Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group Mark Baugher INTERNET-DRAFT Dan Wing EXPIRES: August 2004 Cisco Systems February, 2004 Session Description Protocol Security Descriptions for Media Streams Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document defines a Session Description Protocol (SDP) cryptographic attribute for unicast media streams. The attribute describes a cryptographic key and other parameters, which serve to configure security for a unicast media stream in either a single message or a roundtrip exchange. The attribute can be used with a variety of SDP media transports and this document defines how to use it for the Secure Real-time Transport Protocol (SRTP) unicast media streams. The SDP crypto attribute requires the services of a data security protocol to secure the SDP message. INTERNET-DRAFT SDP Security Descriptions February, 2004 Table of Contents 1. Notational Conventions............................................3 2. Introduction......................................................3 3. SDP "Crypto" Attribute and Parameters.............................5 3.1 Tag.............................................................5 3.2 Crypto-suite....................................................5 3.3 Key Parameters..................................................6 3.4 Session Parameters..............................................6 3.5 Example.........................................................7 4. General Use of the crypto Attribute...............................7 4.1 Use With Offer/Answer...........................................8 4.1.1 Generating the Initial Offer - Unicast Streams............8 4.1.2 Generating the Initial Answer - Unicast Streams...........9 4.1.3 Offerer Processing of the Initial Answer - Unicast Streams10 4.1.4 Modifying the Session....................................10 4.2 Use Outside Offer/Answer.......................................10 4.3 General Backwards Compatibility Considerations.................10 5. SRTP Security Descriptions.......................................11 5.1 SRTP Key Parameter.............................................12 5.2 Crypto-suites..................................................14 5.2.1 AES_CM_128_HMAC_SHA1_80..................................15 5.2.2 AES_CM_128_HMAC_SHA1_32..................................15 5.2.3 F8_128_HMAC_SHA1_80......................................16 5.2.4 Adding new Crypto-suite Definitions......................16 5.3 Session Parameters.............................................16 5.3.1 KDR=n....................................................16 5.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP...................16 5.3.3 UNAUTHENTICATED_SRTP.....................................17 5.3.4 FEC_ORDER=order..........................................17 5.3.5 Window Size Hint (WSH)...................................17 5.3.6 Defining New SRTP Session Parameters.....................18 5.4 SRTP Crypto Context Initialization.............................18 5.5 Removal of Crypto Contexts.....................................20 6. SRTP-Specific Use of the crypto Attribute........................21 6.1 Use with Offer/Answer..........................................21 6.1.1 Generating the Initial Offer - Unicast Streams...........21 6.1.2 Generating the Initial Answer - Unicast Streams..........21 6.1.3 Offerer Processing of the Initial Answer - Unicast Streams22 6.1.4 Modifying the Session....................................22 6.1.5 Offer/Answer Example.....................................23 6.2 SRTP-Specific Use Outside Offer/Answer.........................24 6.3 Support for SIP Forking........................................24 6.4 SRTP-Specific Backwards Compatibility Considerations...........25 6.5 Operation with KEYMGT= and k= lines............................25 7. Security Considerations..........................................26 7.1 Authentication of packets......................................26 7.2 Keystream Reuse................................................26 7.3 Signaling Authentication and Signaling Encryption..............26 8. Grammar..........................................................28 8.1 Generic "Crypto" Attribute Grammar.............................28 Andreasen, Baugher & Wing [Page 2] INTERNET-DRAFT SDP Security Descriptions February, 2004 8.2 SRTP "Crypto" Attribute Grammar................................28 9. IANA Considerations..............................................29 9.1 Registration of the "crypto" attribute.........................29 9.2 New IANA Registries and Registration Procedures................29 9.2.1 Security Descriptions Key Method Registry and Registration30 9.2.2 Security Description Media Stream Transport Registry and Registration.....................................................30 9.3 Initial Registrations..........................................30 9.3.1 Key Method...............................................30 9.3.2 SRTP Media Stream Transport..............................31 10. Acknowledgements................................................32 11. Authors' Addresses..............................................32 12. Normative References............................................32 13. Informative References..........................................33 Intellectual Property Statement.....................................35 Acknowledgement.....................................................36 1. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The terminology in this document conforms to [RFC2828], "Internet Security Glossary". n^r is exponentiation where n is multiplied by itself r times; n and r are integers. 0..k is an integer range of all integers from 0 through k inclusive. Explanatory notes are provided in several places throughout the document; these notes are indented two spaces from the surrounding text. 2. Introduction The Session Description Protocol (SDP) [SDP] describes multimedia sessions, which can be audio, video, whiteboard, fax, modem, and other media streams. Security services such as data origin authentication, integrity and confidentiality are often needed for those streams. The Secure Real-time Transport Protocol (SRTP) [srtp] provides security services for RTP media and is signaled by use of secure RTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in an SDP media (m=) line. However, there are no means within SDP itself to configure SRTP beyond using default values. This document specifies a new SDP attribute called "crypto", which is used to signal and negotiate cryptographic parameters for media streams in general, and SRTP in particular. The definition of the crypto attribute in this document is limited to two-party unicast media streams where each source has a unique cryptographic key; support for multicast media streams or multipoint unicast streams is for further study. Andreasen, Baugher & Wing [Page 3] INTERNET-DRAFT SDP Security Descriptions February, 2004 The crypto attribute is defined in a generic way to enable its use with secure transports besides SRTP that can establish cryptographic parameters with only a single message or in a single round-trip exchange using the offer/answer model [RFC3264]. Extension to other transports, however, is beyond the scope of this document. Each type of secure SDP media transport needs its own specification for the crypto-attribute parameter. These definitions are frequently unique to the particular type of transport and must be specified in an Internet RFC and registered with IANA according to the procedures defined in Section 9. This document defines the security parameters and keying material for SRTP only. It would be self-defeating not to secure cryptographic keys and other parameters at least as well as the data is secured. Data security protocols such as SRTP rely upon a separate key management system to securely establish encryption and/or authentication keys. Key management protocols provide authenticated key establishment (AKE) procedures to authenticate the identity of each endpoint and protect against man-in-the-middle, reflection/replay, connection hijacking and some denial of service attacks [skeme]. Along with the key, an AKE protocol such as MIKEY [mikey], GDOI [GDOI], KINK [kink], IKE [ike] or TLS securely disseminates information describing both the key and the data-security session (for example, whether SRTCP payloads are encrypted or unencrypted in an SRTP session). AKE is needed because it is pointless to provide a key over a medium where an attacker can snoop the key, alter the definition of the key to render it useless, or change the parameters of the security session to gain unauthorized access to session- related information. SDP, however, was not designed to provide AKE services, and the media security descriptions defined in this document do not add AKE services to SDP. This specification is no replacement for a key management protocol or for the conveyance of key management messages in SDP [keymgt]. The SDP security descriptions defined here are suitable for restricted cases only where IPsec, TLS, or some other encapsulating data-security protocol (e.g., SIP secure multiparts) protects the SDP message. This document adds security descriptions to those encrypted and/or authenticated SDP messages through the new SDP "crypto" attribute, which provides the cryptographic parameters of a media stream. The "crypto" attribute can be adapted to any media transport, but its precise definition is frequently unique to a particular transport. In Section 3, we introduce the general SDP crypto attribute, and in Section 4 we define how it is used with and without the offer/answer model. In Section 5, we define the crypto attribute details needed for SRTP, and in Section 6 we define SRTP-specific use of the attribute with and without the offer/answer model. Section 7 Andreasen, Baugher & Wing [Page 4] INTERNET-DRAFT SDP Security Descriptions February, 2004 recites security considerations, and Section 8 gives an Augmented- BNF grammar for the general crypto attribute as well as the SRTP- specific use of the crypto attribute. IANA considerations are provided in Section 9. 3. SDP "Crypto" Attribute and Parameters A new media-level SDP attribute called "crypto" describes the cryptographic suite, key parameters, and session parameters for the preceding unicast media line. The "crypto" attribute MUST only appear at the SDP media level (not the session level). The "crypto" attribute follows the format (see Section 8.1 for the formal ABNF grammar): a=crypto: [] The fields tag, crypto-suite, key-params, and session-params are described in the following sub-sections. Below we show an example of the crypto attribute for the "RTP/SAVP" transport, i.e., the secure RTP extension to the Audio/Video Profile [srtp] (newlines included for formatting reasons only): a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32 The crypto-suite is AES_CM_128_HMAC_SHA1_80, key-params is defined by the text starting with "inline:", and session-params is omitted. 3.1 Tag The tag is a decimal number (see Section 8.1 for details) used as an identifier for a particular crypto attribute. The tag MUST be unique among all crypto attributes for a given media stream. It is used with the offer/answer model (see Section 4.1) to determine which of several offered crypto attributes were chosen by the answerer. In the offer/answer model, the tag is a negotiated parameter. 3.2 Crypto-suite The crypto-suite field is an identifier (see Section 8.1 for details) that describes the encryption and authentication algorithms (e.g., AES_CM_128_HMAC_SHA1_80) for the transport in question. The possible values for the crypto-suite parameter are defined within the context of the transport, i.e., each transport defines a separate namespace for the set of crypto-suites. For example, the crypto-suite "AES_CM_128_HMAC_SHA1_80" defined within the context "RTP/SAVP" transport applies to Secure RTP only; the string may be reused for another transport (e.g., "RTP/SAVPF" [srtpf]), but a separate definition would be needed. Andreasen, Baugher & Wing [Page 5] INTERNET-DRAFT SDP Security Descriptions February, 2004 In the offer/answer model, the crypto-suite is a negotiated parameter. 3.3 Key Parameters The key-params field provides one or more sets of keying material for the crypto-suite in question. The field consists of a method indicator followed by a colon, and the actual keying information as shown below (the formal grammar is provided in Section 8.1): key-params = ":" Keying material might be provided by different means than key- params, however this is out of the scope of this document. Only one method is defined in this document, namely "inline", which indicates that the actual keying material is provided in the key-info field itself. There is a single name space for the key-method, i.e., the key-method is transport independent. New key-methods (e.g., use of a URL) may be defined in an IETF Standards Track RFC in the future. Although the key-method itself may be generic, the accompanying key- info definition is specific not only to the key-method, but also to the transport in question. New key methods MUST be registered with the IANA according to the procedures defined in Section 9.2.1. Key-info is defined as a general character string (see Section 8.1 for details); further transport and key-method specific syntax and semantics MUST be provided in an IETF RFC for each combination of transport and key-method that wants to use it; definitions for SRTP are provided in Section 5. Note that such definitions are provided within the context of both a particular transport (e.g., "RTP/SAVP") and a specific key-method (e.g., "inline"). IANA will register the list of supported key methods for each transport. When multiple keys are included in the key parameters, it MUST be possible to determine which of the keys is being used in a given media packet by a simple inspection of the media packet received; a trial-and-error approach between the possible keys MUST NOT be required. For SRTP, this could for example be achieved by use of Master Key Identifiers (MKI), or <"From", "To"> values [srtp]. In the offer/answer model, the key parameter is a declarative parameter. 3.4 Session Parameters Session parameters are specific to a given transport and use of them is OPTIONAL in the general framework, where they are just defined as a general character string. If session parameters are to be used Andreasen, Baugher & Wing [Page 6] INTERNET-DRAFT SDP Security Descriptions February, 2004 for a given transport, then transport-specific syntax and semantics MUST be provided in an IETF RFC; definitions for SRTP are provided in Section 5. In the offer/answer model, session parameters may be either negotiated or declarative; the definition of specific session parameters MUST indicate whether they are negotiated or declarative. Negotiated parameters apply to data sent in both directions, whereas declarative parameters apply only to media sent by the entity that generated the SDP. Thus, a declarative parameter in an offer applies to media sent by the offerer, whereas a declarative parameter in an answer applies to media sent by the answerer. 3.5 Example The first example shows use of the crypto attribute for the "RTP/SAVP" media transport type (as defined in Section 4). The "a=crypto" line is actually one long line; it is shown as two lines due to page formatting: v=0 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 s=SDP Seminar i=A Seminar on the session description protocol u=http://www.example.com/seminars/sdp.pdf e=j.doe@example.com (Jane Doe) c=IN IP4 161.44.17.12/127 t=2873397496 2873404696 m=video 51372 RTP/SAVP 31 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 m=audio 49170 RTP/SAVP 0 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 m=application 32416 udp wb a=orient:portrait This SDP message describes three media streams, two of which use the "RTP/SAVP" transport. Each has a crypto attribute for the "RTP/SAVP" transport. These secure-RTP specific descriptions are defined in Section 5. 4. General Use of the crypto Attribute In this section, we describe the general use of the crypto attribute outside of any transport or key-method specific rules. Andreasen, Baugher & Wing [Page 7] INTERNET-DRAFT SDP Security Descriptions February, 2004 4.1 Use With Offer/Answer The general offer/answer rules for the crypto attribute are in addition to the rules specified in RFC 3264, which MUST be followed, unless otherwise noted. RFC 3264 defines operation for both unicast and multicast streams; the sections below describe operation for two-party unicast streams only, since support for multicast streams (and multipoint unicast streams) is for further study. 4.1.1 Generating the Initial Offer - Unicast Streams When generating an initial offer for a unicast stream, there MUST be one or more crypto attributes present for each media stream for which security is desired. Each crypto attribute for a given media stream MUST contain a unique tag. The ordering of multiple "a=crypto" lines is significant: The most preferred crypto line is listed first. Each crypto attribute describes the crypto-suite, key(s) and possibly session parameters offered for the media stream. In general, a "more preferred" crypto-suite SHOULD be cryptographically stronger than a "less preferred" crypto-suite. The crypto-suite always applies to media in the directions supported by the media stream (e.g., send and receive). The key(s), however, apply to media in the direction from the offerer to the answerer; if the media stream is marked as "recvonly", a key MUST still be provided. This is done for consistency. Also, in the case of SRTP, for example, secure RTCP will still be flowing in both the send and receive direction for a unidirectional stream. The offer may include session parameters. There are no general offer rules for the session parameters; instead, specific rules may be provided as part of the transport specific definitions of any session parameters. When issuing an offer, the offerer MUST be prepared to support media security in accordance with any of the crypto attributes included in the offer. There are however two problems associated with this. First of all, the offerer does not know which key the answerer will be using for media sent to the offerer. Since media may arrive prior to the answer, delay or clipping can occur. If this is unacceptable to the offerer, the offerer SHOULD use a mechanism outside the scope of this document to prevent the above problem. For example, in SIP [RFC3261], a "security" precondition as defined in [sprecon] could solve the above problem. Another problem can occur when the offerer includes multiple crypto attributes: The offerer may not be able to deduce which of the Andreasen, Baugher & Wing [Page 8] INTERNET-DRAFT SDP Security Descriptions February, 2004 offered crypto attributes was accepted by the answerer until the answer is received, yet media may arrive before the answer. If this is unacceptable to the offerer, the offerer either SHOULD NOT include multiple crypto attributes in the offer, or a mechanism outside the scope of this document SHOULD be used to prevent the above problem (e.g., a "security" precondition). 4.1.2 Generating the Initial Answer - Unicast Streams When the answerer receives the initial offer with one or more crypto attributes for a given unicast media stream, the answerer MUST either accept exactly one of the offered crypto attributes, or the offered stream MUST be rejected. If the answerer wishes to indicate support for other crypto attributes, those can be listed by use of the SDP Simple Capability Declaration [RFC3407] extensions. Only crypto attributes that are valid can be accepted; valid attributes do not violate any of the general rules defined for security descriptions as well as any specific rules defined for the transport and key-method in question. When selecting one of the valid crypto attributes, the answerer SHOULD select the most preferred crypto attribute it can support, i.e., the first valid supported crypto attribute in the list, considering the answerer's capabilities and security policies. If there are one or more crypto attributes in the offer, but none of them are valid, or none of the valid ones are supported, the offered media stream MUST be rejected. When an offered crypto attribute is accepted, the crypto attribute in the answer MUST contain the following: * The tag and crypto-suite from the accepted crypto attribute in the offer (the same crypto-suite MUST be used in the send and receive direction). * The key(s) the answerer will be using for media sent to the offerer. Note that a key MUST be provided, irrespective of any direction attributes in the offer or answer. Furthermore, any session parameters that are negotiated MUST be included in the answer. Declarative session parameters provided by the offerer are not included in the answer, however the answerer may provide its own set of declarative session parameters. Once the answerer has accepted one of the offered crypto attributes, the answerer MAY begin sending media to the offerer in accordance with the selected crypto attribute. Note however, that the offerer Andreasen, Baugher & Wing [Page 9] INTERNET-DRAFT SDP Security Descriptions February, 2004 may not be able to process such media packets correctly until the answer has been received. 4.1.3 Offerer Processing of the Initial Answer - Unicast Streams When the offerer receives the answer, the offerer MUST verify, that one of the initially offered crypto suites and its accompanying tag was accepted and echoed in the answer. Also, the answer MUST include one or more keys, which will be used for media sent from the answerer to the offerer. If the offer contained any mandatory negotiated session parameters (see section 5.3.6), the offerer MUST verify that said parameters are included in the answer. If the answer contains any mandatory declarative session parameters, the offerer MUST be able to support those. If any of the above fails, the negotiation MUST be deemed to have failed. 4.1.4 Modifying the Session Once a media stream has been established, it MAY be modified at any time, as described in RFC 3264, Section 8. Such a modification MAY be triggered by the security service, e.g., in order to perform a re-keying or change the crypto-suite. If media stream security using the general security descriptions defined here is still desired, the crypto attribute MUST be included in these new offer/answer exchanges. The procedures are similar to those defined in Section 4.1.1, 4.1.2, 4.1.3 of this document, subject to the considerations provided in RFC 3264 Section 8. 4.2 Use Outside Offer/Answer The crypto attribute can also be used outside the context of offer/answer where there is no negotiation of the crypto suite, cryptographic key or session parameters. In this case, the sender determines security parameters for the stream. Since there is no negotiation mechanisms, the sender MUST include exactly one crypto attribute and the receiver MUST either accept it or else SHOULD NOT receive the associated stream. The sender SHOULD select the security description that it deems most secure for its purposes. 4.3 General Backwards Compatibility Considerations In the offer/answer model, it is possible that the answerer supports a given secure transport (e.g., "RTP/SAVP") and accepts the offered media stream, yet the answerer does not support the crypto attribute defined in this document and hence ignores it. The offerer can recognize this situation by seeing an accepted media stream in the Andreasen, Baugher & Wing [Page 10] INTERNET-DRAFT SDP Security Descriptions February, 2004 answer that does not include a crypto line. In that case, the security negotiation defined here MUST be deemed to have failed. Similar issues exist when security descriptions are used outside of the offer/answer model. 5. SRTP Security Descriptions In this section, we provide definitions for security descriptions for SRTP media streams. In the next section, we define how to use SRTP security descriptions with and without the offer/answer model. SRTP security descriptions for a media stream MUST only be used for media streams that use the SRTP transport (e.g., "RTP/SAVP" or "RTP/SAVPF") in the media (m=) line and SHALL apply to that media stream only. The following specifies rules for the "RTP/SAVP" profile defined in [srtp], however it is expected that other secure RTP profiles (e.g., "RTP/SAVPF") can use the same rules. There is no assurance that an endpoint is capable of configuring its SRTP service with a particular crypto attribute parameter, but SRTP guarantees minimal interoperability among SRTP endpoints through the default SRTP parameters [srtp]. More capable SRTP endpoints support a variety of parameter values beyond the SRTP defaults and these values can be configured by the SRTP security descriptions defined here. An endpoint that does not support the crypto attribute will ignore it according to the SDP. Hence the endpoint will simply assume use of default SRTP parameters, if it supports SRTP. Such an endpoint will not correctly process the particular media stream. By using the Offer/Answer model, the offerer and answerer can negotiate the crypto parameters to be used before commencement of the multimedia session (see Section 6.1). There are over twenty cryptographic parameters listed in the SRTP specification. Many of these parameters have fixed values for particular cryptographic transforms. At the time of session establishment, moreover, there is usually no need to provide unique settings for many of the SRTP parameters, such as salt length and pseudo-random function (PRF). Thus, it is possible to simplify the list of parameters by defining "cryptographic suites" that fix a set of SRTP parameter values for the security session. This approach is followed by the SRTP security descriptions, which uses the general security description parameters as follows: Andreasen, Baugher & Wing [Page 11] INTERNET-DRAFT SDP Security Descriptions February, 2004 * crypto-suite: Identifies the encryption and authentication transforms * key parameter: SRTP keying material and parameters * session parameters: The following parameters are defined: - KDR: The SRTP Key Derivation Rate is the rate that a pseudo-random function is applied to a master key - UNENCRYPTED_SRTP: SRTP messages are not encrypted - UNENCRYPTED_SRTCP: SRTCP messages are not encrypted - UNAUTHENTICATED_SRTP: SRTP messages are not authenticated - FEC_ORDER: Order of forward error correction (FEC) relative to SRTP services - WSH: Window Size Hint - Extensions: Extension parameters can be defined Please refer to the SRTP specification for a complete list of parameters and their descriptions [Section 8.2, srtp]. The key parameter, the crypto-suite, and the session parameters shown above are described in detail in the following subsections. 5.1 SRTP Key Parameter SRTP security descriptions define use of the "inline" key method as described in the following. Use of any other keying method, e.g., URL, for SRTP security descriptions is for further study. The "inline" type of key contains the keying material (master key and salt) and all policy related to that master key, including how long it can be used (lifetime) and whether or not it uses a master key identifier (MKI) to associate an incoming SRTP packet with a particular master key. Compliant implementations obey the policies associated with a master key, and MUST NOT accept incoming packets that violate the policy (e.g., after the master key lifetime has expired). The key parameter contains one or more cryptographic master keys, each of which MUST be a unique cryptographically random [RFC1750] value with respect to other master keys in the entire SDP message (i.e., including master keys for other streams). Each key follows the format (the formal definition is provided in Section 8.2): "inline:" "|" [lifetime] "|" [MKI ":" length / FromTo] key||salt concatenated master key and salt, base64 encoded (see [RFC3548], Section 3) lifetime master key lifetime (max number of SRTP or SRTCP packets using this master key) MKI:length MKI and length of the MKI field in SRTP packets FromTo <"From", "To"> values, specifying the lifetime for a master key Andreasen, Baugher & Wing [Page 12] INTERNET-DRAFT SDP Security Descriptions February, 2004 The following definition provides an example for AES_CM_128_HMAC_SHA1_80: inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4 The first field ("d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj") of the parameter is the cryptographic master key appended with the master salt; the two are first concatenated and then base64 encoded. The length of the concatenated key and salt is determined by the crypto- suite for which the key applies. If the length (after being decoded from base64) does not match that specified for the crypto-suite, the crypto attribute in question MUST be considered invalid. Each master key and salt MUST be a cryptographically random number and MUST be unique to the entire SDP message. When base64 decoding the key and salt, padding characters (i.e., one or two "=" at the end of the base64 encoded data) are discarded (see [RFC3548] for details). Base64 encoding assumes that the base64 encoding input is an integral number of octets. If a given crypto-suite requires the use of a concatenated key and salt with a length that is not an integral number of octets, said crypto-suite MUST define a padding scheme that results in the base64 input being an integral number of octets. For example, if the length defined was 250 bits, then 6 padding bits would be needed, which could be defined to be the last 6 bits in a 256 bit input. The second field, is the OPTIONAL lifetime of the master key as measured in maximum number of SRTP or SRTCP packets using that master key (i.e., the number of SRTP packets and the number of SRTCP packets each have to be less than the lifetime). The lifetime value MAY be written as a non-zero, positive integer or as a power of 2 (see the grammar in Section 8.2 for details). The "lifetime" value MUST NOT exceed the maximum packet lifetime for the crypto-suite. If the lifetime is too large or otherwise invalid then the entire crypto attribute MUST be considered invalid. The default MAY be implicitly signaled by omitting the lifetime value (i.e., "||"). This is convenient when the SRTP cryptographic key lifetime is the default value. As a shortcut to avoid long decimal values, the syntax of the lifetime allows using the literal "2^", which indicates "two to the power of". The example above, shows a case where the lifetime is specified as 2^20. The following example, which is for the AES_CM_128_HMAC_SHA1_80 crypto-suite, has a default for the lifetime field, which means that SRTP's and SRTCP's default values will be used (see [srtp]): inline:YUJDZGVmZ2hpSktMbW9QUXJzVHVWd3l6MTIzNDU2||1066:4 The example shows a 30-character key and concatenated salt that is base64 encoded: The 30-character key/salt concatenation is expanded to 40 characters by the three-in-four encoding of base64. Andreasen, Baugher & Wing [Page 13] INTERNET-DRAFT SDP Security Descriptions February, 2004 The third field, which is also OPTIONAL, is either the Master Key Identifier (MKI) and its byte length, or a <"From", "To"> value. "MKI" is the master key identifier associated with the SRTP master key. If the MKI is given, then the length of the MKI MUST also be given and separated from the MKI by a colon (":"). The MKI length is the size of the MKI field in the SRTP packet, specified in bytes. If the MKI length is not given or its value exceeds 128 (bytes), then the entire crypto attribute MUST be considered invalid. The substring "1:4" in the first example assigns to the key a master key identifier of 1 that is 4 bytes long, and the second example assigns a 4-byte master key identifier of 1066 to the key. <"From", "To"> specifies the lifetime for a master key, expressed in terms of the ROC and SEQ values inside whose range (including the range ends) the master key is valid. <"From", "To"> is an alternative to the MKI and assumes that a master key is in one-to- one correspondence with the SRTP session key on which the <"From", "To"> range is defined (see [srtp, Section 8.1.1] for details). The following example illustrates the use of the <"From", "To"> parameter: inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|FT=0:0,1:0 As mentioned above, the key parameter can contain one or more master keys. When the key parameter contains more than one master key, all of the master keys in that key parameter MUST either include an MKI or a <"From", "To"> value. Note that it is not permissible to mix the two within a single key parameter (i.e., one crypto attribute); all master keys in a given key parameter must use one or the other (or neither). Furthermore, when using the MKI, the MKI length MUST be the same for all keys in a given crypto attribute. 5.2 Crypto-suites The SRTP crypto-suites define the encryption and authentication transforms to be used for the SRTP media stream. The SRTP specification has defined three crypto-suites, which are described further in the following subsections in the context of the SRTP security descriptions. The table below provides an overview of the crypto-suites and their parameters: Andreasen, Baugher & Wing [Page 14] INTERNET-DRAFT SDP Security Descriptions February, 2004 +---------------------+-------------+--------------+---------------+ | |AES_CM_128_ | AES_CM_128_ | F8_128_ | | |HMAC_SHA1_80 | HMAC_SHA1_32 | HMAC_SHA1_80 | +---------------------+-------------+--------------+---------------+ | Master key length | 128 bits | 128 bits | 128 bits | | Salt value | 112 bits | 112 bits | 112 bits | | Default lifetime | 2^31 packets| 2^31 packets | 2^31 packets | | Cipher | AES Counter | AES Counter | F8 | | | Mode | Mode | | | Encryption key | 128 bits | 128 bits | 128 bits | | MAC | HMAC-SHA1 | HMAC-SHA1 | HMAC-SHA1 | | Authentication tag | 80 bits | 32 bits | 80 bits | | SRTP auth. key | 160 bits | 160 bits | 160 bits | | SRTCP auth. key | 160 bits | 160 bits | 160 bits | +---------------------+-------------+--------------+---------------+ 5.2.1 AES_CM_128_HMAC_SHA1_80 AES_CM_128_HMAC_SHA1_80 is the SRTP default AES Counter Mode cipher and HMAC-SHA1 message authentication having an 80-bit authentication tag. The master-key length is 128 bits and has a default lifetime of a maximum of 2^31 SRTP packets or SRTCP packets, whichever comes first [Page 39, srtp]. Technically, SRTP allows 2^48 SRTP packets or 2^31 SRTCP packets, whichever comes first. SRTP security descriptions, however, simplify the parameters to share a single upper bound of 2^31 packets. It is RECOMMENDED that automated key management allow easy and efficient rekeying at intervals far smaller than 2^31 packets given today's media rates or even HDTV media rates. The SRTP and SRTCP encryption key lengths are 128 bits. The SRTP and SRTCP authentication key lengths are 160 bits (see Security Considerations in Section 7). The master salt value is 112 bits in length and the session salt value is 112 bits in length. The pseudo-random function (PRF) is the default SRTP pseudo-random function that uses AES Counter Mode with a 128-bit key length. The length of the base64 decoded key and salt value for this crypto- suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto attribute is considered invalid. 5.2.2 AES_CM_128_HMAC_SHA1_32 This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except that the authentication tag is 32 bits. The length of the base64-decoded key and salt value for this crypto- suite MUST be 30 characters, i.e., 240 bits; otherwise the crypto attribute is considered invalid. Andreasen, Baugher & Wing [Page 15] INTERNET-DRAFT SDP Security Descriptions February, 2004 5.2.3 F8_128_HMAC_SHA1_80 This crypto-suite is identical to AES_CM_128_HMAC_SHA1_80 except the cipher is F8 [srtp]. The length of the base64 decoded key and salt value for this crypto- suite MUST be 30 characters, i.e. 240 bits; otherwise the crypto attribute is considered invalid. 5.2.4 Adding new Crypto-suite Definitions If new transforms are added to SRTP, new definitions for those transforms SHOULD be given for the SRTP security descriptions and published in an IETF RFC. Sections 5.2.1 through 5.2.3 illustrate how to define crypto-suite values for particular cryptographic transforms. Any new crypto-suites MUST be registered with IANA following the procedures in section 9. 5.3 Session Parameters SRTP security descriptions define a set of "session" parameters, which OPTIONALLY may be used to override SRTP session defaults for the SRTP and SRTCP streams. These parameters configure an RTP session for SRTP services. The session parameters provide session- specific information to establish the SRTP cryptographic context. 5.3.1 KDR=n KDR specifies the Key Derivation Rate, as described in section 4.3.1 of [srtp]. The value n MUST be an integer in the set {1,2,...,24}, which denotes a power of 2 from 2^1 to 2^24, inclusive. The SRTP key derivation rate controls how frequently a new session key is derived from an SRTP master key [srtp]. When the key derivation rate is not specified (i.e., the KDR parameter is omitted), a single initial key derivation is performed [srtp]. In the offer/answer model, KDR is a declarative parameter. 5.3.2 UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP SRTP and SRTCP packet payloads are encrypted by default. The UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP session parameters modify the default behavior of the crypto-suites with which they are used: Andreasen, Baugher & Wing [Page 16] INTERNET-DRAFT SDP Security Descriptions February, 2004 * UNENCRYPTED_SRTCP signals that the SRTCP packet payloads are not encrypted. * UNENCRYPTED_SRTP signals that the SRTP packet payloads are not encrypted. In the offer/answer model, these parameters are negotiated. 5.3.3 UNAUTHENTICATED_SRTP SRTP and SRTCP packet payloads are authenticated by default. The UNAUTHENTICATED_SRTP session parameter signals that SRTP messages are not authenticated. Use of UNAUTHENTICATED_SRTP is NOT RECOMMENDED (see Security Considerations). The SRTP specification requires use of message authentication for SRTCP, but not for SRTP [srtp]. In the offer/answer model, this parameter is negotiated. 5.3.4 FEC_ORDER=order FEC_ORDER signals the use of forward error correction for the RTP packets [rfc2733]. The forward error correction values for "order" are FEC_SRTP, SRTP_FEC, or SPLIT [mikey]. FEC_SRTP signals that FEC is applied before SRTP processing by the sender of the SRTP media and after SRTP processing by the receiver of the SRTP media; FEC_SRTP is the default. SRTP_FEC is the reverse processing. SPLIT signals that the sender performs SRTP encryption, followed by FEC processing, followed by SRTP authentication; processing is reversed on the receiver. In the offer/answer model, FEC_ORDER is a declarative parameter. 5.3.5 Window Size Hint (WSH) SRTP defines the SRTP-WINDOW-SIZE [srtp, section 3.3.2] parameter to protect against replay attacks. The minimum value is 64 [srtp], however this value may be considered too low for some applications (e.g., video). The Window Size Hint (WSH) session parameter provides a hint for how big this window should be to work satisfactorily (e.g., based on sender knowledge of number of packets per second). However, there might be enough information given in SDP attributes like "a=maxprate" and the bandwidth modifiers to allow a receiver to derive the parameter satisfactorily. Consequently, this value is only considered a hint to the receiver of the SDP which MAY choose to ignore the value provided. Andreasen, Baugher & Wing [Page 17] INTERNET-DRAFT SDP Security Descriptions February, 2004 In the offer/answer model, WSH is a declarative parameter. 5.3.6 Defining New SRTP Session Parameters New SRTP session parameters for the SRTP security descriptions can be defined in an IETF RFC and registered with IANA according to the registration procedures defined in Section 9. New SRTP session parameters are by default mandatory. A newly- defined SRTP session parameter that is prefixed with the dash character ("-") however is considered optional and MAY be ignored. If an SDP crypto attribute is received with an unknown session parameter that is not prefixed with a "-" character, that crypto attribute MUST be considered invalid. 5.4 SRTP Crypto Context Initialization In addition to the various SRTP parameters defined above, there are three pieces of information that are critical to the operation of the default SRTP ciphers: * SSRC: Synchronization source * ROC: Roll-over counter for a given SSRC * SEQ: Sequence number for a given SSRC In a unicast session, as defined here, there are three constraints on these values. The first constraint is on the SSRC, which makes an SRTP keystream be unique from other participants. As explained in SRTP, the keystream MUST NOT be reused on two or more different pieces of plaintext. Keystream reuse makes the ciphertext vulnerable. One vulnerability is that known-plaintext fields in one stream can expose portions of the reused keystream and this could further expose more plaintext in other streams. Since all current SRTP encryption transforms use keystreams, key sharing is a general problem [srtp]. SRTP mitigates this problem by including the SSRC of the sender in the keystream. But SRTP does not solve this problem in its entirety because Real-time Transport Protocol has SSRC collisions, which are very rare [rtp], but quite possible. During a collision, two or more SSRCs that share a master key will have identical keystreams for overlapping portions of the RTP sequence-number space. SRTP Security Descriptions avoids keystream reuse by making unique master keys REQUIRED for the sender and receiver of the security description. Thus, the first constraint is satisfied. Also note, that there is a second problem with SSRC collisions: The SSRC is used to identify the crypto context and thereby the cipher, key, ROC, etc. to process incoming packets. In case of SSRC collisions, crypto context identification becomes ambiguous Andreasen, Baugher & Wing [Page 18] INTERNET-DRAFT SDP Security Descriptions February, 2004 and correct packet processing may not occur. Furthermore, if an RTCP BYE packet is to be sent for a colliding SSRC, that packet may also have to be secured. In a (unicast) point-to-multipoint scenario, this can be problematic for the same reasons, i.e., it is not known which of the possible crypto contexts to use. Note that these problems are not unique to the SDP security descriptions; any use of SRTP needs to consider them. The second constraint is that the ROC MUST be zero at the time that each SSRC commences sending packets. Thus, there is no concept of a "late joiner" in SRTP security descriptions, which are constrained to be unicast and pairwise. The ROC and SEQ form a "packet index" in the default SRTP transforms and the ROC is consistently set to zero at session commencement, according to this document. The third constraint is that the initial value of SEQ SHOULD be chosen to be within the range of 0..2^15-1; this avoids an ambiguity when packets are lost at the start of the session. If at the start of a session, an SSRC source might randomly select a high sequence- number value and put the receiver in an ambiguous situation: If initial packets are lost in transit up to the point that the sequence number wraps (i.e., exceeds 2^16-1), then the receiver might not recognize that its ROC needs to be incremented. By restricting the initial SEQ to the range of 0..2^15-1, SRTP packet- index determination will find the correct ROC value, unless all of the first 2^15 packets are lost (which seems, if not impossible, then rather unlikely). See Section 3.3.1 of the SRTP specification regarding packet-index determination [srtp]. The packet index, therefore, depends on the SSRC, the SEQ of an incoming packet and the ROC, which is an SRTP crypto context variable. Thus, SRTP has a big security dependency on SSRC uniqueness. This fact might lead one to consider establishing the SSRC by an entity that keeps these values from colliding. One problem with this approach, however, is that the SSRC belongs to the transport (RTP or SRTP) and not to the signaling. It would be an imposition on RTP and SRTP to require that the SSRC be read and written by an external system such as SDP. Given the above constraints, unicast SRTP crypto contexts can be established without the need to negotiate SSRC values in the SRTP security descriptions. Instead, an approach called "late binding" is RECOMMENDED by this specification. When a packet arrives, the SSRC that is contained in it can be bound to the crypto context at the time of session commencement (i.e., SRTP packet arrival) rather than at the time of session signaling (i.e., receipt of an SDP). With the arrival of the packet containing the SSRC, all the data items needed for the SRTP crypto context are held by the receiver (note that the ROC value by definition is zero; if non-zero values were to be supported, additional signaling would be required). In Andreasen, Baugher & Wing [Page 19] INTERNET-DRAFT SDP Security Descriptions February, 2004 other words, the crypto context for a secure RTP session using late binding is initially identified by the SDP as: <*, address, port> where '*' is a wildcard SSRC, "address" is the local receive address from the "c=" line, and "port" is the local receive port from the "m=" line. When the first packet arrives with ssrcX in its SSRC field, the crypto context is instantiated subject to the following constraints: * Media packets are authenticated: Authentication MUST succeed; otherwise, the crypto context is not instantiated. * Media packets are not authenticated: Crypto context is automatically instantiated. It should be noted, that use of late binding when there is no authentication of the SRTP media packets is subject to numerous security attacks and consequently it is NOT RECOMMENDED (of course, this can be said for unauthenticated SRTP in general). Note that use of late binding without authentication results in local state being created as a result of receiving a packet from any unknown SSRC. UNAUTHENTICATED_SRTP, therefore is NOT RECOMMENDED because it invites easy denial-of-service attack. In contrast, late binding with authentication does not suffer from this weakness. With the constraints and procedures described above, it is not necessary to explicitly signal the SSRC, ROC and SEQ for a unicast SRTP session. 5.5 Removal of Crypto Contexts The mechanism defined above addresses the issue of creating crypto contexts, however in practice, session participants may want to remove crypto contexts prior to session termination. Since a crypto context contains information that can not automatically be recovered (e.g., ROC), it is important that the sender and receiver agree on when a crypto context can be removed, and perhaps more importantly when it cannot. Even when late binding is used for a unicast stream, the ROC is lost and cannot be recovered automatically (unless it is zero) once the crypto context is removed. Andreasen, Baugher & Wing [Page 20] INTERNET-DRAFT SDP Security Descriptions February, 2004 We resolve this problem as follows. When SRTP security descriptions are being used, crypto contexts removal MUST follow the same rules as SSRC removal from the member table [RFC 3550]; note that this can happen as the result of an SRTCP BYE packet or a simple time-out due to inactivity. Inactive session participants that wish to ensure their crypto contexts are not timed out MUST thus send SRTCP packets at regular intervals. 6. SRTP-Specific Use of the crypto Attribute Section 4 describes general use of the crypto attribute, and this section completes it by describing SRTP-specific use. 6.1 Use with Offer/Answer In this section, we describe how the SRTP security descriptions are used with the offer/answer model to negotiate cryptographic capabilities and communicate SRTP master keys. The rules defined below complement the general offer/answer rules defined in Section 4.1, which MUST be followed, unless otherwise specified. Note that the rules below define unicast operation only; support for multicast and multipoint unicast streams is for further study. 6.1.1 Generating the Initial Offer - Unicast Streams When the initial offer is generated, the offerer MUST follow the steps in Section 4.1.1 as well as the following steps. For each unicast media line (m=) using the secure RTP transport where the offerer wants to specify cryptographic parameters, the offerer MUST provide at least one valid SRTP security description ("a=crypto" line), as defined in Section 5. The offerer MAY include one or more other SRTP session parameters as defined in Section 5.3. Note however, that if any SRTP session parameters are included that are not known to the answerer, but are nonetheless mandatory (see Section 5.3.6), the negotiation will fail if the answerer does not support them. 6.1.2 Generating the Initial Answer - Unicast Streams When the initial answer is generated, the answerer MUST follow the steps in Section 4.1.2 as well as the following steps. For each unicast media line which uses the secure RTP transport and contains one or more "a=crypto" lines in the offer, the answerer MUST either accept one (and only one) of the crypto lines for that media stream, or it MUST reject the media stream. Only "a=crypto" lines that are considered valid SRTP security descriptions as defined in Section 5 can be accepted. Furthermore, all parameters (crypto-suite, key parameter, and mandatory session parameters) MUST Andreasen, Baugher & Wing [Page 21] INTERNET-DRAFT SDP Security Descriptions February, 2004 be acceptable to the answerer in order for the offered media stream to be accepted. When the answerer accepts an SRTP unicast media stream with a crypto line, the answerer MUST include one or more master keys appropriate for the selected crypto algorithm; the master key(s) included in the answer MUST be different from those in the offer. When the master key(s) are not shared between the offerer and answerer, SSRC collisions between the offerer and answerer will not lead to keystream reuse, and hence SSRC collisions do not necessarily have to be prevented. Declarative session parameters may be added to the answer as usual, however the answerer SHOULD NOT add any mandatory session parameter (see Section 5.3.6) that might be unknown to the offerer. If the answerer cannot find any valid crypto line that it supports, or if its configured policy prohibits any cryptographic key parameter (e.g., key length) or cryptographic session parameter (e.g., KDR, FEC_ORDER), it MUST reject the media stream, unless it is able to successfully negotiate use of SRTP by other means outside the scope of this document (e.g., by use of MIKEY [mikey]). 6.1.3 Offerer Processing of the Initial Answer - Unicast Streams When the offerer receives the answer, it MUST perform the steps in Section 4.1.3 as well as the following steps for each SRTP media stream it offered with one or more crypto lines in it. If the media stream was accepted and it contains a crypto line, it MUST be checked that the crypto line is valid according to the constraints specified in Section 5. If the offerer either does not support or is not willing to honor one or more of the SRTP parameters in the answer, the offerer MUST consider the crypto line invalid. If the crypto line is not valid, or the offerer's configured policy prohibits any cryptographic key parameter (e.g. key length) or cryptographic session parameter, the SRTP security negotiation MUST be deemed to have failed. 6.1.4 Modifying the Session When a media stream using the SRTP security descriptions has been established, and a new offer/answer exchange is performed, the offerer and answerer MUST follow the steps in Section 4.1.4 as well as the following steps. Andreasen, Baugher & Wing [Page 22] INTERNET-DRAFT SDP Security Descriptions February, 2004 When modifying the session, all negotiated aspects of the SRTP media stream can be modified. For example, a new crypto suite can be used or a new master key can be established. As described in RFC 3264, when doing a new offer/answer exchange there will be a window of time, where the offerer and the answerer must be prepared to receive media according to both the old and the new offer/answer exchange. This requirement applies here as well, however the following should be noted: * When authentication is not being used, it may not be possible for either the offerer or the answerer to determine if a given packet is encrypted according to the old or new offer/answer exchange. RFC 3264 defines a couple of techniques to address this problem, e.g., changing the payload types used and/or the transport addresses. Note however that a change in transport addresses may have an impact on Quality of Service as well as firewall and NAT traversal. The SRTP security descriptions offers two other ways of dealing with this; use the MKI (which adds a few bytes to each SRTP packet) or the <"From","To"> mechanism (which doesn't add bytes to each SRTP packet) as described in Section 5.1. For further details on MKI and "<"From","To">, please refer to [srtp]. * If the answerer changes its master key, the offerer will not be able to process packets secured via this master key until the answer is received. As noted in Section 4.1.1, this could for example be addressed by using a security "precondition" [sprecon]. Finally note, that if the new offer is rejected, the old crypto parameters remain in place. 6.1.5 Offer/Answer Example In this example, the offerer supports two crypto suites (F8 and AES). The a=crypto line is actually one long line, although it is shown as two lines in this document due to page formatting. Andreasen, Baugher & Wing [Page 23] INTERNET-DRAFT SDP Security Descriptions February, 2004 Offerer sends: v=0 o=sam 2890844526 2890842807 IN IP4 10.47.16.5 s=SRTP Discussion i=A discussion of Secure RTP u=http://www.example.com/seminars/srtp.pdf e=marge@example.com (Marge Simpson) c=IN IP4 168.2.17.12 t=2873397496 2873404696 m=audio 49170 RTP/SAVP 0 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 FEC_ORDER=FEC_SRTP a=crypto:2 F8_128_HMAC_SHA1_80 inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4; inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4 FEC_ORDER=FEC_SRTP Answerer replies: v=0 o=jill 25690844 8070842634 IN IP4 10.47.16.5 s=SRTP Discussion i=A discussion of Secure RTP u=http://www.example.com/seminars/srtp.pdf e=homer@example.com (Homer Simpson) c=IN IP4 168.2.17.11 t=2873397526 2873405696 m=audio 32640 RTP/SAVP 0 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 In this case, the session would use the AES_CM_128_HMAC_SHA1_80 crypto suite for the RTP and RTCP traffic. 6.2 SRTP-Specific Use Outside Offer/Answer These are the same as Section 4.2. 6.3 Support for SIP Forking As mentioned earlier, the security descriptions defined here do not support multicast media streams or multipoint unicast streams. However, in the SIP protocol, it is possible to receive several answers to a single offer due to the use of forking (see [SIP]). Receiving multiple answers leads to a couple of problems for the SRTP security descriptions: Andreasen, Baugher & Wing [Page 24] INTERNET-DRAFT SDP Security Descriptions February, 2004 * Different answerers may choose different ciphers, keys, etc., however there is no way for the offerer to associate a particular incoming media packet with a particular answer. * Two or more answerers may pick the same SSRC and hence the SSRC collision problems mentioned earlier may arise. As stated earlier, the above point-to-multipoint cases are outside the scope of the SDP security descriptions. However, there is a way of supporting SIP forking: Change the multipoint scenario resulting from SIP forking into multiple two-party unicast cases. This is done as follows: For each answer received beyond the initial answer, issue a new offer to that particular answerer using a new receive transport address (IP address and port); note that this requires support for the SIP UPDATE method [RFC 3313]. Also, to ensure that two media sessions are not inadvertently established prior to the UPDATE being processed by one of them, use security preconditions [sprecon]. Finally, note that all the answerers will know the key(s) being proposed by the initial offer. If the offerer wants to ensure security with respect to other answerers, a new offer/answer exchange with a new key needs to be performed with the first answerer as well. 6.4 SRTP-Specific Backwards Compatibility Considerations It is possible that the answerer supports the SRTP transport and accepts the offered media stream, yet it does not support the crypto attribute defined here. The offerer can recognize this situation by seeing an accepted SRTP media stream in the answer that does not include a crypto line. In that case, the security negotiation defined here MUST be deemed to have failed. Also, if a media stream with a given SRTP transport (e.g., "RTP/SAVP") is sent to a device that does not support SRTP, that media stream will be rejected. 6.5 Operation with KEYMGT= and k= lines An offer MAY include both "a=crypto" and "a=keymgt" lines [keymgt]. Per SDP rules, the answerer will ignore attribute lines that it does not understand. If the answerer supports both "a=crypto" and "a=keymgt", the answer MUST include either "a=crypto" or "a=keymgt" but not both, as including both is undefined. An offer MAY include both "a=crypto" and "k=" lines [SDP]. Per SDP rules, the answerer will ignore attribute lines it does not understand. If the answerer supports both "a=crypto" and "k=", the Andreasen, Baugher & Wing [Page 25] INTERNET-DRAFT SDP Security Descriptions February, 2004 answer MUST include either "a=crypto" or "k=" but not both, as including both is undefined. 7. Security Considerations Like all SDP messages, SDP messages containing security descriptions, are conveyed in an encapsulating application protocol (e.g., SIP, MGCP, etc.). It is the responsibility of the encapsulating protocol to ensure the protection of the SDP security descriptions. Therefore, the application protocol SHOULD either invoke its own security mechanisms (e.g., secure multiparts) or alternatively utilize a lower-layer security service (e.g., TLS, or IPSec). This security service SHOULD provide strong message authentication and packet-payload encryption as well as effective replay protection. 7.1 Authentication of packets Security descriptions as defined herein signal security services for RTP packets. RTP messages are vulnerable to a variety of attacks such as replay and forging. To limit these attacks, SRTP message integrity mechanisms SHOULD be used (SRTP replay protection is always enabled). 7.2 Keystream Reuse SRTP security descriptions signal configuration parameters for SRTP sessions. Misconfigured SRTP sessions are vulnerable to attacks on their encryption services when running the crypto suites defined in Sections 5.2.1, 5.2.2, and 5.2.3. An SRTP encryption service is "misconfigured" when two or more media streams are encrypted using the same AES keystream. When senders and receivers share derived session keys, SRTP requires that the SSRCs of session participants serve to make their corresponding keystreams unique, which is violated in the case of SSRC collision: SRTP SSRC collision drastically weakens SRTP or SRTCP payload encryption during the time that identical keystreams were used [srtp]. An attacker, for example, might collect SRTP and SRTCP messages and await a collision. This attack on the AES-CM and AES-f8 encryption is avoided entirely when each media stream has its own unique master key in both the send and receive direction. This specification restricts use of SDP security description to unicast point-to-point streams so that keys are not shared between SRTP hosts, and the master keys used in the send and receive direction for a given media stream are unique. 7.3 Signaling Authentication and Signaling Encryption There is no reason to incur the complexity and computational expense of SRTP, however, when its key establishment is exposed to unauthorized parties. In most cases, the SRTP crypto attribute and Andreasen, Baugher & Wing [Page 26] INTERNET-DRAFT SDP Security Descriptions February, 2004 its parameters are vulnerable to denial of service attacks when they are carried in an unauthenticated SDP message. In some cases, the integrity or confidentiality of the RTP stream can be compromised. For example, if an attacker sets UNENCRYPTED for the SRTP stream in an offer, this could result in the answerer not decrypting the encrypted SRTP messages. In the worst case, the answerer might itself send unencrypted SRTP and leave its data exposed to snooping. Thus, MIME secure multiparts, IPsec, TLS, or some other data security service SHOULD be used to provide message authentication for the encapsulating protocol that carries the SDP messages having a crypto attribute (a=crypto). Furthermore, encryption of the encapsulating payload SHOULD be used because a master key parameter (inline) appears in the message. Failure to encrypt the SDP message containing an inline SRTP master key renders the SRTP authentication or encryption service useless in practically all circumstances. Failure to authenticate an SDP message that carries SRTP parameters renders the SRTP authentication or encryption service useless in most practical applications. When the communication path of the SDP message is routed through intermediate systems that inspect parts of the SDP message, security protocols such as IPsec or TLS SHOULD NOT be used for encrypting and/or authenticating the security description. In the case of intermediate-system processing of a message containing SDP security descriptions, the "a=crypto" attributes SHOULD be protected end-to- end so that the intermediate system can neither modify the security description nor access the keying material. Network or transport security protocols that terminate at each intermediate system, therefore, SHOULD NOT be used for protecting SDP security descriptions. A security protocol SHOULD allow the security descriptions to be encrypted and authenticated end-to-end independently of the portions of the SDP message that any intermediate system modifies or inspects: MIME secure multiparts are RECOMMENDED for the protection of SDP messages that are processed by intermediate systems. When the SDP parameters cannot be carried in an encrypted and authenticated SDP message, it is RECOMMENDED that a key management protocol be used instead of the security descriptions defined here (a=crypto). The proposed SDP key-mgmt extension [keymgt] allows authentication and encryption of the key management protocol data independently of the SDP message that carries it. The security of the SDP SRTP attribute, however, is as good as the data security protocol that protects the SDP message. For example, if an IPSec security association exists between the SDP source and destination endpoints, then this solution is more secure than use of the key- mgmt statement in an unauthenticated SDP message, which is vulnerable to tampering. Andreasen, Baugher & Wing [Page 27] INTERNET-DRAFT SDP Security Descriptions February, 2004 8. Grammar In this section we first provide the ABNF grammar for the generic crypto attribute, and then we provide the ABNF grammar for the SRTP specific use of the crypto attribute. 8.1 Generic "Crypto" Attribute Grammar The ABNF grammar for the crypto attribute is defined below: "a=crypto:" tag 1*WSP crypto-suite 1*WSP key-params *(1*WSP session-param) tag = 1*ALPHANUM crypto-suite = 1*(ALPHA / DIGIT / "_") key-params = key-param *(";" key-param) key-param = key-method ":" key-info key-method = "inline" / key-method-ext key-method-ext = 1*(ALPHA / DIGIT / "_") key-info = %x21-3A / %x3C-7E ; visible (printing) characters ; except semi-colon session-param = 1*(VCHAR) ; visible (printing) characters where WSP, ALPHA, DIGIT, and VCHAR are defined in [RFC2234]. 8.2 SRTP "Crypto" Attribute Grammar This section provides an Augmented BNF [RFC2234] grammar for the SRTP-specific use of the SDP crypto attribute: crypto-suite = srtp-crypto-suite key-method = srtp-key-method key-info = srtp-key-info session-param = srtp-session-param srtp-crypto-suite = "AES_CM_128_HMAC_SHA1_32" / "F8_128_HMAC_SHA1_32" / "AES_CM_128_HMAC_SHA1_80" / srtp-crypto-suite-ext srtp-key-method = "inline" srtp-key-info = key-salt "|" [lifetime] "|" [mki / FromTo] key-salt = 1*(base64) ; binary key and salt values ; concatenated together, and then ; base64 encoded [section 6.8 of ; RFC2046] lifetime = ["2^"] 1*(DIGIT) ; see section 5.1 for "2^" mki = mki-value ":" mki-length Andreasen, Baugher & Wing [Page 28] INTERNET-DRAFT SDP Security Descriptions February, 2004 mki-value = 1*DIGIT mki-length = 1*3DIGIT ; range 1..128. FromTo = "FT=" ftval "," ftval ftval = roc ":" seq ; packet index expressed in terms ; of ROC and SEQ. roc = 1*DIGIT ; range 0..2^32-1 seq = 1*DIGIT ; range 0..2^16-1 srtp-session-param = kdr / "UNENCRYPTED_SRTP" / "UNENCRYPTED_SRTCP" / "UNAUTHENTICATED_SRTP" / fec-order / wsh / srtp-session-extension kdr = "KDR=" 1*2(DIGIT) ; range 0..24, power of two fec-order = "FEC_ORDER=" fec-type fec-type = "FEC_SRTP" / "SRTP_FEC" / "SPLIT" wsh = "WSH=" 2*DIGIT ; minimum value is 64 base64 = ALPHA / DIGIT / "+" / "/" / "=" srtp-crypto-suite-ext = 1*(ALPHA / DIGIT / "_") srtp-session-extension = ["-"] 1*(VCHAR) ;visible chars [RFC2234] ; first character must not be dash ("-") 9. IANA Considerations 9.1 Registration of the "crypto" attribute The IANA is hereby requested to register a new SDP attribute as follows: Attribute name: crypto Long form name: Security description cryptographic attribute for media streams Type of attribute: Media-level Subject to charset: No Purpose: Security descriptions Appropriate values: See Section 3 9.2 New IANA Registries and Registration Procedures The following sub-sections define a new IANA registry with associated sub-registries to be used for the SDP security descriptions. The IANA is hereby requested to create an SDP Security Description registry as shown below and further described in the following sections: Andreasen, Baugher & Wing [Page 29] INTERNET-DRAFT SDP Security Descriptions February, 2004 SDP Security Descriptions | +- Key Methods (described in 9.2.1) | +- Media Stream Transports (described in 9.2.2) | +- Transport1 (e.g. SRTP) | | | +- Supported Key Methods (e.g. inline) | | | +- crypto suites | | | +- session parameters | +- Transport2 : : 9.2.1Security Descriptions Key Method Registry and Registration The IANA is hereby requested to create a new subregistry for SDP security description key methods. An IANA key method registration MUST be documented in an IETF Standards Track RFC and it MUST provide the name of the key method in accordance with the grammar for key-method-ext defined in Section 8.1. 9.2.2Security Description Media Stream Transport Registry and Registration The IANA is hereby requested to create a new subregistry for SDP security description Media Stream Transports. An IANA media stream transport registration MUST be documented in an RFC in accordance with the procedures defined in Section 3 and 4 of this document. The registration MUST provide the name of the transport and a list of supported key methods. In addition, each new media stream transport registry must contain a crypto-suite registry and a session parameter registry as well as IANA instructions for how to populate these registries. 9.3 Initial Registrations 9.3.1 Key Method The following security descriptions key methods are hereby registered: inline Andreasen, Baugher & Wing [Page 30] INTERNET-DRAFT SDP Security Descriptions February, 2004 9.3.2 SRTP Media Stream Transport The IANA is hereby requested to create an SDP Security Description Media Stream Transport subregistry for "SRTP". The key methods supported is "inline". The reference for the SDP security description for SRTP is this document. 9.3.2.1 SRTP Crypto Suite Registry and Registration The IANA is hereby requested to create a new subregistry for SRTP crypto suites under the SRTP transport of the SDP Security Descriptions. An IANA SRTP crypto suite registration MUST indicate the crypto suite name in accordance with the grammar for srtp- crypto-suite-ext defined in Section 8.2. The semantics of the SRTP crypto suite MUST be described in an IETF RFC, including the semantics of the "inline" key-method and any special semantics of parameters. The following SRTP crypto suites are hereby registered: AES_CM_128_HMAC_SHA1_80 AES_CM_128_HMAC_SHA1_32 F8_128_HMAC_SHA1_80 The reference for these crypto-suites is provided in this document. 9.3.2.2 SRTP Session Parameter Registration The IANA is hereby requested to create a new subregistry for SRTP session parameters under the SRTP transport of the SDP Security Descriptions. An IANA SRTP session parameter registration MUST indicate the session parameter name (srtp-session-extension as defined in Section 8.2); the name MUST NOT begin with the dash character ("-"). The semantics of the parameter MUST be described in an IETF RFC. If values can be assigned to the parameter, then the format and possible values that can be assigned MUST be described in the IETF RFC as well. Also, it MUST be specified whether the parameter is declarative or negotiated in the offer/answer model. The following SRTP session parameters are hereby registered: SRC KDR UNENCRYPTED_SRTP UNENCRYPTED_SRTCP UNAUTHENTICATED_SRTP FEC_ORDER WSH Andreasen, Baugher & Wing [Page 31] INTERNET-DRAFT SDP Security Descriptions February, 2004 The reference for these parameters is this document. 10. Acknowledgements This document is a product of the IETF MMUSIC working group and has benefited from comments from its participants. This document also benefited from discussions with Elisabetta Cararra, Earl Carter, Bill Foster, Matt Hammer, Cullen Jennings, Paul Kyzivat, David McGrew, Mats Naslund, Dave Oran, Jonathan Rosenberg, Dave Singer, Mike Thomas, Brian Weis, and Magnus Westerlund. These people shared observations, identified errors and made suggestions for improving the specification. Magnus provided many useful comments and Mats made several valuable suggestions on parameters and syntax that are in the current draft. Dave Oran and Mike Thomas encouraged us to bring this work to the IETF for standardization. David McGrew suggested the conservative approach of requiring unique master keys for each unicast SDP media stream as followed in this document. Paul Kyzivat suggested how to handle SIP forking. Jonathan Rosenberg suggested reducing the complexity by specifying only one security parameter for each media stream. 11. Authors' Addresses Flemming Andreasen Cisco Systems, Inc. 499 Thornall Street, 8th Floor Edison, New Jersey 08837 USA fandreas@cisco.com Mark Baugher 5510 SW Orchid Street Portland, Oregon 97219 USA mbaugher@cisco.com Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA dwing@cisco.com 12. Normative References [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003, http://www.ietf.org/rfc/rfc3550.txt. [RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax Specifications: ABNF," RFC 2234, November 1997, http://www.ietf.org/rfc/rfc2234.txt. Andreasen, Baugher & Wing [Page 32] INTERNET-DRAFT SDP Security Descriptions February, 2004 [SDP] M. Handley, V. Jacobson, C. Perkins, "SDP: Session Description Protocol", Work in Progress. [RFC2733] J. Rosenberg, H. Schulzrinne, "An RTP Payload Format for Generic Forward Error Correction", RFC 2733, December 1999, http://www.ietf.org/rfc/rfc2733.txt. [RFC2828] R. Shirey, "Internet Security Glossary", RFC 2828, May 2000, http://www.ietf.org/rfc/rfc2828.txt. [RFC3264] J. Rosenberg, H. Schulzrinne, "An Offer/Answer Model with the Session Description Protocol (SDP)", RFC 3264, June 2202, http://www.ietf.org/rfc/rfc3264.txt. [srtp] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K. Norrman, D. Oran, "The Secure Real-time Transport Protocol", Work in Progress. [RFC1750] D. Eastlake 3rd, S. Crocker, J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994, http://www.ietf.org/rfc/rfc1750.txt. [RFC3548] S. Josefsson, "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. 13. Informative References [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002, http://www.ietf.org/rfc/rfc3407.txt. [Bellovin] Steven M. Bellovin, "Problem Areas for the IP Security Protocols," in Proceedings of the Sixth Usenix Unix Security Symposium, pp. 1-16, San Jose, CA, July 1996. [GDOI] M. Baugher, B. Weis, T. Hardjono, H. Harney, "The Group Domain of Interpretation", RFC 3547, July 2003, http://www.ietf.org/rfc/rfc3547.txt. [kink] M. Thomas, J. Vilhuber, "Kerberized Internet Negotiation of Keys (KINK)", Work in Progress. [ike] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998, http://www.ietf.org/rfc/rfc2409.txt. [ipsec] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998, http://www.ietf.org/rfc/rfc2401.txt. Andreasen, Baugher & Wing [Page 33] INTERNET-DRAFT SDP Security Descriptions February, 2004 [s/mime] Ramsdell B., "S/MIME Version 3 Message Specification", RFC 2633, June 1999, http://www.ietf.org/rfc/rfc2633.txt. [tls] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999, http://www.ietf.org/rfc/rfc2246.txt. [keymgt] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman, "Key Management Extensions for SDP and RTSP", Work in Progress. [mikey] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman, "MIKEY: Multimedia Internet KEYing", Work in Progress. [RFC2045] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996, http://www.ietf.org/rfc/rfc2045.txt. [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2014, November 1997, http://www.ietf.org/rfc/rfc2104.txt. [skeme] H. Krawczyk, "SKEME: A Versatile Secure Key Exchange Mechanism for the Internet", ISOC Secure Networks and Distributed Systems Symposium, San Diego, 1996. [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002, http://www.ietf.org/rfc/rfc3312.txt. [RFC2974] M. Handley, C. Perkins, E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000, http://www.ietf.org/rfc/rfc2974.txt. [srtpf] Ott, J. and E. Carrara, "Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF)", work in progress, October 2003. [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. [sprecon] Andreasen, F., Baugher, M., and D. Wing, "Security Preconditions for Session Description Protocol Media Streams", work in progress, February 2004. Andreasen, Baugher & Wing [Page 34] INTERNET-DRAFT SDP Security Descriptions February, 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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