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ACE Working Group                                            G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: April 14, 2017                                      Ericsson AB
                                                                L. Seitz
                                                        SICS Swedish ICT
                                                        October 11, 2016


                    Object Security of CoAP (OSCOAP)
                 draft-selander-ace-object-security-06

Abstract

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 14, 2017.

Copyright Notice

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

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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  The Object-Security Option  . . . . . . . . . . . . . . . . .   5
   3.  The Security Context  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Security Context Definition . . . . . . . . . . . . . . .   6
     3.2.  Security Context Establishment  . . . . . . . . . . . . .   9
       3.2.1.  Derivation of Sender Key/IV, Recipient Key/IV . . . .   9
       3.2.2.  Sequence Numbers and Replay Window  . . . . . . . . .  10
       3.2.3.  Context Identifier and Sender/Recipient ID  . . . . .  10
   4.  Protected CoAP Message Fields . . . . . . . . . . . . . . . .  11
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Additional Authenticated Data . . . . . . . . . . . . . .  15
   6.  Protecting CoAP Messages  . . . . . . . . . . . . . . . . . .  17
     6.1.  Replay and Freshness Protection . . . . . . . . . . . . .  17
     6.2.  Protecting the Request  . . . . . . . . . . . . . . . . .  18
     6.3.  Verifying the Request . . . . . . . . . . . . . . . . . .  19
     6.4.  Protecting the Response . . . . . . . . . . . . . . . . .  20
     6.5.  Verifying the Response  . . . . . . . . . . . . . . . . .  21
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Sid Registration  . . . . . . . . . . . . . . . . . . . .  24
     9.2.  CoAP Option Number Registration . . . . . . . . . . . . .  24
     9.3.  Media Type Registrations  . . . . . . . . . . . . . . . .  24
     9.4.  CoAP Content Format Registration  . . . . . . . . . . . .  25
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  26
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Overhead . . . . . . . . . . . . . . . . . . . . . .  28
     A.1.  Length of the Object-Security Option  . . . . . . . . . .  28
     A.2.  Size of the COSE Object . . . . . . . . . . . . . . . . .  28
     A.3.  Message Expansion . . . . . . . . . . . . . . . . . . . .  29
     A.4.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  30
     B.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  31
     B.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  32
   Appendix C.  Object Security of Content (OSCON) . . . . . . . . .  34



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     C.1.  Overhead OSCON  . . . . . . . . . . . . . . . . . . . . .  35
     C.2.  MAC Only  . . . . . . . . . . . . . . . . . . . . . . . .  35
     C.3.  Signature Only  . . . . . . . . . . . . . . . . . . . . .  36
     C.4.  Authenticated Encryption with Additional Data (AEAD)  . .  37
     C.5.  Symmetric Encryption with Asymmetric Signature (SEAS) . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   application protocol, designed for constrained nodes and networks
   [RFC7228].  CoAP specifies the use of proxies for scalability and
   efficiency.  At the same time CoAP references DTLS [RFC6347] for
   security.  Proxy operations on CoAP messages require DTLS to be
   terminated at the proxy.  The proxy therefore not only has access to
   the data required for performing the intended proxy functionality,
   but is also able to eavesdrop on, or manipulate any part of the CoAP
   payload and metadata, in transit between client and server.  The
   proxy can also inject, delete, or reorder packages without being
   protected or detected by DTLS.

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

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

   OSCOAP builds on CBOR Object Signing and Encryption (COSE)
   [I-D.ietf-cose-msg], providing end-to-end encryption, integrity, and
   replay protection.  The use of OSCOAP is signaled with the CoAP
   option Object-Security, also defined in this memo.  The solution
   transforms an unprotected CoAP message into a protected CoAP message
   in the following way: the unprotected CoAP message is protected by
   including payload (if present), certain options, and header fields in
   a COSE object.  The message fields that have been encrypted are
   removed from the message whereas the Object-Security option and the
   COSE object are added.  We call the result the "protected" CoAP
   message.  Thus OSCOAP is a security protocol based on the exchange of
   protected CoAP messages (see Figure 1).




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

                        Figure 1: Sketch of OSCOAP

   OSCOAP provides protection of CoAP payload, certain options, and
   header fields, as well as a secure binding between CoAP request and
   response messages, and freshness of requests and responses.  It may
   be used in extremely constrained settings, where DTLS cannot be
   supported.  Alternatively, OSCOAP can be combined with DTLS, thereby
   enabling end-to-end security of CoAP payload, in combination with
   hop-by-hop protection of the entire CoAP message, during transport
   between end-point and intermediary node.  Examples of the use of
   OSCOAP are given in Appendix B.

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

1.1.  Terminology

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

   Readers are expected to be familiar with the terms and concepts
   described in [RFC7252] and [RFC7641].

   Terminology for constrained environments, such as "constrained
   device", "constrained-node network", is defined in [RFC7228].

   Two different scopes of object security are defined:

   o  OSCOAP = object security of CoAP, signaled with the Object-
      Security option.




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   o  OSCON = object security of content, signaled with Content Format/
      Media Type set to application/oscon (defined in Appendix C).

2.  The Object-Security Option

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

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

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

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

                   Figure 2: The Object-Security Option

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

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

   o  If the unprotected message does not have payload, then the COSE
      object is the value of the Object-Security option and the length
      of the Object-Security option is equal to the size of the COSE
      object.  An endpoint receiving a CoAP message without payload,



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      that also contains an empty Object-Security option SHALL treat it
      as malformed and reject it.

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

3.  The Security Context

   OSCOAP uses COSE with an Authenticated Encryption with Additional
   Data (AEAD) algorithm.  The specification requires that client and
   server establish a security context to apply to the COSE objects
   protecting the CoAP messages.  In this section we define the security
   context, and also specify how to establish a security context in
   client and server based on common shared secret material and a key
   derivation function (KDF).

   The EDHOC protocol [I-D.selander-ace-cose-ecdhe] enables the
   establishment of secret material 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.

3.1.  Security Context Definition

   The security context is the set of information elements necessary to
   carry out the cryptographic operations in OSCOAP.  Each security
   context is identified by a Context Identifier.  A Context Identifier
   that is no longer in use can be reassigned to a new security context.

   For each endpoint, the security context is composed by a "Common
   Context", a "Sender Context" and a "Recipient Context".  The Common
   Context includes common security material.  The endpoint protects the
   messages sent using the Sender Context.  The endpoint verifies the
   messages received using the Recipient Context.  In communication
   between two endpoints, the Sender Context of one endpoint matches the
   Recipient Context of the other endpoint, and vice versa.  Note that,
   because of that, the two security contexts identified by the same
   Context Identifiers in the two endpoints are not the same, but they
   are partly mirrored.

   An example is shown in Figure 3.











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

            Figure 3: Retrieval and use of the Security Context

   The Common Context structure contains the following parameters:

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

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

   o  Base Key (base_key).  Byte string containing the key used to
      derive the security context Section 3.2.

   The Sender Context structure contains the following parameters:

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

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

   o  Sender IV.  Byte string containing the fixed portion of IV
      (context IV in [I-D.ietf-cose-msg]) to protect messages to send.



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      Length is determined by Algorithm.  Its value is immutable once
      the security context is established.

   o  Sender Sequence Number.  Non-negative integer enumerating the COSE
      objects that the endpoint sends, associated to the Context
      Identifier.  It is used for replay protection, and to generate
      unique IVs for the AEAD.  Maximum value is determined by
      Algorithm.

   The Recipient Context structure contains the following parameters:

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

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

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

   o  Recipient Sequence Number.  Non-negative integer enumerating the
      COSE objects received, associated to the Context Identifier.  It
      is used for replay protection, and to generate unique IVs for the
      AEAD.  Maximum value is determined by Algorithm.

   o  Replay Window.  The replay protection window for messages
      received, equivalent to the functionality described in
      Section 4.1.2.6 of [RFC6347].

   The 3-tuple (Cid, Sender ID, Sender Sequence Number) is called
   Transaction Identifier (Tid), and SHALL be unique for each COSE
   object and server.  The Tid is used as a unique challenge in the COSE
   object of the protected CoAP request.  The Tid is part of the
   Additional Authenticated Data (AAD, see Section 5) of the protected
   CoAP response message, which is how the challenge becomes signed by
   the server.

   The client and server may change roles while maintaining the same
   security context.  The former server will then make the request using
   the Sender Context, the former client will verify the request using
   its Recipient Context etc.







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3.2.  Security Context Establishment

   This section aims at describing how to establish the security
   context, given some input parameters.  The input parameters, which
   are established in a previous phase, are:

   o  Context Identifier (Cid)

   o  Algorithm (Alg)

   o  Base Key (base_key)

   o  Sender ID

   o  Recipient ID

   o  Replay Window (optionally)

   These are included unchanged in the security context.  We give below
   some indications on how applications should select these parameters.
   Moreover, the following parameters are established as described
   below:

   o  Sender Key

   o  Sender IV

   o  Sender Sequence Number

   o  Recipient Key

   o  Recipient IV

   o  Recipient Sequence Number

   o  Replay Window

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

   Given a common shared secret material and a common key derivation
   function, the client and server can derive the security context
   necessary to run OSCOAP.  The derivation procedure described here
   MUST NOT be executed more than once on a set of common secret
   material.  Also, the same base_key SHOULD NOT be used in different
   security contexts (identified by different Cids).

   The procedure assumes that the common shared secret material is
   uniformly random and that the key derivation function is HKDF



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   [RFC5869].  This is for example the case after having used EDHOC
   [I-D.selander-ace-cose-ecdhe].

   Assumptions:

   o  The hash function, denoted HKDF, is the HMAC based key derivation
      function defined in [RFC5869] with specified hash function

   o  The common shared secret material, denoted base_key, is uniformly
      pseudo-random of length at least equal to the output of the
      specified hash function

   The security context parameters Sender Key/IV, Recipient Key/IV SHALL
   be derived using the HKDF-Expand primitive [RFC5869]:

   output parameter = HKDF-Expand(base_key, info, key_length),

   where:

   o  base_key is defined above

   o  info = Cid || Sender ID/Recipient ID || "IV"/"Key" || Algorithm ||
      key_length

   o  key_length is the key size of the AEAD algorithm

   The Sender/Recipient Key shall be derived using the Cid concatenated
   with the Sender/Recipient ID, the label "Key", the Algorithm and the
   key_length.  The Sender/Recipient IV shall be derived using the Cid
   concatenated with the Sender/Recipient ID, the label "IV", the
   Algorithm and the key_length.

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

3.2.2.  Sequence Numbers and Replay Window

   The values of the Sequence Numbers are initialized to 0 during
   establishment of the security context.  The default Replay Window
   size of 64 is used if no input parameter is provided in the set up
   phase.

3.2.3.  Context Identifier and Sender/Recipient ID

   As mentioned, Cid, Sender ID and Recipient ID are established in a
   previous phase.  How this is done is application specific, but some
   guidelines are given in this section.



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

   In the same phase during which the Cid is established in the
   endpoint, the application informs the endpoint what resource can be
   accessed using the corresponding security context.  The granularity
   of that is decided by the application (resource, host, etc).  The
   endpoint SHALL save the association resource-Cid, in order to be able
   to retrieve the correct security context to access a resource.

   The Sender ID and Recipient ID are also established in the endpoint
   during the previous set up phase.  The application SHOULD make sure
   that these identifiers are locally unique in the set of all endpoints
   using the same security context.  If it is not the case, it is again
   the role of the application to specify how to handle collisions.

   In case of EDHOC [I-D.selander-ace-cose-ecdhe]) the Cid is the hash
   of the messages exchanged.

4.  Protected CoAP Message Fields

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

   The CoAP Payload SHALL be encrypted and integrity protected.

   The CoAP Header fields Version and Code SHALL be integrity protected
   but not encrypted.  The CoAP Message Layer parameters, Type and
   Message ID, as well as Token and Token Length SHALL neither be
   integrity protected nor encrypted.

   Protection of CoAP Options can be summarized as follows:

   o  To prevent information leakage, Uri-Path and Uri-Query SHALL be
      encrypted.  As a consequence, if Proxy-Uri is used, those parts of
      the URI SHALL be removed from the Proxy-Uri. The CoAP Options Uri-
      Host, Uri-Port, Proxy-Uri, and Proxy-Scheme SHALL neither be
      encrypted, nor integrity protected (cf. protection of the
      effective request URI in Section 5.2).

   o  The other CoAP options SHALL be encrypted and integrity protected.





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   A summary of which options are encrypted or integrity protected is
   shown in Figure 4.

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

                   Figure 4: Protection of CoAP Options

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

   The encrypted options are in general omitted from the protected CoAP
   message and not visible to intermediary nodes (see Section 6.2 and
   Section 6.4).  Hence the actions resulting from the use of
   corresponding options is analogous to the case of communicating
   directly with the endpoint.  For example, a client using an ETag
   option will not be served by a proxy.

   However, some options which are encrypted need to be readable in the
   protected CoAP message to support certain proxy functions.  A CoAP
   option which may be both encrypted in the COSE object of the
   protected CoAP message, and also unencrypted as CoAP option in the
   protected CoAP message, is called "duplicate".  The "encrypted" value
   of a duplicate option is intended for the destination endpoint and




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   the "unencrypted" value is intended for a proxy.  The unencrypted
   value is not integrity protected.

   o  The Max-Age option is duplicate.  The unencrypted Max-Age SHOULD
      have value zero to prevent caching of responses.  The encrypted
      Max-Age is used as defined in [RFC7252] taking into account that
      it is not accessible to proxies.

   o  The Observe option is duplicate.  If Observe is used, then the
      encrypted Observe and the unencrypted Observe SHALL have the same
      value.  The Observe option as used here targets the requirements
      on forwarding of [I-D.hartke-core-e2e-security-reqs]
      (Section 2.2.1.2).

   o  The block options Block1 and Block2 are duplicate.  The encrypted
      block options is used for end-to-end secure fragmentation of
      payload into blocks and protected information about the
      fragmentation (block number, last block, etc.).  The MAC from each
      block is included in the calculation of the MAC for the next
      block's (see Section 5.2).  In this way, each block in ordered
      sequence from the first block can be verified as it arrives.  The
      unencrypted block option allows for arbitrary proxy fragmentation
      operations which cannot be verified by the endpoints.  An
      intermediary node can generate an arbitrarily long sequence of
      blocks.  However, since it is possible to protect fragmentation of
      large messages, there SHALL be a security policy defining a
      maximum unfragmented message size such that messages exceeding
      this size SHALL be fragmented by the sending endpoint.  Hence an
      endpoint receiving fragments of a message that exceeds maximum
      message size SHALL discard this message.

   o  The size options Size1 and Size2 are duplicate, analogously to the
      block options.

   Specifications of new CoAP options SHOULD specify how they are
   processed with OSCOAP.  New COAP options SHALL be encrypted and
   integrity protected.  New COAP options SHOULD NOT be duplicate unless
   a forwarding proxy needs to read the option.  If an option is
   registered as duplicate, the duplicate value SHOULD NOT be the same
   as the end-to-end value, unless the proxy is required by
   specification to be able to read the end-to-end value.

5.  The COSE Object

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



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   The mandatory to support AEAD algorithm is AES-CCM-64-64-128 defined
   in Section 10.2 of [I-D.ietf-cose-msg].  For AES-CCM-64-64-128 the
   length of Sender Key and Recipient Key SHALL be 128 bits, the length
   of IV, Sender IV, and Recipient IV SHALL be 7 bytes, and the maximum
   Sender Sequence Number and Recipient Sequence Number SHALL be 2^56-1.
   The IV is constructed using a Partial IV exactly like in Section 3.1
   of [I-D.ietf-cose-msg], i.e. by padding the Sender Sequence Number or
   the Recipient Sequence Number with zeroes and XORing it with the
   Sender IV or Recipient IV, respectively.

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

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

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

   o  The "Headers" field is formed by:

      *  The "protected" field, which SHALL include:

         +  The "Partial IV" parameter.  The value is set to the Sender
            Sequence Number.  The Partial IV is a byte string (type:
            bstr), where the length is the minimum length needed to
            encode the sequence number.  An Endpoint that receives a
            COSE object with a sequence number encoded with leading
            zeroes (i.e. longer than the minimum needed length) SHALL
            reject the corresponding message as malformed.

         +  If the message is a CoAP request, the "kid" parameter.  The
            value is set to the Context Identifier (see Section 3).

         +  Optionally, the parameter called "sid", defined below.  The
            value is set to the Sender ID (see Section 3).  Note that
            since this parameter is sent in clear, privacy issues SHOULD
            be considered by the application defining the Sender ID.

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

   o  The "cipher text" field is computed from the Plaintext (see
      Section 5.1) and the Additional Authenticated Data (AAD) (see




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      Section 5.2) and encoded as a byte string (type: bstr), following
      Section 5.2 of [I-D.ietf-cose-msg].

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

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

                 Table 1: Additional COSE Header Parameter

5.1.  Plaintext

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

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

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Options to Encrypt (if any) ...                            ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1|    Payload (if any) ...                       ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      (only if there
        is payload)

                            Figure 5: Plaintext

5.2.  Additional Authenticated Data

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

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




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

   o  the "external_aad" is a serialized CBOR array (see Figure 8) that
      contains, in the given order:

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

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

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

      *  request-uri: tstr, contains the plaintext "effective" request
         URI composed from the request scheme and Uri-* options
         according to the method described in Section 6.5 of [RFC7252],
         if the message is a CoAP request;

      *  transaction-id: bstr, only included if the message to protect
         or verify is a CoAP response, contains the Transaction
         Identifier (Tid) of the associated CoAP request (see
         Section 3).  Note that the Tid is the 3-tuple (Cid, Sender ID,
         Sender Sequence Number) for the endpoint sending the request
         and verifying the response; which means that for the endpoint
         sending the response, the Tid has value (Cid, Recipient ID,
         seq), where seq is the value of the "Partial IV" in the COSE
         object of the request (see Section 5); and

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

                      external_aad_req = [
                          ver : uint,
                          code : bstr,
                          alg : bstr,
                          request-uri : tstr,
                          ? mac-previous-block : bstr
                      ]

                   Figure 6: external_aad for a request






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                      external_aad_resp = [
                          ver : uint,
                          code : bstr,
                          alg : bstr,
                          transaction-id : bstr,
                          ? mac-previous-block : bstr
                      ]

                   Figure 7: external_aad for a response

            external_aad = external_aad_req / external_aad_resp

                          Figure 8: external_aad

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

6.  Protecting CoAP Messages

6.1.  Replay and Freshness Protection

   In order to protect from replay of messages and verify freshness, a
   CoAP endpoint SHALL maintain a Sender Sequence Number, and a
   Recipient Sequence Number associated to a security context, which is
   identified with a Context Identifier (Cid).  The two sequence numbers
   are the highest sequence number the endpoint has sent and the highest
   sequence number the endpoint has received.  An endpoint uses the
   Sender Sequence Number to protect messages to send and the Recipient
   Sequence Number to verify received messages, as described in
   Section 3.

   Depending on use case and ordering of messages provided by underlying
   layers, an endpoint MAY maintain a sliding replay window for Sequence
   Numbers of received messages associated to each Cid. In case of
   reliable transport, the receiving endpoint MAY require that the
   Sequence Number of a received message equals last Sequence Number +
   1.

   A receiving endpoint SHALL verify that the Sequence Number received
   in the COSE object has not been received before in the security
   context identified by the Cid. The receiving endpoint SHALL also
   reject messages with a sequence number greater than 2^56-1.

   OSCOAP is a challenge-response protocol, where the response is
   verified to match a prior request, by including the unique
   transaction identifier (Tid as defined in Section 3) of the request
   in the Additional Authenticated Data of the response message.




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

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

6.2.  Protecting the Request

   Given an unprotected CoAP request, including header, options and
   payload, the client SHALL perform the following steps to create a
   protected CoAP request using a security context associated with the
   target resource (see Section 3.2.3).

   1.  Increment the Sender Sequence Number by one (note that this means
       that sequence number 0 is never used).  If the Sender Sequence
       Number exceeds the maximum number for the AEAD algorithm, the
       client MUST NOT process any requests with the given security
       context.  The client SHOULD acquire a new security context (and
       consequently inform the server about it) before this happens.
       The latter is out of scope of this memo.

   2.  Compute the COSE object as specified in Section 5

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

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

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

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

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

       *  The CoAP options which are encrypted and not duplicate
          (Section 4) are removed.  Any duplicate option which is
          present has its unencrypted value.  The Object-Security option
          is added.



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

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

6.3.  Verifying the Request

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

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

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

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

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

   4.  Retrieve the Recipient Key.

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

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

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




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6.4.  Protecting the Response

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

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

   1.  Increment the Sender Sequence Number by one (note that this means
       that sequence number 0 is never used).  If the Sender Sequence
       Number exceeds the maximum number for the AEAD algorithm, the
       server MUST NOT process any more responses with the given
       security context.  The server SHOULD acquire a new security
       context (and consequently inform the client about it) before this
       happens.  The latter is out of scope of this memo.

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

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

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

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

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

       *  The CoAP options which are encrypted and not duplicate
          (Section 4) are removed.  Any duplicate option which is
          present has its unencrypted value.  The Object-Security option
          is added.

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




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

6.5.  Verifying the Response

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

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

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

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

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

   4.  Retrieve the Recipient Key.

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

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

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

7.  Security Considerations

   In scenarios with intermediary nodes such as proxies or brokers,
   transport layer security such as DTLS only protects data hop-by-hop.
   As a consequence the intermediary nodes can read and modify
   information.  The trust model where all intermediate nodes are
   considered trustworthy is problematic, not only from a privacy
   perspective, but also from a security perspective, as the



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

   For symmetric encryption it is required to have a unique IV for each
   message, for which the sequence numbers in the COSE message field
   "Partial IV" is used.  The context IVs (Sender IV and Recipient IV)
   SHOULD be established between sender and recipient before the message
   is sent, for example using the method in
   [I-D.selander-ace-cose-ecdhe], to avoid the overhead of sending it in
   each message.

   The MTI AEAD algorithm AES-CCM-64-64-128 is selected for broad
   applicability in terms of message size (2^64 blocks) and maximum no.
   messages (2^56-1).  For 128 bit CCM*, use instead AES-CCM-16-64-128
   [I-D.ietf-cose-msg].



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   If the recipient accepts any sequence number larger than the one
   previously received (less than the maximum sequence number), 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 IVs.  The requirement to change key at
   counter wrap is a complication, but it also forces the user of this
   specification to think about implementing key renewal.

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

   The unencrypted block options allow for arbitrary proxy fragmentation
   operations which 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.

8.  Privacy Considerations

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

   CoAP headers sent in plaintext allow for example matching of CON and
   ACK (CoAP Message Identifier), matching of request and responses
   (Token) and traffic analysis.

9.  IANA Considerations

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







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9.1.  Sid Registration

   IANA is requested to enter a new parameter entitled "sid" to the
   registry "COSE Header Parameters".  The parameter is defined in
   Table 1.

9.2.  CoAP Option Number Registration

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

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

9.3.  Media Type Registrations

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






























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       Type name: application

       Subtype name: cose

       Required parameters: N/A

       Optional parameters: N/A

       Encoding considerations: binary

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

       Interoperability considerations: N/A

       Published specification: [[this document]]

       Applications that use this media type: To be identified

       Fragment identifier considerations: N/A

       Additional information:

       * Magic number(s): N/A

       * File extension(s): N/A

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

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

       Intended usage: COMMON

       Restrictions on usage: N/A

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

       Change Controller: IESG

       Provisional registration? No

9.4.  CoAP Content Format Registration

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





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

10.  Acknowledgments

   Klaus Hartke has independently been working on the same problem and a
   similar solution: establishing end-to-end security across proxies by
   adding a CoAP option.  We are grateful to Malisa Vucinic and Marco
   Tiloca for providing helpful and timely reviews of previous versions
   of the draft.  We are also grateful to Carsten Bormann and Jim Schaad
   for providing input and interesting discussions.

11.  References

11.1.  Normative References

   [I-D.ietf-cose-msg]
              Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              draft-ietf-cose-msg-20 (work in progress), October 2016.

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

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

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

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

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





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11.2.  Informative References

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

   [I-D.hartke-core-e2e-security-reqs]
              Selander, G., Palombini, F., and K. Hartke, "Requirements
              for CoAP End-To-End Security", draft-hartke-core-e2e-
              security-reqs-01 (work in progress), July 2016.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-02 (work in progress), June 2016.

   [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-04 (work in progress), August
              2016.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L., "OSCOAP profile of ACE", draft-seitz-ace-
              oscoap-profile-00 (work in progress), July 2016.

   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-02 (work in progress), July 2016.

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

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







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Appendix A.  Overhead

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

A.1.  Length of the Object-Security Option

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

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

A.2.  Size of the COSE Object

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

   o  the Header parameters,

   o  the Cipher Text (excluding the Tag),

   o  the Tag, and

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

   Let's analyse the contributions one at a time:

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

      *  The size of Cid depends on the number of simultaneous clients,
         as discussed in Section 3.2

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

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




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

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

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

A.3.  Message Expansion

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

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

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

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

            Message Overhead = Cid + Seq + Tag + COSE Overhead

                    Figure 9: OSCOAP message expansion

A.4.  Example

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

   In this example we assume an extreme 4-byte Cid, based on the
   assumption of an ACE deployment with billions of clients requesting
   access to this particular server.  (A typical Cid, will be 1-2 byte
   as is discussed in Appendix A.2.)

   o  Cid: 0xa1534e3c

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



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   o  Seq: 225

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

   o  Tag is 8 bytes

   The COSE object is represented in Figure 10 using CBOR's diagnostic
   notation.

   [
     h'a20444a1534e3c0641e2', # protected:
                                {04:h'a1534e3c',
                                 06:h'e2'}
     {},                      # unprotected: -
     Tag                      # cipher text + 8 byte authentication tag
   ]

                  Figure 10: Example of message expansion

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

   The COSE object encodes to a total size of 22 bytes, which is the
   message expansion in this example.  The COSE overhead in this example
   is 22 - (4 + 1 + 8) = 9 bytes, according to the formula in Figure 9.
   Note that in this example two bytes in the COSE overhead are used to
   encode the length of Cid and the length of Seq.

   Figure 11 summarizes these results.

               +---------+---------+----------+------------+
               |   Tid   |   Tag   | COSE OH  | Message OH |
               +---------+---------+----------+------------+
               | 5 bytes | 8 bytes |  9 bytes |  22 bytes  |
               +---------+---------+----------+------------+

       Figure 11: Message overhead for a 5-byte Tid and 8-byte Tag.

Appendix B.  Examples

   This section gives examples of OSCOAP.  The message exchanges are
   made, based on the assumption that there is a security context
   established between client and server.  For simplicity, these
   examples only indicate the content of the messages without going into
   detail of the COSE message format.





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B.1.  Secure Access to Sensor

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

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

       Figure 12: Indication of CoAP GET protected with OSCOAP.  The
      brackets [ ... ] indicate a COSE object.  The brackets { ... }
                         indicate encrypted data.

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



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

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

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

B.2.  Secure Subscribe to Sensor

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

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



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      |      |      |
      |<-----+      |            Code: 2.05 (Content)
      | 2.05 |      |           Token: 0x83
      |      |      |         Max-Age: 0
      |      |      |         Observe: 1
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c2, {Observe:1,
      |      |      |                   Content-Format:0, "220"}, <Tag>]
     ...    ...    ...
      |      |      |
      |      |<-----+            Code: 2.05 (Content)
      |      | 2.05 |           Token: 0xbe
      |      |      |         Max-Age: 0
      |      |      |         Observe: 2
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c6, {Observe:2,
      |      |      |                   Content-Format:0, "180"}, <Tag>]
      |      |      |
      |<-----+      |            Code: 2.05 (Content)
      | 2.05 |      |           Token: 0x83
      |      |      |         Max-Age: 0
      |      |      |         Observe: 2
      |      |      | Object-Security: -
      |      |      |         Payload: [seq:32c6, {Observe:2,
      |      |      |                   Content-Format:0, "180"}, <Tag>]
      |      |      |

       Figure 13: Indication of CoAP GET protected with OSCOAP.  The
       brackets [ ... ] indicates COSE object.  The bracket { ... }
                         indicates encrypted data.

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

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

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

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




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   Number has not been received before and that the response message is
   generated as a response to the subscribe request.

Appendix C.  Object Security of Content (OSCON)

   OSCOAP protects message exchanges end-to-end between a certain client
   and a certain server, targeting the security requirements for forward
   proxy of [I-D.hartke-core-e2e-security-reqs].  In contrast, many use
   cases require one and the same message to be protected for, and
   verified by, multiple endpoints, see caching proxy section of
   [I-D.hartke-core-e2e-security-reqs].  Those security requirements can
   be addressed by protecting essentially the payload/content of
   individual messages using the COSE format ([I-D.ietf-cose-msg]),
   rather than the entire request/response message exchange.  This is
   referred to as Object Security of Content (OSCON).

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

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

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

   In the case of symmetric encryption, the same key and IV shall not be
   used twice.  Sequence numbers for partial IV as specified for OSCOAP
   may be used for replay protection as described in Section 6.1.  The
   use of time stamps in the COSE header parameter 'operation time'
   [I-D.ietf-cose-msg] for freshness may be used.

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

   The scenarios in Sections 3.3 - 3.5 of
   [I-D.hartke-core-e2e-security-reqs] assume multiple recipients for a
   particular content.  In this case the use of symmetric keys does not



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   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 ciphersuites 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 size of the COSE message for selected algorithms are detailed in
   this section.

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

   o  Cid: 0xa1534e3c

   o  Seq: 0xa3

   For each scheme, we indicate the fixed length of these two parameters
   ("Cid+Seq" column) and of the Tag ("MAC"/"SIG"/"TAG").  The "Message
   OH" column shows the total expansions of the CoAP message size, while
   the "COSE OH" column is calculated from the previous columns
   following the formula in Figure 9.

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

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

C.2.  MAC Only

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

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

   The object in COSE encoding gives:



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            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 14 summarizes these results.

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

      Figure 14: Message overhead for a 5-byte Tid using HMAC 256/64

C.3.  Signature Only

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

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

   The object in COSE encoding gives:

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

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

   Figure 15 summarizes these results.





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

     Figure 15: 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 MAC truncated to 8 bytes.

   It is assumed that the IV is generated from the Sequence Number and
   some previously agreed upon context IV.  This means it is not
   required to explicitly send the whole IV in the message.

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

   The object in COSE encoding gives:

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

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

   Figure 16 summarizes these results.

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

     Figure 16: Message overhead for a 5-byte Tid using AES_128_CCM_8.







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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
         TAG                      # cipher text + truncated 8-byte TAG
       ]
     )

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

   Figure 17 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 17: Message overhead for a 5-byte Tid using AES-CCM
                         countersigned with ECDSA.

Authors' Addresses

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

   Email: goran.selander@ericsson.com








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

   Email: john.mattsson@ericsson.com


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

   Email: francesca.palombini@ericsson.com


   Ludwig Seitz
   SICS Swedish ICT
   Scheelevagen 17
   Lund  22370
   Sweden

   Email: ludwig@sics.se


























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