Network Working Group J. Arkko
Request for Comments: 3329 V. Torvinen
Category: Standards Track G. Camarillo
Ericsson
A. Niemi
T. Haukka
Nokia
January 2003
Security Mechanism Agreement for the
Session Initiation Protocol (SIP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines new functionality for negotiating the security
mechanisms used between a Session Initiation Protocol (SIP) user
agent and its next-hop SIP entity. This new functionality
supplements the existing methods of choosing security mechanisms
between SIP entities.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Motivations . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Design Goals . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Conventions . . . . . . . . . . . . . . . . . . . . . . 3
2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Overview of Operation . . . . . . . . . . . . . . . . . 3
2.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Protocol Operation . . . . . . . . . . . . . . . . . . . 6
2.3.1 Client Initiated . . . . . . . . . . . . . . . . . . 6
2.3.2 Server Initiated . . . . . . . . . . . . . . . . . . 8
2.4 Security Mechanism Initiation. . . . . . . . . . . . . . 9
2.5 Duration of Security Associations. . . . . . . . . . . .10
2.6 Summary of Header Field Use. . . . . . . . . . . . . . .10
Arkko, et. al. Standards Track [Page 1]
RFC 3329 SIP Security Agreement January 2003
3. Backwards Compatibility . . . . . . . . . . . . . . . . . .11
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . .12
4.1 Client Initiated . . . . . . . . . . . . . . . . . . . .12
4.2 Server Initiated . . . . . . . . . . . . . . . . . . . .14
5. Security Considerations . . . . . . . . . . . . . . . . . .15
6. IANA Considerations. . . . . . . . . . . . . . . . . . . . .17
6.1 Registration Information . . . . . . . . . . . . . . . .17
6.2 Registration Template. . . . . . . . . . . . . . . . . .18
6.3 Header Field Names . . . . . . . . . . . . . . . . . . .18
6.4 Response Codes . . . . . . . . . . . . . . . . . . . . .18
6.5 Option Tags. . . . . . . . . . . . . . . . . . . . . . .19
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . .19
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .19
9. Normative References . . . . . . . . . . . . . . . . . . . .19
10. Informative References . . . . . . . . . . . . . . . . . . 20
A. Syntax of ipsec-3gpp . . . . . . . . . . . . . . . . . . . .21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .23
Full Copyright Statement . . . . . . . . . . . . . . . . . . . .24
1. Introduction
Traditionally, security protocols have included facilities to agree
on the used mechanisms, algorithms, and other security parameters.
This is to add flexibility, since different mechanisms are usually
suitable to different scenarios. Also, the evolution of security
mechanisms often introduces new algorithms, or uncovers problems in
existing ones, making negotiation of mechanisms a necessity.
The purpose of this specification is to define negotiation
functionality for the Session Initiation Protocol (SIP) [1]. This
negotiation is intended to work only between a UA and its first-hop
SIP entity.
1.1 Motivations
Without a secured method to choose between security mechanisms and/or
their parameters, SIP is vulnerable to certain attacks.
Authentication and integrity protection using multiple alternative
methods and algorithms is vulnerable to Man-in-the-Middle (MitM)
attacks (e.g., see [4]).
It is also hard or sometimes even impossible to know whether a
specific security mechanism is truly unavailable to a SIP peer
entity, or if in fact a MitM attack is in action.
In certain small networks these issues are not very relevant, as the
administrators of such networks can deploy appropriate software
versions and set up policies for using exactly the right type of
Arkko, et. al. Standards Track [Page 2]
RFC 3329 SIP Security Agreement January 2003
security. However, SIP is also expected to be deployed to hundreds
of millions of small devices with little or no possibilities for
coordinated security policies, let alone software upgrades, which
necessitates the need for the negotiation functionality to be
available from the very beginning of deployment (e.g., see [11]).
1.2 Design Goals
1. The entities involved in the security agreement process need to
find out exactly which security mechanisms to apply, preferably
without excessive additional roundtrips.
2. The selection of security mechanisms itself needs to 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 [8]. This allows the
peers to detect if the initial, unprotected offers were tampered
with.
3. The entities involved in the security agreement process need to be
able to indicate success or failure of the security agreement
process.
4. The security agreement process should not introduce any additional
state to be maintained by the involved entities.
1.3 Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [9].
2. Solution
2.1 Overview of Operation
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 agreement message flow.
Arkko, et. al. Standards Track [Page 3]
RFC 3329 SIP Security Agreement January 2003
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
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.
2.2 Syntax
We define three new SIP header fields, namely Security-Client,
Security-Server and Security-Verify. The notation used in the
Augmented BNF definitions for the syntax elements in this section is
as used in SIP [1], and any elements not defined in this section are
as defined in SIP and the documents to which it refers:
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)
Arkko, et. al. Standards Track [Page 4]
RFC 3329 SIP Security Agreement January 2003
sec-mechanism = mechanism-name *(SEMI mech-parameters)
mechanism-name = ( "digest" / "tls" / "ipsec-ike" /
"ipsec-man" / token )
mech-parameters = ( preference / digest-algorithm /
digest-qop / digest-verify / extension )
preference = "q" EQUAL qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
digest-algorithm = "d-alg" EQUAL token
digest-qop = "d-qop" EQUAL token
digest-verify = "d-ver" EQUAL LDQUOT 32LHEX RDQUOT
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:
Mechanism-name
This token identifies the security mechanism supported by the
client, when it appears in a Security-Client header field; or
by the server, when it appears in a Security-Server or in a
Security-Verify header field. The mechanism-name tokens are
registered with the IANA. This specification defines four
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.
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.
Digest-algorithm
This optional parameter is defined here only for HTTP Digest
[4] in order to prevent the bidding-down attack for the HTTP
Digest algorithm parameter. The content of the field may have
same values as defined in [4] for the "algorithm" field.
Arkko, et. al. Standards Track [Page 5]
RFC 3329 SIP Security Agreement January 2003
Digest-qop
This optional parameter is defined here only for HTTP Digest
[4] in order to prevent the bidding-down attack for the HTTP
Digest qop parameter. The content of the field may have same
values as defined in [4] for the "qop" field.
Digest-verify
This optional parameter is defined here only for HTTP Digest
[4] in order to prevent the bidding-down attack for the SIP
security mechanism agreement (this document). The content of
the field is counted exactly the same way as "request-digest"
in [4] except that the Security-Server header field is included
in the A2 parameter. If the "qop" directive's value is "auth"
or is unspecified, then A2 is:
A2 = Method ":" digest-uri-value ":" security-server
If the "qop" value is "auth-int", then A2 is:
A2 = Method ":" digest-uri-value ":" H(entity-body) ":"
security-server
All linear white spaces in the Security-Server header field
MUST be replaced by a single SP before calculating or
interpreting the digest-verify parameter. Method, digest-uri-
value, entity-body, and any other HTTP Digest parameter are as
specified in [4].
Note that this specification does not introduce any extension or
change to HTTP Digest [4]. This specification only re-uses the
existing HTTP Digest mechanisms to protect the negotiation of
security mechanisms between SIP entities.
2.3 Protocol Operation
This section deals with the protocol details involved in the
negotiation between a SIP UA 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 user agent
client.
2.3.1 Client Initiated
If a client ends up using TLS to contact the server because it has
followed the rules specified in [5], the client MUST NOT use the
security agreement procedure of this specification. If a client ends
Arkko, et. al. Standards Track [Page 6]
RFC 3329 SIP Security Agreement January 2003
up using non-TLS connections because of the rules in [5], the client
MAY use the security agreement of this specification to detect DNS
spoofing, or to negotiate some other security than TLS.
A client wishing to use the security agreement of this specification
MUST add a Security-Client header field to a request addressed to its
first-hop 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 contents of the Security-Client header field may be used by the
server to include any necessary information in its response.
A server receiving an unprotected request that contains a Require or
Proxy-Require header field with the value "sec-agree" MUST respond to
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 server's
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 Proxy-Authenticate header field for
HTTP Digest).
When the client receives a response with a Security-Server header
field, it MUST choose the security mechanism in the server's 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 Proxy-Authenticate header field is
missing). A client detecting such a lack of information in the
Arkko, et. al. Standards Track [Page 7]
RFC 3329 SIP Security Agreement January 2003
response MUST consider the current security agreement 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 server's 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 respond to the client with a 494 (Security
Agreement Required) response. This response MUST include the
server's unmodified list of supported security mechanisms. If the
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 agreement. The user MAY decline to accept a particular
security mechanism, and abort further SIP communications with the
peer.
2.3.2 Server Initiated
A server decides to use the security agreement described in this
document based on local policy. If a server receives a request from
the network interface that is configured to use this mechanism, it
must check that the request has only one Via entry. If there are
Arkko, et. al. Standards Track [Page 8]
RFC 3329 SIP Security Agreement January 2003
several Via entries, the server is not the first-hop SIP entity, and
it MUST NOT use this mechanism. For such a request, the server must
return a 502 (Bad Gateway) response.
A server that decides to use this agreement mechanism MUST challenge
unprotected requests with one Via entry 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. The server MUST also add necessary
information so that the client can initiate the preferred security
mechanism (e.g., a Proxy-Authenticate header field for HTTP Digest).
Clients that support the extension defined in this document SHOULD
add a Supported header field with a value of "sec-agree".
2.4 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 [5]. 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.
If "digest" is chosen, the 494 (Security Agreement Required) response
will contain an HTTP Digest authentication challenge. The client
MUST use the algorithm and qop parameters in the Security-Server
header field to replace the same parameters in the HTTP Digest
challenge. The client MUST also use the digest-verify parameter in
the Security-Verify header field to protect the Security-Server
header field as specified in 2.2.
Arkko, et. al. Standards Track [Page 9]
RFC 3329 SIP Security Agreement January 2003
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. All rules for minimum implementations,
such as mandatory-to-implement algorithms, apply as defined in [2],
[6], and [7].
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 would typically be configured at the same time as
the IKE credentials or manual SAs have been entered.
2.5 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 of
signaling 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 are no 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.
2.6 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.
Arkko, et. al. Standards Track [Page 10]
RFC 3329 SIP Security Agreement January 2003
Header field where proxy ACK BYE CAN INV OPT REG
_________________________________________________________________
Security-Client R ard - o - o o o
Security-Server 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 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.
* 421, 494: A numerical value 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.
3. Backwards Compatibility
The use of this extension in a network interface is a matter of local
policy. Different network interfaces may follow different policies,
and consequently the use of this extension may be situational by
nature. UA and server implementations MUST be configurable to
operate with or without this extension.
Arkko, et. al. Standards Track [Page 11]
RFC 3329 SIP Security Agreement January 2003
A server that is configured to use this mechanism, may also accept
requests from clients that use TLS based on the rules defined in [5].
Requests from clients that do not support this extension, and do not
support TLS, can not be accepted. This obviously breaks
interoperability with some SIP clients. Therefore, this extension
should be used in environments where it is somehow ensured that every
client implements this extension or is able to use TLS. 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.
4. Examples
The following examples illustrate the use of the mechanism defined
above.
4.1 Client Initiated
A UA negotiates the security mechanism to be used with its outbound
proxy without knowing beforehand which mechanisms the proxy supports.
The OPTIONS method can be used here to request the security
capabilities of the proxy. In this way, the security can be
initiated even before the first INVITE is sent via the proxy.
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.
Arkko, et. al. Standards Track [Page 12]
RFC 3329 SIP Security Agreement January 2003
The UAC sends an OPTIONS request to its outbound proxy, indicating at
the same time that it is able to negotiate security mechanisms and
that it supports TLS and HTTP Digest (1).
The outbound proxy responds to the UAC with its own list of security
mechanisms - IPsec and TLS (2). The only common security mechanism
is TLS, so they establish a TLS connection between them. When the
connection is successfully established, the UAC sends an INVITE
request over the TLS connection just established (3). This INVITE
contains the server's 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 3, 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.
(1) OPTIONS sip:proxy.example.com SIP/2.0
Security-Client: tls
Security-Client: digest
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 (6) for the INVITE and the ACK (7) are also sent
over the TLS connection. The ACK will contain the same Security-
Verify header field as the INVITE (3).
Arkko, et. al. Standards Track [Page 13]
RFC 3329 SIP Security Agreement January 2003
4.2 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.
The proxy, following its local policy, does not accept 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.
(1) INVITE sip:uas.example.com SIP/2.0
(2) SIP/2.0 421 Extension Required
Security-Server: ipsec-ike;q=0.1
Security-Server: tls;q=0.2
Arkko, et. al. Standards Track [Page 14]
RFC 3329 SIP Security Agreement January 2003
(4) INVITE sip:uas.example.com SIP/2.0
Security-Verify: ipsec-ike;q=0.1
Security-Verify: tls;q=0.2
5. 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 herein allow current networks using, for
instance, HTTP 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 and replay
protection for the Security-Verify header field.
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 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 TLS [3]). In this situation, an insecure negotiation
procedure allows attackers to trivially force even new devices to use
only integrity protection.
Arkko, et. al. Standards Track [Page 15]
RFC 3329 SIP Security Agreement January 2003
Possible attacks against the security agreement include:
1. 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.
2. 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.
3. 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.
4. Finally, attackers may also try to reply old security agreement
messages. Each security mechanism must provide replay protection.
In particular, HTTP Digest implementations should carefully
utilize existing reply protection options such as including a
time-stamp to the nonce parameter, and using nonce counters [4].
All clients that implement this specification MUST select HTTP
Digest, TLS, IPsec, or any stronger method for the protection of the
second request.
Arkko, et. al. Standards Track [Page 16]
RFC 3329 SIP Security Agreement January 2003
6. IANA Considerations
This specification defines a new mechanism-name namespace in Section
2.2 which requires a central coordinating body. The body responsible
for this coordination is the Internet Assigned Numbers Authority
(IANA).
This document defines four mechanism-names to be initially
registered, namely "digest", "tls", "ipsec-ike", and "ipsec-man". In
addition to these mechanism-names, "ipsec-3gpp" mechanism-name is
also registered (see Appendix A). Following the policies outlined in
[10], further mechanism-names are allocated based on IETF Consensus.
Registrations with the IANA MUST include the mechanism-name token
being registered, and a pointer to a published RFC describing the
details of the corresponding security mechanism.
6.1 Registration Information
IANA registers new mechanism-names at
http://www.iana.org/assignments/sip-parameters under "Security
Mechanism Names". As this document specifies five mechanism-names,
the initial IANA registration for mechanism-names will contain the
information shown in Table 2. It also demonstrates the type of
information maintained by the IANA.
Mechanism Name Reference
-------------- ---------
digest [RFC3329]
tls [RFC3329]
ipsec-ike [RFC3329]
ipsec-man [RFC3329]
ipsec-3gpp [RFC3329]
Table 2: Initial IANA registration.
6.2 Registration Template
To: ietf-sip-sec-agree-mechanism-name@iana.org
Subject: Registration of a new SIP Security Agreement mechanism
Mechanism Name:
(Token value conforming to the syntax described in
Section 2.2.)
Arkko, et. al. Standards Track [Page 17]
RFC 3329 SIP Security Agreement January 2003
Published Specification(s):
(Descriptions of new SIP Security Agreement mechanisms
require a published RFC.)
6.3 Header Field Names
This specification registers three new header fields, namely
Security-Client, Security-Server and Security-Verify. These headers
are defined by the following information, which has been included in
the sub-registry for SIP headers under
http://www.iana.org/assignments/sip-parameters.
Header Name: Security-Client
Compact Form: (none)
Header Name: Security-Server
Compact Form: (none)
Header Name: Security-Verify
Compact Form: (none)
6.4 Response Codes
This specification registers a new response code, namely 494
(Security Agreement Required). The response code is defined by the
following information, which has been included to the sub-registry
for SIP methods and response-codes under
http://www.iana.org/assignments/sip-parameters.
Response Code Number: 494
Default Reason Phrase: Security Agreement Required
6.5 Option Tags
This specification defines a new option tag, namely sec-agree. The
option tag is defined by the following information, which has been
included in the sub-registry for option tags under
http://www.iana.org/assignments/sip-parameters.
Arkko, et. al. Standards Track [Page 18]
RFC 3329 SIP Security Agreement January 2003
Name: sec-agree
Description: This option tag indicates support for the Security
Agreement mechanism. When used in the Require, or
Proxy-Require headers, it indicates that proxy servers
are required to use the Security Agreement mechanism.
When used in the Supported header, it indicates that
the User Agent Client supports the Security Agreement
mechanism. When used in the Require header in the 494
(Security Agreement Required) or 421 (Extension
Required) responses, it indicates that the User Agent
Client must use the Security Agreement Mechanism.
7. Contributors
Sanjoy Sen and Lee Valerius from Nortel Networks have contributed to
the document.
8. Acknowledgements
In addition to the contributors, the authors wish to thank Allison
Mankin, Rolf Blom, James Undery, Jonathan Rosenberg, Hugh Shieh,
Gunther Horn, Krister Boman, David Castellanos-Zamora, 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] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[3] Dierks, T. and C. Allen, P. Kocher, "The TLS Protocol Version
1.0", RFC 2246, January 1999.
[4] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
[5] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[6] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
Arkko, et. al. Standards Track [Page 19]
RFC 3329 SIP Security Agreement January 2003
[7] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[8] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[9] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[10] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
10. Informative References
[11] Garcia-Martin, M., "3rd-Generation Partnership Project (3GPP)
Release 5 requirements on the Session Initiation Protocol
(SIP)", Work in Progress.
[12] 3rd Generation Partnership Project, "Access security for IP-
based services, Release 5", TS 33.203 v5.3.0, September 2002.
[13] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within ESP and
AH", RFC 2403, November 1998.
[14] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
and AH", RFC 2404, November 1998.
[15] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher Algorithms",
RFC 2451, November 1998.
Arkko, et. al. Standards Track [Page 20]
RFC 3329 SIP Security Agreement January 2003
Appendix A. Syntax of ipsec-3gpp
This appendix extends the security agreement framework described in
this document with a new security mechanism: "ipsec-3gpp". This
security mechanism and its associated parameters are used in the 3GPP
IP Multimedia Subsystem [12]. The Augmented BNF definitions below
follow the syntax of SIP [1].
mechanism-name = ( "ipsec-3gpp" )
mech-parameters = ( algorithm / protocol /mode /
encrypt-algorithm / spi /
port1 / port2 )
algorithm = "alg" EQUAL ( "hmac-md5-96" /
"hmac-sha-1-96" )
protocol = "prot" EQUAL ( "ah" / "esp" )
mode = "mod" EQUAL ( "trans" / "tun" )
encrypt-algorithm = "ealg" EQUAL ( "des-ede3-cbc" / "null" )
spi = "spi" EQUAL spivalue
spivalue = 10DIGIT; 0 to 4294967295
port1 = "port1" EQUAL port
port2 = "port2" EQUAL port
port = 1*DIGIT
The parameters described by the BNF above have the following
semantics:
Algorithm
This parameter defines the used authentication algorithm. It
may have a value of "hmac-md5-96" for HMAC-MD5-96 [13], or
"hmac-sha-1-96" for HMAC-SHA-1-96 [14]. The algorithm
parameter is mandatory.
Protocol
This parameter defines the IPsec protocol. It may have a value
of "ah" for AH [6], or "esp" for ESP [7]. If no Protocol
parameter is present, the protocol will be ESP by default.
Mode
This parameter defines the mode in which the IPsec protocol is
used. It may have a value of "trans" for transport mode, or a
value of "tun" for tunneling mode. If no Mode parameter is
present the IPsec protocol is used in transport mode.
Encrypt-algorithm
This parameter defines the used encryption algorithm. It may
have a value of "des-ede3-cbc" for 3DES [15], or "null" for no
encryption. If no Encrypt-algorithm parameter is present,
encryption is not used.
Arkko, et. al. Standards Track [Page 21]
RFC 3329 SIP Security Agreement January 2003
Spi
Defines the SPI number used for inbound messages.
Port1
Defines the destination port number for inbound messages that
are protected.
Port2
Defines the source port number for outbound messages that are
protected. Port 2 is optional.
The communicating SIP entities need to know beforehand which keys to
use. It is also assumed that the underlying IPsec implementation
supports selectors that allow all transport protocols supported by
SIP to be protected with a single SA. The duration of security
association is the same as in the expiration interval of the
corresponding registration binding.
Arkko, et. al. Standards Track [Page 22]
RFC 3329 SIP Security Agreement January 2003
Authors' Addresses
Jari Arkko
Ericsson
Jorvas, FIN 02420
Finland
Phone: +358 40 507 9256
EMail: jari.arkko@ericsson.com
Vesa Torvinen
Ericsson
Joukahaisenkatu 1
Turku, FIN 20520
Finland
Phone: +358 40 723 0822
EMail: vesa.torvinen@ericsson.fi
Gonzalo Camarillo
Advanced Signalling Research Lab.
Ericsson
FIN-02420 Jorvas
Finland
Phone: +358 40 702 3535
EMail: Gonzalo.Camarillo@ericsson.com
Aki Niemi
NOKIA Corporation
P.O.Box 321, FIN 00380
Finland
Phone: +358 50 389 1644
EMail: aki.niemi@nokia.com
Tao Haukka
Nokia Corporation
P.O. Box 50
FIN - 90570 Oulu
Finland
Phone: +358 40 517 0079
EMail: tao.haukka@nokia.com
Arkko, et. al. Standards Track [Page 23]
RFC 3329 SIP Security Agreement January 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Arkko, et. al. Standards Track [Page 24]
RFC, FYI, BCP