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Versions: (draft-arkko-sip-sec-agree) 00 01 02 03 04 RFC 3329

     SIP Working Group                                           Jari Arkko
     INTERNET-DRAFT                                           Vesa Torvinen
     <draft-ietf-sip-sec-agree-00.txt>                             Ericsson
     April 2002                                                  Tao Haukka
                                                                      Nokia
                                                                 Sanjoy Sen
                                                               Lee Valerius
                                                            Nortel Networks


                 Security Mechanism Agreement for SIP Sessions

1.   Status of this Memo

     This document is an Internet-Draft and is in full conformance with all
     provisions of Section 10 of RFC2026. Internet-Drafts are working docu¡
     ments of the Internet Engineering Task Force (IETF), its areas, and
     its working groups.  Note that other groups may also distribute work¡
     ing documents as Internet-Drafts.

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

     The list of current Internet-Drafts may be found at
     http://www.ietf.org/ietf/1id-abstracts.txt

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

     The distribution of this memo is unlimited.  It is filed as <draft-
     ietf-sip-sec-agree-00.txt>, and  expires October, 2002.  Please send
     comments to the author or to SIPPING or SIP working group.

2.   Abstract

     SIP has a number of security mechanisms for hop-by-hop and end-to-end
     protection. Some of the security mechanisms have been built in to the
     SIP protocol, such as HTTP authentication or secure attachments. In
     these mechanisms there are even alternative algorithms and parameters.
     Currently it isn't possible to select which security mechanisms to use
     over a connection. In particular, even if some mechanisms such as
     OPTIONS were used to make this selection, the selection would be vul¡
     nerable against the Bidding-Down attack.  This document defines a
     header for negotiating the security mechanisms within SIP. A SIP
     entity applying this mechanism must always require some minimum secu¡
     rity (i.e. integrity protection) from all communicating parties in
     order to secure the negotiation, but the negotiation can agree on
     which specific minimum security is used.







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3.   Contents


       1.          Status of this Memo..................................1
       2.          Abstract.............................................1
       3.          Contents.............................................2
       4.          Introduction.........................................2
       5.          The Problem..........................................3
       6.          Solution.............................................4
         6.1.      Requirements.........................................4
         6.2.      Different Mechanisms.................................5
         6.3.      Overview of Operations...............................5
         6.4.      Header descriptions..................................7
       7.          Summary of header usage..............................9
       8.          Backwards Compatibility..............................10
       9.          Examples.............................................10
         9.1.      Selecting Between New and Old Mechanisms.............10
         9.2.      Selections Along the Path............................11
       10.         Security Considerations..............................13
       11.         IANA Considerations..................................13
       12.         Modifications........................................14
       13.         Acknowledgments......................................15

4.   Introduction

     Traditionally, security protocols have included facilities to agree on
     the used mechanisms, algorithms, and other security parameters. The
     reason for this is that experience has shown that e.g. algorithm
     development uncovers problems in old algorithms and produces new ones.
     Furthermore, different mechanisms and algorithms are suitable for dif¡
     ferent situations. Typically, protocols also select other parameters
     beyond algorithms at the same time.

     The purpose of this specification is to define a similar negotiation
     functionality in SIP [1]. SIP has some security functionality built-in
     such as HTTP Digest authentication [4], secure attachments such as
     S/MIME [5], and can also use underlying security protocols such as
     IPSec/IKE [2] or TLS [3].  Some of the built-in security functionality
     allows also alternative algorithms and other parameters.  While some
     work within the SIP Working Group has been looking towards reducing
     the number of recommended security solutions (e.g. recommend just one
     lower layer security protocol), we can not expect to cut down the num¡
     ber of items in the whole list to one. There will still be multiple
     security solutions utilized by SIP. Furthermore, it is likely that new
     methods will appear in the future, to complete the methods that exist
     today.

     Chapter 5 shows that without a secured method to choose between secu¡
     rity mechanisms and/or their parameters, SIP is vulnerable to certain
     attacks. As the HTTP authentication RFC [4] points out, authentication
     and integrity protection using multiple alternative methods and algo¡
     rithms is vulnerable to Man-in-the-Middle (MITM) attacks. More seri¡
     ously, it is hard or sometimes even impossible to know whether a SIP
     peer entity is truly unable to perform e.g.  Digest, TLS, or S/MIME,



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     or if a MITM attack is in action. In small networks consisting of
     workstations and servers these issues are not very relevant, as the
     administrators can deploy appropriate software versions and set up
     policies for using exactly the right type of security. However, SIP
     will soon be deployed to hundreds of millions of small devices with
     little or no possibilities for coordinated security policies, let
     alone software upgrades, and this makes these issues much worse. This
     conclusion is also supported by the requirements from 3GPP [6].

     Chapter 6 documents the proposed solution, and chapter 7 gives some
     demonstrative examples.

5.   The Problem


     SIP has alternative security mechanisms such as HTTP authentication /
     integrity protection, lower layer security protocols, and S/MIME. It
     is likely that their use will continue in the future. SIP security is
     developing, and is likely to see also new solutions in the future.

     Deployment of large number of SIP-based consumer devices such as 3GPP
     terminals requires all network devices to be able to accommodate past,
     current and future mechanisms; there is no possiblity for instanta¡
     neous change since the new solutions are coming gradually in as new
     standards and product releases occur. It is sometimes even impossible
     to upgrade some of the devices without getting completely new hard¡
     ware.

     So, the basic security problem that such a large SIP-based network
     must consider, would be on how do security mechanisms get selected? It
     would be desirable to take advantage of new mechanisms as they become
     available in products.

     Firstly, we need to know somehow what security should be applied, and
     preferably find this out without too many additional roundtrips.

     Secondly, selection of security mechanisms MUST be secure.  Tradition¡
     ally, all security protocols use a secure form of negotiation. For
     instance, after establishing mutual keys through Diffie-Hellman, IKE
     sends hashes of the previously sent data -- including the offered
     crypto mechanisms. This allows the peers to detect if the initial,
     unprotected offers were tampered with.

     The security implications of this are subtle, but do have a fundamen¡
     tal importance in building large networks that change over time. Given
     that the hashes are produced also using algorithms agreed in the first
     unprotected messages, one could ask what the difference in security
     really is. Assuming integrity protection is mandatory and only secure
     algorithms are used, we still need to prevent MITM attackers from mod¡
     ifying other parameters, such as whether encryption is provided or
     not.  Let us first assume two peers capable of using both strong and
     weak security. If the initial offers are not protected in any way, any
     attacker can easily "downgrade" the offers by removing the strong
     options. This would force the two peers to use weak security between



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     them. But if the offers are protected in some way -- such as by hash¡
     ing, or repeating them later when the selected security is really on
     -- the situation is different. It would not be sufficient for the
     attacker to modify a single message. Instead, the attacker would have
     to modify both the offer message, as well as the message that contains
     the hash/repetition. More importantly, the attacker would have to
     forge the weak security that is present in the second message, and
     would have to do so in real time between the sent offers and the later
     messages. Otherwise, the peers would notice that the hash is incor¡
     rect. If the attacker is able to break the weak security, the security
     method and/or the algorithm should not be used.

     In conclusion, the security difference is making a trivial attack pos¡
     sible versus demanding the attacker to break algorithms. An example of
     where this has a serious consequence is when a network is first
     deployed with integrity protection (such as HTTP Digest [4]), and then
     later new devices are added that support also encryption (such as
     S/MIME [1]). In this situation, an insecure negotiation procedure
     allows attackers to trivially force even new devices to use only
     integrity protection.

6.   Solution

6.1. Requirements

     The solution to the SIP security negotiation problem should have the
     following properties:

     (a) It allows the selection of security mechanisms, such as lower
     layer security protocols or secure attachments. It also allows the
     selection of individual algorithms and parameters when the security
     functions are integrated in SIP (such as in the case of HTTP authenti¡
     cation or secure attachments).

     (b) It allows both end-to-end and hop-by-hop negotiation.

     (c) It is secure, e.g. prevents the bidding down attack.

     (d) It is capable of running without additional roundtrips.  This is
     important in the cellular environment, where an additional roundtrip
     could delay the call set up for 1000-1500 ms.

     (e) It does not introduce any additional state to servers and proxies.

     Currently, SIP does not have any mechanism which fulfills all the
     requirements above.  The basic SIP features such as OPTIONS and
     Require, Supported headers are capable of informing peers about vari¡
     ous capabilities including security mechanisms. However, the straight¡
     forward use of these features can not guarantee a secured agreement.
     HTTP Digest algorithm lists [4] are not secure for picking among the
     digest integrity algorithms, as is described in the RFC itself.  More
     seriously, they have no provisions for allowing encryption to be nego¡
     tiated. Hence, it would be hard to turn on possible future encryption
     schemes in a secure manner.



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6.2. Different Mechanisms

     A self-describing security mechanism is a security mechanism that,
     when used, contains all necessary information about the method being
     used as well as all of its parameters such as algorithms.

     A non-self-describing security mechanism is a security mechanism that,
     when used, requires that the use of the method or some of its parame¡
     ters have been agreed beforehand.

     Most security mechanisms used with SIP are self-describing.  The use
     of HTTP digest, as well as the chosen algorithm is visible from the
     HTTP authentication headers. The use of S/MIME is indicated by the
     MIME headers, and the CMS structures inside S/MIME describe the used
     algorithms.  TLS is run on a separate port in SIP, and where IPsec/IKE
     is used, IKE negotiates all the necessary parameters.

     The only exception to this list is the use of manually keyed IPsec.
     IPsec headers do not contain information about the used algorithms.
     Furthermore, peers have to set up IPsec Security Associations before
     they can be used to receive traffic. In contrast S/MIME can be
     received even if no Security Association was in place, because the
     application can search for a Security Association (or create a new
     one) after having received a message that contains S/MIME.

     In order to make it possible to negotiate both self-describing and
     non-self-describing security mechanisms, the security agreement scheme
     must allow both sides to decide on the desired security mechanism
     before it is actually used.  This decision can, and must, take place
     on both sides before we can be sure that the negotiation has not been
     tampered by a man-in-the-middle.  This tampering will be detected
     later.

6.3. Overview of Operations

     This specification uses the following approach. The clients and
     servers offer their lists of supported security mechanisms and parame¡
     ters in the first, unprotected messages. They then proceed to turn on
     the selected security, and finally repeat some information under this
     security in order to ensure that the first exchanged lists had not
     been modified. This procedure is stateless for servers (unless the
     used security mechanisms require the server to keep some state).

     The client and the server lists are both static i.e. they do not and
     can not change based on the input from the other side. Nodes MAY, how¡
     ever, maintain several static lists, one for each interface, for exam¡
     ple.

       1. Client ----------client list---------> Server
       2. Client <---------server list---------- Server
       3. Client ------(turn on security)------- Server
       4. Client ----------server list---------> Server
       5. Client <---------ok or error---------- Server




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     The steps are explained below:

     - Step 1: The client MUST announce a list of supported security mecha¡
     nisms in their fist request. The client SHOULD also add the option-tag
     'sec-agree' to the Supported header.

     - Step 2: The server MUST announce a list of supported security mecha¡
     nisms in their first response. The server MUST add its list to the
     response even if there were no common security mechanisms in the
     client's and server's lists, and the list MUST NOT depend on the con¡
     tents of the client's list. The list MUST also be added regardless of
     any potential error codes in the response.

     An error has occurred if the client list is not present in the request
     of Step 1, this request is not OPTIONS, and the server's policy
     requires the use of this specification. This error will be reported by
     returning 421 (Extension Required).  In this case the server MUST also
     include a Require header with an option-tag 'sec-agree' in its
     response.

     - Step 3: The peers MUST initiate the security mechanism, if necessary
     to carry this outside the request in Step 4. For instance, TLS should
     be turned on at this stage.

     - Step 4: The client MUST select and use the first matching security
     mechanism from the server's list. The client MUST also repeat the
     server's list.

     - Step 5: The server MUST check that the server list sent in the
     secured message in Step 4 corresponds to its static list of supported
     security mechanisms.  The server MUST send a positive answer or pro¡
     ceed to execute the requested operation if and only if the list was
     not modified.  If modification of the list is detected, the server
     MUST return a 494 (Security Agreement Failed) response or disconnect.
     The 494 response MUST include server's unmodified list of supported
     security mechanisms. The server MUST NOT copy any Security-Mechanism
     header from the request in Step 4 to the 494 response in Step 5.

     The server MAY decide to use a security mechanism between Steps 1 and
     2 but it MUST do so before processing request in Step 4. The client
     MUST decide to use a security mechanism between Steps 2 and 3.

     Between Steps 1 and 2, the server MAY set up a non-self-describing
     security mechanism if necessary. Note that with this type of security
     mechanisms, the server is necessarily stateful. Between Steps 2 and 4,
     the client MAY set up a non-self-describing security mechanism.

     Note that non-adjacent SIP entities can not use hop-by-hop security
     mechanisms such as TLS or IPsec. If a client receives a list of hop-
     by-hop security mechanisms from a server several hops away, it MUST
     NOT try to use these mechanisms with the first hop proxy. The client
     MAY try to contact the server directly leaving the proxies in between
     away.




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     Once the security has been negotiated between two SIP entities, the
     same SIP entities MAY use the same security when communicating with
     each other in different SIP roles. For example, if a UAC in a end-user
     equipment and a UAS in a proxy negotiate some security, they may try
     to use the same security for terminating requests.

     The results of the security mechanism negotiation MAY be informed to
     the user of an UAC or an UAS. The user MAY decline to accept a partic¡
     ular security mechanism, and abort further SIP communications with the
     peer.

     One SIP request MAY include several independent list.  Only one list
     SHOULD be used between two SIP entities.  This allows a negotiation of
     the first-hop security mechanism while at the same time running e.g. a
     REGISTER with Digest authentication to a server some hops away.

6.4. Header descriptions

     The Security-Mechanism header indicates who wants security towards
     whom, and what kind of security.  The security features are repre¡
     sented using the header syntax described below.

     The following ABNF describes the syntax of this header and extends
     section 25.1 in [1]:

     "Security-Mechanism" HCOLON to-uri from-uri mechanism-list



         to-uri         = "to" EQUAL sip-uri COMMA
         from-uri       = "from" EQUAL sip-uri COMMA
         mechanism-list = 1*(COMMA mechanism [SEMI preference]
                             [SEMI algorithm] [SEMI params])
         mechanism      = "mech" EQUAL ( "digest" / "tls" / "ipsec-ike" /
                          "ipsec-man" / "smime" / token )
         preference     = "pref" EQUAL preference-value
         algorithm      = "alg" EQUAL algorithm-value
         params         = parameter *[SEMI parameter]
         parameter      = param-name EQUAL param-value
         param-name     = token
         param-value    = token / quoted-string


     The meaning of these fields is as follows:

     to-uri         Indicates the desired receiver of the information.
                    The value of this field should be a SIP URI. When
                    sent by a client, the value would typically (but
                    not necessarily) contain just the host and port
                    number parts.

     from-uri       Indicates the sender of the security agreement
                    information. The value of this is also a SIP URI.
                    When sent by a client, the value would typically



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                    (but not necessarily) include a username part.

     mechanism      The security mechanism supported by the SIP entity
                    identified in the from-uri.

                    This specification defines six values:

                    - "tls" for TLS [3],
                    - "digest" for HTTP Digest [4],
                    - "ipsec-ike" for IPsec with IKE [2],
                    - "ipsec-man" for manually keyed IPsec without IKE,
                    - "smime" for S/MIME [5], and
                    - token for extensions

                   To use TLS, the client MUST contact the server side on
                   port 5061. If this connection attempt fails, the
                   security agreement procedure MUST be considered to have
                   failed, and MUST be terminated.

                   To use HTTP Digest, it alone does not fulfill the
                   minimum security requirements of this specification. In
                   order to use HTTP Digest securely, some variant of MIME
                   tunneling SHOULD be used to force the Security-Mechanism
                   header to be integrity protected in the MIME body. Also,
                   if the server decides that the first matching mechanism
                   is HTTP digest in Step 2 of Section 6.3, the server
                   SHOULD include a HTTP authentication challenge in its
                   response. However, HTTP Digest need not to be negotiated
                   if some underlying security is used (e.g. TLS or
                   IPsec). The proxy/server can always challenge the client
                   after some security mechanism is already in place.

                   To use either "ipsec-ike" or "ipsec-man", the client and
                   the server MUST request their IPsec implementations to
                   protect all further communications between the same IP
                   addresses and ports which where used in in the first
                   request from the client and first response from the
                   server. If the IKE connection attempt fails, the
                   agreement procedure MUST be considered to have failed,
                   and MUST be terminated. Clients and servers SHOULD
                   terminate the IPsec protection when it is no longer
                   used.

                   Note also that "ipsec-man" will only work if the
                   communicating SIP entities know which keys and other
                   parameters to use. It is outside the scope of this
                   specification to describe how this information can be
                   made known to the peers.

                   To use S/MIME, the client MUST construct its request in
                   Step 4 (see 6.3) using S/MIME. If the server sees that
                   S/MIME is the selected mechanism in Step 2, it MAY send
                   its own certificate within an S/MIME body in the
                   response of Step 2.



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     preference    A positive integer identifying the preferred order of
                   the mechanisms. Servers MUST use preference numbers in
                   their lists to identify the preferred order of the
                   security mechanisms. Clients MUST NOT use preference
                   numbers in their lists.

     algorithm     An optional algorithm field for those security
                   mechanisms which are not self-describing or which are
                   vulnerable for bidding-down attacks (e.g. HTTP Digest).
                   In the case of HTTP Digest, the same rules apply as
                   defined in [4] for the "algorithm" field in HTTP
                   Digest.

     params        An optional field that allows future extensions. Any
                   unrecognized directive MUST be ignored.

                   Multiple instances of the same header field can appear
                   in SIP messages. Typically, the client inserts its own
                   Security-Mechanism header when it sends a request, and
                   the server/proxy adds its own to the response. The
                   parameters are in all cases set in an appropriate manner
                   to indicate in the "to-uri" paremeter the party who
                   inserted the header. Or rather -- since the client is
                   copying some of the server's responses -- whose security
                   capabilities the header applies to.

7.   Summary of header usage

     The Security-Mechanism header may be used to negotiate the security
     mechanisms between various SIP entities including UAC, UAS, proxy, and
     registrar.  Information about the use of Security-Mechanism header in
     relation to SIP methods and proxy processing is summarized in Table 1.

       Header field           where        proxy ACK BYE CAN INV OPT REG
       _________________________________________________________________
       Security-Mechanism       R           ar    -   -   -   c   c   c
       Security-Mechanism      2xx          a     -   -   -   c   c   c
       Security-Mechanism  401,407,421,494  a     -   -   -   c   c   c

                  Table 1: Summary of header usage.


     The "where" column describes the request and response types in which
     the header field may be used. The header may not appear in other types
     of SIP messages. Values in the where column are:

     - R: Header field may appear in requests.

     - 2xx, 401, etc.: A numerical value or range indicates response codes
     with which the header field can be used.

     The "proxy" column describes the operations a proxy may perform on a
     header field:




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     - a: A proxy can add or concatenate the header field if not present.

     - r: A proxy must be able to read the header field, and thus this
     header field cannot be encrypted.

     The next six columns relate to the presence of a header field in a
     method:

     - c: Conditional; requirements on the header field depend on the con¡
     text of the message.

8.   Backwards Compatibility

     SIP entities which support this specification but whose policy does
     not require its use, SHOULD only start using it if so required by the
     peer. Such SIP entities can thus communicate with other SIP entities
     even if they do not support this specification.

     SIP entities that support this specification and have a policy which
     requires its use MUST insert the Supported and Require headers as
     described in this specification. This makes communications possible
     only with nodes that support this specification. The OPTIONS method
     can still be used with any node.

     Note that the use of this specification together with the Proxy-
     Require header demands the support of this specification from all SIP
     entities along the path.

9.   Examples

9.1. Selecting Between New and Old Mechanisms

     In this example we demonstrate the use of the framework for securing a
     hop using some security mechanism, without knowing beforehand which
     methods the server supports. We assume that the client is not willing
     to reveal any information on what it intends to do, so it uses OPTIONS
     in the first message that is sent in the clear. The example starts by
     a client sending a message to the server, indicating that it is of the
     new variant that supports TLS in Step 1. In Step 2, the server
     responds that with it own list of security mechanisms -- S/MIME or TLS
     in this case -- and the peers start only common security service i.e.
     TLS at Step 3. In Step 4, the client resends the server's Security-
     Mechanism header, which the server verifies, and responds with 200 OK.

       1. Client -> Server:

          OPTIONS server SIP/2.0
          Security-Mechanism: to=sip:server.example.com,
                              from=sip:client.example.com,
                              mech=tls

       2. Server -> Client:

          200 OK



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          Security-Mechanism: to=sip:client.example.com,
                              from=sip:server.example.com,
                              mech=smime;pref=1, mech=tls;pref=2

       3. Security handshake at a lower layer i.e. TLS

       4. Client -> Server:

          INVITE server SIP/2.0
          Security-Mechanism: to=sip:client.example.com,
                              from=sip:server.example.com,
                              mech=smime;pref=1, mech=tls;pref=2

       5. Server -> Client:

          200 OK

     In the example we have omitted the returned values of Security-Mecha¡
     nism in replies for clarity. Typically in SIP the servers do not
     remove header fields as they answer, they only add new headers.

     If this example was run without Security-Mechanism in Step 2, the
     client would not know what kind of security the other one supports,
     and would be forced to error-prone trials.

     More seriously, if the Security-Mechanism was omitted in Step 4, the
     whole process would be prone for MITM attacks. An attacker could spoof
     "ICMP Port Unreachable" message on the trials, or remove the stronger
     security option from the header in Step 1, therefore substantially
     reducing the security.

9.2. Selections Along the Path

     This example attempts to show how selections can be made between a
     client and the first-hop proxy while the actual SIP messages are still
     destinated to a server further on in the network. This example demon¡
     strates how we can securely agree on the security mechanism between
     the client and its first hop proxy, without adding roundtrips.

     In this example, the client sends a REGISTER request to its registrar.
     At the same time, the client negotiates the security with a different
     first hop proxy. There are three alternative security solutions: a)
     TLS, b) IPsec/IKE, and c) HTTP Digest.

     The example starts by a client coming to a new network and learning
     the address of the local proxy. The proxy is of an upgraded version,
     so it supports all security mechanisms. The client supports alterna¡
     tives b) and c).  The client also knows the address of the registrar.
     We assume that some trust has already been established between the
     client and the home, and between the client and the proxy. Perhaps
     this trust is in the form of the nodes belonging under the same PKI,
     or having distributed shared secrets beforehand.





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     In Step 1 the client contacts the proxy using a REGISTER message. We
     omit the details of the communications with the home server in this
     discussion, but the proxy forwards the messages onwards in Step 2.  In
     Step 3, the registrar responds with an end-to-end HTTP Digest authen¡
     tication challenge. Using the same response, the proxy adds an indica¡
     tion that it supports TLS with HTTP Digest, IPsec/IKE, and plain HTTP
     Digest.  In Step 4, the client selects the first method it supports
     (IPsec/IKE in this case), the protection is turned on. In Step 5, the
     client sends the next round of REGISTER messages to the registrar.
     This message includes the repetition of the original security capabil¡
     ities of the proxy. The proxy verifies this list, and forwards the
     request to the registrar. In Step 7, the registrar responds with a 200
     OK.

       1. Client -> Proxy:

          REGISTER server SIP/2.0
          Security-Mechanism: to=sip:proxy.example.com,
               from=sip:client.example.com,
               mech=ipsec-ike, mech=digest;alg=MD5


       2. Proxy communicates with the Server.

       3. Proxy -> Client:

          401 Authentication Required
          (HTTP Digest challenge from the registrar to the client)
          Security-Mechanism: to=sip:client.example.com,
               from=sip:proxy.example.com,
               mech=tls;pref=1, mech=ipsec-ike;pref=2,
               mech=digest;pref=3;alg=MD5

       4. Security handshake at a lower layer i.e. IPsec/IKE

       5. Client -> Proxy:

          REGISTER server SIP/2.0
          Security-Mechanism: to=sip:client.example.com,
               from=sip:proxy.example.com,
               mech=tls;pref=1, mech=ipsec-ike;pref=2,
               mech=digest;pref=3;alg=MD5

       6. Proxy communicates with the Server.

       7. Proxy -> Client:

          200 OK


     As in the previous example, if this was run without Security-Mechanism
     in Step 3, the client would not know what kind of algorithms the
     server supports. In this example we demonstrate also the need for the
     client to send its own mechanism list in Step 1. If this wasn't known



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     to the proxy when it responds in Step 3, it could not have provided a
     suitable HTTP Digest challenge because at that point the proxy would
     not have known if the client supports that.

     As in the previous example, removing the repetition of the Security-
     Mechanism header in Step 5 would open the system to MITM attacks.

10.  Security Considerations

     This specification is about making it possible to select between vari¡
     ous SIP security mechanisms in a secure manner. In particular, the
     method presented here allow current networks using e.g. Digest later
     securely upgrade to e.g. S/MIME without requiring a simultaneous modi¡
     fication in all equipment. The method presented in this specification
     is secure only if the weakest proposed mechanism offers at least
     integrity protection.

     Attackers could try to modify the server's list of security mechanisms
     in the first response. This would be revealed to the server when the
     client returns the received list using the security.

     Attackers could also try to modify the repeated list in the second
     request from the client.  However, if the selected security mechanism
     uses encryption this may not be possible, and if it uses integrity
     protection any modifications will be detected by the server.

     Finally, attackers could try to modify the client's list of security
     mechanisms in the first message. The client selects the security mech¡
     anism based on its own knowledge of its own capabilities and the
     server's list, hence the client's choice would be unaffected by any
     such modification.  However, the server's choice could still be
     affected as described below:

     - If the modification affected the server's choice, the server and
     client would end up choosing different security mechanisms in Step 3
     or 4.)  Since they would be unable to communicate to each other, this
     would be detected as a potential attack. The client would either retry
     or give up in this situation.

     - If the modification did not affect the server's choice, there's no
     effect.

     All clients that implement this specification MUST select HTTP Digest,
     S/MIME, TLS, IPsec, or any stronger method for the protection of the
     second request. If HTTP Digest is used alone, the security agreement
     headers MUST be protected. This can be done with HTTP Digest if com¡
     bined with MIME/SIP tunneling, for example.

11.  IANA Considerations

     This specification defines 'sec-agree' option tag which should be reg¡
     istered in IANA.  The option-tag 'sec-agree' can be used in header
     related to the SIP compatibility mechanisms for extensions (e.g.
     Require and Supported).



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     This specification also defines a new error code, 494 (Security Agree¡
     ment Failed), which should be registered in IANA.

12.  Modifications

     The draft-sip-sec-agree-00.txt version of this specification intro¡
     duced the following modifications:

     - Many editorial changes, restructuring and clarifications.

     - Motivation section has been shortened since this is now a WG item.

     - Clarified that the solution requires always some base level of secu¡
     rity (i.e. integrity) in order to work. Even 'the weak security' must
     not be broken.

     - Text related to alternative solutions shortened and moved to a new
     place.

     - New rules for possible error and special cases has been added, e.g.
     for the case in which an non-adjacent SIP entities try to negotiate
     hop-by-hop security mechanisms.

     - Syntax of the header redesigned. Wanted to get rid of the semantics
     related to the relative position of a header component in the header
     (e.g. first parameters defines the 'from-uri', second the 'to-uri',
     and third the first supported security mechanism). The option tags are
     now used to identify the Security Agreement extension, not the indi¡
     vidual security mechanisms.

     - The semantics of the header slightly changed: the AND operator
     between the indivicual mechanisms is removed because it is really need
     with HTTP Digest only. And even in this case, the negotiation is not
     needed beforehand if some underlying security is used.

     - Options for HTTP Digest algorithms and manually keyed IPsec added.

     - Explicit rules were added to all mechanisms on how they should be
     used, such as TLS to be run on port 5061 etc.

     - References to Enhanced HTTP Digest removed.

     - Example related to 3GPP generalized.

     The draft-arkko-sip-sec-agree-01.txt version of this specification
     introduced the following modifications:

     - Reversed approach to make servers stateless

     - Removed discussion of the use of this for Digest algorithm selec¡
     tion, since Enhanced Digest already has bidding-down protection

     - Renamed org.iana.sip.digest to org.iana.sip.edigest and removed the
     parameters, as we can rely on Enhanced Digest to perform the algorithm



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     selection.

     - Removed agreements for full paths.

     - Simplified syntax

13.  Acknowledgments

     The authors wish to thank Rolf Blom, James Undery, Jonathan Rosenberg,
     Hugh Shieh, Gunther Horn, Krister Boman, David Castellanos-Zamora, Aki
     Niemi, Miguel Garcia, Valtteri Niemi, Martin Euchner, Eric Rescorla,
     and members of the 3GPP SA3 group for interesting discussions in this
     problem space.

     14.  Normative References

     [1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. Peter¡
     son, R. Sparks, M. Handley, E. Schooler "SIP: Session Initiation Pro¡
     tocol", Work In Progress, draft-ietf-sip-rfc2543bis-09.txt, IETF,
     February 2002.

     [2] S. Kent, R. Atkinson, "Security Architecture for the Internet Pro¡
     tocol", RFC 2401, IETF, November 1998.

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

     [4] Franks, J. et al, "HTTP Authentication: Basic and Digest Access
     Authentication", RFC 2617, IETF, June 1999.

     [5] B. Ramsdell and Ed, "S/MIME version 3 message specification," RFC
     2633, IETF, June 1999.

     15.  Non-Normative References

     [6] M. Garcia, D. Mills, G. Bajko, G. Mayer, F. Derome, H. Shieh, A.
     Allen, S. Chotai, K. Drage, J. Bharatia, "3GPP requirements on SIP",
     draft-garcia-sipping-3gpp-reqs-00.txt. Work In Progress, IETF, October
     2001.


     16.  Author's Address

     Jari Arkko, Vesa Torvinen
     Ericsson
     02420 Jorvas
     Finland
     EMail: Jari.Arkko@ericsson.com, Vesa.Torvinen@ericsson.fi

     Tao Haukka
     Nokia
     Finland
     EMail: Tao.Haukka@nokia.com




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     Sanjoy Sen
     Nortel Networks
     2735-B Glenville Drive
     Richardson, TX 75082, USA
     EMail: sanjoy@nortelnetworks.com

     Lee Valerius
     Nortel Networks
     2201 Lakeside Blvd
     Richards, TX 75082, USA
     EMail: valerius@nortelnetworks.com














































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