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A Hitchhiker's Guide to the (Datagram) Transport Layer Security Protocol for Smart Objects and Constrained Node Networks

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Sandeep Kumar , Sye Loong Keoh , Hannes Tschofenig
Last updated 2013-09-04
Replaces draft-tschofenig-lwig-tls-minimal
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LWIG Working Group                                            S.S. Kumar
Internet-Draft                                          Philips Research
Intended status: Informational                                   S. Keoh
Expires: March 8, 2014                             University of Glasgow
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                       September 4, 2013

A Hitchhiker's Guide to the (Datagram) Transport Layer Security Protocol
            for Smart Objects and Constrained Node Networks


   Transport Layer Security (TLS) is a widely used security protocol
   that offers communication security services at the transport layer.
   The initial design of TLS was focused on the protection of
   applications running on top of the Transmission Control Protocol
   (TCP), and was a good match for securing the Hypertext Transfer
   Protocol (HTTP).  Subsequent standardization efforts lead to the
   publication of the Datagram Transport Layer Security (DTLS) protocol,
   which allows the re-use of the TLS security functionality and the
   payloads to be exchanged on top of the User Datagram Protocol (UDP).

   With the work on the Constrained Application Protocol (CoAP), as a
   specialized web transfer protocol for use with constrained nodes and
   constrained networks, DTLS is a preferred communication security

   Smart objects are constrained in various ways (e.g., CPU, memory,
   power consumption) and these limitations may impose restrictions on
   the protocol stack such a device runs. This document only looks at
   the security part of that protocol stacks and the ability to
   customize TLS/DTLS. To offer input for implementers and system
   architects this document illustrates the costs and benefits of
   various TLS/DTLS features for use with smart objects and constraint
   node networks.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119]

Status of this Memo


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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Design Decisions . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Performance Numbers  . . . . . . . . . . . . . . . . . . . . .  7
     4.1. Pre-Shared Key (PSK) based DTLS implementation  . . . . . .  7
       4.1.1. Prototype Environment . . . . . . . . . . . . . . . . .  7
       4.1.2. Code size and Memory Consumption  . . . . . . . . . . .  8
       4.1.3. Communication Overhead  . . . . . . . . . . . . . . . .  8
       4.1.4. Message Delay, Success Rate and Bandwidth . . . . . . . 10
     4.2. Certificate based and Raw-public key based TLS 
          implementation  . . . . . . . . . . . . . . . . . . . . . . 11
       4.2.1. Prototype Environment . . . . . . . . . . . . . . . . . 11
       4.2.2. Code size . . . . . . . . . . . . . . . . . . . . . . . 11
       4.2.3. Raw Public Key Implementation . . . . . . . . . . . . . 12
   5.  Summary and Conclusions  . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13

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   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

1.  Introduction

   The IETF published three versions of Transport Layer Security: TLS
   Version 1.0 [RFC2246], TLS Version 1.1 [RFC4346], and TLS Version 1.2
   [RFC5246]. Section 1.1 of [RFC4346] explains the differences between
   Version 1.0 and Version 1.1; those are small security improvements,
   including the usage of an explicit initialization vector to protect
   against cipher-block-chaining attacks, which all have little to no
   impact on smart object implementations.  Section 1.2 of [RFC5246]
   describes the differences between Version 1.1 and Version 1.2. TLS
   1.2 introduces a couple of major changes with impact to size of an
   implementation. In particular, prior TLS versions hard-coded the
   MD5/SHA-1 combination in the pseudo-random function (PRF). As a
   consequence, any TLS Version 1.0 and Version 1.1 implementation had
   to have MD5 and SHA-1 code even if the remaining cryptographic
   primitives used other algorithms. With TLS Version 1.2 the two had
   been replaced with cipher-suite-specified PRFs. In addition, the TLS
   extensions definition [RFC6066] and various AES ciphersuites
   [RFC3268] got merged into the TLS Version 1.2 specification.

   All three TLS specifications list a mandatory-to-implement
   ciphersuite: for TLS Version 1.0 this was
   TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA, for TLS Version 1.1 it was
   TLS_RSA_WITH_3DES_EDE_CBC_SHA, and for TLS Version 1.2 it is
   TLS_RSA_WITH_AES_128_CBC_SHA. There is, however, an important
   qualification to these compliance statements, namely that they are
   only valid in the absence of an application profile standard
   specifying otherwise. The smart object environment may, for example,
   represent a situation for such an application profile which defines a
   cryptosuite that reduces memory and computation requirements without
   sacrificing security.

   All TLS versions offer a separation between authentication and key
   exchange, and bulk data protection. The former is more costly
   performance- and message-wise. The details of the authentication and
   key exchange, using the TLS Handshake, vary with the chosen
   ciphersuite. With new ciphersuites the TLS feature-set can easily be
   enhanced, in case the already large collection of ciphersuites, see

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   [TLS-IANA], does not match the requirements.

   Once the TLS Handshake has been successfully completed the necessary
   keying material and parameters are setup for usage with the TLS
   Record Layer, which is responsible for bulk data protection. The
   provided security of the TLS Record Layer depends also, but not only,
   on the chosen ciphersuite algorithms; NULL encryption ciphersuites,
   like those specified in RFC 4785 [RFC4785], offer only integrity-
   without confidentiality-protection. Example ciphersuites for the TLS
   Record Layer are RC4 with SHA-1, AES-128 with SHA-1, AES-256 with
   SHA-1, RC4 with SHA-1, RC4 with MD5 It is worth mentioning that TLS
   may also be used without the TLS Record Layer. This has, for example,
   been exercised with the work on the framework for establishing a
   Secure Real-time Transport Protocol (SRTP) security context using the
   Datagram Transport Layer Security (DTLS   [RFC4347]) protocol (DTLS-
   SRTP [RFC5763]).

   It is fair to say that TLS and consequently DTLS offers a fair degree
   of flexibility. What specific security features of TLS are required
   for a specific smart object application scenario depends on various
   factors, including the communication architecture and the threats
   that shall be mitigated.

   The goal of this document is to provide guidance on how to use
   existing DTLS/TLS extensions for smart objects and to explain their
   costs in terms of code size, computational effort, communication
   overhead, and (maybe) energy consumption. The document does not try
   to be exhaustive, as the list of TLS/DTLS extensions is enhanced on a
   frequent basis. Instead we focus on extensions that those working in
   the smart object community often found valuable in their practical
   experience. A non-goal is to propose new extensions to DTLS/TLS to
   provide even better performance characteristics in specific

2.  Overview

   A security solution to be deployed is strongly influenced by the
   communication relationships [RFC4101] between the entities. Having a
   good understanding of these relationships will be essential to define
   the threats and decide on how to customize the security solution.
   Some of these considerations are described in [I-D.gilger-smart-

   Consider the following scenario where a smart-meter transmits its
   energy readings to other parties.  The public utility has to ensure
   that the meter readings it obtained can be attributed to a specific
   meter in a household.  It is simply not acceptable for public utility
   to have any meter readings tampered in transit or by a rogue endpoint

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   (particularly if the attack leads to a disadvantage, for example
   financial loss, for the utility).  Users in a household may want to
   ensure that only certain authorized parties are able to read their
   meter; privacy concerns come to mind.

   In this example, a smart-meter may only ever need to talk to servers
   of a specific utility or even only to a single pre-configured server.
   Clearly, some information has to be pre-provisioned into the device
   to ensure the desired behavior to talk only to selected servers.  The
   meter may come pre-configured with the domain name and certificate
   belonging to the utility.  The device may, however, also be
   configured to accept one or multiple server certificates.  It may
   even be pre-provisioned with the server's raw public key, or a shared
   secret instead of relying on certificates.

   Lowering the flexibility decreases the implementation overhead.  If
   shared secrets are used with TLS-PSK [RFC4279] or raw public keys are
   used with TLS [I-D.ietf-tls-oob-pubkey], fewer lines of code are
   needed than employing X.509 certificate, as will be explained later
   in this document.  A decision for constraining the client-side TLS
   implementation, for example by offering only a single ciphersuite,
   has to be made in awareness of what functionality will be available
   on the TLS server-side.  In certain communication environments it may
   be easy to influence both communication partners while in other cases
   the existing deployment needs to be taken into consideration.

   To illustrate another example, consider an Internet radio, which
   allows a user to connect to available radio stations.  A device like
   this will be more demanding than an IP-enabled weighing scale that
   only connects to the single web server offered by the device
   manufacturer. A threat assessment may even lead to the conclusion
   that TLS support is not necessary at all in this particular case.

   Consider the following extension to our earlier scenario where the
   smart-meter is attached to a home WLAN network and the inter-working
   with WLAN security mechanisms need to be taken care of.  On top of
   the link layer authentication, a transport layer or application layer
   security mechanism needs to be implemented.  Quite likely the
   security mechanisms will be different due to the different credential
   requirements.  While there is a possibility for re-use of
   cryptographic libraries (such as the SHA-1, MD5, or HMAC) the overall
   code footprint will very likely be larger.

   Furthermore, security technology that will be deployed by end-user
   consumer market products and large enterprise customer products will
   need to be customized completely different.  While the security
   building blocks may be reused, there is certainly a big difference
   between in terms of the architecture, the threats and effort that

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   will be spent securing the system. 

3.  Design Decisions

   To evaluate the required TLS functionality a couple of high level
   design decisions have to be made:

   o  What type of protection for the data traffic is required?  Is
      confidentiality protection in addition to integrity protection
      required?  Many TLS ciphersuites also provide a variant for NULL
      encryption [RFC4279].  If confidentiality protection is required,
      a carefully chosen set of algorithms may have a positive impact on
      the code size.   Re-use of crypto-libraries (within TLS but also
      among the entire protocol stack) will also help to reduce the
      overall code size.

   o  What functionality is available in hardware?  For example, certain
      hardware platforms offer support for a random number generator as
      well as cryptographic algorithms (e.g., AES).  These functions can
      be re-used and allow to reduce the amount of required code. Using
      hardware support not only reduces the computation time but can
      also save energy due to the optimized implementation.

   o  What credentials for client and server authentication are
      required: passwords, pre-shared secrets, certificates, raw public
      keys (or a mixture of them)? Is mutual authentication required? Is
      X509 certificate handling necessary? If not, then the ASN.1
      library as well as the certificate parsing and processing can be
      omitted.  If pre-shared secrets are used then the big integer
      implementation can be omitted.

   o  What TLS version and what TLS features, such as session
      resumption, can or have to be used?  In the case of DTLS, generic
      fragmentation and reordering requires large buffers to reassemble
      the messages, which might be too heavy for some devices. 

   o  Is it possible to design only the client-side TLS stack, or
      necessary to provide the server-side implementation as well?
      Handshake messages sent are different sizes for the client and
      server which creates energy consumption differences (due to the
      fact that more power is spent during transmission than while
      receiving data in wireless devices). 

   o  Which side will be more powerful?  Resource-constrained sensor
      nodes running CoAPS might be server only, while nodes running
      HTTPS are most like clients only that post their information to a
      normal Web server.  The constrained side will most likely only
      implement a single ciphersuite.  Flexibility is given to a more

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      powerful counterpart that supports many different ciphersuite for
      various connected devices.

   o  Is it possible to hardwire credentials into the code rather than
      loading them from storage?  If so, then no file handling or
      parsing of the credentials is needed and the credentials are
      already available in a form that they can be used within the TLS

4.  Performance Numbers

   In this section we summarize performance measurements available from
   certain implementation experiences. This section is not supposed to
   be exhaustive as we do not have all measurements available. The
   performances are grouped according to extensions (TLS-PSK, raw-public
   key and certificate based) and further grouped by performance
   measures (memory, code size, communication overhead, etc.). Where
   possible we extract the different building blocks found in TLS and
   present their performance measures individually. 

4.1. Pre-Shared Key (PSK) based DTLS implementation

   This section provides performance numbers for a prototype
   implementation of DTLS-PSK described in [I-D.keoh-lwig-dtls-iot] and 
   evaluates the memory and communication overheads.

4.1.1. Prototype Environment

   The prototype is written in C and runs as an application on Contiki
   OS 2.5 [Dunkels-contiki], an event-driven open source operating
   system for constrained devices. They were tested in the Cooja
   simulator and then ported to run on Redbee Econotag hardware, which
   features a 32-bit CPU, 128 KB of ROM, 96 KB of RAM, and an IEEE
   802.15.4 enabled radio with an AES hardware coprocessor. The
   prototype comprises all necessary functionality to adapt to the roles
   as a domain manager or a joining device.

   The prototype is based on the "TinyDTLS" [Bergmann-Tinydtls] library
   and includes most of the extensions  and the adaptation as follows:

   (1) The cookie mechanism was disabled in order to fit messages to the
       available packet sizes and hence reducing the total number of
       messages when performing the DTLS handshake.

   (2) Separate delivery was used instead of flight grouping of messages
       and redesigned the retransmission mechanism accordingly.

   (3) The "TinyDTLS" AES-CCM module was modified to use the AES

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       hardware coprocessor.

   The following subsections further analyze the memory and
   communication overhead of the solution.

4.1.2. Code size and Memory Consumption

   Table 1 presents the codesize and memory consumption of the prototype
   differentiating (i) the state machine for the handshake, (ii) the
   cryptographic primitives,  and (iii) the DTLS record layer mechanism.

   The use of DTLS appears to incur large memory footprint both in ROM
   and RAM for two reasons. First, DTLS handshake defines many message
   types and this adds more complexity to its corresponding state
   machine. The logic for message re-ordering and retransmission also
   contributes to the complexity of the DTLS state machine. Second, DTLS
   uses SHA2-based crypto suites which is not available from the
   hardware crypto co-processor.

            |                      |      DTLS       |
            |                      +--------+--------+
            |                      |  ROM   |  RAM   |
            | State Machine        |  8.15  |   1.9  |
            | Cryptography         |   3.3  |   1.5  |
            | DTLS Record Layer    |   3.7  |   0.5  |
            | TOTAL                |  15.15 |   3.9  |
                Table 1: Memory Requirements in KB 

4.1.3. Communication Overhead

   The communication overhead is evaluated in this section. In
   particular, the message overhead and the number of exchanged bytes
   under ideal condition without any packet loss is examined.

   Table 2 summarizes the required number of round trips, number of
   messages and the total exchanged bytes for the DTLS-based handshake
   carried out in ideal conditions, i.e., in a network without packet
   losses. DTLS handshake is considered complex as it involves the
   exchange of 12 messages to complete the handshake. Further, DTLS runs
   on top the transport layer, i.e., UDP, and hence this directly

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   increases the overhead due to lower layer per-packet protocol


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            |                               |  DTLS  |
            | No. of Message                |     8  |
            | No. of round trips            |     2  |
            | 802.15.4 headers              |  112B  |
            | 6LowPAN headers               |  320B  |
            | UDP headers                   |   64B  |
            | TOTAL                         |  496B  |
            Table 2: Communication overhead for Network
                     Access and Multicast Key Management 

4.1.4. Message Delay, Success Rate and Bandwidth 

   Section 5.3 provided an evaluation of the protocol in an ideal
   condition, thus establishing the the baseline protocol overhead. The
   prototype was further examined and simulated the protocol behavior by
   tuning the packet loss ratio. In particular, the impact of packet
   loss on message delay, success rate and number of messages exchanged
   in the handshake were examined. 

   Figure 4 shows the percentage of successful handshakes as a function
   of timeouts and packet loss ratios. As expected, a higher packet loss
   ratio and smaller timeout (15s timeout) result in a failure
   probability of completing the DTLS handshake. When the packet loss
   ratio reaches 0.5, practically no DTLS handshake would be

            100 |+
         P      | +
         E   80 |  ++   
         R      |    ++
         C   60 |      +
         E      |       +
         N   40 |        +   
         T      |         ++
         A   20 |            +
         G      |             +++++
         E    0 +------------------++++++++-->
               0 0.1 0.2 0.3 0.4 0.5  

                packet loss ratio (15s timeout)

         Figure 1: Average % of successful handshakes


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   Delays in network access and communication are intolerable since they
   lead to higher resource consumption. As the solution relies on PSK,
   the handshake protocol only incurs a short delay of a few
   milliseconds when there is no packet loss. However, as the packet
   loss ratio increases, the delay in completing the handshake becomes
   significant because loss packets must be retransmitted. Our
   implementation shows that with a packet loss ratio of 0.5, the the
   times to perform network access and multicast key management could
   take up to 24s. 

   Finally, another important criterion is the number of messages
   exchanged in the presence of packet loss. A successful handshake
   could incur up to 35 or more messages to be transmitted when the
   packet loss ratio reaches 0.5. This is mainly because the DTLS
   retransmission is complex and often requires re-sending multiple
   messages even when only a single message has been lost.

4.2. Certificate based and Raw-public key based TLS implementation

4.2.1. Prototype Environment 

   The following code was compiled under Ubuntu Linux using the -Os
   compiler flag setting for a 64-bit AMD machine using a modified
   version of the axTLS embedded SSL implementation. 

4.2.2. Code size

   For the cryptographic support functions these are the binary sizes:

        | Cryptographic functions    |  Code size    |
        | MD5                        |  4,856 bytes  |
        | SHA1                       |  2,432 bytes  |
        | HMAC                       |  2,928 bytes  |
        | RSA                        |  3,984 bytes  |
        | Big Integer Implementation |  8,328 bytes  |
        | AES                        |  7,096 bytes  |
        | RC4                        |  1,496 bytes  |
        | Random Number Generator    |  4,840 bytes  |
        Table 3: Code-size for cryptographic functions

   The TLS library with certificate support consists of the following

   x509 related code: 2,776 bytes
   The x509 related code provides functions to parse certificates, to

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   copy them into the program internal data structures and to perform
   certificate related processing functions, like certificate

   ASN1 Parser: 5,512 bytes
   The ASN1 library contains the necessary code to parse ASN1 data.

   Generic TLS Library: 15,928 bytes
   This library is separated from the TLS client specific code to offer
   those functions that are common with the client and the server-side
   implementation. This includes code for the master secret generation,
   certificate validation and identity verification, computing the
   finished message, ciphersuite related functions, encrypting and
   decrypting data, sending and receiving TLS messages (e.g., finish
   message, alert messages, certificate message, session resumption).

   TLS Client Library: 4,584 bytes
   The TLS client-specific code includes functions that are only
   executed by the client based on the supported ciphersuites, such as
   establishing the connection with the TLS server, sending the
   ClientHello handshake message, parsing the ServerHello handshake
   message, processing the ServerHelloDone message, sending the
   ClientKeyExchange message, processing the CertificateRequest message.

   OS Wrapper Functions:  2,776 bytes
   These functions aim to make development easier (e.g., for failure
   handling with memory allocation and various header definitions) but
   are not absolutely necessary.

   OpenSSL Wrapper Functions: 931 bytes
   The OpenSSL API calls are familiar to many programmers and therefore
   these wrapper functions are provided to simplify application
   development. This library is also not absolutely necessary.

   Certificate Processing Functions: 4,456 bytes
   These functions provide the ability to load certificates from files
   (or to use a default key as a static data structure embedded during
   compile time), to parse them, and populate corresponding data

4.2.3. Raw Public Key Implementation 

   Of course, the use of raw public keys does not only impact the code
   size but also the size of the exchanged messages. When using raw
   public keys (instead of certificates) the "certificate" size was
   reduced from 475 bytes to 163 bytes (using an RSA-based public key).
   Note that the SubjectPublicKeyInfo block does not only contain the

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   raw keys, namely the public exponent and the modulus, but also a
   small ASN.1 header preamble.

   For the raw public key implementation the following components where
   needed (in addition to a subset of the cryptographic support

   Minimal ASN1 Parser:  3,232 bytes
   The necessary support from the ASN1 library now only contains
   functions for parsing of the ASN1 components of the
   SubjectPublicKeyInfo block.

   Generic TLS Library: 16,288 bytes
   This size of this library was slightly enlarged since additional
   functionality for loading keys into the bigint data structure was
   added. On the other hand, code was removed that relates to
   certificate processing and functions to retrieve certificate related
   data (e.g., to fetch the X509 distinguished name or the subject
   alternative name).

   TLS Client Library: 4,528 bytes
   The TLS client-specific code now contains additional code for the raw
   public key support, for example in the ClientHello message. Most
   functions were left unmodified.

5.  Summary and Conclusions

   TLS/DTLS can be tailored to fit the needs of a specific deployment
   environment.  This customization property allows it to be tailored to
   many use cases including smart objects. The communication model and
   the security goals will, however, ultimately decide the resulting
   code size; this is not only true for TLS but for every security
   solution.More flexibility and more features will ultimately translate
   to a bigger footprint.

   There are, however, cases where the security goals ask for a security
   solution other than TLS.  With the wide range of embedded
   applications it is impractical to design for a single security
   architecture or even a single communication architecture.

6.  Security Considerations

   This document discusses various design aspects for reducing the
   footprint of (D)TLS implementations.  As such, it is entirely about


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7.  IANA Considerations

   This document does not contain actions for IANA.

8.  Acknowledgements

   The authors would like to thank the participants of the Smart Object
   Security workshop, March 2012. The authors greatly acknowledge the
   prototyping and implementation efforts by Pedro Moreno-Sanchez and
   Francisco Vidal-Meca who worked as interns at Philips Research.

9.  References

9.1.  Normative References

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
   RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
   (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
   (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
   Security", RFC 4347, April 2006.

   [SHA] National Institute of Standards and Technology, , "Secure Hash
   Standard", FIPS 180-2, Aug 2002.

9.2.  Informative References

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

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
   for Establishing a Secure Real-time Transport Protocol (SRTP)
   Security Context Using Datagram Transport Layer Security (DTLS)",
   RFC 5763, May 2010.

   [RFC4785]  Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
   Ciphersuites with NULL Encryption for Transport Layer Security
   (TLS)", RFC 4785, January 2007.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)

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   Extensions: Extension Definitions", RFC 6066, January 2011.

   [RFC3268]  Chown, P., "Advanced Encryption Standard (AES)
   Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June

   [RFC4101]  Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
   June 2005.

   [RFC4279]  Eronen, P., Ed., and H. Tschofenig, Ed., "Pre-Shared Key
   Ciphersuites for Transport Layer Security (TLS)", RFC 4279, December

   [TLS-IANA] IANA, "Transport Layer Security (TLS) Parameters:",
   Oct 2012.

   [I-D.ietf-tls-oob-pubkey] Wouters, P., Tschofenig, H., Gilmore, J.,
   Weiler, S., and T. Kivinen, "Out-of-Band Public Key Validation for
   Transport Layer Security (TLS)", draft-ietf-tls-oob-pubkey-07 (work
   in progress) February 2013.

   [I-D.keoh-lwig-dtls-iot] Keoh, S., Kumar, S., and Garcia-Morchon, O.,
   "Securing the IP-based Internet of Things with DTLS", draft-keoh-
   lwig-dtls-iot-01, February 2013.

   [I-D.gilger-smart-object-security-workshop] Gilger, J., and
   Tschofenig, H., "Report from the 'Smart Object Security Workshop',
   March 23, 2012, Paris, France", draft-gilger-smart-object-security-
   workshop-01.txt, February 2013.

   [Dunkels-Contiki] Dunkels, A., Gronvall, B., and Voigt, T. "Contiki -
   A Lightweight and Flexible Operating System for Tiny Networked
   Sensors", In Proceedings of the 29th Annual IEEE International
   Conference on Local Computer Networks, IEEE, 2004.

   [Bergmann-Tinydtls] Bergmann, O. "TinyDTLS - A Basic DTLS Server
   Template",, 2012. 


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Authors' Addresses

   Sandeep S. Kumar
   Philips Research
   High Tech Campus 34
   Eindhoven  5656 AE


   Sye Loong Keoh
   University of Glasgow Singapore
   Republic PolyTechnic, 9 Woodlands Ave 9
   Singapore 838964


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo,   02600

   Phone: +358 (50) 4871445

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