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Versions: (draft-selander-ace-object-security) 00 01 02 03 04 05 06 07

CoRE Working Group                                           G. Selander
Internet-Draft                                               J. Mattsson
Intended status: Standards Track                            F. Palombini
Expires: May 24, 2018                                        Ericsson AB
                                                                L. Seitz
                                                        SICS Swedish ICT
                                                       November 20, 2017


     Object Security for Constrained RESTful Environments (OSCORE)
                   draft-ietf-core-object-security-07

Abstract

   This document defines Object Security for Constrained RESTful
   Environments (OSCORE), a method for application-layer protection of
   the Constrained Application Protocol (CoAP), using CBOR Object
   Signing and Encryption (COSE).  OSCORE provides end-to-end
   encryption, integrity and replay protection, as well as a secure
   message binding.  OSCORE is designed for constrained nodes and
   networks and can be used whereever CoAP can be used, and also with
   HTTP.  OSCORE may be used to protect group communications as is
   specified in a separate draft.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 24, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  The CoAP Object-Security Option . . . . . . . . . . . . . . .   5
   3.  The Security Context  . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Security Context Definition . . . . . . . . . . . . . . .   6
     3.2.  Establishment of Security Context Parameters  . . . . . .   9
     3.3.  Requirements on the Security Context Parameters . . . . .  11
   4.  Protected Message Fields  . . . . . . . . . . . . . . . . . .  11
     4.1.  CoAP Payload  . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  CoAP Options  . . . . . . . . . . . . . . . . . . . . . .  13
     4.3.  CoAP Header . . . . . . . . . . . . . . . . . . . . . . .  18
   5.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  19
     5.1.  Kid Context . . . . . . . . . . . . . . . . . . . . . . .  20
     5.2.  Nonce . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.3.  Plaintext . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.4.  Additional Authenticated Data . . . . . . . . . . . . . .  23
   6.  Sequence Numbers, Replay, Message Binding, and Freshness  . .  23
     6.1.  Message Binding . . . . . . . . . . . . . . . . . . . . .  23
     6.2.  AEAD Nonce Uniqueness . . . . . . . . . . . . . . . . . .  24
     6.3.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  24
     6.4.  Replay Protection . . . . . . . . . . . . . . . . . . . .  24
     6.5.  Losing Part of the Context State  . . . . . . . . . . . .  25
   7.  Processing  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     7.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  26
     7.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  27
     7.3.  Protecting the Response . . . . . . . . . . . . . . . . .  28
     7.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  29
   8.  OSCORE Compression  . . . . . . . . . . . . . . . . . . . . .  30
     8.1.  Encoding of the Object-Security Value . . . . . . . . . .  30
     8.2.  Encoding of the OSCORE Payload  . . . . . . . . . . . . .  32
     8.3.  Examples of Compressed COSE Objects . . . . . . . . . . .  32
   9.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  33
   10. Proxy Operations  . . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  CoAP-to-CoAP Forwarding Proxy  . . . . . . . . . . . . .  34
     10.2.  HTTP-to-CoAP Translation Proxy . . . . . . . . . . . . .  34
     10.3.  CoAP-to-HTTP Translation Proxy . . . . . . . . . . . . .  36
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  37
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  38



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   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
     13.1.  COSE Header Parameters Registry  . . . . . . . . . . . .  39
     13.2.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  39
     13.3.  Header Field Registrations . . . . . . . . . . . . . . .  40
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  40
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     15.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  43
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .  43
     B.1.  Secure Access to Sensor . . . . . . . . . . . . . . . . .  43
     B.2.  Secure Subscribe to Sensor  . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  46

1.  Introduction

   The Constrained Application Protocol (CoAP) is a web application
   protocol, designed for constrained nodes and networks [RFC7228].
   CoAP specifies the use of proxies for scalability and efficiency, and
   a mapping to HTTP is also specified [RFC8075].  CoAP [RFC7252]
   references DTLS [RFC6347] for security.  CoAP and HTTP proxies
   require (D)TLS to be terminated at the proxy.  The proxy therefore
   not only has access to the data required for performing the intended
   proxy functionality, but is also able to eavesdrop on, or manipulate
   any part of the message payload and metadata, in transit between the
   endpoints.  The proxy can also inject, delete, or reorder packets
   since they are no longer protected by (D)TLS.

   This document defines the Object Security for Constrained RESTful
   Environments (OSCORE) security protocol, protecting CoAP and CoAP-
   mappable HTTP requests and responses end-to-end across intermediary
   nodes such as CoAP forward proxies and cross-protocol translators
   including HTTP-to-CoAP proxies [RFC8075].  In addition to the core
   CoAP features defined in [RFC7252], OSCORE supports Observe
   [RFC7641], Blockwise [RFC7959], PATCH and FETCH [RFC8132].  An
   analysis of end-to-end security for CoAP messages through some types
   of intermediary nodes is performed in
   [I-D.hartke-core-e2e-security-reqs].  OSCORE protects the Request/
   Response layer only, and not the CoAP Messaging Layer (Section 2 of
   [RFC7252]).  Therefore, any Messaging Layer processing follows
   [RFC7252].  Additionally, since the message formats for CoAP over
   unreliable transport [RFC7252] and for CoAP over reliable transport
   [I-D.ietf-core-coap-tcp-tls] differ only in terms of Messaging Layer,
   OSCORE can be applied to both unreliable and reliable transports.

   OSCORE is designed for constrained nodes and networks and provides an
   in-layer security protocol that does not depend on underlying layers.
   OSCORE can be used anywhere where CoAP or HTTP can be used, including



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   non-IP transports (e.g., [I-D.bormann-6lo-coap-802-15-ie]).  An
   extension of OSCORE may also be used to protect group communication
   for CoAP [I-D.tiloca-core-multicast-oscoap].  The use of OSCORE does
   not affect the URI scheme and OSCORE can therefore be used with any
   URI scheme defined for CoAP or HTTP.  The application decides the
   conditions for which OSCORE is required.

   OSCORE builds on CBOR Object Signing and Encryption (COSE) [RFC8152],
   providing end-to-end encryption, integrity, replay protection, and
   secure the binding of response to request.  A compressed version of
   COSE is used, as discussed in Section 8.  The use of OSCORE is
   signaled with the Object-Security CoAP option or HTTP header, defined
   in Section 2 and Section 10.2.  OSCORE is designed to protect as much
   information as possible, while still allowing proxy operations
   (Section 10).  OSCORE provides protection of message payload, almost
   all CoAP options, and the RESTful method.  The solution transforms a
   message into an "OSCORE message" before sending, and vice versa after
   receiving.  The OSCORE message is related to the original message in
   the following way: the original message is translated to CoAP (if not
   already in CoAP) and the resulting message payload (if present),
   options not processed by a proxy, and the request/response method
   (CoAP Code) are protected in a COSE object.  The message fields of
   the original message that are encrypted are transported in the
   payload of the OSCORE message, and the Object-Security option is
   included, see Figure 1.

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

                   Figure 1: Sketch of CoAP with OSCORE

   OSCORE may be used in very constrained settings, thanks to its small
   message size and the restricted code and memory requirements in
   addition to what is required by CoAP.  OSCORE can be combined with
   transport layer security such as DTLS or TLS, thereby enabling end-
   to-end security of e.g.  CoAP Payload, Options and Code, in
   combination with hop-by-hop protection of the Messaging Layer, during



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   transport between end-point and intermediary node.  Examples of the
   use of OSCORE are given in Appendix B.

   An implementation supporting this specification MAY only implement
   the client part, MAY only implement the server part, or MAY only
   implement one of the proxy parts.  OSCORE is designed to work with
   legacy CoAP-to-CoAP forward proxies [RFC7252], but an OSCORE-aware
   proxy will be more efficient.  HTTP-to-CoAP proxies [RFC8075] and
   CoAP-to-HTTP proxies need to implement respective parts of this
   specification to work with OSCORE (see Section 10).

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 CoAP [RFC7252], Observe [RFC7641], Blockwise [RFC7959],
   COSE [RFC8152], CBOR [RFC7049], CDDL [I-D.ietf-cbor-cddl], and
   constrained environments [RFC7228].

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

2.  The CoAP Object-Security Option

   The CoAP Object-Security option (see Figure 2) indicates that the
   CoAP message is an OSCORE message and that it contains a compressed
   COSE object (see Section 5 and Section 8).  The Object-Security
   option is critical, safe to forward, part of the cache key, and not
   repeatable.

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

                   Figure 2: The Object-Security Option

   The Object-Security option includes the OSCORE flag byte (Section 8),
   the Sender Sequence Number and the Sender ID when present
   (Section 3).  The detailed format is specified in Section 8).  If the



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   OSCORE flag byte is all zero (0x00) the Option value SHALL be empty
   (Option Length = 0).  An endpoint receiving a CoAP message without
   payload, that also contains an Object-Security option SHALL treat it
   as malformed and reject it.

   A successful response to a request with the Object-Security option
   SHALL contain the Object-Security option.  Whether error responses
   contain the Object-Security option depends on the error type (see
   Section 7).

   Since the payload and most options are encrypted Section 4, and the
   corresponding plain text message fields of the original are not
   included in the OSCORE message, the processing of these fields does
   not expand the total message size.

   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, see Section 10.1.  As the compressed COSE
   Object is included in the cache key, messages with the Object-
   Security option will never generate cache hits.  For Max-Age
   processing, see Section 4.2.3.1.

3.  The Security Context

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

3.1.  Security Context Definition

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

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

   An endpoint uses its Sender ID (SID) to derive its Sender Context,
   and the other endpoint uses the same ID, now called Recipient ID
   (RID), to derive its Recipient Context.  In communication between two



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   endpoints, the Sender Context of one endpoint matches the Recipient
   Context of the other endpoint, and vice versa.  Thus, the two
   security contexts identified by the same IDs in the two endpoints are
   not the same, but they are partly mirrored.  Retrieval and use of the
   security context are shown in Figure 3.

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

            Figure 3: Retrieval and use of the Security Context

   The Common Context contains the following parameters:

   o  AEAD Algorithm (alg).  The COSE AEAD algorithm to use for
      encryption.  Its value is immutable once the security context is
      established.

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

   o  Master Secret.  Variable length, uniformly random byte string
      containing the key used to derive traffic keys and IVs.  Its value
      is immutable once the security context is established.

   o  Master Salt (OPTIONAL).  Variable length byte string containing
      the salt used to derive traffic keys and IVs.  Its value is
      immutable once the security context is established.






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   o  Common IV.  Byte string derived from Master Secret and Master
      Salt.  Length is determined by the AEAD Algorithm.  Its value is
      immutable once the security context is established.

   The Sender Context contains the following parameters:

   o  Sender ID.  Byte string used to identify the Sender Context and to
      assure unique nonces.  Maximum length is determined by the AEAD
      Algorithm.  Its value is immutable once the security context is
      established.

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

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

   The Recipient Context contains the following parameters:

   o  Recipient ID.  Byte string used to identify the Recipient Context
      and to assure unique nonces.  Maximum length is determined by the
      AEAD Algorithm.  Its value is immutable once the security context
      is established.

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

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

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

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




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

3.2.  Establishment of Security Context Parameters

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

   o  Master Secret

   o  Sender ID

   o  Recipient ID

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

   o  AEAD Algorithm (alg)

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

   o  Master Salt

      *  Default is the empty string

   o  Key Derivation Function (KDF)

      *  Default is HKDF SHA-256

   o  Replay Window Type and Size

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

   All input parameters need to be known to and agreed on by both
   endpoints, but the replay window may be different in the two
   endpoints.  The replay window type and size is used by the client in
   the processing of the Request-Tag [I-D.ietf-core-echo-request-tag].
   How the input parameters are pre-established, is application
   specific.  The ACE framework may be used to establish the necessary
   input parameters [I-D.ietf-ace-oauth-authz].








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

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

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

   where:

   o  salt is the Master Salt as defined above

   o  IKM is the Master Secret as defined above

   o  info is a CBOR array consisting of:

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

   where:

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

   o  type is "Key" or "IV"

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

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

3.2.2.  Initial Sequence Numbers and Replay Window

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




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3.3.  Requirements on the Security Context Parameters

   As collisions may lead to the loss of both confidentiality and
   integrity, Sender ID SHALL be unique in the set of all security
   contexts using the same Master Secret and Master Salt.  When a
   trusted third party assigns identifiers (e.g., using
   [I-D.ietf-ace-oauth-authz]) or by using a protocol that allows the
   parties to negotiate locally unique identifiers in each endpoint, the
   Sender IDs can be very short.  The maximum length of Sender ID is
   length of nonce subtracted by 6 bytes.  For AES-CCM-16-64-128 the
   maximum length of Sender ID is 7 bytes.  If Sender ID uniqueness
   cannot be guaranteed by construction, Sender IDs MUST be long
   uniformly random distributed byte strings such that the probability
   of collisions is negligible.

   To enable retrieval of the right Recipient Context, the Recipient ID
   SHOULD be unique in the sets of all Recipient Contexts used by an
   endpoint.  The Client MAY provide a "kid context" parameter
   Section 5.1 to help the Server find the right context.

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

4.  Protected Message Fields

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

   The remainder of this section and later sections discuss the behavior
   in terms of CoAP messages.  If HTTP is used for a particular leg in
   the end-to-end path, then this section applies to the conceptual CoAP
   message that is mappable to/from the original HTTP message as
   discussed in Section 10.  That is, an HTTP message is conceptually
   transformed to a CoAP message and then to an OSCORE message, and
   similarly in the reverse direction.  An actual implementation might
   translate directly from HTTP to OSCORE without the intervening CoAP
   representation.

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

   o  Class E: encrypted and integrity protected,




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   o  Class I: integrity protected only, or

   o  Class U: unprotected.

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

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

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

4.1.  CoAP Payload

   The CoAP Payload, if present in the original CoAP message, SHALL be
   encrypted and integrity protected and is thus an Inner message field.
   See Figure 4.

                +------------------+---+---+
                | Field            | E | U |
                +------------------+---+---+
                | Payload          | x |   |
                +------------------+---+---+

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

                   Figure 4: Protection of CoAP Payload

   The sending endpoint writes the payload of the original CoAP message
   into the Plaintext (Section 5.3) input to the COSE object.  The
   receiving endpoint verifies and decrypts the COSE object, and
   recreates the payload of the original CoAP message.






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4.2.  CoAP Options

   A summary of how options are protected is shown in Figure 5.  Note
   that some options may have both Inner and Outer message fields which
   are protected accordingly.  The options which require special
   processing are labelled with asterisks.

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

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

                   Figure 5: Protection of CoAP Options

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








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4.2.1.  Inner Options

   Inner option message fields (class E) are used in a way analogous to
   communicating in a protected manner directly with the other endpoint.

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

   The processing of Inner option message fields by the receiving
   endpoint is specified in Section 7.2 and Section 7.4.

4.2.2.  Outer Options

   Outer option message fields (Class U or I) are used to support proxy
   operations.

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

   The processing of Outer options by the receiving endpoint is
   specified in Section 7.2 and Section 7.4.

   A procedure for integrity-protection-only of Class I option message
   fields is specified in Section 5.4.  New CoAP options which are
   repeatable and of class I MUST specify that proxies MUST NOT change
   the order of the option's occurrences.

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

4.2.3.  Special Options

   Some options require special processing, marked with an asterisk '*'
   in Figure 5; the processing is specified in this section.

4.2.3.1.  Max-Age

   An Inner Max-Age message field is used to specify the freshness (as
   defined in [RFC7252]) of the resource, end-to-end from the server to
   the client, taking into account that the option is not accessible to
   proxies.  The Inner Max-Age SHALL be processed by OSCORE as specified
   in Section 4.2.1.




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   An Outer Max-Age message field is used to avoid unnecessary caching
   of OSCORE error responses at OSCORE unaware intermediary nodes.  A
   server MAY set a Class U Max-Age message field with value zero to
   OSCORE error responses described in Section 6.4, Section 7.2 and
   Section 7.4, which is then processed according to Section 4.2.2.

   Non-error OSCORE responses do not need to include a Max-Age option
   since the responses are non-cacheable by construction (see
   Section 4.3).

4.2.3.2.  The Block Options

   Blockwise [RFC7959] is an optional feature.  An implementation MAY
   support [RFC7252] and the Object-Security option without supporting
   [RFC7959].  The Block options (Block1, Block2, Size1, Size2), when
   Inner message fields, provide secure message fragmentation such that
   each fragment can be verified.  The Block options, when Outer message
   fields, enables hop-by-hop fragmentation of the OSCORE message.
   Inner and Outer block processing may have different performance
   properties depending on the underlying transport.  The end-to-end
   integrity of the message can be verified both in case of Inner and
   Outer Blockwise provided all blocks are received (see
   Section 4.2.3.2.2).

4.2.3.2.1.  Inner Block Options

   The sending CoAP endpoint MAY fragment a CoAP message as defined in
   [RFC7959] before the message is processed by OSCORE.  In this case
   the Block options SHALL be processed by OSCORE as Inner options
   (Section 4.2.1).  The receiving CoAP endpoint SHALL process the
   OSCORE message according to Section 4.2.1 before processing blockwise
   as defined in [RFC7959].

   For concurrent blockwise operations the sending endpoint MUST ensure
   that the receiving endpoint can distinguish between blocks from
   different operations.  One mechanism enabling this is specified in
   [I-D.ietf-core-echo-request-tag].

4.2.3.2.2.  Outer Block Options

   Proxies MAY fragment an OSCORE message using [RFC7959], by
   introducing Block option message fields that are Outer Section 4.2.2
   and not generated by the sending endpoint.  Note that the Outer Block
   options are neither encrypted nor integrity protected.  As a
   consequence, a proxy can maliciously inject block fragments
   indefinitely, since the receiving endpoint needs to receive the last
   block (see [RFC7959]) to be able to compose the OSCORE message and
   verify its integrity.  Therefore, applications supporting OSCORE and



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   [RFC7959] MUST specify a security policy defining a maximum
   unfragmented message size (MAX_UNFRAGMENTED_SIZE) considering the
   maximum size of message which can be handled by the endpoints.
   Messages exceeding this size SHOULD be fragmented by the sending
   endpoint using Inner Block options (Section 4.2.3.2.1).

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

   To allow multiple concurrent request operations to the same server
   (not only same resource), a CoAP proxy SHOULD follow the Request-Tag
   processing specified in section 3.3.2 of
   [I-D.ietf-core-echo-request-tag].

4.2.3.3.  Proxy-Uri

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

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

   Uri-Path and Uri-Query are class E options and SHALL be protected and
   processed as Inner options (Section 4.2.1).

   The Proxy-Uri option of the OSCORE message SHALL be set to the
   composition of Proxy-Scheme, Uri-Host and Uri-Port options (if
   present) as specified in section 6.5 of [RFC7252], and processed as
   an Outer option of Class U (Section 4.2.2).

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




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

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

   During OSCORE processing, Proxy-Uri is split into:

   o  Proxy-Scheme = "coap"

   o  Uri-Host = "example.com"

   o  Uri-Port = "5683"

   o  Uri-Path = "resource"

   o  Uri-Query = "q=1"

   Uri-Path and Uri-Query follow the processing defined in
   Section 4.2.1, and are thus encrypted and transported in the COSE
   object.  The remaining options are composed into the Proxy-Uri
   included in the options part of the OSCORE message, which has value:

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

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

4.2.3.4.  Observe

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

   In order for an OSCORE-unaware proxy to support forwarding of Observe
   messages ([RFC7641]), there SHALL be an Outer Observe option, i.e.,
   present in the options part of the OSCORE message.  The processing of
   the CoAP Code for Observe messages is described in Section 4.3.

   To secure the order of notifications, the client SHALL maintain a
   Notification Number for each Observation it registers.  The
   Notification Number is a non-negative integer containing the largest
   Partial IV of the successfully received notifications for the
   associated Observe registration, see Section 6.4.  The Notification
   Number is initialized to the Partial IV of the first successfully
   received notification response to the registration request.  In
   contrast to [RFC7641], the received Partial IV MUST always be




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   compared with the Notification Number, which thus MUST NOT be
   forgotten after 128 seconds.

   If the verification fails, the client SHALL stop processing the
   response.  The client MAY ignore the Observe option value.

   The Observe option in the CoAP request may be legitimately removed by
   a proxy.  If the Observe option is removed from a CoAP request by a
   proxy, then the server can still verify the request (as a non-Observe
   request), and produce a non-Observe response.  If the OSCORE client
   receives a response to an Observe request without an outer Observe
   value, then it MUST verify the response as a non-Observe response.
   (The reverse case is covered in the verification of the response, see
   Section 7.)

4.2.3.5.  Object-Security

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

4.3.  CoAP Header

   A summary of how the CoAP Header fields are protected is shown in
   Figure 6.

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

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

                Figure 6: Protection of CoAP Header Fields




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

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

   The sending endpoint SHALL write the Code of the original CoAP
   message into the plaintext of the COSE object Section 5.3.  After
   that, the Outer Code of the OSCORE message SHALL be set to 0.02
   (POST) for requests without Observe option, to 0.05 (FETCH) for
   requests with Observe option, and to 2.04 (Changed) for responses.
   Using FETCH with Observe allows OSCORE to be compliant with the
   Observe processing in OSCORE-unaware proxies.  The choice of POST and
   FETCH ([RFC8132]) allows all OSCORE messages to have payload.

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

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

5.  The COSE Object

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

   The AEAD algorithm AES-CCM-16-64-128 defined in Section 10.2 of
   [RFC8152] is mandatory to implement.  For AES-CCM-16-64-128 the
   length of Sender Key and Recipient Key is 128 bits, the length of
   nonce and Common IV is 13 bytes.  The maximum Sender Sequence Number
   is specified in Section 11.

   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.



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

   o  The "protected" field is empty.

   o  The "unprotected" field includes:

      *  The "Partial IV" parameter.  The value is set to the Sender
         Sequence Number.  All leading zeroes SHALL be removed when
         encoding the Partial IV.  The value 0 encodes to the byte
         string 0x00.  This parameter SHALL be present in requests.  In
         case of Observe (Section 4.2.3.4) the Partial IV SHALL be
         present in responses, and otherwise the Partial IV SHOULD NOT
         be present in responses.  (A non-Observe example where the
         Partial IV is included in a response is provided in
         Section 6.5.2.)

      *  The "kid" parameter.  The value is set to the Sender ID.  This
         parameter SHALL be present in requests and SHOULD NOT be
         present in responses.  (An example where the Sender ID is
         included in a response is the extension of OSCORE to group
         communication [I-D.tiloca-core-multicast-oscoap].)

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

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

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

5.1.  Kid Context

   For certain use cases, e.g. deployments where the same "kid" is used
   with multiple contexts, it is necessary or favorable for the sender
   to provide an additional identifier of the security material to use,
   in order for the receiver to retrieve or establish the correct key.
   The "kid context" parameter is used to provide such additional input.
   The "kid context" is implicitly integrity protected, as manipulation
   that leads to the wrong key (or no key) being retrieved which results
   in an error, as described in Section 7.2.

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




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

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

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

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

     Figure 7: Additional common header parameter for the COSE object

5.2.  Nonce

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

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

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

   3.  concatenating the size of the ID (S) with the padded ID and the
       padded Partial IV,

   4.  and then XORing with the Common IV.

   Note that in this specification only algorithms that use nonces equal
   or greater than 7 bytes are supported.

   When observe is not used, the request and the response may use the
   same nonce.  In this way, the Partial IV does not have to be sent in
   responses, which reduces the size.  For processing instructions, see
   Section 7.





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            +---+-----------------------+--+--+--+--+--+
            | S | ID of PIV generator   |  Partial IV  |----+
            +---+-----------------------+--+--+--+--+--+    |
                                                            |
            +------------------------------------------+    |
            |                Common IV                 |->(XOR)
            +------------------------------------------+    |
                                                            |
            +------------------------------------------+    |
            |                  Nonce                   |<---+
            +------------------------------------------+

                      Figure 8: AEAD Nonce Formation

5.3.  Plaintext

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

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

   o  all Inner option message fields (see Section 4.2.1) present in the
      original CoAP message (see Section 4.2).  The options are encoded
      as described in Section 3.1 of [RFC7252], where the delta is the
      difference to the previously included Class E option; and

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

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

                            Figure 9: Plaintext

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







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5.4.  Additional Authenticated Data

   The external_aad SHALL be a CBOR array as defined below:

   external_aad = [
      version : uint,
      alg : int / tstr,
      request_kid : bstr,
      request_piv : bstr,
      options : bstr
   ]

   where:

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

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

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

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

   o  options: contains the Class I options (see Section 4.2.2) present
      in the original CoAP message encoded as described in Section 3.1
      of [RFC7252], where the delta is the difference to the previously
      included class I option.

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

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

6.1.  Message Binding

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





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6.2.  AEAD Nonce Uniqueness

   An AEAD nonce MUST NOT be used more than once per AEAD key.  In order
   to assure unique nonces, each Sender Context contains a Sender
   Sequence Number used to protect requests, and - in case of Observe -
   responses.  If messages are processed concurrently, the operation of
   reading and increasing the Sender Sequence Number MUST be atomic.

   The maximum Sender Sequence Number is algorithm dependent, see
   Section 11, and no greater than 2^40 - 1.  If the Sender Sequence
   Number exceeds the maximum, the endpoint MUST NOT process any more
   messages with the given Sender Context.  The endpoint SHOULD acquire
   a new security context (and consequently inform the other endpoint)
   before this happens.  The latter is out of scope of this document.

6.3.  Freshness

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

   For responses, the message binding guarantees that a response is not
   older than its request.  For responses without Observe, this gives
   strong absolute freshness.  For responses with Observe, the absolute
   freshness gets weaker with time, and it is RECOMMENDED that the
   client regularly restart the observation.

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

6.4.  Replay Protection

   In order to protect from replay of requests, the server's Recipient
   Context includes a Replay Window.  A server SHALL verify that a
   Partial IV received in the COSE object has not been received before.
   If this verification fails the server SHALL stop processing the
   message and, MAY optionally respond with a 4.01 Unauthorized error
   message.  The server MAY set an Outer Max-Age option with value zero.
   The diagnostic payload MAY contain the "Replay protection failed"
   string.  The size and type of the Replay Window depends on the use
   case and lower protocol layers.  In case of reliable and ordered
   transport from endpoint to endpoint, the server MAY just store the
   last received Partial IV and require that newly received Partial IVs
   equals the last received Partial IV + 1.



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   Responses to non-Observe requests are protected against replay as
   they are cryptographically bound to the request.

   In the case of Observe, a client receiving a notification SHALL
   verify that the Partial IV of a received notification is greater than
   the Notification Number bound to that Observe registration.  If the
   verification fails, the client SHALL stop processing the response.
   If the verification succeeds, the client SHALL overwrite the
   corresponding Notification Number with the received Partial IV.

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

6.5.  Losing Part of the Context State

   To prevent reuse of the Nonce with the same key, or from accepting
   replayed messages, a node needs to handle the situation of losing
   rapidly changing parts of the context, such as the request Token,
   Sender Sequence Number, Replay Window, and Notififcation Numbers.
   These are typically stored in RAM and therefore lost in the case of
   an unplanned reboot.

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

   After boot, a node MAY use a partly persistently stored security
   context, but then the node MUST NOT reuse a previous Sender Sequence
   Number and MUST NOT accept previously accepted messages.  Some ways
   to achieve this is described below:

6.5.1.  Sequence Number

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

   o  Each time the Sender Sequence Number is evenly divisible by K,
      where K is a positive integer, store the Sender Sequence Number in
      persistent memory.  After boot, the node initiates the Sender
      Sequence Number to the value stored in persistent memory + K - 1.
      Storing to persistent memory can be costly.  The value K gives a
      trade-off between the number of storage operations and efficient
      use of Sender Sequence Numbers.




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6.5.2.  Replay Window

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

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

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

6.5.3.  Replay Protection of Observe Notifications

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

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

7.  Processing

   This section describes the OSCORE message processing.

7.1.  Protecting the Request

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

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

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

   3.  Compute the AEAD nonce from the Sender ID, Common IV, and Partial
       IV (Sender Sequence Number in network byte order) as described in
       Section 5.2.  Then (in one atomic operation, see Section 6.2)
       increment the Sender Sequence Number by one.

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





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   5.  Format the OSCORE message according to Section 4.  The Object-
       Security option is added, see Section 4.2.2.

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

7.2.  Verifying the Request

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

   1.   Process outer Block options according to [RFC7959], until all
        blocks of the request have been received, see Section 4.2.3.2.

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

   3.   Decompress the COSE Object (Section 8) and retrieve the
        Recipient Context associated with the Recipient ID in the 'kid'
        parameter.  If either the decompression or the COSE message
        fails to decode, or the server fails to retrieve a Recipient
        Context with Recipient ID corresponding to the 'kid' parameter
        received, then the server SHALL stop processing the request.
        If:

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

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

   4.   Verify the 'Partial IV' parameter using the Replay Window, as
        described in Section 6.

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





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

   7.   Decrypt the COSE object using the Recipient Key.

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

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

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

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

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

7.3.  Protecting the Response

   Given a CoAP response, the server SHALL perform the following steps
   to create an OSCORE response.  Note that CoAP error responses derived
   from CoAP processing (point 10. in Section 7.2) are protected, as
   well as successful CoAP responses, while the OSCORE errors (point 3,
   4, and 7 in Section 7.2) do not follow the processing below, but are
   sent as simple CoAP responses, without OSCORE processing.

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

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

   3.  Compute the AEAD nonce

       *  If Observe is used, Compute the AEAD nonce from the Sender ID,
          Common IV, and Partial IV (Sender Sequence Number in network
          byte order).  Then (in one atomic operation, see Section 6.2)
          increment the Sender Sequence Number by one.

       *  If Observe is not used, either the nonce from the request is
          used or a new Partial IV is used.



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   4.  Encrypt the COSE object using the Sender Key. Compress the COSE
       Object as specified in Section 8.  If the nonce was constructed
       from a new Partial IV, this Partial IV MUST be included in the
       message.  If the nonce from the request was used, the Partial IV
       MUST NOT be included in the message.

   5.  Format the OSCORE message according to Section 4.  The Object-
       Security option is added, see Section 4.2.2.

7.4.  Verifying the Response

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

   1.   Process outer Block options according to [RFC7959], until all
        blocks of the OSCORE message have been received, see
        Section 4.2.3.2.

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

   3.   Retrieve the Recipient Context associated with the Token.
        Decompress the COSE Object (Section 8).  If either the
        decompression or the COSE message fails to decode, then go to
        11.

   4.   For Observe notifications, verify the received 'Partial IV'
        parameter against the corresponding Notification Number as
        described in Section 6.  If the client receives a notification
        for which no Observe request was sent, then go to 11.

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

   6.   Compute the AEAD nonce

        1.  If the Observe option and the Partial IV are not present in
            the response, the nonce from the request is used.

        2.  If the Observe option is present in the response, and the
            Partial IV is not present in the response, then go to 11.

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

   7.   Decrypt the COSE object using the Recipient Key.



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        *  If decryption fails, then go to 11.

        *  If decryption succeeds and Observe is used, update the
           corresponding Notification Number, as described in Section 6.

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

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

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

   11.  (Optional) In case any of the previous erroneous conditions
        apply: the client SHALL stop processing the response.

8.  OSCORE Compression

   The Concise Binary Object Representation (CBOR) [RFC7049] combines
   very small message sizes with extensibility.  The CBOR Object Signing
   and Encryption (COSE) [RFC8152] uses CBOR to create compact encoding
   of signed and encrypted data.  COSE is however constructed to support
   a large number of different stateless use cases, and is not fully
   optimized for use as a stateful security protocol, leading to a
   larger than necessary message expansion.  In this section, we define
   a simple stateless compression mechanism for OSCORE called the
   "compressed COSE object", which significantly reduces the per-packet
   overhead.

8.1.  Encoding of the Object-Security Value

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

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

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

                     Figure 10: Object-Security Value



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   o  The first byte (= the OSCORE flag byte) encodes a set of flags and
      the length of the Partial IV parameter.

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

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

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

      *  The sixth least significant bit is reserved for indicating the
         presence of a signature.  This needs to be specified in a
         separate document.  The bit SHALL be set to zero when not in
         use.

      *  The seventh least significant bit is reserved to expand the
         flag byte.  This needs to be specified in a separate document.
         The bit SHALL be set to zero when not in use.

      *  The eighth least significant bit is reserved for indicating if
         a non-compressed COSE object is used.  This needs to be
         specified in a separate document.  The bit SHALL be set to zero
         when not in use.

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

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

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

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

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







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8.2.  Encoding of the OSCORE Payload

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

8.3.  Examples of Compressed COSE Objects

8.3.1.  Example: Requests

   Request with kid = 25 and Partial IV = 5

   Before compression (24 bytes):

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

   After compression (17 bytes):

   Flag byte: 0b00001001 = 0x09

   Option Value: 09 05 25 (3 bytes)

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

   Request with kid = empty string and Partial IV = 0

   After compression (16 bytes):

   Flag byte: 0b00001001 = 0x09

   Option Value: 09 00 (2 bytes)

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

   Request with kid = empty string, Partial IV = 5, and kid context =
   0x44616c656b

   After compression (22 bytes):

   Flag byte: 0b00011001 = 0x19

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

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




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8.3.2.  Example: Response (without Observe)

   Before compression (18 bytes):

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

   After compression (14 bytes):

   Flag byte: 0b00000000 = 0x00

   Option Value: (0 bytes)

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

8.3.3.  Example: Response (with Observe)

   Before compression (21 bytes):

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

   After compression (16 bytes):

   Flag byte: 0b00000001 = 0x01

   Option Value: 01 07 (2 bytes)

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

9.  Web Linking

   The use of OSCORE MAY be indicated by a target attribute "osc" in a
   web link [RFC8288] to a resource.  This attribute is a hint
   indicating that the destination of that link is to be accessed using
   OSCORE.  Note that this is simply a hint, it does not include any
   security context material or any other information required to run
   OSCORE.

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




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

10.  Proxy Operations

   RFC 7252 defines operations for a CoAP-to-CoAP proxy (see Section 5.7
   of [RFC7252]) and for proxying between CoAP and HTTP (Section 10 of
   [RFC7252]).  A more detailed description of the HTTP-to-CoAP mapping
   is provided by [RFC8075].  This section describes the operations of
   OSCORE-aware proxies.

10.1.  CoAP-to-CoAP Forwarding Proxy

   OSCORE is designed to work with legacy CoAP-to-CoAP forward proxies
   [RFC7252], but OSCORE-aware proxies MAY provide certain
   simplifications as specified in this section.

   The targeted proxy operations are specified in Section 2.2.1 of
   [I-D.hartke-core-e2e-security-reqs].  In particular caching is
   disabled since the CoAP response is only applicable to the original
   client's CoAP request.  An OSCORE-aware proxy SHALL NOT cache a
   response to a request with an Object-Security option.  As a
   consequence, the search for cache hits and CoAP freshness/Max-Age
   processing can be omitted.

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

   Proxy processing of the (Outer) Block options is as defined in
   [RFC7959] and [I-D.ietf-core-echo-request-tag].

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

10.2.  HTTP-to-CoAP Translation Proxy

   Section 10.2 of [RFC7252] and [RFC8075] specify the behavior of an
   HTTP-to-CoAP proxy.  As requested in Section 1 of [RFC8075], this
   section describes the HTTP mapping for the OSCORE protocol extension
   of CoAP.

   The presence of the Object-Security option, both in requests and
   responses, is expressed in an HTTP header field named Object-Security
   in the mapped request or response.  The value of the field is:

   o  "" (empty string) if the CoAP Object-Security option is empty, or




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   o  the value of the CoAP Object-Security option Section 8.1 in
      base64url encoding (Section 5 of [RFC4648]) without padding (see
      [RFC7515] Appendix C for implementation notes for this encoding).

   The value of the body is the OSCORE payload Section 8.2.

   Example:

   Mapping and notation here is based on "Simple Form" (Section 5.4.1.1
   of [RFC8075]).

 [HTTP request -- Before object security processing]

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

 [HTTP request -- HTTP Client to Proxy]

   POST http://proxy.url/hc/?target_uri=coap://server.url/ HTTP/1.1
   Object-Security: 09 25
   Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

 [CoAP request -- Proxy to CoAP Server]

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

 [CoAP response -- CoAP Server to Proxy]

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

 [HTTP response -- Proxy to HTTP Client]

   HTTP/1.1 200 OK
   Object-Security: "" (empty string)
   Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]

 [HTTP response -- After object security processing]

   HTTP/1.1 200 OK
   Body: Exterminate! Exterminate!

   Note that the HTTP Status Code 200 in the next-to-last message is the
   mapping of CoAP Code 2.04 (Changed), whereas the HTTP Status Code 200
   in the last message is the mapping of the CoAP Code 2.05 (Content),




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   which was encrypted within the compressed COSE object carried in the
   Body of the HTTP response.

10.3.  CoAP-to-HTTP Translation Proxy

   Section 10.1 of [RFC7252] describes the behavior of a CoAP-to-HTTP
   proxy.  RFC 8075 [RFC8075] does not cover this direction in any more
   detail and so an example instantiation of Section 10.1 of [RFC7252]
   is used below.

   Example:

   [CoAP request -- Before object security processing]

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

   [CoAP request -- CoAP Client to Proxy]

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

   [HTTP request -- Proxy to HTTP Server]

     POST http://server.url/ HTTP/1.1
     Object-Security: 09 25
     Body: 09 07 01 13 61 f7 0f d2 97 b1 [binary]

   [HTTP response -- HTTP Server to Proxy]

     HTTP/1.1 200 OK
     Object-Security: "" (empty string)
     Body: 00 31 d1 fc f6 70 fb 0c 1d d5 ... [binary]

   [CoAP response -- CoAP Server to Proxy]

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

   [CoAP response -- After object security processing]

     2.05 Content
     Payload: Exterminate! Exterminate!





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

11.  Security Considerations

   In scenarios with intermediary nodes such as proxies or brokers,
   transport layer security such as (D)TLS 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.

   (D)TLS protects hop-by-hop the entire message, including header,
   options, and payload.  OSCORE protects end-to-end the payload, and
   all information in the options and header, that is not required for
   proxy operations (see Section 4).  (D)TLS and OSCORE can be combined,
   thereby enabling end-to-end security of the message payload, in
   combination with hop-by-hop protection of the entire message, during
   transport between end-point and intermediary node.  The message
   layer, however, cannot be protected end-to-end through intermediary
   devices since, even if the protocol itself isn't translated, the
   parameters Type, Message ID, Token, and Token Length may be changed
   by a proxy.

   The use of COSE to protect messages as specified in this document
   requires an established security context.  The method to establish
   the security context described in Section 3.2 is based on a common
   shared secret material in client and server, which may be obtained,
   e.g., by using the ACE framework [I-D.ietf-ace-oauth-authz].  An
   OSCORE profile of ACE is described in [I-D.seitz-ace-oscoap-profile].

   Most AEAD algorithms require a unique nonce for each message, for
   which the sender sequence numbers in the COSE message field "Partial
   IV" is used.  If the recipient accepts any sequence number larger
   than the one previously received, then the problem of sequence number
   synchronization is avoided.  With reliable transport, it may be
   defined that only messages with sequence number which are equal to
   previous sequence number + 1 are accepted.  The alternatives to
   sequence numbers have their issues: very constrained devices may not
   be able to support accurate time, or to generate and store large
   numbers of random nonces.  The requirement to change key at counter



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   wrap is a complication, but it also forces the user of this
   specification to think about implementing key renewal.

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

   The inner block options enable the sender to split large messages
   into OSCORE-protected blocks such that the receiving node can verify
   blocks before having received the complete message.  The outer block
   options allow for arbitrary proxy fragmentation operations that
   cannot be verified by the endpoints, but can by policy be restricted
   in size since the encrypted options allow for secure fragmentation of
   very large messages.  A maximum message size (above which the sending
   endpoint fragments the message and the receiving endpoint discards
   the message, if complying to the policy) may be obtained as part of
   normal resource discovery.

12.  Privacy Considerations

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

   The unprotected options (Figure 5) may reveal privacy sensitive
   information.  In particular Uri-Host SHOULD NOT contain privacy
   sensitive information.

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

   Using the mechanisms described in Section 6.5 may reveal when a
   device goes through a reboot.  This can be mitigated by the device
   storing the precise state of sender sequence number and replay window
   on a clean shutdown.

   The length of message fields can reveal information about the
   message.  Applications may use a padding scheme to protect against
   traffic analysis.  As an example, the strings "YES" and "NO" even if
   encrypted can be distinguished from each other as there is no padding



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

13.  IANA Considerations

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

13.1.  COSE Header Parameters Registry

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

   o  Name: kid context

   o  Label: kidctx

   o  Value Type: bstr

   o  Value Registry:

   o  Description: kid context

   o  Reference: Section 5.1 of this document

13.2.  CoAP Option Numbers Registry

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

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





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13.3.  Header Field Registrations

   The HTTP header field Object-Security is added to the Message Headers
   registry:

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

14.  Acknowledgments

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

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

15.  References

15.1.  Normative References

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

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

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

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

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





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

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

   [RFC8075]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for Mapping Implementations: HTTP to
              the Constrained Application Protocol (CoAP)", RFC 8075,
              DOI 10.17487/RFC8075, February 2017,
              <https://www.rfc-editor.org/info/rfc8075>.

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

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

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

15.2.  Informative References

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

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

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-04 (work in progress), October
              2017.




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

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR data structures", draft-ietf-cbor-cddl-00
              (work in progress), July 2017.

   [I-D.ietf-core-coap-tcp-tls]
              Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              draft-ietf-core-coap-tcp-tls-10 (work in progress),
              October 2017.

   [I-D.ietf-core-echo-request-tag]
              Amsuess, C., Mattsson, J., and G. Selander, "Echo and
              Request-Tag", draft-ietf-core-echo-request-tag-00 (work in
              progress), October 2017.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
              and C. Amsuess, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-03 (work in progress),
              October 2017.

   [I-D.seitz-ace-oscoap-profile]
              Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
              profile of the Authentication and Authorization for
              Constrained Environments Framework", draft-seitz-ace-
              oscoap-profile-06 (work in progress), October 2017.

   [I-D.tiloca-core-multicast-oscoap]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Secure group communication for CoAP", draft-tiloca-core-
              multicast-oscoap-04 (work in progress), October 2017.

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






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

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

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

Appendix A.  Test Vectors

   TODO: This section needs to be updated.

Appendix B.  Examples

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

B.1.  Secure Access to Sensor

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




















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

   Figure 11: Secure Access to Sensor.  Square brackets [ ... ] indicate
    content of compressed COSE object.  Curly brackets { ... } indicate
                              encrypted data.

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

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

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

B.2.  Secure Subscribe to Sensor

   This example targets the scenario in Section 3.2 of
   [I-D.hartke-core-e2e-security-reqs] and illustrates a client
   requesting subscription to a blood sugar measurement resource (GET
   /glucose), first receiving the value 220 mg/dl and then a second
   value 180 mg/dl.



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      Client  Proxy  Server
        |       |       |
        +------>|       |            Code: 0.05 (FETCH)
        | FETCH |       |           Token: 0x83
        |       |       |         Observe: 0
        |       |       | Object-Security: [kid:ca,Partial IV:15]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"glucose"}
        |       |       |
        |       +------>|            Code: 0.05 (FETCH)
        |       | FETCH |           Token: 0xbe
        |       |       |         Observe: 0
        |       |       | Object-Security: [kid:ca,Partial IV:15]
        |       |       |         Payload: {Code:0.01,
        |       |       |                   Uri-Path:"glucose"}
        |       |       |
        |       |<------+            Code: 2.04 (Changed)
        |       |  2.04 |           Token: 0xbe
        |       |       |         Observe: 7
        |       |       | Object-Security: [Partial IV:32]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "220"}
        |       |       |
        |<------+       |            Code: 2.04 (Changed)
        |  2.04 |       |           Token: 0x83
        |       |       |         Observe: 7
        |       |       | Object-Security: [Partial IV:32]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "220"}
       ...     ...     ...
        |       |       |
        |       |<------+            Code: 2.04 (Changed)
        |       |  2.04 |           Token: 0xbe
        |       |       |         Observe: 8
        |       |       | Object-Security: [Partial IV:36]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "180"}
        |       |       |
        |<------+       |            Code: 2.04 (Changed)
        |  2.04 |       |           Token: 0x83
        |       |       |         Observe: 8
        |       |       | Object-Security: [Partial IV:36]
        |       |       |         Payload: {Code:2.05,
        |       |       |                   Content-Format:0, "180"}
        |       |       |






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

   The request/response Codes are encrypted by OSCORE and only dummy
   Codes (FETCH/Changed) are visible in the header of the OSCORE
   message.  The options Content-Format (0) and the payload ("220" and
   "180"), are encrypted.

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

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

Authors' Addresses

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com


   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB

   Email: francesca.palombini@ericsson.com


   Ludwig Seitz
   SICS Swedish ICT

   Email: ludwig@sics.se








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