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Versions: (draft-selander-ace-object-security) 00 01 02 03 04

CoRE Working Group                                           G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: January 2, 2018                                     Ericsson AB
                                                                L. Seitz
                                                        SICS Swedish ICT
                                                           July 01, 2017


                    Object Security of CoAP (OSCOAP)
                   draft-ietf-core-object-security-04

Abstract

   This document defines Object Security of CoAP (OSCOAP), a method for
   application layer protection of the Constrained Application Protocol
   (CoAP), using the CBOR Object Signing and Encryption (COSE).  OSCOAP
   provides end-to-end encryption, integrity and replay protection to
   CoAP payload, options, and header fields, as well as a secure message
   binding.  OSCOAP is designed for constrained nodes and networks and
   can be used across intermediaries and over any layer.  The use of
   OSCOAP is signaled with the CoAP option Object-Security, also defined
   in this document.

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 January 2, 2018.

Copyright Notice

   Copyright (c) 2017 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  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 . . . . . . . .   9
     3.3.  Requirements on the Security Context Parameters . . . . .  10
   4.  Protected CoAP Message Fields . . . . . . . . . . . . . . . .  11
     4.1.  CoAP Payload  . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  CoAP Header . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  CoAP Options  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.2.  Additional Authenticated Data . . . . . . . . . . . . . .  19
   6.  Sequence Numbers, Replay, Message Binding, and Freshness  . .  20
     6.1.  AEAD Nonce Uniqueness . . . . . . . . . . . . . . . . . .  20
     6.2.  Replay Protection . . . . . . . . . . . . . . . . . . . .  21
     6.3.  Sequence Number and Replay Window State . . . . . . . . .  21
     6.4.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.5.  Delay and Mismatch Attacks  . . . . . . . . . . . . . . .  23
   7.  Processing  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  23
     7.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  24
     7.3.  Protecting the Response . . . . . . . . . . . . . . . . .  25
     7.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  26
   8.  OSCOAP Compression  . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  Encoding of the Object-Security Option  . . . . . . . . .  27
     8.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  28
   9.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  32
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
     12.1.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  32
     12.2.  Media Type Registrations . . . . . . . . . . . . . . . .  32
     12.3.  CoAP Content Format Registration . . . . . . . . . . . .  33
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  34
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  34



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     14.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  36
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  36
     B.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  36
     B.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  37
   Appendix C.  Object Security of Content (OSCON) . . . . . . . . .  39
     C.1.  Overhead OSCON  . . . . . . . . . . . . . . . . . . . . .  40
     C.2.  MAC Only  . . . . . . . . . . . . . . . . . . . . . . . .  41
     C.3.  Signature Only  . . . . . . . . . . . . . . . . . . . . .  41
     C.4.  Authenticated Encryption with Additional Data (AEAD)  . .  42
     C.5.  Symmetric Encryption with Asymmetric Signature (SEAS) . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   The Constrained Application Protocol (CoAP) 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 [RFC7252] 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 since they are no longer
   protected by DTLS.

   This document 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.  In addition to the core features defined in
   [RFC7252], OSCOAP supports Observe [RFC7641] and Blockwise [RFC7959].

   OSCOAP is designed for constrained nodes and networks and provides an
   in-layer security protocol for CoAP which does not depend on
   underlying layers.  OSCOAP can be used anywhere that CoAP can be
   used, including unreliable transport [RFC7228], reliable transport
   [I-D.ietf-core-coap-tcp-tls], and non-IP transport
   [I-D.bormann-6lo-coap-802-15-ie].  OSCOAP may also be used to protect
   group communication for CoAP [I-D.tiloca-core-multicast-oscoap].  The
   use of OSCOAP does not affect the URI scheme and OSCOAP can therefore
   be used with any URI scheme defined for CoAP.  The application
   decides the conditions for which OSCOAP is required.

   OSCOAP builds on CBOR Object Signing and Encryption (COSE)
   [I-D.ietf-cose-msg], providing end-to-end encryption, integrity,



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   replay protection, and secure message binding.  A compressed version
   of COSE is used, see Section 8.  The use of OSCOAP is signaled with
   the CoAP option Object-Security, defined in Section 2.  OSCOAP
   provides protection of CoAP payload, certain options, and header
   fields.  The solution transforms a CoAP message into an "OSCOAP
   message" before sending, and vice versa after receiving.  The OSCOAP
   message is a CoAP message related to the original CoAP message in the
   following way: the original CoAP message is protected by including
   payload (if present), certain options, and header fields in a COSE
   object.  The message fields that have been encrypted are removed from
   the message whereas the Object-Security option and the compressed
   COSE object are added, see Figure 1.

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

                        Figure 1: Sketch of OSCOAP

   OSCOAP may be used in very constrained settings, thanks to its small
   message size, its restricted code and memory requirements, and is
   independent of underlying layer below CoAP.  OSCOAP can be combined
   with DTLS, thereby enabling end-to-end security of e.g.  CoAP payload
   and options, 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), which 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
   words may also appear in this document in lowercase, absent their
   normative meanings.




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   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252], Observe [RFC7641], Blockwise [RFC7959],
   COSE [I-D.ietf-cose-msg], CBOR [RFC7049], CDDL
   [I-D.greevenbosch-appsawg-cbor-cddl], and constrained environments
   [RFC7228].

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

2.  The Object-Security Option

   The Object-Security option (see Figure 2) indicates that OSCOAP is
   used to protect the CoAP message exchange.  The Object-Security
   option is critical, safe to forward, part of the cache key, not
   repeatable, and opaque.

        +-----+---+---+---+---+-----------------+--------+--------+
        | 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

   A successful response to a request with the Object-Security option
   SHALL contain the Object-Security option.  A CoAP endpoint 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 is that messages with the Object-Security option
   will never generate cache hits.  For Max-Age processing, see
   Section 4.3.1.1.

   The protection is achieved by means of a COSE object (see Section 5),
   which is compressed and then included in the OSCOAP message.  The
   placement of the COSE object depends on whether the method/response
   code allows payload (see [RFC7252]):

   o  If the method/response code allows payload, then the compressed
      COSE object Section 8 is the payload of the OSCOAP message, and
      the Object-Security option has length zero.  An endpoint receiving
      a CoAP message with payload, that also contains a non-empty
      Object-Security option SHALL treat it as malformed and reject it.





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   o  If the method/response code does not allow payload, then the
      compressed COSE object Section 8 is the value of the Object-
      Security option and the length of the Object-Security option is
      equal to the size of the compressed 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.

   The size of the COSE object depends on whether the method/response
   code allows payload, if the message is a request or response, on the
   set of options that are included in the original message, the AEAD
   algorithm, the length of the information identifying the security
   context, and the length of the sequence number.

3.  The Security Context

   OSCOAP uses COSE with an Authenticated Encryption with Additional
   Data (AEAD) algorithm between a CoAP client and a CoAP server.  An
   implementation supporting this specification MAY only implement the
   client part or MAY only implement the server part.

   This 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.  For each endpoint,
   the security context is composed of 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.  Clients need
   to be able to retrieve the correct security context to use.

   An endpoint uses its Sender ID (SID) to derive its Sender Context,
   and the other endpoint uses the same ID, now called Recipient ID
   (RID), to derive its Recipient Context.  In communication between two
   endpoints, the Sender Context of one endpoint matches the Recipient
   Context of the other endpoint, and vice versa.  Thus the two security
   contexts identified by the same IDs in the two endpoints are not the
   same, but they are partly mirrored.  Retrieval and use of the
   security context are shown in Figure 3.



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                  .------------.           .------------.
                  |  Common,   |           |  Common,   |
                  |  Sender,   |           |  Recipient,|
                  |  Recipient |           |  Sender    |
                  '------------'           '------------'
                      Client                   Server
                         |                       |
   Retrieve context for  | OSCOAP request:       |
    target resource      | [Token = Token1,      |
   Protect request with  |  kid = SID, ...]      |
     Sender Context      +---------------------->| Retrieve context with
                         |                       |  RID = kid
                         |                       | Verify request with
                         |                       |  Recipient Context
                         | OSCOAP 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  Algorithm (Alg).  Value that identifies the COSE AEAD algorithm to
      use for encryption.  Its value is immutable once the security
      context is established.

   o  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.

   o  Master Salt (OPTIONAL).  Variable length byte string containing
      the salt 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 Sender
      Context.  Its value is immutable once the security context is
      established.

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




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   o  Sender IV.  Byte string containing the IV to protect messages to
      send.  Derived from Common Context and Sender ID.  Length is
      determined by Algorithm.  Its value is immutable once the security
      context is established.

   o  Sequence Number.  Non-negative integer used to protect requests
      and observe responses to send.  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
      Recipient Context.  Its value is immutable once the security
      context is established.

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

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

   o  Replay Window.  The replay window to verify requests and observe
      responses received.

   When it is understood which context is referred to (Sender Context or
   Recipient Context), the term "Context IV" is used to denote the IV
   currently used with this context.

   An endpoint may free up memory by not storing the Sender Key, Sender
   IV, Recipient Key, and Recipient IV, deriving them from the Common
   Context when needed.  Alternatively, an endpoint may free up memory
   by not storing the Master Secret and Master Salt after the other
   parameters have been derived.

   The endpoints MAY interchange the client and server roles while
   maintaining the same security context.  When this happens, the former
   server still protects messages to send using its Sender Context, and
   verifies messages 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.





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3.2.  Derivation of Security Context Parameters

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

   o  Master Secret

   o  Sender ID

   o  Recipient ID

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

   o  AEAD Algorithm (Alg)

      *  Default is AES-CCM-64-64-128 (COSE abbreviation: 12)

   o  Master Salt

      *  Default is the empty string

   o  Key Derivation Function (KDF)

      *  Default is HKDF SHA-256

   o  Replay Window Type and Size

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

   How the input parameters are pre-established, is application
   specific.  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.

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

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

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



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   where:

   o  salt is the Master Salt as defined above

   o  IKM is the Master Secret is defined above

   o  info is a CBOR array consisting of:

      info = [
          id : bstr,
          alg : int,
          type : tstr,
          L : int
      ]

      * id is the Sender ID or Recipient ID

      * type is "Key" or "IV"

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

   For example, if the algorithm AES-CCM-64-64-128 (see Section 10.2 in
   [I-D.ietf-cose-msg]) is used, the value for L is 16 for keys and 7
   for IVs.

3.2.2.  Initial Sequence Numbers and Replay Window

   The Sequence Number is initialized to 0.  The supported types of
   replay protection and replay window length is application specific
   and depends on the lower layers.  Default is DTLS-type replay
   protection with a window size of 32 initiated as described in
   Section 4.1.2.6 of [RFC6347].

3.3.  Requirements on the Security Context Parameters

   As collisions may lead to the loss of both confidentiality and
   integrity, Sender ID SHALL be unique in the set of all security
   contexts using the same Master Secret.  Normally (e.g. when using
   EDHOC [I-D.selander-ace-cose-ecdhe]) Sender IDs can be very short.
   Note that Sender IDs of different lengths can be used with the same
   Master Secret.  E.g. the SID with value 0x00 is different from the
   SID with the value 0x0000.  If Sender ID uniqueness cannot be
   guaranteed, random Sender IDs MUST be used.  Random Sender IDs MUST
   be long enough so that the probability of collisions is negligible.






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   To enable retrieval of the right Recipient Context, the Recipient ID
   SHOULD be unique in the sets of all Recipient Contexts used by an
   endpoint.

   The same Master Salt MAY be used with several Master Secrets.

4.  Protected CoAP Message Fields

   OSCOAP transforms a CoAP message into an OSCOAP message, and vice
   versa.  This section defines how the CoAP message fields are
   protected.  Note that OSCOAP protects messages from the CoAP
   Requests/Responses layer only, and not from the Messaging layer
   (Section 2 of [RFC7252]): this means that RST and ACK empty messages
   are not protected, while ACK with piggybacked responses are protected
   using the process defined in this document.  All the messages
   mentioned in this document refer to CON, NON and non-empty ACK
   messages.

   OSCOAP protects as much of the original CoAP message as possible,
   while still allowing forward proxy operations
   [I-D.hartke-core-e2e-security-reqs].  Message fields may either be

   o  Class E: encrypted and integrity protected,

   o  Class I: integrity protected only, or

   o  Class U: unprotected.

   This section also outlines how the message fields are transferred, a
   detailed description of the processing is provided in Section 7.
   Message fields of the original CoAP message are either transferred in
   the header/options part of the OSCOAP message, or in the plaintext of
   the COSE object.  Depending on which, the location of the message
   field in the OSCOAP message is called "outer" or "inner":

   o  Inner message field: message field included in the plaintext of
      the COSE object of the OSCOAP message (see Section 5.1).  The
      inner message fields are by definition encrypted and integrity
      protected by the COSE object (Class E).

   o  Outer message field: message field included in the header or
      options part of the OSCOAP message.  The outer message fields are
      not encrypted and thus visible to an intermediary, but may be
      integrity protected by including the message field values in the
      Additional Authenticated Data (AAD) of the COSE object (see
      Section 5.2).  I.e. outer message fields may be Class I or Class
      U.




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   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 (Class
   E), and thus is an inner message field.

   The sending endpoint writes the payload of the original CoAP message
   into the plaintext of the COSE object.

   The receiving endpoint verifies and decrypts the COSE object, and
   recreates the payload of the original CoAP message.

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 in
   general 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 (Class I) to
   prevent an intermediary from changing, e.g. from GET to DELETE.  The
   CoAP version number MUST be integrity protected to prevent potential
   future version-based attacks (Class I).  Note that while the version
   number is not sent in each CoAP message over reliable transport
   [I-D.ietf-core-coap-tcp-tls], its value is known to client and
   server.

   The other CoAP header fields SHALL neither be integrity protected nor
   encrypted (Class U).  All CoAP header fields are thus outer message
   fields.

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

4.3.  CoAP Options

   Most options are encrypted and integrity protected (Class E), and
   thus inner message fields.  But to allow certain proxy operations,
   some options have outer values, i.e. are present as options in the
   OSCOAP message.  Certain options may have both an inner value and a



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   potentially different outer value, where the inner value is intended
   for the destination endpoint and the outer value is intended for a
   proxy.

   A summary of how options are protected and processed is shown in
   Figure 4.  Options within each class are protected and processed in a
   similar way, but certain options which require special processing as
   indicated by a * in Figure 4 and described in the processing of the
   respective option.

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

        E=Encrypt and Integrity Protect, I=Integrity Protect only,
        U=Unprotected, *=Special

                   Figure 4: Protection of CoAP Options

   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 define how they are
   processed with OSCOAP.  New COAP options SHOULD be of class E and
   SHOULD NOT have outer values unless a forwarding proxy needs to read
   that option value.  If a certain option has both inner and outer
   values, the two values SHOULD NOT be the same.



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4.3.1.  Class E Options

   For options in class E (see Figure 4) the option value in the
   original CoAP message, if present, SHALL be encrypted and integrity
   protected between the endpoints.  Hence the actions resulting from
   the use of such options is analogous to communicating in a protected
   manner directly with the endpoint.  For example, a client using an
   If-Match option will not be served by a proxy.

   The sending endpoint SHALL write the class E option from the original
   CoAP message into the plaintext of the COSE object.

   Except for the special options (* in Figure 4), the sending endpoint
   SHALL NOT use the outer options of class E.  However, note that an
   intermediary may, legitimately or not, add, change or remove the
   value of an outer option.

   Except for the special options, the receiving endpoint SHALL discard
   any outer options of class E from the OSCOAP message and SHALL write
   the Class E options present in the plaintext of the COSE object into
   the decrypted CoAP message.

4.3.1.1.  Max-Age

   An inner Max-Age option, like other class E options, 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.  To avoid
   unnecessary caching, a server MAY add an outer Max-Age option with
   value zero to OSCOAP responses (see Section 5.6.1 of [RFC7252]).  The
   outer Max-Age option is not integrity protected.

4.3.1.2.  The Block Options

   Blockwise [RFC7959] is an optional feature.  An implementation MAY
   comply with [RFC7252] and the Object-Security option without
   implementing [RFC7959].

   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.








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4.3.1.2.1.  Inner Block Options

   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, block size, last block).  In
   this case, the sending CoAP endpoint fragments the CoAP message as
   defined in [RFC7959] before the message is processed by OSCOAP.  The
   receiving CoAP endpoint first processes the OSCOAP message before
   processing blockwise as defined in [RFC7959].

   Applications using OSCOAP with inner Block options MUST specify 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.

   For blockwise request operations (using Block1) the client MUST use
   and process the Request-Tag as defined in Section 3 of
   [I-D.amsuess-core-repeat-request-tag].  In particular, the rules in
   section 3.3.1 of [I-D.amsuess-core-repeat-request-tag] MUST be
   followed, which guarantee that a specific request body is assembled
   only from the corresponding request blocks.

   For blockwise response operations (using Block2) the server MUST use
   and process the ETag as defined in Section 4 of
   [I-D.amsuess-core-repeat-request-tag].

4.3.1.2.2.  Outer Block Options

   A CoAP proxy may do block fragmentation on any CoAP message
   (including OSCOAP messages) as defined in [RFC7959], and thereby
   decompose it into multiple blocks using outer Block options.  The
   outer block options are thus neither encrypted nor integrity
   protected.

   To allow multiple concurrent request operations to the same server
   (not only same resource), a CoAP proxy should use and process the
   Request-Tag as specified in section 3.3.2 of
   [I-D.amsuess-core-repeat-request-tag]; an OSCOAP server that supports
   outer Block options MUST support the Request-Tag option.

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




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4.3.2.  Class I Options

   A Class I option is an outer option and hence visible in the options
   part of the OSCOAP message.  Except for special options described in
   the subsections, for options in Class I (see Figure 4) the option
   value SHALL be integrity protected between the endpoints, see
   (Section 5.2).  Unless otherwise specified, the sending endpoint
   SHALL encode the Class I options in the OSCOAP message as described
   in Section 4.3.4.

4.3.2.1.  Observe

   Observe [RFC7641] is an optional feature.  An implementation MAY
   support [RFC7252] and the Object-Security option without supporting
   [RFC7641].  The Observe option as used here targets the requirements
   on forwarding of [I-D.hartke-core-e2e-security-reqs]
   (Section 2.2.1.2).

   In order for a proxy to support forwarding of Observe messages, there
   must be an Observe option present in options part of the OSCOAP
   message ([RFC7641]), so Observe must have an outer value:

   o  The Observe option of the original CoAP request SHALL be encoded
      in the OSCOAP request as described in Section 4.3.4.

   To secure the order of the notifications, responses with the Observe
   option SHALL be integrity protected in the following way:

   o  The Observe option SHALL be included in the external_aad of the
      response (see Section 5.2), with value set to the 3 least
      significant bytes of the Sequence Number of the response.

   The Observe option in the CoAP request SHALL NOT be integrity
   protected, since it may be legitimately removed by a proxy.

   If the Observe option is removed from a CoAP request by a proxy, then
   the server can still verify the request (as a non-Observe request),
   and produce a non-Observe response.  If the OSCOAP client receives a
   response to an Observe request without an outer Observe value, then
   it MUST verify the response as a non-Observe response, i.e. not
   include the Sequence Number of the response in the external_aad.

4.3.3.  Class U Options

   Options in Class U have outer values and are used to support forward
   proxy operations.  Unless otherwise specified, the sending endpoint
   SHALL encode the Class U options in the options part of the OSCOAP
   message as described in Section 4.3.4.



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4.3.3.1.  Uri-Host, Uri-Port, and Proxy-Scheme

   The sending endpoint SHALL copy Uri-Host, Uri-Port, and Proxy-Scheme
   from the original CoAP message to the options part of the OSCOAP
   message.  When Uri-Host, Uri-Port, or Proxy-Scheme options are
   present, Proxy-Uri is not used [RFC7252].

4.3.3.2.  Proxy-Uri

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

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

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

   The value of the Proxy-Uri option of the OSCOAP message MUST be
   replaced with Proxy-Scheme, Uri-Host and Uri-Port options (if
   present) composed according to section 6.5 of [RFC7252] and MUST be
   processed as a class U option, see Section 4.3.3.

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

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

   During OSCOAP processing, Proxy-Uri is split into:

   o  Proxy-Scheme = "coap"

   o  Uri-Host = "example.com"

   o  Uri-Port = "5863"

   o  Uri-Path = "resource"

   o  Uri-Query = "q=1"

   Uri-Path and Uri-Query follow the processing defined in
   Section 4.3.1, and are thus encrypted and transported in the COSE
   object.  The remaining options are composed into the Proxy-Uri
   included in the options part of the OSCOAP message, which has value:



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

4.3.4.  Outer Options in the OSCOAP Message

   All options with outer values present in the OSCOAP message,
   including the Object-Security option, SHALL be encoded as described
   in Section 3.1 of [RFC7252], where the delta is the difference to the
   previously included outer option value.

5.  The COSE Object

   This section defines how to use COSE [I-D.ietf-cose-msg] to wrap and
   protect data in the original CoAP message.  OSCOAP uses the untagged
   COSE_Encrypt0 structure with an Authenticated Encryption with
   Additional Data (AEAD) algorithm.  The key lengths, IV lengths, and
   maximum sequence number are algorithm dependent.

   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 is 128 bits, the length of
   nonce, Sender IV, and Recipient IV is 7 bytes.  The maximum Sequence
   Number is specified in Section 10.

   The nonce is constructed as described in Section 3.1 of
   [I-D.ietf-cose-msg], i.e. by padding the partial IV (Sequence Number
   in network byte order) with zeroes and XORing it with the Context IV
   (Sender IV or Recipient IV), with the following addition: The most
   significant bit in the first byte of the Context IV SHALL be flipped
   for responses, in case there is a unique response (not Observe).  In
   this way, the same sequence number can be reused for requests and
   corresponding responses, which reduces the size of the responses in
   the most common case.  For detailed processing instructions, see
   Section 7.

   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.

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

   o  The "protected" field is empty.

   o  The "unprotected" field includes:

      *  The "Partial IV" parameter.  The value is set to the Sequence
         Number.  The Partial IV SHALL be of minimum length needed to
         encode the sequence number.  This parameter SHALL be present in



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         requests.  In case of Observe (Section 4.3.2.1) the Partial IV
         SHALL be present in the response, and otherwise the Partial IV
         SHALL NOT be present in the response.

      *  The "kid" parameter.  The value is set to the Sender ID (see
         Section 3).  This parameter SHALL be present in requests and
         SHALL NOT be present in responses.

   o  The "ciphertext" field is computed from the Plaintext (see
      Section 5.1) and the Additional Authenticated Data (AAD) (see
      Section 5.2) following Section 5.2 of [I-D.ietf-cose-msg].

   The encryption process is described in Section 5.3 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 Class E option values Section 4.3.1 present in the original
      CoAP message (see Section 4.3).  The options are encoded as
      described in Section 3.1 of [RFC7252], where the delta is the
      difference to the previously included Class E option; and

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Class E options (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 external_aad SHALL be a CBOR array as defined below:








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   external_aad = [
      ver : uint,
      code : uint,
      options : bstr,
      alg : int,
      request_kid : bstr,
      request_seq : bstr
   ]

   where:

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

   o  code: contains is the CoAP Code of the original CoAP message, as
      defined in Section 3 of [RFC7252].

   o  options: contains the Class I options Section 4.3.2 present in the
      original CoAP message encoded as described in Section 3.1 of
      [RFC7252], where the delta is the difference to the previously
      included class I option

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

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

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

6.  Sequence Numbers, Replay, Message Binding, and Freshness

   Sequence numbers and replay window are initialized as defined in
   Section 3.2.2.

6.1.  AEAD Nonce Uniqueness

   An AEAD nonce MUST NOT be used more than once per AEAD key.  In order
   to assure unique nonces, each Sender Context contains a Sequence
   Number used to protect requests, and - in case of Observe -
   responses.  The maximum sequence number is algorithm dependent, see
   Section 10.  If the Sequence Number exceeds the maximum sequence
   number, the endpoint MUST NOT process any more messages with the
   given Sender Context.  The endpoint SHOULD acquire a new security
   context (and consequently inform the other endpoint) before this
   happens.  The latter is out of scope of this document.




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6.2.  Replay Protection

   In order to protect from replay of messages, each Recipient Context
   contains a Replay Window used to verify request, and - in case of
   Observe - responses.  A receiving endpoint SHALL verify that a
   Sequence Number (Partial IV) received in the COSE object has not been
   received before in the Recipient Context.  For requests, if this
   verification fails and the message received is a CON message, the
   server SHALL respond with a 4.00 Bad Request error message.  The
   diagnostic payload MAY contain the "Replay protection failed" string.
   For responses, if this verification fails and the message received is
   a CON message, the client SHALL respond with an empty ACK and stop
   processing the response.

   The size and type of the Replay Window depends on the use case and
   lower protocol layers.  In case of reliable and ordered transport
   from endpoint to endpoint, the recipient MAY just store the last
   received sequence number and require that newly received Sequence
   Numbers equals the last received Sequence Number + 1.

6.3.  Sequence Number and Replay Window State

   To prevent reuse of the Nonce/Sequence Number with the same key, or
   from accepting replayed messages, a node needs to handle the
   situation of suddenly losing sequence number and replay window state
   in RAM, e.g. as a result of a reboot.

   After boot, a node MAY reject to use existing security contexts from
   before it booted and MAY establish a new security context with each
   party it communicates, e.g. using EDHOC
   [I-D.selander-ace-cose-ecdhe].  However, establishing a fresh
   security context may have a non-negligible cost in terms of e.g.
   power consumption.

   If a stored security context is to be used after reboot, then the
   node MUST NOT reuse a previous Sequence Number and MUST NOT accept
   previously accepted messages.

6.3.1.  The Basic Case

   To prevent reuse of Sequence Number, the node MAY perform the
   following procedure during normal operations:

   o  Before sending a message, the client stores in persistent memory a
      sequence number associated to the stored security context higher
      than any sequence number which has been or are being sent using
      this security context.  After boot, the client does not use any




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      lower sequence number in a request than what was persistently
      stored with that security context.

      *  Storing to persistent memory can be costly.  Instead of storing
         a sequence number for each request, the client may store Seq +
         K to persistent memory every K requests, where Seq is the
         current sequence number and K > 1.  This is a trade-off between
         the number of storage operations and efficient use of sequence
         numbers.

   To prevent accepting replay of previously received messages, the node
   MAY perform the following procedure:

   o  After boot, before verifying a message using a security context
      stored before boot, the server synchronizes the replay window so
      that no old messages are being accepted.  The server uses the
      Repeat option [I-D.amsuess-core-repeat-request-tag] for
      synchronizing the replay window: For each stored security context,
      the first time after boot the server receives an OSCOAP request,
      it generates a pseudo-random nonce and responds with the Repeat
      option set to the nonce as described in
      [I-D.amsuess-core-repeat-request-tag].  If the server receives a
      repeated OSCOAP request containing the Repeat option and the same
      nonce, and if the server can verify the request, then the sequence
      number obtained in the repeated message is set as the lower limit
      of the replay window.

6.3.2.  The Observe Case

   To prevent reuse of Sequence Number in case of Observe, the node MAY
   perform the following procedure during normal operations:

   o  Before sending a notification, the server stores in persistent
      memory a sequence number associated to the stored security context
      higher than any sequence number for which a notification has been
      or are being sent using this security context.  After boot, the
      server does not use any lower sequence number in an Observe
      response than what was persistently stored with that security
      context.

      *  Storing to persistent memory can be costly.  Instead of storing
         a sequence number for each notification, the server may store
         Seq + K to persistent memory every K requests, where Seq is the
         current sequence number and K > 1.  This is a trade-off between
         the number of storage operations and efficient use of sequence
         numbers.





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   Note that a client MAY continue an ongoing observation after reboot
   using a stored security context.  With Observe, the client can only
   verify the order of the notifications, as they may be delayed.  If
   the client wants to synchronize with a server resource it MAY restart
   an observation.

6.4.  Freshness

   For responses without Observe, OSCOAP provides absolute freshness.
   For requests, and responses with Observe, OSCOAP 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].

6.5.  Delay and Mismatch Attacks

   In order to prevent response delay and mismatch attacks
   [I-D.mattsson-core-coap-actuators] from on-path attackers and
   compromised proxies, OSCOAP binds responses to the request by
   including the request's ID (Sender ID or Recipient ID) and sequence
   number in the AAD of the response.  The server therefore needs to
   store the request's ID (Sender ID or Recipient ID) and sequence
   number until all responses have been sent.

7.  Processing

7.1.  Protecting the Request

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

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

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

   3.  Compose the AEAD nonce by XORing the Context IV (Sender IV) with
       the partial IV (Sequence Number in network byte order).

   4.  Encrypt the COSE object using the Sender Key. Compress the COSE
       Object as specified in Section 8.

   5.  Format the OSCOAP message according to Section 4.  The Object-
       Security option is added, see Section 4.3.4.




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   6.  Store the association Token - Security Context.  The client SHALL
       be able to find the Recipient Context from the Token in the
       response.

   7.  Increment the Sequence Number by one.

7.2.  Verifying the Request

   A server receiving a request containing the Object-Security option
   SHALL perform the following steps:

   1.  Process outer Block options according to [RFC7959], until all
       blocks of the request have been received, see Section 4.3.1.2.

   2.  Decompress the COSE Object (Section 8) and retrieve the Recipient
       Context associated with the Recipient ID in the 'kid' parameter.
       If the request is a CON message, and:

       *  either the decompression or the COSE message fails to decode,
          the server SHALL respond with a 4.02 Bad Option error message.
          The diagnostic payload SHOULD contain the string "Failed to
          decode COSE".

       *  the server fails to retrieve a Recipient Context with
          Recipient ID corresponding to the 'kid' parameter received,
          the server SHALL respond with a 4.01 Unauthorized error
          message.  The diagnostic payload MAY contain the string
          "Security context not found".

   If the request is a NON message and either the decompression or the
   COSE message fails to decode, or the server fails to retrieve a
   Recipient Context with Recipient ID corresponding to the 'kid'
   parameter received, then the server SHALL stop processing the
   request.

   1.  Verify the Sequence Number in the 'Partial IV' parameter, as
       described in Section 6.

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

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

   4.  Decrypt the COSE object using the Recipient Key.





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       *  If decryption fails, the server MUST stop processing the
          request and, if the request is a CON message, the server MUST
          respond with a 4.00 Bad Request error message.  The diagnostic
          payload MAY contain the "Decryption failed" string.

       *  If decryption succeeds, update the Recipient Replay Window, as
          described in Section 6.

   5.  Add decrypted options and payload to the decrypted request,
       processing the E options as described in (Section 4).  The
       Object-Security option is removed.

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

7.3.  Protecting the Response

   Given a CoAP response, the server SHALL perform the following steps
   to create an OSCOAP response:

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

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

   3.  Compose the AEAD nonce

       *  If Observe is not used, compose the AEAD nonce by XORing the
          Context IV (Sender IV with the most significant bit in the
          first byte flipped) with the padded Partial IV parameter from
          the request.

       *  If Observe is used, compose the AEAD nonce by XORing the
          Context IV (Sender IV) with the Partial IV of the response
          (Sequence Number in network byte order).

   4.  Encrypt the COSE object using the Sender Key. Compress the COSE
       Object as specified in Section 8.

   5.  Format the OSCOAP message according to Section 4.  The Object-
       Security option is added, see Section 4.3.4.

   6.  If Observe is used, increment the Sequence Number by one.








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7.4.  Verifying the Response

   A client receiving a response containing the Object-Security option
   SHALL perform the following steps:

   1.  Process outer Block options according to [RFC7959], until all
       blocks of the OSCOAP message have been received, see
       Section 4.3.1.2.

   2.  Retrieve the Recipient Context associated with the Token.
       Decompress the COSE Object (Section 8).  If the response is a CON
       message and either the decompression or the COSE message fails to
       decode, then the client SHALL send an empty ACK back and stop
       processing the response.  If the response is a NON message and
       any of the previous conditions appear, then the client SHALL
       simply stop processing the response.

   1.  For Observe notifications, verify the Sequence Number in the
       'Partial IV' parameter as described in Section 6.

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

   3.  Compose the AEAD nonce

       *  If the Observe option is not present in the response, compose
          the AEAD nonce by XORing the Context IV (Recipient IV with the
          the most significant bit in the first byte flipped) with the
          padded Partial IV parameter from the request.

       *  If the Observe option is present in the response, compose the
          AEAD nonce by XORing the Context IV (Recipient IV) with the
          padded Partial IV parameter from the response.

   4.  Decrypt the COSE object using the Recipient Key.

       *  If decryption fails, the client MUST stop processing the
          response and, if the response is a CON message, the client
          MUST respond with an empty ACK back.

       *  If decryption succeeds and Observe is used, update the
          Recipient Replay Window, as described in Section 6.

   5.  Add decrypted options or payload to the decrypted response
       overwriting any outer E options (see Section 4).  The Object-
       Security option is removed.





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       *  If Observe is used, replace the Observe value with the 3 least
          significant bytes in the sequence number.

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

8.  OSCOAP Compression

   The Concise Binary Object Representation (CBOR) [RFC7049] combines
   very small message sizes with extensibility.  The CBOR Object Signing
   and Encryption (COSE) [I-D.ietf-cose-msg] uses CBOR to create compact
   encoding of signed and encrypted data.  COSE is however constructed
   to support a large number of different stateless use cases, and is
   not fully optimized for use as a stateful security protocol, leading
   to a larger than necessary message expansion.  In this section we
   define a simple stateless compression mechanism for OSCOAP, which
   significantly reduces the per-packet overhead.

8.1.  Encoding of the Object-Security Option

   The value of the Object-Security option SHALL be encoded as follows:

   o  The first byte MUST encode a set of flags and the length of the
      Partial IV parameter.

      *  The three least significant bits encode the Partial IV size.
         If their value is 0, the Partial IV is not present in the
         compressed message.

      *  The fourth least significant bit is set to 1 if the kid is
         present in the compressed message.

      *  The fifth-eighth least significant bits (= most significant
         half-byte) are reserved and SHALL be set to zero when not in
         use.

   o  The following n bytes (n being the value of the Partial IV size in
      the first byte) encode the value of the Partial IV, if the Partial
      IV is present (size not 0).

   o  The following byte encodes the size of the kid parameter, if the
      kid is present (flag bit set to 1)

   o  The following m bytes (m given by the previous byte) encode the
      value of the kid, if the kid is present (flag bit set to 1)

   o  The remainining bytes encode the ciphertext.





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   The presence of Partial IV and kid in requests and responses is
   specified in Section 5, and summarized in Figure 6.

              7 6 5 4 3 2 1 0
             +-+-+-+-+-+-+-+-+  k: kid flag bit
             |0 0 0 0|k|pivsz|  pivsz: Partial IV size (3 bits)
             +-+-+-+-+-+-+-+-+

           +-------+---------+------------+-----------+
           |       | Request | Resp with- | Resp with |
           |       |         | out observe| observe   |
           +-------+---------+------------+-----------+
           |     k |    1    |     0      |      0    |
           | pivsz |  > 0    |     0      |    > 0    |
           +-------+---------+------------+-----------+

                Figure 6: Flag byte for OSCOAP compression

8.2.  Examples

   This section provides examples of COSE Objects before and after
   OSCOAP compression.

8.2.1.  Example: Request

   Before compression:

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

   0x83 40 a2 04 41 25 06 41 05 4e ae a0 15 56 67 92
   4d ff 8a 24 e4 cb 35 b9 (24 bytes)

   After compression:

   First byte: 0b00001001 = 0x09

   0x09 05 01 25 ae a0 15 56 67 92 4d ff 8a 24 e4 cb
   35 b9 (18 bytes)

8.2.2.  Example: Response (without Observe)

   Before compression:





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

   0x83 40 a0 4e ae a0 15 56 67 92 4d ff 8a 24 e4 cb
   35 b9 (18 bytes)

   After compression:

   First byte: 0b00000000 = 0x00

   0x00 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
   (15 bytes)

8.2.3.  Example: Response (with Observe)

   Before compression:

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

   0x83 40 a1 06 41 07 4e ae a0 15 56 67 92 4d ff
   8a 24 e4 cb 35 b9 (21 bytes)

   After compression:

   First byte: 0b00000001 = 0x01

   0x01 07 ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9
   (16 bytes)

9.  Web Linking

   The use of OSCOAP MAY be indicated by a target attribute "osc" in a
   web link [RFC5988] to a CoAP resource.  This attribute is a hint
   indicating that the destination of that link is to be accessed using
   OSCOAP.  Note that this is simply a hint, it does not include any
   security context material or any other information required to run
   OSCOAP.

   A value MUST NOT be given for the "osc" attribute; any present value
   MUST be ignored by parsers.  The "osc" attribute MUST NOT appear more




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   than once in a given link-value; occurrences after the first MUST be
   ignored by parsers.

10.  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
   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 in client and server, which may be
   obtained e.g. by using EDHOC [I-D.selander-ace-cose-ecdhe] or the 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 number of messages (2^56).  Compatibility with
   CCM* is achieved by using the algorithm AES-CCM-16-64-128
   [I-D.ietf-cose-msg].




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   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 maximum sequence number to guarantee nonce uniqueness
   (Section 6.1) is algorithm dependent.  Using AES_CCM, with the
   maximum sequence number SHALL be 2^(min(nonce length in bits, 56) -
   1) - 1.  The "-1" in the exponent stems from the same partial IV and
   flipped bit of IV (Section 5) is used in request and response.  The
   compression algorithm (Section 8) assumes that the partial IV is 56
   bits or less (which is the reason for min(,) in the exponent).

   The inner block options enable the sender to split large messages
   into OSCOAP-protected blocks such that the receiving node can verify
   blocks before having received the complete message.  The outer block
   options allow for arbitrary proxy fragmentation operations that
   cannot be verified by the endpoints, but can by policy be restricted
   in size since the encrypted options allow for 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.



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11.  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.

   The unprotected options (Figure 4) may reveal privacy sensitive
   information.  In particular Uri-Host SHOULD NOT contain privacy
   sensitive information.

   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.

   Using the mechanisms described in Section 6.3 reveals when a device
   goes through a reboot.  This can be mitigated by the device storing
   the precise state of sender sequence number and recipient replay
   window on a clean shutdown.

12.  IANA Considerations

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

12.1.  CoAP Option Numbers Registry

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

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

12.2.  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]] (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:
          Goeran Selander <goran.selander@ericsson.com>

       Intended usage: COMMON

       Restrictions on usage: N/A

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

12.3.  CoAP Content Format Registration

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

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




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13.  Acknowledgments

   The following individuals provided input to this document: Christian
   Amsuess, Tobias Andersson, Carsten Bormann, Joakim Brorsson, Thomas
   Fossati, Martin Gunnarsson, Klaus Hartke, Jim Schaad, Marco Tiloca,
   and Malisa Vu&#269;ini&#263;.

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

14.  References

14.1.  Normative References

   [I-D.amsuess-core-repeat-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Repeat And
              Request-Tag", draft-amsuess-core-repeat-request-tag-00
              (work in progress), July 2017.

   [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>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [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>.

   [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>.

   [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>.







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   [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>.

14.2.  Informative References

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

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-greevenbosch-appsawg-
              cbor-cddl-10 (work in progress), March 2017.

   [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-02 (work in progress), January 2017.

   [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-06 (work in progress), March 2017.

   [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-09 (work in progress), May
              2017.

   [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.




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   [I-D.seitz-ace-oscoap-profile]
              Seitz, L., Gunnarsson, M., and F. Palombini, "OSCOAP
              profile of ACE", draft-seitz-ace-oscoap-profile-03 (work
              in progress), June 2017.

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-06 (work in progress), April 2017.

   [I-D.tiloca-core-multicast-oscoap]
              Tiloca, M., Selander, G., and F. Palombini, "Secure group
              communication for CoAP", draft-tiloca-core-multicast-
              oscoap-01 (work in progress), March 2017.

   [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.  Test Vectors

   TODO: This section needs to be updated.

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

   This example targets the scenario in Section 3.1 of
   [I-D.hartke-core-e2e-security-reqs] and illustrates a client
   requesting the alarm status from a server.









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

   Figure 7: Secure Access to Sensor.  Square brackets [ ... ] indicate
      a COSE object.  Curly brackets { ... } indicate encrypted data.

   Since the method (GET) doesn't allow payload, the Object-Security
   option carries the COSE object as its value.  Since the response code
   (Content) allows payload, the COSE object is carried as the CoAP
   payload.

   The COSE header of the request contains an identifier (5f),
   indicating which security context was used to protect the message and
   a sequence number (42).  The option Uri-Path ("alarm_status") and
   payload ("OFF") are encrypted.

   The server verifies that the sequence number has not been received
   before.  The client verifies that the response is bound to the
   request.

B.2.  Secure Subscribe to Sensor

   This example targets the scenario in Section 3.2 of
   [I-D.hartke-core-e2e-security-reqs] and illustrates a client
   requesting subscription to a blood sugar measurement resource (GET




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   /glucose), first receiving the value 220 mg/dl and then a second
   value 180 mg/dl.

 Client  Proxy  Server
    |      |      |
    +----->|      |            Code: 0.01 (GET)
    | GET  |      |           Token: 0x83
    |      |      |         Observe: 0
    |      |      | Object-Security: [kid:ca, seq:15,
    |      |      |                   {Uri-Path:"glucose"}]
    |      |      |         Payload: -
    |      |      |
    |      +----->|            Code: 0.01 (GET)
    |      | GET  |           Token: 0xbe
    |      |      |         Observe: 0
    |      |      | Object-Security: [kid:ca, seq:15,
    |      |      |                   {Uri-Path:"glucose"}]
    |      |      |         Payload: -
    |      |      |
    |      |<-----+            Code: 2.05 (Content)
    |      | 2.05 |           Token: 0xbe
    |      |      |         Observe: 000032
    |      |      | Object-Security: -
    |      |      |         Payload: [seq:32, {Content-Format:0, "220"}]
    |      |      |
    |<-----+      |            Code: 2.05 (Content)
    | 2.05 |      |           Token: 0x83
    |      |      |         Observe: 000032
    |      |      | Object-Security: -
    |      |      |         Payload: [seq:32, {Content-Format:0, "220"}]
   ...    ...    ...
    |      |      |
    |      |<-----+            Code: 2.05 (Content)
    |      | 2.05 |           Token: 0xbe
    |      |      |         Observe: 000036
    |      |      | Object-Security: -
    |      |      |         Payload: [seq:36, {Content-Format:0, "180"}]
    |      |      |
    |<-----+      |            Code: 2.05 (Content)
    | 2.05 |      |           Token: 0x83
    |      |      |         Observe: 000036
    |      |      | Object-Security: -
    |      |      |         Payload: [seq:36, {Content-Format:0, "180"}]
    |      |      |

      Figure 8: Secure Subscribe to Sensor.  Square brackets [ ... ]
    indicate a COSE object.  Curly brackets { ... } indicate encrypted
                                   data.



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   Since the method (GET) doesn't allow payload, the Object-Security
   option carries the COSE object as its value.  Since the response code
   (Content) allows payload, the COSE object is carried as the CoAP
   payload.

   The COSE header of the request contains an identifier (ca),
   indicating the security context used to protect the message and a
   Sequence Number (15).  The COSE header of the responses contains
   sequence numbers (32 and 36).  The options Content-Format (0) and the
   payload ("220" and "180"), are encrypted.  The Observe option is
   integrity protected.  The shown Observe values (000032 and 000036)
   are the ones that the client will see after OSCOAP processing.

   The server verifies that the sequence number has not been received
   before.  The client verifies that the sequence number has not been
   received before and that the responses are bound to the request.

Appendix C.  Object Security of Content (OSCON)

   TODO: This section needs to be updated.

   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
   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 a CoAP message into an "OSCON message" in the
   following way: the payload of the original CoAP message is wrapped by
   a COSE object, which replaces the payload and this then becomes the
   OSCON message.

   The original 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 original CoAP message includes a Content-Format option, then the
   COSE object shall include a protected 'content type' field, whose
   value is set to the original message Content-Format value.  The
   Content-Format option of the OSCOON message shall be replaced with
   "application/oscon" (Section 12)

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



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   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.  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.

   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.

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




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   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
              ]
            )

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

   Figure 9 summarizes these results.

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

       Figure 9: 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:






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             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 10 summarizes these results.

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

     Figure 10: Message overhead for a 5-byte Tid using 64 byte ECDSA
                                signature.

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
    ciphertext               # ciphertext including truncated 8-byte TAG
  ]
)

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

   Figure 11 summarizes these results.




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

     Figure 11: 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:

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

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

   Figure 12 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 12: Message overhead for a 5-byte Tid using AES-CCM
                         countersigned with ECDSA.

Authors' Addresses

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com



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   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB

   Email: francesca.palombini@ericsson.com


   Ludwig Seitz
   SICS Swedish ICT

   Email: ludwig@sics.se



































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