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ACE Working Group                                            G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: January 8, 2017                                     Ericsson AB
                                                                L. Seitz
                                                        SICS Swedish ICT
                                                           July 07, 2016


                    Object Security of CoAP (OSCOAP)
                 draft-selander-ace-object-security-05

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 January 8, 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 Derivation . . . . . . . . . . . . . . .   8
   4.  Protected CoAP Message Fields . . . . . . . . . . . . . . . .  10
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Additional Authenticated Data . . . . . . . . . . . . . .  14
   6.  Protecting CoAP Messages  . . . . . . . . . . . . . . . . . .  15
     6.1.  Replay and Freshness Protection . . . . . . . . . . . . .  15
     6.2.  Protecting the Request  . . . . . . . . . . . . . . . . .  16
     6.3.  Verifying the Request . . . . . . . . . . . . . . . . . .  17
     6.4.  Protecting the Response . . . . . . . . . . . . . . . . .  17
     6.5.  Verifying the Response  . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  CoAP Option Number Registration . . . . . . . . . . . . .  21
     9.2.  Media Type Registrations  . . . . . . . . . . . . . . . .  21
     9.3.  CoAP Content Format Registration  . . . . . . . . . . . .  22
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Overhead . . . . . . . . . . . . . . . . . . . . . .  24
     A.1.  Length of the Object-Security Option  . . . . . . . . . .  25
     A.2.  Size of the COSE Object . . . . . . . . . . . . . . . . .  25
     A.3.  Message Expansion . . . . . . . . . . . . . . . . . . . .  26
     A.4.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  27
     B.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  27
     B.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  29
   Appendix C.  Object Security of Content (OSCON) . . . . . . . . .  31
     C.1.  Overhead OSCON  . . . . . . . . . . . . . . . . . . . . .  32
     C.2.  MAC Only  . . . . . . . . . . . . . . . . . . . . . . . .  32
     C.3.  Signature Only  . . . . . . . . . . . . . . . . . . . . .  33
     C.4.  Authenticated Encryption with Additional Data (AEAD)  . .  34



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     C.5.  Symmetric Encryption with Asymmetric Signature (SEAS) . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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 reliable and unreliable transport, 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 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.
   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 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.

   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 example of option length is given in Appendix A.



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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 keying material and a key
   derivation function (KDF).

   The EDHOC protocol [I-D.selander-ace-cose-ecdhe] enables the
   establishment of forward secret keying material, 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 has a "Sender" part and a
   "Receiver" part.  The endpoint protects the messages sent using the
   Sender part of the context.  The endpoint verifies the message
   received using the Receiver part.  In communication between two
   endpoints, the Sender part of one endpoint matches the Receiver part
   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,   |            |  Receiver, |
                  |  Receiver  |            |  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
                         |                       |  Receiver
                         | response:             | Protect response with
                         |  [Token = Token1, ...]|  Sender
   Retrieve context with |<----------------------+
    Token = Token1       |                       |
   Verify request with   |                       |
    Receiver             |                       |

            Figure 3: Retrieval and use of the Security Context

   The security context structure contains the following parameters:

   o  Context Identifier (Cid).  Variable length byte string that
      identifies the security context.  Immutable.

   o  Algorithm (Alg).  Value that identifies the COSE AEAD algorithm to
      use for encryption.  Immutable.

   o  Sender Key. Byte string containing the symmetric key to protect
      messages to send.  Length is determined by Algorithm.  Immutable.

   o  Sender IV.  Byte string containing the static IV to protect
      messages to send.  Length is determined by Algorithm.  Immutable.

   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.  Initialized to 0.  Maximum value is
      determined by Algorithm.

   o  Receiver Key. Byte string containing the symmetric key to verify
      messages received.  Length is determined by the Algorithm.
      Immutable.




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   o  Receiver IV.  Byte string containing the static IV to verify
      messages received.  Length is determined by Algorithm.  Immutable.

   o  Receiver 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.  Initialized to 0.  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 default window size is 64.

   The ordered pair (Cid, 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 using the same security
   context.  The former server will then make the request using the
   Sender part of the context, the former client will verify the request
   using its Receiver part of the context etc.

3.2.  Security Context Derivation

   Given a shared secret keying material and a common key derivation
   function, the client and server can derive the security context
   necessary to run OSCOAP.  The procedure described here assumes that
   the keying material is uniformly random and that the key derivation
   function is HKDF [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 shared secret keying material, denoted traffic_secret_0, is
      uniformly pseudo-random of length at least equal to the output of
      the specified hash function

   The security context parameters SHALL be derived using the HKDF-
   Expand primitive [RFC5869]:

   Key = HKDF-Expand(traffic_secret_0, info, key_length),



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   where:

   o  traffic_secret_0 is defined above

   o  info = "Party U Key" / "Party U IV" / "Party V Key" / "Party V IV"

   o  key_length is the key size of the AEAD algorithm

   The party being initially client SHALL use "Party U" info to derive
   Sender keying material and "Party V" info to derive Receiver keying
   material, and vice versa for the server.

   With the mandatory OSCOAP 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 static IVs is 56 bits.

   The Context Identifier SHALL be unique for all security contexts used
   by the party being server.  This can be achieved by the server, or
   trusted third party, assigning identifiers in a non-colliding way.
   In case it is acceptable for the application that the client and
   server switch roles, the application SHALL also ensure that the
   Context Identifier is unique for all contexts used by the party being
   the client.  This can be achieved by storing the Cid paired with some
   sort of communication identifier (e.g. the server's address).

   The size of Cid depends on the number of simultaneous clients, and
   must be chosen so that the server can uniquely identify the
   requesting client.  Cids of different lengths can be used by
   different clients.  In the case of an ACE-based authentication and
   authorization model [I-D.ietf-ace-oauth-authz], the Authorization
   Server can define the context identifier of all clients interacting
   with a particular server, in which case the size of Cid can be
   proportional to the logarithm of the number of authorized clients.
   It is RECOMMENDED to start assigning Cids of length 1 byte (0x00,
   0x01, ..., 0xff), and then when all 1 byte Cids are in use, start
   handling out Cids with a length of two bytes (0x0000, 0x0001, ...,
   0xffff), and so on.  Note that a Cid with the value 0x00 is
   considered different from the Cid with the value 0x0000.

   In case of EDHOC, party V (typically the server) can use the key
   identifier of its ephemeral public key (kid_ev, Section 1.1 of
   [I-D.selander-ace-cose-ecdhe]) to label the derived keying material,
   traffic_secret_0, and to identify the security context derived from
   traffic_secret_0.  In this case, Cid would be assigned the value
   kid_ev.

   Alternatively, the derivation scheme above MAY be used to derive a
   random context identifier (using info = "Context Identifier".  In



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   this case key_length SHALL be sufficiently large so that accidental
   collisions are negligible given the number of security contexts being
   derived in this way.

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.

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




















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   +----+---+---+---+---+----------------+--------+--------+---+---+---+
   | No.| C | U | N | R | Name           | Format | Length | E | I | D |
   +----+---+---+---+---+----------------+--------+--------+---+---+---+
   |  1 | x |   |   | x | If-Match       | opaque | 0-8    | x | x |   |
   |  3 | x | x | - |   | Uri-Host       | string | 1-255  |   |   |   |
   |  4 |   |   |   | x | ETag           | opaque | 1-8    | x | x |   |
   |  5 | x |   |   |   | If-None-Match  | empty  | 0      | x | x |   |
   |  6 |   | x | - |   | Observe        | uint   | 0-3    | x | x | x |
   |  7 | x | x | - |   | Uri-Port       | uint   | 0-2    |   |   |   |
   |  8 |   |   |   | x | Location-Path  | string | 0-255  | x | x |   |
   | 11 | x | x | - | x | Uri-Path       | string | 0-255  | x | x |   |
   | 12 |   |   |   |   | Content-Format | uint   | 0-2    | x | x |   |
   | 14 |   | x | - |   | Max-Age        | uint   | 0-4    | x | x | x |
   | 15 | x | x | - | x | Uri-Query      | string | 0-255  | x | x |   |
   | 17 | x |   |   |   | Accept         | uint   | 0-2    | x | x |   |
   | 20 |   |   |   | x | Location-Query | string | 0-255  | x | x |   |
   | 23 | x | x | - | - | Block2         | uint   | 0-3    | x | x | x |
   | 27 | x | x | - | - | Block1         | uint   | 0-3    | x | 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, I=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.

   Specifications of new CoAP options SHOULD specify how they are
   processed with OSCOAP.  New COAP options SHOULD be encrypted and
   integrity protected.  New COAP options SHALL be integrity protected
   unless a proxy needs to change the option, and SHALL be encrypted
   unless a proxy needs to read the option.

   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 present 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



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   protected CoAP message, is called "duplicate".  The "encrypted" value
   of a duplicate option is intended for the destination endpoint and
   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 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 enable end-to-end secure fragmentation of payload
      into blocks and protected information about the fragmentation
      (block number, last block, etc.) such that 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.

   Specifications of new CoAP options SHALL define if the new option is
   duplicate and how it is processed with OSCOAP.  New COAP options
   SHOULD NOT be duplicate.

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_Encrypted structure with an Authenticated Encryption
   with Additional Data (AEAD) algorithm.

   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 Receiver Key SHALL be 128 bits, the length
   of IV, Sender IV, and Receiver IV SHALL be 7 bytes, and the maximum
   Sender Sequence Number and Receiver Sequence Number SHALL be 2^56-1.
   The IV is constructed using a Partial Initialization Vector exactly



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   like in Section 3.1 of [I-D.ietf-cose-msg], i.e. by padding the
   Sender Sequence Number or the Receiver Sequence Number with zeroes
   and XORing it with the static Sender IV or Receiver 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_Encrypted structure (Section 2. of
   [I-D.ietf-cose-msg]).  If the COSE object has a different structure,
   the receiver 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_Encrypted 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 Initialization Vector" 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).

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

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

5.1.  Plaintext

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

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




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   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 (see Figure 6):

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

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

   o  the "external_aad" includes, in the given order:

      *  the CoAP version number and Code of the message formatted as
         two bytes, see Figure 6.  This corresponds to the first two
         bytes of the CoAP header in the unprotected message with Type
         and Token Length bits set to 0 in the case of CoAP over UDP,
         but the same format is also used in case of CoAP over TCP;

      *  The Algorithm from the security context used for the exchange;

      *  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;

      *  the Transaction Identifier (Tid) of the associated CoAP
         request, if the message is a CoAP response (see Section 3), and

      *  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 [I-D.ietf-core-block].





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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Ver|0 0 0 0 0 0|      Code     |      Alg      |      ...      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~   request URI (if request) / request Tid (if response)        ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~   MAC of previous block (if Block1 or Block2 present)         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 6: Additional Authenticated Data

   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 Receiver
   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 Receiver
   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:

   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 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 static IV (Sender
          IV) with the partial IV (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 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 unprotected CoAP message has no Payload, then the value
          of the Object-Security option is the COSE object.  If the
          unprotected CoAP message has Payload, then the Object-Security




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          option is empty and the Payload of the protected CoAP message
          is the COSE object.

   The Client SHALL be able to find the correct security context 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 fragment to compute the following.

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

   4.  Retrieve the Receiver 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 Receiver 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.

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.





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   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 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 static IV (Sender
          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 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 unprotected CoAP message has no Payload, then the value
          of the Object-Security option is the COSE object.  If the
          unprotected CoAP message has Payload, then the Object-Security
          option is empty, and the Payload of the protected CoAP message
          is the COSE object.

   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.







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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 fragment to compute the following.

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

   4.  Retrieve the Receiver 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 Receiver 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
   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.




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   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.

   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 protected 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 keying 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-ocsoap-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 static IVs (Sender IV and Receiver IV)
   SHOULD be established between sender and receiver 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.

   If the receiver accepts any sequence number larger than the one
   previously received, 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



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   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.

9.1.  CoAP Option Number Registration

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

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

9.2.  Media Type Registrations

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




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       Type name: application

       Subtype name: 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

9.3.  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 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-14 (work in progress), June 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>.

11.2.  Informative References








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   [I-D.bormann-6lo-coap-802-15-ie]
              Bormann, D., "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-block]
              Bormann, D. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-20 (work in progress), April 2016.

   [I-D.seitz-ace-ocsoap-profile]
              Seitz, L., "OSCOAP profile of ACE", draft-seitz-ace-
              ocsoap-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>.

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.





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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 unprotected CoAP message has
   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 has 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 static IV it is not required to send the
         whole IV in the message.

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

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





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   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 7: OSCOAP message expansion

A.4.  Example

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

   In this example we assume an 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.)

   o  Seq: 225

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

   o  Tag is 8 bytes




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

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

                  Figure 8: Example of message expansion

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

   The COSE object encodes to a total size of 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 7.
   Note that in this example two bytes in the COSE overhead are used to
   encode the length of Cid and the length of Seq.

   Figure 9 summarizes these results.

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

        Figure 9: 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.

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



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

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

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






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   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>]
      |      |      |
      |<-----+      |            Code: 2.05 (Content)
      | 2.05 |      |           Token: 0x83
      |      |      |         Max-Age: 0



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

   Since the unprotected request message (GET) has 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
   Number has not been received before and that the response message is
   generated as a response to the subscribe request.






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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 receivers 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.




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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 7.

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

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

C.2.  MAC Only

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

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

   The object in COSE encoding gives:






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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 12 summarizes these results.

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

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

C.3.  Signature Only

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

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

   The object in COSE encoding gives:

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

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

   Figure 13 summarizes these results.





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

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

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 static 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_Encrypted_Tagged structure is used (Section 5.2 of
   [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:

     993(                         # COSE_Encrypted_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 14 summarizes these results.

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

     Figure 14: 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_Encrypted_Tagged message (see Section 3 of
   [I-D.ietf-cose-msg]).

   The object in COSE encoding gives:

     993(                         # COSE_Encrypted_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 15 summarizes these results.

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

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

Authors' Addresses

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

   Email: goran.selander@ericsson.com








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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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