draft-ietf-mmusic-kmgmt-ext-08.txt   draft-ietf-mmusic-kmgmt-ext-09.txt 
Internet Engineering Task Force J. Arkko Internet Engineering Task Force J. Arkko
MMUSIC Working Group E. Carrara MMUSIC Working Group E. Carrara
INTERNET-DRAFT F. Lindholm INTERNET-DRAFT F. Lindholm
Expires: February 2004 M. Naslund Expires: April 2004 M. Naslund
K. Norrman K. Norrman
Ericsson Ericsson
August 2003 October 2003
Key Management Extensions for Session Description Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP) Protocol (SDP) and Real Time Streaming Protocol (RTSP)
<draft-ietf-mmusic-kmgmt-ext-08.txt> <draft-ietf-mmusic-kmgmt-ext-09.txt>
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts. groups may also distribute working documents as Internet-Drafts.
skipping to change at page 1, line 42 skipping to change at page 1, line 42
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html http://www.ietf.org/shadow.html
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This document defines general extensions for SDP and RTSP to carry This document defines general extensions for SDP and RTSP to carry
the security information needed by a key management protocol, in messages as specified by a key management protocol, in order to
order to secure the media. These extensions are presented as a secure the media. These extensions are presented as a framework, to
framework, to be used by one or more key management protocols. As be used by one or more key management protocols. As such, its use is
such, its use is meaningful only when it is completed by the key meaningful only when it is completed by the key management protocol
management protocol in use. in use.
General guidelines are also given on how the framework should be used General guidelines are also given on how the framework should be used
together with SIP and RTSP. together with SIP and RTSP.
TABLE OF CONTENTS TABLE OF CONTENTS
1. Introduction.....................................................2 1. Introduction.....................................................2
1.1. Notational Conventions.........................................3 1.1. Notational Conventions.........................................3
2. Extensions to SDP and RTSP.......................................4 2. Extensions to SDP and RTSP.......................................4
2.1. SDP Extensions.................................................4 2.1. SDP Extensions.................................................4
2.2. RTSP Extensions................................................4 2.2. RTSP Extensions................................................5
3. Usage with SIP and RTSP..........................................5 3. Usage with SIP and RTSP..........................................5
3.1. General SDP processing.........................................5 3.1. General SDP processing.........................................6
3.2. SIP usage......................................................7 3.2. SIP usage......................................................7
3.3. RTSP usage.....................................................8 3.3. RTSP usage.....................................................8
3.4. Example scenarios..............................................9 3.4. Bidding-down attack prevention.................................9
4. Adding further Key management protocols.........................11 3.5. Example scenarios.............................................10
5. Security Considerations.........................................12 4. Adding further Key management protocols.........................13
6. IANA Considerations.............................................13 5. Security Considerations.........................................13
6.1. SDP Attribute Registration....................................13 6. IANA Considerations.............................................14
6.2. Protocol Identifier Registration..............................13 6.1. SDP Attribute Registration....................................14
8. Acknowledgments.................................................14 6.2. RTSP Header Registration......................................15
9. Author's Addresses..............................................14 6.3. Protocol Identifier Registration..............................15
10. References.....................................................15 7. Acknowledgments.................................................16
10.1. Normative References.........................................15 8. Author's Addresses..............................................16
10.2. Informative References.......................................15 9. References......................................................17
9.1. Normative References..........................................17
9.2. Informative References........................................17
1. Introduction 1. Introduction
[Editor remark] All instances of RFC xxxx should be replaced with [Editor remark] All instances of RFC xxxx should be replaced with
the RFC number of this document, when published. Furthermore, all the RFC number of this document, when published. Furthermore, all
instances of RFC yyyy should be replaced with the RFC number of instances of RFC yyyy should be replaced with the RFC number of
the MIKEY (Multimedia Internet KEYing) document [MIKEY], when the MIKEY (Multimedia Internet KEYing) document [MIKEY], when
published. published.
There has recently been work to define a security framework for the There has recently been work to define a security framework for the
protection of real-time applications running over RTP, [SRTP]. protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management infrastructure to However, a security protocol needs a key management solution to
exchange keys and security parameters, manage and refresh keys, etc. exchange keys and security parameters, manage and refresh keys, etc.
A key management protocol is executed prior to the security protocol A key management protocol is executed prior to the security protocol
execution. The key management protocol's main goal is to, in a secure execution. The key management protocol's main goal is to, in a secure
and reliable way, establish a security association for the security and reliable way, establish a security association for the security
protocol. This includes one or more cryptographic keys and the set of protocol. This includes one or more cryptographic keys and the set of
necessary parameters for the security protocol, e.g., cipher and necessary parameters for the security protocol, e.g., cipher and
authentication algorithm to be used. The key management protocol has authentication algorithm to be used. The key management protocol has
similarities with, e.g., SIP [SIP] and RTSP [RTSP] in the sense that similarities with, e.g., SIP [SIP] and RTSP [RTSP] in the sense that
it negotiates necessary information in order to be able to setup the it negotiates necessary information in order to be able to setup the
skipping to change at page 3, line 43 skipping to change at page 3, line 45
applying end-to-end protection of the SDP (instead, hop-by-hop applying end-to-end protection of the SDP (instead, hop-by-hop
security mechanisms can be used which may be useful if security mechanisms can be used which may be useful if
intermediate proxies needs access to the SDP). intermediate proxies needs access to the SDP).
Currently in SDP [SDPnew], one field exists to transport keys, i.e. Currently in SDP [SDPnew], one field exists to transport keys, i.e.
the "k=" field. However, this is not enough for a key management the "k=" field. However, this is not enough for a key management
protocol as there are many more parameters that need to be protocol as there are many more parameters that need to be
transported. The approach here is to use and extend the SDP transported. The approach here is to use and extend the SDP
description to transport the key management offer/answer and also to description to transport the key management offer/answer and also to
associate it with the media sessions. SIP uses the offer/answer model associate it with the media sessions. SIP uses the offer/answer model
[OAM] whereby extensions to SDP will be enough. However, RTSP [OAM] whereby extensions to SDP will be enough. However, RTSP [RTSP]
[RTSP]does not use the offer/answer model with SDP, so a new header does not use the offer/answer model with SDP, so a new header is
is introduced to convey key management data. introduced to convey key management data.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Extensions to SDP and RTSP 2. Extensions to SDP and RTSP
This section describes common attributes that are to be included in This section describes common attributes that are to be included in
an SDP description or in an RTSP header when an integrated key an SDP description or in an RTSP header when an integrated key
management protocol is used. The attribute values MUST follow the management protocol is used. The attribute values MUST follow the
general SDP or RTSP guidelines (see [SDPnew] and [RTSP]). general SDP or RTSP guidelines (see [SDPnew] and [RTSP]).
For both SDP and RTSP, the general method of adding the key For both SDP and RTSP, the general method of adding the key
management protocol is to introduce new attributes, one identifier to management protocol is to introduce new attributes, one identifier to
identify the specific key management protocol, and one data field identify the specific key management protocol, and one data field
where the key management protocol data is placed. The key management where the key management protocol data is placed. The key management
protocol data contains the necessary information to establish the protocol data contains the necessary information to establish the
security protocol, e.g., keys and cryptographic parameters. All security protocol, e.g., keys and cryptographic parameters. All
parameters and keys are protected by the key management. parameters and keys are protected by the key management protocol.
The key management data SHALL be base64 encoded and comply with the
base64 grammar as defined in [SDPnew]. The key management protocol
identifier, KMID, is defined as below (where ALPHA and DIGIT are as
defined in [RFC2234]).
KMID = 1*(ALPHA/DIGIT)
Values for the identifier, KMID, are registered and defined in
accordance to Section 6. Note that the KMID will be case sensitive
and it is therefore RECOMMENDED that values registered are lower case
letters.
2.1. SDP Extensions 2.1. SDP Extensions
This section provides an Augmented Backus-Naur Form (ABNF) grammar This section provides an Augmented Backus-Naur Form (ABNF) grammar
(as used in [SDPnew]) for the key management extensions to SDP. (as used in [SDPnew]) for the key management extensions to SDP.
Note that the new definitions are compliant with the definition of an Note that the new definitions are compliant with the definition of an
attribute field, i.e. attribute field, i.e.
attribute = (att-field ":" att-value) | att-field attribute = (att-field ":" att-value) | att-field
One new attribute for SDP is defined: One new attribute for SDP is defined:
key-mgmt = "key-mgmt: " prtcl-id keymgmt-data key-mgmt = "key-mgmt: " prtcl-id keymgmt-data
prtcl-id = non-ws-string prtcl-id = KMID
; e.g. "mikey" ; e.g. "mikey"
keymgmt-data = text keymgmt-data = text
where non-ws-string and text are as defined in SDP [SDPnew]. The where KMID is as previously defined, "text" is as defined in SDP
attribute may be used at session level, media level, or at both [SDPnew]. Prtcl-id refers to the set of values defined for KMID in
Section 6. "text" is consistent with the requirement of base64-
encoded data, and KMID is consistent with the standard definition of
non-ws-string.
The attribute may be used at session level, media level, or at both
levels. An attribute defined at media level overrides an attribute levels. An attribute defined at media level overrides an attribute
defined at session level. Note that the prtcl-id name will be case defined at session level. Section 3 describes in detail how the
sensitive and it is therefore RECOMMENDED that attributes registered
are in lower case letters. Section 3 describes in detail how the
attributes are used and how the SDP is handled in different usage attributes are used and how the SDP is handled in different usage
scenarios. scenarios.
2.2. RTSP Extensions 2.2. RTSP Extensions
To support the needed attribute, the following RTSP header is To support the needed attribute, the following RTSP header is
defined: defined:
KeyMgmt = "keymgmt" ":" 1#key-mgmt-spec KeyMgmt = "keymgmt" ":" 1#key-mgmt-spec
key-mgmt-spec = "prot" "=" token ";" "data" "=" quoted-string key-mgmt-spec = "prot" "=" KMID ";" "data" "=" quoted-string
"token" and "quoted-string" are as defined in the RTSP specification
[RTSP]. where KMID is as previously defined and "quoted-string" as defined in
the RTSP specification [RTSP]. "quoted-string" is consistent with the
requirement of base64-encoded data, and KMID is consistent with the
standard definition of token.
The KeyMgmt header should be possible to use in the messages The KeyMgmt header should be possible to use in the messages
described in the table below. described in the table below.
Method Direction Requirement Method Direction Requirement
DESCRIBE C->S required DESCRIBE C->S optional
SETUP C->S required SETUP C->S required
ANNOUNCE C->S, S->C optional (required: if re-key is supported) ANNOUNCE C->S, S->C optional (required: if re-key is supported)
Note: Section 3 describes in detail how the RTSP extensions are used. Note: Section 3 describes in detail how the RTSP extensions are used.
3. Usage with SIP and RTSP 3. Usage with SIP and RTSP
This section gives recommendations of how/when to include the defined This section gives recommendations of how/when to include the defined
key management attribute when SIP and/or RTSP are used together with key management attribute when SIP and/or RTSP are used together with
SDP. SDP.
skipping to change at page 5, line 35 skipping to change at page 6, line 7
* It MUST be possible to execute the key management protocol in at * It MUST be possible to execute the key management protocol in at
most one roundtrip in case the answerer accepts the offer. most one roundtrip in case the answerer accepts the offer.
* It MUST be possible from the SIP/SDP and RTSP application, using * It MUST be possible from the SIP/SDP and RTSP application, using
the key management API, to receive key management data, and the key management API, to receive key management data, and
information of whether a message is accepted or not. information of whether a message is accepted or not.
Today, the MIKEY protocol [MIKEY] has adopted the key management Today, the MIKEY protocol [MIKEY] has adopted the key management
extensions to work together with SIP and RTSP. Other protocols MAY extensions to work together with SIP and RTSP. Other protocols MAY
use the described attribute and header, e.g. Kerberos [KERB]. use the described attribute and header, e.g. Kerberos [KERB], however
this is subject to future standardization.
3.1. General SDP processing 3.1. General SDP processing
When an SDP message is created, the following procedure should be When an SDP message is created, the following procedure SHALL be
applied: applied:
* The identifier of the key management protocol used (e.g. MIKEY or * The identifier of the key management protocol used MUST be placed
Kerberos) MUST be placed in the prtcl-id field. in the prtcl-id field. The protocol identifier values are
specified by IANA (see Section 6).
* The list of protocol identifiers is provided by the SDP application
to (each) key management protocol, as defined in Section 3.4. (to
defeat bidding-down attacks).
* The keymgmt-data field MUST be created as follows. The key * The keymgmt-data field MUST be created as follows. The key
management protocol MUST be used to create the key management management protocol MUST be used to create the key management
message. This message SHALL be base64 encoded [RFC3548] by the SDP message. This message SHALL be base64 encoded [RFC3548] by the SDP
application and then encapsulated in the keymgmt-data attribute. application and then encapsulated in the keymgmt-data attribute.
The data may e.g. be a MIKEY message (see [MIKEY], Section 7) or The data may e.g. be a MIKEY message (see [MIKEY], Section 7).
Kerberos ticket.
A received SDP message that contains the key management attributes A received SDP message that contains the key management attributes
SHOULD be processed in the following manner: SHALL be processed in the following manner:
* The key management protocol is identified according to the prtcl-id * The key management protocol is identified according to the prtcl-id
field. field. The protocol identifier values are specified by IANA
(Section 6).
* The key management data from the keymgmt-data field MUST be * The key management data from the keymgmt-data field MUST be
extracted, base64 decoded to reconstruct the original message, and extracted, base64 decoded to reconstruct the original message, and
then passed to the key management protocol. Note that depending on then passed to the key management protocol.
key management protocol, some extra parameters might of course be
requested by the specific API, such as the source/destination
network address/port(s) for the specified media (however, this
will be implementation specific depending on the actual API).
* Depending on the outcome of the key management processing (i.e. * The list of protocol identifiers is provided by the SDP application
whether it was successful or not), the processing can proceed to the key management protocol, as defined in Section 3.4. Note
according to normal rules (e.g. according to the offer/answer that depending on key management protocol, some extra parameters
model, see also Section 3.2). might also be requested by the specific API, such as the
source/destination network address/port(s) for the specified media
(however, this will be implementation specific depending on the
actual API). The extra parameters that a key management protocol
might need (other than the ones defined here) SHOULD be
documented, describing their use, as well as the interaction of
that key management protocol with SDP and RTSP.
* If the key management processing is successful, then the answerer
sends back the answer. Otherwise, if the key management rejects
the offer, an error is sent back ("606 Not Acceptable") and the
session is aborted. See Section 3.2 for further details.
Note that the key management attribute MAY be repeated more than once Note that the key management attribute MAY be repeated more than once
(e.g., one at session level and one at media level). Consequently, (e.g., one at session level and one at media level). Consequently,
the process is repeated for each key management attribute detected. the process is repeated for each key management attribute detected.
However, in case of failure of the key management (on either session However, in case of failure of the key management (on either session
or media level), the session setup SHALL be aborted (see also Section or media level), the session setup SHALL be aborted (see also Section
3.2 and Section 3.3 for more details). 3.2 and Section 3.3 for more details).
If more than one key management protocol is supported, multiple In the Offer/Answer case, and in general when there is an answer, if
more than one key management protocol is supported, multiple
instances of the key management attribute MAY be included in the instances of the key management attribute MAY be included in the
initial offer, each transporting a different key management data, initial offer, each transporting a different key management data,
thus indicating supported alternatives. thus indicating supported alternatives.
If the sender includes more than one key management protocol If the sender includes more than one key management protocol
attribute at session level (analogous for the media level), these attribute at session level (analogous for the media level), these
SHOULD be listed in order of preference (the first being the SHOULD be listed in order of preference (the first being the
preferred). The receiver selects the key management protocol it preferred). The receiver selects the key management protocol it
wishes to use and includes only that attribute in the answer. If the wishes to use and includes only that attribute in the answer. If the
receiver does not support any of the sender's suggested key receiver does not support any of the sender's suggested key
management protocols, the receiver returns an error message (see management protocols, the receiver returns an error message (see
section 3.2 and section 3.3), whereby the sender MUST abort the section 3.2 and section 3.3), whereby the sender MUST abort the
current setup procedure. current setup procedure.
Note that the placement of multiple key management offers in a single Note that the placement of multiple key management offers in a single
message has the disadvantage that the message expands and the message has the disadvantage that the message expands and the
computational workload for the offerer will increase drastically. computational workload for the offerer will increase drastically.
Unless the guidelines of Section 3.4 are followed, multiple lines may
open up for bidding-down attacks.
The possibility to support multiple key management protocols may The following Sections describe the specific use with SIP and RTSP
introduce bidding down attacks. To avoid this, the list of respectively. There are of course other cases where SDP is used, such
identifiers of the proposed key management protocols MUST be as SAP and HTTP. If SDP is transported in an Offer-Answer model
authenticated. The authentication MUST be done separately by each key fashion, then the guidelines in Section 3.2 can be used. However, for
management protocol (see e.g. Section 7.1 in [MIKEY]). one-way SDP distribution (i.e. without back channel), the above
guidelines can be used though with certain restrictions. First, the
Accordingly, it MUST be specified (in the key management protocol key management protocol MUST support one-way messages, and secondly,
specification itself or in a companion document) how the list of key only one key management protocol SHALL be offered (i.e. no
management protocol identifiers can be authenticated from the offerer negotiation will be possible).
to the answerer by the specific key management protocol. Note that
even if only one key management protocol is used, that still MUST
authenticate its own protocol identifier.
The list of protocol identifiers MUST be given to the selected key
management protocol by the SDP application with ";" separated
identifiers. All the offered protocol identifiers MUST be included,
in the same order as they appear in the corresponding SDP
description.
The protocol list can formally be described as
prtcl-list = prtcl-id *(";" prtcl-id)
prtcl-id = non-ws-string
For example, if the SDP is:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey <data1>
a=key-mgmt:keyp1 <data2>
a=key-mgmt:keyp2 <data3>
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
The protocol list, "mikey;keyp1;keyp2", would be generated from
the SDP description and used as input to each specified key
management protocol (together with the data for that protocol).
If more than one protocol is supported by the offerer, it is This document does NOT address one-to-many distribution scenarios, as
RECOMMENDED that all acceptable protocols are included in the first this would require different types of key management protocols. The
offer, rather than making single, subsequent alternative offers in support for such scenarios is for future standardization.
response to error messages, see "Security Considerations".
3.2. SIP usage 3.2. SIP usage
When used with SIP and the offer/answer model, the offerer SHOULD When used with SIP and the offer/answer model, the offerer SHOULD
include the key management data within an offer that contains the include the key management data within an offer that contains the
media description it should apply to. The answerer MUST check with media description it should apply to. At the answerer's side, the key
the key management protocol if the attribute values are valid, and management protocol checks the validity of the key management
then obtain from the key management the data to include in the message, together with the availability of the offered attribute
answer. values, and then provides the key management data to be included in
the answer.
If the offer is not accepted, the answerer SHOULD return a "606 Not If the offer is not accepted, the answerer SHOULD return a "606 Not
Acceptable" message, including one or more Warning headers (at least Acceptable" message, including one or more Warning headers (e.g. a
a 306 "Attribute not understood"). The session is then aborted (and 306 "Attribute not understood" when one of the parameters is not
it is up to local policy or end user to decide how to continue). supported). The session is then aborted (and it is up to local policy
or end user to decide how to continue).
Re-keying can be handled as a new offer, i.e. a re-INVITE should be Re-keying can be handled as a new offer, i.e. a re-INVITE should be
sent with the new proposed parameters. The answerer treats this as a sent with the new proposed parameters. The answerer treats this as a
new offer where the key management is the issue of change. In new offer where the key management is the issue of change. In
general, the re-INVITE (and the key exchange) must be finalized general, the re-INVITE (and the key exchange) must be finalized
before the security protocol can change the keys. The same key before the security protocol can change the keys. The same key
management protocol used in the original INVITE SHALL also be used in management protocol used in the original INVITE SHALL also be used in
the re-INVITE carrying re-keying. If the re-INVITE carrying re-keying the re-INVITE carrying re-keying. If the re-INVITE carrying re-keying
fails (e.g., the authentication verification fails), the answerer fails (e.g., the authentication verification fails), the answerer
SHOULD send a "606 Not Acceptable" message, including one or more SHOULD send a "606 Not Acceptable" message, including one or more
Warning headers (at least a 306). The offer MUST then abort the Warning headers (at least a 306). The offerer MUST then abort the
security setup. security setup.
3.3. RTSP usage 3.3. RTSP usage
RTSP does not use the offer/answer model, as SIP does. This causes RTSP does not use the offer/answer model, as SIP does. This causes
some problems, as it is not possible (without abusing RTSP) to send some problems, as it is not possible (without abusing RTSP) to send
back an answer to the server (as the server will in most cases be the back an answer to the server (as the server will in most cases be the
one initiating the security parameter exchange). To solve this, a new one initiating the security parameter exchange). To solve this, a new
header has been introduced (Section 2.2). This also assumes that the header has been introduced (Section 2.2). This also assumes that the
key management also has some kind of binding to the media, so that key management also has some kind of binding to the media, so that
the response to the server will be processed as required. the response to the server will be processed as required.
The initial key management message from a server should be sent to To obtain a session description, the client initially contacts the
the client using SDP. When responding to this, the client uses the server via a DESCRIBE message (according to RTSP, a media description
new RTSP header to send back an answer (included in the SETUP could also be obtained by other means e.g. using http). The initial
message). If a server receives a SETUP message in which it expects a key management message from the RTSP server is sent to the client in
key management message, but none is included, a 403 Forbidden SHOULD the SDP of the 200 OK in response to the DESCRIBE. When responding to
be returned to the client, whereby the current setup MUST be aborted. this, the client uses the new RTSP header to send back an answer
(included in the SETUP message). If a server receives a SETUP message
in which it expects a key management message, but none is included, a
403 Forbidden SHOULD be returned to the client, whereby the current
setup MUST be aborted.
The processing of creating a key management header in RTSP SHOULD be The processing of creating a key management header in RTSP SHALL be
as follow: as follow:
* The identifier of the key management protocol used (e.g. MIKEY or * The identifier of the key management protocol used (e.g. MIKEY)
Kerberos) MUST be placed in the "prot" field of the header. MUST be placed in the "prot" field of the header. The prot values
are specified by IANA (Section 6).
* The list of protocol identifiers is sent by the RTSP application to
(each) key management protocol as described in Section 3.4. (to
defeat bidding-down attacks).
* The keymgmt-data field MUST be created as follows. The key * The keymgmt-data field MUST be created as follows. The key
management protocol MUST be used to create the key management management protocol MUST be used to create the key management
message. This message SHALL be base64 encoded by the SDP message. This message SHALL be base64 encoded by the SDP
application and then encapsulated in the "data" field of the application and then encapsulated in the "data" field of the
header. The data may e.g. be a MIKEY message (see [MIKEY], Section header. The data may e.g. be a MIKEY message (see [MIKEY], Section
7) or Kerberos ticket. 7).
A received key management header SHOULD be processed in the following A received key management header SHALL be processed in the following
manner: manner:
* The key management protocol is identified according to the "prot" * The key management protocol is identified according to the "prot"
field. field.
* The key management data from the "data" field MUST be extracted, * The key management data from the "data" field MUST be extracted,
base64 decoded to reconstruct the original message, and then base64 decoded to reconstruct the original message, and then
passed to the key management protocol. Note that depending on the passed to the key management protocol. Note that depending on the
key management protocol, some extra parameters might of course be key management protocol, some extra parameters might of course be
requested by the specific API, such as the source/destination requested by the specific API, such as the source/destination
network address/port(s) for the specified media (however, this network address/port(s) for the specified media (however, this
will be implementation specific depending on the actual API). will be implementation specific depending on the actual API).
* Depending on the outcome of the key management processing (i.e. * Depending on the outcome of the key management processing (i.e.
whether it was successful or not), the processing can proceed whether it was successful or not), the processing can proceed
according to normal rules (see also below). according to normal rules (see also below).
The server MAY provide re-keying/updating facilities by sending a new The server MAY provide re-keying/updating facilities by sending a new
key management message in an ANNOUNCE messages. The ANNOUNCE message key management message in an ANNOUNCE message. The ANNOUNCE message
contains an SDP message including the key management parameters. The contains an SDP message including the key management parameters. The
response message is put in the new RTSP header in the response from response message is put in the new RTSP header in the response from
the client to the server. Note that the ANNOUNCE messages MUST be the client to the server. Note that the ANNOUNCE messages MUST be
supported if this feature is to be used. supported if this feature is to be used.
3.4. Example scenarios 3.4. Bidding-down attack prevention
The possibility to support multiple key management protocols may,
unless properly handled, introduce so-called bidding-down attacks.
Specifically, a man-in-the-middle could "peel off" cryptographically
strong offers (deleting key-mgmt lines from the message), leaving
only weaker ones as the responderĘs choice. To avoid this, the list
of identifiers of the proposed key management protocols MUST be
authenticated. The authentication MUST be done separately by each key
management protocol (see e.g. Section 7.1 in [MIKEY]).
Accordingly, it MUST be specified (in the key management protocol
specification itself or in a companion document) how the list of key
management protocol identifiers can be processed to be authenticated
from the offerer to the answerer by the specific key management
protocol. Note that even if only one key management protocol is used,
that still MUST authenticate its own protocol identifier.
The list of protocol identifiers MUST then be given to each of the
selected (offered) key management protocols by the application with
";" separated identifiers. All the offered protocol identifiers MUST
be included, in the same order as they appear in the corresponding
SDP description.
The protocol list can formally be described as
prtcl-list = KMID *(";" KMID)
where KMID is as defined in Section 2.
For example, if the offered protocols are MIKEY and two yet-to-be-
invented protocols KEYP1, KEYP2, the SDP is:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey <data1>
a=key-mgmt:keyp1 <data2>
a=key-mgmt:keyp2 <data3>
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
The protocol list, "mikey;keyp1;keyp2", would be generated from
the SDP description and used as input to each specified key
management protocol (together with the data for that protocol).
Each of the three protocols includes this protocol identifier
list in its authentication coverage (according to its protocol
specification).
If more than one protocol are supported by the offerer, it is
RECOMMENDED that all acceptable protocols are included in the first
offer, rather than making single, subsequent alternative offers in
response to error messages, see "Security Considerations".
3.5. Example scenarios
Example 1 (SIP) Example 1 (SIP)
A SIP call is taking place between Alice and Bob. Alice sends an A SIP call is taking place between Alice and Bob. Alice sends an
Invite message consisting of the following offer: Invite message consisting of the following offer:
v=0 v=0
o=alice 2891092738 2891092738 IN IP4 w-land.example.com o=alice 2891092738 2891092738 IN IP4 w-land.example.com
s=Cool stuff s=Cool stuff
e=alice@w-land.example.com e=alice@w-land.example.com
skipping to change at page 11, line 35 skipping to change at page 13, line 4
keymgmt: prot=mikey; data="skaoqDeMkdwRW278HjKVB..." keymgmt: prot=mikey; data="skaoqDeMkdwRW278HjKVB..."
The server processes the request including checking the validity of The server processes the request including checking the validity of
the key management header. the key management header.
RTSP/1.0 200 OK RTSP/1.0 200 OK
CSeq: 313 CSeq: 313
Session: 12345678 Session: 12345678
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057; Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057;
server_port=5000-5001 server_port=5000-5001
The RTSP then proceeds as usual (with e.g. a SETUP message for the The RTSP then proceeds as usual (with e.g. a SETUP message for the
video followed by a PLAY message). video followed by a PLAY message).
4. Adding further Key management protocols 4. Adding further Key management protocols
This framework cannot be used with all key management protocols. The This framework cannot be used with all key management protocols. The
key management protocol needs to comply with the requirements key management protocol needs to comply with the requirements
described in Section 3. To be able to use a key management protocol described in Section 3. In addition to this, the following needs to
with this framework, the following MUST be specified: be defined:
* the key management protocol identifier that should be used in the * The key management protocol identifier to be used as the protocol
protocol identifier fields in both SDP and RTSP (e.g. "mikey" for identifier should be registered at IANA according to Section 6
MIKEY). (e.g. "mikey" for MIKEY).
* the information the key management needs from SDP and RTSP (Section * The information that the key management needs from SDP and RTSP,
3 gives a guideline of what SDP and RTSP needs from the key and vice versa, as described in Section 3. The exact API is
management). The exact API is implementation specific, but it implementation specific, but it SHOULD at least support to
SHOULD at least support to exchange the specified information. exchange the specified information.
Note that in particular, the key management MUST always be given * The key management protocol to be added MUST be such, that the
the protocol identifier(s) of the key management protocol(s) processing in Section 3 (describing its interactions with SDP and
included in the offer in the correct order as they appear. RTSP) can be applied. Note in particular, Section 3.4 requires
each key management protocol to specify how the list of protocol
identifiers is authenticated inside that key management protocol.
The key management MUST always be given the protocol identifier(s)
of the key management protocol(s) included in the offer in the
correct order as they appear.
The key management data MUST be base64 encoded in the SDP and RTSP Finally, it is obviously crucial to analyze possible security
fields. Therefore, considerations of possible conversion from the implications induced by the introduction of a new key management
normal key management representation to base64 SHOULD be taken into protocol in the described framework.
account.
5. Security Considerations 5. Security Considerations
The nature of this document is to allow SDP and RTSP to support As a general practice, it is a good thing, not only to try to secure
negotiation of the security of the media sessions. It is therefore the session, but also to secure the session setup. However, the
not a primary intention of this document to describe possible security of the session setup might not possible on an end-to-end
security solutions or to define possible security problems. The basis, but the setup may require to be protected on a hop-by-hop
defined SDP and RTSP extensions are not believed to introduce any new basis (this is generally the case for SIP/SDP when intermediate
security risks to SDP and RTSP, if used as specified. proxies needs to obtain information about the sessions etc, c.f.
[E2M]). In fact, the focus of this framework is mainly when end-to-
end protection of the session setup is not used, but where the media
streams needs to be end-to-end protected. General security
considerations for the session setup can be found in SDP [SDPnew],
SIP [SIP], and RTSP [RTSP].
Note that the purpose of the key management fields is to provide The security will also depend on the encapsulated level of security
information to secure the media streams. Under the assumption that the key management protocol offers. It follows that, under the
the key management schemes are secure, the SDP can be passed along assumption that the key management schemes are secure, the SDP can be
unprotected without affecting the key management, and the media passed along unprotected without affecting the key management as
streams will still be secure even if some attackers gained knowledge such, and the media streams will still be secure even if some
of the SDP contents. attackers gained knowledge of the SDP contents. Further security
considerations can be found for each key management protocol (for
MIKEY these can be found in [MIKEY]).
However, if the SDP messages are not sent authenticated between the However, if the SDP messages are not sent authenticated between the
parties, it is possible for an active attacker to change attributes parties, it is possible for an active attacker to change attributes
without being detected. As the key management protocol may without being detected. As the key management protocol may
(indirectly) rely on some of the session information from SDP (e.g., (indirectly) rely on some of the session information from SDP (e.g.,
address information), an attack on SDP may have indirect consequences address information), an attack on SDP may have indirect consequences
on the key management. Even if the key management protocol does not on the key management. Even if the key management protocol does not
rely on parameters of SDP and will not be affected by manipulation of rely on parameters of SDP and will not be affected by manipulation of
these, different DoS attacks aimed at SDP (e.g. the SIMCAP these, different DoS attacks aimed at SDP (e.g. the SIMCAP
extensions) may lead to undesired interruption in the setup. extensions) may lead to undesired interruption in the setup.
In general, it is therefore a good thing, not only to try to secure The use of multiple key management protocols in the same offer may
the session, but also to secure the session setup. However, the open up the possibility of a bidding-down attack, as specified in
security of the session setup might not possible on an end-to-end Section 3.4. To exclude such possibility, the authentication of the
basis, but may require to be protected on a hop-by-hop basis (this is protocol identifier list is used. Note though, that the security
generally the case for SIP/SDP when intermediate proxies needs to level of the authenticated protocol identifier will be as high (or
obtain information about the sessions etc). In fact, the focus of low), as the "weakest" protocol. Therefore, it is discouraged to
this framework is mainly when end-to-end protection of the session offer protocols with too different security levels.
setup is not used, but where the media streams needs to be end-to-end
protected.
Note that it is impossible to assure the authenticity of a declined Note that it is impossible to assure the authenticity of a declined
offer, since even if it comes from the true respondent, the fact that offer, since even if it comes from the true respondent, the fact that
the answerer declines the offer usually means that he does not the answerer declines the offer usually means that he does not
support the protocol(s) offered, and consequently cannot be expected support the protocol(s) offered, and consequently cannot be expected
to authenticate the response either. This means that if the initiator to authenticate the response either. This means that if the initiator
is unsure of which protocol(s) the responder supports, we RECOMMEND is unsure of which protocol(s) the responder supports, we RECOMMEND
that the initiator offers all acceptable protocols in a single offer. that the initiator offers all acceptable protocols in a single offer.
If not, this opens up the possibility for a "man-in-the-middle" If not, this opens up the possibility for a "man-in-the-middle"
(MITM) to affect the outcome of the eventually agreed upon protocol, (MITM) to affect the outcome of the eventually agreed upon protocol,
by faking unauthenticated error messages until the initiator by faking unauthenticated error messages until the initiator
eventually offers a protocol "to the liking" of the MITM. This is not eventually offers a protocol "to the liking" of the MITM. This is not
really a security problem, but rather a mild form of denial of really a security problem, but rather a mild form of denial of
service that can be avoided by following the above recommendation. service that can be avoided by following the above recommendation. In
the case that the response declines any security (therefore there is
impossibility of authenticating it), the session setup SHALL be
aborted.
6. IANA Considerations 6. IANA Considerations
6.1. SDP Attribute Registration 6.1. SDP Attribute Registration
A new SDP attribute needs to be registered for the purpose of key A new SDP attribute needs to be registered for the purpose of key
management protocol integration with SDP. management protocol integration with SDP.
Contact: Fredrik Lindholm Contact: Fredrik Lindholm
mailto: fredrik.lindholm@ericsson.com mailto: fredrik.lindholm@ericsson.com
tel: +46 8 58531705 tel: +46 8 58531705
SDP Attribute ("att-field"): SDP Attribute ("att-field"):
Name: key-mgmt Name: key-mgmt
Long form: key management protocol Long form: key management protocol
Type of name: att-field Type of name: att-field
Type of attribute: Media and session level Type of attribute: Media and session level
Purpose: See RFC xxxx, Section 2. Purpose: See RFC xxxx, Section 2.
Reference: RFC xxxx, Section 2.1 Reference: RFC xxxx, Section 2.1
Values: See registrations below Values: See Section 6.3
6.2. Protocol Identifier Registration 6.2. RTSP Header Registration
This document defines one new name space associated with the above A new RTSP Header needs to be registered for the purpose of key
registered key-mgmt attribute i.e., the protocol identifier (see also management protocol integration with RTSP.
Section 2.1 and Section 2.2).
A new registry needs to be set up for "prtcl-id" parameter of the Following the guidelines of [RTSP], the registration is defined as
"key-mgmt" attribute, with the following registration created follows:
initially: "mikey".
Header name: keymgmt
Header syntax: see RFC xxxx, Section 2.2
Intended usage: see RFC xxxx, Section 2.2
Proxy treatment: Proxies SHOULD forward the header
Purpose: see RFC xxxx, Section 2
6.3. Protocol Identifier Registration
This document defines one new name space associated with the protocol
identifier, KMID, defined in Section 2 to be used with the above
registered key-mgmt attributes in SDP and RTSP.
A new registry needs to be set up for the KMID parameter, with the
following registration created initially: "mikey".
Contact: Fredrik Lindholm Contact: Fredrik Lindholm
mailto: fredrik.lindholm@ericsson.com mailto: fredrik.lindholm@ericsson.com
tel: +46 8 58531705 tel: +46 8 58531705
Value name: mikey Value name: mikey
Long name: Multimedia Internet KEYing Long name: Multimedia Internet KEYing
Purpose: Usage of MIKEY with the key-mgmt attribute Purpose: Usage of MIKEY with the key-mgmt attribute
Reference: Section 7 in RFC yyyy Reference: Section 7 in RFC yyyy
Further entries may be registered according to the "Specification
Note that this registration will imply that the protocol identifier,
KMID, name space will be shared between SDP and RTSP.
Further values may be registered according to the "Specification
Required" policy as defined in [RFC2434]. Each new registration needs Required" policy as defined in [RFC2434]. Each new registration needs
to indicate the parameter name and the syntax of possible additional to indicate the parameter name, and register it within IANA. Note
arguments. Note that the parameter name is case sensitive and it is that the parameter name is case sensitive and it is recommended that
recommended that the name should be in lower case letters. For each the name should be in lower case letters. For each new registration,
new registration, it is mandatory that a permanent, stable, and it is mandatory that a permanent, stable, and publicly accessible
publicly accessible document exists that specifies the semantics of document exists that specifies the semantics of the registered
the registered parameter, the syntax and semantics of its parameters parameter and the requested details of interaction between the key
as well as all the requested details of interaction between the key management protocol and SDP, as specified in RFC xxxx.
management protocol and SDP, as specified in this document.
8. Acknowledgments The registration itself of new values should be sent to IANA.
Registrations should include the following information:
* Contact: the contact name and email address
* Value name: the name of the value being registered (which MUST
comply with the KMID as defined in Section 2)
* Long Name: long-form name in English (optional)
* Purpose: short explanation of the purpose of the registered name.
* Reference: a reference to the specification (e.g. RFC number) of
the draft providing the usage guidelines in accordance to Section
4 (and also complying to the specified requirements).
Thanks to: Rolf Blom, Magnus Westerlund, and the rest involved in the 7. Acknowledgments
MMUSIC WG and the MSEC WG.
A special thanks to Joerg Ott and Colin Perkins. Thanks to: Rolf Blom and Magnus Westerlund. A special thanks to Joerg
Ott and Colin Perkins.
9. Author's Addresses 8. Author's Addresses
Jari Arkko Jari Arkko
Ericsson Ericsson
02420 Jorvas Phone: +358 40 5079256 02420 Jorvas Phone: +358 40 5079256
Finland Email: jari.arkko@ericsson.com Finland Email: jari.arkko@ericsson.com
Elisabetta Carrara Elisabetta Carrara
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040 SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@ericsson.com Sweden EMail: elisabetta.carrara@ericsson.com
skipping to change at page 15, line 5 skipping to change at page 17, line 5
Mats Naslund Mats Naslund
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739 SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@ericsson.com Sweden EMail: mats.naslund@ericsson.com
Karl Norrman Karl Norrman
Ericsson Research Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502 SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@ericsson.com Sweden EMail: karl.norrman@ericsson.com
10. References 9. References
10.1. Normative References 9.1. Normative References
[OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with [OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
the Session Description Protocol (SDP)", IETF, RFC 3264. the Session Description Protocol (SDP)", IETF, RFC 3264.
[RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time [RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
Streaming Protocol (RTSP)", IETF, RFC 2326. Streaming Protocol (RTSP)", IETF, RFC 2326.
[RFC2119] Bradner, S. "Key words for use in RFCs to Indicate [RFC2119] Bradner, S. "Key words for use in RFCs to Indicate
Requirement Levels", IETF, RFC 2119. Requirement Levels", IETF, RFC 2119.
[SDPnew] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session [SDPnew] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", Internet Draft, IETF, Work in progress Description Protocol", Internet Draft, IETF, Work in progress
(MMUSIC), draft-ietf-mmusic-sdp-new-13.txt. (MMUSIC), draft-ietf-mmusic-sdp-new-13.txt.
[SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J., [SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
"SIP: Session Initiation Protocol", IETF, RFC 3261. "SIP: Session Initiation Protocol", IETF, RFC 3261.
[RFC2234] Crocker, D. and Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2434] Narten, T. and Alvestrand, H., "Guidelines for Writing an [RFC2434] Narten, T. and Alvestrand, H., "Guidelines for Writing an
IANA Considerations Section in RFCs", IETF, RFC 2434. IANA Considerations Section in RFCs", IETF, RFC 2434.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", IETF, RFC 3548. Encodings", IETF, RFC 3548.
10.2. Informative References 9.2. Informative References
[E2M] Ono, K. and Tachimoto, S., "End-to-middle security in the
Session Initiation Protocol (SIP)", Internet Draft, IETF, Work in
Progress.
[KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication [KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication
Service (V5)", IETF, RFC 1510. Service (V5)", IETF, RFC 1510.
[MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and [MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
Norrman, K., "MIKEY: Multimedia Internet KEYing", IETF, RFC yyyy, Norrman, K., "MIKEY: Multimedia Internet KEYing", IETF, RFC yyyy,
[Internet Draft, Work in progress (MSEC)]. [Internet Draft, Work in progress (MSEC)].
[SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M, [SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M,
Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol", Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol",
skipping to change at page 16, line 21 skipping to change at page 18, line 29
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This Internet-Draft expires in February 2004. This Internet-Draft expires in April 2004.
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/