Internet Engineering Task Force Baugher (Cisco) MSEC Working Group Carrara (Ericsson) INTERNET-DRAFT EXPIRES: January 2005 July 2004 The Use of TESLA in SRTP Status of this memo By submitting this Internet-Draft, the authors certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, the authors accept the provisions of Section 3 of RFC 3667 (BCP 78). 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/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This memo describes the use of the Timed Efficient Stream loss- tolerant Authentication (TESLA) transform within the Secure Real- time Transport Protocol (SRTP), to provide data origin authentication for multicast and broadcast data streams. INTERNET-DRAFT TESLA-SRTP July, 2004 TABLE OF CONTENTS 1. Introduction...................................................2 1.1. Notational Conventions.......................................3 2. SRTP...........................................................3 3. TESLA..........................................................4 4. Usage of TESLA within SRTP.....................................4 4.1. The TESLA extension..........................................4 4.2. SRTP Packet Format...........................................5 4.3. Extension of the SRTP Cryptographic Context..................7 4.4. SRTP Processing..............................................8 4.4.1 Sender Processing...........................................8 4.4.2 Receiver Processing.........................................9 4.5. SRTCP Packet Format.........................................10 4.6. TESLA MAC...................................................12 4.7. PRFs........................................................12 5. TESLA Bootstrapping...........................................13 6. SRTP TESLA Default parameters.................................13 6.2 Transform-dependent Parameters for TESLA MAC.................14 7. Security Considerations.......................................14 8. IANA Considerations...........................................15 9. Acknowledgements..............................................15 10. Author's Addresses...........................................15 11. References...................................................16 1. Introduction Multicast and broadcast communication introduce some new security challenges compared to unicast communication. Many multicast and broadcast applications need "data origin authentication" (DOA), or "source authentication", in order to guarantee that a received message had originated from a given source, and was not manipulated during the transmission. In unicast communication, a pairwise security association between one sender and one receiver can provide data origin authentication using symmetric-key cryptography (such as a message authentication code, MAC). When the communication is strictly pairwise, the sender and receiver agree upon a key that is known only to them. In groups, however, a key is shared among more than two members, and this symmetric-key approach does not guarantee data origin authentication. When there is a group security association [gkmarch] instead of a pairwise security association, any of the members can alter the packet and impersonate any other member. The MAC in this case only guarantees that the packet was not manipulated by an attacker outside the group (and hence not in possession of the Baugher, Carrara [Page 2] INTERNET-DRAFT TESLA-SRTP July, 2004 group key), and that the packet was sent by a source within the group. Some applications cannot tolerate source ambiguity and must discern the true sender from any other group member. A common way to solve the problem is by use of asymmetric cryptography, such as digital signatures. This method, unfortunately, suffers from high overhead, in terms of time (to sign and verify) and bandwidth (to convey the signature in the packet). Several schemes have been proposed to provide efficient data origin authentication in multicast and broadcast scenarios. The Timed Efficient Stream loss-tolerant Authentication (TESLA), is one such scheme. This memo specifies TESLA authentication for SRTP. SRTP TESLA can provide data origin authentication to RTP applications that use group security associations (such as multicast RTP applications) so long as receivers abide by the TESLA security invariants [TESLA]. 1.1. Notational Conventions The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This specification assumes the reader familiar with both SRTP and TESLA. Few of their details are explained in this document, and the reader can find them in their respective specifications [RFC3711], [TESLA]. This specification uses the same definitions as TESLA for common terms and assumes that the reader is familiar with the TESLA algorithms and protocols [TESLA]. 2. SRTP The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile of RTP, which can provide confidentiality, message authentication, and replay protection to the RTP traffic and to the RTP control protocol, the Real-time Transport Control Protocol (RTCP). Note, the term SRTP may often used to indicate SRTCP as well. SRTP is a framework that allows new security functions and new transforms to be added. SRTP currently does not define any mechanism to provide data origin authentication for group security associations. Fortunately, it is straightforward to add TESLA to the SRTP cryptographic framework. Baugher, Carrara [Page 3] INTERNET-DRAFT TESLA-SRTP July, 2004 The TESLA extension to SRTP is defined in this specification, which assumes that the reader is familiar with the SRTP specification [RFC3711], its packet structure, and processing rules. 3. TESLA TESLA provides delayed per-packet data authentication and is specified in [TESLA]. This specification assumes that the reader is familiar with TESLA [TESLA]. In addition to its SRTP data-packet definition given here, TESLA needs an initial synchronization protocol and initial bootstrapping procedure. The synchronization protocol allows the sender and the receiver to compare their clocks and determine an upper bound of the difference. The synchronization protocol is outside the scope of this document. TESLA also requires an initial bootstrapping procedure to exchange needed parameters and the initial commitment to the key chain [TESLA]. For SRTP, it is assumed that the bootstrapping is performed out-of-band, possibly using the key management protocol that is exchanging the security parameters for SRTP, e.g. [GDOI], [MIKEY]. Initial bootstrapping of TESLA is outside the scope of this document. 4. Usage of TESLA within SRTP The present specification is an extension to the SRTP specification [RFC3711] and describes the use of TESLA with only a single key chain, and the delayed-authentication TESLA elements of procedure [TESLA]. 4.1. The TESLA extension TESLA is an OPTIONAL authentication transform for SRTP. When used, TESLA adds the fields showed in Figure 1 per-packet. The fields added by TESLA are called "TESLA authentication extensions" altogether, whereas "authentication tag" or "integrity protection tag" indicate the normal SRTP integrity protection tag, when the SRTP master key is shared by more than two endpoints [RFC3711]. Baugher, Carrara [Page 4] INTERNET-DRAFT TESLA-SRTP July, 2004 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | I | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Disclosed Key ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ TESLA MAC ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: The "TESLA authentication extension". I: 32 bit, MANDATORY Identifier of the time interval I, corresponding to the key K_i that is used to calculate the TESLA MAC present in the current packet (and in the packets sent in the current time interval I). Disclosed Key: variable length, MANDATORY The disclosed key (K_i-d), that can be used to authenticate previous packets from earlier time intervals [TESLA]. TESLA MAC (Message Authentication Code): variable length, MANDATORY The MAC computed using the key K'_i (derived from K_i) [TESLA], which is disclosed in a subsequent packet (in the Disclosed Key field). The MAC coverage is defined in Section 4.6. 4.2. SRTP Packet Format Figure 2 illustrates the format of the SRTP packet when TESLA is applied. When applied to RTP, the TESLA authentication extension SHALL be inserted before the (optional) SRTP MKI and (recommended) authentication tag. Baugher, Carrara [Page 5] INTERNET-DRAFT TESLA-SRTP July, 2004 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+ |V=2|P|X| CC |M| PT | sequence number | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | timestamp | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | synchronization source (SSRC) identifier | | | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | | contributing source (CSRC) identifiers | | | | .... | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | RTP extension (OPTIONAL) | | | +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | payload ... | | | | | +-------------------------------+ | | | | | RTP padding | RTP pad count | | | +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | I | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ | | ~ Disclosed Key ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ TESLA MAC ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+ | ~ MKI ~ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ MAC ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +- Encrypted Portion TESLA Authenticated Portion ---+ | | Authenticated Portion ---+ Figure 2. The format of the SRTP packet when TESLA is applied. Note that it is OPTIONAL to apply TESLA, i.e. the TESLA fields are OPTIONAL. As in SRTP, the "Encrypted Portion" of an SRTP packet consists of the encryption of the RTP payload (including RTP padding when present) of the equivalent RTP packet. The "Authenticated Portion" of an SRTP packet consists of the RTP header, the Encrypted Portion of the SRTP packet, and the TESLA authentication extension. Note that the definition is extended from [RFC3711] by the inclusion of the TESLA authentication extension. Baugher, Carrara [Page 6] INTERNET-DRAFT TESLA-SRTP July, 2004 The "TESLA Authenticated Portion" of an SRTP packet consists of the RTP header, the Encrypted Portion of the SRTP packet, and the TESLA I field. 4.3. Extension of the SRTP Cryptographic Context When TESLA is used, the definition of cryptographic context in Section 3.2 of SRTP SHALL include the following extensions. Transform-independent Parameter a flag indicating the use of TESLA in SRTP. When this bit is set, the following TESLA transform-dependent parameters define the particular TESLA configuration. Transform-dependent Parameters 1. an identifier for the PRF, f, implementing the one-way function F(x) in TESLA (to derive the keys in the chain), e.g. to indicate HMAC-SHA1, see Section 6.2 for the default value. 2. a non-negative integer n_c, determining the length of the F output, i.e. the length of the keys in the chain (that is also the key disclosed in an SRTP packet), see Section 6.2 for the default value. 3. an identifier for the PRF, f', implementing the one-way function F'(x) in TESLA (to derive the keys for the TESLA MAC, from the keys in the chain), e.g. to indicate HMAC-SHA1, see Section 6.2 for the default value. 4. a non-negative integer n_f, determining the length of the output of F', i.e. of the key for the TESLA MAC, see Section 6.2 for the default value. 5. an identifier for the TESLA MAC, that accepts the output of F'(x) as its key, e.g. to indicate HMAC-SHA1, see Section 6.2 for the default value. 6. a non-negative integer n_m, determining the length of the output of the TESLA MAC, see Section 6.2 for the default value. 7. the beginning of the session T_0, 8. the interval duration T_int (in msec), 9. the key disclosure delay d (in number of intervals) Baugher, Carrara [Page 7] INTERNET-DRAFT TESLA-SRTP July, 2004 10. non-negative integer n_c, determining the length of the key chain, which is determined based up the expected duration of the stream. 11. the initial key of the chain to which the sender has committed himself. F(x) is used to compute a keychain of keys in SRTP TESLA, as defined in Section 6. Also according to TESLA, F'(x) computes a TESLA MAC key with inputs as defined in Section 6. Section 6 of this document defines the default values for the transform-independent and transform-specific TESLA parameters. 4.4. SRTP Processing The SRTP packet processing is described in Section 3.3 of the SRTP specification [RFC3711]. The use of TESLA slightly changes the processing, as the SRTP MAC is checked upon packet arrival for DoS prevention, but the current packet is not TESLA-authenticated. Each packet is buffered until a subsequent packet discloses its TESLA key. The TESLA verification itself consists of some steps, such as tests of TESLA security invariants, that are described in Section 3.5-3.7 of [TESLA]. The words "TESLA computation" and "TESLA verification" hereby imply all those steps, which are not all spelled out in the following. In particular, notice that the TESLA verification implies checking the safety condition (Section 3.5 of [TESLA]). If the safe condition does not hold, the packet MUST be discarded. 4.4.1 Sender Processing The sender processing is as described in Section 3.3 of [RFC3711], up to step 5 included. After that the following process is followed: 6. When TESLA is applied, identify the key in the TESLA chain to be used in the current time interval, and the TESLA MAC key derived from it. Execute the TESLA computation to obtain the TESLA authentication extension for the current packet, by appending the current interval time (as I field), the disclosed key of the chain for an earlier packet, and the TESLA MAC under the current key from the chain. This step uses the related TESLA parameters from the crypto context as for Step 4. 7. If the MKI indicator in the SRTP crypto context is set to one, append the MKI to the packet. Baugher, Carrara [Page 8] INTERNET-DRAFT TESLA-SRTP July, 2004 8. When TESLA is applied, compute the authentication tag as described in step 7 of Section 3.3 of the SRTP specification, but with coverage as defined in this specification (see Section 4.6). 9. If necessary, update the ROC (step 8 in Section 3.3 of [RFC3711]). 4.4.2 Receiver Processing The receiver processing is as described in Section 3.3 of [RFC3711], up to step 4 included. To authenticate and replay-protect the current packet, the processing is the following: First check if the packet has been replayed (as for Section 3.3 of [RFC3711]). The SRTP replay list contains SRTP indices of recently received packets that have been authenticated by TESLA. (I.e. replay list updates MUST NOT be based on SRTP MAC.) If the packet is judged to be replayed, then the packet MUST be discarded, and the event SHOULD be logged. Next, perform verification of the SRTP integrity protection tag (note, not the TESLA MAC), if present, using the rollover counter from the current packet, the authentication algorithm indicated in the cryptographic context, and the session authentication key. If the verification is unsuccessful, the packet MUST be discarded from further processing and the event SHOULD be logged. If the verification is successful, remove and store the MKI (if present) and authentication tag fields from the packet. The packet is buffered, awaiting disclosure of the TESLA key in a subsequent packet. TESLA authentication is performed on a packet when the key is disclosed in a subsequent packet. When such key is disclosed, perform the TESLA verification of the packet using the rollover counter from the packet, the TESLA security parameters from the cryptographic context, and the disclosed key. If the verification is unsuccessful, the packet MUST be discarded from further processing and the event SHOULD be logged. If the TESLA verification is successful, remove the TESLA authentication extension from the packet. To decrypt the current packet, the processing is the following: Decrypt the Encrypted Portion of the packet, using the decryption algorithm indicated in the cryptographic context, the session Baugher, Carrara [Page 9] INTERNET-DRAFT TESLA-SRTP July, 2004 encryption key and salt (if used) found in Step 4 with the index from Step 2. (Note that the order of decryption and TESLA verification is not mandated. It is up to the application if to perform decryption immediately after the successful SRTP integrity protection verification and then get informed if the TESLA authentication for that packet has failed, or if to wait and TESLA- verify the packet before further processing). Update the rollover counter and highest sequence number, s_l, in the cryptographic context, using the packet index estimated in Step 2. If replay protection is provided, also update the Replay List (i.e., the Replay List is updated after the TESLA authentication is successfully verified). 4.5. SRTCP Packet Format Figure 3 illustrates the format of the SRTCP packet when TESLA is applied. The TESLA authentication extension SHALL be inserted before the MKI and authentication tag. Recall from [RFC3711] that in SRTCP the MKI is OPTIONAL, while the E-bit, the SRTCP index, and the authentication tag are MANDATORY. As in SRTP, the "Encrypted Portion" of an SRTCP packet consists of the encryption of the RTCP payload of the equivalent compound RTCP packet, from the first RTCP packet, i.e., from the ninth (9) octet to the end of the compound packet. The "Authenticated Portion" of an SRTCP packet consists of the entire equivalent (eventually compound) RTCP packet, the E flag, the SRTCP index (after any encryption has been applied to the payload), and the TESLA extension. Note that the definition is extended from [RFC3711] by the inclusion of the TESLA authentication extension. We define the "TESLA Authenticated Portion" of an SRTCP packet as consisting of the RTCP header (first 8 bytes), the Encrypted Portion of the SRTCP packet, and the I field. Processing of an SRTCP packets is similar to the SRTP processing (Section 4.3), but there are SRTCP-specific changes described in Section 3.4 of the SRTP specification [RFC3711] and in Section 4.6 of this memo. Baugher, Carrara [Page 10] INTERNET-DRAFT TESLA-SRTP July, 2004 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+ |V=2|P| RC | PT=SR or RR | length | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | SSRC of sender | | | +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | | ~ sender info ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ report block 1 ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ report block 2 ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ ... ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | |V=2|P| SC | PT=SDES=202 | length | | | | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | | | SSRC/CSRC_1 | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ SDES items ~ | | | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | | ~ ... ~ | | +>+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | | |E| SRTCP index | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | I | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ | | ~ Disclosed Key ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ~ TESLA MAC ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<|-+ | ~ SRTCP MKI ~ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | : authentication tag : | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | +-- Encrypted Portion TESLA Authenticated Portion -----+ | | Authenticated Portion -------+ Figure 3. The format of the SRTCP packet when TESLA is applied. Note that it is OPTIONAL to apply TESLA, i.e. the TESLA fields are OPTIONAL. Baugher, Carrara [Page 11] INTERNET-DRAFT TESLA-SRTP July, 2004 4.6. TESLA MAC Let M' denote packet data to be TESLA-authenticated. In the case of SRTP, M' SHALL consist of the SRTP TESLA Authenticated Portion (RTP header, SRTP Encrypted Portion, and I field) shown in Figure 1 or Figure 2) of the packet concatenated with the ROC of the same packet: M' = ROC || TESLA Authenticated Portion. In the case of SRTCP, M' SHALL consist of the SRTCP TESLA Authenticated Portion only (RTCP header, SRTCP Encrypted Portion, and I field). The normal authentication tag SHALL be applied with the same coverage as specified in [RFC3711], i.e.: - for SRTP: Authenticated Portion || ROC (with the extended definition of SRTP Authentication Portion as for Section 4.2) - for SRTCP: Authenticated Portion (with the extended definition of SRTCP Authentication Portion as for Section 4.2). The pre-defined authentication transform in SRTP, HMAC-SHA1 [RFC2104], is also used to generate the TESLA MAC. For SRTP (respectively SRTCP), the HMAC SHALL be applied to the key in the TESLA chain corresponding to a particular time interval, and M' as specified above. The HMAC output SHALL then be truncated to the n_m left-most bits. Default values are in Section 6.2. 4.7. PRFs TESLA requires two pseudo-random functions (PRFs), f and f', to implement * one one-way function F(x) to derive the key chain, and * one one-way function F'(x) to derive (from each key of the chain) the key that is actually used to calculate the TESLA MAC. When TESLA is used within SRTP, the default choice of the two PRFs SHALL be HMAC-SHA1. Default values are in Section 6.2. Other PRFs can be chosen, and their use SHALL follow the common guidelines in [RFC3711] when adding new security parameters. Baugher, Carrara [Page 12] INTERNET-DRAFT TESLA-SRTP July, 2004 5. TESLA Bootstrapping The extensions to the SRTP cryptographic context include a set of TESLA parameters that are listed in section 4.3 of this document. Furthermore, TESLA MUST be bootstrapped at session set-up (for the parameter exchange and the initial key commitment) through a regular data authentication system (a digital signature algorithm is RECOMMENDED). Key management procedures can take care of this bootstrapping prior to the commencement of an SRTP session where TESLA authentication is used. The bootstrapping mechanism is out of scope for this document. A critical factor for the security of TESLA is that the sender and receiver need to be loosely synchronized. TESLA assumes that the local internal clocks do not drift too much during the session. Use of TESLA in SRTP assumes that the time synchronization is guaranteed by out-of-band schemes (e.g. key management), i.e. it is not in the scope of SRTP. 6. SRTP TESLA Default parameters Key management procedures establish SRTP TESLA operating parameters listed in section 4.3 of this document. The operating parameters appear in the SRTP cryptographic context and have the following default values. In the future, an Internet RFC MAY define alternative settings for SRTP TESLA that are different than those specified here. In particular, it should be noted that the settings defined in this memo can have a large impact on bandwidth, as it adds 38 bytes to each packet (when the field length values are the default ones). For certain applications, this overhead may represent more than a 50% increase in packet size. Alternative settings might seek to reduce the number and length of various TESLA fields and outputs. No such optimizations are considered in this memo. It is RECOMMENDED that the SRTP MAC be truncated to 32 bits since the SRTP MAC provides only group authentication and serves only as protection against external DoS. 6.1 Transform-independent Parameters The value of the flag indicating the use of TESLA in SRTP is by default zero (TESLA not used). Baugher, Carrara [Page 13] INTERNET-DRAFT TESLA-SRTP July, 2004 6.2 Transform-dependent Parameters for TESLA MAC The default values for the security parameters are listed in the following. "OWF" denotes a one-way function. Parameter Mandatory-to-support Default --------- -------------------- ------- TESLA KEYCHAIN OWF (F(x)) HMAC-SHA1 HMAC-SHA1 OUTPUT LENGTH 160 160 TESLA MAC KEY OWF (F'(F(x))) HMAC-SHA1 HMAC-SHA1 OUTPUT LENGTH n_f 160 160 TESLA MAC HMAC-SHA1 HMAC-SHA1 (TRUNCATED) OUTPUT LENGTH n_m 80 80 As shown above, TESLA implementations MUST support HMAC-SHA1 for the TESLA MAC, the MAC key generator, and the TESLA keychain generator one-way function. The TESLA keychain generator is recursively defined as follows [TESLA]. K_i=HMAC_SHA1(K_{i+1},0), i=0..N-1 where N-1=n_c from the cryptographic context. The TESLA MAC key generator is defined as follows [TESLA]. K'_i=HMAC_SHA1(K_i,1) The TESLA MAC uses a truncated output of ten bytes [RFC2104] and is defined as follows. HMAC_SHA1(K'_i, M') where M' is as specified in Section 4.6. 7. Security Considerations Denial of Service (DoS) attacks when delayed authentication is used are discussed in [PCST]. TESLA requires receiver's buffering before authentication, therefore the receiver can suffer a denial of service attack due to a flood of bogus packets. To address this problem, the current specification REQUIRES the use of a 32-bit SRTP MAC in addition to TESLA MAC. The shorter size of the SRTP MAC is Baugher, Carrara [Page 14] INTERNET-DRAFT TESLA-SRTP July, 2004 here motivated by the fact that that MAC served purely for DoS prevention from attackers external to the group. The use of TESLA in SRTP defined in this specification is subject to the security considerations discussed in the SRTP specification [RFC3711] an in the TESLA specification [TESLA]. In particular, it must be noted that the all TESLA security is dependent on the computation of the "safety condition" as defined in Section 3.5 of [TESLA]. SRTP TESLA depends on the effective security of the systems that perform bootstrapping (time synchronization) and key management. These systems are external to SRTP and are not considered in this specification. 8. IANA Considerations No IANA registration is required. 9. Acknowledgements The authors would like to thanks Ran Canetti, Karl Norrman, Mats N„slund, and Fredrik Lindholm for their valuable help. 10. Author's Addresses Questions and comments should be directed to the authors and msec@ietf.org: Mark Baugher Cisco Systems, Inc. 5510 SW Orchid Street Phone: +1 408-853-4418 Portland, OR 97219 USA Email: mbaugher@cisco.com Elisabetta Carrara Ericsson Research SE-16480 Stockholm Phone: +46 8 50877040 Sweden EMail: elisabetta.carrara@ericsson.com Baugher, Carrara [Page 15] INTERNET-DRAFT TESLA-SRTP July, 2004 11. References Normative [PCST] Perrig, A., Canetti, R., Song, D., Tygar, D., "Efficient and Secure Source Authentication for Multicast", in Proc. of Network and Distributed System Security Symposium NDSS 2001, pp. 35-46, 2001. [RFC1305] Mills D., Network Time Protocol (Version 3) Specification, Implementation and Analysis, RFC 1305, March, 1992. http://www.ietf.org/rfc/rfc1305.txt [RFC3711] Baugher, McGrew, Naslund, Carrara, Norrman, "The Secure Real-time Transport Protocol", RFC 3711, March 2004. [TESLA] Perrig, Canetti, Song, Tygar, Briscoe, "TESLA: Multicast Source Authentication Transform Introduction", October 2002, draft- ietf-msec-tesla-intro-02.txt. Informative [gkmarch] Baugher, Canetti, Dondeti, Lindholm, "MSEC Group Key Management Architecture", June 2004, . [GDOI] Baugher, Weis, Hardjono, Harney, "The Group Domain of Interpretation", RFC 3547, July 2003. [MIKEY] Arkko et al., "MIKEY: Multimedia Internet KEYing", December 2003, Copyright Notice Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. This draft expires in January 2005. Baugher, Carrara [Page 16]