Options for Securing RTP Sessions
draft-ietf-avtcore-rtp-security-options-09
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7201.
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Authors | Magnus Westerlund , Colin Perkins | ||
Last updated | 2013-12-19 (Latest revision 2013-11-12) | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Reviews | |||
Additional resources | Mailing list discussion | ||
Stream | WG state | Submitted to IESG for Publication | |
Document shepherd | Roni Even | ||
Shepherd write-up | Show Last changed 2013-10-31 | ||
IESG | IESG state | Became RFC 7201 (Informational) | |
Consensus boilerplate | Yes | ||
Telechat date | (None) | ||
Responsible AD | Richard Barnes | ||
Send notices to | avtcore-chairs@tools.ietf.org, draft-ietf-avtcore-rtp-security-options@tools.ietf.org | ||
IANA | IANA review state | IANA OK - No Actions Needed |
draft-ietf-avtcore-rtp-security-options-09
#x27;t support group keys. It is designed for centralized conferencing, but can also be used in sessions where end- points connect to a conference bridge or a gateway, and need to be provisioned with the keys each participant on the bridge or gateway uses to avoid decryption and encryption cycles on the bridge or gateway. This can enable interworking between DTLS-SRTP and other keying systems where either party can set the key (e.g., interworking with security descriptions). The mechanism is based on establishing an additional EKT key which everyone uses to protect their actual session key. The actual session key is sent in a expanded authentication tag to the other session participants. This key is only sent occasionally or periodically depending on use cases and depending on what requirements exist for timely delivery or notification. The only known deployment of EKT so far are in some Cisco video conferencing products. 3.1.5. Key Management for SRTP: Other systems The ZRTP [RFC6189] key-management system for SRTP was proposed as an alternative to DTLS-SRTP. ZRTP provides best effort encryption independent of the signalling protocol and utilizes key continuity, Short Authentication Strings, or a PKI for authentication. ZRTP wasn't adopted as an IETF standards track protocol, but was instead published as an informational RFC. Commercial implementations exist. Westerlund & Perkins Expires May 16, 2014 [Page 15] Internet-Draft Options for Securing RTP Sessions November 2013 Additional proprietary solutions are also known to exist. 3.2. RTP Legacy Confidentiality Section 9 of the RTP standard [RFC3550] defines a DES or 3DES based encryption of RTP and RTCP packets. This mechanism is keyed using plain text keys in SDP [RFC4566] using the "k=" SDP field. This method can provide confidentiality but, as discussed in Section 9 of [RFC3550], it has extremely weak security properties and is not to be used. 3.3. IPsec IPsec [RFC4301] can be used in either tunnel or transport mode to protect RTP and RTCP packets in transit from one network interface to another. This can be sufficient when the network interfaces have a direct relation, or in a secured environment where it can be controlled who can read the packets from those interfaces. The main concern with using IPsec to protect RTP traffic is that in most cases using a VPN approach that terminates the security association at some node prior to the RTP end-point leaves the traffic vulnerable to attack between the VPN termination node and the end-point. Thus usage of IPsec requires careful thought and design of its usage so that it meets the security goals. A important question is how one ensures the IPsec terminating peer and the ultimate destination are the same. Applications can have issues using existing APIs with determining if IPsec is being used or not, and when used who the authenticated peer entity is. IPsec with RTP is more commonly used as a security solution between infrastructure nodes that exchange many RTP sessions and media streams. The establishment of a secure tunnel between such nodes minimizes the key-management overhead. 3.4. RTP over TLS over TCP Just as RTP can be sent over TCP [RFC4571], it can also be sent over TLS over TCP [RFC4572], using TLS to provide point-to-point security services. The security properties TLS provides are confidentiality, integrity protection and possible source authentication if the client or server certificates are verified and provide a usable identity. When used in multi-party scenarios using a central node for media distribution, the security provide is only between the central node and the peers, so the security properties for the whole session are dependent on what trust one can place in the central node. Westerlund & Perkins Expires May 16, 2014 [Page 16] Internet-Draft Options for Securing RTP Sessions November 2013 RTSP 1.0 [RFC2326] and 2.0 [I-D.ietf-mmusic-rfc2326bis] specifies the usage of RTP over the same TLS/TCP connection that the RTSP messages are sent over. It appears that RTP over TLS/TCP is also used in some proprietary solutions that uses TLS to bypass firewalls. 3.5. RTP over Datagram TLS (DTLS) Datagram Transport Layer Security (DTLS) [RFC6347] is a based on TLS [RFC5246], but designed to work over a unreliable datagram oriented transport rather than requiring reliable byte stream semantics from the transport protocol. Accordingly, DTLS can provide point-to-point security for RTP flows analogous to that provided by TLS, but over an datagram transport such as UDP. The two peers establish an DTLS association between each other, including the possibility to do certificate-based source authentication when establishing the association. All RTP and RTCP packets flowing will be protected by this DTLS association. Note that using DTLS for RTP flows is different to using DTLS-SRTP key management. DTLS-SRTP uses the same key-management steps as DTLS, but uses SRTP for the per packet security operations. Using DTLS for RTP flows uses the normal datagram TLS data protection, wrapping complete RTP packets. When using DTLS for RTP flows, the RTP and RTCP packets are completely encrypted with no headers in the clear; when using DTLS-SRTP, the RTP headers are in the clear and only the payload data is encrypted. DTLS can use similar techniques to those available for DTLS-SRTP to bind a signalling-side agreement to communicate to the certificates used by the end-point when doing the DTLS handshake. This enables use without having a certificate-based trust chain to a trusted certificate root. There does not appear to be significant usage of DTLS for RTP. 3.6. Media Content Security/Digital Rights Management Mechanisms have been defined that encrypt only the media content, operating within the RTP payload data and leaving the RTP headers and RTCP unaffected. There are several reasons why this might be appropriate, but a common rationale is to ensure that the content stored by RTSP streaming servers has the media content in a protected format that cannot be read by the streaming server (this is mostly done in the context of Digital Rights Management). These approaches then use a key-management solution between the rights provider and the consuming client to deliver the key used to protect the content and do not give the media server access to the security context. Such methods have several security weaknesses such as the fact that Westerlund & Perkins Expires May 16, 2014 [Page 17] Internet-Draft Options for Securing RTP Sessions November 2013 the same key is handed out to a potentially large group of receiving clients, increasing the risk of a leak. Use of this type of solution can be of interest in environments that allow middleboxes to rewrite the RTP headers and select which streams are delivered to an end-point (e.g., some types of centralised video conference systems). The advantage of encrypting and possibly integrity protecting the payload but not the headers is that the middlebox can't eavesdrop on the media content, but can still provide stream switching functionality. The downside of such a system is that it likely needs two levels of security: the payload level solution to provide confidentiality and source authentication, and a second layer with additional transport security ensuring source authentication and integrity of the RTP headers associated with the encrypted payloads. This can also results in the need to have two different key-management systems as the entity protecting the packets and payloads are different with different set of keys. The aspect of two tiers of security are present in ISMACryp (see Section 3.6.1) and the deprecated 3GPP Packet Based Streaming Service Annex.K [T3GPP.26.234R8] solution. 3.6.1. ISMA Encryption and Authentication The Internet Streaming Media Alliance (ISMA) has defined ISMA Encryption and Authentication 2.0 [ISMACryp2]. This specification defines how one encrypts and packetizes the encrypted application data units (ADUs) in an RTP payload using the MPEG-4 Generic payload format [RFC3640]. The ADU types that are allowed are those that can be stored as elementary streams in an ISO Media File format based file. ISMACryp uses SRTP for packet level integrity and source authentication from a streaming server to the receiver. Key-management for a ISMACryp based system can be achieved through Open Mobile Alliance (OMA) Digital Rights Management 2.0 [OMADRMv2], for example. 4. Securing RTP Applications In the following we provide guidelines for how to choose appropriate security mechanisms for RTP applications. 4.1. Application Requirements This section discusses a number of application requirements that need be considered. An application designer choosing security solutions requires a good understanding of what level of security is needed and what behaviour they strive to achieve. Westerlund & Perkins Expires May 16, 2014 [Page 18] Internet-Draft Options for Securing RTP Sessions November 2013 4.1.1. Confidentiality When it comes to confidentiality of an RTP session there are several aspects to consider: Probability of compromise: When using encryption to provide media confidentiality, it is necessary to have some rough understanding of the security goal and how long one expect the protected content to remain confidential. National or other regulations might provide additional requirements on a particular usage of an RTP. From that, one can determine which encryption algorithms are to be used from the set of available transforms. Potential for other leakage: RTP based security in most of its forms simply wraps RTP and RTCP packets into cryptographic containers. This commonly means that the size of the original RTP payload is visible to observers of the protected packet flow. This can provide information to those observers. A well-documented case is the risk with variable bit-rate speech codecs that produce different sized packets based on the speech input [RFC6562]. Potential threats such as these need to be considered and, if they are significant, then restrictions will be needed on mode choices in the codec, or additional padding will need to be added to make all packets equal size and remove the informational leakage. Another case is RTP header extensions. If SRTP is used, header extensions are normally not protected by the security mechanism protecting the RTP payload. If the header extension carries information that is considered sensitive, then the application needs to be modified to ensure that mechanisms used to protect against such information leakage are employed. Who has access: When considering the confidentiality properties of a system, it is important to consider where the media handled in the clear. For example, if the system is based on an RTP mixer that needs the keys to decrypt the media, process, and repacketize it, then is the mixer providing the security guarantees expected by the other parts of the system? Furthermore, it is important to consider who has access to the keys. The policies for the handling of the keys, and who can access the keys, need to be considered along with the confidentiality goals. As can be seen the actual confidentiality level has likely more to do with the application's usage of centralized nodes, and the details of the key-management solution chosen, than with the actual choice of encryption algorithm (although, of course, the encryption algorithm needs to be chosen appropriately for the desired security level). Westerlund & Perkins Expires May 16, 2014 [Page 19] Internet-Draft Options for Securing RTP Sessions November 2013 4.1.2. Integrity Protection against modification of content by a third party, or due to errors in the network, is another factor to consider. The first aspect that one considers is what resilience one has against modifications to the content. Some media types are extremely sensitive to network bit errors, whereas others might be able to tolerate some degree of data corruption. Equally important is to consider the sensitivity of the content, who is providing the integrity assertion, what is the source of the integrity tag, and what are the risks of modifications happening prior to that point where protection is applied? These issues affect what cryptographic algorithm is used, and the length of the integrity tags, and whether the entire payload is protected. RTP applications that rely on central nodes need to consider if hop- by-hop integrity is acceptable, or if true end-to-end integrity protection is needed? Is it important to be able to tell if a middlebox has modified the data? There are some uses of RTP that require trusted middleboxes that can modify the data in a way that doesn't break integrity protection as seen by the receiver, for example local advertisement insertion in IPTV systems; there are also uses where it is essential that such in-network modification be detectable. RTP can support both, with appropriate choices of security mechanisms. Integrity of the data is commonly closely tied to the question of source authentication. That is, it becomes important to know who makes an integrity assertion for the data. 4.1.3. Source Authentication Source authentication is about determining who sent a particular RTP or RTCP packet. It is normally closely tied with integrity, since a receiver generally also wants to ensure that the data received is what the source really sent, so source authentication without integrity is not particularly useful. Similarly, integrity protection without source authentication is also not particularly useful; a claim that a packet is unchanged that cannot itself be validated as from the source (or some from other known and trusted party) is meaningless. Source authentication can be asserted in several different ways: Base level: Using cryptographic mechanisms that give authentication with some type of key-management provide an implicit method for source authentication. Assuming that the mechanism has sufficient strength to not be circumvented in the time frame when you would Westerlund & Perkins Expires May 16, 2014 [Page 20] Internet-Draft Options for Securing RTP Sessions November 2013 accept the packet as valid, it is possible to assert a source- authenticated statement; this message is likely from a source that has the cryptographic key(s) to this communication. What that assertion actually means is highly dependent on the application and how it handles the keys. If only the two peers have access to the keys, this can form a basis for a strong trust relationship that traffic is authenticated coming from one of the peers. However, in a multi-party scenario where security contexts are shared among participants, most base-level authentication solutions can't even assert that this packet is from the same source as the previous packet. Binding the source and the signalling: A step up in the assertion that can be done in base-level systems is to tie the signalling to the key-exchange. Here, the goal is to at least be able to assert that the source of the packets is the same entity that the receiver established the session with. How feasible this is depends on the properties of the key-management system, the ability to tie the signalling to a particular source, and the degree of trust the receiver places on the different nodes involved. For example, systems where the key-exchange is done using the signalling systems, such as Security Descriptions [RFC4568], enable a direct binding between signalling and key-exchange. In such systems, the actual security depends on the trust one can place in the signalling system to correctly associate the peer's identity with the key-exchange. Using Identities: If the applications have access to a system that can provide verifiable identities, then the source authentication can be bound to that identity. For example, in a point-to-point communication even symmetric key crypto, where the key-management can assert that the key has only been exchanged with a particular identity, can provide a strong assertion about the source of the traffic. SIP identity [RFC4474] provides one example of how this can be done, and could be used to bind DTLS-SRTP certificates to the identity provider's public key to authenticate the source of a DTLS-SRTP flow. Note that all levels of the system need to have matching capability to assert identity. If the signalling can assert that only a given entity in a multiparty session has a key, then the media layer might be able to provide guarantees about the identity of the media sender. However, using an signalling authentication mechanism built on a group key can limit the media layer to asserting only group membership. Westerlund & Perkins Expires May 16, 2014 [Page 21] Internet-Draft Options for Securing RTP Sessions November 2013 4.1.4. Identity There exist many different types of identity systems with different properties (e.g., SIP identity [RFC4474]). In the context of RTP applications, the most important property is the possibility to perform source authentication and verify such assertions in relation to any claimed identities. What an identity really is can also vary but, in the context of communication, one of the most obvious is the identity of the human user one communicates with. However, the human user can also have additional identities in a particular role. For example, the human Alice, can also be a police officer and in some cases her identity as police officer will be more relevant then that she is Alice. This is common in contact with organizations, where it is important to prove the persons right to represent the organization. Some examples of identity mechanisms that can be used: Certificate based: A certificate is used to prove the identity, by having access to the private part of the certificate one can perform signing to assert ones identity. Any entity interested in verifying the assertion then needs the public part of the certificate. By having the certificate, one can verify the signature against the certificate. The next step is to determine if one trusts the certificate's trust chain. Commonly by provisioning the verifier with the public part of a root certificate, this enables the verifier to verify a trust chain from the root certificate down to the identity certificate. However, the trust is based on all steps in the certificate chain being verifiable and trusted. Thus provisioning of root certificates and the ability to revoke compromised certificates are aspects that will require infrastructure. Online Identity Providers: An online identity provider (IdP) can authenticate a user's right to use an identity, then perform assertions on their behalf or provision the requester with short- term credentials to assert their identity. The verifier can then contact the IdP to request verification of a particular identity. Here the trust is highly dependent on how much one trusts the IdP. The system also becomes dependent on having access to the relevant IdP. In all of the above examples, an important part of the security properties are related to the method for authenticating the access to the identity. 4.1.5. Privacy RTP applications need to consider what privacy goals they have. As RTP applications communicate directly between peers in many cases, Westerlund & Perkins Expires May 16, 2014 [Page 22] Internet-Draft Options for Securing RTP Sessions November 2013 the IP addresses of any communication peer will be available. The main privacy concern with IP addresses is related to geographical location and the possibility to track a user of an end-point. The main way of avoid such concerns is the introduction of relay (e.g., a TURN server [RFC5766]) or centralized media mixers or forwarders that hides the address of a peer from any other peer. The security and trust placed in these relays obviously needs to be carefully considered. RTP itself can contribute to enabling a particular user to be tracked between communication sessions if the CNAME is generated according to the RTP specification in the form of user@host. Such RTCP CNAMEs are likely long term stable over multiple sessions, allowing tracking of users. This can be desirable for long-term fault tracking and diagnosis, but clearly has privacy implications. Instead cryptographically random ones could be used as defined by Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs) [RFC7022]. If there exist privacy goals, these need to be considered, and the system designed with them in mind. In addition certain RTP features might have to be configured to safeguard privacy, or have requirements on how the implementation is done. 4.2. Application Structure When it comes to RTP security, the most appropriate solution is often highly dependent on the topology of the communication session. The signalling also impacts what information can be provided, and if this can be instance specific, or common for a group. In the end the key- management system will highly affect the security properties achieved by the application. At the same time, the communication structure of the application limits what key management methods are applicable. As different key-management have different requirements on underlying infrastructure it is important to take that aspect into consideration early in the design. 4.3. Automatic Key Management The Guidelines for Cryptographic Key Management [RFC4107] provide an overview of why automatic key management is important. They also provide a strong recommendation on using automatic key management. Most of the security solutions reviewed in this document provide or support automatic key management, at least to establish session keys. In some more long term use cases, credentials might in certain cases need to be be manually deployed. Westerlund & Perkins Expires May 16, 2014 [Page 23] Internet-Draft Options for Securing RTP Sessions November 2013 For SRTP an important aspect of automatic key management is to ensure that two time pads do not occur, in particular by preventing multiple end points using the same session key and SSRC. In these cases automatic key management methods can have strong dependencies on signalling features to function correctly. If those dependencies can't be fulfilled, additional constrains on usage, e.g., per-end point session keys, might be needed to avoid the issue. When selecting security mechanisms for an RTP application it is important to consider the properties of the key management. Using key management that is both automatic and integrated will provide minimal interruption for the user, and is important to ensure that security can, and will remain, to be on by default. 4.4. End-to-End Security vs Tunnels If the security mechanism only provides a secured tunnel, for example like some common uses of IPSec Section 3.3, it is important to consider the full end-to-end properties of the system. How does one ensure that the path from the endpoint to the local tunnel ingress/ egress is secure and can be trusted (and similarly for the other end of the tunnel)? How does one handle the source authentication of the peer, as the security protocol identifies the other end of the tunnel. These are some of the issues that arise when one considers a tunnel based security protocol rather than an end-to-end. Even with clear requirements and knowledge that one still can achieve the security properties using a tunnel based solution, one ought to prefer to use end-to-end mechanisms, as they are much less likely to violate any assumptions made about deployment. These assumptions can also be difficult to automatically verify. 4.5. Plain Text Keys Key management solutions that use plain text keys, like SDP Security Descriptions (Section 3.1.3), require care to ensure a secure transport of the signalling messages that contain the plain text keys. For plain text keys the security properties of the system depend on how securely the plain text keys are protected end-to-end between the sender and receiver(s). Not only does one need to consider what transport protection is provided for the signalling message including the keys, but also the degree to which any intermediaries in the signalling are trusted. Untrusted intermediaries can perform man in the middle attacks on the communication, or can log the keys with the result in encryption being compromised significantly after the actual communication occurred. Westerlund & Perkins Expires May 16, 2014 [Page 24] Internet-Draft Options for Securing RTP Sessions November 2013 4.6. Interoperability Few RTP applications exist as independent applications that never interoperate with anything else. Rather, they enable communication with a potentially large number of other systems. To minimize the number of security mechanisms that need to be implemented, it is important to consider if one can use the same security mechanisms as other applications. This can also reduce problems of determining what security level is actually negotiated in a particular session. The desire to be interoperable can, in some cases, be in conflict with the security requirements of an application. To meet the security goals, it might be necessary to sacrifice interoperability. Alternatively, one can implement multiple security mechanisms, this however introduces the complication of ensuring that the user understands what it means to use a particular security system. In addition, the application can then become vulnerable to bid-down attack. 5. Examples In the following we describe a number of example security solutions for applications using RTP services or frameworks. These examples are provided to illustrate the choices available. They are not normative recommendations for security. 5.1. Media Security for SIP-established Sessions using DTLS-SRTP The IETF evaluated media security for RTP sessions established using point-to-point SIP sessions in 2009. A number of requirements were determined, and based on those, the existing solutions for media security and especially the keying methods were analysed. The resulting requirements and analysis were published in [RFC5479]. Based on this analysis and working group discussion, DTLS-SRTP was determined to be the best solution. Westerlund & Perkins Expires May 16, 2014 [Page 25] Internet-Draft Options for Securing RTP Sessions November 2013 The security solution for SIP using DTLS-SRTP is defined in the Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS) [RFC5763]. On a high level the framework uses SIP with SDP offer/answer procedures to exchange the network addresses where the server end-point will have a DTLS-SRTP enable server running. The SIP signalling is also used to exchange the fingerprints of the certificate each end-point will use in the DTLS establishment process. When the signalling is sufficiently completed, the DTLS- SRTP client performs DTLS handshakes and establishes SRTP session keys. The clients also verify the fingerprints of the certificates to verify that no man in the middle has inserted themselves into the exchange. DTLS has a number of good security properties. For example, to enable a man in the middle someone in the signalling path needs to perform an active action and modify both the signalling message and the DTLS handshake. There also exists solutions that enables the fingerprints to be bound to identities. SIP Identity provides an identity established by the first proxy for each user [RFC4474]. This reduces the number of nodes the connecting user User Agent has to trust to include just the first hop proxy, rather than the full signalling path. The biggest security weakness of this system is its dependency on the signalling. SIP signalling passes multiple nodes and there is usually no message security deployed, only hop-by-hop transport security, if any, between the nodes. 5.2. Media Security for WebRTC Sessions Web Real-Time Communication (WebRTC) [I-D.ietf-rtcweb-overview] is a solution providing JavaScript web applications with real-time media directly between browsers. Media is transported using RTP protected using a mandatory application of SRTP [RFC3711], with keying done using DTLS-SRTP [RFC5764]. The security configuration is further defined in the WebRTC Security Architecture [I-D.ietf-rtcweb-security-arch]. A hash of the peer's certificate is provided to the JavaScript web application, allowing that web application to verify identity of the peer. There are several ways in which the certificate hashes can be verified. An approach identified in the WebRTC security architecture [I-D.ietf-rtcweb-security-arch] is to use an identity provider. In this solution the Identity Provider, which is a third party to the web application, signs the DTLS-SRTP hash combined with a statement on the validity of the user identity that has been used to sign the hash. The receiver of such an identity assertion can then independently verify the user identity to ensure that it is the identity that the receiver intended to communicate with, and that the Westerlund & Perkins Expires May 16, 2014 [Page 26] Internet-Draft Options for Securing RTP Sessions November 2013 cryptographic assertion holds; this way a user can be certain that the application also can't perform a MITM and acquire the keys to the media communication. Other ways of verifying the certificate hashes exist, for example they could be verified against a hash carried in some out of band channel (e.g., compare with a hash printed on a business card), or using a verbal short authentication string (e.g., as in ZRTP [RFC6189]), or using hash continuity. In the development of WebRTC there has also been attention given to privacy considerations. The main RTP-related concerns that have been raised are: Location Disclosure: As ICE negotiation [RFC5245] provides IP addresses and ports for the browser, this leaks location information in the signalling to the peer. To prevent this one can block the usage of any ICE candidate that isn't a relay candidate, i.e. where the IP and port provided belong to the service providers media traffic relay. Prevent tracking between sessions: static RTP CNAMEs and DTLS-SRTP certificates provide information that is re-used between session instances. Thus to prevent tracking, such information is ought not be re-used between sessions, or the information ought not sent in the clear. Note, that generating new certificates each time prevents continuity in authentication, however, as WebRTC users are expected to use multiple devices to access the same communication service, such continuity can't be expected anyway, instead the above described identity mechanism has to be relied on. Note: The above cases are focused on providing privacy from other parties, not on providing privacy from the web server that provides the WebRTC Javascript application. 5.3. IP Multimedia Subsystem (IMS) Media Security In IMS, the core network is controlled by a single operator, or by several operators with high trust in each other. Except for some types of accesses, the operator is in full control, and no packages are routed over the Internet. Nodes in the core network offer services such as voice mail, interworking with legacy systems (PSTN, GSM, and 3G), and transcoding. End-points are authenticated during the SIP registration using either IMS-AKA (using SIM credentials) or SIP Digest (using password). In IMS media security [T3GPP.33.328], end-to-end encryption is therefore not seen as needed or desired as it would hinder for example interworking and transcoding, making calls between Westerlund & Perkins Expires May 16, 2014 [Page 27] Internet-Draft Options for Securing RTP Sessions November 2013 incompatible terminals impossible. Because of this IMS media security mostly uses end-to-access-edge security where SRTP is terminated in the first node in the core network. As the SIP signaling is trusted and encrypted (with TLS or IPsec), security descriptions [RFC4568] is considered to give good protection against eavesdropping over the accesses that are not already encrypted (GSM, 3G, LTE). Media source authentication is based on knowledge of the SRTP session key and trust in that the IMS network will only forward media from the correct end-point. For enterprises and government agencies, which might have weaker trust in the IMS core network and can be assumed to have compatible terminals, end-to-end security can be achieved by deploying their own key management server. Work on Interworking with WebRTC is currently ongoing; the security will still be end-to-access-edge, but using DTLS-SRTP [RFC5763] instead of security descriptions. 5.4. 3GPP Packet Based Streaming Service (PSS) The 3GPP Release 11 PSS specification of the Packet Based Streaming Service (PSS) [T3GPP.26.234R11] defines, in Annex R, a set of security mechanisms. These security mechanisms are concerned with protecting the content from being copied, i.e. Digital Rights Management. To meet these goals with the specified solution, the client implementation and the application platform are trusted to protect against access and modification by an attacker. PSS is RTSP 1.0 [RFC2326] controlled media streaming over RTP. Thus an RTSP client whose user wants to access a protected content will request a session description (SDP [RFC4566]) for the protected content. This SDP will indicate that the media is ISMACryp 2.0 [ISMACryp2] protected media encoding application units (AUs). The key(s) used to protect the media are provided in either of two ways. If a single key is used then the client uses some DRM system to retrieve the key as indicated in the SDP. Commonly OMA DRM v2 [OMADRMv2] will be used to retrieve the key. If multiple keys are to be used, then an additional RTSP stream for key-updates in parallel with the media streams is established, where key updates are sent to the client using Short Term Key Messages defined in the "Service and Content Protection for Mobile Broadcast Services" section of the OMA Mobile Broadcast Services [OMABCAST]. Worth noting is that this solution doesn't provide any integrity verification method for the RTP header and payload header information, only the encoded media AU is protected. 3GPP has not defined any requirement for supporting any solution that could Westerlund & Perkins Expires May 16, 2014 [Page 28] Internet-Draft Options for Securing RTP Sessions November 2013 provide that service. Thus, replay or insertion attacks are possible. Another property is that the media content can be protected by the ones providing the media, so that the operators of the RTSP server has no access to unprotected content. Instead all that want to access the media is supposed to contact the DRM keying server and if the device is acceptable they will be given the key to decrypt the media. To protect the signalling, RTSP 1.0 supports the usage of TLS. This is, however, not explicitly discussed in the PSS specification. Usage of TLS can prevent both modification of the session description information and help maintain some privacy of what content the user is watching as all URLs would then be confidentiality protected. 5.5. RTSP 2.0 Real-time Streaming Protocol 2.0 [I-D.ietf-mmusic-rfc2326bis] offers an interesting comparison to the PSS service (Section 5.4) that is based on RTSP 1.0 and service requirements perceived by mobile operators. A major difference between RTSP 1.0 and RTSP 2.0 is that 2.0 is fully defined under the requirement to have mandatory to implement security mechanism. As it specifies how one transport media over RTP it is also defining security mechanisms for the RTP transported media streams. The security goals for RTP in RTSP 2.0 is to ensure that there is confidentiality, integrity and source authentication between the RTSP server and the client. This to prevent eavesdropping on what the user is watching for privacy reasons and to prevent replay or injection attacks on the media stream. To reach these goals, the signalling also has to be protected, requiring the use of TLS between the client and server. Using TLS-protected signalling the client and server agree on the media transport method when doing the SETUP request and response. The secured media transport is SRTP (SAVP/RTP) normally over UDP. The key management for SRTP is MIKEY using RSA-R mode. The RSA-R mode is selected as it allows the RTSP Server to select the key despite having the RTSP Client initiate the MIKEY exchange. It also enables the reuse of the RTSP servers TLS certificate when creating the MIKEY messages thus ensuring a binding between the RTSP server and the key exchange. Assuming the SETUP process works, this will establish a SRTP crypto context to be used between the RTSP Server and the Client for the RTP transported media streams. 6. IANA Considerations This document makes no request of IANA. Westerlund & Perkins Expires May 16, 2014 [Page 29] Internet-Draft Options for Securing RTP Sessions November 2013 Note to RFC Editor: this section can be removed on publication as an RFC. 7. Security Considerations This entire document is about security. Please read it. 8. Acknowledgements We thank the IESG for their careful review of [I-D.ietf-avt-srtp-not-mandatory] which led to the writing of this memo. John Mattsson has contributed the IMS Media Security example (Section 5.3). The authors wished to thank Christian Correll, Dan Wing, Kevin Gross, Alan Johnston, Michael Peck, Ole Jacobsen, and John Mattsson for review and proposals for improvements of the text. 9. Informative References [I-D.ietf-avt-srtp-not-mandatory] Perkins, C. and M. Westerlund, "Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single Media Security Solution", draft-ietf-avt-srtp-not-mandatory-14 (work in progress), October 2013. [I-D.ietf-avtcore-aria-srtp] Kim, W., Lee, J., Kim, D., Park, J., and D. Kwon, "The ARIA Algorithm and Its Use with the Secure Real-time Transport Protocol(SRTP)", draft-ietf-avtcore-aria-srtp-05 (work in progress), September 2013. [I-D.ietf-avtcore-srtp-aes-gcm] McGrew, D. and K. Igoe, "AES-GCM and AES-CCM Authenticated Encryption in Secure RTP (SRTP)", draft-ietf-avtcore-srtp- aes-gcm-10 (work in progress), September 2013. [I-D.ietf-avtcore-srtp-ekt] McGrew, D. and D. Wing, "Encrypted Key Transport for Secure RTP", draft-ietf-avtcore-srtp-ekt-01 (work in progress), October 2013. [I-D.ietf-mmusic-rfc2326bis] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M. Stiemerling, "Real Time Streaming Protocol 2.0 (RTSP)", draft-ietf-mmusic-rfc2326bis-38 (work in progress), October 2013. Westerlund & Perkins Expires May 16, 2014 [Page 30] Internet-Draft Options for Securing RTP Sessions November 2013 [I-D.ietf-rtcweb-overview] Alvestrand, H., "Overview: Real Time Protocols for Brower- based Applications", draft-ietf-rtcweb-overview-08 (work in progress), September 2013. [I-D.ietf-rtcweb-security-arch] Rescorla, E., "WebRTC Security Architecture", draft-ietf- rtcweb-security-arch-07 (work in progress), July 2013. [ISMACryp2] Internet Streaming Media Alliance (ISMA), "ISMA Encryption and Authentication, Version 2.0 release version", November 2007. [OMABCAST] Open Mobile Alliance, "OMA Mobile Broadcast Services V1.0", February 2009. [OMADRMv2] Open Mobile Alliance, "OMA Digital Rights Management V2.0", July 2008. [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, August 1989. [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [RFC3365] Schiller, J., "Strong Security Requirements for Internet Engineering Task Force Standard Protocols", BCP 61, RFC 3365, August 2002. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC3640] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., and P. Gentric, "RTP Payload Format for Transport of MPEG-4 Elementary Streams", RFC 3640, November 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. Westerlund & Perkins Expires May 16, 2014 [Page 31] Internet-Draft Options for Securing RTP Sessions November 2013 [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, June 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 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. [RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 4474, August 2006. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. Carrara, "Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC 4567, July 2006. [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (SDP) Security Descriptions for Media Streams", RFC 4568, July 2006. [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection- Oriented Transport", RFC 4571, July 2006. [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)", RFC 4572, July 2006. [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, 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. Westerlund & Perkins Expires May 16, 2014 [Page 32] Internet-Draft Options for Securing RTP Sessions November 2013 [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. [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007. [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. [RFC5197] Fries, S. and D. Ignjatic, "On the Applicability of Various Multimedia Internet KEYing (MIKEY) Modes and Extensions", RFC 5197, June 2008. [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet, "Requirements and Analysis of Media Security Management Protocols", RFC 5479, April 2009. [RFC5669] Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The SEED Cipher Algorithm and Its Use with the Secure Real- Time Transport Protocol (SRTP)", RFC 5669, August 2010. [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)", RFC 5763, May 2010. [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. Westerlund & Perkins Expires May 16, 2014 [Page 33] Internet-Draft Options for Securing RTP Sessions November 2013 [RFC6043] Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based Modes of Key Distribution in Multimedia Internet KEYing (MIKEY)", RFC 6043, March 2011. [RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure RTP", RFC 6188, March 2011. [RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media Path Key Agreement for Unicast Secure RTP", RFC 6189, April 2011. [RFC6267] Cakulev, V. and G. Sundaram, "MIKEY-IBAKE: Identity-Based Authenticated Key Exchange (IBAKE) Mode of Key Distribution in Multimedia Internet KEYing (MIKEY)", RFC 6267, June 2011. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012. [RFC6509] Groves, M., "MIKEY-SAKKE: Sakai-Kasahara Key Encryption in Multimedia Internet KEYing (MIKEY)", RFC 6509, February 2012. [RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of Variable Bit Rate Audio with Secure RTP", RFC 6562, March 2012. [RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure Real-time Transport Protocol (SRTP)", RFC 6904, April 2013. [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, "Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)", RFC 7022, September 2013. [T3GPP.26.234R11] 3GPP, "Technical Specification Group Services and System Aspects; Transparent end-to-end Packet-switched Streaming Service (PSS); Protocols and codecs", 3GPP TS 26.234 11.1.0, September 2012. [T3GPP.26.234R8] 3GPP, "Technical Specification Group Services and System Aspects; Transparent end-to-end Packet-switched Streaming Service (PSS); Protocols and codecs", 3GPP TS 26.234 8.4.0, September 2009. [T3GPP.26.346] Westerlund & Perkins Expires May 16, 2014 [Page 34] Internet-Draft Options for Securing RTP Sessions November 2013 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs", 3GPP TS 26.346 10.7.0, March 2013. [T3GPP.33.246] 3GPP, "3G Security; Security of Multimedia Broadcast/ Multicast Service (MBMS)", 3GPP TS 33.246 12.1.0, December 2012. [T3GPP.33.328] 3GPP, "IP Multimedia Subsystem (IMS) media plane security", 3GPP TS 33.328 12.1.0, December 2012. Authors' Addresses Magnus Westerlund Ericsson Farogatan 6 SE-164 80 Kista Sweden Phone: +46 10 714 82 87 Email: magnus.westerlund@ericsson.com Colin Perkins University of Glasgow School of Computing Science Glasgow G12 8QQ United Kingdom Email: csp@csperkins.org URI: http://csperkins.org/ Westerlund & Perkins Expires May 16, 2014 [Page 35]