On the applicability of various MIKEY modes and extensions
draft-ietf-msec-mikey-applicability-08.txt

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Abstract

Multimedia Internet Keying - MIKEY - is a key management protocol that can be used for real-time applications. In particular, it has been defined focusing on the support of the Secure Real-time Transport Protocol. MIKEY itself is standardized within RFC3830 and defines four key distribution methods. Moreover, it is defined to allow extensions of the protocol. As MIKEY becomes more and more accepted, extensions to the base protocol arose, especially in terms of additional key distribution methods, but also in terms of payload enhancements.

This document provides an overview about the MIKEY base document in general as well as the existing extensions for MIKEY, which have been defined or are in the process of definition. It is intended as additional source of information for developers or architects to provide more insight in use case scenarios and motivations as well as advantages and disadvantages for the different key distribution schemes. The use cases discussed in this document are strongly related to dedicated SIP call scenarios providing challenges for key management in general among them media before SDP answer, forking, and shared key conferencing.

Table of Contents

1.  Introduction
2.  Terminology and Definitions
3.  MIKEY Overview
    3.1.  Pre-shared key protected distribution
    3.2.  Public Key encrypted key distribution
    3.3.  Diffie-Hellman key agreement protected with digital signatures
    3.4.  Unprotected key distribution
    3.5.  Diffie-Hellman key agreement protected with pre-shared secrets
    3.6.  SAML assisted DH-key agreement
    3.7.  Asymmetric key distribution with in-band certificate exchange
4.  Further MIKEY Extensions
    4.1.  ECC algorithms support
        4.1.1.  Elliptic Curve Integrated Encryption Scheme application in MIKEY
        4.1.2.  Elliptic Curve Menezes-Qu-Vanstone Scheme application in MIKEY
    4.2.  New MIKEY Payload for bootstrapping TESLA
    4.3.  MBMS extensions to the Key ID information type
    4.4.  OMA BCAST MIKEY General Extension Payload Specification
    4.5.  Supporting Integrity Transform carrying the Rollover Counter
5.  Selection and interworking of MIKEY modes
    5.1.  MIKEY and Early Media
    5.2.  MIKEY and Forking
    5.3.  MIKEY and Call Transfer/Redirect/Retarget
    5.4.  MIKEY and Shared Key Conferencing
    5.5.  MIKEY Mode Summary
6.  Transport of MIKEY messages
7.  MIKEY alternatives for SRTP security parameter negotiation
8.  Summary of MIKEY related IANA Registrations
9.  Security Considerations
10.  IANA Considerations
11.  Acknowledgments
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

Key distribution describes the process of delivering cryptographic keys to the required parties. MIKEY [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.), the Multimedia Internet Keying, has been defined focusing on support for the establishment of security context for the Secure Real-time Transport Protocol [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.). Note that RFC3830 is not restricted to be used for SRTP only, as it features a generic approach and allows for extensions to the key distribution schemes. Thus, it may also be used for security parameter negotiation for other protocols.

For MIKEY, meanwhile seven key distribution methods are described as there are:

• Symmetric key distribution as defined in [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) (MIKEY-PSK)
• Asymmetric key distribution as defined in [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) (MIKEY-RSA)
• Diffie-Hellman key agreement protected by digital signatures as defined in [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) (MIKEY-DHSIGN)
• Unprotected key distribution (MIKEY-NULL)
• Diffie-Hellman key agreement protected by symmetric pre-shared keys as defined in [RFC4650] (Euchner, M., “HMAC-Authenticated Diffie-Hellman for Multimedia Internet KEYing (MIKEY),” September 2006.) (MIKEY-DHHMAC)
• SAML assisted Diffie-Hellman key agreement as defined (not available as seperate document, but discussions are reflected within this document (MIKEY-DHSAML))
• Asymmetric key distribution (based on asymmetric encryption) with in-band certificate provision as defined in [RFC4738] (Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, “MIKEY-RSA-R: An Additional Mode of Key Distribution in Multimedia Internet KEYing (MIKEY),” November 2006.) (MIKEY-RSA-R)

Note that the latter three modes are extensions to MIKEY as there have been scenarios where none of the first four modes defined in [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) fits perfectly. There are further extensions to MIKEY comprising algorithm enhancements and a new payload definition supporting other protocols than SRTP.

Algorithm extensions are defined in the following document:

• ECC algorithms for MIKEY as defined in [I‑D.ietf‑msec‑mikey‑ecc] (Milne, A., “ECC Algorithms for MIKEY,” June 2007.)

Payload extensions are defined in the following documents:

• Bootstrapping TESLA, defining a new payload for the Timed Efficient Stream Loss-tolerant Authentication (TESLA) protocol [RFC4082] (Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, “Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction,” June 2005.) as defined in [RFC4442] (Fries, S. and H. Tschofenig, “Bootstrapping Timed Efficient Stream Loss-Tolerant Authentication (TESLA),” March 2006.)
• The Key ID information type for the general extension payload as defined in [RFC4563] (Carrara, E., Lehtovirta, V., and K. Norrman, “The Key ID Information Type for the General Extension Payload in Multimedia Internet KEYing (MIKEY),” June 2006.)
• OMA BCAST MIKEY General Extension Payload Specification, as defined in [RFC4909] (Dondeti, L., Castleford, D., and F. Hartung, “Multimedia Internet KEYing (MIKEY) General Extension Payload for Open Mobile Alliance BCAST LTKM/STKM Transport,” June 2007.)
• Integrity Transform Carrying Roll-over Counter for SRTP, as defined in [RFC4771] (Lehtovirta, V., Naslund, M., and K. Norrman, “Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP),” January 2007.). Note that this is rather an extension to SRTP and requires MIKEY to carry a new parameter, but is stated here for completeness.

This document provides an overview about RFC3830 and the relations to the different extensions to provide a framework when using MIKEY. It is intended as additional source of information for developers or architects to provide more insight in use case scenarios and motivations as well as advantages and disadvantages for the different key distribution schemes. The use cases discussed in this document are inspired by specific protocol workings of SIP that have proved to be problematic for a general key distribution mechanisms in general. These protocol workings are described in detail in Wing et al. [I‑D.ietf‑sip‑media‑security‑requirements] (Wing, D., Fries, S., Tschofenig, H., and F. Audet, “Requirements and Analysis of Media Security Management Protocols,” January 2009.) to include the following:

• Early Media respectively Media before SDP answer
• Forking
• Call Transfer/Redirect/Retarget
• Shared Key Conferencing

2.  Terminology and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

The following definitions have been taken from [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.):

(Data) Security Protocol:

the security protocol used to protect the actual data traffic. Examples of security protocols are IPsec and SRTP.

Data SA

Data Security Association information for the security protocol, including a TEK and a set of parameters/policies.

CS

Crypto Session, uni- or bi-directional data stream(s), protected by a single instance of a security protocol.

CSB

Crypto Session Bundle, collection of one or more Crypto Sessions, which can have common TGKs (see below) and security parameters.

CS ID

Crypto Session ID, unique identifier for the CS within a CSB.

CSB ID

Crypto Session Bundle ID, unique identifier for the CSB.

TGK

TEK Generation Key, a bit-string agreed upon by two or more parties, associated with CSB. From the TGK, Traffic-encrypting Keys can then be generated without needing further communication.

TEK

Traffic-Encrypting Key, the key used by the security protocol to protect the CS (this key may be used directly by the security protocol or may be used to derive further keys depending on the security protocol). The TEKs are derived from the CSB's TGK.

TGK re-keying

the process of re-negotiating/updating the TGK (and consequently future TEK(s)).

Initiator

the initiator of the key management protocol, not necessarily the initiator of the communication.

Responder

the responder in the key management protocol.

Salting key

a random or pseudo-random (see [RAND, HAC]) string used to protect against some off-line pre-computation attacks on the underlying security protocol.

HDR

denotes the protocol header

PRF(k,x)

a keyed pseudo-random function

E(k,m)

encryption of m with the key k

RAND

Random value

T

Timestamp

CERTx

the certificate of x

SIGNx

the signature from x using the private key of x

PKx

the public key of x

IDx

the identity of x

[]

an optional piece of information

{}

denotes zero or more occurrences

||

concatenation

|

OR (selection operator)

^

exponentiation

XOR

exclusive or

The following definition has been added to the ones from [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.):

SSRC

Synchronization Source Identifier

KEMAC

MIKEY Key Data Transport Payload, containing a set of encrypted sub-payloads and a MAC.

V

MIKEY Verification Message

SP

Security Parameter

Forking

The ability of a SIP proxy to replicate an incoming request to multiple outgoing requests in order to efficiently find the called party for rendezvous. SIP forking can be done in serial (depth-first search), or in parallel (breadth-first search).

Redirect

The ability of a SIP proxy to send a final response that redirects the caller to send a request to an alternate location.

Re-target

The ability of a SIP proxy to re-write the Request-URI thereby altering the destination of the request without explicitly notifying the user agent client.

3.  MIKEY Overview

This section will provide an overview about MIKEY. MIKEY focuses on the setup of cryptographic context to secure multimedia sessions in a heterogeneous environment. MIKEY is mainly intended to be used for peer-to-peer, simple one-to-many, and small-size (interactive) groups. One objective of MIKEY is to produce a Data security association (SA) for the security protocol, including a traffic-encrypting key (TEK), which is derived from a TEK Generation Key (TGK), and used as input for the security protocol.

MIKEY supports the possibility of establishing keys and parameters for more than one security protocol (or for several instances of the same security protocol) at the same time. The concept of Crypto Session Bundle (CSB) is used to denote a collection of one or more Crypto Sessions that can have common TGK and security parameters, but which obtain distinct TEKs from MIKEY.

MIKEY as defined in RFC3830 may proceed with one roundtrip at most, using a so-called Initiator message for the forward direction and a Responder message for the backward direction. Note that there exist MIKEY schemes, which may proceed within a half roundtrip (e.g., based on a pre-shared key), while other schemes require a full roundtrip (e.g., Diffie Hellman based schemes). The main objective of the Initiator's message (I_MESSAGE) is to transport one or more TGKs (carried in the KEMAC field) and a set of security parameters (SPs) to the Responder in a secure manner. As the verification message from the Responder is optional for some schemes, the Initiator indicates whether it requires a verification message or not from the Responder.

The focus of the following subsections lies on the key distribution methods as well as the discussion about advantages and disadvantages of the different schemes. Note that the MIKEY key distribution schemes rely on loosely synchronized clocks. If clock synchronization is not available, the replay handling of MIKEY (cf.[RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.)) may not work. A secure network clock synchronization protocol should realize this. RFC3830 recommends the ISO time synchronization protocol [ISO_sec_time] (, “"ISO/IEC 18014 Information technology - Security techniques - Time-stamping services, Part 1-3.",” 2002.). The format applied to the timestamps submitted in the MIKEY have to match the NTP format described in [RFC1305] (Mills, D., “Network Time Protocol (Version 3) Specification, Implementation,” March 1992.). In other cases, such as of a SIP endpoint, clock synchronization by deriving time from a trusted outbound proxy may be appropriate.

The different MIKEY related schemes are compared regarding following criteria:

• Mandatory for implementation: provides information, if RFC3830 requires the implementation of this scheme.
• Scalability: describes the technical feasibility to easily deploy a solution based on the considered scheme
• Dependency on PKI: states if the support of a PKI is required to support this scheme. Note, that PKI here relates to PKI services like key generation, distribution and revocation.
• Provision of Perfect Forward Secrecy (PFS): Describes the support of PFS, which is, according to RFC4949 [RFC4949] (Shirey, R., “Internet Security Glossary, Version 2,” August 2007.) the property that compromising the long-term keying material does not compromise session keys that were previously derived from the long-term material.
• Key generation involvement: Describes if both or just one of the participants are actively involved in key generation. The option to involve both parties in the key generation is interesting to avoid that one communication partner generates (intentionally or unintentionally) weak parameters, which in turn may result in weak keys. It also provides to the freshness of keys.
• Support of group keying: Feasibility of the MIKEY option to be used also for group keying, e.g., in conferencing scenarios.

If MIKEY is used for SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) bootstrapping, it also uses the SSRC to associate security policies with actual sessions. The SSRC identifies the synchronization source. The value is chosen randomly, with the intent that no two synchronization sources within the same SRTP session will have the same SSRC. Although the probability of multiple sources choosing the same identifier is low, all (S)RTP implementations must be prepared to detect and resolve collisions. Nevertheless in multimedia communication scenarios supporting forking (see Section 5.2 (MIKEY and Forking)), collisions may occur leading to so-called two-time pads, i.e., the same key is used for media streams to different destinations. Note that two time pads may also occur for media streams to the same destination.This is outlined in [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.).

3.1.  Pre-shared key protected distribution

This option of the key management uses a pre-shared secret key to derive key material for integrity protection and encryption to protect the actual exchange of key material. Note that the pre-shared secret is agreed upon before the session, e.g., by out-of-band means. The response message is optional and may be used for mutual authentication (proof of possession of the pre-shared secret) or error signaling.

Initiator                                  Responder

I_MESSAGE =
HDR, T, RAND, [IDi],[IDr],
    {SP}, KEMAC                --->
                                           R_MESSAGE =
                              [<---]       HDR, T, [IDr], V

The advantages of this approach lay in the fact that there is no dependency on a PKI (Public Key Infrastructure), the solution consumes low bandwidth and enables high performance, and is all in all a simple straightforward master key provisioning. The disadvantages are that perfect forward secrecy is not provided and key generation is just performed by the initiator. Furthermore, the approach is not scalable to larger configurations but is acceptable in small-sized groups. Note that according to [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) this option is mandatory to implement.

3.2.  Public Key encrypted key distribution

Using the asymmetric option of the key management, the initiator generates the key material (TGK's) to be transmitted and sends it encrypted with a so-called envelope key, which in turn is encrypted with the receiver's public key. The envelope key, env-key, which is a random number, is used to derive the auth-key and the enc-key. Moreover, the envelope key may be used as a pre-shared key to establish further crypto sessions. The response message is optional and may be used for mutual authentication or error signaling.

Initiator                                    Responder

I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
  [IDr], {SP}, KEMAC, [CHASH],
  PKE, SIGNi                   --->
                                            R_MESSAGE =
                              [<---]         HDR, T, [IDr], V

An advantage of this approach is that it allows the usage of self-signed certificates, which in turn can avoid a full blown PKI. Note that using self-signed certificates may result in limited scalability and also require additional means for authentication such as exchange of fingerprints of the certificates or similar techniques. The disadvantages comprise the necessity of a PKI for fully scalability, the performance of the key generation just by the initiator, and no provision of perfect forward secrecy. Additionally, the responder certificate needs to be available in advance at the sender's side. Furthermore, the verification of certificates may not be done in real-time. This could be the case in scenarios where the revocation status of certificates is checked through a further component. Depending on the initiator role this scheme may can also be applied in group based communication, where a central server distributes the group key protected with the public keys of the associated clients. Note, according to [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) this option is mandatory to implement.

3.3.  Diffie-Hellman key agreement protected with digital signatures

The Diffie-Hellman option of the key management enables a shared secret establishment between initiator and responder in a way where both parties contribute to the shared secret. The Diffie-Hellman key agreement is authenticated (and integrity protected) using digital signatures.

Initiator                                 Responder

I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
     [IDr], {SP}, DHi, SIGNi   --->
                                          R_MESSAGE =
                               <---        HDR, T, [IDr|CERTr],
                                            IDi, DHr, DHi, SIGNr

[RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) does mandate the support of RSA as specific asymmetric algorithm for the signature generation. Additionally the algorithm used for signature or public key encryption is defined by, and dependent on the certificate used. Besides the use of X.509v3 certificates it is mandatory to support the Diffie-Hellmann group "OAKLEY5" [RFC2412] (Orman, H., “The OAKLEY Key Determination Protocol,” November 1998.) (other DH groups may be used as well. The advantages of this approach are a fair, mutual key agreement (both parties provide to the key), and perfect forward secrecy, and the absence of the need to fetch a certificate in advance as needed for the MIKEY-RSA method depicted above. Moreover, it also provides the option to use self-signed certificates to avoid a PKI deployment. Note that, depending on the security policy, self-signed certificates may not be suitable for every use case.

Negatively to remark is that this approach scales mainly to point-to-point and depends on PKI for full scalability. Multiparty conferencing is not supported using just MIKEY-DHSIGN. Nevertheless, the established Diffie-Hellman-Secret may serve as a pre-shared key to bootstrap group-related security parameter. Furthermore, as for the MIKEY-RSA mode described above, the verification of certificates may not be necessarily done in real-time. This could be the case in scenarios where the revocation status of certificates is checked through a further component. Note, according to [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) it is optional to implement this scheme.

3.4.  Unprotected key distribution

RFC3830 also supports a mode to provide a key in an unprotected manner (MIKEY-NULL). This is based on the symmetric key encryption option depicted in Section 3.1 (Pre-shared key protected distribution) but is used with the NULL encryption and the NULL authentication algorithm. It may be compared with the plain approach in sdescriptions [RFC4568] (Andreasen, F., Baugher, M., and D. Wing, “Session Description Protocol (SDP) Security Descriptions for Media Streams,” July 2006.). MIKEY-NULL completely relies on the security of the underlying layer, e.g., provided by TLS. This option should be used with caution as it does not protect the key management.

Based on the missing cryptographic protection of this method, it is obvious that perfect forward secrecy is not provided. As it is based on the pre-shared secret mode only the initiator provides to the key management. The method itself is highly scalable but again, without proper protection through an underlying security layers it is not advisable for use.

3.5.  Diffie-Hellman key agreement protected with pre-shared secrets

This is an additional option which has been defined in [RFC4650] (Euchner, M., “HMAC-Authenticated Diffie-Hellman for Multimedia Internet KEYing (MIKEY),” September 2006.). In contrast to the method described in Section 3.3 (Diffie-Hellman key agreement protected with digital signatures) here the Diffie-Hellmann key agreement is authenticated (and integrity protected) using a pre-shared secret and keyed hash function.

Initiator                                  Responder

I_MESSAGE =
    HDR, T, RAND, [IDi],
    IDr, {SP}, DHi, KEMAC      --->
                                          R_MESSAGE =
                               <---           HDR, T,[IDr], IDi,
                                              DHr, DHi, KEMAC

TGK = g^(xi * yi)                        TGK = g^(xi * yi)

For the integrity protection of the Diffie-Hellman key agreement [RFC4650] (Euchner, M., “HMAC-Authenticated Diffie-Hellman for Multimedia Internet KEYing (MIKEY),” September 2006.) mandates the use of HMAC SHA-1. Regarding Diffie-Hellman groups [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) is referenced. Thus, it is mandatory to support the Diffie-Hellman group "OAKLEY5" [RFC2412] (Orman, H., “The OAKLEY Key Determination Protocol,” November 1998.). This option has also several advantages, as there are the fair mutual key agreement, the perfect forward secrecy, and no dependency on a PKI and PKI standards. Moreover, this scheme has a sound performance and reduced bandwidth requirements compared to MIKEY-DH-SIGN and provides a simple and straightforward master key provisioning. The establishment of shared secrets and the lack of support for group keying is a disadvantage.

This mode of operation provides an efficient scheme in deployments where there is a central trusted server that is provisioned with shared secrets for many clients. Such setups could for example be enterprise PBXs, service provider proxies, etc. In contrast to the plain pre-shared key encryption based mode, described in Section 3.1 (Pre-shared key protected distribution), this mode offers perfect forward secrecy as well as active involvement in the key generation of both parties involved.

3.6.  SAML assisted DH-key agreement

There has been a longer discussion during IETF meetings and also on the IETF MSEC mailing about a SAML assisted DH approach. This idea has not been submitted as a separate draft. Nevertheless, the discussion is reflected here as it is targeted to fulfill general requirements on key management approaches. Those requirements can be summarized as:

1. Mutual authentication of involved parties
2. Both parties involved contribute to the session key generation
3. Provide perfect forward secrecy
4. Support distribution of group session keys
5. Provide liveliness tests when involved parties do not have a reliable clock
6. Support of limited parties involved

To fulfill all of the requirements, it was proposed to use a classic Diffie-Hellman key agreement protocol for key establishment in conjunction with a User Agents (UA's) SIP server signed element, authenticating the Diffie-Hellman key and the ID using the SAML (Security Association Markup Language, [SAML_overview] (Huges, J. and E. Maler, “"Security Assertion Markup Language (SAML) 2.0 Technical Overview, Working Draft",” 2005.)) approach. Here the client's public Diffie-Hellman-credentials are signed by the server to form a SAML assertion (referred to as CRED below), which may be used for later sessions with other clients. This assertion needs at least to convey the ID, public DH key, expiry, and the signature from the server. It provides the involved clients with mutual authentication and message integrity of the key management messages exchanged.

Initiator                             Responder

I_MESSAGE =
HDR, T, RAND1, [CREDi],
IDr, {SP}                      --->
                                      R_MESSAGE =
                               <---   HDR, T, [CREDr], IDi, DHr,
                                      RAND2, (SP)
       TGK = HMACx(RAND1|RAND2), where x = g^(xi * xr).

Additionally the scheme proposes a second roundtrip to avoid the dependence on synchronized clocks and provide liveliness checks. This is achieved by exchanging nonces, protected with the session key. The second roundtrip can also be used for distribution of group keys or to leverage a weak DH key for a stronger session key. The trigger for the second round trip would be handled via SP, the Security Policy communicated via MIKEY.

Initiator                             Responder

I_MESSAGE =
HDR, SIGN(ENC(RAND3))          --->
                                      R_MESSAGE =
                               <---   SIGN(ENC(RAND4))

Note if group keys are to be provided RAND would be substituted by that group key.

With the second roundtrip, this approach also provides an option for all of the other key distribution methods, when liveliness checks are needed. The drawback of the second roundtrip is that these messages need to be integrated into the call flow of the signaling protocol. In straight forward call one roundtrip may be enough to setup a session. Thus this second roundtrip would require additional messages to be exchanged.

Regarding the different criteria discussed in the introduction of this section, the advantages of this approach are a fair, mutual key agreement (both parties provide to the key), perfect forward secrecy. Through the second roundtrip, the dependency on synchronized clocks can be avoided. Moreover, this second roundtrip enables the distribution of a group key and thus enhances the scalability from mainly point-to-point to also multiparty conferencing. The usage of SAML assisted DH may decrease the hidden latency cost through the credential validation necessary to be done for the signed DH scheme described in Section 3.3 (Diffie-Hellman key agreement protected with digital signatures). If the UA received its SAML assertion from its domain's SIP server, it is trusting the server implicitly thus it may extend that trust to relying on it to validate the other party's SAML assertion. This not only eliminates the hidden validation latency, but also its computational cost to the UA.

Negatively to remark is that this proposal does have one significant security risk. The UA's SIP server can cheat and create an extra authentication object for the UA where it has the Diffie-Hellman private key. With this, the (SIP) server issuing the SAML assertion can successfully launch a MITM attack against two of its UAs. Also two SIP servers can collude so that either can successfully launch a MITM attack against their UAs. A UA can block this attack if its Diffie-Hellman key is authenticated by a trustworthy third party and this whole object is signed by the SIP server. Moreover, this approach uses two roundtrips, increasing the necessary bandwidth and also the setup time, which may be crucial for many scenarios. For the credential generation usually a seperate component (server) is necessary, so server less call setup is not supported.

3.7.  Asymmetric key distribution with in-band certificate exchange

This is an additional option which has been defined in [RFC4738] (Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, “MIKEY-RSA-R: An Additional Mode of Key Distribution in Multimedia Internet KEYing (MIKEY),” November 2006.). It describes the asymmetric key distribution with optional in-band certificate exchange.

Initiator                             Responder

I_MESSAGE =
HDR, T, [IDi|CERTi], [IDr],
      {SP}, [RAND], SIGNi      --->
                                      R_MESSAGE =
                               <---   HDR, [GenExt(CSB-ID)], T,
                                        RAND, [IDr|CERTr], [SP],
                                        KEMAC, SIGNr

This option has some advantages compared to the asymmetric key distribution stated in Section 3.2 (Public Key encrypted key distribution). Here, the sender and receiver do not need to know the certificate of the other peer in advance as it may be sent in the MIKEY initiator message (if the receiver knows the certificate in advance, RFC3830's MIKEY-RSA mode may be used instead). Thus, the receiver of this message can utilize the received key material to encrypt the session parameter and send them back as part of the MIKEY response message. The certificate check may be done depending on the signing authority. If the certificate is signed by a publicly accepted authority the certificate validation can be done in a straightforward manner, by using the commonly known certificate authority's public key. In the other case additional steps may be necessary. The disadvantage is that no perfect forward secrecy is provided.

This mode is meant to provide an easy option for certificate provisioning when PKI is present and/or required. Specifically in SIP, session invitations can be retargeted or forked. MIKEY modes that require the Initiator to target a single well known Responder may be impractical here as they may require multiple roundtrips to do key negotiation. By allowing the Responder to generate secret material used for key derivation this mode allows for an efficient key delivery scheme. Note that the Initiator can contribute to the public entropy since CSB-ID and RAND payloads in unicast use cases. This mode is also useful in multicast scenarios where multiple clients are contacting a known server and are downloading the key. Responder workload is reduced in these scenarios compared to MIKEY in public key mode. This is due to the fact that the RSA asymmetric encryption requires less effort compared to the decryption using the private key (The public key is usually shorter than the private key, hence less performance for encryption compared to decryption). This obviously means that the Initiator workload is increased by the same amount. Examples of deployments where this mode can be used are enterprises with PKI, service provider setups where the service provider decides to provision certificates to its users, etc.

4.  Further MIKEY Extensions

This section will provide an overview about further MIKEY [RFC3830] (Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, “MIKEY: Multimedia Internet KEYing,” August 2004.) extensions for crypto algorithms, generic payload enhancements, as well as enhancements to support the negotiation of security parameters for other security protocols than SRTP. These extensions have been defined in several additional documents.

4.1.  ECC algorithms support

[I‑D.ietf‑msec‑mikey‑ecc] (Milne, A., “ECC Algorithms for MIKEY,” June 2007.) proposes extensions to the authentication, encryption and digital signature methods described for use in MIKEY, employing elliptic-curve cryptography (ECC). These extensions are defined to align MIKEY with other ECC implementations and standards.

The motivation for supporting ECC within the MIKEY stems from the following advantages:

• ECC modes are more and more added to security protocols
• ECC support requires considerably smaller keys by keeping the same security level compared to other asymmetric techniques (like RSA). Elliptic curve algorithms are capable of providing security consistent with AES keys of 128, 192, and 256 bits without extensive growth in asymmetric key sizes.
• As stated in [I‑D.ietf‑msec‑mikey‑ecc] (Milne, A., “ECC Algorithms for MIKEY,” June 2007.) implementations have shown that elliptic curve algorithms can significantly improve performance and security-per-bit over other recommended algorithms.

These advantages make the usage of ECC especially interesting for embedded devices, which may have only limited performance and storage capabilities.

[I‑D.ietf‑msec‑mikey‑ecc] (Milne, A., “ECC Algorithms for MIKEY,” June 2007.) proposes several ECC based mechanisms to enhance the MIKEY key distribution schemes, as there are:

• Use of ECC methods extending the Diffie-Hellman key exchange: MIKEY-DHSIGN with ECDSA or ECGDSA
• Use of ECC methods extending the Diffie-Hellman key exchange: MIKEY-DHSIGN with ECDH
• Use of Elliptic Curve Integrated Encryption Scheme (MIKEY-ECIES)
• Use of Elliptic Curve Scheme Menezes-Qu-Vanstone (MIKEY-ECMQV)

The following subsections will provide more detailed information about the message exchanges for MIKEY-ECIES and MIKEY-ECMQV.

4.1.1.  Elliptic Curve Integrated Encryption Scheme application in MIKEY

The following figure shows the message exchange for the MIKEY-ECIES scheme:

Initiator                                       Responder

I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
    [IDr], {SP}, KEMAC,
    [CHASH], PKE, SIGNi        --->
                                                R_MESSAGE =
                              [<---]            HDR, T, [IDr], V

4.1.2.  Elliptic Curve Menezes-Qu-Vanstone Scheme application in MIKEY

The following figure shows the message exchange for the MIKEY-ECMQV scheme:

Initiator                                      Responder

I_MESSAGE =
HDR, T, RAND, [IDi|CERTi],
   [IDr], {SP},
   ECCPTi, SIGNi               --->
                                               R_MESSAGE =
                              [<---]           HDR, T, [IDr], V

4.2.  New MIKEY Payload for bootstrapping TESLA

TESLA [RFC4082] (Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, “Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction,” June 2005.) is a protocol for providing source authentication in multicast scenarios. TESLA is an efficient protocol with low communication and computation overhead, which scales to large numbers of receivers, and also tolerates packet loss. TESLA is based on loose time synchronization between the sender and the receivers. Source authentication is realized in TESLA by using Message Authentication Code (MAC) chaining. The use of TESLA within the Secure Real-time Transport Protocol (SRTP) has been published in [RFC4383] (Baugher, M. and E. Carrara, “The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Secure Real-time Transport Protocol (SRTP),” February 2006.) targeting multicast authentication in scenarios, where SRTP is applied to protect the multimedia data. This solution assumes that TESLA parameters are made available by out-of-band mechanisms.

[RFC4442] (Fries, S. and H. Tschofenig, “Bootstrapping Timed Efficient Stream Loss-Tolerant Authentication (TESLA),” March 2006.) specifies payloads for MIKEY to bootstrap TESLA for source authentication of secure group communications using SRTP. TESLA may be bootstrapped using one of the MIKEY key management approaches described above by sending the MIKEY message via unicast, multicast or broadcast. This approach provides the necessary parameter payload extensions for the usage of TESLA in SRTP. Nevertheless, if the parameter set is also sufficient for other TESLA use cases, it can be applied as well.

4.3.  MBMS extensions to the Key ID information type

This extension specifies a new Type (the Key ID Information Type) for the General Extension Payload. This is used in, e.g., the Multimedia Broadcast/Multicast Service (MBMS) specified in the 3rd Generation Partnership Project (3GPP). MBMS requires the use of MIKEY to convey the keys and related security parameters needed to secure the multimedia that is multicast or broadcast.

One of the requirements that MBMS puts on security is the ability to perform frequent updates of the keys. The rationale behind this is that it will be costly for subscribers to re-distribute the decryption keys to non-subscribers. The cost for re-distributing the keys using the unicast channel should be higher than the cost of purchasing the keys for this scheme to have an effect. To achieve this, MBMS uses a three-level key management, to distribute group keys to the clients, and be able to re-key by pushing down a new group key. MBMS has the need to identify, which types of keys are involved in the MIKEY message and their identity.

[RFC4563] (Carrara, E., Lehtovirta, V., and K. Norrman, “The Key ID Information Type for the General Extension Payload in Multimedia Internet KEYing (MIKEY),” June 2006.) specifies a new Type for the General Extension Payload in MIKEY, to identify the type and identity of involved keys. Moreover, as MBMS uses MIKEY both as a registration protocol and a re-key protocol, this RFC specifies the necessary additions that allow MIKEY to function both as a unicast and multicast re-key protocol in the MBMS setting.

4.4.  OMA BCAST MIKEY General Extension Payload Specification

The document [RFC4909] (Dondeti, L., Castleford, D., and F. Hartung, “Multimedia Internet KEYing (MIKEY) General Extension Payload for Open Mobile Alliance BCAST LTKM/STKM Transport,” June 2007.) specifies a new general extension payload type for use in the Open Mobile Alliance's (OMA) Browser and Content Broadcast (BCAST) group. OMA BCAST's service and content protection specification uses short term key message and long term key message payloads that in certain broadcast distribution systems are carried in MIKEY. The document defines a general extensions payload to allow possible extensions to MIKEY without defining a new payload. The general extension payload can be used in any MIKEY message and is part of the authenticated or signed data part. Note, that only a parameter description is included, but no key information.

4.5.  Supporting Integrity Transform carrying the Rollover Counter

The document [RFC4771] (Lehtovirta, V., Naslund, M., and K. Norrman, “Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP),” January 2007.) defines a new integrity transform for SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) providing the option to also transmit the Roll Over Counter (ROC) as part of dedicated SRTP packets. This extension has been defined for the use in the 3GPP multicast/broadcast service. While the communicating parties did agree on a starting ROC, in some cases the receiver may not be able to synchronize his ROC with the one used by the sender even if it is signaled to him out of band. Here the new extension provides the possibility for the receiver to re-synchronize to the sender's ROC. To signal the use of the new integrity transform new definitions for certain MIKEY payloads need to be done. These new definition comprise the integrity transforms itself as well as new integrity transform parameter. Moreover, the document specifies additional parameter, to enable the usage of different integrity transforms for SRTP and SRTCP.

5.  Selection and interworking of MIKEY modes

While MIKEY and its extensions provide a variety of choice in terms of modes of operation an implementation may choose to simplify its behavior. This can be achieved by operating in a single mode of operation when in Initiator's role. Where PKI is available and/or required an implementation may choose for example to start all sessions in RSA-R mode and it would be trivial for it to act as a Responder in public key mode. If envelope keys are cached it can then also choose to do re-keying in shared key mode. In general, modes of operation where the Initiator generates keying material are useful when two peers are aware of each other before the MIKEY communication takes place. If a peer chooses not to operate in the public key mode it may reject the certificate of the Initiator. The same applies to peers that choose to operate in one of the DH modes exclusively.

Forward MIKEY modes, where the initiator provides the key material, like public key or shared key mode when used in SIP/SDP may lead to complications in some calls scenarios, for example forking scenarios where key derivation material gets distributed to multiple parties. As mentioned earlier this may be impractical as some of the destinations may not have the resources to validate the message and may cause the initiator to drop the session invitation. Even in the case all parties involved have all the prerequisites for interpreting the MIKEY message received there is a possible problem with multiple responders starting media sessions using the same key. While the SSRCs will be different in most of the cases they are only sixteen bits long and there is a high probability of a two-time pad problem. As suggested earlier forward modes are most useful when the two peers are aware of each other before the communication takes place (as is the case in key renewal scenarios when costly public key operations can be avoided by using the envelope key).

The following list gives an idea how the different MIKEY modes may be used or combined, depending on available key material at the initiator side.

1. If the Initiator has a PSK with the Responder, it uses the PSK mode.
2. If the Initiator has a PSK with the Responder, but needs PFS or knows that the responder has a policy that both parties should provide entropy to the key, then it uses the DH-HMAC mode.
3. If the Initiator has the RSA key of the Responder, it uses the RSA mode to establish the TGK. Note that the TGK may be used as PSK together with Option 1 for further key management operations.
4. If the Initiator does not expect the receiver to have his certificate he may use RSA-R. Using RSA-R he can provide the initiators certificate information in-band to the receiver. Moreover, the initiator may also provide a random number which can be used by the receiver for key generation. Thus both parties can be involved in the key management. Note that in this mode, after establishing the TGK, it may be used as PSK with other MIKEY modes.
5. The Initiator uses DH-SIGN when PFS is required by his policy and he knows that the responder has a policy that both parties should provide entropy. Note that also in this mode, after establishing the TGK, it may be used as PSK with other MIKEY modes.
6. If no PSK or certificate is available at the initiators side (and likewise at the receivers side) but lower level security (like TLS or IPsec) is in place the user may use the unprotected mode of MIKEY.It has to obeyed, that this enables intermediate nodes like proxies to actually get the exchanged master key in plain. This may not be intended, especially in cases, where the intermediate node is not trusted.

Besides the available key material choosing between the different modes of MIKEY depends strongly on the use case. This section will depict dedicated scenarios to discuss the feasibility of the different modes in these scenarios. A comparison of the different modes of operation regarding the influences and requirements to the deploying infrastructure as well as the cryptographic strength can be found in [I‑D.ietf‑sip‑media‑security‑requirements] (Wing, D., Fries, S., Tschofenig, H., and F. Audet, “Requirements and Analysis of Media Security Management Protocols,” January 2009.) The following list provides the most prominent call scenarios and are matter of further discussion:

• Early Media
• Forking
• Call Transfer/Redirect/Retarget
• Shared key conferencing

5.1.  MIKEY and Early Media

The term early media describes two different scenarios. The first one relates to the case where media data are received before the actual SDP signaling answer has been received. This may arise through the different latency on the signaling and media path. This case is often referred to as media before signaling answer. The second scenario describes the case were media data are send from the callee before sending the final SIP 200 OK nessage. This situation appears usually in call center scenarios, when queueing a waiting loop or when providing personal ring tones.

In early media scenarios, SRTP data may be received before the answer over the SIP signaling arrives. The two MIKEY modes, which only require one message to be transported (Section 3.1 (Pre-shared key protected distribution) and Section 3.2 (Public Key encrypted key distribution)), work nicely in early media situations, as both, sender and receiver have all the necessary parameters in place before actually sending/receiving encrypted data. The other modes, featuring either Diffie-Hellman key agreement (Section 3.3 (Diffie-Hellman key agreement protected with digital signatures), Section 3.5 (Diffie-Hellman key agreement protected with pre-shared secrets), and Section 3.6 (SAML assisted DH-key agreement)) or the enhanced asymmetric variant (Section 3.7 (Asymmetric key distribution with in-band certificate exchange)) suffer from the requirements that the initiator has to wait for the response before being able to decrypt the incoming SRTP media. In fact, even if early media is not used, in other words if media is not sent before the SDP answer a similar problem may arise from the fact that SIP/SDP signaling has to traverse multiple proxies on its way back and media may arrive before the SDP answer. It is expected that this delay would be significantly shorter than in the case of early media though.

It is worth mentioning here that security descriptions [RFC4568] (Andreasen, F., Baugher, M., and D. Wing, “Session Description Protocol (SDP) Security Descriptions for Media Streams,” July 2006.) has basically the same problem as the initiating end needs the SDP answer before it can start decrypting SRTP media.

To eliminate the early media problem there are further approaches to describe security preconditions [RFC5027] (Andreasen, F. and D. Wing, “Security Preconditions for Session Description Protocol (SDP) Media Streams,” October 2007.), i.e., certain preconditions need to be met to enable voice data encryption. One example is for instance that a scenario where a provisional response, containing the required MIKEY parameter, is sent before encrypted media is processed. However, this approach is not widely accepted as early media is still considered a valid use case.

5.2.  MIKEY and Forking

In SIP forking scenarios a SIP proxy server sends an INVITE request to more than one location. This means that also the MIKEY payload, which is part of the SDP is sent to several (different) locations. MIKEY modes supporting signatures may be used in forking scenarios (Section 3.3 (Diffie-Hellman key agreement protected with digital signatures) and Section 3.7 (Asymmetric key distribution with in-band certificate exchange)) as here the receiver can validate the signature. There are limitations with the symmetric key encryption as well as the asymmetric key encryption modes (Section 3.1 (Pre-shared key protected distribution) and Section 3.2 (Public Key encrypted key distribution)). This is due to the fact that in symmetric encryption the recipient needs to possess the symmetric key before handling the MIKEY data. For asymmetric MIKEY modes, if the sender is aware of the forking he may not know in advance to which location the INVITE is forked and thus may not use the right receiver certificate to encrypt the MIKEY envelope key. Note, the sender may include several MIKEY containers into the same INVITE message to cope with forking, but this requires the knowledge of all forking targets in advance and also requires the possession of the target certificates. It is out of the scope of MIKEY to specify behavior in such a case. DH modes or the Section 3.7 (Asymmetric key distribution with in-band certificate exchange) do not have this problem. In scenarios, where the sender is not aware of forking, only the intended receiver is able to decrypt the MIKEY container.

If forking is combined with early media the situation gets aggravated. If MIKEY modes requiring a full roundtrip are used, like the signed Diffie-Hellman, multiple responses may overload the end device. An example is forking to 30 destinations (group pickup), while MIKEY is used with the signed Diffie-Hellman mode together with security preconditions. Here, every target would answer with a provisional response, leading to 30 signature validations and Diffie-Hellman calculations at the senders site. This may lead to a prolonged media setup delay.

Moreover, depending on the MIKEY mode chosen, a two-time pad may occur in dependence of the negotiated key material and the SSRC. For the non Diffie-Hellman modes other than RSA-R, a two-time pad may occur when multiple receivers pick the same SSRC.

5.3.  MIKEY and Call Transfer/Redirect/Retarget

In a SIP environment MIKEY exchange is tied to SDP offer/answer and irrespective of the implementation model used for call transfer the same properties and limitations of MIKEY modes apply as in a normal call setup scenarios.

In certain SIP scenarios the functionality of redirect is supported. In redirect scenarios the call initiator gets a response that the called party for instance has temporarily moved and may be reached at a different destination. The caller can now perform a call establishment with the new destination. Depending on the originally chosen MIKEY mode, the caller may not be able to perform this mode with the new destination. To be more precise MIKEY-PSK, and MIKEY-DHHMAC require a pre-shared secret in advance. MIKEY-RSA requires the knowledge about the target's certificate. Thus, these modes may influence the ability of the caller to initiate a session.

Another functionality, which may be supported in SIP is retargeting. In contrast to redirect, the call initiator does not get a response about the different target. The SIP proxy sends the request to a different target about receiving a redirect response from the originally called target. This most likely will lead to problems when using MIKEY modes requiring a pre-shared key (MIKEY-PSK, MIKEY-DHHMAC) or were the caller used asymmetric key encryption (MIKEY-RSA) because the key management was originally targeted to a different destination.

5.4.  MIKEY and Shared Key Conferencing

First of all, not all modes of MIKEY support shared key conferencing. Mainly the Diffie Hellman modes cannot be used straight forward for conferencing as this mechanism results in a pair wise shared secret key. All other modes can be applied in conferencing scenarios by obeying the initiator and responder role, i.e., the half roundtrip modes need to be initiated by the conferencing unit, to be able to distribute the conferencing key. The remaining full roundtrip mode, MIKEY RSA-R will be initiated by the client, while the conferencing unit provides the conferencing key based on the received certificate.

An example conferencing architecture is defined in the IETF's XCON WG. The scope of this working group relates to mechanism for membership and authorization control, a mechanism to manipulate and describe media "mixing" or "topology" for multiple media types (audio, video, text), a mechanism for notification of conference related events/changes (for example a floor change), and a basic floor control protocol. A document describing possible use case scenarios is available in [RFC4597] (Even, R. and N. Ismail, “Conferencing Scenarios,” August 2006.).

5.5.  MIKEY Mode Summary

The following two tables summarize the discussion from the subsections before. The first table matches the scenarios discussed in this section to the different MIKEY modes.

MIKEY             Early    Secure      Retarget   Redirect   Shared
mode              Media    Forking                           Key Conf
---------------------------------------------------------------------
PSK  (3.1)         Yes                                        Yes *
RSA  (3.2)         Yes                                        Yes *
DH-SIGN (3.3)                Yes*         Yes       Yes
Unprotected (3.4)  Yes
DH-HMAC (3.5)
RSA-R  (3.7)                 Yes          Yes       Yes       Yes *

* = may lead to additional parameter distribution

The following table maps the MIKEY modes to key management related properties.

MIKEY             Manual    Needs      PFS    Key Generation
mode              Keys      PKI	              Involvement
--------------------------------------------------------------
PSK  (3.1)         Yes      No          No     Initiator
RSA  (3.2)         No       Yes         No     Initiator
DH-SIGN (3.3)      No       Yes         Yes    Both
Unprotected (3.4)  No       No          No     Initiator
DH-HMAC (3.5)      Yes      No          Yes    Both
RSA-R  (3.7)       No       Yes         No     Both*

* = assumed the Initiator provides the (optional) RAND value

6.  Transport of MIKEY messages

MIKEY defines message formats to transport key information and security policies between communicating entities. It does not define the embedding of these messages into the used signaling protocol. This definition is provided in separate documents, depending on the used signaling protocol. Nevertheless, MIKEY can also be transported over plain UDP or TCP to port 2269.

Several IETF defined protocols utilize the Session Description Protocol (SDP, [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.)) to transport the session parameters. Examples are the Session Initiation Protocol (SIP, [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) or the Gateway Control Protocol (GCP, [RFC3525] (Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, “Gateway Control Protocol Version 1,” June 2003.)). The transport of MIKEY messages as part of SDP is described in [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),” July 2006.). Here, the complete MIKEY message is base64 encoded and transmitted as part of the SDP part of the signaling protocol message. Note, as several key distribution messages may be transported within one SDP container, [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),” July 2006.) also comprises an integrity protection regarding all supplied key distribution attempts. Thus, bidding down attacks will be recognized.

MIKEY is also applied in ITU-T protocols like H.323, which is used to establish communication sessions similar to SIP. For H.323 a security framework exists, which is defined in H.235. Within this framework H.235.7 [H.235.7] (, “"ITU-T Recommendation H.235.7: Usage of the MIKEY Key Management Protocol for the Secure Real Time Transport Protocol (SRTP) within H.235",” 2005.) describes the usage of MIKEY and SRTP in the context of H.323. In contrast to SIP H.323 uses ASN.1 (Abstract Syntax Notation). Thus there is no need to encode the MIKEY container as base64. Within H.323 the MIKEY container is binary encoded.

7.  MIKEY alternatives for SRTP security parameter negotiation

Besides MIKEY there exists several approaches to handle the security parameter establishment. This is due to the fact, that some limitations in certain scenarios have been seen. Examples are early media and forking situations as described in Section 5 (Selection and interworking of MIKEY modes). The following list provides a short summary about possible alternatives:

• sdescription - [RFC4568] (Andreasen, F., Baugher, M., and D. Wing, “Session Description Protocol (SDP) Security Descriptions for Media Streams,” July 2006.) describes a key management scheme, which uses SDP for transport and completely relies on underlying protocol security. For transport the documents defines a SDP attribute transmitting all necessary SRTP parameter in clear. For security it references TLS and S/MIME. In contrast to MIKEY the SRTP parameter in the initiator to responder direction is actually sent in the message from the initiator to the responder rather than vice versa. This may lead to problems in early media scenarios.
• sdescription with early media support - [I‑D.wing‑mmusic‑sdes‑early‑media] (Raymond, R. and D. Wing, “Security Descriptions Extension for Early Media,” October 2005.) enhances the above scheme with the possibility to also be usable in early media scenarios, when security preconditions is not used.
• Encrypted Key Transport for Secure RTP - [I‑D.mcgrew‑srtp‑ekt] (McGrew, D., Andreasen, F., Wing, D., and L. Dondeti, “Encrypted Key Transport for Secure RTP,” October 2009.) is an extension to SRTP that provides for the secure transport of SRTP master keys, Rollover Counters, and other information, within SRTCP. This facility enables SRTP to work for decentralized conferences with minimal control, and to handle situations caused by SIP forking and early media. It may also be used in conjunction with MIKEY.
• Diffie Hellman support in SDP - [I‑D.baugher‑mmusic‑sdp‑dh] (Baugher, M. and D. McGrew, “Diffie-Hellman Exchanges for Multimedia Sessions,” February 2006.) defines a new SDP attribute for exchanging Diffie-Hellman public keys. The attribute is an SDP session-level attribute for describing DH keys, and there is a new media-level parameter for describing public keying material for SRTP key generation.
• DTLS-SRTP describing SRTP extensions for DTLS - [I‑D.ietf‑avt‑dtls‑srtp] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.) describes a method of using DTLS key management for SRTP by using a new extension that indicates that SRTP is to be used for data protection, and which establishes SRTP keys.
• ZRTP - [I‑D.zimmermann‑avt‑zrtp] (Zimmermann, P., Johnston, A., and J. Callas, “ZRTP: Media Path Key Agreement for Unicast Secure RTP,” April 2010.) This document defines ZRTP as RTP header extensions for a Diffie-Hellman exchange to agree on a session key and parameters for establishing SRTP sessions. The ZRTP protocol is completely self-contained in RTP and does not require support in the signaling protocol or assume a PKI.

There has been a longer discussion regarding a preferred key management approach in the IETF coping with the different scenarios and requirements continuously sorting out key management approaches. During IETF 68 three options were considered: MIKEY in an updated version (referred to as MIKEYv2); ZRTP; and DTLS-SRTP. The potential key management protocol for the standards track for media security was voted in favor of DTLS-SRTP. Thus, the reader is pointed to the appropriate resources for further information on DTLS-SRTP [I‑D.ietf‑avt‑dtls‑srtp] (McGrew, D. and E. Rescorla, “Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP),” February 2009.). Note that MIKEY has already been deployed for setting up SRTP security context and is also targeted for use in MBMS applications.

8.  Summary of MIKEY related IANA Registrations

For MIKEY and the extensions to MIKEY IANA registrations have been made. Here only a link to the appropriate IANA registration is provided to avoid inconsistencies. The IANA registrations for MIKEY payloads can be found under http://www.iana.org/assignments/mikey-payloads These registrations comprise the MIKEY base registrations as well as registrations made by MIKEY extensions regarding the payload.

The IANA registrations for MIKEY port numbers can be found under http://www.iana.org/assignments/port-numbers (search for MIKEY).

9.  Security Considerations

This document does not define extensions to existing protocols. It rather provides an overview about the set of MIKEY modes and available extensions and provides information about the applicability of the different modes in different scenarios to support the decision making for network architects regarding the appropriate MIKEY scheme or extension to be used in a dedicated target scenario. Choosing between the different schemes described in this document strongly influences the security of the target system as the different schemes provide different level of security and also require different infrastructure support.

As this document bases on the MIKEY base specification as well as the different specifications of the extensions the reader is referred to the original documents for the specific security considerations.

10.  IANA Considerations

This document does not require any IANA registration.

11.  Acknowledgments

The authors would like to thank Lakshminath Dondeti for his document reviews and for his guidance.

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses

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