SIP Working Group Jari Arkko
INTERNET-DRAFT Vesa Torvinen
<draft-ietf-sip-sec-agree-04.txt> Gonzalo Camarillo
June 2002 Ericsson
Expires: December 2002 Tao Haukka
Nokia
Sanjoy Sen
Nortel Networks
Security Mechanism Agreement for SIP Sessions
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 documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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The list of current Internet-Drafts can be accessed at
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Abstract
SIP has a number of security mechanisms. Some of them have been built
in to the SIP protocol, such as HTTP authentication or secure
attachments. These mechanisms have even alternative algorithms and
parameters. SIP does not currently provide any mechanism for
selecting which security mechanisms to use between two entities. In
particular, even if some mechanisms such as OPTIONS were used to make
this selection, the selection would be vulnerable against the
Bidding-Down attack. This document defines three header fields for
negotiating the security mechanisms within SIP between a SIP entity
and its next SIP hop. A SIP entity applying this mechanism must
always require some minimum security (i.e. integrity protection) from
all communicating parties in order to secure the negotiation, but the
negotiation can agree on which specific security is used.
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TABLE OF CONTENTS
1. Introduction....................................................2
2. The Problem.....................................................3
3. Solution........................................................4
3.1. Requirements...............................................4
3.2. Overview of Operations.....................................5
3.3. Syntax.....................................................6
3.4. Protocol Operation.........................................7
3.4.1 Client Initiated........................................7
3.4.2 Server Initiated........................................8
3.5. Security Mechanism Initiation..............................9
3.6. Duration of the Security Association......................10
3.7. Summary of Header Field Use...............................10
4. Backwards Compatibility........................................11
5. Examples.......................................................11
5.1. Client Initiated..........................................10
5.2. Server Initiated..........................................12
5.3. Security Negotiation between Proxies......................13
6. Security Considerations........................................13
7. IANA Considerations............................................15
8. Acknowledgments................................................15
9. Normative References...........................................15
10. Non-Normative References.......................................16
11. AuthorsÆs Addresses............................................16
1. Introduction
Traditionally, security protocols have included facilities to agree
on the used mechanisms, algorithms, and other security parameters.
The reason for this is that algorithm development typically uncovers
problems in old algorithms and sometimes even produces new problems.
Furthermore, different mechanisms and algorithms are suitable for
different 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 (e.g. HTTP Digest authentication [4]), it can utilize secure
attachments (e.g. S/MIME [5]), and it can also use underlying
security protocols (e.g. 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 (i.e., recommend just one lower layer security protocol),
we can not expect to cut down the number 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 complement the methods that exist today.
Chapter 2 shows that without a secured method to choose between
security 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 algorithms is vulnerable to Man-in-the-Middle (MitM)
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attacks. More seriously, 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) 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 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 [7].
Chapter 6 documents the proposed solution, and chapter 7 gives some
demonstrative examples.
2. Problem Description
SIP has alternative security mechanisms such as HTTP authentication
with 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 possibility for
instantaneous 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 hardware.
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.
Traditionally, 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
fundamental 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 modifying 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
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by removing the strong options. This would force the two peers to use
weak security between them. But if the offers are protected in some
way -- such as by hashing, 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 incorrect. 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
possible 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.
3. Solution
3.1 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 HTTP digest. 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 authentication).
(b) It allows next-hop security negotiation.
(c) It is secure (i.e., prevents the bidding down attack.)
(d) It is capable of running without additional roundtrips. This is
important in the cellular environment, where link delays are
relatively high, and an additional roundtrip could delay the call
set up further.
(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
various 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 HTTP
authentication RFC [4] itself. More seriously, they have no
provisions for allowing encryption to be negotiated. Hence, it would
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be hard to turn on possible future encryption schemes in a secure
manner.
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
parameters 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, we need another requirement
on the security agreement scheme:
(f) 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.
3.2. Overview of Operations
The message flow below illustrates how the mechanism defined in this
document works:
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
Figure 1: Security negotiation message flow
Step 1: Clients wishing to use this specification can send a list of
their supported security mechanisms along the first request to the
server.
Step 2: Servers wishing to use this specification can challenge the
client to perform the security agreement procedure. The security
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mechanisms and parameters supported by the server are sent along in
this challenge.
Step 3: The client then proceeds to select the highest-preference
security mechanism they have in common and to turn on the selected
security.
Step 4: The client contacts the server again, now using the selected
security mechanism. The server's list of supported security
mechanisms is returned as a response to the challenge.
Step 5: The server verifies its own list of security mechanisms in
order to ensure that the original list 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 cannot change based on the input from the other side). Nodes may,
however, maintain several static lists, one for each interface, for
example.
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. The client would set
up the non-self-describing security mechanism between Steps 2 and 4.
3.3. Syntax
We define three new SIP header fields, namely Security-Client,
Security-Server and Security-Verify. Their BNF syntax is provided
below:
security-client = "Security-Client" HCOLON
sec-mechanism *(COMMA sec-mechanism)
security-server = "Security-Server" HCOLON
sec-mechanism *(COMMA sec-mechanism)
security-verify = "Security-Verify" HCOLON
sec-mechanism *(COMMA sec-mechanism)
sec-mechanism = mechanism-name *(SEMI mech-parameters)
mechanism-name = ( "digest-integrity" / "tls" / "ipsec-ike" /
"ipsec-man" / "smime" / token )
mech-parameters = ( preference / algorithm / extension )
preference = "q" EQUAL qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
algorithm = "alg" EQUAL token
extension = generic-param
Note that qvalue is already defined in the SIP BNF [1]. We have
copied its definitions here for completeness.
The parameters described by the BNF above have the following
semantics:
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Mechanism-name: It identifies the security mechanism supported by
the client, when it appears in a Security-Client header fields, or
by the server, when it appears in a Security-Server or in a
Security-Verify header field. This specification defines five
values:
- "tls" for TLS [3].
- "digest-integrity" for HTTP Digest [4] using additional
integrity protection for the Security-Verify header field. The
additional integrity protection consists of using the qop
parameter to protect a MIME body (e.g., "message/sip") that
contains the Security-Verify header field.
- "ipsec-ike" for IPsec with IKE [2].
- "ipsec-man" for manually keyed IPsec without IKE.
- "smime" for S/MIME [5].
Preference: The "q" value indicates a relative preference for the
particular mechanism. The higher the value the more preferred the
mechanism is. All the security mechanisms MUST have different "q"
values. It is an error to provide two mechanisms with the same "q"
value.
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 RFC 2617 [4] for the
"algorithm" field in HTTP Digest.
3.4. Protocol Operation
This section deals with the protocol details involved in the
negotiation between a SIP entity and its next-hop SIP entity.
Throughout the text the next-hop SIP entity is referred to as the
first-hop proxy or outbound proxy. However, the reader should bear in
mind that a user agent server can also be the next-hop for a proxy
or, in absence of proxies, for a user agent client. Note as well that
a proxy can also have an outbound proxy.
3.4.1 Client Initiated
A client wishing to establish some type of security with its first-
hop proxy MUST add a Security-Client header field to a request
addressed to this proxy (i.e., the destination of the request is the
first-hop proxy). This header field contains a list of all the
security mechanisms that the client supports. The client SHOULD NOT
add preference parameters to this list. The client MUST add both a
Require and Proxy-Require header field with the value "sec-agree" to
its request.
The Security-Client header field is used by the server to include any
necessary information in its response. For example, if digest-
integrity is the chosen mechanism, the server includes an HTTP
authentication challenge in the response. If S/MIME is chosen, the
appropriate certificate is included.
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A server receiving an unprotected request that contains a Require or
Proxy-Require header field with the value "sec-agree" MUST challenge
the client with a 494 (Security Agreement Required) response. The
server MUST add a Security-Server header field to this response
listing the security mechanisms that the server supports. The server
MUST add its list to the response even if there are no common
security mechanisms in the client's and server's lists. The serverÆs
list MUST NOT depend on the contents of the client's list.
The server MUST compare the list received in the Security-Client
header field with the list to be sent in the Security-Server header
field. When the client receives this response, it will choose the
common security mechanism with the highest "q" value. Therefore, the
server MUST add the necessary information so that the client can
initiate that mechanism (e.g., a WWW-Authenticate header field for
digest-integrity).
When the client receives a response with a Security-Server header
field, it MUST choose the security mechanism in the serverÆs list
with the highest "q" value among all the mechanisms that are known to
the client. Then, it MUST initiate that particular security mechanism
as described in Section 3.5. This initiation may be carried out
without involving any SIP message exchange (e.g., establishing a TLS
connection).
If an attacker modified the Security-Client header field in the
request, the server may not include in its response the information
needed to establish the common security mechanism with the highest
preference value (e.g., the WWW-authenticate header field is
missing). A client detecting such a lack of information in the
response MUST consider the current security negotiation process
aborted, and MAY try to start it again by sending a new request with
a Security-Client header field as described above.
All the subsequent SIP requests sent by the client to that server
SHOULD make use of the security mechanism initiated in the previous
step. These requests MUST contain a Security-Verify header field that
mirrors the serverÆs list received previously in the Security-Server
header field. These requests MUST also have both a Require and Proxy-
Require header fields with the value "sec-agree".
The server MUST check that the security mechanisms listed in the
Security-Verify header field of incoming requests correspond to its
static list of supported security mechanisms.
Note that, following the standard SIP header field comparison rules
defined in [1], both lists have to contain the same security
mechanisms in the same order to be considered equivalent. In
addition, for each particular security mechanism, its parameters in
both lists need to have the same values.
The server can proceed processing a particular request if, and only
if, the list was not modified. If modification of the list is
detected, the server MUST challenge the client with a 494 (Security
Agreement Required). This response MUST include a challenge with
server's unmodified list of supported security mechanisms. If the
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list was not modified, and the server is a proxy, it MUST remove the
"sec-agree" value from both the Require and Proxy-Require header
fields, and then remove the header fields if no values remain.
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 and its
outbound proxy negotiate some security, they may try to use the same
security for incoming requests (i.e., the UA will be acting as a
UAS).
The user of a UA SHOULD be informed about the results of the security
mechanism negotiation. The user MAY decline to accept a particular
security mechanism, and abort further SIP communications with the
peer.
3.4.2 Server Initiated
A server decides to use the security negotiation described in this
document based on local policy. A server that decides to use this
negotiation MUST challenge unprotected requests regardless of the
presence or the absence of any Require, Proxy-Require or Supported
header fields in incoming requests.
A server that by policy requires the use of this specification and
receives a request that does not have the sec-agree option tag in a
Require, Proxy-Require or Supported header field MUST return a 421
(Extension Required) response. If the request had the sec-agree
option tag in a Supported header field, it MUST return a 494
(Security Agreement Required) response. In both situation the server
MUST also include in the response a Security-Server header field
listing its capabilities and a Require header field with an option-
tag "sec-agree" in it. All the Via header field entries in the
response except the topmost value MUST be removed. This ensures that
the previous hop is the one processing the response (see example in
Section 5.3).
Clients that support the extension defined in this document MAY add a
Supported header field with a value of "sec-agree". In addition to
this, clients SHOULD add a Security-Client header field so that they
can save a round trip in case the server decides to challenge the
request.
3.5. Security mechanism initiation
Once the client chooses a security mechanism from the list received
in the Security-Server header field from the server, it initiates
that mechanism. Different mechanisms require different initiation
procedures.
If TLS is chosen, the client uses the procedures of Section 8.1.2 of
[1] to determine the URI to be used as an input to the DNS procedures
of [6]. However, if the URI is a sip URI, it MUST treat the scheme as
if it were sips, not sip. If the URI scheme is not sip, the request
MUST be sent using TLS.
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If digest-integrity is chosen, the 494 (Security Agreement Required)
response will contain an HTTP Digest authentication challenge. The
client MUST use the qop parameter to protect a MIME body (e.g.,
"message/sip") that contains the Security-Verify header field in the
request. Currently, only the qop value Æauth-intÆ is able to provide
required protection. Note that digest alone without placing Security-
Verify header in the body would not fulfill the minimum security
requirements of this specification.
To use "ipsec-ike", the client attempts to establish an IKE
connection to the host part of the Request-URI in the first request
to the server. If the IKE connection attempt fails, the agreement
procedure MUST be considered to have failed, and MUST be terminated.
Note 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.
In both IPsec-based mechanisms, it is expected that appropriate
policy entries for protecting SIP have been configured or will be
created before attempting to use the security agreement procedure,
and that SIP communications use port numbers and addresses according
to these policy entries. It is outside the scope of this
specification to describe how this information can be made known to
the peers, but it could be typically configured at the same time as
the IKE credentials or manual SAs have been entered.
To use S/MIME, the client MUST construct its request using S/MIME.
The client may have received the serverÆs certificate in an S/MIME
body in the 494 (Security Agreement Required) response. Note that
S/MIME can only be used if the next hop SIP entity is a UA.
3.6. Duration of Security Associations
Once a security mechanism has been negotiated, both the server and
the client need to know until when it can be used. All the mechanisms
described in this document have a different way to signal the end of
a security association. When TLS is used, the termination of the
connection indicates that a new negotiation is needed. IKE negotiates
the duration of a security association. If the credentials provided
by a client using digest-integrity are not longer valid, the server
will re-challenge the client. It is assumed that when IPsec-man is
used, the same out-of-band mechanism used to distribute keys is used
to define the duration of the security association.
3.7. Summary of Header Field Use
The header fields defined in this document may be used to negotiate
the security mechanisms between a UAC and other SIP entities
including UAS, proxy, and registrar. Information about the use of
headers in relation to SIP methods and proxy processing is summarized
in Table 1.
Header field where proxy ACK BYE CAN INV OPT REG
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_________________________________________________________________
Security-Client R ard - o - o o o
Security-Server 401,407,421,494 - o - o o o
Security-Verify R ard - o - o o o
Header field where proxy SUB NOT PRK IFO UPD MSG
_________________________________________________________________
Security-Client R ard o o - o o o
Security-Server 401,407,421,494 o o - o o o
Security-Verify R ard o o - o o o
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.
- 401, 407 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:
- 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.
- d: A proxy can delete a header field value.
The next six columns relate to the presence of a header field in a
method:
- o: The header field is optional.
4. Backwards Compatibility
A server that, by local policy, decides to use the negotiation
mechanism defined in this document, will not accept requests from
clients that do not support this extension. This obviously breaks
interoperability with every plain SIP client. Therefore, this
extension should be used in environments where it is somehow ensured
that every client implements this extension. This extension may also
be used in environments where insecure communication is not
acceptable if the option of not being able to communicate is also
accepted.
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5. Examples
The following examples illustrate the use of the mechanism defined
above.
5.1. Client Initiated
A UA negotiates the security mechanism to be used with its outbound
proxy without knowing beforehand which mechanisms the proxy supports.
UAC Proxy UAS
| | |
|----(1) OPTIONS---->| |
| | |
|<-----(2) 494-------| |
| | |
|<=======TLS========>| |
| | |
|----(3) INVITE----->| |
| |----(4) INVITE--->|
| | |
| |<---(5) 200 OK----|
|<---(6) 200 OK------| |
| | |
|------(7) ACK------>| |
| |-----(8) ACK----->|
| | |
| | |
| | |
| | |
Figure 2: Negotiation initiated by the client
The UAC sends an OPTIONS request to its outbound proxy, indicating
that it is able to negotiate security mechanisms and that it supports
TLS and digest-integrity (Step 1 of figure 1). The outbound proxy
challenges the UAC with its own list of security mechanisms û IPsec
and TLS (Step 2 of figure 1). The only common security mechanism is
TLS, so they establish a TLS connection between them (Step 3 of
figure 1). When the connection is successfully established, the UAC
sends an INVITE over the TLS connection just established (Step 4 of
figure 1). This INVITE contains the serverÆs security list. The
server verifies it, and since it matches its static list, it
processes the INVITE and forwards it to the next hop.
If this example was run without Security-Server header in Step 2, the
UAC would not know what kind of security the other one supports, and
would be forced to error-prone trials.
More seriously, if the Security-Verify 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
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stronger security option from the header in Step 1, therefore
substantially reducing the security.
(1) OPTIONS sip:proxy.example.com SIP/2.0
Security-Client: tls
Security-Client: digest-integrity
Require: sec-agree
Proxy-Require: sec-agree
(2) SIP/2.0 494 Security Agreement Required
Security-Server: ipsec-ike;q=0.1
Security-Server: tls;q=0.2
(3) INVITE sip:proxy.example.com SIP/2.0
Security-Verify: ipsec-ike;q=0.1
Security-Verify: tls;q=0.2
Route: sip:callee@domain.com
Require: sec-agree
Proxy-Require: sec-agree
The 200 OK response for the INVITE and the ACK are also sent over the
TLS connection. The ACK (7) will contain the same Security-Verify
header field as the INVITE (3).
5.2. Server Initiated
In this example of figure 3 the client sends an INVITE towards the
callee using an outbound proxy. This INVITE does not contain any
Require header field.
UAC Proxy UAS
| | |
|-----(1) INVITE---->| |
| | |
|<-----(2) 421-------| |
| | |
|------(3) ACK------>| |
| | |
|<=======IKE========>| |
| | |
|-----(4) INVITE---->| |
| |----(5) INVITE--->|
| | |
| |<---(6) 200 OK----|
|<----(7) 200 OK-----| |
| | |
|------(8) ACK------>| |
| |-----(9) ACK----->|
| | |
| | |
Figure 3: Server initiated security negotiation
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The proxy, following its local policy, challenges the INVITE. It
returns a 421 (Extension Required) with a Security-Server header
field that lists IPsec-IKE and TLS. Since the UAC supports IPsec-IKE
it performs the key exchange and establishes a security association
with the proxy. The second INVITE (4) and the ACK (8) contain a
Security-Verify header field that mirrors the Security-Server header
field received in the 421. The INVITE (4), the 200 OK (7) and the ACK
(8) are sent using the security association that has been
established.
5.3 Security Negotiation between Proxies
The example in Figure 4 shows a security negotiation between two
adjacent proxies. P1 forwards an INVITE (2) to P2. P2, by policy,
requires that a security negotiation takes place before accepting any
request. Therefore, it challenges P1 with a 421 (Extension Required)
response (3). P2 removes all the Via entries except the topmost one
(i.e., P1) so that P1 itself processes the response rather than
forwarding it to the UAC. This 421 response contains a Security-
Server header field listing the server's capabilities and a Require
header field with an option-tag "sec-agree" in it. P2 includes "TLS"
and "ipsec-ike" in the Security-Server header field. P1 sends an ACK
(4) for the response and proceeds to establish a TLS connection,
since this is the only security mechanism supported by P1. Once the
TLS connection is established, session establishment proceeds
normally. Messages (5), (8) and (11) are sent using the just
established TLS connection. Messages (5) and (11) contain a Security-
Verify header field that P2 removes before forwarding them to the
UAS. Note that, following normal SIP procedures, P1 uses a different
branch ID for INVITE (5) than the one it used for INVITE (2).
UAC P1 P2 UAS
| | | |
|-(1) INVITE->| | |
| |-(2) INVITE->| |
| | | |
| |<--(3) 421---| |
| | | |
| |---(4) ACK-->| |
| | | |
| |<====TLS====>| |
| | | |
| |-(5) INVITE->| |
| | |-(6) INVITE->|
| | | |
| | |<-(7) 200 OK-|
| |<-(8) 200 OK-| |
|<-(9) 200 OK-| | |
| | | |
|--(10) ACK-->| | |
| |--(11) ACK-->| |
| | |--(12) ACK-->|
| | | |
Figure 4: Negotiation between two proxies
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6. Security Considerations
This specification is about making it possible to select between
various SIP security mechanisms in a secure manner. In particular,
the method presented here allow current networks using, for instance,
Digest, to be securely upgraded to, for instance, IPsec without
requiring a simultaneous modification 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
mechanism 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 of figure 1. 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
with integrity, TLS, IPsec, or any stronger method for the protection
of the second request.
7. IANA Considerations
This specification defines three new header fields, namely Security-
Client, Security-Server and Security-Verify that should be included
in the registry for SIP header fields maintained by IANA.
This specification defines the 'sec-agree' SIP option tag which
should be registered in IANA.
This specification also defines a new SIP status code, 494 (Security
Agreement Failed), which should be registered in IANA.
8. Acknowledgments
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The authors wish to thank Lee Valerius, Allison Mankin, 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.
9. Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, E. Schooler "SIP: Session Initiation
Protocol", Work in Progress, draft-ietf-sip-rfc2543bis-09.txt, IETF,
February 2002.
[2] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", 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.
[6] H. Schulzrinne and J. Rosenberg, "SIP: Locating SIP servers",
Work in Progress, draft-ietf-sip-srv-06.txt, IETF, February 2002.
10. Non-Normative References
[7] 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.
11. Authors's Addresses
Jari Arkko
Ericsson
02420 Jorvas
Finland
EMail: Jari.Arkko@ericsson.com
Vesa Torvinen
Ericsson
02420 Jorvas
Finland
EMail: Vesa.Torvinen@ericsson.fi
Gonzalo Camarillo
Ericsson
02420 Jorvas
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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Tao Haukka
Nokia
Finland
EMail: Tao.Haukka@nokia.com
Sanjoy Sen
Nortel Networks
2735-B Glenville Drive
Richardson, TX 75082, USA
EMail: sanjoy@nortelnetworks.com
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