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Network Working Group                                           D. Meyer
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                          P. Saint-Andre
Expires: December 31, 2009                                         Cisco
                                                           June 29, 2009


 XTLS: End-to-End Encryption for the Extensible Messaging and Presence
          Protocol (XMPP) Using Transport Layer Security (TLS)
                   draft-meyer-xmpp-e2e-encryption-02

Status of this Memo

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   This Internet-Draft will expire on December 31, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   This document specifies "XTLS", a protocol for end-to-end encryption



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   of Extensible Messaging and Presence Protocol (XMPP) traffic.  XTLS
   is an application-level usage of Transport Layer Security (TLS) that
   is set up using the XMPP Jingle extension for session negotiation and
   transported using any streaming transport as the data delivery
   mechanism.  Thus XTLS treats the end-to-end exchange of XML stanzas
   as a virtual transport and uses TLS to secure that transport,
   enabling XMPP entities to communicate in a way that is designed to
   ensure the confidentiality and integrity XML stanzas.  The protocol
   can be used for secure end-to-end messaging as well as other XMPP
   applications, such as file transfer.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Approach . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  XTLS Protocol Flow . . . . . . . . . . . . . . . . . . . . . .  4
   4.  End-to-End Streams over XTLS Protocol Flow . . . . . . . . . . 11
   5.  Bootstrapping Trust on First Communication . . . . . . . . . . 15
     5.1.  Exchanging Certificates  . . . . . . . . . . . . . . . . . 16
     5.2.  Verification of Non-Human Parties  . . . . . . . . . . . . 17
   6.  Session Termination  . . . . . . . . . . . . . . . . . . . . . 18
   7.  Determining Support  . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     8.1.  Mandatory-to-Implement Technologies  . . . . . . . . . . . 19
     8.2.  Certificates . . . . . . . . . . . . . . . . . . . . . . . 19
     8.3.  Denial of Service  . . . . . . . . . . . . . . . . . . . . 20
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  XML Schema  . . . . . . . . . . . . . . . . . . . . . 22
   Appendix B.  Copying Conditions  . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

















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1.  Introduction

   End-to-end encryption of traffic sent over the Extensible Messaging
   and Presence Protocol (XMPP) is a desirable goal.  Requirements and a
   threat analysis for XMPP encryption are provided in [E2E-REQ].  This
   document explores the possibility of using the Transport Layer
   Security [TLS] to meet those requirements.

   TLS is the most widely implemented protocol for securing network
   traffic.  In addition to applications in the email infrastructure,
   the World Wide Web [HTTP-TLS], and datagram transport for multimedia
   session negotiation [DTLS], TLS is used in XMPP to secure TCP
   connections from client to server and from server to server, as
   specified in [XMPP-CORE].  Therefore TLS is already familiar to XMPP
   developers.

   This specification, called "XTLS", defines a method whereby any XMPP
   entity that supports the XMPP Jingle negotiation framework [JINGLE]
   can use TLS semantics for end-to-end encryption, whether the
   application data is sent over a streaming transport (like TCP) or a
   datagram transport (like UDP).  The basic use case is to tunnel XMPP
   stanzas between two IM users for end-to-end secure chat using end-to-
   end XML streams.  However, XTLS is not limited to encryption of one-
   to-one text chat, since it can be used between two XMPP clients for
   encryption of any XMPP payloads, between an XMPP client and a remote
   XMPP service (i.e., a service with which a client does not have a
   direct XML stream, such as a [MUC] chatroom), or between two remote
   XMPP services.  Furthermore, XTLS can be used for encrypted file
   transfer using [JINGLE-FILE], for encrypted voice or video sessions
   using [JINGLE-RTP] and [DTLS-SRTP], and other applications.

   Note: The following capitalized keywords are to be interpreted as
   described in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL
   NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED";
   "MAY", "OPTIONAL".


2.  Approach

   In broad outline, XTLS takes the following approach to end-to-end
   encryption of XMPP traffic:

   1.  We assume that all XMPP entities will have X.509 certificates;
       realistically these certificates are likely to be self-signed and
       automatically generated by an XMPP client, however certificates
       issued by known certification authorities are encouraged to
       overcome problems with self-signed certificates.




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   2.  We use the XMPP Jingle extensions as the negotiation framework
       (see [JINGLE]).
   3.  We use the concept of Jingle security preconditions to ensure
       that the negotiated transport will be encrypted before used for
       sending application data.
   4.  When an entity wishes to encrypt its communications with a second
       entity, it sends a Jingle session-initiate request that specifies
       the desired application type, a possible transport, and a TLS
       security precondition that includes the sender's X.509
       fingerprint and optionally hints about the sender's supported TLS
       methods.
   5.  If both parties support XTLS, the first data sent over the
       negotiated transport is TLS handshake data, not application data.
       Once the TLS handshake has finished, the parties can then send
       application data over the now-encrypted transport (called an
       "XTLS tunnel").
   6.  The simplest scenario is end-to-end encryption of traditional
       XMPP text chat using end-to-end XML streams as the application
       and in-band bytestreams [IBB] as the transport.
   7.  If the parties have previously negotiated an XTLS tunnel, during
       the TLS negotiation each party simply needs to verify that the
       other party is presenting the same certificate as used in
       previous sessions.
   8.  If the parties have not previously negotiated an XTLS tunnel,
       they need to bootstrap trust in their certificates; to do so, it
       is encouraged to use secure remote passwords rather than leap-of-
       faith.

   We expand on this approach in the following section.

   More complex scenarios are theoretically supported (e.g., encrypted
   file transfer using SOCKS5 bytestreams and encrypted voice chat using
   DTLS-SRTP) but have not yet been fully defined.

   XTLS theoretically can be used to establish a TLS-encrypted streaming
   transport or a DTLS-encrypted datagram transport, but integration
   with DTLS [DTLS] has not yet been prototyped so use with streaming
   transports is the more stable scenario.


3.  XTLS Protocol Flow

   The basic flow for an XTLS session is as follows, where traffic
   represented by single dashes (---) is sent over the XMPP signalling
   channel and traffic represented by double lines (===) is sent over
   the negotiated transport.





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   Initiator                   Responder
     |                            |
     |  session-initiate          |
     |  (with security info)      |
     |--------------------------->|
     |  ack                       |
     |<---------------------------|
     |  session-accept            |
     |<---------------------------|
     |  ack                       |
     |--------------------------->|
     |  open transport            |
     |<==========================>|
     |  TLS ClientHello           |
     |===========================>|
     |  TLS ServerHello, [...]    |
     |<===========================|
     |  TLS [...], Finished       |
     |===========================>|
     |  TLS [...], Finished       |
     |<===========================|
     |  application data          |
     |<==========================>|
     |  session-terminate         |
     |<---------------------------|
     |  ack                       |
     |--------------------------->|
     |                            |

   To simplify the description we assume here that the parties already
   trust each other's certificates.  See discussion under Section 5 for
   information about bootstrapping of certificate trust when the parties
   first negotiate the use of an XTLS tunnel.

   First the initiator sends a Jingle session-initiate request (here the
   simple case of an end-to-end text chat session using in-band
   bytestreams [IBB]).  This request includes a <security/> element that
   contains the fingerprint of the certificate that the initiator will
   use during the TLS negotiation and a list of TLS methods the
   initiator supports (here certificate-based authentication [X509] and
   TLS with Secure Remote Passwords [TLS-SRP]).  Note that this
   information is exchanged over the insecure server-based connection.
   The purpose of the exchange is to gather information about which TLS
   method should be used in the TLS handshake, e.g. if a client cannot
   verify the fingerprint of the peer it MAY omit the X.509 method.  If
   both clients can verify the fingerprint of the other, it is likely
   that X.509 certificate-based authentication will succeed (unless the
   data is altered); if one client cannot verify the fingerprint the



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   client MAY prompt the user for a password for TLS-SRP based
   authentication (see Section 5 for details).

   <iq from='romeo@montague.lit/orchard'
       id='xn28s7gk'
       to='juliet@capulet.lit/balcony'
       type='set'>
     <jingle xmlns='urn:xmpp:jingle:1'>
             action='session-initiate'
             initiator='romeo@montague.lit/orchard'
             sid='a73sjjvkla37jfea'>
       <content creator='initiator' name='xmlstream'>
         <description xmlns='urn:xmpp:jingle:apps:xmlstream:0'/>
         <transport xmlns='urn:xmpp:jingle:transports:ibb:0'
                    block-size='4096'
                    sid='ch3d9s71'/>
         <security xmlns='urn:xmpp:jingle:security:xtls:0'>
           <fingerprint algo='sha1'>RomeoX509CertSHA1Hash</fingerprint>
           <method name='x509'/>
           <method name='srp'/>
         </security>
       </content>
     </jingle>
   </iq>

   The responder immediately acknowledges receipt of the session-
   initiate by sending an IQ stanza of type "result" (not shown here).

   Depending on the application type, a user agent controlled by a human
   user might need to wait for the user to affirm a desire to proceed
   with the session before continuing.  When the user agent has received
   such affirmation (or if the user agent can automatically proceed for
   any reason, e.g. because no human intervention is expected or because
   a human user has configured the user agent to automatically accept
   sessions with a given entity), it returns a Jingle session-accept
   message.  This message will typically contain the offered application
   type, transport method, and a <security/> element that includes the
   fingerprint of the responder's X.509 certificate as well as the
   responder's supported TLS methods.












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   <iq from='juliet@capulet.com/balcony'
       id='hf64hl'
       to='romeo@montague.net/orchard'
       type='set'>
     <jingle xmlns='urn:xmpp:jingle:1'>
             action='session-accept'
             initiator='romeo@montague.lit/orchard'
             sid='a73sjjvkla37jfea'>
       <content creator='initiator' name='xmlstream'>
         <description xmlns='urn:xmpp:jingle:apps:xmlstream:0'/>
         <transport xmlns='urn:xmpp:jingle:transports:ibb:0'
                    block-size='4096'
                    sid='ch3d9s71'/>
         <security xmlns='urn:xmpp:jingle:security:xtls:0'/>
           <fingerprint algo='sha1'>JulietX509CertSHA1Hash</fingerprint>
           <method name='x509'/>
           <method name='srp'/>
         </security>
       </content>
     </jingle>
   </iq>

   The following rules apply to the responder's handling of the session-
   initiate message:

   1.  If the responder does not support XTLS it will silently ignore
       the <security/> element in the offer and therefore will return a
       session-accept message without a <security/> element.
   2.  If the responder supports XTLS it MUST return a session-accept
       message that contains a <security/> element.
   3.  If the responder thinks it will be able to verify the initiator's
       certificate, it MUST include the fingerprint for the responder's
       certificate in the <security/> element of the session-accept
       message.  This is the "happy path" and will occur when the
       parties have already verified each other's certificates.
   4.  If the responder thinks it will not be able to verify the
       initiator's certificate, it MAY omit the fingerprint for the
       responder's certificate in the <security/> element of the
       session-accept message.  This indicates that certificate-based
       authentication is not possible.  In this case the responder
       SHOULD signal that it wishes to use some other authentication
       method, such as secure remote passwords (see discussion under
       Section 5).
   5.  If the responding client cannot verify the initiator's
       certificate, it SHOULD ask the responding user if a password was
       exchanged between the parties that can be used for TLS-SRP.  If
       this is not the case, setting up a mutually-authenticated link
       will fail and the responder MAY terminate the session.



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       Alternatively it could send its own fingerprint knowing it cannot
       authenticate the initiator, in which case the responder has to
       trust that there is no man-in-the-middle (see discussion under
       Section 5).

   When the responder sends the session-accept message, the initiator
   acknowledges receipt by sending an IQ stanza of type "result" (not
   shown here).

   The following rules apply to the initiator's handling of the session-
   accept message:

   1.  If the initiator receives a session-accept without a <security/>
       element, setting up a secure transport layer has failed.  The
       initiator MAY terminate the session at this point or instead
       proceed without securing the transport.  The client SHOULD ask
       the initiating user how to processed.  This depends on the Jingle
       application and the initiator's preferences: it makes no sense to
       use end-to-end XML streams without encryption, but the initiator
       might continue a file transfer without encryption.
   2.  If the initiating client cannot verify the responder's
       certificate it SHOULD ask the initiating user if a password was
       exchanged between the parties that can be used for TLS-SRP.  If
       this is not the case, setting up a mutually-authenticated link
       will fail and the responder MAY terminate the session or proceed
       with leap-of-faith (see discussion under Section 5).

   The initiator can now determine if X.509 certificate-based
   authentication will work or if TLS-SRP will be used.  It sends an
   additional security-info message to the responder to signal its
   choice.  This step is not really necessary because the responder will
   see the initiator's choice in the first message of the TLS handshake,
   but it can assist an implementation in setting up its TLS library
   properly.  Because in this section we assume that the parties already
   have validated each other's certificates, the security method
   signalled here is "x509".















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   <iq from='romeo@montague.lit/orchard'
       id='hf749j'
       to='juliet@capulet.lit/balcony'
       type='set'>
     <jingle xmlns='urn:xmpp:jingle:1'>
             action='security-info'
             initiator='romeo@montague.lit/orchard'
             sid='a73sjjvkla37jfea'>
       <content creator='initiator' name='xmlstream'>
         <security xmlns='urn:xmpp:jingle:security:xtls:0'>
           <method name='x509'/>
         </security>
       </content>
     </jingle>
   </iq>

   The responder acknowledges receipt by sending an IQ stanza of type
   "result" (not shown here).

   Parallel to the security-info exchange, the clients negotiate a
   transport for the Jingle session (here the transport is an in-band
   bytestream as defined in [IBB], for which the Jingle negotiation
   process is specified in [XEP-0261]; however other transports could be
   used, for example SOCKS5 bytestreams as defined in [XEP-0065] and
   negotiated for Jingle as specified in [XEP-0260]).  Because the
   parties wish to establish end-to-end encryption, they do not send
   application data over the transport until the transport has been
   secured.  Therefore the first data that they exchange over the
   transport consists of the standard four-way TLS handshake, encoded in
   accordance with the negotiated transport method.

      Note: Each transport MUST define a specific time when both clients
      know that the transport is secured.  When XTLS is not used, the
      Jingle implementation would signal to the using application that
      the transport is open when the session-accept is sent or received,
      or when connectivity checks determine media can flow over one of
      the transport candidates.  When XTLS is used, the Jingle
      implementation starts a TLS handshake on the transport and signals
      to the using application that the transport is open only after the
      TLS handshake has finished successfully.

   During the TLS handshake, the responder MUST take the role of the TLS
   server and the initiator MUST take the role of the TLS client.
   Because the transport is an in-band bytestream, the TLS handshake
   data is prepared as described in [IBB] (i.e., Base64-encoded).  First
   the initiator (acting as the TLS client) constructs a TLS
   ClientHello, encodes it according to IBB, and sends it to the
   responder.



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   <iq from='romeo@montague.net/orchard'
       id='vh38s618'
       to='juliet@capulet.com/balcony'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='0'
           sid='vj3hs98y'>
       Base64-encoded-TLS-data
     </data>
   </iq>

   The responder (acting as the TLS server) then acknowledges receipt by
   sending an IQ stanza of type "result" (not shown here).

   The responder then constructs an appropriate TLS message or messages,
   such as a ServerHello and a CertificateRequest.

      Note: The responder MUST send a CertificateRequest to the
      initiator.

   <iq from='juliet@capulet.com/balcony'
       id='xyw516d0'
       from='romeo@montague.net/orchard'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='0'
           sid='vj3hs98y'>
       Base64-encoded-TLS-data
     </data>
   </iq>

   (Because in-band bytestreams are bidirectional and this data is sent
   from the responder to the initiator, the IBB 'seq' attribute has a
   value of zero, not 1.)

   The initiator then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   After some number of TLS messages, the initiator eventually sends a
   TLS Finished message to the responder.











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   <iq from='romeo@montague.net/orchard'
       id='s91vd527'
       to='juliet@capulet.com/balcony'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='3'
           sid='vj3hs98y'>
       Base64-encoded-TLS-data
     </data>
   </iq>

   The responder then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   The responder then also sends a TLS Finished message.

   <iq from='juliet@capulet.com/balcony'
       id='z71gs73t'
       from='romeo@montague.net/orchard'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='3'
           sid='vj3hs98y'>
       Base64-encoded-TLS-data
     </data>
   </iq>

   The initiator then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   If the TLS negotiation has finished successfully, then the Jingle
   implementation shall signal to the using application that the
   transport has been secured and is ready to be used.  The parties now
   have a secure channel for the end-to-end exchange of application data
   using XMPP as the virtual transport; we call such a channel an XTLS
   TUNNEL.


4.  End-to-End Streams over XTLS Protocol Flow

   For end-to-end encryption of XMPP stanzas (<message/>, <presence/>,
   and <iq/>), the application data is an end-to-end XML stream.  After
   the XTLS tunnel is established, the peers open an XML stream over the
   tunnel to exchange stanzas.  In this example, the tunnel is
   established using a transport of IBB, but any streaming transport
   could be used.

   First the initiator constructs an initial stream header.



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   <stream:stream
           xmlns='jabber:client'
           xmlns:stream='http://etherx.jabber.org/streams'
           from='romeo@montague.lit/orchard'
           to='juliet@capulet.lit/balcony'
           version='1.0'>

   Note: In accordance with [XMPP-CORE], the initial stream header
   SHOULD include the 'to' and 'from' attributes, which SHOULD specify
   the full JIDs of the clients.  The initiator SHOULD include the
   version='1.0' flag as shown in the previous example.

   The initiator then transforms the stream header into TLS data,
   encodes the data into IBB, and sends an IQ-set to the responder.

   <iq from='romeo@montague.net/orchard'
       id='ur73n153'
       to='juliet@capulet.com/balcony'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='4'
           sid='vj3hs98y'>
       Base64-TLS-data-of-the-stream-header
     </data>
   </iq>

   The responder then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   The responder then constructs a response stream header back to the
   initiator.

   <stream:stream
           xmlns='jabber:client'
           xmlns:stream='http://etherx.jabber.org/streams'
           from='juliet@capulet.lit/balcony'
           id='hs91gh1836d8s717'
           to='romeo@montague.lit/orchard'
           version='1.0'>

   The responder then sends the response stream header over the XTLS
   tunnel.









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   <iq from='juliet@capulet.com/balcony'
       id='pd61g397'
       to='romeo@montague.net/orchard'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='4'
           sid='vj3hs98y'>
       Base64-TLS-data-of-the-responce-stream-header
     </data>
   </iq>

   The initiator then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   Once the XML stream is established over the XTLS tunnel, either
   entity then can send XMPP message, presence, and IQ stanzas, with or
   without 'to' and 'from' addresses.

   For example, the initiator could construct an XMPP message.

   <message from='romeo@montague.lit/orchard'
            to='juliet@capulet.lit/balcony'>
     <body>
       M&apos;lady, I would be pleased to make your acquaintance.
     </body>
   </message>

   The initiator then sends the message over the XTLS tunnel.

   <iq from='romeo@montague.net/orchard'
       id='iq7dh294'
       to='juliet@capulet.com/balcony'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='5'
           sid='vj3hs98y'>
       Base64-TLS-data
     </data>
   </iq>

   The responder then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   The responder could then construct a reply.

   <message from='juliet@capulet.lit/balcony'
            to='romeo@montague.lit/orchard'>
     <body>Art thou not Romeo, and a Montague?</body>



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   </message>

   The responder then sends the reply over the XTLS tunnel.

   <iq from='juliet@capulet.com/balcony'
       id='hr91hd63'
       to='romeo@montague.net/orchard'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='5'
           sid='vj3hs98y'>
       Base64-TLS-data
     </data>
   </iq>

   The initiator then acknowledges receipt by sending an IQ stanza of
   type "result" (not shown here).

   To close the end-to-end XML stream, either party (here the responder)
   constructs a closing </stream:stream> element.

   </stream:stream>

   The client sends the closing element to the peer over the XTLS
   tunnel.

   <iq from='juliet@capulet.com/balcony'
       id='kr91n475'
       to='romeo@montague.net/orchard'
       type='set'>
     <data xmlns='http://jabber.org/protocol/ibb'
           seq='6'
           sid='vj3hs98y'>
       Base64-TLS-data
     </data>
   </iq>

   The peer then acknowledges receipt by sending an IQ stanza of type
   "result" (not shown here).

   However, even after the end-to-end XML stream is terminated, the
   negotiated Jingle transport (here an in-band bytestream) continues
   and could be re-used.  To completely terminate the Jingle session,
   the terminating party would then also send a Jingle session-terminate
   message.






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   <iq from='juliet@capulet.lit/balcony'
       id='psy617r4'
       to='romeo@montague.lit/orchard'
       type='set'>
     <jingle xmlns='urn:xmpp:jingle:1'
             action='session-terminate'
             initiator='romeo@montague.lit/orchard'
             sid='851ba2'/>
   </iq>

   The other party then acknowledges the Jingle session-terminate by
   sending an IQ stanza of type "result" (not shown here).


5.  Bootstrapping Trust on First Communication

   When two parties first attempt to use XTLS, their certificates might
   not be accepted (e.g., because they are self-signed or issued by
   unknown certification authorities).  Therefore each party needs to
   accept the other's certificate for use in future communication
   sessions.  There are several ways to do so:

   o  Leap of faith.  The recipient can hope that there is no man-in-
      the-middle during the first communication session.  If the
      certificate does not change in future sessions, the recipient at
      least knows that it is talking with the same entity it talked with
      during the first session.  However, that entity might be a man-in-
      the-middle rather than the assumed communication partner.
      Therefore, leap of faith is discouraged.
   o  Check fingerprints.  The parties could validate the certificate
      fingerprints via some trusted means outside the XMPP band, such as
      in person, via encrypted email, or over the phone.  This is not
      user-friendly because certificate fingerprints consist of long
      strings of letters and numbers.  As a result, few humans routinely
      check certificate fingerprints in protocols such as Secure Shell
      (ssh).
   o  One-time password.  The parties can exchange a user-friendly
      password known only to themselves and verify it out of band before
      the TLS handshake finishes.  For this purpose, it is REQUIRED for
      implementations to support at least one TLS cipher that uses
      Secure Remote Password (SRP) as defined in [TLS-SRP].
   o  Channel binding.  It is possible that a future version of this
      specification will describe how to use an appropriate Simple
      Authentication and Security Layer (SASL) mechanism, such as
      [SCRAM], to authenticate the XTLS tunnel after the TLS handshake
      finishes; such a method would use the concept of channel bindings
      as described in [RFC5056].




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   If the parties use a password or SASL channel binding to bootstrap
   trust, the process needs to be completed only once.  After the
   clients have authenticated with the shared secret, they can exchange
   their certificates for future communication.

5.1.  Exchanging Certificates

   To retrieve the certificate of the peer for future communications, a
   client SHOULD request the certificate according to [XEP-0189] over
   the secure connection.  This works only if XTLS was used to set up an
   end-to-end secure XML stream; exchanging certificates if XTLS was
   used for other purposes like file transfer is not possible.  A client
   MUST NOT request the certificate over the insecure stream-based on
   the connection to the XMPP server.

   <iq from='romeo@montague.lit/orchard'
       id='hf7634k4'
       to='juliet@capulet.lit/balcony'
       type='get'>
     <pubkeys xmlns='urn:xmpp:pubkey:0'/>
   </iq>

   The peer MUST return its own client certificate.  If the user has
   different clients with different client certificates and one user
   certificate, the user certificate SHOULD also be returned.  The user
   certificate allows it to verify other client certificates using
   public key retrieval as described in [XEP-0189].
























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   <iq from='juliet@capulet.com/balcony'
       id='hf7634k4'
       to='romeo@montague.lit/orchard'
       type='result'>
     <pubkeys xmlns='urn:xmpp:pubkey:0'>
       <keyinfo>
         <x509cert>
   MIICCTCCAXKgAwIBAgIJALhU0Id6xxwQMA0GCSqGSIb3DQEBBQUAMA4xDDAKBgNV
   BAMTA2ZvbzAeFw0wNzEyMjgyMDA1MTRaFw0wODEyMjcyMDA1MTRaMA4xDDAKBgNV
   BAMTA2ZvbzCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEA0DPcfeJzKWLGE22p
   RMINLKr+CxqozF14DqkXkLUwGzTqYRi49yK6aebZ9ssFspTTjqa2uNpw1U32748t
   qU6bpACWHbcC+eZ/hm5KymXBhL3Vjfb/dW0xrtxjI9JRFgrgWAyxndlNZUpN2s3D
   hKDfVgpPSx/Zp8d/ubbARxqZZZkCAwEAAaNvMG0wHQYDVR0OBBYEFJWwFqmSRGcx
   YXmQfdF+XBWkeML4MD4GA1UdIwQ3MDWAFJWwFqmSRGcxYXmQfdF+XBWkeML4oRKk
   EDAOMQwwCgYDVQQDEwNmb2+CCQC4VNCHesccEDAMBgNVHRMEBTADAQH/MA0GCSqG
   SIb3DQEBBQUAA4GBAIhlUeGZ0d0msNVxYWAXg2lRsJt9INHJQTCJMmoUeTtaRjyp
   ffJtuopguNNBDn+MjrEp2/+zLNMahDYLXaTVmBf6zvY0hzB9Ih0kNTh23Fb5j+yK
   QChPXQUo0EGCaODWhfhKRNdseUozfNWOz9iTgMGw8eYNLllQRL//iAOfOr/8
         </x509cert>
       </keyinfo>
     </pubkeys>
   </iq>

5.2.  Verification of Non-Human Parties

   If one of the parties is a "bot" (e.g., an automated service or a
   device such as a set-top box), the password exchange is a bit more
   complicated.  It is similar to Bluetooth peering if the user has
   access to both clients at the same time.  One of the following
   scenarios might apply:

   o  The bot can be controlled via a remote control input device.  The
      human user can enter the same password or "PIN" on both the bot
      and the XMPP client.
   o  If the bot has no user input but does have a small display, it
      could display a random password.  The human user can then enter
      the provided password on the XMPP client.
   o  The bot might not have enough buttons for input and might not have
      an output screen.  In that case the password is fixed.  Similar to
      Bluetooth peering with simple devices such as a headset, the
      password will be written in the manual or printed on the device.
      For security reasons the device SHOULD NOT use password-based
      authentication without any user input.  Many Bluetooth devices
      have at least one button to set the device into peering mode.
   o  A bot may be associated with a web service and could display a
      random password when the user has logged in to the web site using
      HTTPS.  This assumes that an attacker cannot at the same time both
      control over the web server and perform a man-in-the-middle attack



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      on the XMPP channel.  If the web service knows the GPG key of the
      user it could send an encrypted email.

   A user might have different X.509 certificates for each device.
   [XEP-0189] can be used to manage the user's certificates.  A client
   SHOULD check the peer's PubSub node for certificates.  This makes it
   possible to use the password method only once between two users even
   if one or both users switch clients.  A user can also communicate
   with a friend's bots: they first open a secure link between two chat
   clients with a password and exchange the user certificates.  After
   that each device of a user can verify all devices of the other
   without the need of a password.

   The retrieved certificate from the PubSub node might be signed by a
   certification authority that the client can verify.  In that case the
   client MAY skip the password authentication and rely on the X.509
   certificate chain.  The client SHOULD ask the user if the certificate
   is acceptable or if a password exchange is desired.


6.  Session Termination

   If either client cannot verify the certificate of the peer or
   receives an invalid message on the TLS layer, it MUST terminate the
   Jingle session immediately by sending a Jingle session-terminate
   message that includes a Jingle reason of <security-error/>.

   <iq from='romeo@montague.lit/orchard'
       id='hz81vf48'
       to='juliet@capulet.lit/balcony'
       type='set'>
     <jingle xmlns='urn:xmpp:jingle:1'
             action='session-terminate'
             initiator='romeo@montague.lit/orchard'
             sid='a73sjjvkla37jfea'>
       <reason><security-error/></reason>
     </jingle>
   </iq>

   The other party then acknowledges the session-terminate by sending an
   IQ stanza of type "result" (not shown here), and the Jingle session
   is finished.


7.  Determining Support

   If an entity wishes to request the use of XTLS, it SHOULD first
   determine whether the intended responder supports the protocol.  This



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   can be done directly via [XEP-0030] or indirectly via [XEP-0115].

   If an entity supports XTLS, it MUST report that by including a
   service discovery feature of "urn:xmpp:jingle:security:xtls:1" in
   response to disco#info requests.

   <iq from='romeo@montague.lit/orchard'
       id='disco1'
       to='juliet@capulet.lit/chamber'
       type='get'>
     <query xmlns='http://jabber.org/protocol/disco#info'/>
   </iq>


   <iq from='juliet@capulet.lit/chamber'
       id='disco1'
       to='romeo@montague.lit/orchard'
       type='result'>
     <query xmlns='http://jabber.org/protocol/disco#info'>
       <feature var='urn:xmpp:jingle:security:xtls:1'/>
       <feature var='urn:xmpp:jingle:apps:xmlstream:1'/>
     </query>
   </iq>

   Both service discovery and entity capabilities information could be
   corrupted or intercepted; for details, see under Section 8.3.


8.  Security Considerations

   This entire document addresses security.  Particular security-related
   issues are discussed in the following sections.

8.1.  Mandatory-to-Implement Technologies

   An implementation MUST at a minimum support the "srp" and "x509"
   methods.  A future version of this specification will document
   mandatory-to-implement TLS ciphers.

8.2.  Certificates

   As noted, XTLS can be used between XMPP clients, between an XMPP
   client and a remote XMPP service (i.e., a service with which a client
   does not have a direct XML stream), or between remote XMPP services.
   Therefore, a party to an XTLS bytestream will present either a client
   certificate or a server certificate as appropriate.  Such
   certificates MUST be generated and validated in accordance with the
   certificate guidelines guidelines provided in [XMPP-CORE].



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   A future version of this specification might provide additional
   guidelines regarding certificate validation in the context of client-
   to-client encryption.

8.3.  Denial of Service

   Currently XMPP stanzas such as Jingle negotiation messages and
   service discovery exchanges are not encrypted or signed.  As a
   result, it is possible for an attacker to intercept these stanzas and
   modify them, thus convincing one party that the other party does not
   support XTLS and therefore denying the parties an opportunity to use
   XTLS.

   This is a more general problem with XMPP technologies and needs to be
   addressed at the core XMPP layer.


9.  IANA Considerations

   It might be helpful to create a registry of TLS methods that can be
   used in the context of XTLS (e.g., "openpgp" for use of [RFC5081],
   "srp" for use of [TLS-SRP], and "x509" for use of [TLS] with
   certificates).  The registry could be maintained by the IANA or by
   the XMPP Registrar (see [XEP-0053]).  A future version of this
   specification will provide more detailed information about the
   registration requirements.


10.  References

10.1.  Normative References

   [E2E-REQ]  Saint-Andre, P., "Requirements for End-to-End Encryption
              in the Extensible Messaging and Presence Protocol (XMPP)",
              draft-saintandre-xmpp-e2e-requirements-01 (work in
              progress), June 2009.

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

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

   [IBB]      Karneges, J., "In-Band Bytestreams (IBB)", XSF XEP 0047,
              March 2009.

   [JINGLE]   Ludwig, S., Beda, J., Saint-Andre, P., McQueen, R., Egan,
              S., and J. Hildebrand, "Jingle", XSF XEP 0166, June 2009.



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   [XMPP-CORE]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-00 (work
              in progress), June 2009.

10.2.  Informative References

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

   [DTLS-SRTP]
              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-ietf-avt-dtls-srtp-07 (work in progress),
              February 2009.

   [HTTP-TLS]
              Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [JINGLE-FILE]
              Saint-Andre, P., "Jingle File Transfer", XSF XEP 0234,
              February 2009.

   [JINGLE-RTP]
              Ludwig, S., Saint-Andre, P., Egan, S., McQueen, R., and D.
              Cionoiu, "Jingle RTP Sessions", XSF XEP 0167, June 2009.

   [MUC]      Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
              July 2008.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              November 2007.

   [TLS-SRP]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

   [SCRAM]    Menon-Sen, A., Melnikov, A., Newman, C., and N. Williams,
              "Salted Challenge Response (SCRAM) SASL Mechanism",
              draft-newman-auth-scram-13 (work in progress), May 2009.

   [X509]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key



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              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [XEP-0030]
              Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
              Andre, "Service Discovery", XSF XEP 0030, June 2008.

   [XEP-0053]
              Saint-Andre, P., "XMPP Registrar Function", XSF XEP 0053,
              October 2008.

   [XEP-0065]
              Smith, D., Miller, M., and P. Saint-Andre, "SOCKS5
              Bytestreams", XSF XEP 0065, May 2007.

   [XEP-0115]
              Hildebrand, J., Saint-Andre, P., Troncon, R., and J.
              Konieczny, "Entity Capabilities", XSF XEP 0115,
              February 2008.

   [XEP-0189]
              Paterson, I., Saint-Andre, P., and D. Meyer, "Public Key
              Publishing", XSF XEP 0189, March 2009.

   [XEP-0260]
              Saint-Andre, P. and D. Meyer, "Jingle SOCKS5 Bytestreams
              Transport Method", XSF XEP 0260, February 2009.

   [XEP-0261]
              Saint-Andre, P., "Jingle In-Band Bytestreams Transport",
              XSF XEP 0261, February 2009.


Appendix A.  XML Schema

   The XML schema will be provided in a later version of this document.


Appendix B.  Copying Conditions

   Regarding this entire document or any portion of it, the authors make
   no guarantees and are not responsible for any damage resulting from
   its use.  The authors grant irrevocable permission to anyone to use,
   modify, and distribute it in any way that does not diminish the
   rights of anyone else to use, modify, and distribute it, provided
   that redistributed derivative works do not contain misleading author
   or version information.  Derivative works need not be licensed under
   similar terms.



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Authors' Addresses

   Dirk Meyer
   Universitaet Bremen TZI

   Email: dmeyer@tzi.de


   Peter Saint-Andre
   Cisco

   Email: psaintan@cisco.com







































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