Network Working Group S. Fries Request for Comments: 5197 Siemens Category: Informational D. Ignjatic Polycom June 2008
On the Applicability of Various Multimedia Internet KEYing (MIKEY) Modes and Extensions
Status of This Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
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 (SRTP). MIKEY itself is standardized within RFC 3830 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 arise, 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 an 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 Session Description Protocol (SDP) answer, forking, and shared key conferencing.
Key distribution describes the process of delivering cryptographic keys to the required parties. MIKEY [RFC3830], the Multimedia Internet Keying, has been defined focusing on support for the establishment of security context for the Secure Real-time Transport Protocol [RFC3711]. Note that RFC 3830 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:
o Symmetric key distribution as defined in [RFC3830] (MIKEY-PSK)
o Asymmetric key distribution as defined in [RFC3830] (MIKEY-RSA)
o Diffie-Hellman key agreement protected by digital signatures as defined in [RFC3830] (MIKEY-DHSIGN)
o Unprotected key distribution (MIKEY-NULL)
o Diffie-Hellman key agreement protected by symmetric pre-shared keys as defined in [RFC4650] (MIKEY-DHHMAC)
o Security Assertion Markup Language (SAML) assisted Diffie-Hellman key agreement as defined (not available as a separate document, but discussions are reflected within this document (MIKEY-DHSAML))
o Asymmetric key distribution (based on asymmetric encryption) with in-band certificate provision as defined in [RFC4738] (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] fits perfectly. There are further extensions to MIKEY comprising algorithm enhancements and a new payload definition supporting protocols other than SRTP.
Algorithm extensions are defined in the following document:
o Elliptic Curve Cryptography (ECC) algorithms for MIKEY as defined in [MSEC-MIKEY]
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Payload extensions are defined in the following documents:
o Bootstrapping TESLA, defining a new payload for the Timed Efficient Stream Loss-tolerant Authentication (TESLA) protocol [RFC4082] as defined in [RFC4442]
o The Key ID information type for the general extension payload as defined in [RFC4563]
o Open Mobile Alliance (OMA) Broadcast (BCAST) MIKEY General Extension Payload Specification as defined in [RFC4909]
o Integrity Transform Carrying Roll-over Counter for SRTP as defined in [RFC4771]. 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 RFC 3830 and the relations to the different extensions to provide a framework when using MIKEY. It is intended as an 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. [SIP-MEDIA] and include the following:
o Early Media (i.e., media that arrives before the SDP answer)
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].
The following definitions have been taken from [RFC3830]:
(Data) Security Protocol: the security protocol used to protect the actual data traffic. Examples of security protocols are IPsec and SRTP.
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Data SA Data Security Association information for the security protocol, including a TEK and a set of parameters/ policies.
CS Crypto Session, uni- or bidirectional 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 [RFC4086]) string used to protect against some off-line pre-computation attacks on the underlying security protocol.
HDR the protocol header
PRF(k,x) a keyed pseudo-random function
E(k,m) encryption of m with the key k
RAND random value
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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
{} zero or more occurrences
|| concatenation
| OR (selection operator)
^ exponentiation
XOR exclusive or
The following definitions have been added to the ones from [RFC3830]:
SSRC Synchronization Source Identifier
KEMAC MIKEY Key Data Transport Payload, containing a set of encrypted sub-payloads and a Message Authentication Code (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.
Retarget 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.
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 that obtain distinct TEKs from MIKEY.
MIKEY as defined in RFC 3830 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 that 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 or not it requires a verification message 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]) may not work. This is due to the fact that MIKEY does not use a challenge-response mechanism for replay handling; instead, timestamps are used together with message caching. Thus, the required synchronization depends on the number of messages that can be cached on either side. Therefore, MIKEY recommends adjusting the cache size depending on the clock skew in the deployment environment. Moreover, RFC 3830 recommends the ISO time synchronization protocol [ISO_sec_time]. If replay handling is not available, an attacker may be able to replay an older message that he eavesdropped earlier, leading to different TGKs on both sides. As these are fed to the application utilizing MIKEY (e.g., SRTP or TESLA), both sides may rely on different keys and thus may be unable to communicate with
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each other. The format applied to the timestamps submitted in MIKEY have to match the NTP format described in [RFC1305]. 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 the following criteria:
o Mandatory for implementation: provides information, if RFC 3830 requires the implementation of this scheme.
o Scalability: describes the technical feasibility to easily deploy a solution based on the considered scheme.
o 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.
o Provision of Perfect Forward Secrecy (PFS): describes the support of PFS, which is, according to RFC 4949 [RFC4949], the property that compromising the long-term keying material does not compromise session keys that were previously derived from the long-term material.
o Key generation involvement: describes if both or just one of the participants is actively involved in key generation. The option to involve both parties in the key generation is considered here as it addresses several points:
* If both sides contribute public entropy, it is ensured that each side can guarantee that keys are fresh to avoid replay attacks.
* Involvement of both sides avoids that one side generates (intentionally or unintentionally) weak (predictable) nonces, which in turn may result in weak keys.
o 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] 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
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forking (see Section 5.2) or retargeting (see Section 5.3) collisions may occur leading to so-called two-time pads; i.e., the same key is used for media streams to different destinations. This occurs if two branches have the same TEK (based on the MIKEY key establishment) and choose the same 32-bit SSRC for the SRTP streams. The SRTP key derivation will then produce the same session keys (as the input values are the same) and also derive the same initialization vector per packet, as the SSRCs are the same. Note that two time pads may also occur for media streams to the same destination. This is outlined in [RFC3711].
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 responder message is optional and may be used for mutual authentication (proof of possession of the pre-shared secret) or error signaling.
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], this option is mandatory to implement.
Using the asymmetric option of the key management, the Initiator generates the key material (TGKs) 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
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establish further crypto sessions. The responder message is optional and may be used for mutual authentication or error signaling.
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 full 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 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 that according to [RFC3830], 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 the 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.
[RFC3830] does mandate the support of RSA as a 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-Hellman group "OAKLEY5" [RFC2412]. It is also possible to use other Diffie-Hellman groups within MIKEY. This can be done by defining a new mapping sub- payload and the associated policy payload according to [RFC3830]. The advantages of this approach are a fair, mutual key agreement (both parties provide to the key), 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 necessarily be done in real time. This could be the case in scenarios where the revocation status of certificates is checked through a further component. Note that, according to [RFC3830], it is optional to implement this scheme.
RFC 3830 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 but is used with the NULL encryption and the NULL authentication algorithms. It may be compared with the plain approach in SDP security descriptions [RFC4568]. 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 contributes to the key management. The method itself is highly scalable, but again, without proper protection through an underlying security layer, it is not advisable for use.
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3.5. Diffie-Hellman Key Agreement Protected with Pre-Shared Secrets
This is an additional option, which has been defined in [RFC4650]. In contrast to the method described in Section 3.3, here the Diffie- Hellman key agreement is authenticated (and integrity protected) using a pre-shared secret and keyed hash function.
For the integrity protection of the Diffie-Hellman key agreement, [RFC4650] mandates the use of HMAC SHA-1. Regarding Diffie-Hellman groups, [RFC3830] is referenced. Thus, it is mandatory to support the Diffie-Hellman group "OAKLEY5" [RFC2412]. It is also possible to use other Diffie-Hellman groups within MIKEY. This can be done by defining a new mapping sub-payload and the associated policy payload according to RFC 3830. 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 Private Branch Exchanges (PBXs), service provider proxies, etc. In contrast to the plain pre-shared key encryption-based mode, described in Section 3.1, this mode offers perfect forward secrecy as well as active involvement in the key generation of both parties involved.
There has been a longer discussion during IETF meetings and also on the IETF MSEC mailing list about a SAML-assisted DH approach. This idea has not been submitted as a separate document. Nevertheless, the discussion is reflected here as it is targeted to fulfill general
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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
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 Agent's (UA's) SIP server signed element, authenticating the Diffie-Hellman key and the ID using the SAML (Security Assertion Markup Language [SAML_overview]) 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 roundtrip would be handled via SP, the security policy communicated via MIKEY.
Note that 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 a straight-forward call, one roundtrip may be enough to set up 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), and 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. 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 eliminates not only 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 Man-in-the-Middle (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 separate component (server) is necessary, so serverless call setup is not supported.
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3.7. Asymmetric Key Distribution with In-Band Certificate Exchange
This is an additional option, which has been defined in [RFC4738]. It describes the asymmetric key distribution with optional in-band certificate exchange.
This option has some advantages compared to the asymmetric key distribution stated in Section 3.2. 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, RFC 3830'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 responder 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 key material since the key is derived from 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 significantly 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). Examples of deployments where
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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] extensions for crypto algorithms and generic payload enhancements, as well as enhancements to support the negotiation of security parameters for security protocols other than SRTP. These extensions have been defined in several additional documents.
[MSEC-MIKEY] 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 MIKEY stems from the following advantages:
o ECC modes are more and more added to security protocols.
o 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 Advanced Encryption Standard (AES) keys of 128, 192, and 256 bits without extensive growth in asymmetric key sizes.
o As stated in [MSEC-MIKEY], 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.
[MSEC-MIKEY] proposes several ECC-based mechanisms to enhance the MIKEY key distribution schemes:
o Use of ECC methods extending the Diffie-Hellman key exchange: MIKEY-DHSIGN with ECDSA or ECGDSA
o Use of ECC methods extending the Diffie-Hellman key exchange: MIKEY-DHSIGN with ECDH
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o Use of Elliptic Curve Integrated Encryption Scheme (MIKEY-ECIES)
o Use of Elliptic Curve Menezes-Qu-Vanstone Scheme(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:
TESLA [RFC4082] 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] 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.
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[RFC4442] 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] 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] specifies a new general extension payload type for use in the Open Mobile Alliance (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 extension 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.
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4.5. Supporting Integrity Transform Carrying the Rollover Counter
The document [RFC4771] defines a new integrity transform for SRTP [RFC3711] providing the option to also transmit the Roll Over Counter (ROC) as part of dedicated SRTP packets. This extension has been defined for 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 definitions comprise the integrity transform itself as well as a new integrity transform parameters. 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 choices 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 the 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. It is outside the scope of MIKEY or MIKEY extensions if the caching of envelope keys is allowed. This is a matter of the configuration of the involved components. This local configuration is also outside the scope of MIKEY. 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 call 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 in which 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
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32 bits long and there is a high probability of a two-time pad problem. This is due to the support of scenarios like forking (see also Section 5.2) or retargeting (see also Section 5.3), where a two- time pad occurs if two branches have the same TEK (based on the MIKEY key establishment) and choose the same 32-bit SSRC for the SRTP streams and transmit SRTP packets. 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 responder to have his certificate, he may use RSA-R. Using RSA-R, he can provide the Initiator's certificate information in-band to the receiver. Moreover, the Initiator may also provide a random number that can be used by the receiver for key generation. Thus, both parties can be involved in the key management. But as the inclusion of the random number cannot be forced by the Initiator, true PFS cannot be provided. 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 Initiator's side (and likewise at the responder's side) but lower-level security (like TLS or IPsec) is in place the user may use the unprotected mode of MIKEY. It has to considered that using the unprotected mode 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.
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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 [SIP-MEDIA]. The following list provides the most prominent call scenarios and are matter of further discussion:
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 message. This situation appears usually in call center scenarios, when queuing 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 and Section 3.2), 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, Section 3.5, and Section 3.6) or the enhanced asymmetric variant (Section 3.7), 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.
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It is worth mentioning here that security descriptions [RFC4568] have basically the same problem as the initiating end needs the SDP answer before it can start decrypting SRTP media.
To cope with the early media problem, there are further approaches to describe security preconditions [RFC5027]; i.e., certain preconditions need to be met to enable voice data encryption. One example, for instance, is that a scenario where a provisional response, containing the required MIKEY parameter, is sent before encrypted media is processed.
In SIP forking scenarios, a SIP proxy server sends an INVITE request to more than one location. This means also that 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 and Section 3.7) 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 and Section 3.2). 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 that 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. MIKEY Diffie Hellman modes or MIKEY-RSA_R Section 3.7 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 sender's 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.
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 scenario.
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 that 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 where the caller used asymmetric key encryption (MIKEY- RSA) because the key management was originally targeted to a different destination.
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 roles; 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 a 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].
The following two tables summarize the discussion from the previous subsections. The first table matches the scenarios discussed in this section to the different MIKEY modes.
* In centralized conferencing, the media mixer needs to send the MIKEY Initiator message.
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]) to transport the session parameters. Examples are the Session Initiation Protocol (SIP, [RFC3261] or the Gateway Control Protocol (GCP, [RFC5125]). The transport of MIKEY messages as part of SDP is described in [RFC4567]. Here, the
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complete MIKEY message is base64 encoded and transmitted as part of the SDP part of the signaling protocol message. Note that as several key distribution messages may be transported within one SDP container, [RFC4567] also comprises an integrity protection regarding all supplied key distribution attempts. Thus, bidding-down attacks will be recognized. Regarding RTSP, [RFC4567] defines header extensions allowing the transport of MIKEY messages. Here, the initial messages uses SDP, while the remaining part of the key management is performed using the header extensions.
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] 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 exist 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. The following list provides a short summary about possible alternatives:
o sdescription - [RFC4568] describes a key management scheme, which uses SDP for transport and completely relies on underlying protocol security. For transport, the document defines an 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.
o sdescription with early media support - [WING-MMUSIC] enhances the above scheme with the possibility to also be usable in early media scenarios, when security preconditions are not used.
o Encrypted Key Transport for Secure RTP - [MCGREW-SRTP] 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.
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o Diffie-Hellman support in SDP - [BAUGHER] 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.
o DTLS-SRTP describing SRTP extensions for DTLS - [AVT-DTLS] 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 that establishes SRTP keys.
o ZRTP - [ZIMMERMANN] 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 long 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 [AVT-DTLS]. Note that MIKEY has already been deployed for setting up SRTP security context and is also targeted for use in MBMS applications.
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.
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
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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 levels of security and also require different infrastructure support.
As this document is based 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.
[AVT-DTLS] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for Secure Real-time Transport Protocol (SRTP)", Work in Progress, February 2008.
[BAUGHER] Baugher, M. and D. McGrew, "Diffie-Hellman Exchanges for Multimedia Sessions", Work in Progress, February 2006.
[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.
[ISO_sec_time] ""ISO/IEC 18014 Information technology - Security techniques - Time-stamping services, Part 1- 3.http://www.oasis-open.org/committees/ documents.php?wg_abbrev=security"", 2002.
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[MCGREW-SRTP] McGrew, D., "Encrypted Key Transport for Secure RTP", Work in Progress, March 2007.
[MSEC-MIKEY] Milne, A., "ECC Algorithms for MIKEY", Work in Progress, June 2007.
[RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992.
[RFC2412] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412, November 1998.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Secure Real-time Transport Protocol (SRTP)", RFC 4383, February 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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
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[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.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, July 2006.
[RFC4597] Even, R. and N. Ismail, "Conferencing Scenarios", RFC 4597, August 2006.
[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for Multimedia Internet KEYing (MIKEY)", RFC 4650, September 2006.
[RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-RSA-R: An Additional Mode of Key Distribution in Multimedia Internet KEYing (MIKEY)", RFC 4738, November 2006.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)", RFC 4771, January 2007.
[RFC4909] Dondeti, L., Castleford, D., and F. Hartung, "Multimedia Internet KEYing (MIKEY) General Extension Payload for Open Mobile Alliance BCAST LTKM/STKM Transport", RFC 4909, June 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007.
[RFC5027] Andreasen, F. and D. Wing, "Security Preconditions for Session Description Protocol (SDP) Media Streams", RFC 5027, October 2007.
[SAML_overview] Huges, J. and E. Maler, "Security Assertion Markup Language (SAML) 2.0 Technical Overview, Working Draft", 2005.
[SIP-MEDIA] Wing, D., Fries, S., Tschofenig, H., and F. Audet, "Requirements and Analysis of Media Security Management Protocols", Work in Progress, June 2008.
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[WING-MMUSIC] Raymond, R. and D. Wing, "Security Descriptions Extension for Early Media", Work in Progress, October 2005.
[ZIMMERMANN] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media Path Key Agreement for Secure RTP", Work in Progress, June 2008.
Dragan Ignjatic Polycom 3605 Gilmore Way Burnaby, BC V5G 4X5 Canada
EMail: dignjatic@polycom.com
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