[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 draft-ietf-core-object-security

ACE Working Group                                            G. Selander
Internet-Draft                                               J. Mattsson
Intended Status: Standards Track                            F. Palombini
Expires: December 31, 2015                                      Ericsson
                                                                L. Seitz
                                                        SICS Swedish ICT

                                                           June 29, 2015


                   Object Security for CoAP (OSCOAP)
                 draft-selander-ace-object-security-02
Abstract

   This memo presents OSCOAP, a scheme for protection of request and
   response messages of the Constrained Application Protocol (CoAP),
   using data object security.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   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".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html


Copyright and License Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



Selander, et al.       Expires December 31, 2015                [Page 1]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Background  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3. End-to-end Security in Presence of Intermediary Nodes . . . . .  6
   4. Secure Message  . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1 Secure Message format  . . . . . . . . . . . . . . . . . . .  8
       4.1.1 Secure Message Header  . . . . . . . . . . . . . . . . .  8
       4.1.2 Secure Message Body  . . . . . . . . . . . . . . . . . .  9
         4.1.2.1 Secure Signed Message Body . . . . . . . . . . . . .  9
         4.1.2.2 Secure Encrypted Message Body  . . . . . . . . . . .  9
       4.1.3 Secure Message Tag . . . . . . . . . . . . . . . . . . .  9
   5. Message Protection  . . . . . . . . . . . . . . . . . . . . . . 10
     5.1 CoAP Message Protection (Mode:COAP)  . . . . . . . . . . . . 10
       5.1.1 The Sig Option . . . . . . . . . . . . . . . . . . . . . 10
         5.1.1.1 Option Structure . . . . . . . . . . . . . . . . . . 10
         5.1.1.2 Integrity Protection and Verification  . . . . . . . 11
         5.1.1.3 SSM Body . . . . . . . . . . . . . . . . . . . . . . 11
       5.1.2 The Enc Option . . . . . . . . . . . . . . . . . . . . . 12
         5.1.2.1 Option Structure . . . . . . . . . . . . . . . . . . 12
         5.1.2.2 Encryption and Decryption  . . . . . . . . . . . . . 13
         5.1.2.3 SEM Body . . . . . . . . . . . . . . . . . . . . . . 13
         5.1.2.4 CoAP Message with Enc Option . . . . . . . . . . . . 13
       5.1.3 SM Tag . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.2 Payload Only Protection (Mode:PAYL)  . . . . . . . . . . . . 14
     5.3 Replay Protection and Freshness  . . . . . . . . . . . . . . 15
       5.3.1 Replay Protection  . . . . . . . . . . . . . . . . . . . 15
       5.3.2 Freshness  . . . . . . . . . . . . . . . . . . . . . . . 15
   6. Security Considerations . . . . . . . . . . . . . . . . . . . . 15
   7. Privacy Considerations  . . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 19
     10.2  Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A. Which CoAP Header Fields and Options to Protect  . . . 20
     A.1 CoAP Header Fields . . . . . . . . . . . . . . . . . . . . . 20
     A.2 CoAP Options . . . . . . . . . . . . . . . . . . . . . . . . 20



Selander, et al.       Expires December 31, 2015                [Page 2]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


       A.2.1 Integrity Protection . . . . . . . . . . . . . . . . . . 20
         A.2.1.1 Proxy-Scheme . . . . . . . . . . . . . . . . . . . . 21
         A.2.1.2 Uri-*  . . . . . . . . . . . . . . . . . . . . . . . 21
       A.2.2 Encryption . . . . . . . . . . . . . . . . . . . . . . . 22
       A.2.3 Summary  . . . . . . . . . . . . . . . . . . . . . . . . 22
   Appendix B.  JOSE Profile of SM  . . . . . . . . . . . . . . . . . 23
     B.1  JWS as Secure Signed Message  . . . . . . . . . . . . . . . 23
     B.2  JWE as Secure Encrypted Message . . . . . . . . . . . . . . 24
     B.3 "seq" (Sequence Number) Header Parameter . . . . . . . . . . 24
     B.4 "cid" (Context Identifier) Header Parameter  . . . . . . . . 24
   Appendix C.  Compact Secure Message  . . . . . . . . . . . . . . . 24
   Appendix D.  COSE Profile of SM  . . . . . . . . . . . . . . . . . 26
     D.1  COSE_Sign or COSE_mac as Secure Signed Message  . . . . . . 26
       D.1.1  COSE_Sign as Secure Signed Message  . . . . . . . . . . 27
       D.1.2  COSE_mac as Secure Signed Message . . . . . . . . . . . 27
     D.2  COSE_encrypt as Secure Encrypted Message  . . . . . . . . . 28
     D.3 COSE optimizations . . . . . . . . . . . . . . . . . . . . . 29
   Appendix E.  Comparison of message sizes . . . . . . . . . . . . . 30
     E.1 SSM: Message Authentication Code . . . . . . . . . . . . . . 30
     E.2 SSM: Digital Signature . . . . . . . . . . . . . . . . . . . 31
     E.3 SEM: Authenticated Encryption  . . . . . . . . . . . . . . . 32
     E.4 SEM: Symmetric Encryption + Digital Signature  . . . . . . . 34
   Appendix F.  Examples  . . . . . . . . . . . . . . . . . . . . . . 36
     F.1 CoAP Message Protection  . . . . . . . . . . . . . . . . . . 36
       F.1.1 Integrity Protection of CoAP Message Exchange  . . . . . 36
       F.1.2 Additional Encryption of CoAP Message  . . . . . . . . . 38
     F.2 Payload Protection . . . . . . . . . . . . . . . . . . . . . 39
       F.2.1 Proxy Caching  . . . . . . . . . . . . . . . . . . . . . 39
       F.2.2 Publish-Subscribe  . . . . . . . . . . . . . . . . . . . 40
       F.2.3 Transporting Authorization Information . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42




















Selander, et al.       Expires December 31, 2015                [Page 3]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


1.  Introduction

   The Constrained Application Protocol CoAP [RFC7252] was designed with
   a constrained RESTful environment in mind.  CoAP references DTLS
   [RFC6347] for securing the message exchanges.  Two commonly used
   features of CoAP are store-and-forward and publish-subscribe
   exchanges, which are problematic to secure with DTLS and transport
   layer security.  As DTLS offers hop-by-hop security, in case of
   store-and-forward exchanges it necessitates a trusted intermediary.
   On the other hand, securing publish-subscribe CoAP exchanges with
   DTLS requires the use of the keep-alive mechanism which incurs
   additional overhead and actually takes away most of the benefits of
   asynchronous communication.

   The pervasive monitoring debate has illustrated the need to protect
   data also from trustworthy intermediary nodes as they can be
   compromised.  The community has reacted strongly to the revelations,
   and new solutions must consider this attack [RFC7258] and include
   encryption by default.

   This memo presents OSCOAP, a data object based communication security
   solution complementing DTLS and supporting secure messaging end-to-
   end across intermediary nodes.  OSCOAP may be used in very
   constrained settings where DTLS cannot be supported.  OSCOAP can also
   be combined with DTLS thus enabling, for example, end-to-end security
   of CoAP payload in combination with hop-by-hop protection of the
   entire CoAP message during transport between end-point and
   intermediary node.

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.

   Certain security-related terms are to be understood in the sense
   defined in RFC 4949 [RFC4949].  These terms include, but are not
   limited to, "authentication", "authorization", "confidentiality",
   "(data) integrity", "message authentication code", and "verify".  For
   "signature", see below.

   RESTful terms, such as "resource" or "representation", are to be
   understood as used in HTTP [RFC7231] and CoAP.

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



Selander, et al.       Expires December 31, 2015                [Page 4]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   JSON Web Signature (JWS), JOSE Header, JWS Payload, and JWS Signature
   are defined in [RFC7515].  JSON Web Encryption (JWE), JWE AAD, JWE
   Ciphertext, and JWE Authentication Tag are defined in [RFC7516].

   Secure Message (SM), Secure Signed Message (SSM), and Secure
   Encrypted Message (SEM) are message formats defined in this memo.
   The Compact Secure Message (CSM) format is defined in Appendix C.
   The Sig and Enc options are CoAP options defined in this memo.

   Note that "signature" as in "JSON Web Signature" and the derived
   terms "Secure Signed Message" and "Sig" option may refer either to
   digital signature using private key of an asymmetric key pair, or
   Message Authentication Code using a shared key.  In other occurrences
   we use the term as defined in [RFC4949], meaning digital signature.

   Excluded Authenticated Data (EAD) is defined in this memo (see
   Sections 4.1.2).  Transaction Identifier (TID) is defined in this
   memo (see Section 4.1.1).

   COSE is defined in [I-D.schaad-cose-msg].

2. Background

   The background for this work is provided by the use cases and problem
   description in [I-D.ietf-ace-usecases] and [I-D.gerdes-ace-actors].
   The focus of this memo is on end-to-end security in constrained
   environments in the presence of intermediary nodes.

   For constrained-node networks there may be several reasons for
   messages to be cached or stored in one node and later forwarded.  For
   example, connectivity between the nodes may be intermittent, or some
   node may be sleeping at the time when the message should have been
   forwarded (see e.g. [I-D.ietf-ace-usecases] sections 2.1.1,  and
   2.5.1).  Also, the architectural model or protocol applied may
   require an intermediary node which breaks security on transport layer
   (see e.g. [I-D.ietf-ace-usecases] sections 2.1.1, and 2.5.2).
   Examples of intermediary nodes include forward proxies, reverse
   proxies, pub-sub brokers, HTTP-CoAP cross-proxies, and SMS servers.

   On a high level, end-to-end security in this setting encompasses:

      1. Protection against eavesdropping and manipulation of resource
         representations in intermediary nodes;

      2. Protection against message replay;

      3. Protection of authorization information ("access tokens") in
         transport from an Authorization Server to a Resource Server via



Selander, et al.       Expires December 31, 2015                [Page 5]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


         a Client, or other intermediary nodes which could gain from
         changing the information;

      4. Allowing a client to verify that a response comes from a
         certain server and is the response to a particular request;

      5. Protection of the RESTful method used by the client, or the
         response code used by the server. For example if a malicious
         proxy replaces the client requested GET with a DELETE this must
         be detected by the server;

      6. Protection against eavesdropping of meta-data of the request or
         response, including CoAP options such as for example Uri-Path
         and Uri-Query, which may reveal some information on what is
         requested.

   From the listed examples, there are two main categories of security
   requirements and corresponding solutions.  The first category deals
   essentially with protecting the CoAP payload (1-3).

   The second category deals with protecting an entire CoAP message,
   targeting also CoAP options and header fields (4-6).  The next
   section formulates security requirements for the two categories,
   which correspond to two modes of OSCOAP denoted Mode:PAYL and
   Mode:COAP, respectively.

3. End-to-end Security in Presence of Intermediary Nodes

   For high-level security requirements related to resource access, see
   section 8.7 of [I-D.gerdes-ace-actors].  This section defines the
   specific requirements that address the two categories of examples
   identified in the previous section, taking into account potential
   intermediary nodes.

   In the case of CoAP payload only protection (Mode:PAYL), the end-to-
   end security requirements apply to payload data, such as Resource
   Representations:

      a. The payload shall be integrity protected and should be
         encrypted end-to-end from sender to receiver.

      b. It shall be possible for an intended receiver to detect if it
         has received this message previously, i.e. replay protection.

   Note that a Mode:PAYL message may have multiple recipients. For
   example, in the case of a proxy that is caching responses used to
   serve multiple clients, or in a publish-subscribe setting with
   multiple subscribers to a given publication.



Selander, et al.       Expires December 31, 2015                [Page 6]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   In the case of protecting specific Client-Server CoAP message
   exchanges (Mode:COAP), potentially passing via intermediary nodes,
   there are additional end-to-end security requirements:

      c. The CoAP options which are not intended to be changed by an
         intermediary node shall be integrity protected between Client
         and Server.

      d. The CoAP options which are not intended to be read by an
         intermediary node shall be encrypted between Client and Server.

      e. The CoAP header field "Code" shall be integrity protected
         between Client and Server.

      f. A Client shall be able to verify that a message is the response
         to a particular request the Client made.

   The requirements listed above can be met by encryption, integrity
   protection and replay protection.  What differs between the modes is
   the actual data that is protected, i.e. CoAP payload data only or
   also other CoAP message data.  This memo specifies a common "Secure
   Message" format that can be used to wrap either payload only
   (Mode:PAYL) or also additional selected CoAP message fields
   (Mode:COAP), and be sent as part of the message.


4. Secure Message

   There exist already standardized and draft content formats for
   cryptographically protected data such as CMS [RFC5652], JWS
   [RFC7515], JWE [RFC7516], and COSE [I-D.schaad-cose-msg].

   Current CMS and JWx objects are undesirably large for very
   constrained devices, and can lead to packet fragmentation in
   constrained-node networks due to limited frame sizes, and to problems
   with limited storage capacity on constrained devices due to limited
   RAM.  First estimates with COSE render more compact objects, see
   Appendix E for a discussion of message format overhead and minimum
   message expansion.  For example, COSE header for a Message
   Authentication Code object encodes to 37 bytes, while the same header
   with JWS results in 74 bytes.

   Thus, the candidate message format for use in OSCOAP is COSE [I-
   D.schaad-cose-msg].  Pending a stable version of COSE this draft uses
   multiple formats and their terminology to illustrate how the message
   format is applied and processed.  It is the intention to replace
   these with a profile of one single compact secure message format in a
   future version of this draft.



Selander, et al.       Expires December 31, 2015                [Page 7]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   None of the message formats listed about provide support for replay
   protection, but it is noted section 10.10 of [RFC7515] that one way
   to thwart replay attacks is to include a unique transaction
   identifier and have the recipient verify that the message has not
   been previously received or acted upon.

   We use the term Secure Message (SM) format to refer to a content
   format for cryptographically protected data which includes a unique
   transaction identifier, and some other common data as specified in
   Section 4.1.1.

   This memo uses JOSE content formats as a model to specify format and
   processing of messages.  The terms Secure Signed Message (SSM) format
   and Secure Encrypted Message (SEM) format to refer to Secure Message
   formats supporting integrity protection only and additional
   encryption, analogous to JWS and JWE, respectively.  Appendix B shows
   how to profile JOSE objects to become Secure Message formats.
   Appendix C shows how to profile COSE objects to become Secure Message
   formats.


4.1 Secure Message format

   A Secure Message (SM) SHALL consist of Header, Body and Tag.

4.1.1 Secure Message Header

   The following parameters SHALL be included in the SM Header:

      o Algorithm.  This parameter identifies the cryptographic
        algorithm(s) used to protect the Secure Message.  In case of SSM
        it has the same semantics as the JOSE Header Parameter "alg"
        defined in Section 4.1.1 of [RFC7515].  In case of SEM,  "direct
        key agreement" (corresponding to the JWE "alg" = "dir") is
        assumed, and the encryption algorithm corresponds to the JOSE
        Header Parameter "enc" (Section 4.1.2 of [RFC7516]).  However,
        the cipher suites are not limited to AEAD algorithms but also
        include symmetric key encryption combined with private key
        signature.

      o Context Identifier.  This parameter identifies the sender and
        the security context/key(s) used together with the Algorithm to
        protect the message.  For Mode:COAP, the Context Identifier
        typically identifies the sending party and different resources
        are identified by their Uri-Path.  For Mode:PAYL, the Context
        Identifier may identify the resource itself.  The structure of
        this identifier is unspecified.




Selander, et al.       Expires December 31, 2015                [Page 8]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


      o Sequence Number.  The Sequence Number parameter enumerates the
        Secure Messages protected using the security context identified
        by the Context Identifier, and is used for replay protection and
        uniqueness of nonce.  The start sequence number SHALL be 0.  For
        a given key, any Sequence Number MUST NOT be used more than
        once.

   The ordered sequence (Sequence Number, Context Identifier) is called
   Transaction Identifier (TID), and SHALL be unique for each SM.

4.1.2 Secure Message Body

   Analogously to JWS and JWE, the SM Body contains what is being
   protected.  The SM Body is different for SSM and SEM.

   In order to obtain a compact representation, certain data is
   integrity protected but excluded from the Secure Message.  Such data
   is referred to as Excluded Authenticated Data (EAD).  To further
   reduce message size, the unencrypted part of the SM Body may be
   "detached" from the Secure Message, see sections 4.1.2.1 and 4.1.2.2.

   The assumption behind excluding integrity protected data from the SM,
   or detaching integrity protected but not encrypted parts of the SM
   during transport, is that the data in question is known to the
   receiver, e.g. because it is established beforehand or because it is
   transported as part of the CoAP message carrying the Secure Message.


4.1.2.1 Secure Signed Message Body

   For SSM, the Body consists of the payload data which is integrity
   protected, analogously to the JWS Payload.  Detached Content is
   defined to mean that the Body is removed from the Secure Message,
   analogously to Appendix F of [RFC7515].  Hence a SSM with Detached
   Content consists of Header and Tag.

4.1.2.2 Secure Encrypted Message Body

   Analogously to JWE, the terms Plaintext, Ciphertext and Additional
   Authenticated Data (AAD) are used for the SEM.  The Body of a SEM
   consists of Ciphertext, the encrypted Plaintext as defined by the
   Algorithm, and Additional Authenticated Data (AAD) which is integrity
   protected by the Algorithm as defined by the Cipher Suite.  For SEM
   Detached Content is defined to mean that the AAD is removed from the
   Secure Message.  Hence a SEM with Detached Content consists of the
   Header, Ciphertext and Tag.

4.1.3 Secure Message Tag



Selander, et al.       Expires December 31, 2015                [Page 9]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   The SM Tag consists of the Signature / Message Authentication Code
   value as defined by the Algorithm, calculated over the SM Header, SM
   Body and EAD (if present).  The content of EAD depends on the Mode,
   see 5.1.3 and 5.2


5. Message Protection

   This section describes what is protected in a Secure Message and how
   it depends on the defined Modes "COAP" and "PAYL".  The use of
   Mode:COAP is signaled with the presence of the options Sig or Enc
   defined in this section.  The differences in SM Body and SM Tag as a
   function of Mode are described below.

   Both formats SSM and SEM defined in the previous section are
   applicable to both Modes.  For any Secure Message Mode, the SEM
   format SHALL be used by default.  Examples of SSM and SEM are given
   in Appendix F.

5.1 CoAP Message Protection (Mode:COAP)

   Referring to examples 4-6 in Section 2 and requirements a-f in
   Section 3, this section presents how to protect individual CoAP
   messages including options and header fields, as well as request-
   response message exchanges, using the Secure Message format.  This is
   called Mode:COAP.  An endpoint receiving a CoAP request containing a
   Secure Message with Mode:COAP MUST respond with a CoAP message
   containing a Secure Message with Mode:COAP.

   Since slightly different message formats are used for integrity
   protection only (SSM), and additional encryption (SEM), these cases
   are treated separately.

5.1.1 The Sig Option

   In order to integrity protect CoAP message exchanges including
   options and headers, a new CoAP option is introduced: the Sig option,
   containing a SSM Mode:COAP object.  Endpoints supporting this scheme
   MUST check for the presence of a Sig option, and verify the SSM as
   described in Section 5.1.1.2 before accepting a message as valid.

5.1.1.1 Option Structure

   The Sig option indicates that certain CoAP Header Fields, Options,
   and Payload (if present) are integrity and replay protected using a
   Secure Signed Message (SSM).  The Sig option SHALL contain a SSM with
   Detached Content (see Section 4.1.2.1).




Selander, et al.       Expires December 31, 2015               [Page 10]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   This option is critical, safe to forward, it is not part of a cache
   key, and it is not repeatable.  Table 1 illustrates the structure of
   this option.


   +-----+---+---+---+---+---------+--------+-----------+
   | No. | C | U | N | R | Name    | Format | Length *) |
   +-----+---+---+---+---+---------+--------+-----------+
   | TBD | x |   | x |   | Sig     | opaque |  12-TBD   |
   +-----+---+---+---+---+---------+--------+-----------+

         C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                       Table 1: The Sig Option


    *) Length is essentially Length(SSM Header) + Length(SSM Tag).  The
   minimum length is estimated in Appendix E.  The maximum length
   depends on actual message format selected and is TBD.


5.1.1.2 Integrity Protection and Verification

   A CoAP endpoint composing a message with the Sig option SHALL process
   the SSM and produce the SSM Tag, as defined in 5.1.1.3 and 5.1.3,
   analogously to the specification for producing a JWS object as
   described in Section 5.1 of [RFC7515] (cf. Appendix B).  In addition,
   the sending endpoint SHALL process the Sequence Number as described
   in Section 5.3.

   A CoAP endpoint receiving a message containing the Sig option SHALL
   first recreate the SSM Body as described in Section 5.1.1.3, and then
   verify the SSM Tag as described in Section 5.1.3, analogously to the
   specification for verifying a JWS object as described in Section 5.2
   of [RFC7515] (cf. Appendix B).  In addition, the receiving endpoint
   SHALL process the Sequence Number as described in Section 5.3.

   NOTE: The explicit steps of the protection and verification procedure
   will be included in a future version of this draft.

5.1.1.3 SSM Body

   The SSM Body of SHALL consist of the following data, in this order:

      o the 8-bit CoAP header field Code;

      o all CoAP options present which are marked as IP in Table 3
        (Appendix A), in the order as given by the option number (each



Selander, et al.       Expires December 31, 2015               [Page 11]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


        Option with Option Header including delta to previous IP-marked
        Option which is present); and

      o the CoAP Payload (if any).


5.1.2 The Enc Option

   In order to encrypt and integrity protect CoAP messages, a new CoAP
   option is introduced: the Enc option, indicating the presence of a
   SEM Mode:COAP object in the CoAP message, containing the encrypted
   part of the CoAP message.  Endpoints supporting this scheme MUST
   check for the presence of an Enc option, and verify the SEM as
   described in 5.1.2.2 before accepting a message as valid.


5.1.2.1 Option Structure

   The Enc option indicates that certain CoAP Options and Payload (if
   present) are encrypted, integrity and replay protected using a Secure
   Encrypted Message (SEM) with Detached Content (see Section 4.1.2.2).
   The structure of a CoAP message with an Enc option is described in
   Section 5.1.2.4.

   This option is critical, safe to forward, it is not part of a cache
   key, and it is not repeatable.  Table 2 illustrates the structure of
   this option.


   +-----+---+---+---+---+---------+--------+-------------+
   | No. | C | U | N | R | Name    | Format | Length *)   |
   +-----+---+---+---+---+---------+--------+-------------+
   | TBD | x |   | x |   | Enc     | opaque | 0 or 12-TBD |
   +-----+---+---+---+---+---------+--------+-------------+

         C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                       Table 2: The Enc Option


    *) Length indicates in this case the additional length added to the
   total length of all CoAP options.  If the CoAP message has Payload,
   then the Enc option is empty, otherwise it contains the SEM (see
   Section 5.1.2.4).  In the latter case, the SEM Ciphertext contains
   the encrypted CoAP Options (see Section 5.1.2.3), which are thus
   excluded from plaintext part of the message.  Hence the additional
   length is essentially Length(SEM Header) + Length(SEM Tag).  The
   minimum length is estimated in Appendix E.  The maximum length



Selander, et al.       Expires December 31, 2015               [Page 12]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   depends on actual message format selected and is TBD.

5.1.2.2 Encryption and Decryption

   A CoAP endpoint composing a message with the Enc option SHALL process
   the SEM and produce the SEM Ciphertext and SEM Tag, as defined in
   5.1.2.3 and 5.1.3, analogously to the specification for producing a
   JWE object as described in Section 5.1 of [RFC7516] (cf. Appendix B).
    In addition, the sending endpoint SHALL process the Sequence Number
   as described in Section 5.3.

   A CoAP endpoint receiving a message containing the Enc option SHALL
   first recreate the SEM Body as described in Section 5.1.2.3, and then
   decrypt and verify the SEM analogously to the specification for
   verifying a JWE object as describe in Section 5.2 of [RFC7516] (cf.
   Appendix B).  In addition, the receiving endpoint SHALL process the
   Sequence Number as described in Section 5.3.

   NOTE: The explicit steps of the protection and verification procedure
   will be included in a future version of this draft.

5.1.2.3 SEM Body

   The SEM Plaintext SHALL consist of the following data, formatted as a
   CoAP message without Header consisting of:

      o all CoAP Options present which are marked as E in Table 3 (see
        Appendix A), in the order as given by the Option number (each
        Option with Option Header including delta to previous E-marked
        Option); and

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


   The SEM Additional Authenticated Data SHALL consist of the following
   data, in this order:

      o the 8-bit CoAP header field Code;

      o all CoAP options present which are marked as IP and not marked
        as E in Table 2 (see Appendix A), in the order as given by the
        Option number (each Option with Option Header including delta to
        previous such Option).

5.1.2.4 CoAP Message with Enc Option

   An unprotected CoAP message is encrypted and integrity protected by



Selander, et al.       Expires December 31, 2015               [Page 13]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   means of an Enc option and a SEM.  The structure and format of the
   protected CoAP message being sent instead of the unprotected CoAP
   message is now described.

   The protected CoAP message is formatted as an ordinary CoAP message,
   with the following Header, Options and Payload:

      o The CoAP header SHALL be the same as the unprotected CoAP
        message.

      o The CoAP options SHALL consist of the unencrypted options of the
        unprotected CoAP message, and the Enc option.  The options shall
        be formatted as in a CoAP message (each Option with Options
        Header including delta to previous unencrypted Option).

      o If the unprotected CoAP message has no Payload then the Enc
        option SHALL contain the SEM with Detached Content.  If the
        unprotected CoAP message has Payload, then the SEM option SHALL
        be empty and the Payload of the CoAP message SHALL be the SEM
        with Detached Content.  The Payload is prefixed by the one-byte
        Payload Marker (0xFF).

5.1.3 SM Tag

   This section describes the SM Tag for Mode:COAP, which applies both
   to SEM and SSM.  The SM Tag is defined in 4.1.3.  If the message is a
   CoAP Request, then EAD SHALL be empty.  If the message is a CoAP
   Response, then EAD SHALL consist of the TID of the associated CoAP
   Request.


5.2 Payload Only Protection (Mode:PAYL)

   Referring to examples 1-3 in Section 2 and requirements a and b in
   Section 3, the case of only protecting CoAP payload using the Secure
   Message format is now discussed.  This is called Mode:PAYL.

   The sending endpoint SHALL wrap the Payload, and the receiving
   endpoint unwrap the Payload in the relevant SM format (SSM or SEM)
   Mode:PAYL.  The SSM (SEM) SHALL be protected (encrypted) and verified
   (decrypted) as described in 5.1.1.2 (5.1.2.2), including replay
   protection as described in section 5.3.

   NOTE: The explicit steps of the protection and verification procedure
   will be included in a future version of this draft.

   For Mode:PAYL, the EAD SHALL be empty.  Hence, the SM Tag is
   calculated over the SM Header and SM Body.



Selander, et al.       Expires December 31, 2015               [Page 14]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   A CoAP message where the Payload is wrapped as a Mode:PAYL object is
   indicated by setting the option Content-Format to application/smpayl.
    A CoAP client may request a response containing such a payload
   wrapping by setting the option Accept to application/smpayl.  (See
   Section 8.)


5.3 Replay Protection and Freshness

   In order to protect from replay of messages and verify freshness of
   responses, a CoAP endpoint supporting OSCOAP SHALL maintain
   Transaction Identifiers (TIDs) of sent and received Secure Messages
   (see section 4.1.1).

5.3.1 Replay Protection

   An endpoint SHALL maintain a TID and associated security
   context/key(s) for each other endpoint it receives messages from, and
   one TID and associated security context/key(s) for protecting sent
   messages.  Depending on use case, an endpoint MAY maintain a sliding
   receive window for Sequence Numbers associated to TIDs in received
   messages, equivalent to the functionality described in section
   4.1.2.6 of [RFC6347].

   Before composing a new message a sending endpoint SHALL step the
   Sequence Number of the associated send TID and SHALL include it in
   the SM Header parameter Sequence Number as defined in section 4.1.1.
   However, if the Sequence Number counter wraps, the client must first
   acquire a new TID and associated security context/key(s).  The latter
   is out of scope of this memo.

   A receiving endpoint SHALL verify that the Sequence Number received
   in the SM Header is greater than the Sequence Number in the TID for
   received messages (or within the sliding window and not previously
   received) and update the TID (window) accordingly.

5.3.2 Freshness

   If a CoAP server receives a valid Secure Message request in
   Mode:COAP, then the response SHALL include the TID of the request as
   EAD, as defined in section 5.1.3.  If the CoAP client receives a
   Secure Message response in Mode:COAP, then the client SHALL verify
   the signature by reconstructing SM Body and using the TID of its own
   associated request as EAD, as defined in section 5.1.3.


6. Security Considerations




Selander, et al.       Expires December 31, 2015               [Page 15]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   In scenarios with proxies, gateways, or caching, DTLS only protects
   data hop-by-hop meaning that these intermediary nodes can read and
   modify information.  The trust model where all participating 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 weak.

   DTLS protects the entire CoAP message including Header, Options and
   Payload, whereas Mode:COAP  protects the message fields described in
   Appendix A.  The cost for DTLS providing this protection is the
   overhead in e.g. additional messages, processing, memory incurred by
   the DTLS Handshake protocol, which can be omitted in use cases where
   key establishment can be provided by other means.

   Mode:COAP provides point to point encryption, integrity and replay
   protection, and freshness of response.  Payload as well as relevant
   options and header field Code are protected.

   Mode:PAYL only protects payload and only gives replay protection (not
   freshness), but allows additional use cases such as point to multi-
   point interactions including publish-subscribe, reverse proxies and
   proxy caching of responses.  In case of symmetric keys the receiver
   does not get data origin authentication, which requires a digital
   signature using a private asymmetric key.  Mode:PAYL SHALL NOT be
   used in cases where the CoAP header field Code needs to be integrity
   protected.

   Blockwise transfers in CoAP [I-D.ietf-core-coap-block] can be applied
   both to Mode:COAP and Mode:PAYL.  With Mode:COAP each block and the
   Block options are integrity protected.  Hence each individual block
   can be securely verified by the receiver, retransmission securely
   requested etc.  With Mode:PAYL the entire payload is encapsulated in
   a Secure Message which is partitioned into blocks which are sent with
   unprotected CoAP.  The receiver is able to verify the integrity of
   the payload but only after the last block containing the
   signature/MAC is received, and if the verification fails the entire
   message needs to be resent.  However, if the verification succeeds,
   then the transmission in Mode:PAYL has less computational and packet
   overhead since only one signature/MAC was generated and sent.  As
   CoAP blockwise transfer with Mode:PAYL is prone to Denial of Service
   attacks, it should only be used for exchanges where this threat can
   be mitigated, for example within a local area network where link-
   layer security is activated.




Selander, et al.       Expires December 31, 2015               [Page 16]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   The Version header field is not integrity protected to allow
   backwards compatibility with future versions of CoAP.  Considering
   this, it may in theory be possible to launch a cross-version attack,
   e.g. something analogous to a bidding down attack.  Future updates of
   CoAP would need to take this into account.

   The use of sequence numbers for replay protection introduces the
   problem related to wrapping of the counter.  The alternatives also
   have issues: very constrained devices may not be able to support
   accurate time or generate and store large numbers of random nonces.
   The requirement to change key at counter wrap is a complication, but
   it also forces the user of this specification to think about
   implementing key renewal.

   Independently of message format, and whether the target is CoAP
   message protection or payload only protection, this specification
   needs to be complemented with a procedure whereby the client and the
   server establish the keys used for wrapping and unwrapping the Secure
   Message.  One way to address key establishment is to assume that
   there is a trusted third party which can support client and server,
   such as the Authorization Server in [I-D.gerdes-ace-actors].  The
   Authorization Server may, for example, authenticate the client on
   behalf of the server, or provide cryptographic keys or credentials to
   the client and/or server which can be used in the Secure Message
   exchange.  Similarly, the Authorization Server may, on behalf of the
   server, notify the client of server supported ciphers, in order to
   facilitate the usage of OSCOAP in deployments with multiple supported
   cryptographic algorithms.

   The security contexts required for SSM and SEM are different.  For a
   SSM, the security context is essentially Algorithm, Context
   Identifier, Sequence Number and Key.  For a SEM it is also required
   to have a unique Initialization Vector for each message.  The
   Initialization Vector SHALL be the concatenation of a Salt (4 bytes
   unsigned integer) and the Sequence Number.  The Salt SHOULD be
   established between sender and receiver before the message is sent,
   to avoid the overhead of sending it in each message.  For example,
   the Salt may be established by the same means as keys are
   established.  For a SEM, the security context is essentially
   Algorithm, Context Identifier, Salt, Sequence Number and Key.


7. Privacy Considerations

   End-to-end integrity protection provides certain privacy properties,
   e.g. protection of communication with sensor and actuator from
   manipulation which may affect the personal sphere.  End-to-end
   encryption of payload and certain CoAP options provides additional



Selander, et al.       Expires December 31, 2015               [Page 17]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   protection as to the content and nature of the message exchange.

   The headers sent in plaintext allow for example matching of CON and
   ACK (CoAP Message Identifier), matching of request and response
   (Token).  Plaintext options could also reveal information, e.g.
   lifetime of measurement (Max-age), or that this message contains one
   data point in a sequence (Observe).


8.  IANA Considerations

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

   The following entry is added to the CoAP Option Numbers registry:

                        +--------+---------+-------------------+
                        | Number | Name    |     Reference     |
                        +--------+---------+-------------------+
                        |  TBD   | Sig     | [[this document]] |
                        +--------+---------+-------------------+
                        |  TBD   | Enc     | [[this document]] |
                        +--------+---------+-------------------+




   This document registers the following value in the CoAP Content
   Format registry established by [RFC7252].


      Media Type: application/smpayl

      Encoding: -

      Id: 70

      Reference: [this document]


9.  Acknowledgements

   Klaus Hartke has independently been working on the same problem and a
   similar solution: establishing end-to-end security across proxies by
   adding a CoAP option.  We are grateful to Malisa Vucinic for
   providing helpful and timely comments.





Selander, et al.       Expires December 31, 2015               [Page 18]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


10.  References

10.1  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6347]  Rescorla, E., and N. Modadugu,  "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014.

   [RFC7258]  Farrell, S., and H. Tschofenig, "Pervasive Monitoring Is
              an Attack", RFC 7258, May 2014.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura,  "JSON Web
              Signature (JWS)", RFC 7515, May 2015.

   [RFC7516]  Jones, M., and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, May 2015.


10.2  Informative References

   [I-D.gerdes-ace-actors]
              Gerdes, S., Seitz, L., G. Selander, Bormann, C. "An
              Arhitecture for Authorization in Constrained
              Environments", draft-gerdes-ace-actors-05 (work in
              progress), April 2015.

   [I-D.ietf-ace-usecases]
              Seitz, L., Gerdes, S., Selander, G., Mani, M., and S.
              Kumar, "ACE use cases", draft-ietf-ace-usecases-04 (work
              in progress), March 2015.

   [I-D.schaad-cose-msg]
              Schaad, J., "CBOR Encoded Message Syntax", draft-schaad-
              cose-msg-00 (work in progress), June 2015.

   [I-D.ietf-core-coap-block]
              Bormann, C., and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-17 (work in progress), July 2014.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
              36, RFC 4949, August 2007.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,



Selander, et al.       Expires December 31, 2015               [Page 19]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


              RFC 5652, September 2009.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen,  "Terminology for
              Constrained-Node Networks", RFC 7228, May 2014.

   [RFC7231]  Fielding, R., and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

Appendix A. Which CoAP Header Fields and Options to Protect

   In the case of CoAP Message Protection (Mode:COAP) as much as
   possible of the CoAP message is protected.  However, not all CoAP
   header fields or options can be encrypted and integrity protected,
   because some are intended to be read or changed by an intermediary
   node.

A.1 CoAP Header Fields

   The CoAP Message Layer parameters, Type and Message ID, as well as
   Token and Token Length may be changed by a proxy and thus SHALL
   neither be integrity protected nor encrypted.  Example 5 in Section 2
   shows that the Code SHALL be integrity protected.  The Version
   parameter SHALL neither be integrity protected nor encrypted (see
   Section 6).

A.2 CoAP Options

   This section describes what options need to be integrity protected
   and encrypted.  On a high level, all CoAP options must be encrypted
   by default, unless intended to be read by an intermediate node; and
   integrity protected, unless intended to be changed by an intermediate
   node.

   However, some special considerations are necessary because CoAP
   defines certain legitimate proxy operations, because the security
   information itself may be transported as an option, and because
   different processing is performed for SSM and SEM.

A.2.1 Integrity Protection

   CoAP options which are not intended to be changed by an intermediate
   node MUST be integrity protected:

      o CoAP options which are Safe-to-Forward SHALL be integrity
        protected, the only exception being the security options Enc and
        Sig.  See Table 3.

      o Block1, Block2 are Unsafe but not intended to be modified by



Selander, et al.       Expires December 31, 2015               [Page 20]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


        intermediaries and hence SHALL be integrity protected.

   CoAP options which are intended to be modified by a proxy can be
   divided into two categories, those that are intended to change in a
   predictable way, and those which are not.  The following options are
   of the latter kind and SHALL NOT be integrity protected:

      o Max-Age, Observe: These options may be modified by a proxy in a
        way that is not predictable for client and server.

   The remaining options may be modified by a proxy, but when they are,
   the change is predictable.  Therefore it is possible to define
   "invariants" which can be integrity protected.

A.2.1.1 Proxy-Scheme

   A Forward Proxy is intended to replace the URI scheme with the
   content of the Proxy-Scheme option.  The Proxy-Scheme option is
   defined to be an invariant with respect to the following processing:

      o If there is a Proxy-Scheme present, then the client MUST
        integrity protect the Proxy-Scheme option.

      o If there is no Proxy-Scheme option present the client SHALL
        integrity protect the Proxy-Scheme option set to the URI scheme
        used in the message sent.

      o The server SHALL insert the Proxy-Scheme option with the name of
        the URI scheme the message was received with before verifying
        the integrity.


A.2.1.2 Uri-*

   For options related to URI of resource (Uri-Host, Uri-Port, Uri-Path,
   Uri-Query, Proxy-Uri) a Forward Proxy is intended to replace the Uri-
   * options with the content of the Proxy-Uri option.

   The Proxy-Uri option is defined to be an invariant with respect to
   the following processing (applying to a SSM, for SEM see next
   section):

      o If there is a Proxy-Uri present, then the client MUST integrity
        protect the Proxy-Uri option and the Uri-* options MUST NOT be
        integrity protected.

      o If there is no Proxy-Uri option present, then the client SHALL
        compose the full URI from Uri-* options according to the method



Selander, et al.       Expires December 31, 2015               [Page 21]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


        described in section 6.5 of [RFC7252].  The SM Tag is calculated
        on the following message, modified compared to what is sent:

         o All Uri-* options removed

         o A Proxy-Uri option with the full URI included

      o The server SHALL compose the URI from the Uri-* options
        according to the method described in section 6.5 of [RFC7252].
        The so obtained URI is placed into a Proxy-Uri option, which is
        included in the integrity verification.

A.2.2 Encryption

   All CoAP options MUST be encrypted, except the options below which
   MUST NOT be encrypted:

      o Max-Age, Observe:  This information is intended to be read by a
        proxy.

      o Enc, Sig: These are the security-providing options.

      o Uri-Host, Uri-Port:  This information can be inferred from
        destination IP address and port.

      o Proxy-Uri, Proxy-Scheme:  This information is intended to be
        read by a proxy.

   In the case of a SEM, the Proxy-Uri MUST only contain Uri-Host and
   Uri-Port and MUST NOT contain Uri-Path and Uri-Query because the
   latter options are not intended to be revealed to a Forward Proxy.

A.2.3 Summary

   Table 3 summarizes which options are encrypted and integrity
   protected, if present.

   In a SSM, options marked with "a" and "b" are composed into a URI as
   described above and included as the Proxy-Uri option which is part of
   the SSM Body.  In a SEM, options marked "a" are composed into a URI
   as described above and included as the Proxy-Uri option in the SEM
   Additional Authenticated Data.


   +-----+---+---+---+---+----------------+--------+--------+--------+
   | No. | C | U | N | R | Name           | Format | Length | E | IP |
   +-----+---+---+---+---+----------------+--------+--------+--------+
   |   1 | x |   |   | x | If-Match       | opaque | 0-8    | x | x  |



Selander, et al.       Expires December 31, 2015               [Page 22]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   |   3 | x | x | - |   | Uri-Host       | string | 1-255  |   | a  |
   |   4 |   |   |   | x | ETag           | opaque | 1-8    | x | x  |
   |   5 | x |   |   |   | If-None-Match  | empty  | 0      | x | x  |
   |   6 |   | x | - |   | Observe        | uint   | 0-3    |   |    |
   |   7 | x | x | - |   | Uri-Port       | uint   | 0-2    |   | a  |
   |   8 |   |   |   | x | Location-Path  | string | 0-255  | x | x  |
   |  11 | x | x | - | x | Uri-Path       | string | 0-255  | x | b  |
   |  12 |   |   |   |   | Content-Format | uint   | 0-2    | x | x  |
   |  14 |   | x | - |   | Max-Age        | uint   | 0-4    |   |    |
   |  15 | x | x | - | x | Uri-Query      | string | 0-255  | x | b  |
   |  17 | x |   |   |   | Accept         | uint   | 0-2    | x | x  |
   |  20 |   |   |   | x | Location-Query | string | 0-255  | x | x  |
   |  23 | x | x | - |   | Block2         | uint   | 0-3    | x | x  |
   |  27 | x | x | - |   | Block1         | uint   | 0-3    | x | x  |
   |  28 |   |   | x |   | Size2          | uint   | 0-4    | x | x  |
   |  35 | x | x | - |   | Proxy-Uri      | string | 1-1034 |   | x  |
   |  39 | x | x | - |   | Proxy-Scheme   | string | 1-255  |   | x  |
   |  60 |   |   | x |   | Size1          | uint   | 0-4    | x | x  |
   +-----+---+---+---+---+----------------+--------+--------+--------+

         C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable,
         E=Encrypt, IP=Integrity Protect.

         Table 3: Protected CoAP options in Mode=COAP.


Appendix B.  JOSE Profile of SM

   This section defines profiles of JWS and JWE complying with the
   Secure Message format (see Section 4.1).  The use of compact
   serialization is assumed.

B.1  JWS as Secure Signed Message

   The JOSE Header of JWS contains the mandatory parameter "alg",
   defined in Section 4.1.1 of [RFC7515], which corresponds to the
   parameter Algorithm of the Secure Message.

   A JWS is a Secure Message if the JOSE Header includes

      o the new parameter "cid" defined in B.4, and

      o the new parameter "seq" defined in B.3.


   An SSM with Detached Content corresponds to a JWS with JOSE Header
   and JWS Signature; i.e. no JWS Payload.




Selander, et al.       Expires December 31, 2015               [Page 23]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


B.2  JWE as Secure Encrypted Message

   In case of JWE, the SM Header parameters of a JWE consists of the
   JOSE Header Parameters and JWE Initialization Vector (IV).

   The JOSE Header of JWE contains the mandatory parameter "enc",
   defined in Section 4.1.2 of [RFC7516], which corresponds to the
   parameter Algorithm of the Secure Message.  The JOSE Header also
   contains the mandatory parameter "alg", the key encryption algorithm,
   which in the current version of the draft is assumed to be equal to
   "dir" (constant).  It is also assumed that plaintext compression
   (zip) is not used.

   A JWE is a Secure Message if the JOSE Header includes

      o the new parameter "cid" defined in B.4, and

      o the IV contains the Sequence Number and a Salt (see Section 6).

   An SEM with Detached Content corresponds to a JWE with JOSE Header,
   JWE Initialization Vector, JWE Ciphertext and JWE Authentication Tag;
   i.e. no JWE AAD.


B.3 "seq" (Sequence Number) Header Parameter

   The Sequence Number, corresponding to the Secure Message parameter
   with the same name (Section 4.1.1), SHALL be an integer represented
   as a byte string.  Only the significant bytes are sent (initial bytes
   with zeros are removed).  The start sequence number SHALL be 0.  For
   a given key, "seq" MUST NOT be used more than once.

   The parameter "seq" SHALL be marked as critical using the "crit"
   header parameter (see section 4.1.11 of  [RFC7515]), meaning that if
   a receiver does not understand this parameter it must reject the
   message.


B.4 "cid" (Context Identifier) Header Parameter

   The Context Identifier, corresponding to the Secure Message parameter
   with the same name (Section 4.1.1), SHALL be a unique byte string
   identifying the security context of the sending party.  The parameter
   "cid" SHALL be marked as critical.


Appendix C.  Compact Secure Message




Selander, et al.       Expires December 31, 2015               [Page 24]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   For constrained environments it is important that the message
   expansion due to security overhead is kept at a minimum.  As an
   attempt to assess what this minimum expansion could be, this section
   defines an optimized bespoke Secure Message format (Section 4.1)
   called the Compact Secure Message (CSM) format.  This is intended as
   a benchmark for COSE [I-D.schaad-cose-msg].

   The Compact Secure Message (CSM) format is depicted in Figure 4.


     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      ALG      |   CL    |  SL |             CID               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                            SEQ                                ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                            Body                               ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                            Tag                                ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 4: Compact Secure Message format

   The CSM Header (see Section 4.1.1.) consists of 2 bytes of fixed
   length parameters and two variable length parameters, Context
   Identifier (CID) and Sequence Number (SEQ).  The Header parameters
   are (compare Table 5):


      o Algorithm (ALG).  This parameter consists of an encoding of the
        ciphersuite used in the Secure Message.  The encoding is TBD.

      o CID Length (CL).  This parameter consist of a length indication
        of the header parameter Context Identifier.  The actual length
        of CID is CL + 1 bytes.

      o SEQ Length (SL).  This parameter consist of a length indication
        of the header parameter Sequence Number.  The actual length of
        SEQ is SL + 1 bytes.

      o Context Identifier (CID).  This parameter identifies the
        security context/key(s) used to protect the Secure Message.
        Only the significant bytes are sent (initial bytes with zeros
        are removed).

      o Sequence Number (SEQ).  This parameter consists of the sequence
        number used by the sender of the Secure Message.  Only the



Selander, et al.       Expires December 31, 2015               [Page 25]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


        significant bytes are sent (initial bytes with zeros are
        removed).


           +------+---------------------------+--------------------+
           | Name | Parameter                 | Length             |
           +------+---------------------------+--------------------+
           |  ALG | Algorithm                 | 8 bits             |
           +------+---------------------------+--------------------+
           |   CL | Context Identifier Length | 5 bits             |
           +------+---------------------------+--------------------+
           |   SL | Sequence Number Length    | 3 bits             |
           +------+---------------------------+--------------------+
           |  CID | Context Identifier        | CL + 1: 1-32 bytes |
           +------+---------------------------+--------------------+
           |  SEQ | Sequence Number           | SL + 1: 1-8 bytes  |
           +------+---------------------------+--------------------+

                  Table 5: CSM Header Parameters.
                  The minimum CSM Header is 4 bytes.

   The TID consists of the concatenation of SEQ and CID, in that order,
   formatted as in the CSM format (initial bytes with zeros are
   removed).

   The content of CSM Body depends on whether it is a SSM or a SEM (see
   Section 4.1.2) which is determined by the Algorithm.  This version of
   the draft focuses on Secure Message with Detached Content.  Hence,
   the SSM Body is empty and the SEM Body consists of the Ciphertext.
   In the former case, the length of the CSM Body is 0.  In the latter
   case, the length of the CSM Body equals the sum of the lengths of the
   present CoAP options marked encrypted in Table 3 and the length of
   the payload of the unprotected CoAP message.

   The CSM Tag contains the MAC/Signature as determined from the
   Algorithm.  The length is determined by ALG.


Appendix D.  COSE Profile of SM

   This section defines a profile of the 00-version of COSE [I-D.schaad-
   cose-msg] complying with the Secure Message format (see Section 4.1)
   and supporting the two modes of operation Mode:COAP and Mode:PAYL.
   In the last subsection we elaborate on possible optimizations.

D.1  COSE_Sign or COSE_mac as Secure Signed Message

   SSM corresponds to COSE_MSG msg_type 1 (COSE_Sign) or 3 (COSE_mac).



Selander, et al.       Expires December 31, 2015               [Page 26]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


D.1.1  COSE_Sign as Secure Signed Message

   A COSE_MSG of type COSE_Sign is a Secure Message if its fields are
   defined as follows (see example in Appendix E.2).

   The "Headers" field of COSE_Sign MUST contain the field "protected"
   and this field MUST include the new "seq" parameter corresponding to
   the parameter Sequence Number of the Secure Message (see section
   4.1.1).

   The mandatory "signatures" array contains one "COSE_signature" item
   which contains a "protected" field and the mandatory "signature"
   field. The "protected" field includes:

      o the "alg" parameter which corresponds to the parameter Algorithm
        of the Secure Message (see section 4.1.1);

      o the new "cid" parameter which corresponds to the parameter
        Context Identifier of the Secure Message (see section 4.1.1);

   The mandatory "signature" field contains the computed signature
   value.

   A SSM with digital signature and Detached Content corresponds to
   COSE_sign with "Headers" and "signatures" fields; i.e. no "payload"
   field.



D.1.2  COSE_mac as Secure Signed Message

   A COSE_MSG of type COSE_mac is a Secure Message if its fields are
   defined as follows (see example in Appendix E.1).

   The "Headers" field of COSE_mac object MUST contain the "protected"
   field, the "recipient" field and the mandatory "tag" field.  The
   "protected" field MUST include:

      o the "alg" parameter which corresponds to the parameter Algorithm
        of the Secure Message (see section 4.1.1);

      o the new "seq" parameter corresponding to the parameter Sequence
        Number of the Secure Message (see section 4.1.1).

   The "recipients" array contains one "COSE_encrypt_a" item (section 5
   of [I-D.schaad-cose-msg]), which contains an "unprotected" field that
   includes:




Selander, et al.       Expires December 31, 2015               [Page 27]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


      o the "alg" parameter corresponding to the key encryption
        algorithm, which in the current version of the draft is assumed
        to be equal to "dir" (constant). (Appendix A of [I-D.schaad-
        cose-msg]);

      o the new "cid" parameter which corresponds to the parameter
        Context Identifier of the Secure Message (see section 4.1.1);

   The mandatory "tag" field contains the MAC value.

   A SSM with MAC and Detached Content corresponds to a COSE_sign with
   "Headers", "recipients" and "tag" fields; i.e. no "payload" field.


D.2  COSE_encrypt as Secure Encrypted Message

   SEM with AEAD algorithm corresponds to COSE_MSG msg_type 2
   (COSE_encrypt).  A COSE_MSG of type COSE_encrypt [I-D.schaad-cose-
   msg] is a Secure Message if its fields are defined as follows (see
   example in Appendix E.3).

   The "Headers" field of COSE_encrypt MUST contain the "protected"
   field, the "recipient" field, the "cipherText" field and depending on
   the algorithm used, the "iv" field.

   The "iv" corresponds to the Initialization Vector, which contains a
   Salt (see Section 6) and Sequence Number as defined in section 4.1.1.
   For some algorithms, it is mandatory to include the "iv" field and
   hence the Salt is sent in each message.

   The "protected" field includes:

      o the "alg" parameter which corresponds to the parameter Algorithm
        of the Secure Message (see section 4.1.1);

      o the new "seq" parameter corresponding to the parameter Sequence
        Number of the Secure Message (see section 4.1.1). This parameter
        is present only if the "iv" field is not present in the
        COSE_encrypt structure.

   The "recipients" array contains one "COSE_encrypt_a" item as defined
   in section 5 of [I-D.schaad-cose-msg], which contains an
   "unprotected" field that includes:

      o the "alg" parameter corresponding to the key encryption
        algorithm, which in the current version of the draft is assumed
        to be equal to "dir" (constant). (Appendix A of [I-D.schaad-
        cose-msg]);



Selander, et al.       Expires December 31, 2015               [Page 28]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


      o the new "cid" parameter which corresponds to the parameter
        Context Identifier of the Secure Message (see section 4.1.1);

   The "cipherText" field contains the encrypted plain text, as defined
   in section 5 of [I-D.schaad-cose-msg].

   A SEM with Detached Content corresponds to a COSE_encrypt with
   "Headers", "recipients", optionally "iv" and "cipherText" fields;
   i.e. no "aad" field.


D.3 COSE optimizations

   This section lists potential optimizations of COSE [I-D.schaad-cose-
   msg] for the purpose of reducing message size and improving
   performance in constrained node networks.  The message sizes
   resulting from the first two optimizations are presented in Appendix
   E (as "modified COSE").

      1. For COSE_encrypt and COSE_mac, there is a 'recipient' field
         (see section 6 of [I-D.schaad-cose-msg]).  This field is
         intended for a setting when the sender is aware of the
         recipients of the message, and can wrap keys for these
         recipients.  This is not necessarily true in the use cases
         targeting constrained devices and thus one possible
         optimization is to remove the 'recipient' field.  The Context
         Identifier "cid" can be carried in the Header, preferably in
         the protected field, to avoid both protected and unprotected
         fields causing additional overhead.  (An alternative is to
         define "tid", Transaction Identifier, as an array consisting of
         "seq" and "cid".)

      2. Analogous to other key values, one-byte keys/labels can be
         assigned to the new parameters defined in this document and
         cipher suites adapted to constrained device processing.  For
         example: "cid" = 11, "seq" = 12, and "AES-CCM" = 14.

      3. The combination of secret key encryption and digital signature
         is well founded in the use cases.  A solution based on wrapping
         one COSE message into another creates substantial overhead (see
         difference between modified COSE and CSM in Table 11 of
         Appendix E.4).  A valuable optimization would be to define
         combined cipher suites and security contexts, and corresponding
         "alg" and "cid" parameters.  An example would be 128-bit AES
         and particular curve parameters for a 64 bytes ECDSA signature.

      4. Digitally signed messages have the largest absolute overhead
         due to the size of the signature (see Appendices E.2 and E.4).



Selander, et al.       Expires December 31, 2015               [Page 29]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


         Whereas certain MACs can be securely truncated, signatures
         cannot.  Signature schemes with message recovery allow some
         remedy since they allow part of the message to be recovered
         from the signature itself and thus need not be sent.  The
         effective size of the signature could in this way be
         considerably reduced, which would have a large impact on the
         message size (compare size of signature and total overhead in
         Tables 9 and 11).  A valuable optimization is thus to support
         signature schemes with message recovery.


Appendix E.  Comparison of message sizes

   This section gives some examples of overhead incurred with JOSE, with
   the current proposal for COSE at the time of writing [I-D.schaad-
   cose-msg], and with CSM.  CSM should be viewed as a lower bound for
   COSE.  Message sizes are also listed for a modified version of COSE
   implementing some of the optimizations described in Appendix D.3.

   Motivated by the use cases, there are four different kinds of
   protected messages that need to be supported: message authentication
   code, digital signature, authenticated encryption, and symmetric
   encryption + digital signature.  The latter is relevant e.g. for
   proxy-caching and publish-subscribe with untrusted intermediary (see
   Appendix F.2).  The sizes estimated for selected algorithms are
   detailed in the subsections.

   The size of the header is shown separately from the size of the
   MAC/signature, since JWS/JWE has no provisions for truncating it.
   Compact serialization for both JWS and JWE is assumed.  For CSM the
   encoding of algorithms is assumed as in COSE.  An 8-byte Context
   Identifier and a 3-byte Sequence Number are used throughout all
   examples.  To make it easier to read, COSE objects are represented
   using CBOR's diagnostic notation rather than a binary dump.

E.1 SSM: Message Authentication Code

   This example is based on HMAC-SHA256, with truncation to 16 bytes.
   For JWS the following header is used:

   {"alg":"HS256","cid":0xa1534e3c5fdc09bd,"seq":0x112233}

   which encodes to a size of 74 bytes in Base64url, and the 32 bytes of
   HS256 MAC encode to 43 bytes.  The concatenation marks add 2 bytes to
   that in the total overhead.

   The same object in COSE gives:




Selander, et al.       Expires December 31, 2015               [Page 30]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   {1:3, 2:{1:4, "seq":h'112233'}, 9:[{3:{"cid":h'a1534e3c5fdc09bd', 1:-
   6}}], 10:MAC}, where MAC is the truncated 16-byte MAC.

   The COSE object encodes to a total size of 53 bytes.

   In a modified version of COSE, with no 'recipient' field (see section
   6 of [I-D.schaad-cose-msg])and protected "cid" in the header, 1-byte
   key values are assigned to "cid" and "seq", for exemple: "cid" = 11
   and "seq" = 12. The equivalent COSE object would be:

   {1:3, 2:{1:4, 12:h'112233', 11:h'a1534e3c5fdc09bd'}, 10:MAC}, where
   MAC is the truncated 16-byte MAC.

   This modified COSE object encodes to a total size of 40 bytes.

   For CSM the same header is represented by 13 bytes.

   Table 6 summarizes these results.

               +--------+----------------+----------------+
               | Scheme | Header  | MAC  | Total Overhead |
               +--------+----------------+----------------+
               |   JWS  |  74 B   | 43 B |    119 bytes   |
               +--------+---------+------+----------------+
               |  COSE  |  37 B   | 16 B |     53 bytes   |
               +--------+---------------------------------+
               |mod-COSE|  24 B   | 16 B |     40 bytes   |
               +--------+---------------------------------+
               |   CSM  |  13 B   | 16 B |     29 bytes   |
               +--------+---------------------------------+

               Table 6: Comparison of JWS, COSE, modified COSE and CSM
               for HMAC-SHA256.

E.2 SSM: Digital Signature

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

   For JWS the following header is used:

   {"alg":"ECDSA","cid":0xa1534e3c5fdc09bd,"seq":0x112233}

   which encodes to a size of 74 bytes in Base64url, and the 64 bytes of
   signature encode to 86 bytes.  The concatenation marks add 2 bytes to
   that in the total overhead.

   The same object in COSE gives:




Selander, et al.       Expires December 31, 2015               [Page 31]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   {1:1, 2:{"seq":h'112233'}, 5:[{2:{1:-7,"cid":h'a1534e3c5fdc09bd'},
   6:SIG}]}, where SIG is the 64-byte signature.

   The COSE object encodes to a total size of 100 bytes.

   In a modified version of COSE, 1-byte key values are assigned to
   "cid" and "seq", for exemple: "cid" = 11 and "seq" = 12.  The
   equivalent COSE object would be:

   {1:1, 2:{12:h'112233'}, 5:[{2:{1:-7,11:h'a1534e3c5fdc09bd'},  6:SIG},
   where SIG is the 64-byte signature.

   The COSE object encodes to a total size of 94 bytes.

   For CSM the same header is represented by 13 bytes.

   Table 7 summarizes these results.


               +--------+----------------+----------------+
               | Scheme | Header  | Tag  | Total Overhead |
               +--------+----------------+----------------+
               |   JWS  |  74 B   | 86 B |    162 bytes   |
               +--------+---------+------+----------------+
               |  COSE  |  36 B   | 64 B |    100 bytes   |
               +--------+---------------------------------+
               |mod-COSE|  30 B   | 64 B |     94 bytes   |
               +--------+---------------------------------+
               |   CSM  |  13 B   | 64 B |     77 bytes   |
               +--------+---------------------------------+

               Table 7: Comparison of JWS, COSE, modified COSE and CSM
               for 64 byte ECDSA signature.


E.3 SEM: Authenticated Encryption

   This example is based on both AES-128-CCM-8 and AES-128-GCM.  Since
   the former is not supported by JOSE, we use the latter for comparison
   between JOSE and COSE.

   For JWE it is assumed that the IV is generated from the Sequence
   Number and some previously agreed upon Salt.  This means it is not
   required to explicitly send the whole IV in the CSM format, but also
   that the JWE and COSE formats may omit the Sequence Number.

   The JWE header




Selander, et al.       Expires December 31, 2015               [Page 32]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   {"alg":"dir","cid":0xa1534e3c5fdc09bd,"enc":"A128GCM"}

   encodes to a size of 72 bytes in Base64url, while the necessary 12
   byte IV for GCM mode is expanded to 16 bytes by encoding.  The 16
   bytes of the authentication tag expand to 22 bytes.  The
   concatenation marks add 3 bytes to the total overhead.

   The corresponding COSE object is:

   {1:2, 2:{1:1}, 7:IV, 4:TAG, 9:[{3:{"cid":h'a1534e3c5fdc09bd', 1:-
   6}}]}, where IV is the 12-byte IV and TAG the 16-byte authentication
   tag

   The COSE object encodes to a total size of 59 bytes.

   Table 8 summarizes these results.


            +--------+----------------+-------+----------------+
            | Scheme | Header  | IV   |  Tag  | Total Overhead |
            +--------+----------------+-------+----------------+
            |   JWE  |  72 B   | 16 B |  22 B |    113 bytes   |
            +--------+---------+------+-------+----------------+
            |  COSE  |  31 B   | 12 B |  16 B |     59 bytes   |
            +--------+------------------------+----------------+

               Table 8: Comparison of JWE and COSE
               for AES-GCM.

   The same calculation have been done using CCM mode instead of GCM,
   and adding a 3-byte "seq".  The COSE object is represented as
   follows:

   {1:2, 2:{1:"AES-CCM","seq":h'112233'}, 4:TAG,
   9:[{3:{"cid":h'a1534e3c5fdc09bd', 1:-6}}]}, where TAG is the 16-byte
   authentication tag.

   The COSE object encodes to a total size of 52 bytes.

   In a modified version of COSE, the 'recipient' field is removed (see
   section 6 of [I-D.schaad-cose-msg])and "cid" is protected in the
   header. 1-byte key values are assigned to "cid", "seq" and "AES-CCM",
   for exemple: "cid" = 11, "seq" = 12 and "AES-CCM" = 14.  The
   equivalent COSE object would be:

   {1:2, 2:{1:14,12:h'112233'}, 4:TAG, 9:[{3:{11:h'a1534e3c5fdc09bd',
   1:-6}}]}, where TAG is the truncated 8-byte authentication tag.




Selander, et al.       Expires December 31, 2015               [Page 33]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   This modified COSE object encodes to a total size of 39 bytes.

   For CSM, the corresponding header for AES-128-CCM-8, including the 8
   byte Sequence Number, is represented by 13 bytes and the tag is
   truncated to 8 Bytes.

   Table 9 summarizes these results.


            +--------+---------+-------+----------------+
            | Scheme | Header  |  Tag  | Total Overhead |
            +--------+---------+-------+----------------+
            |  COSE  |  44 B   |  16 B |     60 bytes   |
            +--------+---------+-------+----------------+
            |mod-COSE|  31 B   |   8 B |     39 bytes   |
            +--------+-----------------+----------------+
            |   CSM  |  13 B   |   8 B |     21 bytes   |
            +--------+-----------------+----------------+

               Table 9: Comparison of COSE, modified COSE and CSM
               for AES-CCM.

E.4 SEM: Symmetric Encryption + Digital Signature

   This example is based on AES-128 and ECDSA with 64 bytes signature.
   JOSE and COSE require this to be a nested encapsulation of one object
   into another, here illustrated with a digitally signed AEAD protected
   object.

   For JWS the following header is used:

   {"alg":"ECDSA","cid":0xa1534e3c5fdc09bd,"seq":0x112233}

   which encodes to a size of 74 bytes in Base64url, and the 64 bytes of
   signature encode to 86 bytes.  The concatenation marks add 2 bytes to
   that in the total overhead.

   The payload of the JWS object is a JWE object with the following
   header:

   {"alg":"dir","cid":0xa1534e3c5fdc09bd,"enc":"A128GCM"}

   which encodes to a size of 72 bytes in Base64url, while the necessary
   12 byte IV for GCM mode is expanded to 16 bytes by encoding.  The 16
   bytes of the authentication tag expand to 22 bytes.  The
   concatenation marks add 3 bytes to the total overhead.

   The total size of the JWS object is 275 bytes.



Selander, et al.       Expires December 31, 2015               [Page 34]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   The same object in COSE gives:

   {1:1, 2:{"seq":h'112233'}, 4:{1:2, 2:{1:1}, 7:IV, 4:TAG,
   9:[{3:{"cid":h'a1534e3c5fdc09bd', 1:-6}}]}, 5:[{2:{1:-
   7,"cid":h'a1534e3c5fdc09bd'},  6:SIG}]}, where SIG is the 64-byte
   signature, IV is the 12-byte IV and TAG the 16-byte authentication
   tag.

   The COSE object encodes to a total size of 160 bytes.

   Table 10 summarizes these results.


            +--------+---------+-------+---------+----------------+
            | Scheme | Header  |  Sig  | Payload | Total Overhead |
            +--------+---------+-------+---------+----------------+
            |   JWS  |  74 B   |  86 B |  113 B  |    275 bytes   |
            +--------+---------+-------+---------+----------------+
            |  COSE  |  37 B   |  64 B |   59 B  |    160 bytes   |
            +--------+---------+-------+---------+----------------+

               Table 10: Comparison of JWS and COSE
               for nested AES-GCM within ECDSA.


   The same calculation have been done using CCM mode instead of GCM,
   and adding a 3-byte "seq" in the protected header.

   The COSE object is represented as follows:

   {1:1, 2:{"seq":h'112233'}, 4:{1:2, 2:{1:"AES-CCM","seq":h'112233'},
   4:TAG, 9:[{3:{"cid":h'a1534e3c5fdc09bd', 1:-6}}]}, 5:[{2:{1:-
   7,"cid":h'a1534e3c5fdc09bd'},  6:SIG}]}, where SIG is the 64-byte
   signature and TAG is the 16-byte authentication tag.

   The COSE object encodes to a total size of 153 bytes.

   In a modified version of COSE, the 'recipient' field is removed (see
   section 6 of [I-D.schaad-cose-msg])and "cid" is protected in the
   header. 1-byte key values are assigned to "cid", "seq" and "AES-CCM",
   for exemple: "cid" = 11, "seq" = 12 and "AES-CCM" = 14.  The
   equivalent COSE object would be:

   {1:1, 2:{12:h'112233'}, 4:{1:2, 2:{1:14,12:h'112233'}, 4:TAG,
   9:[{3:{11:h'a1534e3c5fdc09bd', 1:-6}}]}, 5:[{2:{1:-
   7,11:h'a1534e3c5fdc09bd'},  6:SIG}]}, where SIG is the 64-byte
   signature and TAG is the 8-byte truncated authentication tag.




Selander, et al.       Expires December 31, 2015               [Page 35]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   This modified COSE object encodes to a total size of 134 bytes.

   For CSM we assume that an (AES, ECDSA) cipher suite has been defined,
   and that the "cid" identifies the context used by both the
   algorithms.  Then the corresponding header is represented by 13
   bytes, and the signature by 64 bytes.

   Table 11 summarizes these results.


            +--------+---------+-------+---------+----------------+
            | Scheme | Header  |  Sig  | Payload | Total Overhead |
            +--------+---------+-------+---------+----------------+
            |  COSE  |  36 B   |  64 B |   52 B  |    153 bytes   |
            +--------+---------+-------+---------+----------------+
            |mod-COSE|  30 B   |  64 B |   39 B  |    134 bytes   |
            +--------+-----------------+---------+----------------+
            |   CSM  |  13 B   |  64 B |    0 B  |     77 bytes   |
            +--------+-----------------+---------+----------------+

               Table 11: Comparison of nested AES-CCM within
               ECDSA (COSE, modified COSE) and combined
               AES-ECDSA (CSM).


Appendix F.  Examples

   This section gives examples of how to use the new options and message
   formats defined in this memo.

F.1 CoAP Message Protection

   This section illustrates Mode:COAP.  The message exchange assumes
   there is a security context established between client and server.
   One key is used for each direction of the message transfer.  The
   intermediate node detects that the CoAP message contains a SM
   Mode:COAP object (Sig or Enc option is set) and thus forwards the
   message as it cannot serve a cached response.


F.1.1 Integrity Protection of CoAP Message Exchange

   Here is an example of a PUT request/response message exchange passing
   an intermediate node protected with the Sig option.  The example
   illustrates a client closing a lock and getting a confirmation that
   the lock is closed.  Code, Uri-Path and Payload of the request and
   Code of the response are integrity protected (and other message
   fields, see Appendix A).



Selander, et al.       Expires December 31, 2015               [Page 36]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


    Client  Proxy  Server
       |      |      |
       |      |      |
       |      |      |
       +----->|      |      Code: 0.03 (PUT)
       | PUT  |      |     Token: 0x8c
       |      |      |  Uri-Path: lock
       |      |      |       Sig: SSM {"seq":"00000142",
       |      |      |            "cid":"a1534e3c5fdc09bd", ...}
       |      |      |   Payload: 1
       |      |      |
       |      +----->|      Code: 0.03 (PUT)
       |      | PUT  |    Token: 0x7b
       |      |      |  Uri-Path: lock
       |      |      |       Sig: SSM {"seq":"00000142",
       |      |      |            "cid":"a1534e3c5fdc09bd", ...}
       |      |      |   Payload: 1
       |      |      |
       |      |      |
       |      |<-----+      Code: 2.04 (Changed)
       |      | 2.04 |     Token: 0x7b
       |      |      |       Sig: SSM {"seq":"000000a6",
       |      |      |            "cid":"5fdc09bda1534e3c", ...}
       |      |      |
       |<-----+      |      Code: 2.04 (Changed)
       | 2.04 |      |     Token: 0x8c
       |      |      |       Sig: SSM {"seq":"000000a6",
       |      |      |            "cid":"5fdc09bda1534e3c", ...}
       |      |      |

            Figure 8: CoAP PUT protected with Sig/SSM (Mode:COAP)

   The Context Identifier is an identifier indicating which security
   context was used to integrity protect the message, and may be used as
   an identifier for a secret key or a public key.  (It may e.g. be the
   hash of a public key.)

   The server and client can verify that the Sequence Number has not
   been received and used with this key before, and since Mode is COAP,
   the client can additionally verify the freshness of the response,
   i.e. that the response message is generated as an answer to the
   received request message (see Section 5.3).

   The SSM also contains the Tag as specified in the Algorithm (not
   shown in the Figure).

   This example deviates from encryption (SEM) by default (see Section
   6) just to illustrate the Sig option.  If there is no compelling



Selander, et al.       Expires December 31, 2015               [Page 37]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   reason why the CoAP message should be in plaintext, then the Enc
   option MUST be used.

F.1.2 Additional Encryption of CoAP Message

   Here is an example of a GET request/response message exchange passing
   an intermediate node protected with the Enc option.  The example
   illustrates a client requesting a blood sugar measurement resource
   (GET /glucose) and receiving the value 220 mg/dl.  Uri-Path and
   Payload are encrypted and integrity protected.  Code is integrity
   protected only (see Appendix A).

    Client  Proxy  Server
       |      |      |
       |      |      |
       |      |      |
       +----->|      |      Code: 0.01 (GET)
       | GET  |      |     Token: 0x83
       |      |      |       Enc: SEM {"seq":"000015b7",
       |      |      |            "cid":"34e3c5fdca1509bd",
       |      |      |            ["glucose" ... ], ...}
       |      |      |
       |      +----->|      Code: 0.01 (GET)
       |      | GET  |     Token: 0xbe
       |      |      |       Enc: SEM {"seq":"000015b7",
       |      |      |            "cid":"34e3c5fdca1509bd",
       |      |      |            ["glucose" ... ], ...}
       |      |      |
       |      |      |
       |      |<-----+      Code: 2.05 (Content)
       |      | 2.05 |     Token: 0xbe
       |      |      |       Enc:
       |      |      |   Payload: SEM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            [... 220], ...}
       |      |      |
       |<-----+      |      Code: 2.05 (Content)
       | 2.05 |      |     Token: 0x83
       |      |      |       Enc:
       |      |      |   Payload: SEM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            [... 220], ...}
       |      |      |

            Figure 9: CoAP GET protected with Enc/SEM (Mode:COAP).
            The bracket [ ... ] indicates encrypted data.

   Since the request message (GET) does not support payload, the SEM is



Selander, et al.       Expires December 31, 2015               [Page 38]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   carried in the Enc option.  Since the response message (Content)
   supports payload, the Enc option is empty and the SEM is carried in
   the payload.

   The Context Identifier is a hint to the receiver indicating which
   security context was used to encrypt and integrity protect the
   message, and may be used as an identifier for the AEAD secret key.
   One key is used for each direction of the message transfer.

   The server and client can verify that the Sequence Number has not
   been received and used with this key before, and since Mode:COAP the
   client can additionally verify the freshness of the response, i.e.
   that the response message is generated as an answer to the received
   request message (see Section 5.3).

   The SEM also contains the Tag as specified by the Algorithm (not
   shown in the Figure).


F.2 Payload Protection

   This section gives examples that illustrate Mode:PAYL.  This mode
   assumes that only the intended receiver(s) has the relevant security
   context related to the resource.  In case of a closed group of
   recipients of the same object, e.g. in Information-Centric Networking
   or firmware update distribution, it may be necessary to support
   symmetric key encryption in combination with digital signature.

F.2.1 Proxy Caching

   This examples applies e.g. to closed user groups of a single data
   source. The example outlines how a proxy forwarding request and
   response of one client can cache a response whose payload is a SEM
   object, and serve this response to another client request, such that
   both clients can verify integrity and non-replay.

    Client1 Proxy  Server

       |      |      |
       |      |      |
       +----->|      |      Code: 0.01 (GET)
       | GET  |      |     Token: 0x83
       |      |      | Proxy-Uri: example.com/temp
       |      |      |
       |      |      |
       |      +----->|      Code: 0.01 (GET)
       |      | GET  |     Token: 0xbe
       |      |      |  Uri-Host: example.com



Selander, et al.       Expires December 31, 2015               [Page 39]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


       |      |      |  Uri-Path: temp
       |      |      |
       |      |      |
       |      |<-----+      Code: 2.05 (Content)
       |      | 2.05 |     Token: 0xbe
       |      |      |   Payload: SEM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            ["471 F"], ...}
       |      |      |
       |<-----+      |      Code: 2.05 (Content)
       | 2.05 |      |     Token: 0x83
       |      |      |   Payload: SEM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
              |      |            ["471 F"], ...}
   Client2    |      |
              |      |
       |      |      |
       |      |      |
       +----->|      |      Code: 0.01 (GET)
       | GET  |      |     Token: 0xa1
       |      |      | Proxy-Uri: example.com/temp
       |      |      |
       |<-----+      |      Code: 2.05 (Content)
       | 2.05 |      |     Token: 0xa1
       |      |      |   Payload: SEM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            ["471 F"], ...}

        Figure 10: Proxy caching protected with SEM (Mode:PAYL)



F.2.2 Publish-Subscribe

   This example outlines a publish-subscribe setting where the payload
   is integrity and replay protected end-to-end between Publisher and
   Subscriber.  The example illustrates a subscription registration and
   a new publication of birch pollen count of 300 per cubic meters.  The
   PubSub Broker can define the Observe count arbitrarily (as could any
   intermediary node, even in Mode:COAP), but cannot manipulate the
   Sequence Number without being noticed.

   Sub-    PubSub- Pub-
   scriber Broker  lisher

       |      |      |
       +----->|      |      Code: 0.01 (GET)
       | GET  |      |     Token: 0x72



Selander, et al.       Expires December 31, 2015               [Page 40]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


       |      |      |  Uri-Path: ps
       |      |      |  Uri-Path: birch-pollen
       |      |      |   Observe: 0 (register)
       |      |      |
       |      |      |
       |<-----+      |      Code: 2.05 (Content)
       | 2.05 |      |     Token: 0x72
       |      |      |   Observe: 1
       |      |      |   Payload: SSM {"seq":"000015b7",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            ["270"], ...}
       |      |      |
       |      |      |
       |      |      |
       |      |<-----+      Code: 0.03 (PUT)
       |      | PUT  |     Token: 0x1f
       |      |      |  Uri-Path: ps
       |      |      |  Uri-Path: birch-pollen
       |      |      |   Payload: SSM {"seq":"000015b8",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            ["300"], ...}
       |      |      |
       |      +----->|      Code: 2.04 (Changed)
       |      | 2.04 |     Token: 0x1f
       |      |      |
       |      |      |
       |<-----+      |      Code: 2.05 (Content)
       | 2.05 |      |     Token: 0x72
       |      |      |   Observe: 2
       |      |      |   Payload: SSM {"seq":"000015b8",
       |      |      |            "cid":"c09bda155fd34e3c",
       |      |      |            ["300"], ...}

        Figure 11: Publish-subscribe protected with SSM (Mode:PAYL)

   This example deviates from encryption (SEM) by default (see Section
   6) just to illustrate the SSM in Mode:PAYL.  If there is no
   compelling reason why the payload should be in plaintext, then SEM
   MUST be used.

F.2.3 Transporting Authorization Information

   This example outlines the transportation of authorization information
   from a node producing (Authorization Server, AS) to a node consuming
   (Resource Server, RS) such information.  Authorization information
   may for example be an authorization decision with respect to a Client
   (C) accessing a Resource to be enforced by RS.  See [I-D.seitz-ace-
   core-authz] and Section 8.4-8.6 of [I-D.gerdes-ace-actors].



Selander, et al.       Expires December 31, 2015               [Page 41]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


   Here, C is clearly not trusted with modifying the information, but
   may need to be involved in mediating the authorization information to
   the RS, for example, because AS and RS does not have direct
   connectivity.  So end-to-end security is required and object security
   ("access tokens") is the natural candidate.

   This example considers the authorization information to be
   encapsulated in a SEM Mode:PAYL object, generated by AS.  How C
   accesses the SEM is out of scope for this example, it may e.g. be
   using CoAP.  C then requests RS to configure the authorization
   information in the SEM by doing POST to /authorize.  This particular
   resource has a default access policy that only new messages signed by
   AS are authorized.  RS thus verifies the integrity and sequence
   number by using the existing security context for the AS, and
   responds accordingly, a) or b), see Figure 12.

    Authz           Resource
    Server  Client  Server
       |      |      |
       |      |      |        Client access Access Token:
       +- - ->|      |     SEM {"seq":"00000142",
       |      |      |          "cid":"c09bda1534e3c5fdc09bd", ...}
       |      |      |
       |      |      |
       |      +----->|      Code: 0.02 (POST)
       |      | POST |     Token: 0xac
       |      |      |  Uri-Path: authorize
       |      |      |   Payload: SEM {"seq":"00000142",
       |      |      |            "cid":"c09bda1534e3c5fdc09bd", ...}

   a)
       |      |      |
       |      |<-----+      Code: 2.04 (Changed)
       |      | 2.04 |     Token: 0xac
       |      |      |

   b)
       |      |      |
       |      |<-----+      Code: 4.01 (Unauthorized)
       |      | 4.01 |     Token: 0xac
       |      |      |

        Figure 12: Protected Transfer of Access Token = SEM (Mode:PAYL)



Authors' Addresses




Selander, et al.       Expires December 31, 2015               [Page 42]


INTERNET DRAFT          Object Security for ACE            June 29, 2015


        Goeran Selander
        Ericsson
        Farogatan 6
        16480 Kista
        SWEDEN
        EMail: goran.selander@ericsson.com

        John Mattsson
        Ericsson
        Farogatan 6
        16480 Kista
        SWEDEN
        EMail: john.mattsson@ericsson.com

        Francesca Palombini
        Ericsson
        Farogatan 6
        16480 Kista
        SWEDEN
        EMail: francesca.palombini@ericsson.com

        Ludwig Seitz
        SICS Swedish ICT AB
        Scheelevagen 17
        22370 Lund
        SWEDEN
        EMail: ludwig@sics.se
























Selander, et al.       Expires December 31, 2015               [Page 43]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/