8a24e4cb35b9'
         ]

       After compression (14 bytes):

         Flag byte: 0b00000000 = 0x00

         Option Value: (0 bytes)

         Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)

   5.  Response with ciphertext = 0xaea0155667924dff8a24e4cb35b9 and
       Partial IV = 0x07

       Before compression (21 bytes):

         [
         h'',
         { 6:h'07' },
         h'aea0155667924dff8a24e4cb35b9'
         ]

       After compression (16 bytes):

         Flag byte: 0b00000001 = 0x01

         Option Value: 01 07 (2 bytes)

         Payload: ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 (14 bytes)






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7.  Message Binding, Sequence Numbers, Freshness and Replay Protection

7.1.  Message Binding

   In order to prevent response delay and mismatch attacks
   [I-D.mattsson-core-coap-actuators] from on-path attackers and
   compromised intermediaries, OSCORE binds responses to the requests by
   including the kid and Partial IV of the request in the AAD of the
   response.  The server therefore needs to store the kid and Partial IV
   of the request until all responses have been sent.

7.2.  Sequence Numbers

   An AEAD nonce MUST NOT be used more than once per AEAD key.  The
   uniqueness of (key, nonce) pairs is shown in Appendix D.3, and in
   particular depends on a correct usage of Partial IVs.  If messages
   are processed concurrently, the operation of reading and increasing
   the Sender Sequence Number MUST be atomic.

   The maximum Sender Sequence Number is algorithm dependent (see
   Section 12), and SHALL be less than 2^40.  If the Sender Sequence
   Number exceeds the maximum, the endpoint MUST NOT process any more
   messages with the given Sender Context.  If necessary, the endpoint
   SHOULD acquire a new security context before this happens.  The
   latter is out of scope of this document.

7.3.  Freshness

   For requests, OSCORE provides only the guarantee that the request is
   not older than the security context.  For applications having
   stronger demands on request freshness (e.g., control of actuators),
   OSCORE needs to be augmented with mechanisms providing freshness, for
   example as specified in [I-D.ietf-core-echo-request-tag].

   Assuming an honest server, the message binding guarantees that a
   response is not older than its request.  For responses that are not
   notifications (i.e. when there is a single response to a request),
   this gives absolute freshness.  For notifications, the absolute
   freshness gets weaker with time, and it is RECOMMENDED that the
   client regularly re-register the observation.  Note that the message
   binding does not guarantee that misbehaving server created the
   response before receiving the request, i.e. it does not verify server
   aliveness.

   For requests and notifications, OSCORE also provides relative
   freshness in the sense that the received Partial IV allows a
   recipient to determine the relative order of requests or responses.




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7.4.  Replay Protection

   In order to protect from replay of requests, the server's Recipient
   Context includes a Replay Window.  A server SHALL verify that a
   Partial IV received in the COSE object has not been received before.
   If this verification fails the server SHALL stop processing the
   message, and MAY optionally respond with a 4.01 Unauthorized error
   message.  Also, the server MAY set an Outer Max-Age option with value
   zero.  The diagnostic payload MAY contain the "Replay detected"
   string.  The size and type of the Replay Window depends on the use
   case and the protocol with which the OSCORE message is transported.
   In case of reliable and ordered transport from endpoint to endpoint,
   e.g.  TCP, the server MAY just store the last received Partial IV and
   require that newly received Partial IVs equals the last received
   Partial IV + 1.  However, in case of mixed reliable and unreliable
   transports and where messages may be lost, such a replay mechanism
   may be too restrictive and the default replay window be more suitable
   (see Section 3.2.2).

   Responses that are not notifications (with or without Partial IV) are
   protected against replay as they are bound to the request and the
   fact that only a single response is accepted.  Note that the Partial
   IV is not used for replay protection in this case.

   A client receiving a notification SHALL compare the Partial IV of a
   received notification with the Notification Number associated to that
   Observe registration.  A client MUST consider the notification with
   the highest Partial IV as the freshest, regardless of the order of
   arrival.  If the verification of the response succeeds, and the
   received Partial IV was greater than the Notification Number then the
   client SHALL overwrite the corresponding Notification Number with the
   received Partial IV (see step 7 of Section 8.4.  The client MUST stop
   processing notifications with a Partial IV which has been previously
   received.  The client MAY process only notifications which have
   greater Partial IV than the Notification Number.

   If messages are processed concurrently, the Partial IV needs to be
   validated a second time after decryption and before updating the
   replay protection data.  The operation of validating the Partial IV
   and updating the replay protection data MUST be atomic.

7.5.  Losing Part of the Context State

   To prevent reuse of an AEAD nonce with the same key, or from
   accepting replayed messages, an endpoint needs to handle the
   situation of losing rapidly changing parts of the context, such as
   the request Token, Sender Sequence Number, Replay Window, and




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   Notification Numbers.  These are typically stored in RAM and
   therefore lost in the case of an unplanned reboot.

   After boot, an endpoint MAY reject to use pre-existing security
   contexts, and MAY establish a new security context with each endpoint
   it communicates with.  However, establishing a fresh security context
   may have a non-negligible cost in terms of, e.g., power consumption.

   After boot, an endpoint MAY use a partly persistently stored security
   context, but then the endpoint MUST NOT reuse a previous Sender
   Sequence Number and MUST NOT accept previously accepted messages.
   Some ways to achieve this are described in the following sections.

7.5.1.  Sequence Number

   To prevent reuse of Sender Sequence Numbers, an endpoint MAY perform
   the following procedure during normal operations:

   o  Before using a Sender Sequence Number that is evenly divisible by
      K, where K is a positive integer, store the Sender Sequence Number
      in persistent memory.  After boot, the endpoint initiates the
      Sender Sequence Number to the value stored in persistent memory +
      K.  Storing to persistent memory can be costly.  The value K gives
      a trade-off between the number of storage operations and efficient
      use of Sender Sequence Numbers.

7.5.2.  Replay Window

   To prevent accepting replay of previously received requests, the
   server MAY perform the following procedure after boot:

   o  For each stored security context, the first time after boot the
      server receives an OSCORE request, the server responds with the
      Echo option [I-D.ietf-core-echo-request-tag] to get a request with
      verifiable freshness.  The server MUST use its Partial IV when
      generating the AEAD nonce and MUST include the Partial IV in the
      response.

   If the server using the Echo option can verify a second request as
   fresh, then the Partial IV of the second request is set as the lower
   limit of the replay window.

7.5.3.  Replay Protection of Observe Notifications

   To prevent accepting replay of previously received notification
   responses, the client MAY perform the following procedure after boot:





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   o  The client rejects notifications bound to the earlier
      registration, removes all Notification Numbers and re-registers
      using Observe.

8.  Processing

   This section describes the OSCORE message processing.

8.1.  Protecting the Request

   Given a CoAP request, the client SHALL perform the following steps to
   create an OSCORE request:

   1.  Retrieve the Sender Context associated with the target resource.

   2.  Compose the Additional Authenticated Data and the plaintext, as
       described in Section 5.4 and Section 5.3.

   3.  Encode the Partial IV (Sender Sequence Number in network byte
       order) and increment the Sender Sequence Number by one.  Compute
       the AEAD nonce from the Sender ID, Common IV, and Partial IV as
       described in Section 5.2.

   4.  Encrypt the COSE object using the Sender Key. Compress the COSE
       Object as specified in Section 6.

   5.  Format the OSCORE message according to Section 4.  The OSCORE
       option is added (see Section 4.1.2).

   6.  Store the attribute-value pair (Token, {Security Context, PIV})
       in order to be able to find the Recipient Context and the
       request_piv from the Token in the response.

8.2.  Verifying the Request

   A server receiving a request containing the OSCORE option SHALL
   perform the following steps:

   1.   Process Outer Block options according to [RFC7959], until all
        blocks of the request have been received (see Section 4.1.3.3).

   2.   Discard the message Code and all non-special Inner option
        message fields (marked in Figure 5 with 'x' in column E only)
        present in the received message.  For example, an If-Match Outer
        option is discarded, but an Uri-Host Outer option is not
        discarded.





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   3.   Decompress the COSE Object (Section 6) and retrieve the
        Recipient Context associated with the Recipient ID in the 'kid'
        parameter.  If either the decompression or the COSE message
        fails to decode, or the server fails to retrieve a Recipient
        Context with Recipient ID corresponding to the 'kid' parameter
        received, then the server SHALL stop processing the request.
        If:

        *  either the decompression or the COSE message fails to decode,
           the server MAY respond with a 4.02 Bad Option error message.
           The server MAY set an Outer Max-Age option with value zero.
           The diagnostic payload SHOULD contain the string "Failed to
           decode COSE".

        *  the server fails to retrieve a Recipient Context with
           Recipient ID corresponding to the 'kid' parameter received,
           the server MAY respond with a 4.01 Unauthorized error
           message.  The server MAY set an Outer Max-Age option with
           value zero.  The diagnostic payload SHOULD contain the string
           "Security context not found".

   4.   Verify the 'Partial IV' parameter using the Replay Window, as
        described in Section 7.4.

   5.   Compose the Additional Authenticated Data, as described in
        Section 5.4.

   6.   Compute the AEAD nonce from the Recipient ID, Common IV, and the
        'Partial IV' parameter, received in the COSE Object.

   7.   Decrypt the COSE object using the Recipient Key, as per
        [RFC8152] Section 5.3.  (The decrypt operation includes the
        verification of the integrity.)

        *  If decryption fails, the server MUST stop processing the
           request and MAY respond with a 4.00 Bad Request error
           message.  The server MAY set an Outer Max-Age option with
           value zero.  The diagnostic payload MAY contain the
           "Decryption failed" string.

        *  If decryption succeeds, update the Replay Window, as
           described in Section 7.

   8.   For each decrypted option, check if the option is also present
        as an Outer option: if it is, discard the Outer.  For example:
        the message contains a Max-Age Inner and a Max-Age Outer option.
        The Outer Max-Age is discarded.




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   9.   Add decrypted code, options and payload to the decrypted
        request.  The OSCORE option is removed.

   10.  The decrypted CoAP request is processed according to [RFC7252].

8.3.  Protecting the Response

   If a CoAP response is generated in response to an OSCORE request, the
   server SHALL perform the following steps to create an OSCORE
   response.  Note that CoAP error responses derived from CoAP
   processing (point 10. in Section 8.2) are protected, as well as
   successful CoAP responses, while the OSCORE errors (point 3, 4, and 7
   in Section 8.2) do not follow the processing below, but are sent as
   simple CoAP responses, without OSCORE processing.

   1.  Retrieve the Sender Context in the Security Context used to
       verify the request.

   2.  Compose the Additional Authenticated Data and the plaintext, as
       described in Section 5.4 and Section 5.3.

   3.  Compute the AEAD nonce

       *  For Observe notifications, encode the Partial IV (Sender
          Sequence Number in network byte order) and increment the
          Sender Sequence Number by one.  Compute the AEAD nonce from
          the Sender ID, Common IV, and Partial IV as described in
          Section 5.2.

       *  For responses that are not Observe notifications, either use
          the nonce from the request, or compute a new nonce from the
          Sender ID, Common IV, and a new Partial IV as described in
          Section 5.2, and increment the Sender Sequence Number by one.

   4.  Encrypt the COSE object using the Sender Key. Compress the COSE
       Object as specified in Section 6.  If the AEAD nonce was
       constructed from a new Partial IV, this Partial IV MUST be
       included in the message.  If the AEAD nonce from the request was
       used, the Partial IV MUST NOT be included in the message.

   5.  Format the OSCORE message according to Section 4.  The OSCORE
       option is added (see Section 4.1.2).

8.4.  Verifying the Response

   A client receiving a response containing the OSCORE option SHALL
   perform the following steps:




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   1.   Process Outer Block options according to [RFC7959], until all
        blocks of the OSCORE message have been received (see
        Section 4.1.3.3).

   2.   Discard the message Code and all non-special Class E options
        from the message.  For example, ETag Outer option is discarded,
        Max-Age Outer option is not discarded.

   3.   Retrieve the Recipient Context associated with the Token.
        Decompress the COSE Object (Section 6).  If either the
        decompression or the COSE message fails to decode, then go to
        11.

   4.   If the Observe option is present in the response, but the
        request was not an Observe registration, then go to 11.  If a
        Partial IV is required (i.e. an Observe option is included or
        the Notification number for the observation has already been
        initiated), but not present in the response, then go to 11.  For
        Observe notifications, verify the received 'Partial IV'
        parameter against the corresponding Notification Number as
        described in Section 7.4.

   5.   Compose the Additional Authenticated Data, as described in
        Section 5.4.

   6.   Compute the AEAD nonce

        *  If the Partial IV are not present in the response, the nonce
           from the request is used.

        *  If the Partial IV is present in the response, compute the
           nonce from the Recipient ID, Common IV, and the 'Partial IV'
           parameter, received in the COSE Object.

   7.   Decrypt the COSE object using the Recipient Key, as per
        [RFC8152] Section 5.3.  (The decrypt operation includes the
        verification of the integrity.)  If decryption fails, then go to
        11.

   8.   If the response is a notification, initiate or update the
        corresponding Notification Number, as described in Section 7.
        Otherwise, delete the attribute-value pair (Token, {Security
        Context, PIV}).

   9.   For each decrypted option, check if the option is also present
        as an Outer option: if it is, discard the Outer.  For example:
        the message contains a Max-Age Inner and a Max-Age Outer option.
        The Outer Max-Age is discarded.



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   10.  Add decrypted code, options and payload to the decrypted
        request.  The OSCORE option is removed.

   11.  The decrypted CoAP response is processed according to [RFC7252].

   12.  In case any of the previous erroneous conditions apply: the
        client SHALL stop processing the response.

   An error condition occurring while processing a response in an
   observation does not cancel the observation.  A client MUST NOT react
   to failure in step 7 by re-registering the observation immediately.

9.  Web Linking

   The use of OSCORE MAY be indicated by a target attribute "osc" in a
   web link [RFC8288] to a resource, e.g. using a link-format document
   [RFC6690] if the resource is accessible over CoAP.

   The "osc" attribute is a hint indicating that the destination of that
   link is only accessible using OSCORE, and unprotected access to it is
   not supported.  Note that this is simply a hint, it does not include
   any security context material or any other information required to
   run OSCORE.

   A value MUST NOT be given for the "osc" attribute; any present value
   MUST be ignored by parsers.  The "osc" attribute MUST NOT appear more
   than once in a given link-value; occurrences after the first MUST be
   ignored by parsers.

   The example in Figure 11 shows a use of the "osc" attribute: the
   client does resource discovery on a server, and gets back a list of
   resources, one of which includes the "osc" attribute indicating that
   the resource is protected with OSCORE.  The link-format notation (see
   Section 5. of [RFC6690]) is used.

                      REQ: GET /.well-known/core

                      RES: 2.05 Content
                         </sensors/temp>;osc,
                         </sensors/light>;if="sensor"

                          Figure 11: The web link

10.  CoAP-to-CoAP Forwarding Proxy

   CoAP is designed for proxy operations (see Section 5.7 of [RFC7252]).
   Security requirements for forwarding are presented in Section 2.2.1
   of [I-D.hartke-core-e2e-security-reqs].



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   OSCORE is designed to work with legacy CoAP proxies.  Since a CoAP
   response is only applicable to the original CoAP request, caching is
   in general not useful.  In support of legacy proxies, OSCORE defines
   special Max-Age processing, see Section 4.1.3.1.  An OSCORE-aware
   proxy SHOULD NOT cache a response to a request with an OSCORE option

   Proxy processing of the (Outer) Proxy-Uri option is as defined in
   [RFC7252].

   Proxy processing of the (Outer) Block options is as defined in
   [RFC7959].

   Proxy processing of the (Outer) Observe option is as defined in
   [RFC7641].  OSCORE-aware proxies may look at the Partial IV value
   instead of the Outer Observe option.

11.  HTTP Operations

   The CoAP request/response model may be mapped to HTTP and vice versa
   as described in Section 10 of [RFC7252].  The HTTP-CoAP mapping is
   further detailed in [RFC8075].  This section defines the components
   needed to map and transport OSCORE messages over HTTP hops.  By
   mapping between HTTP and CoAP and by using cross-protocol proxies
   OSCORE may be used end-to-end between e.g. an HTTP client and a CoAP
   server.  Examples are provided at the end of the section.

11.1.  The HTTP OSCORE Header Field

   The HTTP OSCORE Header Field (see Section 13.4) is used for carrying
   the content of the CoAP OSCORE option when transporting OSCORE
   messages over HTTP hops.

   The HTTP OSCORE header field is only used in POST requests and 200
   (OK) responses.  When used, the HTTP header field Content-Type is set
   to 'application/oscore' (see Section 13.5) indicating that the HTTP
   body of this message contains the OSCORE payload (see Section 6.2}.
   No additional semantics is provided by other message fields.

   Using the Augmented Backus-Naur Form (ABNF) notation of [RFC5234],
   including the following core ABNF syntax rules defined by that
   specification: ALPHA (letters) and DIGIT (decimal digits), the HTTP
   OSCORE header field value is as follows.

   base64url-char = ALPHA / DIGIT / "-" / "_"

   OSCORE = 2*base64url-char





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   The HTTP OSCORE header field is not appropriate to list in the
   Connection header field (see Section 6.1 of [RFC7230]) since it is
   not hop-by-hop.  The HTTP OSCORE header field is not appropriate to
   list in a Vary response header field (see Section 7.1.4 of [RFC7231])
   since a cached response would in general not be useful for other
   clients.  The HTTP OSCORE header field is not useful in trailers (see
   Section 4.1 of [RFC7230]).

   Intermediaries are in general not allowed to insert, delete, or
   modify the OSCORE header.  Changes to the HTTP OSCORE header field
   will in general violate the integrity of the OSCORE message resulting
   in an error.  For the same reason the HTTP OSCORE header field is in
   general not preserved across redirects.  A CoAP-to-HTTP proxy
   receiving a request for redirect may copy the HTTP OSCORE header
   field to the new request, although the condition for this being
   successful is that the server to which the OSCORE message is
   redirected needs to be a clone of the server for which the OSCORE
   message was intended (same target resource, same OSCORE security
   context etc.).  If an HTTP/OSCORE client receives a redirect it
   should instead generate a new OSCORE request for the server it was
   redirected to.

11.2.  CoAP-to-HTTP Mapping

   Section 10.1 of [RFC7252] describes the fundamentals of the CoAP-to-
   HTTP cross-protocol mapping process.  The additional rules for OSCORE
   messages are:

   o  The HTTP OSCORE header field value is set to

      *  AA if the CoAP OSCORE option is empty, otherwise

      *  the value of the CoAP OSCORE option (Section 6.1) in base64url
         (Section 5 of [RFC4648]) encoding without padding.
         Implementation notes for this encoding are given in Appendix C
         of [RFC7515].

   o  The HTTP Content-Type is set to 'application/oscore' (see
      Section 13.5), independent of CoAP Content-Format.

11.3.  HTTP-to-CoAP Mapping

   Section 10.2 of [RFC7252] and [RFC8075] specify the behavior of an
   HTTP-to-CoAP proxy.  The additional rules for HTTP messages with the
   OSCORE header field are:

   o  The CoAP OSCORE option is set as follows:




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      *  empty if the value of the HTTP OSCORE header field is a single
         zero byte (0x00) represented by AA, otherwise

      *  the value of the HTTP OSCORE header field decoded from
         base64url (Section 5 of [RFC4648]) without padding.
         Implementation notes for this encoding are given in Appendix C
         of [RFC7515].

   o  The CoAP Content-Format option is omitted, the content format for
      OSCORE (Section 13.6) MUST NOT be used.

11.4.  HTTP Endpoints

   Restricted to subsets of HTTP and CoAP supporting a bijective
   mapping, OSCORE can be originated or terminated in HTTP endpoints.

   The sending HTTP endpoint uses [RFC8075] to translate the HTTP
   message into a CoAP message.  The CoAP message is then processed with
   OSCORE as defined in this document.  The OSCORE message is then
   mapped to HTTP as described in Section 11.2 and sent in compliance
   with the rules in Section 11.1.

   The receiving HTTP endpoint maps the HTTP message to a CoAP message
   using [RFC8075] and Section 11.3.  The resulting OSCORE message is
   processed as defined in this document.  If successful, the plaintext
   CoAP message is translated to HTTP for normal processing in the
   endpoint.

11.5.  Example: HTTP Client and CoAP Server

   This section is giving an example of how a request and a response
   between an HTTP client and a CoAP server could look like.  The
   example is not a test vector but intended as an illustration of how
   the message fields are translated in the different steps.

   Mapping and notation here is based on "Simple Form" (Section 5.4.1 of
   [RFC8075]).

   [HTTP request -- Before client object security processing]

     GET http://proxy.url/hc/?target_uri=coap://server.url/orders
      HTTP/1.1









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   [HTTP request -- HTTP Client to Proxy]

     POST http://proxy.url/hc/?target_uri=coap://server.url/ HTTP/1.1
     Content-Type: application/oscore
     OSCORE: CSU
     Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

   [CoAP request -- Proxy to CoAP Server]

     POST coap://server.url/
     OSCORE: 09 25
     Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

   [CoAP request -- After server object security processing]

     GET coap://server.url/orders

   [CoAP response -- Before server object security processing]

     2.05 Content
     Content-Format: 0
     Payload: Exterminate! Exterminate!

   [CoAP response -- CoAP Server to Proxy]

     2.04 Changed
     OSCORE: [empty]
     Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]

   [HTTP response -- Proxy to HTTP Client]

     HTTP/1.1 200 OK
     Content-Type: application/oscore
     OSCORE: AA
     Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]

   [HTTP response -- After client object security processing]

     HTTP/1.1 200 OK
     Content-Type: text/plain
     Body: Exterminate! Exterminate!

   Note that the HTTP Status Code 200 in the next-to-last message is the
   mapping of CoAP Code 2.04 (Changed), whereas the HTTP Status Code 200
   in the last message is the mapping of the CoAP Code 2.05 (Content),
   which was encrypted within the compressed COSE object carried in the
   Body of the HTTP response.




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11.6.  Example: CoAP Client and HTTP Server

   This section is giving an example of how a request and a response
   between a CoAP client and an HTTP server could look like.  The
   example is not a test vector but intended as an illustration of how
   the message fields are translated in the different steps

   [CoAP request -- Before client object security processing]

     GET coap://proxy.url/
     Proxy-Uri=http://server.url/orders

   [CoAP request -- CoAP Client to Proxy]

     POST coap://proxy.url/
     Proxy-Uri=http://server.url/
     OSCORE: 09 25
     Payload: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

   [HTTP request -- Proxy to HTTP Server]

     POST http://server.url/ HTTP/1.1
     Content-Type: application/oscore
     OSCORE: CSU
     Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

   [HTTP request -- After server object security processing]

     GET http://server.url/orders HTTP/1.1

   [HTTP response -- Before server object security processing]

     HTTP/1.1 200 OK
     Content-Type: text/plain
     Body: Exterminate! Exterminate!

   [HTTP response -- HTTP Server to Proxy]

     HTTP/1.1 200 OK
     Content-Type: application/oscore
     OSCORE: AA
     Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]

   [CoAP response -- Proxy to CoAP Client]

     2.04 Changed
     OSCORE: [empty]
     Payload: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]



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   [CoAP response -- After client object security processing]

     2.05 Content
     Content-Format: 0
     Payload: Exterminate! Exterminate!

   Note that the HTTP Code 2.04 (Changed) in the next-to-last message is
   the mapping of HTTP Status Code 200, whereas the CoAP Code 2.05
   (Content) in the last message is the value that was encrypted within
   the compressed COSE object carried in the Body of the HTTP response.

12.  Security Considerations

   An overview of the security properties is given in Appendix D.

12.1.  End-to-end Protection

   In scenarios with intermediary nodes such as proxies or gateways,
   transport layer security such as (D)TLS only protects data hop-by-
   hop.  As a consequence, the intermediary nodes can read and modify
   any information.  The trust model where all intermediary nodes are
   considered trustworthy is problematic, not only from a privacy
   perspective, but also from a security perspective, as the
   intermediaries are free to delete resources on sensors and falsify
   commands to actuators (such as "unlock door", "start fire alarm",
   "raise bridge").  Even in the rare cases where all the owners of the
   intermediary nodes are fully trusted, attacks and data breaches make
   such an architecture brittle.

   (D)TLS protects hop-by-hop the entire message.  OSCORE protects end-
   to-end all information that is not required for proxy operations (see
   Section 4).  (D)TLS and OSCORE can be combined, thereby enabling end-
   to-end security of the message payload, in combination with hop-by-
   hop protection of the entire message, during transport between end-
   point and intermediary node.  In particular when OSCORE is used with
   HTTP, the additional TLS protection of HTTP hops is recommended, e.g.
   between an HTTP endpoint and a proxy translating between HTTP and
   CoAP.

   The consequences of unprotected message fields are analyzed in
   Appendix D.4.  Error messages occurring during CoAP processing are
   protected end-to-end.  Error messages occurring during OSCORE
   processing are not always possible to protect, e.g. if the receiving
   endpoint cannot locate the right security context.  It may still be
   favorable to send an unprotected error message, e.g. to prevent
   extensive retransmissions, so unprotected error messages are allowed
   as specified.  Similar to error messages, signaling messages are not
   always possible to protect as they may be intended for an



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   intermediary.  Applications using unprotected error and signaling
   messages need to consider the threat that these messages may be
   spoofed.

12.2.  Security Context Establishment

   The use of COSE_Encrypt0 and AEAD to protect messages as specified in
   this document requires an established security context.  The method
   to establish the security context described in Section 3.2 is based
   on a common Master Secret and unique Sender IDs.  The necessary input
   parameters may be pre-established or obtained using a key
   establishment protocol augmented with establishment of Sender/
   Recipient ID such as the OSCORE profile of the ACE framework
   [I-D.ietf-ace-oscore-profile].  This procedure must ensure that the
   requirements of the security context parameters are complied with
   Section 3.3 for the intended use and also in error situations.  It is
   recommended to use a key establishment protocol which provides
   forward secrecy whenever possible.  Considerations for the deploying
   OSCORE with a fixed Master Secret are given in Appendix B.

12.3.  Master Secret

   OSCORE uses HKDF [RFC5869] and the established input parameters to
   derive the security context.  The required properties of the security
   context parameters are discussed in Section 3.3, in this section we
   focus on the Master Secret.  HKDF denotes in this specification the
   composition of the expand and extract functions as defined in
   [RFC5869] and the Master Secret is used as Input Key Material (IKM).

   Informally, HKDF takes as source an IKM containing some good amount
   of randomness but not necessarily distributed uniformly (or for which
   an attacker has some partial knowledge) and derive from it one or
   more cryptographically strong secret keys [RFC5869].

   Therefore, the main requirement for the OSCORE Master Secret, in
   addition to being secret, is that it is has a good amount of
   randomness.  The selected key establishment schemes must ensure that
   the necessary properties for the Master Secret are fulfilled.  For
   pre-shared key deployments and key transport solutions such as
   [I-D.ietf-ace-oscore-profile], the Master Secret can be generated
   offline using a good random number generator.

12.4.  Replay Protection

   Most AEAD algorithms require a unique nonce for each message, for
   which the sender sequence numbers in the COSE message field 'Partial
   IV' is used.  If the recipient accepts any sequence number larger
   than the one previously received, then the problem of sequence number



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   synchronization is avoided.  With reliable transport, it may be
   defined that only messages with sequence number which are equal to
   previous sequence number + 1 are accepted.  The alternatives to
   sequence numbers have their issues: very constrained devices may not
   be able to support accurate time, or to generate and store large
   numbers of random nonces.  The requirement to change key at counter
   wrap is a complication, but it also forces the user of this
   specification to think about implementing key renewal.

12.5.  Client Aliveness

   A verified OSCORE request enables the server to verify the identity
   of the entity who generated the message.  However, it does not verify
   that the client is currently involved in the communication, since the
   message may be a delayed delivery of a previously generated request
   which now reaches the server.  To verify the aliveness of the client
   the server may use the Echo option in the response to a request from
   the client (see [I-D.ietf-core-echo-request-tag]).

12.6.  Cryptographic Considerations

   The maximum sender sequence number is dependent on the AEAD
   algorithm.  The maximum sender sequence number is 2^40 - 1, or any
   algorithm specific lower limit, after which a new security context
   must be generated.  The mechanism to build the nonce (Section 5.2)
   assumes that the nonce is at least 56 bits, and the Partial IV is at
   most 40 bits.  The mandatory-to-implement AEAD algorithm AES-CCM-
   16-64-128 is selected for compatibility with CCM*.

   In order to prevent cryptanalysis when the same plaintext is
   repeatedly encrypted by many different users with distinct keys, the
   nonce is formed by mixing the sequence number with a secret per-
   context initialization vector (Common IV) derived along with the keys
   (see Section 3.1 of [RFC8152]), and by using a Master Salt in the key
   derivation (see [MF00] for an overview).  The Master Secret, Sender
   Key, Recipient Key, and Common IV must be secret, the rest of the
   parameters may be public.  The Master Secret must have a good amount
   of randomness (see Section 12.3)).

12.7.  Message Segmentation

   The Inner Block options enable the sender to split large messages
   into OSCORE-protected blocks such that the receiving endpoint can
   verify blocks before having received the complete message.  The Outer
   Block options allow for arbitrary proxy fragmentation operations that
   cannot be verified by the endpoints, but can by policy be restricted
   in size since the Inner Block options allow for secure fragmentation
   of very large messages.  A maximum message size (above which the



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   sending endpoint fragments the message and the receiving endpoint
   discards the message, if complying to the policy) may be obtained as
   part of normal resource discovery.

12.8.  Privacy Considerations

   Privacy threats executed through intermediary nodes are considerably
   reduced by means of OSCORE.  End-to-end integrity protection and
   encryption of the message payload and all options that are not used
   for proxy operations, provide mitigation against attacks on sensor
   and actuator communication, which may have a direct impact on the
   personal sphere.

   The unprotected options (Figure 5) may reveal privacy sensitive
   information, see Appendix D.4.  CoAP headers sent in plaintext allow,
   for example, matching of CON and ACK (CoAP Message Identifier),
   matching of request and responses (Token) and traffic analysis.
   OSCORE does not provide protection for HTTP header fields which are
   not both CoAP-mappable and class E.  The HTTP message fields which
   are visible to on-path entity are only used for the purpose of
   transporting the OSCORE message, whereas the application layer
   message is encoded in CoAP and encrypted.

   Unprotected error messages reveal information about the security
   state in the communication between the endpoints.  Unprotected
   signaling messages reveal information about the reliable transport
   used on a leg of the path.  Using the mechanisms described in
   Section 7.5 may reveal when a device goes through a reboot.  This can
   be mitigated by the device storing the precise state of sender
   sequence number and replay window on a clean shutdown.

   The length of message fields can reveal information about the
   message.  Applications may use a padding scheme to protect against
   traffic analysis.

13.  IANA Considerations

   Note to RFC Editor: Please replace all occurrences of "[[this
   document]]" with the RFC number of this specification.

   Note to IANA: Please note all occurrences of "TBDx" in this
   specification should be assigned the same number.

13.1.  COSE Header Parameters Registry

   The 'kid context' parameter is added to the "COSE Header Parameters
   Registry":




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   o  Name: kid context

   o  Label: TBD2

   o  Value Type: bstr

   o  Value Registry:

   o  Description: Identifies the kid context

   o  Reference: Section 5.1 of this document

   Note to IANA: Label assignment in (Integer value between 1 and 255)
   is requested.  (RFC Editor: Delete this note after IANA assignment)

13.2.  CoAP Option Numbers Registry

   The OSCORE option is added to the CoAP Option Numbers registry:

             +--------+-----------------+-------------------+
             | Number | Name            | Reference         |
             +--------+-----------------+-------------------+
             |  TBD1  | OSCORE          | [[this document]] |
             +--------+-----------------+-------------------+

13.3.  CoAP Signaling Option Numbers Registry

   The OSCORE option is added to the CoAP Signaling Option Numbers
   registry:

     +------------+--------+---------------------+-------------------+
     | Applies to | Number | Name                | Reference         |
     +------------+--------+---------------------+-------------------+
     | 7.xx (any) |  TBD1  | OSCORE              | [[this document]] |
     +------------+--------+---------------------+-------------------+

13.4.  Header Field Registrations

   The HTTP OSCORE header field is added to the Message Headers
   registry:

    +----------------------+----------+----------+-------------------+
    | Header Field Name    | Protocol | Status   | Reference         |
    +----------------------+----------+----------+-------------------+
    | OSCORE               | http     | standard | [[this document]] |
    +----------------------+----------+----------+-------------------+





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13.5.  Media Type Registrations

   This section registers the 'application/oscore' media type in the
   "Media Types" registry.  These media types are used to indicate that
   the content is an OSCORE message.  The OSCORE body cannot be
   understood without the OSCORE header field value and the security
   context.












































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     Type name: application

     Subtype name: oscore

     Required parameters: N/A

     Optional parameters: N/A

     Encoding considerations: binary

     Security considerations: See the Security Considerations section
     of [[This document]].

     Interoperability considerations: N/A

     Published specification: [[This document]]

     Applications that use this media type: IoT applications sending
     security content over HTTP(S) transports.

     Fragment identifier considerations: N/A

     Additional information:

     *  Deprecated alias names for this type: N/A

     *  Magic number(s): N/A

     *  File extension(s): N/A

     *  Macintosh file type code(s): N/A

     Person & email address to contact for further information:
     iesg@ietf.org

     Intended usage: COMMON

     Restrictions on usage: N/A

     Author: Goeran Selander, goran.selander@ericsson.com

     Change Controller: IESG

     Provisional registration?  No







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13.6.  CoAP Content-Formats Registry

   Note to IANA: ID assignment in the 10000-64999 range is requested.
   (RFC Editor: Delete this note after IANA assignment)

   This section registers the media type 'application/oscore' media type
   in the "CoAP Content-Format" registry.  This Content-Format for the
   OSCORE payload is defined for potential future use cases and SHALL
   NOT be used in the OSCORE message.  The OSCORE payload cannot be
   understood without the OSCORE option value and the security context.

    +----------------------+----------+----------+-------------------+
    | Media Type           | Encoding |   ID     |     Reference     |
    +----------------------+----------+----------+-------------------+
    | application/oscore   |          |   TBD3   | [[this document]] |
    +----------------------+----------+----------+-------------------+

14.  References

14.1.  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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [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>.

   [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>.




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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>.

   [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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [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>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

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

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.




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   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

14.2.  Informative References

   [I-D.bormann-6lo-coap-802-15-ie]
              Bormann, C., "Constrained Application Protocol (CoAP) over
              IEEE 802.15.4 Information Element for IETF", draft-
              bormann-6lo-coap-802-15-ie-00 (work in progress), April
              2016.

   [I-D.hartke-core-e2e-security-reqs]
              Selander, G., Palombini, F., and K. Hartke, "Requirements
              for CoAP End-To-End Security", draft-hartke-core-e2e-
              security-reqs-03 (work in progress), July 2017.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-05 (work in progress), March 2018.

   [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)", draft-ietf-ace-oauth-authz-11
              (work in progress), March 2018.

   [I-D.ietf-ace-oscore-profile]
              Seitz, L., Palombini, F., Gunnarsson, M., and G. Selander,
              "OSCORE profile of the Authentication and Authorization
              for Constrained Environments Framework", draft-ietf-ace-
              oscore-profile-01 (work in progress), March 2018.

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-ietf-cbor-cddl-02
              (work in progress), February 2018.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Echo and
              Request-Tag", draft-ietf-core-echo-request-tag-01 (work in
              progress), March 2018.




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   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Secure group communication for CoAP", draft-ietf-core-
              oscore-groupcomm-01 (work in progress), March 2018.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
              and C. Amsuess, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-05 (work in progress), March
              2018.

   [MF00]     McGrew, D. and S. Fluhrer, "Attacks on Encryption of
              Redundant Plaintext and Implications on Internet
              Security", the Proceedings of the Seventh Annual Workshop
              on Selected Areas in Cryptography (SAC 2000), Springer-
              Verlag. , 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

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

   [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>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.



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   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

Appendix A.  Scenario Examples

   This section gives examples of OSCORE, targeting scenarios in
   Section 2.2.1.1 of [I-D.hartke-core-e2e-security-reqs].  The message
   exchanges are made, based on the assumption that there is a security
   context established between client and server.  For simplicity, these
   examples only indicate the content of the messages without going into
   detail of the (compressed) COSE message format.

A.1.  Secure Access to Sensor

   This example illustrates a client requesting the alarm status from a
   server.

      Client  Proxy  Server
        |       |       |
        +------>|       |            Code: 0.02 (POST)
        | POST  |       |           Token: 0x8c
        |       |       |          OSCORE: [kid:5f,Partial IV:42]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"alarm_status"}
        |       |       |
        |       +------>|            Code: 0.02 (POST)
        |       | POST  |           Token: 0x7b
        |       |       |          OSCORE: [kid:5f,Partial IV:42]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"alarm_status"}
        |       |       |
        |       |<------+            Code: 2.04 (Changed)
        |       |  2.04 |           Token: 0x7b
        |       |       |          OSCORE: -
        |       |       |         Payload: {Code:2.05, "OFF"}
        |       |       |
        |<------+       |            Code: 2.04 (Changed)
        |  2.04 |       |           Token: 0x8c
        |       |       |          OSCORE: -
        |       |       |         Payload: {Code:2.05, "OFF"}
        |       |       |

   Figure 12: Secure Access to Sensor.  Square brackets [ ... ] indicate
    content of compressed COSE object.  Curly brackets { ... } indicate
                              encrypted data.




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   The request/response Codes are encrypted by OSCORE and only dummy
   Codes (POST/Changed) are visible in the header of the OSCORE message.
   The option Uri-Path ("alarm_status") and payload ("OFF") are
   encrypted.

   The COSE header of the request contains an identifier (5f),
   indicating which security context was used to protect the message and
   a Partial IV (42).

   The server verifies the request as specified in Section 8.2.  The
   client verifies the response as specified in Section 8.4.

A.2.  Secure Subscribe to Sensor

   This example illustrates a client requesting subscription to a blood
   sugar measurement resource (GET /glucose), first receiving the value
   220 mg/dl and then a second value 180 mg/dl.

      Client  Proxy  Server
        |       |       |
        +------>|       |            Code: 0.05 (FETCH)
        | FETCH |       |           Token: 0x83
        |       |       |         Observe: 0
        |       |       |          OSCORE: [kid:ca,Partial IV:15]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"glucose"}
        |       |       |
        |       +------>|            Code: 0.05 (FETCH)
        |       | FETCH |           Token: 0xbe
        |       |       |         Observe: 0
        |       |       |          OSCORE: [kid:ca,Partial IV:15]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"glucose"}
        |       |       |
        |       |<------+            Code: 2.05 (Content)
        |       |  2.05 |           Token: 0xbe
        |       |       |         Observe: 7
        |       |       |          OSCORE: [Partial IV:32]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "220"}
        |       |       |
        |<------+       |            Code: 2.05 (Content)
        |  2.05 |       |           Token: 0x83
        |       |       |         Observe: 7
        |       |       |          OSCORE: [Partial IV:32]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "220"}
       ...     ...     ...



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        |       |       |
        |       |<------+            Code: 2.05 (Content)
        |       |  2.05 |           Token: 0xbe
        |       |       |         Observe: 8
        |       |       |          OSCORE: [Partial IV:36]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "180"}
        |       |       |
        |<------+       |            Code: 2.05 (Content)
        |  2.05 |       |           Token: 0x83
        |       |       |         Observe: 8
        |       |       |          OSCORE: [Partial IV:36]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "180"}
        |       |       |

      Figure 13: Secure Subscribe to Sensor.  Square brackets [ ... ]
   indicate content of compressed COSE object header.  Curly brackets {
                      ... } indicate encrypted data.

   The dummy Codes (FETCH/Content) are visible in the header of the
   OSCORE message to allow intermediary processing of Observe.  The
   options Content-Format (0) and the payload ("220" and "180"), are
   encrypted.

   The COSE header of the request contains an identifier (ca),
   indicating the security context used to protect the message and a
   Partial IV (15).  The COSE headers of the responses contains Partial
   IVs (32 and 36).

   The server verifies that the Partial IV has not been received before.
   The client verifies that the responses are bound to the request and
   that the Partial IVs are greater than any Partial IV previously
   received in a response bound to the request.

Appendix B.  Deployment Examples

   Two examples complying with the requirements on the security context
   parameters (Section 3.3) are given in this section.

B.1.  Master Secret Used Once

   For settings where the Master Secret is only used during deployment,
   the uniqueness of the AEAD nonce may be assured by persistent storage
   of the security context as described in this specification (see
   Section 7.5).  For many IoT deployments, a 128 bit uniformly random
   Master Key is sufficient for encrypting all data exchanged with the
   IoT device throughout its lifetime.



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B.2.  Master Secret Used Multiple Times

   One Master Secret can be used to derive multiple security contexts if
   unique Master Salts can be guaranteed.  This may be useful e.g. in
   case of recommissioning with reused Master Secret.  In order to
   prevent reuse of AEAD nonce and key, which would compromise the
   security, the Master Salt must never be used twice, even if the
   device is reset, recommissioned or in error cases.  Examples of
   failures include derivation of pseudorandom master salt from a static
   seed, or a deterministic seeding procedure with inputs that are
   repeated or can be replayed.  Techniques for persistent storage of
   security state may be used also in this case, to ensure uniqueness of
   Master Salt.

   Assuming the Master Salts are indeed unique (or stochastically
   unique) we give an example of a procedure which may be implemented in
   client and server to establish the OSCORE security context based on
   pre-established input parameters (see Section 3.2) except for the
   Master Salt, which is transported in kid context parameter (see
   Section 5.1) of the request.

   1.  In order to establish a security context with a server for the
       first time, or a new security context replacing an old security
       context, the client generates a (pseudo-)random uniformly
       distributed 64-bit Master Salt and derives the security context
       as specified in Section 3.2.  The client protects a request with
       the new Sender Context and sends the message with kid context set
       to the Master Salt.

   2.  The server, receiving an OSCORE request with a non-empty kid
       context derives the new security context using the received kid
       context as Master Salt.  The server processes the request as
       specified in this document using the new Recipient Context.  If
       the processing of the request completes without error, the server
       responds with an Echo option as specified in
       [I-D.ietf-core-echo-request-tag].  The response is protected with
       the new Sender Context.

   3.  The client, receiving a response with an Echo option to a request
       which used a new security context, verifies the response using
       the new Recipient Context, and if valid repeats the request with
       the Echo option (see [I-D.ietf-core-echo-request-tag]) using the
       new Sender Context.  Subsequent message exchanges (unless
       superseded) are processed using the new security context without
       including the Master Salt in the kid context.

   4.  The server, receiving a request with a kid context and a valid
       Echo option (see [I-D.ietf-core-echo-request-tag]), repeats the



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       processing described in step 2.  If it completes without error,
       then the new security context is established, and the request is
       valid.  If the server already had an old security context with
       this client that is now replaced by the new security context.

   If the server receives a request without kid context from a client
   with which no security context is established, then the server
   responds with a 4.01 Unauthorized error message with diagnostic
   payload containing the string "Security context not found".  This
   could be the result of the server having lost its security context or
   that a new security context has not been successfully established,
   which may be a trigger for the client to run this procedure.

Appendix C.  Test Vectors

   This appendix includes the test vectors for different examples of
   CoAP messages using OSCORE.

C.1.  Test Vector 1: Key Derivation with Master Salt

   Given a set of inputs, OSCORE defines how to set up the Security
   Context in both the client and the server.  The default values are
   used for AEAD Algorithm and KDF.

C.1.1.  Client

   Inputs:

   o  Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)

   o  Master Salt: 0x9e7ca92223786340 (8 bytes)

   o  Sender ID: 0x (0 byte)

   o  Recipient ID: 0x01 (1 byte)

   From the previous parameters,

   o  info (for Sender Key): 0x84400A634b657910 (8 bytes)

   o  info (for Recipient Key): 0x8441010A634b657910 (9 bytes)

   o  info (for Common IV): 0x84400a6249560d (7 bytes)

   Outputs:

   o  Sender Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)




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   o  Recipient Key: 0xe534a26a64aa3982e988e31f1e401e65 (16 bytes)

   o  Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)

C.1.2.  Server

   Inputs:

   o  Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)

   o  Master Salt: 0x9e7ca92223786340 (64 bytes)

   o  Sender ID: 0x01 (1 byte)

   o  Recipient ID: 0x (0 byte)

   From the previous parameters,

   o  info (for Sender Key): 0x8441010A634b657910 (9 bytes)

   o  info (for Recipient Key): 0x84400A634b657910 (8 bytes)

   o  info (for Common IV): 0x84400a6249560d (7 bytes)

   Outputs:

   o  Sender Key: 0xe534a26a64aa3982e988e31f1e401e65 (16 bytes)

   o  Recipient Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)

   o  Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)

C.2.  Test Vector 2: Key Derivation without Master Salt

   Given a set of inputs, OSCORE defines how to set up the Security
   Context in both the client and the server.  The default values are
   used for AEAD Algorithm, KDF, and Master Salt.

C.2.1.  Client

   Inputs:

   o  Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)

   o  Sender ID: 0x00 (1 byte)

   o  Recipient ID: 0x01 (1 byte)




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   From the previous parameters,

   o  info (for Sender Key): 0x8441000A634b657910 (9 bytes)

   o  info (for Recipient Key): 0x8441010A634b657910 (9 bytes)

   o  info (for Common IV): 0x84400a6249560d (7 bytes)

   Outputs:

   o  Sender Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)

   o  Recipient Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)

C.2.2.  Server

   Inputs:

   o  Master Secret: 0x0102030405060708090a0b0c0d0e0f10 (16 bytes)

   o  Sender ID: 0x01 (1 byte)

   o  Recipient ID: 0x00 (1 byte)

   From the previous parameters,

   o  info (for Sender Key): 0x8441010A634b657910 (9 bytes)

   o  info (for Recipient Key): 0x8441000A634b657910 (9 bytes)

   o  info (for Common IV): 0x84400a6249560d (7 bytes)

   Outputs:

   o  Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  Recipient Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)

   o  Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)

C.3.  Test Vector 3: OSCORE Request, Client

   This section contains a test vector for an OSCORE protected CoAP GET
   request using the security context derived in Appendix C.1.  The
   unprotected request only contains the Uri-Path option.




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   Unprotected CoAP request:
   0x440149c60000f2a7396c6f63616c686f737483747631 (22 bytes)

   Common Context:

   o  AEAD Algorithm: 10 (AES-CCM-16-64-128)

   o  Key Derivation Function: HKDF SHA-256

   o  Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)

   Sender Context:

   o  Sender ID: 0x00 (1 byte)

   o  Sender Key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)

   o  Sender Sequence Number: 20

   The following COSE and cryptographic parameters are derived:

   o  Partial IV: 0x14 (1 byte)

   o  kid: 0x00 (1 byte)

   o  external_aad: 0x8501810a4100411440 (9 bytes)

   o  AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)

   o  plaintext: 0x01b3747631 (5 bytes)

   o  encryption key: 0xf8f3b887436285ed5a66f6026ac2cdc1 (16 bytes)

   o  nonce: 0xd0a1949aa253278f34c528d2d8 (13 bytes)

   From the previous parameter, the following is derived:

   o  OSCORE option value: 0x091400 (3 bytes)

   o  ciphertext: 0x55b3710d47c611cd3924838a44 (13 bytes)

   From there:

   o  Protected CoAP request (OSCORE message): 0x44026dd30000acc5396c6f6
      3616c686f7374d305091400ff55b3710d47c611cd3924838a44 (37 bytes)






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C.4.  Test Vector 4: OSCORE Request, Client

   This section contains a test vector for an OSCORE protected CoAP GET
   request using the security context derived in Appendix C.2.  The
   unprotected request only contains the Uri-Path option.

   Unprotected CoAP request:
   0x440149c60000f2a7396c6f63616c686f737483747631 (22 bytes)

   Common Context:

   o  AEAD Algorithm: 10 (AES-CCM-16-64-128)

   o  Key Derivation Function: HKDF SHA-256

   o  Common IV: 0x01727733ab49ead385b18f7d91 (13 bytes)

   Sender Context:

   o  Sender ID: 0x (0 bytes)

   o  Sender Key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)

   o  Sender Sequence Number: 20

   The following COSE and cryptographic parameters are derived:

   o  Partial IV: 0x14 (1 byte)

   o  kid: 0x (0 byte)

   o  external_aad: 0x8501810a40411440 (8 bytes)

   o  AAD: 0x8368456e63727970743040488501810a40411440 (20 bytes)

   o  plaintext: 0x01b3747631 (5 bytes)

   o  encryption key: 0x7230aab3b549d94c9224aacc744e93ab (16 bytes)

   o  nonce: 0x01727733ab49ead385b18f7d85 (13 bytes)

   From the previous parameter, the following is derived:

   o  OSCORE option value: 0x0914 (2 bytes)

   o  ciphertext: 0x6be9214aad448260ff1be1f594 (13 bytes)

   From there:



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   o  Protected CoAP request (OSCORE message): 0x44023bfc000066ef396c6f6
      3616c686f7374d2050914ff6be9214aad448260ff1be1f594 (36 bytes)

C.5.  Test Vector 5: OSCORE Response, Server

   This section contains a test vector for an OSCORE protected 2.05
   Content response to the request in Appendix C.3.  The unprotected
   response has payload "Hello World!" and no options.  The protected
   response does not contain a kid nor a Partial IV.  Note that some
   parameters are derived from the request.

   Unprotected CoAP response:
   0x644549c60000f2a7ff48656c6c6f20576f726c6421 (21 bytes)

   Common Context:

   o  AEAD Algorithm: 10 (AES-CCM-16-64-128)

   o  Key Derivation Function: HKDF SHA-256

   o  Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)

   Sender Context:

   o  Sender ID: 0x01 (1 byte)

   o  Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  Sender Sequence Number: 0

   The following COSE and cryptographic parameters are derived:

   o  external_aad: 0x8501810a4100411440 (9 bytes)

   o  AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)

   o  plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)

   o  encryption key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  nonce: 0xd0a1949aa253278f34c528d2d8 (13 bytes)

   From the previous parameter, the following is derived:

   o  OSCORE option value: 0x (0 bytes)

   o  ciphertext: 0xe4e8c28c41c8f31ca56eec24f6c71d94eacbcdffdc6d (22
      bytes)



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   From there:

   o  Protected CoAP response (OSCORE message): 0x64446dd30000acc5d008ff
      e4e8c28c41c8f31ca56eec24f6c71d94eacbcdffdc6d (33 bytes)

C.6.  Test Vector 6: OSCORE Response with Partial IV, Server

   This section contains a test vector for an OSCORE protected 2.05
   Content response to the request in Appendix C.3.  The unprotected
   response has payload "Hello World!" and no options.  The protected
   response does not contain a kid, but contains a Partial IV.  Note
   that some parameters are derived from the request.

   Unprotected CoAP response:
   0x644549c60000f2a7ff48656c6c6f20576f726c6421 (21 bytes)

   Common Context:

   o  AEAD Algorithm: 10 (AES-CCM-16-64-128)

   o  Key Derivation Function: HKDF SHA-256

   o  Common IV: 0xd1a1949aa253278f34c528d2cc (13 bytes)

   Sender Context:

   o  Sender ID: 0x01 (1 byte)

   o  Sender Key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  Sender Sequence Number: 0

   The following COSE and cryptographic parameters are derived:

   o  Partial IV: 0x00 (1 byte)

   o  external_aad: 0x8501810a4100411440 (9 bytes)

   o  AAD: 0x8368456e63727970743040498501810a4100411440 (21 bytes)

   o  plaintext: 0x45ff48656c6c6f20576f726c6421 (14 bytes)

   o  encryption key: 0xd904cb101f7341c3f4c56c300fa69941 (16 bytes)

   o  nonce: 0xd0a1949aa253278e34c528d2cc (13 bytes)

   From the previous parameter, the following is derived:




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   o  OSCORE option value: 0x0100 (2 bytes)

   o  ciphertext: 0xa7e3ca27f221f453c0ba68c350bf652ea096b328a1bf (22
      bytes)

   From there:

   o  Protected CoAP response (OSCORE message): 0x64442b130000b29ed20801
      00ffa7e3ca27f221f453c0ba68c350bf652ea096b328a1bf (35 bytes)

Appendix D.  Overview of Security Properties

D.1.  Supporting Proxy Operations

   CoAP is designed to work with intermediaries reading and/or changing
   CoAP message fields and performing supporting operations in
   constrained environments, e.g. forwarding and cross-protocol
   translations.

   Securing CoAP on transport layer protects the entire message between
   the endpoints in which case CoAP proxy operations are not possible.
   In order to enable proxy operations, security on transport layer
   needs to be terminated at the proxy in which case the CoAP message in
   its entirety is unprotected in the proxy.

   Requirements for CoAP end-to-end security are specified in
   [I-D.hartke-core-e2e-security-reqs].  The client and server are
   assumed to be honest, but proxies and gateways are only trusted to
   perform their intended operations.  Forwarding is specified in
   Section 2.2.1 of [I-D.hartke-core-e2e-security-reqs].  HTTP-CoAP
   translation is specified in [RFC8075].  Intermediaries translating
   between different transport layers are intended to perform just that.

   By working at the CoAP layer, OSCORE enables different CoAP message
   fields to be protected differently, which allows message fields
   required for proxy operations to be available to the proxy while
   message fields intended for the other endpoint remain protected.  In
   the remainder of this section we analyze how OSCORE protects the
   protected message fields and the consequences of message fields
   intended for proxy operation being unprotected.

D.2.  Protected Message Fields

   Protected message fields are included in the Plaintext (Section 5.3)
   and the Additional Authenticated Data (Section 5.4) of the
   COSE_Encrypt0 object using an AEAD algorithm.





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   OSCORE depends on a pre-established random Master Secret
   (Section 12.3) which can be used to derive keys, and a construction
   for making (key, nonce) pairs unique (Appendix D.3).  Assuming this
   is true, and the keys are used for no more data than indicated in
   Section 7.2, OSCORE should provide the following guarantees:

   o  Confidentiality: An attacker should not be able to determine the
      plaintext contents of a given OSCORE message or determine that
      different plaintexts are related (Section 5.3).

   o  Integrity: An attacker should not be able to craft a new OSCORE
      message with protected message fields different from an existing
      OSCORE message which will be accepted by the receiver.

   o  Request-response binding: An attacker should not be able to make a
      client match a response to the wrong request.

   o  Non-replayability: An attacker should not be able to cause the
      receiver to accept a message which it has already accepted.

   In the above, the attacker is anyone except the endpoints, e.g. a
   compromised intermediary.  Informally, OSCORE provides these
   properties by AEAD-protecting the plaintext with a strong key and
   uniqueness of (key, nonce) pairs.  AEAD encryption [RFC5116] provides
   confidentiality and integrity for the data.  Response-request binding
   is provided by including the kid and Partial IV of the request in the
   AAD of the response.  Non-replayability of requests and notifications
   is provided by using unique (key, nonce) pairs and a replay
   protection mechanism (application dependent, see Section 7.4).

   OSCORE is susceptible to a variety of traffic analysis attacks based
   on observing the length and timing of encrypted packets.  OSCORE does
   not provide any specific defenses against this form of attack but the
   application may use a padding mechanism to prevent an attacker from
   directly determine the length of the padding.  However, information
   about padding may still be revealed by side-channel attacks observing
   differences in timing.

D.3.  Uniqueness of (key, nonce)

   In this section we show that (key, nonce) pairs are unique as long as
   the requirements Section 3.3 and Section 7.2 are followed.

   Fix a security context and an endpoint, called the encrypting
   endpoint.  Endpoints may alternate between client and server roles,
   but each endpoint encrypts with the Sender Key of its Sender Context.
   Sender Keys are (stochastically) unique since they are derived with
   HKDF from unique Sender IDs, so messages encrypted by different



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   endpoints use different keys.  It remains to prove that the nonces
   used by the fixed endpoint are unique.

   Since the Common IV is fixed, the nonces are determined by a Partial
   IV (PIV) and the Sender ID of the endpoint generating that Partial IV
   (ID_PIV).  The nonce construction (Section 5.2) with the size of the
   ID_PIV (S) creates unique nonces for different (ID_PIV, PIV) pairs.

   For requests and responses with Partial IV (e.g.  Observe
   notifications):

   o  ID_PIV = Sender ID of the encrypting endpoint

   o  PIV = current Partial IV of the encrypting endpoint

   Since the encrypting endpoint steps the Partial IV for each use, the
   nonces used are all unique as long as the number of encrypted
   messages is kept within the required range (Section 7.2).

   For responses without Partial IV (i.e. single response to a request):

   o  ID_PIV = Sender ID of the endpoint generating the request

   o  PIV = Partial IV of the request

   Since the Sender IDs are unique, ID_PIV is different from the Sender
   ID of the encrypting endpoint.  Therefore, the nonce is different
   compared to nonces where the encrypting endpoint generated the
   Partial IV.  Since the Partial IV of the request is verified for
   replay (Section 7.4) associated to this Recipient Context, PIV is
   unique for this ID_PIV.

   The argumentation also holds for group communication as specified in
   [RFC7390] (see [I-D.ietf-core-oscore-groupcomm]).

D.4.  Unprotected Message Fields

   This section lists and discusses issues with unprotected message
   fields.

D.4.1.  CoAP Code

   The CoAP Code of an OSCORE message is POST or FETCH for requests and
   with corresponding response codes.  Since the use of Observe is
   indicated with the Outer Observe option, no additional information is
   revealed by having a special codes for Observe messages.  A change of
   code does not affect the method of the end-to-end message but may be




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   a denial service attack caused by error in the OSCORE processing.
   Other aspects of Observe are discussed in Appendix D.4.3.

D.4.2.  CoAP Header Fields

   o  Version.  The CoAP version [RFC7252] is not expected to be
      sensitive to disclose.  Currently there is only one CoAP version
      defined.  A change of this parameter is potentially a denial of
      service attack.  Future versions of CoAP need to analyze attacks
      to OSCORE protected messages due to an adversary changing the CoAP
      version.

   o  Token/Token Length.  The Token field is a client-local identifier
      for differentiating between concurrent requests [RFC7252].  An
      eavesdropper reading the token can match requests to responses
      which can be used in traffic analysis.  CoAP proxies are allowed
      to change Token and Token Length between UDP hops.  However,
      modifications of Token and Token Length during a UDP hop may
      become a denial of service attack, since it may prevent the client
      to identify to which request the response belongs or to find the
      correct information to verify integrity of the response.

   o  Type/Message ID.  The Type/Message ID fields [RFC7252] reveal
      information about the UDP transport binding, e.g. an eavesdropper
      reading the Type or Message ID gain information about how UDP
      messages are related to each other.  CoAP proxies are allowed to
      change Type and Message ID.  These message fields are not present
      in CoAP over TCP, and does not impact the request/response
      message.  A change of these fields in a UDP hop is a denial of
      service attack similar to changing UDP header fields.

   o  Length.  This field contain the length of the message [RFC8323]
      which may be used for traffic analysis.  These message fields are
      not present in CoAP over UDP, and does not impact the request/
      response message.  A change of Length is a denial of service
      attack similar to changing TCP header fields.

D.4.3.  CoAP Options

   o  Max-Age. The Outer Max-Age is set to zero to avoid unnecessary
      caching of OSCORE error responses.  Changing this value thus may
      cause unnecessary caching.  No additional information is carried
      with this option.

   o  Proxy-Uri/Proxy-Scheme/Uri-Host/Uri-Port.  With OSCORE, the Proxy-
      Uri option does not contain the Uri-Path/Uri-Query parts of the
      URI.  Proxy-Uri/Proxy-Scheme/Uri-Host/Uri-Port cannot be integrity
      protected since they are allowed to be changed by a forward proxy.



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      Depending on content, the Uri-Host may either reveal information
      equivalent to that of the IP address or more privacy-sensitive
      information, which is discouraged in Section 4.1.3.2.

   o  Observe.  The Outer Observe option is intended for an OSCORE-
      unaware proxy to support forwarding of Observe messages.  Removing
      this option in the request turns the notification request into a
      normal request, which is allowed for a proxy and server and
      understood by the client but changes the performed operation from
      a request for notifications to a plain request, but the client
      cannot tell what party removed the option.

   Removing this option in the response may lead to notifications not
   being forwarded or cause a denial of service.  The Outer option value
   indicates a relative order of notifications as read and written by
   the proxy and a change of that may affect proxy operations and
   potentially lead to denial of service.  Since OSCORE provides
   absolute ordering of notifications it is not possible for an
   intermediary to spoof reordering (see Section 4.1.3.4).  The size and
   distributions of notifications over time may reveal information about
   the content or nature of the notifications.

   o  Block1/Block2/Size1/Size2.  The Outer Block options enables
      fragmentation of OSCORE messages in addition to segmentation
      performed by the Inner Block options.  The presence of these
      options indicates a large message being sent and the message size
      can be estimated and used for traffic analysis.  Manipulating
      these options is a potential denial of service attack, e.g.
      injection of alleged Block fragments.  The specification of
      MAX_UNFRAGMENTED_SIZE (Section 4.1.3.3.2), at which the messages
      will be dropped, is intended as one measure to mitigate this kind
      of attack.

   o  No-Response.  The Outer No-Response option is used to support
      proxy functionality, specifically to avoid error transmissions
      from proxies to clients, and to avoid bandwidth reduction to
      servers by proxies applying congestion control when not receiving
      responses.  Modifying or introducing this option is a potential
      denial of service attack against the proxy operations, but since
      the option has an Inner value its use can be securely agreed
      between the endpoints.  The presence of this option is not
      expected to reveal any sensitive information about the message
      exchange.

   o  OSCORE.  The OSCORE option contains information about the
      compressed COSE header.  A change of this field may result in not
      being able to verify the OSCORE message.




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D.4.4.  HTTP Message Fields

   In contrast to CoAP, where OSCORE does not protect header fields to
   enable CoAP-CoAP proxy operations, the use of OSCORE with HTTP is
   restricted to transporting a protected CoAP message over an HTTP hop.
   Any unprotected HTTP message fields may reveal information about the
   transport of the OSCORE message and enable various denial of service
   attacks.  It is recommended to additionally use TLS [RFC5246] for
   HTTP hops, which enables encryption and integrity protection of
   headers, but still leaves some information for traffic analysis.

Appendix E.  CDDL Summary

   Data structure definitions in the present specification employ the
   CDDL language for conciseness and precision.  CDDL is defined in
   [I-D.ietf-cbor-cddl], which at the time of writing this appendix is
   in the process of completion.  As the document is not yet available
   for a normative reference, the present appendix defines the small
   subset of CDDL that is being used in the present specification.

   Within the subset being used here, a CDDL rule is of the form "name =
   type", where "name" is the name given to the "type".  A "type" can be
   one of:

   o  a reference to another named type, by giving its name.  The
      predefined named types used in the present specification are:
      "uint", an unsigned integer (as represented in CBOR by major type
      0); "int", an unsigned or negative integer (as represented in CBOR
      by major type 0 or 1); "bstr", a byte string (as represented in
      CBOR by major type 2); "tstr", a text string (as represented in
      CBOR by major type 3);

   o  a choice between two types, by giving both types separated by a
      "/";

   o  an array type (as represented in CBOR by major type 4), where the
      sequence of elements of the array is described by giving a
      sequence of entries separated by commas ",", and this sequence is
      enclosed by square brackets "[" and "]".  Arrays described by an
      array description contain elements that correspond one-to-one to
      the sequence of entries given.  Each entry of an array description
      is of the form "name : type", where "name" is the name given to
      the entry and "type" is the type of the array element
      corresponding to this entry.







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Acknowledgments

   The following individuals provided input to this document: Christian
   Amsuess, Tobias Andersson, Carsten Bormann, Joakim Brorsson, Esko
   Dijk, Thomas Fossati, Martin Gunnarsson, Klaus Hartke, Jim Schaad,
   Peter van der Stok, Dave Thaler, Marco Tiloca, William Vignat, and
   Malisa Vucinic.

   Ludwig Seitz and Goeran Selander worked on this document as part of
   the CelticPlus project CyberWI, with funding from Vinnova.

Authors' Addresses

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com


   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB

   Email: francesca.palombini@ericsson.com


   Ludwig Seitz
   RISE SICS

   Email: ludwig.seitz@ri.se
















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