SIPPING K. Ono
Internet-Draft S. Tachimoto
Expires: December 22, 2003 NTT Corporation
June 23, 2003
End-to-middle security in the Session Initiation Protocol(SIP)
draft-ono-sipping-end2middle-security-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on December 22, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
End-to-end encryption for confidentiality services can conflict with
some of the features provided by intermediaries. For example, if a
SIP UA encrypts the message body by using S/MIME for end-to-end
security, it cannot use features that the proxy employs to inspect
the message body contained in the request. This situation requires
securing information passed between the UA and an intermediary proxy,
also called "end-to-middle security", which can work with end-to-end
security. This document describes a method of achieving
end-to-middle security, allowing a SIP UA to disclose message data to
selected intermediaries and protect the data from being seen by other
intermediaries. It describes how to apply S/MIME CMS EnvelopedData
body for use in end-to-middle security.
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Conventions used in this document
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 RFC-2119 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problems with the Existing Situations . . . . . . . . . . . . 4
3. Requirements for a Solution . . . . . . . . . . . . . . . . . 6
3.1 Requirements from UA's Perspective . . . . . . . . . . . . . . 6
3.2 Requirements from Proxy's Perspective . . . . . . . . . . . . 6
4. Overview of Proposed Mechanism . . . . . . . . . . . . . . . . 8
4.1 UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4 Summary of Header Field Use . . . . . . . . . . . . . . . . . 11
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1 Request example . . . . . . . . . . . . . . . . . . . . . . . 13
5.2 Response example . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . 22
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1. Introduction
The Session Initiation Protocol (SIP) [2] supports hop-by-hop
security using TLS [3] and end-to-end security using S/MIME [4].
This assumes a SIP UA trusts all proxy servers in a request path to
decide whether or not to inspect the message bodies contained in a
message.
However, there is a model where trusted and partially-trusted proxy
servers are mixed through a message path. The partially-trusted proxy
servers are only trusted in terms of the SIP routing. Hop-by-hop
confidentiality services using TLS are not suitable for this model.
End-to-end confidentiality services using S/MIME are also not
suitable when the intermediaries provide features based on reading
the message bodies and/or headers. This problem is described in
Section 23 of [2].
One example of such a feature is NAT/firewall control, where a midcom
[5] agent co-located with a proxy server controls a NAT/firewall
based on certain SDP attributes in a SIP transaction. Another example
of such a feature is the archiving of instant messaging traffic,
where the archiving function co-located with a proxy server logs the
message bodies in the MESSAGE [6] method. In these cases, a UA might
want to protect the message bodies and/or headers from proxy servers
excluding a selected proxy, which provides these features.
Such a proxy is not always the first hop for the UA. These situations
require security between the UA and the intermediary proxy for the
message bodies and/or message headers. We call this "end-to-middle
security".
End-to-middle security consists of authentication, message integrity,
and message confidentiality. As for authentication, HTTP digest
authentication described in [2] is used for user-to-proxy and
proxy-to-user authentication. The authenticating proxy is not limited
to the first hop for the UA. Thus, HTTP digest authentication can be
used for end-to-middle security. Digital signatures in a Public Key
Infrastructure, that is S/MIME CMS [7] SignedData body with
certificate, can also be used for authentication. As for message
integrity, S/MIME CMS SignedData body can be used. S/MIME CMS
SignedData body is created with the original data and the
originator's private key, and anyone can verify the integrity using
the originator's public key and the certificate. Thus, S/MIME CMS
SignedData body can be used for end-to-middle security at the same
time as end-to-end security. However, proxy servers usually transfer
SIP messages without interpreting the S/MIME bodies. This document
mainly discusses the message confidentiality and integrity of
end-to-middle security.
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2. Problems with the Existing Situations
We describe here examples of models in which trusted and
partially-trusted proxy servers are mixed along a message path. These
situations demonstrate the reasons for requiring end-to-middle
security.
In the following example, Proxy#1 server is the home proxy server of
User#1 using UA#1. User#1 communicates with User#2 through Proxy#1
and Proxy#2 as shown in Figure 1 . UA#1 has already known the
public key certificate of Proxy#1, and it allows Proxy#1 to inspect
the message bodies in a request for some purpose. However, User#1
does not know whether Proxy#2 is trustworthy, and thus wants to
protect the message bodies in the request. Thus, there is the
problem of granting a trusted intermediary permission to inspect
message bodies while preserving their confidentiality with respect to
other intermediaries.
Even if UA#1's request message authorizes a selected proxy (Proxy#1)
to see the message body, UA#1 is unable to authorize the same proxy
to see the message body in the response from UA#2. Thus, there is the
problem of designating and sharing a key that can be reused as a CEK
for bidirectional exchanges of S/MIME-secured messages within SIP.
Note: This document describes the two different problems and
solutions. It might be a good idea to break this document into two
separate drafts.
Home network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+
User#1------| | |-----| |-----| |-----| |-- User#2
| +-----+ +-----+ | +-----+ +-----+
| UA#1 Proxy#1 | Proxy#2 UA#2
+---------------------+
Figure 1: Deployment example#1
In the next example, User#1 connects UA#1 to a proxy server in a
visited network, e.g. a hotspot service or a roaming service. Since
User#1 wants to utilize certain home network services, UA#1 connects
to a home proxy server, Proxy#1. However, UA#1 has to connect
Proxy#1 via the proxy server of the visited network (Proxy A),
because User#1 has to follow the policy of that network. Proxy A may
perform access control based on the destination addresses of calls.
User#1 trusts Proxy A to route requests, but not to inspect the
message bodies they contained. User#1 trusts Proxy#1 both to route
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requests and to inspect the message bodies for some purpose.
The same problems exist as those in the first example.
Visited network
+---------------------+
| +-----+ +-----+ | +-----+ +-----+ +-----+
User#1 -- | | |-----| |-----| |-----| |-----| |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| UA#1 Proxy A | Proxy#1 Proxy#2 UA#2
+---------------------+
Figure 2: Deployment example#2
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3. Requirements for a Solution
These requirements are similar to the general requirements described
in the Internet Draft of the Session Policy Requirements [8]. The
differences are that in this document a UA explicitly authorizes the
use of the features provided by intermediaries. In [8], the
intermediaries imposes a session policy without user authorization.
This document describes security issues related to authorizing an
intermediary to see message contents.
3.1 Requirements from UA's Perspective
1. The solution SHOULD work even with SIP end-to-end encryption for
confidentiality service enabled.
2. It SHOULD work even with SIP end-to-end integrity service
enabled.
3. It SHOULD have a minimal impact on the way to handle messages
with S/MIME bodies.
4. It SHOULD allow a UA to request selected proxy servers to view
selected message body. In addition, the request itself SHOULD be
secure.
5. It SHOULD allow a UA to request the UA on the opposite-side to
impose the same proxy policy on the same proxy server. In
addition, the request itself SHOULD be secure.
It is not appropriate for the UA on the opposite-side to have
knowledge of the public key certificate of the proxy server on
the originating network. This last requirement can be modified
into the following:
+ The solution SHOULD allow a UA to request the opposite-side
UA to reuse a content-encryption-key in subsequent messages
during a dialog.
+ It SHOULD allow a UA to request a selected proxy server to
keep a content-encryption-key in a message during a dialog.
+ The above requests themselves SHOULD be secure.
3.2 Requirements from Proxy's Perspective
1. The solution SHOULD have no impact on proxy servers that do not
provide features based on S/MIME bodies in terms of handling the
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existing SIP headers.
2. It SHOULD have less impact on proxy servers that provide features
based on S/MIME bodies.
When a proxy server receives an S/MIME message, it should be
able to quickly and easily determine the necessity to
investigate the S/MIME body. This last requirement can be
modified into the following:
+ It SHOULD allow proxy servers to quickly and easily
determine whether to handle S/MIME bodies and, if so, how
and which one.
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4. Overview of Proposed Mechanism
The proposed mechanism uses a new SIP header, "Proxy-Policy". The
"Proxy-Policy" header indicates that a UA wants selected proxy
servers to view selected S/MIME bodies. The proxy servers view the
"Proxy-Policy" to determine whether to handle the S/MIME bodies and
if so, which one.
Note: An alternative mechanism would be to add a parameter in the
"Content-Disposition" header. However, since proxy servers usually
do not inspect the "Content-Disposition" header, it is not as good
as using an additional SIP header.
In addition, the proposed mechanism employs the "unprotectedAttrs"
attribute in the S/MIME CMS EnvelopedData body to expresses a
sender's preferences to reuse a content-encryption-key in subsequent
messages during a dialog.
Note: An alternative mechanism would be to add a parameter in the
"Proxy-Policy" header. However, since the key reuse is executed
after the investigation of CMS data, it is not necessary to be set
at the SIP layer. In addition, since [9] has already defined the
reuse method of content-encryption-key using the
"unprotectedAttrs" attribute in the EnvelopedData, it is better to
use the existing mechanism.
We assume that UA#1 requires Proxy#1 to view the message body's SDP
in order to control a firewall for the session in the situation shown
in Figure 1.
Since UA#1 requires end-to-end and end-to-middle confidentiality for
the content of the SDP, it uses S/MIME CMS EnvelopedData for multiple
recipients and sets the "Content-ID" header to identify the content.
The "recipientInfos" data of the EnvelopedData contains the encrypted
content-encryption-keys for each recipient of the same encrypted
content. One of the "RecepientInfo" attributes is for Proxy#1, while
another is for UA#2. UA#1 may use the "unprotectedAttrs" attribute to
request UA#2 to reuse the content-encryption-key instead of a public
key to encrypt a content-encryption-key of a response.
+----------------+ +-------+ +-----+ +-----+
|E-CEK(C) with |--->|C, |---->| |----->|C, |
|E-UA#2key(CEK)& | | | | | |CEK |
|E-P#1key(CEK) | |CEK | | | | |
+----------------+ +-------+ +-----+ +-----+
UA#1 Proxy#1(P#1) Proxy#2 UA#2
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C: Content of a request
CEK: Content-encryption-key
E-CEK(C): Content encrypted using CEK
E-Xkey(CEK): CEK encrypted using X's public key.
Figure 3: Overview of message with CMS EnvelopedData
+----------------+ +-------+ +-----+ +----------------+
|C' |<---|C', |<----| |<-----|E-CEK'(C') with |
|CEK' | |CEK' | | | |E-CEK(CEK')& |
+----------------+ +-------+ +-----+ +----------------+
UA#1 Proxy#1(P#1) Proxy#2 UA#2
C': Content of a response
CEK': Content-encryption-key.
Figure 4: Overview of subsequent message with CMS EnvelopedData
Issue:How does this mechanism apply for the case when Proxy#2
needs to inspect the message body contained in the request to
UA#2?
When UA#1 requires the message integrity for end-to-end and
end-to-middle security, it uses the S/MIME CMS SignedData for the
"message/sipfrag"[10] Content-type. When it requires confidentiality
and integrity for the message, it uses the S/MIME SignedData of the
S/MIME EnvelopedData for the message.
4.1 UAC Behavior
When a UAC sends a request and it requires end-to-end and
end-to-middle confidentiality of the message bodies and/or headers ,
it uses S/MIME to encrypt them. In the above examples, UA#1 uses S/
MIME EnvelopedData for UA#2 and Proxy#1. At the SIP layer, UA#1
requires Proxy#1 to decrypt selected content and to view the content
by using the "Proxy-Policy" header. Proxy#1 then provides some
feature based on the decrypted content.
When the UAC needs Proxy#1 to inspect the message bodies and/or
headers in the response, it SHOULD request the UAS to encrypt the
response by using the same content-encryption-key as for the request.
The UAC uses the "unprotectedAttrs" attribute to stipulate reuse of
the content-encryption-key and indicate its identifier.
Note: The "unprotectedAttrs" data is not protected, so it should
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be protected using S/MIME SignedData.
When the UAC sends a request and needs the end-to-end and
end-to-middle integrity for the message bodies and/or headers, it
uses S/MIME to attach a digital signature. In the above examples, it
uses the S/MIME CMS SignedData of the contents. At the SIP layer,
UA#1 requires Proxy#1 to validate the integrity of the selected
content by employing the "Proxy-Policy" header.
When the UAC receives a response that uses S/MIME, it decrypts and/or
validates the S/MIME bodies as usual. If it receives a response that
uses S/MIME EnvelopedData with the "KEKRecipentInfo" type of
"RecepientInfo" attribute, it should decrypt the "RecipentInfo" by
using the same content-encryption-key as for the sending request.
4.2 UAS Behavior
When a UAS sends a response for the request with this mechanism,
using the same type of S/MIME CMS data is recommended. For example,
if the UAS receives an INVITE request in which the SDP is encrypted
by using S/MIME EnvelopedData, the recommended response would be a
"200 OK" containing the encrypted SDP based on the the S/MIME
EnvelopedData.
In particular, when the S/MIME EnvelopedData of the request contains
the "unprotectedAttrs" attribute specifying reuse of the
content-encryption-key, the UAS SHOULD encrypt a
content-encryption-key with the content-encryption-key that was used
in the request, instead of a public key of the UAC. The UAS SHOULD
use the S/MIME EnvelopedData to contain the encrypted SDP in the "200
OK" response. In addition, the UAS SHOULD set the same proxy server
as in the request and the Content-id of the encrypted SDP in the
"Proxy-Policy" header.
If the UAS encrypted the SDP with a content-encryption-key that was
itself encrypted with the content-encryption-key in the request, the
proxy server selected by the UAC can view the SDP in the "200 OK"
response.
Note: In the case when the response does not contain a message
body, even if the request contains a message body and was
encrypted by using S/MIME EnvelopedData, the UAS does not need to
use the S/MIME EnvelopedData.
When the UAS receives a request that uses S/MIME, it decrypts and/or
validates the S/MIME bodies as usual.
When the UAS sends a response for the request without this mechanism
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and needs end-to-end and end-to-middle confidentiality of the message
bodies and/or headers , it uses S/MIME to encrypt them. When the UAC
sends a request and needs end-to-end and end-to-middle integrity of
the message bodies and/or headers, it uses S/MIME to attach a digital
signature. This is the same way the UAC normally performs with this
mechanism.
4.3 Proxy Behavior
When a proxy supporting this mechanism receives a message, the proxy
server inspects the "Proxy-Policy" header. If the header includes the
processing server's own name, the proxy server inspects the specified
Content-ID.
When the specified content is S/MIME EnvelopedData, the proxy server
inspects it and tries to decrypt the "RecipientInfo" attribute. If
the proxy fails to decrypt that, it should cancel the subsequent
procedure and respond with a 493 (Undecipherable) response if it is a
request, or any existing dialog MAY be terminated. If the proxy
succeeds in this decryption, it inspects the "unprotectedAttrs" data
of the S/MIME EnvelopedData. If the attribute gives the key's
identifier, the proxy must keep the content-encryption-key with its
identifier during the dialog. When it receives subsequent messages in
the dialog, it tries to decrypt the "KEKRecipientInfo" type of
"RecepientInfo" attribute by using this content-encryption-key.
When the specified content is S/MIME SignedData, the proxy server
inspects it and validates the digital signature. If the verification
is unsuccessful, the proxy server should reject the subsequent
procedure and respond with a 403 (Forbidden) response if the message
is a request, or any existing dialog MAY be terminated.
When the proxy server transfers the request, it modifies the routing
headers normally. It does not need to modify the S/MIME body.
If a proxy does not support this mechanism and receives a message
with the "Proxy-Policy" header, the proxy ignores the header and
perform as usual.
4.4 Summary of Header Field Use
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [11].
Proxy-Policy = HCOLON proxy-uri *( SEMI (proxy-policy-param) )
proxy-uri = ( name-addr / addr-spec )
proxy-policy-param = content-id ( SEMI policy )
content-id = "cid" EQUAL cid
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cid = LDQUOT dot-atom "@" (dot-atom / host) RDQUOT
dot-atom = atom *( "." atom )
atom = 1*( alphanum / "-" / "!" / "%" / "*" / "_" / "+" / "'" / "`" / "~" )
policy = "policy" EQUAL ( token )
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
-----------------------------------------------------
Proxy-Policy R adr o o o o o o
Proxy-Policy 200-699 adr - o - o o o
Proxy-Policy 1xx adr - - - o - -
Header field where proxy SUB NOT PRK IFO UPD MSG
-----------------------------------------------------
Proxy-Policy R adr o o - o o o
Proxy-Policy 200-699 adr o o - o o o
Table 1: Summary of header field use
The "where" column gives the request and response types in which the
header field can be used. The values in the "where" column are as
follows:
* R: The header field may appear in requests
* 200-699: A numeral range indicates response codes with which
the header field can be used.
* a: A proxy can add or concatenate the header field if it is not
present.
* r: A proxy must be able to read the header field, and thus it
cannot be encrypted.
* d: A proxy can delete a header field value.
* o: The header field is optional.
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5. Examples
The following examples illustrate the use of the mechanism defined in
the previous section.
5.1 Request example
In the following example, a UA needs the confidentiality of the SDP
in INVITE message and the UA allows a proxy server to view the SDP in
INVITE request. In addition, the UA needs to protect the policy for
the proxy server. In the example encrypted message below, the text
boxed in asterisks ("*") is encrypted:
INVITE alice@atlanta.example.com --> ss1.atlanta.example.com
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
Route: <sip:ss1.atlanta.example.com;lr>
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Date: Fri, 20 June 2003 13:02:03 GMT
Proxy-Policy: ss1.atlanta.example.com; cid="2UWQFN309shb3@atlanta.example.com"
Content-Type: multipart/singed; protocol="application/pkcs7-signature"; micalg=sha1; boundary=boundary2
Content-Length: 878
--boundary1
Content-Type: multipart/mixed; boundary=boundary2
Content-Length: 568
--boundary2
Content-Type: message/sipfrag
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Date: Fri, 20 June 2003 13:02:03 GMT
Proxy-Policy: ss1.atlanta.example.com; cid="2UWQFN309shb3@atlanta.example.com"
--boundary2
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;name=smime.p7m
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Content-Transfer-Encoding: base64
Content-Disposition: session; filename=smime.p7m;handling=required
Content-ID: <2UWQFN309shb3@atlanta.example.com>
Content-Length: 231
******************************************************************
* (encryptedContentInfo) *
* Content-Type: application/sdp *
* Content-Length: 151 *
* *
* v=0 *
* o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com*
* s=- *
* c=IN IP4 192.0.2.101 *
* t=0 0 *
* m=audio 49172 RTP/AVP 0 *
* a=rtpmap:0 PCMU/8000 *
* *
* (recipientInfos) *
* RecepientInfo[0] for ss1.atlanta.example.com public key *
* RecepientInfo[1] for bob's public key *
* *
* (unprotectedAttr) *
* CEKReference *
******************************************************************
--boundary2--
--boundary1--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s;handling=required
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary1--
5.2 Response example
In the following example, a UA sends a response with this mechanism.
In the example encrypted message below, the text boxed in asterisks
("*") is encrypted:
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200 OK bob@biloxi.example.com --> ss2.biloxi.example.com
SIP/2.0 200 OK
Via: SIP/2.0/TCP
ss2.biloxi.example.com:5060;branch=z9hG4bK721e418c4.1
;received=192.0.2.222
Via: SIP/2.0/TCP ss1.atlanta.example.com:5060;branch=z9hG4bK2d4790.1
;received=192.0.2.111
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
Record-Route: <sip:ss2.biloxi.example.com;lr>,
<sip:ss1.atlanta.example.com;lr>
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=314159
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
--boundary41
Content-Type: multipart/mixed; boundary=boundary2
Content-Length: 468
--boundary42
Content-Type: message/sipfrag
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=314159
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 2 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
Date: Fri, 20 June 2003 13:02:03 GMT
Proxy-Policy: ss1.atlanta.example.com; cid="2UWQFN309shb3@biloxi.example.com"
--boundary2
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: session; filename=smime.p7m;handling=required
Content-ID: <2UWQFN309shb3@biloxi.example.com>
Content-Length: 211
******************************************************************
* (encryptedContentInfo) *
* Content-Type: application/sdp *
* Content-Length: 147 *
* *
* v=0 *
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* o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com *
* s=- *
* c=IN IP4 192.0.2.201 *
* t=0 0 *
* m=audio 3456 RTP/AVP 0 *
* a=rtpmap:0 PCMU/8000 *
* *
* (recipientInfos) *
* RecepientInfo[0] for KEKidentifier *
******************************************************************
--boundary42--
--boundary41--
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s;handling=required
hhhHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary41--
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6. Security Considerations
This specification is about applying S/MIME-secured messages for use
in end-to-middle security. It is also applying the CEK reuse method
defined in [9]. This requires the same security consideration as
those of [4] and [9].
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7. IANA Considerations
This document requires a new header fields, namely "Proxy-Policy".
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8. Acknowledgments
Thanks to Rohan Mahy and Cullen Jennings for their initial support of
this concept, and to Jon Peterson for his helpful comments.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] 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.
[3] Allen, C. and T. Dierks, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1992.
[5] Srisuresh, P., Kuthan, J., Rosenberg, J., Brim, S., Molitor, A.
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[6] Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C.
and D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[7] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[8] Rosenberg, J., "Requirements for Session Policy for the Session
Initiation Protocol (SIP)",
draft-rosenberg-sipping-session-policy-req-00 (work in
progress), December 2002.
[9] Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
Keys", RFC 3185, October 2001.
[10] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[11] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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Authors' Addresses
Kumiko Ono
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: ono.kumiko@lab.ntt.co.jp
Shinya Tachimoto
Network Service Systems Laboratories
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-shi, Tokyo 180-8585
Japan
EMail: tachimoto.shinya@lab.ntt.co.jp
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Intellectual Property Statement
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