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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: June 22, 2017                                       Ericsson AB
                                                                L. Seitz
                                                        SICS Swedish ICT
                                                       December 19, 2016


                    Object Security of CoAP (OSCOAP)
                   draft-ietf-core-object-security-01

Abstract

   This memo defines Object Security of CoAP (OSCOAP), a method for
   application layer protection of message exchanges with the
   Constrained Application Protocol (CoAP), using the CBOR Object
   Signing and Encryption (COSE) format.  OSCOAP provides end-to-end
   encryption, integrity and replay protection to CoAP payload, options,
   and header fields, as well as a secure binding between CoAP request
   and response messages.  The use of OSCOAP is signaled with the CoAP
   option Object-Security, also defined in this memo.

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 http://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 June 22, 2017.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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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 . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  The Object-Security Option  . . . . . . . . . . . . . . . . .   5
   3.  The Security Context  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Security Context Definition . . . . . . . . . . . . . . .   6
     3.2.  Derivation of Security Context Parameters . . . . . . . .   8
       3.2.1.  Derivation of Sender Key/IV, Recipient Key/IV . . . .  10
       3.2.2.  Context Identifier  . . . . . . . . . . . . . . . . .  11
       3.2.3.  Sender ID and Recipient ID  . . . . . . . . . . . . .  11
       3.2.4.  Sequence Numbers and Replay Window  . . . . . . . . .  11
   4.  Protected CoAP Message Fields . . . . . . . . . . . . . . . .  11
     4.1.  CoAP Payload  . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  CoAP Header . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  CoAP Options  . . . . . . . . . . . . . . . . . . . . . .  13
       4.3.1.  Class E Options . . . . . . . . . . . . . . . . . . .  15
       4.3.2.  Class A Options . . . . . . . . . . . . . . . . . . .  17
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.2.  Additional Authenticated Data . . . . . . . . . . . . . .  19
   6.  Protecting CoAP Messages  . . . . . . . . . . . . . . . . . .  21
     6.1.  Replay and Freshness Protection . . . . . . . . . . . . .  21
     6.2.  Protecting the Request  . . . . . . . . . . . . . . . . .  22
     6.3.  Verifying the Request . . . . . . . . . . . . . . . . . .  23
     6.4.  Protecting the Response . . . . . . . . . . . . . . . . .  24
     6.5.  Verifying the Response  . . . . . . . . . . . . . . . . .  25
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  28
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  28
     9.2.  COSE Header Parameters Registry . . . . . . . . . . . . .  29
     9.3.  Media Type Registrations  . . . . . . . . . . . . . . . .  29
     9.4.  CoAP Content Format Registration  . . . . . . . . . . . .  30
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     11.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Appendix A.  Overhead . . . . . . . . . . . . . . . . . . . . . .  33
     A.1.  Length of the Object-Security Option  . . . . . . . . . .  33
     A.2.  Size of the COSE Object . . . . . . . . . . . . . . . . .  33



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     A.3.  Message Expansion . . . . . . . . . . . . . . . . . . . .  34
     A.4.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  35
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  36
     B.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  36
     B.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  38
   Appendix C.  Object Security of Content (OSCON) . . . . . . . . .  39
     C.1.  Overhead OSCON  . . . . . . . . . . . . . . . . . . . . .  40
     C.2.  MAC Only  . . . . . . . . . . . . . . . . . . . . . . . .  41
     C.3.  Signature Only  . . . . . . . . . . . . . . . . . . . . .  42
     C.4.  Authenticated Encryption with Additional Data (AEAD)  . .  43
     C.5.  Symmetric Encryption with Asymmetric Signature (SEAS) . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   application protocol, designed for constrained nodes and networks
   [RFC7228].  CoAP specifies the use of proxies for scalability and
   efficiency.  At the same time CoAP references DTLS [RFC6347] for
   security.  Proxy operations on CoAP messages require DTLS 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 CoAP
   payload and metadata, in transit between client and server.  The
   proxy can also inject, delete, or reorder packages without being
   protected or detected by DTLS.

   This memo defines Object Security of CoAP (OSCOAP), a data object
   based security protocol, protecting CoAP message exchanges end-to-
   end, across intermediary nodes.  An analysis of end-to-end security
   for CoAP messages through intermediary nodes is performed in
   [I-D.hartke-core-e2e-security-reqs], this specification addresses the
   forwarding case.

   The solution provides an in-layer security protocol for CoAP which
   does not depend on underlying layers and is therefore favorable for
   providing security for "CoAP over foo", e.g.  CoAP messages passing
   over both unreliable and reliable transport
   [I-D.ietf-core-coap-tcp-tls], CoAP over IEEE 802.15.4 IE
   [I-D.bormann-6lo-coap-802-15-ie].

   OSCOAP builds on CBOR Object Signing and Encryption (COSE)
   [I-D.ietf-cose-msg], providing end-to-end encryption, integrity, and
   replay protection.  The use of OSCOAP is signaled with the CoAP
   option Object-Security, also defined in this memo.  The solution
   transforms an unprotected CoAP message into a protected CoAP message
   in the following way: the unprotected CoAP message is protected by
   including payload (if present), certain options, and header fields in



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   a COSE object.  The message fields that have been encrypted are
   removed from the message whereas the Object-Security option and the
   COSE object are added.  We call the result the "protected" CoAP
   message.  Thus OSCOAP is a security protocol based on the exchange of
   protected CoAP messages (see Figure 1).

          Client                                           Server
             |  request:                                     |
             |    GET example.com                            |
             |    [Header, Token, Options:{...,              |
             |     Object-Security:COSE object}]             |
             +---------------------------------------------->|
             |  response:                                    |
             |    2.05 (Content)                             |
             |    [Header, Token, Options:{...,              |
             |     Object-Security:-}, Payload:COSE object]  |
             |<----------------------------------------------+
             |                                               |

                        Figure 1: Sketch of OSCOAP

   OSCOAP provides protection of CoAP payload, certain options, and
   header fields, as well as a secure binding between CoAP request and
   response messages.  OSCOAP provides replay protection, but like DTLS,
   OSCOAP only provides relative freshness in the sense that the
   sequence numbers allows a recipient to determine the relative order
   of messages.  For applications having stronger demands on freshness
   (e.g. control of actuators), OSCOAP needs to be augmented with
   mechanisms providing absolute freshness
   [I-D.mattsson-core-coap-actuators].

   OSCOAP may be used in extremely constrained settings, where DTLS
   cannot be supported.  Alternatively, OSCOAP can be combined with
   DTLS, thereby enabling end-to-end security of CoAP payload, in
   combination with hop-by-hop protection of the entire CoAP message,
   during transport between end-point and intermediary node.  Examples
   of the use of OSCOAP are given in Appendix B.

   The message protection provided by OSCOAP can alternatively be
   applied only to the payload of individual messages.  We call this
   object security of content (OSCON) and it is defined in Appendix C.

1.1.  Terminology

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




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   words may also appear in this document in lowercase, absent their
   normative meanings.

   Readers are expected to be familiar with the terms and concepts
   described in [RFC7252] and [RFC7641].  Readers are also expected to
   be familiar with [RFC7049] and understand
   [I-D.greevenbosch-appsawg-cbor-cddl].  Terminology for constrained
   environments, such as "constrained device", "constrained-node
   network", is defined in [RFC7228].

2.  The Object-Security Option

   The Object-Security option indicates that OSCOAP is used to protect
   the CoAP message exchange.  The protection is achieved by means of a
   COSE object included in the protected CoAP message, as detailed in
   Section 5.

   The Object-Security option is critical, safe to forward, part of the
   cache key, and not repeatable.  Figure 2 illustrates the structure of
   the Object-Security option.

   A CoAP proxy SHOULD NOT cache a response to a request with an Object-
   Security option, since the response is only applicable to the
   original client's request.  The Object-Security option is included in
   the cache key for backward compatibility with proxies not recognizing
   the Object-Security option.  The effect of this is that messages with
   the Object-Security option will never generate cache hits.  To
   further prevent caching, a Max-Age option with value zero SHOULD be
   added to the protected CoAP responses.

        +-----+---+---+---+---+-----------------+--------+--------+
        | No. | C | U | N | R | Name            | Format | Length |
        +-----+---+---+---+---+-----------------+--------+--------|
        | TBD | x |   |   |   | Object-Security | opaque | 0-     |
        +-----+---+---+---+---+-----------------+--------+--------+
             C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                   Figure 2: The Object-Security Option

   The length of the Object-Security option depends on whether the
   unprotected message allows payload, on the set of options that are
   included in the unprotected message, the length of the integrity tag,
   and the length of the information identifying the security context.

   o  If the unprotected message allows payload, then the COSE object is
      the payload of the protected message (see Section 6.2 and
      Section 6.4), and the Object-Security option has length zero.  An
      endpoint receiving a CoAP message with payload, that also contains



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      a non-empty Object-Security option SHALL treat it as malformed and
      reject it.

   o  If the unprotected message does not allow payload, then the COSE
      object is the value of the Object-Security option and the length
      of the Object-Security option is equal to the size of the COSE
      object.  An endpoint receiving a CoAP message without payload,
      that also contains an empty Object-Security option SHALL treat it
      as malformed and reject it.

   Note that according to [RFC7252], new Methods and Response Codes
   should specify if the payload is optional, required or not allowed
   (Section 12.1.2) in the message, and in case this is not defined the
   sender must not include a payload (Section 5.5).  Thus, in this case,
   the COSE object MUST be the value of the Object-Security option.

   More details about the message overhead caused by the Object-Security
   option are given in Appendix A.

3.  The Security Context

   OSCOAP uses COSE with an Authenticated Encryption with Additional
   Data (AEAD) algorithm.  The specification requires that client and
   server establish a security context to apply to the COSE objects
   protecting the CoAP messages.  In this section we define the security
   context, and also specify how to derive the initial security contexts
   in client and server based on common 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 OSCOAP.  Each security
   context is identified by a Context Identifier.  A Context Identifier
   that is no longer in use can be reassigned to a new security context.

   For each endpoint, the security context is composed by a "Common
   Context", a "Sender Context" and a "Recipient Context".  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.  Each endpoint
   has a unique ID used to derive its Sender Context, this identifier is
   called "Sender ID".  The Recipient Context is derived with the other
   endpoint's ID, which is called "Recipient ID".  The Recipient ID is
   thus the ID of the endpoint from which a CoAP message is received.
   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



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   Context Identifiers in the two endpoints are not the same, but they
   are partly mirrored.  Retrieval and use of the security context are
   shown in Figure 3."

                  .-Cid = Cid1-.           .-Cid = Cid1-.
                  |  Common,   |           |  Common,   |
                  |  Sender,   |           |  Recipient,|
                  |  Recipient |           |  Sender    |
                  '------------'           '------------'
                      Client                   Server
                         |                       |
   Retrieve context for  | request:              |
    target resource      | [Token = Token1,      |
   Protect request with  |  Cid = Cid1, ...]     |
     Sender Context      +---------------------->| Retrieve context with
                         |                       |  Cid = Cid1
                         |                       | Verify request with
                         |                       |  Recipient Context
                         | response:             | Protect response with
                         | [Token = Token1, ...] |  Sender Context
   Retrieve context with |<----------------------+
    Token = Token1       |                       |
   Verify request with   |                       |
    Recipient Context    |                       |

            Figure 3: Retrieval and use of the Security Context

   The Common Context contains the following parameters:

   o  Context Identifier (Cid).  Variable length byte string that
      identifies the security context.  Its value is immutable once the
      security context is established.

   o  Algorithm (Alg).  Value that identifies the COSE AEAD algorithm to
      use for encryption.  Its value is immutable once the security
      context is established.

   o  Base Key (master_secret).  Variable length, uniformly random byte
      string containing the key used to derive traffic keys and IVs.
      Its value is immutable once the security context is established.

   The Sender Context contains the following parameters:

   o  Sender ID.  Variable length byte string identifying the endpoint
      itself.  Its value is immutable once the security context is
      established.





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   o  Sender Key. Byte string containing the symmetric key to protect
      messages to send.  Length is determined by Algorithm.  Its value
      is immutable once the security context is established.

   o  Sender IV.  Byte string containing the fixed context IV
      [I-D.ietf-cose-msg]) to protect messages to send.  Length is
      determined by Algorithm.  Its value is immutable once the security
      context is established.

   o  Sender Sequence Number.  Non-negative integer enumerating the COSE
      objects that the endpoint sends using the context.  Used as
      partial IV [I-D.ietf-cose-msg] to generate unique nonces for the
      AEAD.  Maximum value is determined by Algorithm.

   The Recipient Context contains the following parameters:

   o  Recipient ID.  Variable length byte string identifying the
      endpoint messages are received from.  Its value is immutable once
      the security context is established.

   o  Recipient Key. Byte string containing the symmetric key to verify
      messages received.  Length is determined by the Algorithm.  Its
      value is immutable once the security context is established.

   o  Recipient IV.  Byte string containing the context IV to verify
      messages received.  Length is determined by Algorithm.  Its value
      is immutable once the security context is established.

   o  Recipient Replay Window.  The replay protection window for
      messages received.

   The 3-tuple (Cid, Sender ID, Partial IV) is called Transaction
   Identifier (Tid), and SHALL be unique for each Base Key. The Tid is
   used as a unique challenge in the COSE object of the protected CoAP
   request.  The Tid is part of the Additional Authenticated Data (AAD,
   see Section 5) of the protected CoAP response message, which is how
   responses are bound to requests.

3.2.  Derivation of Security Context Parameters

   This section describes how to derive the initial parameters in the
   security context, given a small set of input parameters.  We also
   give indications on how applications should select the input
   parameters.

   The following input parameters SHALL be pre-established:

   o  Context Identifier (Cid)



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   o  Base Key (master_secret)

   o  AEAD Algorithm (Alg)

      *  Default is AES-CCM-64-64-128 (value 12)

   The following input parameters MAY be pre-established:

   o  Sender ID

      *  Defaults are 0x00 for the endpoint intially being client, and
         0x01 for the endpoint initially being server

   o  Recipient ID

      *  Defaults are 0x01 for the endpoint intially being client, and
         0x00 for the endpoint initially being server

   o  Key Derivation Function (KDF)

      *  Default is HKDF SHA-256

   o  Replay Window Size

      *  Default is 64

   The endpoints MAY interchange the CoAP client and server roles while
   maintaining the same security context.  When this happens, the former
   server still protects the message to send using the Sender Context,
   and verifies the message received using its Recipient Context.  The
   same is also true for the former client.  The endpoints MUST NOT
   change the Sender/Recipient ID.  In other words, changing the roles
   does not change the set of keys to be used.

   The input parameters are included unchanged in the security context.
   From the input parameters, the following parameters are derived:

   o  Sender Key, Sender IV, Sender Sequence Number

   o  Recipient Key, Recipient IV, Recipient Sequence Number

   The EDHOC protocol [I-D.selander-ace-cose-ecdhe] enables the
   establishment of input parameters with the property of forward
   secrecy, and negotiation of KDF and AEAD, it thus provides all
   necessary pre-requisite steps for using OSCOAP as defined here.






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3.2.1.  Derivation of Sender Key/IV, Recipient Key/IV

   Given the input parameters, the client and server can derive all the
   other parameters in the security context.  The derivation procedure
   described here MUST NOT be executed more than once using the same
   master_secret and Cid. The same master_secret SHOULD NOT be used with
   more than one Cid.

   The KDF MUST be one of the HKDF [RFC5869] algorithms defined in COSE.
   The KDF HKDF SHA-256 is mandatory to implement.  The security context
   parameters Sender Key/IV, Recipient Key/IV SHALL be derived using
   HKDF, and consists of the composition of the HKDF-Extract and HKDF-
   Expand steps ({{RFC5869}):

     output parameter = HKDF(master_secret, salt, info, output_length),

   where:

   o  master_secret is defined above

   o  salt is a string of zeros of the length of the hash function
      output in octets

   o  info is a serialized CBOR array consisting of:

      info = [
          cid : bstr,
          id : bstr,
          alg : int,
          out_type : tstr,
          out_len : uint
      ]

      - id is the Sender ID or Recipient ID

      - out_type is "Key" or "IV"

      - out_len is the key/IV size of the AEAD algorithm

   o  output_length is the size of the AEAD key/IV in bytes encoded as
      an 8-bit unsigned integer

   For example, if the algorithm AES-CCM-64-64-128 (see Section 10.2 in
   [I-D.ietf-cose-msg]) is used, output_length for the keys is 128 bits
   and output_length for the IVs is 56 bits.






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3.2.2.  Context Identifier

   As mentioned, Cid is pre-established.  How this is done is
   application specific, but it is RECOMMENDED that the application uses
   64-bits long pseudo-random Cids, in order to have globally unique
   Context Identifiers.  Cid SHOULD be unique in the sets of all
   security contexts used by all the endpoints.  If it is not the case,
   it is the role of the application to specify how to handle
   collisions.

   If the application has total control of both clients and servers,
   shorter unique Cids MAY be used.  Note that Cids of different lengths
   can be used by different clients and that e.g. a Cid with the value
   0x00 is different from the Cid with the value 0x0000.

   In the same phase during which the Cid is established in the
   endpoint, the application informs the endpoint what resources can be
   accessed using the corresponding security contexts.  Resources that
   are accessed with OSCOAP are called "protected" resources.  The set
   of resources that can be accessed using a certain security context is
   decided by the application (resource, host, etc.).  The client SHALL
   save the association resource-Cid, in order to be able to retrieve
   the correct security context to access a protected resource.  The
   server SHALL save the association resource-Cid, in order to determine
   whether a particular resource may be accessed using a certain Cid.

3.2.3.  Sender ID and Recipient ID

   The Sender ID and Recipient ID SHALL be unique in the set of all
   endpoints using the same security context.  Collisions may lead to
   the loss of both confidentiality and integrity.  If random IDs are
   used, they MUST be long enough so that the probability of collisions
   is negligible.

3.2.4.  Sequence Numbers and Replay Window

   The Sender Sequence Number is initialized to 0.  The Recipient Replay
   Window is initiated as described in Section 4.1.2.6 of [RFC6347].

4.  Protected CoAP Message Fields

   OSCOAP transforms an unprotected CoAP message into a protected CoAP
   message, and vice versa.  This section defines how the unprotected
   CoAP message fields are protected.  OSCOAP protects as much of the
   unprotected CoAP message as possible, while still allowing forward
   proxy operations [I-D.hartke-core-e2e-security-reqs].





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   This section also outlines how the message fields are processed and
   transferred, a detailed description is provided in Section 6.
   Message fields of the unprotected CoAP message are either transferred
   in the header/options part of the protected CoAP message, or in the
   plaintext of the COSE object.  Depending on which, the location of
   the message field in the protected CoAP message is called "outer" or
   "inner":

   o  Inner message field = message field included in the plaintext of
      the COSE object of the protected CoAP message (see Section 5.1)

   o  Outer message field = message field included in the header or
      options part of the protected CoAP message

   The inner message fields are encrypted and integrity protected by the
   COSE object.  The outer message fields are sent in plain text but may
   be integrity protected by including the message field values in the
   AAD of the COSE object (see Section 5.2).

   Note that, even though the message formats are slightly different,
   OSCOAP complies with CoAP over unreliable transport [RFC7252] as well
   as CoAP over reliable transport [I-D.ietf-core-coap-tcp-tls].

4.1.  CoAP Payload

   The CoAP Payload SHALL be encrypted and integrity protected, and thus
   is an inner message field.

   The sending endpoint writes the payload of the unprotected CoAP
   message into the plaintext of the COSE object (see Section 6.2 and
   Section 6.4).

   The receiving endpoint verifies and decrypts the COSE object, and
   recreates the payload of the unprotected CoAP message (see
   Section 6.3 and Section 6.5).

4.2.  CoAP Header

   Many CoAP header fields are required to be read and changed during a
   normal message exchange or when traversing a proxy and thus cannot be
   protected between the endpoints, e.g.  CoAP message layer fields such
   as Message ID.

   The CoAP header field Code MUST be sent in plaintext to support
   RESTful processing, but MUST be integrity protected to prevent an
   intermediary from changing, e.g. from GET to DELETE.  The CoAP
   version number SHALL be integrity protected to prevent potential
   future version-based attacks.  Note that while the version number is



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   not sent in each CoAP message over reliable transport
   [I-D.ietf-core-coap-tcp-tls], its value is known to client and
   server.

   Other CoAP header fields SHALL neither be integrity protected nor
   encrypted.  The CoAP header fields are thus outer message fields.

   The sending endpoint SHALL copy the header fields from the
   unprotected CoAP message to the protected CoAP message.  The
   receiving endpoint SHALL copy the header fields from the protected
   CoAP message to the unprotected CoAP message.  Both sender and
   receiver inserts the CoAP version number and header field Code in the
   AAD of the COSE object (see section Section 5.2).

4.3.  CoAP Options

   As with the message fields described in the previous sections, CoAP
   options may be encrypted and integrity protected, integrity protected
   only, or neither encrypted nor integrity protected.

   Most options are encrypted and integrity protected (see Figure 4),
   and thus inner message fields.  But to allow certain proxy
   operations, some options have outer values and require special
   processing.  Indeed, certain options may or must have both an inner
   value and a potentially different outer value, where the inner value
   is intended for the destination endpoint and the outer value is
   intended for the proxy.
























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     +----+---+---+---+---+----------------+--------+--------+---+---+
     | No.| C | U | N | R | Name           | Format | Length | E | A |
     +----+---+---+---+---+----------------+--------+--------+---+---+
     |  1 | x |   |   | x | If-Match       | opaque | 0-8    | x |   |
     |  3 | x | x | - |   | Uri-Host       | string | 1-255  |   | x |
     |  4 |   |   |   | x | ETag           | opaque | 1-8    | x |   |
     |  5 | x |   |   |   | If-None-Match  | empty  | 0      | x |   |
     |  6 |   | x | - |   | Observe        | uint   | 0-3    | * |   |
     |  7 | x | x | - |   | Uri-Port       | uint   | 0-2    |   | x |
     |  8 |   |   |   | x | Location-Path  | string | 0-255  | x |   |
     | 11 | x | x | - | x | Uri-Path       | string | 0-255  | x |   |
     | 12 |   |   |   |   | Content-Format | uint   | 0-2    | x |   |
     | 14 |   | x | - |   | Max-Age        | uint   | 0-4    | * |   |
     | 15 | x | x | - | x | Uri-Query      | string | 0-255  | x |   |
     | 17 | x |   |   |   | Accept         | uint   | 0-2    | x |   |
     | 20 |   |   |   | x | Location-Query | string | 0-255  | x |   |
     | 23 | x | x | - | - | Block2         | uint   | 0-3    | * |   |
     | 27 | x | x | - | - | Block1         | uint   | 0-3    | * |   |
     | 28 |   |   | x |   | Size2          | unit   | 0-4    | * |   |
     | 35 | x | x | - |   | Proxy-Uri      | string | 1-1034 |   | * |
     | 39 | x | x | - |   | Proxy-Scheme   | string | 1-255  |   | x |
     | 60 |   |   | x |   | Size1          | uint   | 0-4    | * |   |
     +----+---+---+---+---+----------------+--------+--------+---+---+
           C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
       E=Encrypt and Integrity Protect, A=Integrity Protect, *=Special

                   Figure 4: Protection of CoAP Options

   A summary of how options are protected and processed is shown in
   Figure 4.  The CoAP options are partitioned into two classes:

   o  E - options which are encrypted and integrity protected, and

   o  A - options which are only integrity protected.

   Options within each class are protected and processed in a similar
   way, but certain options which require special processing as
   described in the subsections and indicated by a '*' in Figure 4.

   Unless specified otherwise, CoAP options not listed in Figure 4 SHALL
   be encrypted and integrity protected and processed as class E
   options.

   Specifications of new CoAP options SHOULD specify how they are
   processed with OSCOAP.  New COAP options SHOULD be of class E and
   SHOULD NOT have outer options unless a forwarding proxy needs to read
   an option value.  If a certain option is both inner and outer, the




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   two values SHOULD NOT be the same, unless a proxy is required by
   specification to be able to read the end-to-end value.

4.3.1.  Class E Options

   For options in class E (see Figure 4) the option value in the
   unprotected CoAP message, if present, SHALL be encrypted and
   integrity protected between the endpoints, and thus is not visible to
   or possible to change by intermediary nodes.  Hence the actions
   resulting from the use of such options is analogous to communicating
   in a protected manner with the endpoint.  For example, a client using
   an ETag option will not be served by a proxy.

   The sending endpoint SHALL write the class E option from the
   unprotected CoAP message into the plaintext of the COSE object (see
   Section 6.2 and Section 6.4).

   Except for the special options described in the subsections, the
   sending endpoint SHALL NOT use the outer options of class E.
   However, note that an intermediary may, legimitimately or not, add,
   change or remove the value of an outer option.

   Execept for the Block options Section 4.3.1.3, the receiving endpoint
   SHALL discard any outer options of class E from the protected CoAP
   message and SHALL replace it with the value from the COSE object when
   present (see Section 6.3 and Section 6.5).

4.3.1.1.  Max-Age

   An inner Max-Age option is used as defined in [RFC7252] taking into
   account that it is not accessible to proxies.

   Since OSCOAP binds CoAP responses to requests, a cached response
   would not be possible to use for any other request.  Therefore, there
   SHOULD be an outer Max-Age option with value zero to prevent caching
   of responses (see Section 5.6.1 of [RFC7252]).

   The outer Max-Age option SHALL NOT be encrypted and SHALL NOT be
   integrity protected.

4.3.1.2.  Observe

   The Observe option as used here targets the requirements on
   forwarding of [I-D.hartke-core-e2e-security-reqs] (Section 2.2.1.2).

   An inner Observe option is used between endpoints.  In order for a
   proxy to support forwarding of notifications, there SHALL be an outer
   Observe option.  To simplify the processing in the server, the outer



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   option SHOULD have the same value as the inner Observe option.  The
   outer Observe option MAY have different values than the inner, but
   the order of the different values is SHALL be the same as for the
   inner Observe option.

   The outer Observe option SHALL neither be encrypted nor integrity
   protected.

4.3.1.3.  The Block Options

   The Block options (Block1, Block2, Size1 and Size2) MAY be either
   only inner options, only outer options or both inner and outer
   options.  The inner and outer options are processed independently.

   The inner block options are used for endpoint-to-endpoint secure
   fragmentation of payload into blocks and protection of information
   about the fragmentation (block number, last block, etc.).
   Additionally, a proxy may arbitrarily do fragmentation operations on
   the protected CoAP message, adding outer block options that are not
   intended to be verified by any endpoint or proxy.

   There SHALL be a security policy defining a maximum unfragmented
   message size for inner Block options such that messages exceeding
   this size SHALL be fragmented by the sending endpoint.

   In addition to the processing defined for the inner Block options
   inherent to class E options, the AEAD Tag from each block SHALL be
   included in the calculation of the Tag for the next block (see
   Section 5.2), so that each block in the order being sent can be
   verified as it arrives.

   The protected CoAP message may be fragmented by the sending endpoint
   or proxy as defined in [RFC7959], in which case the outer Block
   options are being used.  The outer Block options SHALL neither be
   encrypted nor integrity protected.

   An endpoint receiving a message with an outer Block option SHALL
   first process this option according to [RFC7959], until all blocks of
   the protected CoAP message has been received, or the cumulated
   message size of the exceeds the maximum unfragmented message size.
   In the latter case the message SHALL be discarded.  In the former
   case, the processing of the protected CoAP message continues as
   defined in this document (see Section 6.3 and Section 6.5).

   If the unprotected CoAP message contains Block options, the receiving
   endpoint processes this according to {{RFC7959}.





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4.3.2.  Class A Options

   Options in this class are used to support forward proxy operations.
   Class A options SHALL only have outer values and SHALL NOT be
   encrypted.  In order for the destination endpoint to verify the Uri,
   class A options SHALL be integrity protected.

   Uri-Host, Uri-Port, Proxy-Scheme and Proxy-Uri are class A options.
   When Uri-Host, Uri-Port, Proxy-Scheme options are present, Proxy-Uri
   is not used [RFC7252].  Proxy-Uri is processed like the other class A
   options after a pre-processing step (see Section 4.3.2.1.

   Except for Proxi-Uri, the sending endpoint SHALL copy the class A
   option from the unprotected CoAP message to the protected CoAP
   message.  The class A options are inserted in the AAD of the COSE
   object (see unencrypted-Uri Section 5.2).

4.3.2.1.  Proxy-Uri

   Proxy-Uri, when present, is split by OSCOAP into class A options and
   privacy sensitive class E options, which are processed accordingly.
   When Proxy-Uri is used in the unprotected CoAP message, Uri-* are not
   present [RFC7252].

   The sending endpoint SHALL first decompose the Proxy-Uri value of the
   unprotected CoAP message into the unencrypted-Uri (Section 5.2) and
   Uri-Path/Query options according to section 6.4 of [RFC7252].

   Uri-Path and Uri-Query are class E options and SHALL be protected and
   processed as if obtained from the unprotected CoAP message, see
   Section 4.3.1.

   The value of the Proxy-Uri option of the protected CoAP message SHALL
   be replaced with unencrypted-Uri and SHALL be protected and processed
   as a class A option, see Section 4.3.2.

5.  The COSE Object

   This section defines how to use the COSE format [I-D.ietf-cose-msg]
   to wrap and protect data in the unprotected CoAP message.  OSCOAP
   uses the COSE_Encrypt0 structure with an Authenticated Encryption
   with Additional Data (AEAD) algorithm.

   The AEAD algorithm AES-CCM-64-64-128 defined in Section 10.2 of
   [I-D.ietf-cose-msg] is mandatory to implement.  For AES-CCM-64-64-128
   the length of Sender Key and Recipient Key SHALL be 128 bits, the
   length of nonce, Sender IV, and Recipient IV SHALL be 7 bytes, and
   the maximum Sequence Number SHALL be 2^56-1.  The nonce is



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   constructed as described in Section 3.1 of [I-D.ietf-cose-msg], i.e.
   by padding the Partial IV (Sequence Number) with zeroes and XORing it
   with the context IV (Sender IV or Recipient IV).

   Since OSCOAP only makes use of a single COSE structure, there is no
   need to explicitly specify the structure, and OSCOAP uses the
   untagged version of the COSE_Encrypt0 structure (Section 2. of
   [I-D.ietf-cose-msg]).  If the COSE object has a different structure,
   the recipient MUST reject the message, treating it as malformed.

   OSCOAP introduces a new COSE Header Parameter, the Sender Identifier:

   sid:  This parameter is used to identify the sender of the message.
      Applications MUST NOT assume that 'sid' values are unique.  This
      is not a security critical field.  For this reason, it can be
      placed in the unprotected headers bucket.

    +------+-------+------------+----------------+-------------------+
    | name | label | value type | value registry | description       |
    +------+-------+------------+----------------+-------------------+
    | sid  |  TBD  |    bstr    |                | Sender Identifier |
    +------+-------+------------+----------------+-------------------+

                 Table 1: Additional COSE Header Parameter

   We denote by Plaintext the data that is encrypted and integrity
   protected, and by Additional Authenticated Data (AAD) the data that
   is integrity protected only, in the COSE object.

   The fields of COSE_Encrypt0 structure are defined as follows (see
   example in Appendix C.4).

   o  The "Headers" field is formed by:

      *  The "protected" field, which SHALL include:

         +  The "Partial IV" parameter.  The value is set to the Sender
            Sequence Number.  The Partial IV is a byte string (type:
            bstr), and SHOULD be of minimum length needed to encode the
            sequence number.

         +  The "kid" parameter.  The value is set to the Context
            Identifier (see Section 3).  This parameter is optional if
            the message is a CoAP response.

         +  Optionally, the parameter called "sid", defined below.  The
            value is set to the Sender ID (see Section 3).  Note that




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            since this parameter is sent in clear, privacy issues SHOULD
            be considered by the application defining the Sender ID.

      *  The "unprotected" field, which SHALL be empty.

   o  The "ciphertext" field is computed from the Plaintext (see
      Section 5.1) and the Additional Authenticated Data (AAD) (see
      Section 5.2) and encoded as a byte string (type: bstr), following
      Section 5.2 of [I-D.ietf-cose-msg].

5.1.  Plaintext

   The Plaintext is formatted as a CoAP message without Header (see
   Figure 5) consisting of:

   o  all CoAP Options present in the unprotected message which are
      encrypted (see Section 4), in the order as given by the Option
      number (each Option with Option Header including delta to previous
      included encrypted option); and

   o  the CoAP Payload, 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Options to Encrypt (if any) ...                            ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1|    Payload (if any) ...                       ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      (only if there
        is payload)

                            Figure 5: Plaintext

5.2.  Additional Authenticated Data

   The Additional Authenticated Data ("Enc_structure") as described is
   Section 5.3 of [I-D.ietf-cose-msg] includes:

   o  the "context" parameter, which has value "Encrypted"

   o  the "protected" parameter, which includes the "protected" part of
      the "Headers" field;

   o  the "external_aad" is a serialized CBOR array Figure 6 where the
      exact content is different in requests (external_aad_req) and
      repsonses (external_aad_resp).  It contains:



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      *  ver: uint, contains the CoAP version number, as defined in
         Section 3 of [RFC7252]

      *  code: uint, contains is the CoAP Code of the unprotected CoAP
         message, as defined in Section 3 of [RFC7252].

      *  alg: int, contains the Algorithm from the security context used
         for the exchange (see Section 3.1);

      *  unencrypted-uri: tstr with tag URI, contains the part of the
         URI which is not encrypted, and is composed of the request
         scheme (Proxy-Scheme if present), Uri-Host and Uri-Port (if
         present) options according to the method described in
         Section 6.5 of [RFC7252], if the message is a CoAP request;

      *  cid : bstr, contains the cid for the request (which is same as
         the cid for the response).

      *  id : bstr, is the identifier for the endpoint sending the
         request and verifying the response; which means that for the
         endpoint sending the response, the id has value Recipient ID,
         while for the endpoint receiving the response, id has the value
         Sender ID.

      *  seq : bstr, is the value of the "Partial IV" in the COSE object
         of the request (see Section 5).

      *  tag-previous-block: bstr, contains the AEAD Tag of the message
         containing the previous block in the sequence, as enumerated by
         Block1 in the case of a request and Block2 in the case of a
         response, if the message is fragmented using a block option
         [RFC7959].



















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            external_aad = external_aad_req / external_aad_resp

            external_aad_req = [
               ver : uint,
               code : uint,
               alg : int,
               unencrypted-uri : uri,
               ? tag-previous-block : bstr
            ]

            external_aad_resp = [
               ver : uint,
               code : uint,
               alg : int,
               cid : bstr,
               id : bstr,
               seq : bstr,
               ? tag-previous-block : bstr
            ]

                   Figure 6: External AAD (external_aad)

   The encryption process is described in Section 5.3 of
   [I-D.ietf-cose-msg].

6.  Protecting CoAP Messages

6.1.  Replay and Freshness Protection

   In order to protect from replay of messages and verify freshness, a
   CoAP endpoint SHALL maintain a Sender Sequence Number and a Recipient
   Replay Window in the security context.  An endpoint uses the Sender
   Sequence Number to protect messages to send and the Recipient Replay
   Window to verify received messages, as described in Section 3.

   A receiving endpoint SHALL verify that the Sequence Number (Partial
   IV) received in the COSE object has not been received before in the
   security context identified by the Cid. The size of the Replay Window
   depends on the use case and lower protocol layers.  In case of
   reliable and ordered transport, the recipient MAY just store the last
   received sequence number and require that newly received Sequence
   Numbers equals the last received Recipient Sequence Number + 1.

   The receiving endpoint SHALL reject messages with a sequence number
   greater than the maximum value of the Partial IV.  This maximum value
   is algorithm specific, for example for AES-CCM-64-64-128 it is
   2^56-1.




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   OSCOAP responses are verified to match a prior request, by including
   the unique transaction identifier (Tid as defined in Section 3) of
   the request in the Additional Authenticated Data of the response
   message.  In case of CoAP observe, each notification MUST be verified
   using the Tid of the observe registration, so the Tid of the
   registration needs to be cached by the observer until the observation
   ends.

   If a CoAP server receives a request with the Object-Security option,
   then the server SHALL include the Tid of the request in the AAD of
   the response, as described in Section 6.4.

   If the CoAP client receives a response with the Object-Security
   option, then the client SHALL verify the integrity of the response,
   using the Tid of its own associated request in the AAD, as described
   in Section 6.5.

6.2.  Protecting the Request

   Given an unprotected CoAP request, including header, options and
   payload, the client SHALL perform the following steps to create a
   protected CoAP request using a security context associated with the
   target resource (see Section 3.2.2).

   When using Uri-Host or Proxy-Uri in the construction of the request,
   the <host> value MUST be a reg-name ([RFC3986]), and not an IP-
   literal or IPv4address, for canonicalization of the destination
   address.

   1.  Compute the COSE object as specified in Section 5

       *  the AEAD nonce is created by XORing the Sender IV (context IV)
          with the Sender Sequence Number (partial IV).

       *  If the block option is used, the AAD includes the AEAD Tag
          from the previous block sent (from the second block and
          following) Section 5.2.  This means that the endpoint MUST
          store the Tag of each last-sent block to compute the
          following.

       *  Note that the 'sid' field containing the Sender ID is included
          in the COSE object (Section 5) if the application needs it.

   2.  Format the protected CoAP message as an ordinary CoAP message,
       with the following Header, Options, and Payload, based on the
       unprotected CoAP message:

       *  The CoAP header is the same as the unprotected CoAP message.



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       *  If present, the CoAP option Proxy-Uri is decomposed as
          described in Section 4.3.2.1.

       *  The CoAP options which are of class E (Section 4) are removed.
          The Object-Security option is added.

       *  If the message type of the unprotected CoAP message does not
          allow Payload, then the value of the Object-Security option is
          the COSE object.  If the message type of the unprotected CoAP
          message allows Payload, then the Object-Security option is
          empty and the Payload of the protected CoAP message is the
          COSE object.

   3.  Store the association Token - Cid. The Client SHALL be able to
       find the correct security context used to protect the request and
       verify the response with use of the Token of the message
       exchange.

   4.  Increment the Sender Sequence Number by one.  If the Sender
       Sequence Number exceeds the maximum number for the AEAD
       algorithm, the client MUST NOT process any more requests with the
       given security context.  The client SHOULD acquire a new security
       context (and consequently inform the server about it) before this
       happens.  The latter is out of scope of this memo.

6.3.  Verifying the Request

   A CoAP server receiving an unprotected CoAP request to access a
   protected resource (as defined Section 3.2.2) SHALL reject the
   message with error code 4.01 (Unauthorized).

   A CoAP server receiving a message containing the Object-Security
   option and a outer Block option SHALL first process this option
   according to [RFC7959], until all blocks of the protected CoAP
   message has been received, see Section 4.3.1.3.

   A CoAP server receiving a message containing the Object-Security
   option SHALL perform the following steps, using the security context
   identified by the Context Identifier in the "kid" parameter in the
   received COSE object:

   1.  Verify the Sequence Number in the Partial IV parameter, as
       described in Section 6.1.  If it cannot be verified that the
       Sequence Number has not been received before, the server MUST
       stop processing the request.

   2.  Recreate the Additional Authenticated Data, as described in
       Section 5.



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       *  If the block option is used, the AAD includes the AEAD Tag
          from the previous block received (from the second block and
          following) Section 5.2.  This means that the endpoint MUST
          store the Tag of each last-received block to compute the
          following.

       *  Note that the server's <host> value MUST be a reg-name
          ([RFC3986]), and not an IP-literal or IPv4address.

   3.  Compose the AEAD nonce by XORing the Recipient IV (context IV)
       with the padded Partial IV parameter, received in the COSE
       Object.

   4.  Retrieve the Recipient Key.

   5.  Verify and decrypt the message.  If the verification fails, the
       server MUST stop processing the request.

   6.  If the message verifies, update the Recipient Replay Window, as
       described in Section 6.1.

   7.  Restore the unprotected request by adding any decrypted options
       or payload from the plaintext.  Any outer E options (Section 4)
       are overwritten.  The Object-Security option is removed.

6.4.  Protecting the Response

   A server receiving a valid request with a protected CoAP message
   (i.e. containing an Object-Security option) SHALL respond with a
   protected CoAP message.

   Given an unprotected CoAP response, including header, options, and
   payload, the server SHALL perform the following steps to create a
   protected CoAP response, using the security context identified by the
   Context Identifier of the received request:

   1.  Compute the COSE object as specified in Section Section 5

       *  The AEAD nonce is created by XORing the Sender IV (context IV)
          and the padded Sender Sequence Number.

       *  If the block option [RFC7959] is used, the AAD includes the
          AEAD Tag from the previous block sent (from the second block
          and following) Section 5.2.  This means that the endpoint MUST
          store the Tag of each last-sent block to compute the
          following.  Note that this applies even for random access of
          blocks, i.e. when blocks are not requested in the order of
          their relative number (NUM).



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   2.  Format the protected CoAP message as an ordinary CoAP message,
       with the following Header, Options, and Payload based on the
       unprotected CoAP message:

       *  The CoAP header is the same as the unprotected CoAP message.

       *  The CoAP options which are of class E are removed, except any
          special option (labelled '*') that is present which has its
          outer value (Section 4).  The Object-Security option is added.

       *  If the message type of the unprotected CoAP message does not
          allow Payload, then the value of the Object-Security option is
          the COSE object.  If the message type of the unprotected CoAP
          message allows Payload, then the Object-Security option is
          empty and the Payload of the protected CoAP message is the
          COSE object.

   3.  Increment the Sender Sequence Number by one.  If the Sender
       Sequence Number exceeds the maximum number for the AEAD
       algorithm, the server MUST NOT process any more responses with
       the given security context.  The server SHOULD acquire a new
       security context (and consequently inform the client about it)
       before this happens.  The latter is out of scope of this memo.

   Note the differences between generating a protected request, and a
   protected response, for example whether "kid" is present in the
   header, or whether Destination URI or Tid is present in the AAD, of
   the COSE object.

6.5.  Verifying the Response

   A CoAP client receiving a message containing the Object-Security
   option SHALL perform the following steps, using the security context
   identified by the Token of the received response:

   1.  If the message contain an outer Block option the client SHALL
       process this option according to [RFC7959], until all blocks of
       the protected CoAP message has been received, see
       Section 4.3.1.3.

   2.  Verify the Sequence Number in the Partial IV parameter as
       described in Section 6.1.  If it cannot be verified that the
       Sequence Number has not been received before, the client MUST
       stop processing the response.

   3.  Recreate the Additional Authenticated Data as described in
       Section 5.




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       *  If the block option is used, the AAD includes the AEAD Tag
          from the previous block received (from the second block and
          following) Section 5.2.  This means that the endpoint MUST
          store the Tag of each last-received block to compute the
          following.

   4.  Compose the AEAD nonce by XORing the Recipient IV (context IV)
       with the Partial IV parameter, received in the COSE Object.

   5.  Retrieve the Recipient Key.

   6.  Verify and decrypt the message.  If the verification fails, the
       client MUST stop processing the response.

   7.  If the message verifies, update the Recipient Replay Window, as
       described in Section 6.1.

   8.  Restore the unprotected response by adding any decrypted options
       or payload from the plaintext.  Any class E options (Section 4)
       are overwritten.  The Object-Security option is removed.

7.  Security Considerations

   In scenarios with intermediary nodes such as proxies or brokers,
   transport layer security such as DTLS only protects data hop-by-hop.
   As a consequence the intermediary nodes can read and modify
   information.  The trust model where all intermediate 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.

   DTLS protects hop-by-hop the entire CoAP message, including header,
   options, and payload.  OSCOAP protects end-to-end the payload, and
   all information in the options and header, that is not required for
   forwarding (see Section 4).  DTLS and OSCOAP can be combined, thereby
   enabling end-to-end security of CoAP payload, in combination with
   hop-by-hop protection of the entire CoAP message, during transport
   between end-point and intermediary node.

   The CoAP message layer, however, cannot be protected end-to-end
   through intermediary devices since the parameters Type and Message
   ID, as well as Token and Token Length may be changed by a proxy.
   Moreover, messages that are not possible to verify should for
   security reasons not always be acknowledged but in some cases be



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   silently dropped.  This would not comply with CoAP message layer, but
   does not have an impact on the application layer security solution,
   since message layer is excluded from that.

   The use of COSE to protect CoAP 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 shared secret material and key derivation function in client
   and server.  EDHOC [I-D.selander-ace-cose-ecdhe] describes an
   augmented Diffie-Hellman key exchange to produce forward secret
   keying material and agree on crypto algorithms necessary for OSCOAP,
   authenticated with pre-established credentials.  These pre-
   established credentials may, in turn, be provisioned using a trusted
   third party such as described in the OAuth-based ACE framework
   [I-D.ietf-ace-oauth-authz].  An OSCOAP profile of ACE is described in
   [I-D.seitz-ace-oscoap-profile].

   The mandatory-to-implement AEAD algorithm AES-CCM-64-64-128 is
   selected for broad applicability in terms of message size (2^64
   blocks) and maximum no. messages (2^56-1).  Compatibility with CCM*
   is achieved by using the algorithm AES-CCM-16-64-128
   [I-D.ietf-cose-msg].

   Most AEAD algorithms require a unique nonce for each message, for
   which the 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
   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.

   The encrypted block options enable the sender to split large messages
   into protected blocks such that the receiving node can verify blocks
   before having received the complete message.  In order to protect
   from attacks replacing blocks from a different message with the same
   block number between same endpoints and same resource at roughly the
   same time, the AEAD Tag from the message containing one block is
   included in the external_aad of the message containing the next
   block.

   The unencrypted 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 encrypted options allow for



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   secure fragmentation of very large messages.  A maximum message size
   (above which the 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.

   Applications need to use a padding scheme if the content of a message
   can be determined solely from the length of the payload.  As an
   example, the strings "YES" and "NO" even if encrypted can be
   distinguished from each other as there is no padding supplied by the
   current set of encryption algorithms.  Some information can be
   determined even from looking at boundary conditions.  An example of
   this would be returning an integer between 0 and 100 where lengths of
   1, 2 and 3 will provide information about where in the range things
   are.  Three different methods to deal with this are: 1) ensure that
   all messages are the same length.  For example using 0 and 1 instead
   of 'yes' and 'no'.  2) Use a character which is not part of the
   responses to pad to a fixed length.  For example, pad with a space to
   three characters.  3) Use the PKCS #7 style padding scheme where m
   bytes are appended each having the value of m.  For example,
   appending a 0 to "YES" and two 1's to "NO".  This style of padding
   means that all values need to be padded.

8.  Privacy Considerations

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

   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.

9.  IANA Considerations

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

9.1.  CoAP Option Numbers Registry

   The Object-Security option is added to the CoAP Option Numbers
   registry:








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             +--------+-----------------+-------------------+
             | Number | Name            | Reference         |
             +--------+-----------------+-------------------+
             |  TBD   | Object-Security | [[this document]] |
             +--------+-----------------+-------------------+

9.2.  COSE Header Parameters Registry

   The "sid" parameter is added to the COSE Header Parameter Registry:

    +------+-------+------------+----------------+-------------------+
    | name | label | value type | value registry | description       |
    +------+-------+------------+----------------+-------------------+
    |  sid |  TBD  |    bstr    |                | Sender Identifier |
    +------+-------+------------+----------------+-------------------+

9.3.  Media Type Registrations

   The "application/oscon" media type is added to the Media Types
   registry:































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       Type name: application

       Subtype name: oscon

       Required parameters: N/A

       Optional parameters: N/A

       Encoding considerations: binary

       Security considerations: See Appendix C of [[this document]].

       Interoperability considerations: N/A

       Published specification: [[this document]]

       Applications that use this media type: To be identified

       Fragment identifier considerations: N/A

       Additional information:

       * 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

9.4.  CoAP Content Format Registration

   The "application/oscon" content format is added to the CoAP Content
   Format registry:






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         +-------------------+----------+----+-------------------+
         | Media type        | Encoding | ID | Reference         |
         +-------------------+----------+----+-------------------+
         | application/oscon | -        | 70 | [[this document]] |
         +-------------------+----------+----+-------------------+

10.  Acknowledgments

   The following individuals provided input to this document: Carsten
   Bormann, Joakim Brorsson, Martin Gunnarsson, Klaus Hartke, Jim
   Schaad, Marco Tiloca, and Malisa Vucinic.

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

11.  References

11.1.  Normative References

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-24 (work in progress), November 2016.

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

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

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

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7959>.





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   [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,
              <http://www.rfc-editor.org/info/rfc3986>.

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

11.2.  Informative References

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-04 (work in progress), October 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-01 (work in progress), July 2016.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., and F.
              Palombini, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-02 (work in progress),
              November 2016.

   [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.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-04 (work in progress), October 2016.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L. and F. Palombini, "OSCOAP profile of ACE",
              draft-seitz-ace-oscoap-profile-01 (work in progress),
              October 2016.








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   [I-D.ietf-core-coap-tcp-tls]
              Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              draft-ietf-core-coap-tcp-tls-05 (work in progress),
              October 2016.

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Vigano, C. and H. Birkholz, "CBOR data definition language
              (CDDL): a notational convention to express CBOR data
              structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
              in progress), September 2016.

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

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

Appendix A.  Overhead

   OSCOAP transforms an unprotected CoAP message to a protected CoAP
   message, and the protected CoAP message is larger than the
   unprotected CoAP message.  This appendix illustrates the message
   expansion.

A.1.  Length of the Object-Security Option

   The protected CoAP message contains the COSE object.  The COSE object
   is included in the payload if the message type of the unprotected
   CoAP message allows payload or else in the Object-Security option.
   In the former case the Object-Security option is empty.  So the
   length of the Object-Security option is either zero or the size of
   the COSE object, depending on whether the CoAP message allows payload
   or not.

   Length of Object-Security option = { 0, size of COSE Object }

A.2.  Size of the COSE Object

   The size of the COSE object is the sum of the sizes of

   o  the Header parameters,




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   o  the Cipher Text (excluding the Tag),

   o  the Tag, and

   o  data incurred by the COSE format itself (including CBOR encoding).

   Let's analyze the contributions one at a time:

   o  The header parameters of the COSE object are the Context
      Identifier (Cid) and the Sequence Number (Seq) (also part of the
      Transaction Identifier (Tid)) if the message is a request, and Seq
      only if the message is a response (see Section 5).

      *  The size of Cid is recommended to be 64 bits, but may be
         shorter, as discussed in Section 3.2.2

      *  The size of Seq is variable, and increases with the number of
         messages exchanged.

      *  As the AEAD nonce is generated from the padded Sequence Number
         and a previously agreed upon context IV it is not required to
         send the whole nonce in the message.

   o  The Cipher Text, excluding the Tag, is the encryption of the
      payload and the encrypted options Section 4, which are present in
      the unprotected CoAP message.

   o  The size of the Tag depends on the Algorithm.  For example, for
      the algorithm AES-CCM-64-64-128, the Tag is 8 bytes.

   o  The overhead from the COSE format itself depends on the sizes of
      the previous fields, and is of the order of 10 bytes.

A.3.  Message Expansion

   The message expansion is not the size of the COSE object.  The
   ciphertext in the COSE object is encrypted payload and options of the
   unprotected CoAP message - the plaintext of which is removed from the
   protected CoAP message.  Since the size of the ciphertext is the same
   as the corresponding plaintext, there is no message expansion due to
   encryption; payload and options are just represented in a different
   way in the protected CoAP message:

   o  The encrypted payload is in the payload of the protected CoAP
      message

   o  The encrypted options are in the Object-Security option or within
      the payload.



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   Therefore the OSCOAP message expansion is due to Cid (if present),
   Seq, Tag, and COSE overhead:

            Message Overhead = Cid + Seq + Tag + COSE Overhead

                    Figure 7: OSCOAP message expansion

A.4.  Example

   This section gives an example of message expansion in a request with
   OSCOAP.

   In this example we assume an 8-byte Cid.

   o  Cid: 0xa1534e3c9cecad84

   In the example the sequence number is 225, requiring 1 byte to
   encode.  (The size of Seq could be larger depending on how many
   messages that has been sent as is discussed in Appendix A.2.)

   o  Seq: 225

   The example is based on AES-CCM-64-64-128.

   o  Tag is 8 bytes

   The COSE object is represented in Figure 8 using CBOR's diagnostic
   notation.

      [
        h'a20448a1534e3c9cecad840641e2', / protected:
                                            {04:h'a1534e3c9cecad84',
                                             06:h'e2'} /
        {},                              / unprotected: - /
        Ciph + Tag                       / ciphertext + 8 byte
                                            authentication tag /
      ]

                  Figure 8: Example of message expansion

   Note that the encrypted CoAP options and payload are omitted since we
   target the message expansion (see Appendix A.3).  Therefore the size
   of the COSE Cipher Text equals the size of the Tag, which is 8 bytes.

   The COSE object encodes to a total size of 26 bytes, which is the
   message expansion in this example.  The COSE overhead in this example
   is 26 - (8 + 1 + 8) = 9 bytes, according to the formula in Figure 7.




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   Note that in this example two bytes in the COSE overhead are used to
   encode the length of Cid and the length of Seq.

   Figure 9 summarizes these results.

          +---------+---------+---------+----------+------------+
          |   Cid   |   Seq   |   Tag   | COSE OH  | Message OH |
          +---------+---------+---------+----------+------------+
          | 8 bytes | 1 byte  | 8 bytes |  9 bytes |  22 bytes  |
          +---------+---------+---------+----------+------------+

    Figure 9: Message overhead for a 8-byte Cid, 1-byte Seq and 8-byte
                                   Tag.

Appendix B.  Examples

   This section gives examples of OSCOAP.  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 COSE message format.

B.1.  Secure Access to Sensor

   Here is an example targeting the scenario in the Section 2.2.1. -
   Forwarding of [I-D.hartke-core-e2e-security-reqs].  The example
   illustrates a client requesting the alarm status from a server.  In
   the request, CoAP option Uri-Path is encrypted and integrity
   protected, and the CoAP header fields Code and Version are integrity
   protected (see Section 4).  In the response, the CoAP Payload is
   encrypted and integrity protected, and the CoAP header fields Code
   and Version are integrity protected.



















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      Client  Proxy  Server
         |      |      |
         +----->|      |            Code: 0.01 (GET)
         | GET  |      |           Token: 0x8c
         |      |      | Object-Security: [cid:5fdc, seq:42,
         |      |      |                   {Uri-Path:"alarm_status"},
         |      |      |                   <Tag>]
         |      |      |         Payload: -
         |      |      |
         |      +----->|            Code: 0.01 (GET)
         |      | GET  |           Token: 0x7b
         |      |      | Object-Security: [cid:5fdc, seq:42,
         |      |      |                   {Uri-Path:"alarm_status"},
         |      |      |                   <Tag>]
         |      |      |         Payload: -
         |      |      |
         |      |<-----+            Code: 2.05 (Content)
         |      | 2.05 |           Token: 0x7b
         |      |      |         Max-Age: 0
         |      |      | Object-Security: -
         |      |      |         Payload: [seq:56, {"OFF"}, <Tag>]
         |      |      |
         |<-----+      |            Code: 2.05 (Content)
         | 2.05 |      |           Token: 0x8c
         |      |      |         Max-Age: 0
         |      |      | Object-Security: -
         |      |      |         Payload: [seq:56, {"OFF"}, <Tag>]
         |      |      |

       Figure 10: Indication of CoAP GET protected with OSCOAP.  The
      brackets [ ... ] indicate a COSE object.  The brackets { ... }
                         indicate encrypted data.

   Since the unprotected request message (GET) has no payload, the
   Object-Security option carries the COSE object as its value.  Since
   the unprotected response message (Content) has payload ("OFF"), the
   COSE object (indicated with [ ... ]) is carried as the CoAP payload.

   The COSE header of the request contains a Context Identifier
   (cid:5fdc), indicating which security context was used to protect the
   message and a Sequence Number (seq:42).

   The option Uri-Path (alarm_status) and payload ("OFF") are formatted
   as indicated in Section 5, and encrypted in the COSE Cipher Text
   (indicated with { ... }).

   The server verifies that the Sequence Number has not been received
   before (see Section 6.1).  The client verifies that the Sequence



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   Number has not been received before and that the response message is
   generated as a response to the sent request message (see
   Section 6.1).

B.2.  Secure Subscribe to Sensor

   Here is an example targeting the scenario in the Forwarding with
   observe case of [I-D.hartke-core-e2e-security-reqs].  The example
   illustrates a client requesting subscription to a blood sugar
   measurement resource (GET /glucose), and first receiving the value
   220 mg/dl, and then a second reading with value 180 mg/dl.  The CoAP
   options Observe, Uri-Path, Content-Format, and Payload are encrypted
   and integrity protected, and the CoAP header field Code is integrity
   protected (see Section 4).

   Client  Proxy  Server
      |      |      |
      +----->|      |            Code: 0.01 (GET)
      | GET  |      |           Token: 0x83
      |      |      |         Observe: 0
      |      |      | Object-Security: [cid:ca, seq:15b7, {Observe:0,
      |      |      |                   Uri-Path:"glucose"}, <Tag>]
      |      |      |         Payload: -
      |      |      |
      |      +----->|            Code: 0.01 (GET)
      |      | GET  |           Token: 0xbe
      |      |      |         Observe: 0
      |      |      | Object-Security: [cid:ca, seq:15b7, {Observe:0,
      |      |      |                   Uri-Path:"glucose"}, <Tag>]
      |      |      |         Payload: -
      |      |      |
      |      |<-----+            Code: 2.05 (Content)
      |      | 2.05 |           Token: 0xbe
      |      |      |         Max-Age: 0
      |      |      |         Observe: 1
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c2, {Observe:1,
      |      |      |                   Content-Format:0, "220"}, <Tag>]
      |      |      |
      |<-----+      |            Code: 2.05 (Content)
      | 2.05 |      |           Token: 0x83
      |      |      |         Max-Age: 0
      |      |      |         Observe: 1
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c2, {Observe:1,
      |      |      |                   Content-Format:0, "220"}, <Tag>]
     ...    ...    ...
      |      |      |



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      |      |<-----+            Code: 2.05 (Content)
      |      | 2.05 |           Token: 0xbe
      |      |      |         Max-Age: 0
      |      |      |         Observe: 2
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c6, {Observe:2,
      |      |      |                   Content-Format:0, "180"}, <Tag>]
      |      |      |
      |<-----+      |            Code: 2.05 (Content)
      | 2.05 |      |           Token: 0x83
      |      |      |         Max-Age: 0
      |      |      |         Observe: 2
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c6, {Observe:2,
      |      |      |                   Content-Format:0, "180"}, <Tag>]
      |      |      |

       Figure 11: Indication of CoAP GET protected with OSCOAP.  The
       brackets [ ... ] indicates COSE object.  The bracket { ... }
                         indicates encrypted data.

   Since the unprotected request message (GET) allows no payload, the
   COSE object (indicated with [ ... ]) is carried in the Object-
   Security option value.  Since the unprotected response message
   (Content) has payload, the Object-Security option is empty, and the
   COSE object is carried as the payload.

   The COSE header of the request contains a Context Identifier
   (cid:ca), indicating which security context was used to protect the
   message and a Sequence Number (seq:15b7).

   The options Observe, Content-Format and the payload are formatted as
   indicated in Section 5, and encrypted in the COSE ciphertext
   (indicated with { ... }).

   The server verifies that the Sequence Number has not been received
   before (see Section 6.1).  The client verifies that the Sequence
   Number has not been received before and that the response message is
   generated as a response to the subscribe request.

Appendix C.  Object Security of Content (OSCON)

   OSCOAP protects message exchanges end-to-end between a certain client
   and a certain server, targeting the security requirements for forward
   proxy of [I-D.hartke-core-e2e-security-reqs].  In contrast, many use
   cases require one and the same message to be protected for, and
   verified by, multiple endpoints, see caching proxy section of
   [I-D.hartke-core-e2e-security-reqs].  Those security requirements can



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   be addressed by protecting essentially the payload/content of
   individual messages using the COSE format ([I-D.ietf-cose-msg]),
   rather than the entire request/response message exchange.  This is
   referred to as Object Security of Content (OSCON).

   OSCON transforms an unprotected CoAP message into a protected CoAP
   message in the following way: the payload of the unprotected CoAP
   message is wrapped by a COSE object, which replaces the payload of
   the unprotected CoAP message.  We call the result the "protected"
   CoAP message.

   The unprotected payload shall be the plaintext/payload of the COSE
   object.  The 'protected' field of the COSE object 'Headers' shall
   include the context identifier, both for requests and responses.  If
   the unprotected CoAP message includes a Content-Format option, then
   the COSE object shall include a protected 'content type' field, whose
   value is set to the unprotected message Content-Format value.  The
   Content-Format option of the protected CoAP message shall be replaced
   with "application/oscon" (Section 9)

   The COSE object shall be protected (encrypted) and verified
   (decrypted) as described in ([I-D.ietf-cose-msg]).

   Most AEAD algorithms require a unique nonce for each message.
   Sequence numbers for partial IV as specified for OSCOAP may be used
   for replay protection as described in Section 6.1.  The use of time
   stamps in the COSE header parameter 'operation time'
   [I-D.ietf-cose-msg] for freshness may be used.

   OSCON shall not be used in cases where CoAP header fields (such as
   Code or Version) or CoAP options need to be integrity protected or
   encrypted.  OSCON shall not be used in cases which require a secure
   binding between request and response.

   The scenarios in Sections 3.3 - 3.5 of
   [I-D.hartke-core-e2e-security-reqs] assume multiple recipients for a
   particular content.  In this case the use of symmetric keys does not
   provide data origin authentication.  Therefore the COSE object should
   in general be protected with a digital signature.

C.1.  Overhead OSCON

   In general there are four different kinds of modes that need to be
   supported: message authentication code, digital signature,
   authenticated encryption, and symmetric encryption + digital
   signature.  The use of digital signature is necessary for
   applications with many legitimate recipients of a given message, and
   where data origin authentication is required.



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   To distinguish between these different cases, the tagged structures
   of COSE are used (see Section 2 of [I-D.ietf-cose-msg]).

   The sizes of COSE messages for selected algorithms are detailed in
   this section.

   The size of the header is shown separately from the size of the MAC/
   signature.  A 4-byte Context Identifier and a 1-byte Sequence Number
   are used throughout all examples, with these values:

   o  Cid: 0xa1534e3c

   o  Seq: 0xa3

   For each scheme, we indicate the fixed length of these two parameters
   ("Cid+Seq" column) and of the Tag ("MAC"/"SIG"/"TAG").  The "Message
   OH" column shows the total expansions of the CoAP message size, while
   the "COSE OH" column is calculated from the previous columns
   following the formula in Figure 7.

   Overhead incurring from CBOR encoding is also included in the COSE
   overhead count.

   To make it easier to read, COSE objects are represented using CBOR's
   diagnostic notation rather than a binary dump.

C.2.  MAC Only

   This example is based on HMAC-SHA256, with truncation to 8 bytes
   (HMAC 256/64).

   Since the key is implicitly known by the recipient, the
   COSE_Mac0_Tagged structure is used (Section 6.2 of
   [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:

            996(                         # COSE_Mac0_Tagged
              [
                h'a20444a1534e3c0641a3', # protected:
                                           {04:h'a1534e3c',
                                            06:h'a3'}
                {},                      # unprotected
                h'',                     # payload
                MAC                      # truncated 8-byte MAC
              ]
            )




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   This COSE object encodes to a total size of 26 bytes.

   Figure 12 summarizes these results.

          +------------------+-----+-----+---------+------------+
          |     Structure    | Tid | MAC | COSE OH | Message OH |
          +------------------+-----+-----+---------+------------+
          | COSE_Mac0_Tagged | 5 B | 8 B |   13 B  |    26 B    |
          +------------------+-----+-----+---------+------------+

      Figure 12: Message overhead for a 5-byte Tid using HMAC 256/64

C.3.  Signature Only

   This example is based on ECDSA, with a signature of 64 bytes.

   Since only one signature is used, the COSE_Sign1_Tagged structure is
   used (Section 4.2 of [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:

             997(                         # COSE_Sign1_Tagged
               [
                 h'a20444a1534e3c0641a3', # protected:
                                            {04:h'a1534e3c',
                                             06:h'a3'}
                 {},                      # unprotected
                 h'',                     # payload
                 SIG                      # 64-byte signature
               ]
             )

   This COSE object encodes to a total size of 83 bytes.

   Figure 13 summarizes these results.

         +-------------------+-----+------+---------+------------+
         |     Structure     | Tid |  SIG | COSE OH | Message OH |
         +-------------------+-----+------+---------+------------+
         | COSE_Sign1_Tagged | 5 B | 64 B |   14 B  |  83 bytes  |
         +-------------------+-----+------+---------+------------+

     Figure 13: Message overhead for a 5-byte Tid using 64 byte ECDSA
                                signature.







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C.4.  Authenticated Encryption with Additional Data (AEAD)

   This example is based on AES-CCM with the Tag truncated to 8 bytes.

   Since the key is implicitly known by the recipient, the
   COSE_Encrypt0_Tagged structure is used (Section 5.2 of
   [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:

     993(                         # COSE_Encrypt0_Tagged
       [
         h'a20444a1534e3c0641a3', # protected:
                                    {04:h'a1534e3c',
                                     06:h'a3'}
         {},                      # unprotected
         TAG                      # ciphertext + truncated 8-byte TAG
       ]
     )

   This COSE object encodes to a total size of 25 bytes.

   Figure 14 summarizes these results.

        +----------------------+-----+-----+---------+------------+
        |       Structure      | Tid | TAG | COSE OH | Message OH |
        +----------------------+-----+-----+---------+------------+
        | COSE_Encrypt0_Tagged | 5 B | 8 B |   12 B  |  25 bytes  |
        +----------------------+-----+-----+---------+------------+

     Figure 14: Message overhead for a 5-byte Tid using AES_128_CCM_8.

C.5.  Symmetric Encryption with Asymmetric Signature (SEAS)

   This example is based on AES-CCM and ECDSA with 64 bytes signature.
   The same assumption on the security context as in Appendix C.4.  COSE
   defines the field 'counter signature w/o headers' that is used here
   to sign a COSE_Encrypt0_Tagged message (see Section 3 of
   [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:










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     993(                         # COSE_Encrypt0_Tagged
       [
         h'a20444a1534e3c0641a3', # protected:
                                    {04:h'a1534e3c',
                                     06:h'a3'}
         {9:SIG},                 # unprotected:
                                     09: 64 bytes signature
         TAG                      # ciphertext + truncated 8-byte TAG
       ]
     )

   This COSE object encodes to a total size of 92 bytes.

   Figure 15 summarizes these results.

    +----------------------+-----+-----+------+---------+------------+
    |       Structure      | Tid | TAG | SIG  | COSE OH | Message OH |
    +----------------------+-----+-----+------+---------+------------+
    | COSE_Encrypt0_Tagged | 5 B | 8 B | 64 B |   15 B  |    92 B    |
    +----------------------+-----+-----+------+---------+------------+

        Figure 15: Message overhead for a 5-byte Tid using AES-CCM
                         countersigned with ECDSA.

Authors' Addresses

   Goeran Selander
   Ericsson AB
   Farogatan 6
   Kista  SE-16480 Stockholm
   Sweden

   Email: goran.selander@ericsson.com


   John Mattsson
   Ericsson AB
   Farogatan 6
   Kista  SE-16480 Stockholm
   Sweden

   Email: john.mattsson@ericsson.com









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   Francesca Palombini
   Ericsson AB
   Farogatan 6
   Kista  SE-16480 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com


   Ludwig Seitz
   SICS Swedish ICT
   Scheelevagen 17
   Lund  22370
   Sweden

   Email: ludwig@sics.se



































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