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TLS Working Group                                   N. Mavrogiannopoulos
Internet-Draft                                                    RedHat
Intended status: Standards Track                           H. Tschofenig
Expires: May 17, 2017                                                ARM
                                                              T. Fossati
                                                                   Nokia
                                                       November 13, 2016


 Datagram Transport Transport Layer Security (DTLS) Transport-Agnostic
                     Security Association Extension
                 draft-mavrogiannopoulos-tls-cid-00

Abstract

   This memo proposes a new Datagram Transport Transport Layer Security
   (DTLS) extension that provides the ability to negotiate, during
   handshake, a transport independent identifier that is unique per
   security association.  This identifier effectively decouples the DTLS
   session from the underlying transport protocol, allowing the same
   security association to be migrated across different instances of the
   same transport, or to a completely different transport.

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 17, 2017.

Copyright Notice

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

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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Transport Agnostic Security Associatiation Extension  . . . .   4
     3.1.  Extended Client Hello . . . . . . . . . . . . . . . . . .   4
     3.2.  Extended Server Hello . . . . . . . . . . . . . . . . . .   5
     3.3.  Wire Format Changes . . . . . . . . . . . . . . . . . . .   6
   4.  Clashing HOTP CIDs  . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   DTLS security context demultiplexing is done via the 5-tuple.
   Therefore, the security association needs to be re-negotiated from
   scratch whenever the transport identifiers change.  For example, when
   moving the network attachment from WLAN to a cellular connection, or
   when the IP address of the IoT devices changes during a sleep cycle.
   A NAT device may also modify the source UDP port after a short idle
   period.  In such cases, there is not enough information in the DTLS
   record header for a server that is handling multiple concurrent
   sessions to associate the new address to an existing client.

   This memo proposes a new TLS extension [RFC6066] that provides the
   ability to negotiate, at handshake time, a transport independent
   identifier that is unique per security association.  We call this
   identifier Connection ID (CID).  Its function is to effectively
   decouple the DTLS session from the underlying transport protocol,
   allowing the same DTLS security association to be migrated across
   different instances of the same transport, or even to a completely
   different transport - e.g., from UDP to GSM-SMS as showed in
   Figure 1.






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                                        00
                                        /\
                                        :
    IP                    UDP           : DTLS Record Header
    +-----+-----+-------+ +-----+-----+ : +---------+-------+------
    | src | dst | proto | | src | dst | : | Seq#i   |  CID  | ...
    +-----+-----+-------+ +-----+-----+ : +---------+-------+------
    `----------------+----------------' :              ^
                      `................ : .............'
     <Handover event>                   :
                      GSM-SMS           : DTLS Record Header
                      +-------+-------+ : +---------+-------+-----
                      | tp-oa | tp-da | : | Seq#i+1 |  CID  | ...
                      +-------+-------+ : +---------+-------+-----
                                        :
                                        \/
                                        00

              Figure 1: Transparent Handover of DTLS Session

   We present two methods for producing the CID: the first uses a single
   value generated unilaterally by the server which is fixed throughout
   the session, whereas the second provides a sequence of identifiers
   that are created using a HMAC-based OTP algorithm [RFC4226] keyed
   with the session shared secret.  The latter allows a client to shift
   to a new identifier, for example when switching networks, and is
   intended as a mechanism to counteract tracking.  However, it must be
   noted that this is not generally applicable as a tracking-protection
   measure: in fact, it becomes totally ineffective when the client is
   oblivious of changes in the underlying transport identifiers (e.g.,
   on NAT rebind after timeout), and also does not guarantee unique
   identifiers (see Section 4 for further details).  Both methods
   generate a CID that is 32-bits in size, like the Security Parameter
   Index (SPI) in IPsec [RFC4301].

   Similar approaches to support transparent handover of a DTLS session
   have been described in [I-D.barrett-mobile-dtls] and [DTLSMOB].

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].







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3.  Transport Agnostic Security Associatiation Extension

   In order to negotiate a Transport Agnostic Security Association,
   clients include an extension of type "ta_sa" in the extended client
   hello (Section 3.1).  Servers that receive an extended hello
   containing a "ta_sa" extension MAY agree to use a Transport Agnostic
   Security Association by including an extension of type "ta_sa" in the
   extended server hello (Section 3.2).

   If both server and client agree, the DTLSCiphertext format does
   change after the DTLS connection state is updated; i.e.: for the
   sending side, after the ChangeCipherSpec message is sent, for the
   receiving sides, after the ChangeCipherSpec is received.

   The DTLSCiphertext format is changed for both the client and the
   server.  However, only a client can initiate a switch to an unused
   'cid' value; a server MUST utilize the same value seen on the last
   valid message received by the client.  A server which receives a
   'cid' value which is not expected (e.g., a value outside its
   advertised window) MAY ignore the packet.

            struct {
                 ContentType type;
                 ProtocolVersion version;
                 uint16 epoch;
                 uint48 sequence_number;
                 uint32 cid;                          // New field
                 uint16 length;
                 select (CipherSpec.cipher_type) {
                   case block:  GenericBlockCipher;
                   case aead:   GenericAEADCipher;
                 } fragment;
            } DTLSCiphertext;

                   Figure 2: Modified DTLS Record Format

3.1.  Extended Client Hello

   In order to negotiate a Transport Agnostic Security Association,
   clients include an extension of type "ta_sa" in the extended client
   hello.  The "extension_data" field of this extension SHALL contain
   the ClientSecAssocData structure in Figure 3.

   In case the fixed(0) type has been negotiated, the 'cid' of the
   packets after ChangeCipherSpec is sent explicitly by the server.

   In case the hotp(1) type has been negotiated, the initial 'cid' is
   calculated using the HOTP algorithm ([RFC4226]) as follows:



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   o  A 20-byte string is generated using a [RFC5705] exporter.  The key
      material exporter uses the label "EXPORTER-ta-security-
      association-hotp" without the quotes, and without any context
      value.
   o  The initial 'cid' equals to the first HOTP value (i.e., the 31-bit
      value of Sbits in [RFC4226] notation), generated by using the
      previously exported value as K.

   Subsequent values of the HOTP algorithm can be used in place of the
   initial, as long as they fall into the negotiated window_size (see
   Figure 4).

            enum {
                fixed(0), hotp(1), (255)
            } SecAssocType;

            struct {
                 SecAssocType types<1..2^8-1>;
            } ClientSecAssocData;

                     Figure 3: ta_sa extension, client

3.2.  Extended Server Hello

   Servers that receive an extended hello containing a "ta_sa" extension
   MAY agree to use a Transport Agnostic Security Association by
   including an extension of type "ta_sa", with "extension_data" being
   ServerSecAssocData in the extended server hello (Figure 4).

            struct {
                 SecAssocType type;
                 select (type) {
                     case fixed:
                         struct {
                             uint32 cid_value;
                         };
                     case hotp:
                         struct {
                             uint16 window_size;
                         };
                 };
            } ServerSecAssocData;

                     Figure 4: ta_sa extension, server

   In case the fixed(0) type is chosen, 'cid_value' contains the value
   to be used as 'cid'.  In case hotp(1) type is chosen, 'window_size'




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   must be greater or equal to 1, indicating the number of HOTP values
   that the server can recognize for this particular client.

3.3.  Wire Format Changes

   How to signal the modified wire format to the receiving end is
   currently an open problem.

   Note that moving the cid after the length field and computing the
   difference between the UDP datagram's and DTLS record's lengths is
   not an option because there is no guarantee that UDP datagrams carry
   one and one only DTLS record (Section 4.1.1. of [RFC6347]).

   Ideally, we would just bump the version number, but there seems to be
   limited room for maneuver given the way TLS encodes version
   information in the record header, and also given that we want CID to
   work with DTLS 1.2 and later.

   More discussion needed to sort out this point.

4.  Clashing HOTP CIDs

   HOTP behaves like a PRF, thus uniformly distributing the produced
   CIDs across the 32-bit space.  Table 1 presents the probability to
   end up with two separate sessions having the same HOTP CID when the
   number of concurrent sessions is increased.

        +----------+---------------------------------------------+
        | Sessions | Collision probability                       |
        +----------+---------------------------------------------+
        | 10       | 1.16415320717e-08, or about 1 in 85,899,347 |
        | 100      | 1.16415254059e-06, or about 1 in 858,994    |
        | 1000     | 0.000116408545826, or about 1 in 8,590      |
        | 10000    | 0.011574031737, or about 1 in 86            |
        | 100000   | 0.687813095694, or about 1 in 1             |
        | 1000000  | 1.0, or about 1 in 1                        |
        +----------+---------------------------------------------+

                                  Table 1

   The takeaway is that 32-bits are probably too few for highly loaded
   servers that want to do HOTP as their primary CID allocation
   strategy.  An alternative would be for the server to stop negotiating
   'hotp' and fall back to 'fixed' when the number of active sessions
   crosses some threshold; another would be to increase the CID space to
   40 or 48 bits when HOTP is used.





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5.  Security Considerations

   TODO

6.  IANA Considerations

   This document adds a new extension for DTLS: ts_sa(TODO).  This
   extension MUST only be used with DTLS, and not with TLS.  This
   extension is assigned from the TLS ExtensionType registry defined in
   [RFC5246].

7.  Acknowledgments

   Thanks to Achim Krauss, Carsten Bormann, Kai Hudalla, Simon Bernard,
   Stephen Farrell, for helpful comments and discussions that have
   shaped the document.

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI).  This support does not imply
   endorsement.

8.  References

8.1.  Normative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-18 (work in progress),
              October 2016.

   [I-D.rescorla-tls-dtls13]
              Rescorla, E. and H. Tschofenig, "The Datagram Transport
              Layer Security (DTLS) Protocol Version 1.3", draft-
              rescorla-tls-dtls13-00 (work in progress), October 2016.

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

   [RFC4226]  M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
              O. Ranen, "HOTP: An HMAC-Based One-Time Password
              Algorithm", RFC 4226, DOI 10.17487/RFC4226, December 2005,
              <http://www.rfc-editor.org/info/rfc4226>.






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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

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

8.2.  Informative References

   [DTLSMOB]  Seggelmann, R., Tuexen, M., and E. Rathgeb, "DTLS
              Mobility", 2012.

   [I-D.barrett-mobile-dtls]
              Williams, M. and J. Barrett, "Mobile DTLS", draft-barrett-
              mobile-dtls-00 (work in progress), March 2009.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

Authors' Addresses

   Nikos Mavrogiannopoulos
   RedHat

   EMail: nmav@redhat.com


   Hannes Tschofenig
   ARM

   EMail: hannes.tschofenig@arm.com








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   Thomas Fossati
   Nokia

   EMail: thomas.fossati@nokia.com















































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