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RFC 4738

MIKEY-RSA-R: An Additional Mode of Key Distribution in Multimedia Internet KEYing (MIKEY)

Pages: 19
Proposed Standard
Updates:  3830

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Network Working Group                                        D. Ignjatic
Request for Comments: 4738                                       Polycom
Updates: 3830                                                 L. Dondeti
Category: Standards Track                                       QUALCOMM
                                                                F. Audet
                                                                  P. Lin
                                                           November 2006

          MIKEY-RSA-R: An Additional Mode of Key Distribution
                 in Multimedia Internet KEYing (MIKEY)

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 IETF Trust (2006).


The Multimedia Internet Keying (MIKEY) specification describes several modes of key distribution solution that address multimedia scenarios (e.g., SIP calls and Real Time Streaming Protocol (RTSP) sessions) using pre-shared keys, public keys, and optionally a Diffie-Hellman key exchange. In the public-key mode, the Initiator encrypts a random key with the Responder's public key and sends it to the Responder. In many communication scenarios, the Initiator may not know the Responder's public key, or in some cases the Responder's ID (e.g., call forwarding) in advance. We propose a new MIKEY mode that works well in such scenarios. This mode also enhances the group key management support in MIKEY; it supports member-initiated group key download (in contrast to group manager pushing the group keys to all members). This document updates RFC 3830 with the RSA-R mode.
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Table of Contents

1. Introduction ....................................................3 1.1. Terminology Used in This Document ..........................3 2. Motivation ......................................................3 2.1. Description of the MIKEY Modes .............................3 2.2. Use Case Motivating the Proposed Mode ......................5 3. A New MIKEY-RSA Mode: MIKEY-RSA-R ...............................5 3.1. Outline ....................................................5 3.2. Group Communication Using the MIKEY RSA-R Mode .............6 3.3. Preparing RSA-R Messages ...................................6 3.4. Components of the I_MESSAGE ................................6 3.5. Processing the I_MESSAGE ...................................8 3.6. Components of the R_MESSAGE ................................9 3.7. Processing the R_MESSAGE ..................................10 3.8. Certificate Handling ......................................10 3.9. Additions to RFC 3830 Message Types and Other Values ......11 3.9.1. Modified Table 6.1a from RFC 3830 ..................11 3.9.2. Modified Table 6.12 from RFC 3830 ..................12 3.9.3. Modified Table 6.15 from RFC 3830 ..................12 4. Applicability of the RSA-R and RSA Modes .......................13 4.1. Limitations ...............................................13 5. Security Considerations ........................................14 5.1. Impact of the Responder Choosing the TGK ..................15 5.2. Updates to Security Considerations in RFC 3830 ............15 6. IANA Considerations ............................................15 7. Acknowledgments ................................................16 8. References .....................................................16 8.1. Normative References ......................................16 8.2. Informative References ....................................16
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1. Introduction

The MIKEY protocol [RFC3830] has three different methods for key transport or exchange: a pre-shared key mode (PSK), a public-key (RSA) mode, and an optional Diffie-Hellman exchange (DHE) mode. In addition, there is also an optional DH-HMAC mode [RFC4650], bringing the total number of modes to four. The primary motivation for the MIKEY protocol design is low-latency requirements of real-time communication, and thus all the exchanges finish in one-half to 1 roundtrip; note that this offers no room for security parameter negotiation of the key management protocol itself. In this document, we note that the MIKEY modes defined in [RFC3830] and [RFC4650] are insufficient to address some deployment scenarios and common use cases, and we propose a new mode called MIKEY-RSA in Reverse mode, or simply MIKEY-RSA-R. This document updates RFC 3830 with the addition of this new mode to that specification.

1.1. Terminology 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 BCP 14, RFC 2119 [RFC2119]. Furthermore, this document reuses the terminology of the MIKEY specification [RFC3830].

2. Motivation

As noted in the introduction, the MIKEY specification and other proposals define four different modes of efficient key management for real-time applications. Those modes differ from each other in either the authentication method of choice (public-key, or symmetric shared key-based), or the key establishment method of choice (key download, or key agreement using a Diffie-Hellman exchange). We summarize these modes below, including their advantages and shortcomings. We then discuss the use cases where these modes are unusable or inefficient.

2.1. Description of the MIKEY Modes

The PSK mode requires that the Initiator and the Responder have a common secret key established offline. In this mode, the Initiator selects a TEK Generation Key (TGK), encrypts it with a key derived from the PSK, and sends it to the Responder as part of the first message, namely, I_MESSAGE. The I_MESSAGE is replay protected with timestamps, and integrity protected with another key derived from the PSK. An optional Verification message from the Responder to the
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   Initiator provides mutual authentication.  This mode does not scale
   well as it requires pre-establishment of a shared key between
   communicating parties; for example, consider the use cases where any
   user may want to communicate to any other user in an Enterprise or
   the Internet at large.  The RSA mode might be more suitable for such

   In the RSA mode, the Initiator selects a TGK, encrypts and
   authenticates it with an envelope key, and sends it to the Responder
   as part of the I_MESSAGE.  The Initiator includes the envelope key,
   encrypted with the Responder's public key, in the I_MESSAGE.  The
   I_MESSAGE is replay protected with timestamps, and signed with the
   Initiator's public key.  The Initiator's ID, Certificate (CERT), and
   the Responder's ID may be included in the I_MESSAGE.  If the
   Initiator knows several public keys of the Responder, it can indicate
   the key used in the optional CHASH payload.  An optional Verification
   message from the Responder to the Initiator provides mutual
   authentication.  The RSA mode works well if the Initiator knows the
   Responder's ID and the corresponding CERT (or can obtain the CERT
   independent of the MIKEY protocol).  RFC 3830 suggests that an
   Initiator, in the event that it does not have the Responder's CERT,
   may obtain the CERT from a directory agent using one or more
   roundtrips.  However, in some cases, the Initiator may not even know
   the Responder's ID in advance, and because of that or for other
   reasons cannot obtain the Responder's CERT.

   In addition to the case where the Responder may have several IDs,
   some applications may allow for the Responder's ID to change
   unilaterally, as is typical in telephony (e.g., forwarding).  In
   those cases and in others, the Initiator might be willing to let the
   other party establish identity and prove it via an Initiator-trusted
   third party (e.g., a Certification Authority (CA)).

   The DH mode or the DH-HMAC mode of MIKEY might be useful in cases
   where the Initiator does not have access to the Responder's exact
   identity and/or CERT.  In these modes, the two parties engage in an
   authenticated DH exchange to derive the TGK.  On the downside, the DH
   modes have higher computational and communication overhead compared
   to the RSA and the PSK modes.  More importantly, these modes are
   unsuitable for group key distribution.  The DH-HMAC mode also
   requires establishment of PSKs between all possible communicating
   entities and thus has similar scaling issues as any PSK-based key
   management protocol.

   In summary, in some communication scenarios -- where the Initiator
   might not have the correct ID and/or the CERT of the Responder --
   none of the MIKEY modes described in [RFC3830] or [RFC4650] are
   suitable and efficient for multimedia session key establishment.
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2.2. Use Case Motivating the Proposed Mode

In addition to the issues listed above, there are some types of applications that motivate the new MIKEY mode design proposed in this document. Note that in the MIKEY-RSA mode (as in case of the PSK mode), the Initiator proposes the session security policy and chooses the TGK. However, it is also possible that the Initiator wants to allow the Responder to specify the security policy and send the TGK. Consider for example, the case of a conferencing scenario where the convener sends an invitation to a group of people to attend a meeting. The procedure then might be for the invitees to request group key material from the convener by sending a MIKEY I_MESSAGE. Thus, in the MIKEY definition of initiators and responders, the Initiator is asking the Responder for keying material. Note that this mode of operation is in line with the MSEC group key management architecture [RFC4046].


3.1. Outline

The proposed MIKEY mode requires 1 full roundtrip. The Initiator sends a signed I_MESSAGE to the intended Responder requesting the Responder to send the traffic keying material. The I_MESSAGE MAY contain the Initiator's CERT or a link (URL) to the CERT, and similarly the Responder's reply, R_MESSAGE, MAY contain the Responder's CERT or a link to it. The Responder can use the Initiator's public key from the CERT in the I_MESSAGE to send the encrypted TGK in the R_MESSAGE. Upon receiving the R_MESSAGE, the Initiator can use the CERT in the R_MESSAGE to verify whether the Responder is in fact the party that it wants to communicate to, and accept the TGK. We refer to this protocol as MIKEY-RSA in the reverse mode, or simply as MIKEY-RSA-R. The MIKEY-RSA-R mode exchange is defined as follows: Initiator Responder --------- --------- I_MESSAGE = HDR, T, [RAND], [IDi|CERTi], [IDr], {SP}, SIGNi R_MESSAGE = HDR, [GenExt(CSB_ID)], T, [RAND], [IDr|CERTr], [SP], KEMAC, PKE, SIGNr Figure 1: MIKEY-RSA-R Unicast Mode
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3.2. Group Communication Using the MIKEY RSA-R Mode

For group conferencing using MIKEY RSA-R mode, the members receive an invitation to initiate MIKEY with the group key server to download the secure session information. In this case, the Responder is either the group sender or group key server. Group members request group policy and keying material as MIKEY RSA-R Initiators. Initiators MUST NOT send the SP payload. The Responder sends all the payloads necessary to distribute the secure group policy as well as payloads used in the group key derivation: specifically, the SP payload is used to convey the session policy, and the GenExt(CSB-ID), TGK, and the RAND payloads selected by the Responder and included in the R_Message are used to compute the Secure Realtime Transport Protocol (SRTP) session keys. MIKEY RSA-R for group communication: Initiator Responder --------- --------- I_MESSAGE = HDR, T, [RAND], [IDi|CERTi], [IDr], SIGNi R_MESSAGE = HDR, GenExt(CSB_ID), T, RAND, [IDr|CERTr], SP, KEMAC, PKE, SIGNr Figure 2: MIKEY-RSA-R in Group Mode Note that the SP payload in the I_MESSAGE is not present. In the R_MESSAGE, the CSB_ID, RAND, and SP payloads are not optional.

3.3. Preparing RSA-R Messages

Preparation and parsing of RSA-R messages are as described in Sections 5.2 and 5.3 of RFC 3830. Error handling is described in Section 5.1.2 and replay protection guidelines are in Section 5.4 of RFC 3830. In the following, we describe the components of RSA-R messages and specify message processing and parsing rules in addition to those in RFC 3830.

3.4. Components of the I_MESSAGE

MIKEY-RSA-R requires a full roundtrip to download the TGKs. The I_MESSAGE MUST have the MIKEY HDR and the timestamp payload for replay protection. The HDR field contains a CSB_ID (Crypto Session Bundle ID) randomly selected by the Initiator. The V bit MUST be set to '1' and ignored by the Responder, as a response is MANDATORY in this mode. The Initiator SHOULD indicate the number of CSs supported, and SHOULD fill in the CS ID map type and CS ID info
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   fields for the RTP/RTCP streams it originates.  This is because the
   sender of the streams chooses the SSRC that is carried in the CS ID
   info field; see Section 6.1.1 of RFC 3830.  The exception to
   Initiators not specifying SSRC values is to allow the Responder to
   pick them to avoid SSRC collisions.  Initiators of MIKEY messages
   that do not originate RTP streams MUST specify a '0' as the number of
   CSs supported.  This typically applies to group communication and to
   the entities in the listen-only mode.

   The I_MESSAGE MUST be signed by the Initiator following the procedure
   to sign MIKEY messages specified in RFC 3830.  The SIGNi payload
   contains this signature.  Thus, the I_MESSAGE is integrity and replay

   The RAND payload SHOULD be included in the I_MESSAGE when MIKEY-RSA-R
   mode is used for unicast communication.  The reason for recommending
   the inclusion of the RAND payload in the I_MESSAGE for unicast
   communication is to allow the Initiator to contribute entropy to the
   key derivation process (in addition to the CSB_ID).  When the RAND
   payload is not included, the Initiator will be relying on the
   Responder to supply all the entropy for SRTP key generation, which is
   in fact similar (but with the reversal of roles) to the MIKEY-RSA
   mode, where the Responder supplies all the entropy.

   The RAND payload MAY be included when MIKEY-RSA-R is used to
   establish group keys.  However, the RAND payload in the I_MESSAGE
   MUST NOT be used for MIKEY key generation, in case of group
   communication.  The Responder MUST include a RAND payload in the
   R_MESSAGE for TEK generation from a TGK when MIKEY-RSA-R is used for
   group communication.

   IDi and CERTi SHOULD be included, but they MAY be left out when it is
   expected that the peer already knows the Initiating party's ID (or
   can obtain the certificate in some other manner).  For example, this
   could be the case if the ID is extracted from SIP.  For certificate
   handling, authorization, and policies, see Sections 4.3 and 6.7 of
   RFC 3830.  If CERTi is included, it MUST correspond to the private
   key used to sign the I_MESSAGE.

   If the Responder has multiple identities, the Initiator MAY also
   include the specific identity, IDr, of the Responder with whom
   communication is desired.  If the Initiator's policy does not allow
   acceptance of an R_MESSAGE from any entity other than one that can
   assert a specific identity, the Initiator MUST include that specific
   identity in an IDr payload in the I_MESSAGE.
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   The Initiator MAY also send security policy (SP) payload(s)
   containing all the security policies that it supports.  If the
   Responder does not support any of the policies included, it SHOULD
   reply with an Error message of type "Invalid SPpar" (Error no. 10).
   The Responder has the option not to send the Error message in MIKEY
   if a generic session establishment failure indication is deemed
   appropriate and communicated via other means (see Section 4.1.2 of
   [RFC4567] for additional guidance).

   SIGNi is a signature covering the Initiator's MIKEY message,
   I_MESSAGE, using the Initiator's signature key (see Section 5.2 of
   RFC 3830 for the exact definition).  The signature assures the
   Responder that the claimed Initiator has indeed generated the
   message.  This automatically provides message integrity as well.

3.5. Processing the I_MESSAGE

Upon receiving an I_MESSAGE of the RSA-R format, the Responder MUST respond with one of the following messages: o The Responder SHOULD send an Error message "Message type not supported" (Error no. 13), if it cannot correctly parse the received MIKEY message. Error message format is as specified in Section 5.1.2 of RFC 3830. Error no. 13 is not defined in RFC 3830, and so RFC 3830 compliant implementations MAY return "an unspecified error occurred" (Error no. 12). o The Responder MUST send an R_MESSAGE, if SIGNi can be correctly verified and the timestamp is current; if an SP payload is present in the I_MESSAGE the Responder MUST return one of the proposed security policies that matches the Responder's local policy. o If a RAND payload is present in the I_MESSAGE, both sides use that RAND payload as the RAND value in the MIKEY key computation. In case of multicast, if a RAND payload is present in the I_MESSAGE, the Responder SHOULD ignore the payload. In any case, the R_MESSAGE for multicast communication MUST contain a RAND payload and that RAND payload is used for key computation. o The rest of the error message rules are as described in Section 5.1.2 of RFC 3830, and message processing rules are as described in Section 5.3 of RFC 3830.
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3.6. Components of the R_MESSAGE

The HDR payload in the R_MESSAGE is formed following the procedure described in RFC 3830. Specifically, the CSB_ID in the HDR payload MUST be the same as the one in the HDR of the I_MESSAGE. The Responder MUST fill in the number of CSs and the CS ID map type and CS ID info fields of the HDR payload. For group communication, all the members MUST use the same CSB_ID and CS ID in computing the traffic keying material. Therefore, for group key establishment, the Responder MUST include a General Extension Payload containing a new CSB_ID in the R_MESSAGE. If a new CSB_ID is present in the R_MESSAGE, the Initiator and the Responder MUST use that value in key material computation. Furthermore, the CS ID map type and CS ID map info MUST be populated by the Responder. The General Extension Payload carrying a CSB_ID MUST NOT be present in case of unicast communication. The T payload is exactly the same as that received in the I_MESSAGE. If the I_MESSAGE did not include the RAND payload, it MUST be present in the R_MESSAGE. In case it has been included in the I_MESSAGE, it MUST NOT be present in the R_MESSAGE. In group communication, the Responder always sends the RAND payload and in unicast communication, either the Initiator or the Responder (but not both) generate and send the RAND payload. IDr and CERTr SHOULD be included, but they MAY be left out when it can be expected that the peer already knows the other party's ID (or can obtain the certificate in some other manner). For example, this could be the case if the ID is extracted from SIP. For certificate handling, authorization, and policies, see Section 4.3. of RFC 3830. If CERTr is included, it MUST correspond to the private key used to sign the R_MESSAGE. An SP payload MAY be included in the R_MESSAGE. If an SP payload was in the I_MESSAGE, then the R_MESSAGE MUST contain an SP payload specifying the security policies of the secure RTP session being negotiated. More specifically, the Initiator may have provided multiple options, but the Responder MUST choose one option per Security Policy Parameter. The KEMAC payload contains a set of encrypted sub-payloads and a MAC: KEMAC = E(encr_key, IDr || {TGK}) || MAC. The first payload (IDr) in KEMAC is the identity of the Responder (not a certificate, but generally the same ID as the one specified in the certificate). Each of the following payloads (TGK) includes a TGK randomly and independently chosen by the Responder (and possible other related
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   parameters, e.g., the key lifetime).  The encrypted part is then
   followed by a MAC, which is calculated over the KEMAC payload.  The
   encr_key and the auth_key are derived from the envelope key, env_key,
   as specified in Section 4.1.4. of RFC 3830.  The payload definitions
   are specified in Section 6.2 of RFC 3830.

   The Responder encrypts and integrity protects the TGK with keys
   derived from a randomly or pseudo-randomly chosen envelope key, and
   encrypts the envelope key itself with the public key of the
   Initiator.  The PKE payload contains the encrypted envelope key,
   env_key: PKE = E(PKi, env_key).  PKi denotes the Initiator's public
   key.  Note that, as suggested in RFC 3830, the envelope key MAY be
   cached and used as the PSK for re-keying.

   To compute the signature that goes in the SIGNr payload, the
   Responder MUST sign:

   R_MESSAGE (excluding the SIGNr payload itself) || IDi || IDr || T.

   Note that the added identities and timestamp are identical to those
   transported in the ID and T payloads.

3.7. Processing the R_MESSAGE

In addition to the processing rules in RFC 3830, the following rules apply to processing of the R_MESSAGE of MIKEY RSA-R mode. If the I_MESSAGE contained a RAND payload, the Initiator MUST silently discard an R_MESSAGE that contains a RAND payload. Similarly, if the I_MESSAGE did not contain a RAND payload, the Initiator MUST silently discard an R_MESSAGE that does not contain a RAND payload. If the SP payload contains a policy not specified in the SP message, if present, in the I_MESSAGE, such an R_MESSAGE MUST be discarded silently.

3.8. Certificate Handling

If a Certificate payload is present, the X.509v3 URL Cert type from Table 6.7.b [RFC3830] is the default method in RSA-R mode and MUST be implemented. The HTTP URL to fetch a certificate as specified in RFC 2585 [RFC2585] MUST be supported. Devices are not required to support the FTP URLs. When retrieving data from the URL, application/pkix-cert MIME type with X.509 certificates DER-encoded MUST be supported.
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   The RECOMMENDED way of doing certificate validation is by using OCSP
   as specified by RFC 2560 [RFC2560].  When OCSP is used and nextUpdate
   time is present in the response, it defines how long the certificate
   can be considered valid and cached.  If OCSP is not supported or
   nextUpdate time is not present in the response, the certificate cache
   timeout is a matter of local policy.

   The communicating peers (such as SIP User Agents for instance) MAY
   choose to create a URL pointing to certificate files residing on
   themselves or by appending their ID and a ".cer" extension to a
   provisioned root path to the certificate.  Other methods MAY also be
   used, subject to local policy.

3.9. Additions to RFC 3830 Message Types and Other Values

This document introduces two new message types (Table 6.1a of RFC 3830), an Error no (Table 6.12 of RFC 3830), and a general extension payload (Table 6.15 of RFC 3830). This section specifies those additions.

3.9.1. Modified Table 6.1a from RFC 3830

Modified Table 6.1a from RFC 3830: Data type | Value | Comment ------------------------------------------------------------------ Pre-shared | 0 | Initiator's pre-shared key message PSK ver msg | 1 | Verification message of a Pre-shared key msg Public key | 2 | Initiator's public-key transport message PK ver msg | 3 | Verification message of a public-key message D-H init | 4 | Initiator's DH exchange message D-H resp | 5 | Responder's DH exchange message Error | 6 | Error message DHHMAC init | 7 | DH HMAC message 1 DHHMAC resp | 8 | DH HMAC message 2 RSA-R I_MSG | 9 | Initiator's RSA-R public-key message (NEW) RSA-R R_MSG | 10 | Responder's RSA-R public-key message (NEW) Figure 3: Table 6.1a from RFC 3830 (Revised)
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3.9.2. Modified Table 6.12 from RFC 3830

Modified Table 6.12 from RFC 3830: Error no | Value | Comment ------------------------------------------------------- Auth failure | 0 | Authentication failure Invalid TS | 1 | Invalid timestamp Invalid PRF | 2 | PRF function not supported Invalid MAC | 3 | MAC algorithm not supported Invalid EA | 4 | Encryption algorithm not supported Invalid HA | 5 | Hash function not supported Invalid DH | 6 | DH group not supported Invalid ID | 7 | ID not supported Invalid Cert | 8 | Certificate not supported Invalid SP | 9 | SP type not supported Invalid SPpar | 10 | SP parameters not supported Invalid DT | 11 | not supported Data type Unspecified error | 12 | an unspecified error occurred Unsupported | | message type | 13 | unparseable MIKEY message (NEW) Figure 4: Table 6.12 from RFC 3830 (Revised)

3.9.3. Modified Table 6.15 from RFC 3830

Modified Table 6.15 from RFC 3830: Type | Value | Comments --------------------------------------- Vendor ID | 0 | Vendor specific byte string SDP IDs | 1 | List of SDP key mgmt IDs (allocated for use in | | [RFC4567]) TESLA I-Key| 2 | [RFC4442] Key ID | 3 | information on type and identity of keys | | [RFC4563]) CSB_ID | 4 | Responder's modified CSB_ID (group mode) Figure 5: Table 6.15 from RFC 3830 (Revised)
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4. Applicability of the RSA-R and RSA Modes

MIKEY-RSA-R mode and RSA mode are both very useful: deciding on which mode to use depends on the application. The RSA-R mode is useful when you have reasons to believe that the Responder may be a different party than the one to which the MIKEY I_MESSAGE was sent. This is quite common in telephony and multimedia applications where the session or the call can be retargeted or forwarded. When the security policy allows it, leaving some flexibility for the Initiator to see who the Responder may turn out to be, before making the decision to continue or discontinue the session, may be appropriate. In such cases, the main objective of the Initiator's RSA-R message is to present its public key/ certificate to the Responder, and wait for a Responder to present its identity. The second scenario is when the Initiator already has the Responder's certificate but wants to allow the Responder to come up with all the keying material. This is applicable in conferences where the Responder is the key distributor and the Initiators contact the Responder to initiate key download. Notice that this is quite similar to the group key download model as specified in GDOI [RFC3547], GSAKMP [RFC4535], and GKDP [GKDP] protocols (also see [RFC4046]). The catch, however, is that the participating entities must know that they need to contact a well-known address as far as that conferencing group is concerned. Note that they only need the Responder's address, not necessarily its CERT. If the group members have the Responder's CERT, there is no harm; they simply do not need the CERT to compose the I_MESSAGE. The RSA mode is useful when the Initiator knows the Responder's identity and CERT. This mode is also useful when the key exchange is happening in an established session with a Responder (for example, when switching from a non-secure mode to a secure mode), and when the policy is such that it is only appropriate to establish a MIKEY session with the Responder that is targeted by the Initiator.

4.1. Limitations

The RSA-R mode may not easily support 3-way calling, under the assumptions that motivated the design. An extra message may be required compared to the MIKEY-RSA mode specified in RFC 3830. Consider that A wants to talk to B and C, but does not have B's or C's CERT. A might contact B and request that B supply a key for a 3-way call. Now if B knows C's CERT, then B can simply use the MIKEY-RSA mode (as defined in RFC 3830) to send the TGK to C. If not, then the solution is not straightforward. For instance, A might
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   ask C to contact B or itself to get the TGK, in effect initiating a
   3-way exchange.  It should be noted that 3-way calling is typically
   implemented using a bridge, in which case there are no issues (it
   looks like 3 point-to-point sessions, where one end of each session
   is a bridge mixing the traffic into a single stream).

5. Security Considerations

We offer a brief overview of the security properties of the exchange. There are two messages: the I_MESSAGE and the R_MESSAGE. The I_MESSAGE is a signed request by an Initiator requesting the Responder to select a TGK to be used to protect multimedia (e.g., Secure RTP or SRTP [RFC3711]) sessions. The message is signed, which assures the Responder that the claimed Initiator has indeed generated the message. This automatically provides message integrity as well. There is a timestamp in the I_MESSAGE, which when generated and interpreted in the context of the MIKEY specification assures the Responder that the request is live and not a replay. Indirectly, this also provides protection against a denial of service (DoS) attack in that the I_MESSAGE must itself be signed. The Responder, however, would have to verify the Initiator's signature and the timestamp, and thus would spend significant computing resources. It is possible to mitigate this by caching recently received and verified requests. Note that the I_MESSAGE in this method basically equals DoS protection properties of the DH method and not the public-key method as there are no payloads encrypted by the Responder's public key in the I_MESSAGE. If IDr is not included in the I_MESSAGE, the Responder will accept the message and a response (and state) would be created for the malicious request. The R_MESSAGE is quite similar to the I_MESSAGE in the MIKEY-RSA mode and has all the same security properties. When using the RSA-R mode, the Responder may be a different party than the one to which the MIKEY I_MESSAGE was sent. It is the responsibility of the Initiator to verify that the identity of the Responder is acceptable (based on its local policy) if it changes from the party to which the MIKEY I_MESSAGE was sent, and to take appropriate action based on the outcome. In some cases, it could be appropriate to accept a Responder's identity if it can be strongly authenticated; in other cases, a blacklist or a whitelist may be appropriate.
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   When both unicast and multicast streams need to be negotiated, it is
   RECOMMENDED to use multiple instances of MIKEY-RSA-R rather than a
   single instance in group mode.  This is to avoid potential key reuse
   with counter mode.

5.1. Impact of the Responder Choosing the TGK

In the MIKEY-RSA or PSK modes, the Initiator chooses the TGK, and the Responder has the option to accept the key or not. In the RSA-R mode for unicast communication, the RECOMMENDED mode of operation is for the Initiator and the Responder to contribute random information in generating the TEK (RAND from the Initiator and the TGK from the Responder). For group communication, the sender (MIKEY Responder) will choose the TGK and the RAND; note that it is in the interest of the sender to provide sufficient entropy to TEK generation since the TEK protects data sent by the Responder. Thus, in case of unicast communication, the RSA-R mode is slightly better than the RSA mode in that it allows the Initiator as well as the Responder to contribute entropy to the TEK generation process. This comes at the expense of the additional message. However, as noted earlier, the new mode needs the additional message to allow simpler provisioning.

5.2. Updates to Security Considerations in RFC 3830

MIKEY requires clock synchronization, and a secure network clock synchronization protocol SHOULD be used, e.g., [ISO3] or secure NTP [NTPv4]. RFC 3830 has additional notes on the security properties of the MIKEY protocol, key derivation functions, and other components.

6. IANA Considerations

The following IANA assignments were added to the MIKEY registry: Added to "Error payload name spaces:" Unsupported message type ------- 13 Added to "Common Header payload name spaces:" RSA-R I_MSG ------------- 9 RSA-R R_MSG ------------- 10
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      Added to "General Extensions payload name spaces:"

         CSB_ID ----------------- 4

7. Acknowledgments

Many thanks to Mark Baugher, Steffen Fries, Russ Housley, Cullen Jennings, and Vesa Lehtovirta for their reviews of earlier version of this document.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key Infrastructure Operational Protocols: FTP and HTTP", RFC 2585, May 1999. [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004.

8.2. Informative References

[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group Domain of Interpretation", RFC 3547, July 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm, "Multicast Security (MSEC) Group Key Management Architecture", RFC 4046, April 2005. [RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for Multimedia Internet KEYing (MIKEY)", RFC 4650, September 2006.
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   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP:  Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [GKDP]     Dondeti, L., "GKDP: Group Key Distribution Protocol", Work
              in Progress, March 2006.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.

   [RFC4442]  Fries, S. and H. Tschofenig, "Bootstrapping Timed
              Efficient Stream Loss-Tolerant Authentication (TESLA)",
              RFC 4442, March 2006.

   [RFC4563]  Carrara, E., Lehtovirta, V., and K. Norrman, "The Key ID
              Information Type for the General Extension Payload in
              Multimedia Internet KEYing (MIKEY)", RFC 4563, June 2006.

   [NTPv4]    Burbank, J., "The Network Time Protocol Version 4 Protocol
              Specification", Work in Progress, May 2006.

   [ISO3]     ISO, "ISO/IEC 18014 Information technology - Security
              techniques - Time-stamping services, Part 1-3", 2002.
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Authors' Addresses

Dragan Ignjatic Polycom 1000 W. 14th Street North Vancouver, BC V7P 3P3 Canada Phone: +1 604 982 3424 EMail: Lakshminath Dondeti QUALCOMM 5775 Morehouse drive San Diego, CA 92121 US Phone: +1 858 845 1267 EMail: Francois Audet Nortel 4655 Great America Parkway Santa Clara, CA 95054 US Phone: +1 408 495 3756 EMail: Ping Lin Nortel 250 Sidney St. Belleville, Ontario K8P3Z3 Canada Phone: +1 613 967 5343 EMail:
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