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Versions: 00 01

TLS Working Group                                   N. Mavrogiannopoulos
Internet-Draft                                                    RedHat
Intended status: Standards Track                           H. Tschofenig
Expires: November 17, 2017                                           ARM
                                                              T. Fossati
                                                                   Nokia
                                                            May 16, 2017


 Datagram Transport Transport Layer Security (DTLS) Transport-Agnostic
                     Security Association Extension
                   draft-mavrogiannopoulos-tls-cid-01

Abstract

   This memo proposes a new Datagram Transport Transport Layer Security
   (DTLS) extension for DTLS 1.2 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 November 17, 2017.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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.  Handling unknown CIDs . . . . . . . . . . . . . . . . . .   6
     3.4.  Wire Format Changes . . . . . . . . . . . . . . . . . . .   6
     3.5.  De-duplication Algorithm  . . . . . . . . . . . . . . . .   7
   4.  Clashing HOTP CIDs  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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] for DTLS 1.2 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 a per-session shared secret (see Section 3.1 for details).  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 by third party observers.  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.

            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

   See Section 3.3 for details on how to handle receipt of unknown or
   unexpected CIDs.

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.





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

   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




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

3.3.  Handling unknown CIDs

   Server might need to deal with unknown or unexpected CIDs.

   An unknown CID is one that is not found in the server's lookup table.
   This could happen because:

   o  Server reboots and looses its state; or
   o  There is a genuine bug in either client or server code (or even
      somewhere on the network path).

   Either way, the server will not be able to locate a suitable context
   to use for sending an (encrypted) Alert back to the sender.
   Therefore, the server should simply discard records containing an
   unknown CID.

   A CID might be unexpected if it existed but it's been already
   shifted.  This situation may arise if the packet has been delayed by
   the network.  Server MAY ignore a record carrying an unexpected CID.

   Ignoring unknown or unexpected CIDs should also reduce the attack
   surface.

3.4.  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.







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3.5.  De-duplication Algorithm

   The following algorithm assumes that receivers have an unambiguous
   way to tell that the wire format is the one described in Section 3.
   As suggested in Section 3.4, this could be signalled by a different
   version number { TBD, TBD }.

   In order to enqueue an incoming record to the right security context,
   a receiver SHALL extract the 4-tuple from the UDP packet and the
   ContentType of the TLS record.  Then, if ContentType is
   change_cipher_spec or handshake, the receiver SHALL use the sender
   address to lookup or create (if ContentType is handshake and
   HandshakeType is one of ClientHello or ServerHello) the session
   context.  If ContentType is one of application_data or alert,
   receiver SHALL inspect the ProtocolVersion field and: if it is { 3, 3
   } (i.e., TLS v1.2), use the 4-tuple to lookup the session context, if
   it is { TBD, TBD }, extract the CID from the record and use it to
   lookup the session context.

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 and/or the length of the CIDs sequence
   is increased.

   +-----------------------+-------------------------------------------+
   | Sessions x            | Collision probability                     |
   | window_size           |                                           |
   +-----------------------+-------------------------------------------+
   | 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




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   40 or 48 bits when HOTP is used; yet another would be to allow the
   length to be negotiable.

5.  Security Considerations

   CID does not affect the running protocol in any way other than adding
   an un-authenticated field to the record header.  As such, this
   identifier has no effect on the overall security of the session with
   respect to authentication, confidentiality and integrity.  On the
   other hand, since this identifier is not authenticated, it should not
   be used in any way that assumes it is, nor be assumed to be secret or
   unknown to an adversary.  In general, this identifier should not be
   relied on more than the IP address or UDP port numbers are.

   To address the privacy concerns of using a fixed identifier for the
   lifetime of a session which may roam through multiple networks, we
   have introduced the hotp identifier type.  This type of identifier
   gives the client a chance to switch its ts_sa identity when also
   switching its transport identifiers or network attachment (assuming
   that client is made aware of the change before it sends a new DTLS
   record).  The choice of which type of identifier to use is a trade-
   off between the request for privacy stated by the client and the
   ability of the server to control the identifiers in use at each point
   in time, as explained in Section 4.

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 Kraus, 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







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8.1.  Normative References

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
              April 2017.

   [I-D.rescorla-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-rescorla-tls-dtls13-01 (work in progress),
              March 2017.

   [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>.

   [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.






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   [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


   Thomas Fossati
   Nokia

   EMail: thomas.fossati@nokia.com

























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