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Versions: 00 01 02 03 04 05 RFC 4383

   Internet Engineering Task Force                     Baugher (Cisco)
   MSEC Working Group                               Carrara (Ericsson)
   INTERNET-DRAFT
   EXPIRES: August 2004                                  February 2004


                        The Use of TESLA in SRTP
                   <draft-ietf-msec-srtp-tesla-00.txt>



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
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   reference material or cite them other than as "work in progress".

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   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.














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   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..................6
    4.4. SRTP Processing..............................................7
    4.4.1 Sender Processing...........................................8
    4.4.2 Receiver Processing.........................................8
    4.5. SRTCP Packet Format..........................................9
    4.6. TESLA MAC...................................................11
    4.7. PRFs........................................................11
    5. TESLA Bootstrapping...........................................12
    6. SRTP TESLA Default parameters.................................12
    6.1 Transform-independent Parameter: SRTP MAC with TESLA MAC.....13
    6.2 Transform-dependent Parameters for TESLA MAC.................13
    7. Security Considerations.......................................14
    8. IANA Considerations...........................................14
    9. Acknowledgements..............................................14
    10. Author's Addresses...........................................15
    11. References...................................................15
    Intellectual Property Right Considerations.......................16
    Full Copyright Statement.........................................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 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



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   by an attacker outside the group (and hence not in possession of the
   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 [TESLA1,
   TESLA2].


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.  Almost none of their details will be explained, and the
   reader can find them in their respective specifications [SRTP,
   TESLA1, TESLA2].  Also, this specification uses the same definitions
   as TESLA for common terms.


2. SRTP

   The Secure Real-time Transport Protocol (SRTP) [SRTP] 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).

   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.



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   The TESLA extension to SRTP is defined in this specification, which
   assumes that the reader is familiar with the SRTP specification
   [SRTP], its packet structure, and processing rules.


3. TESLA

   TESLA provides delayed per-packet data authentication and is
   specified in two documents, an introductory overview [TESLA1] and a
   second specification that defines signaling and data packet
   parameters [TESLA2].  This specification assumes that the reader is
   familiar with these two documents.

   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
   [TESLA2].  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
   [SRTP] and describes the use of TESLA with only a single key chain,
   and the delayed-authentication TESLA elements of procedure [TESLA1,
   TESLA2].


4.1. The TESLA extension

   TESLA is an OPTIONAL authentication algorithm 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 integrity protection tag when the SRTP
   master key is shared by more than two endpoints [SRTP].





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   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Id                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                         Disclosed Key                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                           TESLA MAC                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: The "TESLA authentication extension".


   Id: identifier of K_i, MANDATORY
       The identifier of the key that was used to calculate the MAC
       present in the packet during interval i.

   Disclosed Key: variable length, MANDATORY
       The disclosed key, that can be used to authenticate previous
       packets from earlier time intervals, i.e. K_{i-d}.

   TESLA MAC (Message Authentication Code): variable length, MANDATORY
       The MAC computed using K'_i, which is disclosed in a subsequent
       packet.  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.

   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
   [SRTP] by the inclusion of the TESLA authentication extension.

   The "TESLA Authenticated Portion" of an SRTP packet consists of the
   RTP header, the Encrypted Portion of the SRTP packet, the TESLA Id
   field, and the TESLA disclosed key.





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     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 | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| |                            Id                                 | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~                      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.


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:

      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. HMAC-
        SHA1,




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      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),

      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. HMAC-SHA1,

      4. a non-negative integer n_f, determining the length of the
        output of F', i.e. of the key for the TESLA MAC,

      5. an identifier for the TESLA MAC, that accepts the output of
        F'(x) as its key,

      6. a non-negative integer n_m, determining the length of the
        output of the TESLA MAC,

      7. the identifier id_j of a specific time interval I_j,

      8. an NTP timestamp TI_j describing the beginning of I_j,

      9. an NTP timestamp T_int describing the interval duration,

      10. the key-disclosure interval, d,

      11. the id_n of the final key in the keychain, K_n,

      12. the interval d_n of the last key chain element.

   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.

    Note that the replay list is now containing indices of recently
    received packets that have been authenticated by TESLA. I.e. replay
    list updates MUST NOT be based on SRTP MAC.

   These parameters are all "transform-specific" parameters.  There is
   one transform-independent parameter that declares that SRTP message
   authentication is extended with TESLA DOA authentication.  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 [SRTP]. The use of TESLA slightly changes the



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   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 4
   of [TESLA1]. The words "TESLA computation" and "TESLA verification"
   hereby imply all those steps, which are not all spelled out in the
   following.


4.4.1 Sender Processing

   The sender processing is as described in Section 3.3 of [SRTP], 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
   key Id, 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 is set to one, append the MKI to the packet.

   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 9 in Section 3.3 of [SRTP]).


4.4.2 Receiver Processing

   The receiver processing is as described in Section 3.3 of [SRTP], 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
     [SRTP]). 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



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     the verification is unsuccessful, the packet MUST be discarded
     from further processing and the event SHOULD be logged.

     If the verification is successful, remove 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
     encryption key and salt (if used) found in Step 4 with the index
     from Step 2.

   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 [SRTP] that in
   SRTCP the MKI is OPTIONAL, while the E-bit, the SRTCP index, and the
   authentication tag are MANDATORY.













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   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                         | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| |                          Id (OPTIONAL)                        | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| ~                         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.


   As in SRTP, the "Encrypted Portion" of an SRTCP packet consists of
   the encryption of the RTCP payload of the equivalent compound RTCP




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   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
   [SRTP] 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, the Id field, and the TESLA disclosed key.

   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 [SRTP] and in Section 4.6 of
   this memo.


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, TESLA Id, and disclosed key) 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,
   TESLA Id, and disclosed key).

   The normal authentication tag SHALL be applied with the same
   coverage as specified in [SRTP], i.e. Authenticated Portion || ROC
   for SRTP, and Authenticated Portion for SRTCP.

   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



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   * 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 [SRTP] when adding new security parameter.


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.
   Key management procedures establish these parameters prior to the
   commencement of an SRTP session where TESLA authentication is used.

   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, i.e. it is not in the scope of SRTP.  The
   TESLA overview specification [TESLA2] describes some methods, which
   might be accomplished as part of SRTP key management.  At least one
   SRTP key management protocol, MIKEY, requires time synchronization
   [MIKEY].


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.






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6.1 Transform-independent Parameter: SRTP MAC with TESLA MAC

   Section 3.2.1 of the SRTP specification identifies "message
   authentication" as one of the transform-independent parameters.  By
   default, this is HMAC-SHA1 for SRTP.  With the addition of TESLA,
   SRTP message authentication becomes a compound parameter since it is
   necessary to identify two message authentication algorithms, one for
   the SRTP MAC and one for the TESLA MAC.  Thus, the use or non-use of
   TESLA SHALL be indicated by the presence of a TESLA bit in the SRTP
   cryptographic context.  When this bit is set, the SRTP
   implementation MUST inspect the TESLA transform-dependent parameters
   to determine the particular TESLA configuration.

   It is RECOMMENDED that the SRTP MAC be truncated to four bytes since
   the SRTP MAC provides only group authentication and serves only as
   protection against DoS.


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

   id_j

   TI_j
   T_int

   id_n
   d_n

   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.




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                     K_i=HMAC_SHA1(K_{i+1},0), i=0..N-1

   The TESLA MAC key generator is defined as follows.

                           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.

   The TESLA interval parameters are id_j and id_n, both are 32 bits in
   length.  The times associated with the intervals are TI_j, T_int,
   and d_n, which are 64-bit values in Network Time Protocol (NTP)
   format.

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 four-byte
   SRTP MAC in addition to TESLA MAC.  The shorter size of the SRTP MAC
   is here motivated by the fact that that MAC served purely for DoS
   prevention from attackers external to the group.

   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 Karl Norrman, Mats Näslund, and Ran
   Canetti, for their valuable help.







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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


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.

   [SRTP] Baugher, McGrew, Carrara, Naslund, Norrman, "The Secure Real-
   time Transport Protocol", July 2003, <draft-ietf-avt-srtp-09.txt>.

   [TESLA1] Perrig, Canetti, Song, Tygar, Briscoe, "TESLA: Multicast
   Source Authentication Transform Introduction", October 2002, draft-
   ietf-msec-tesla-intro-01.txt.

   [TESLA2] Perrig, Canetti, Whillock, "TESLA: Multicast Source
   Authentication Transform Specification", October 2002, draft-ietf-
   msec-tesla-spec-00.txt

   Informative

   [gkmarch] Baugher, Canetti, Dondeti, Lindholm, "MSEC Group Key
   Management Architecture", January 2003, <draft-ietf-msec-gkmarch-
   07.txt>.

   [GDOI] Baugher, Weis, Hardjono, Harney, "The Group Domain of
   Interpretation", RFC 3547, July 2003.

   [MESP] Baugher, Canetti, Cheng, Rohatgi, "MESP: A Multicast
   Framework for the IPsec ESP", March 2003, <draft-ietf-msec-mesp-
   01.txt>.




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   [MIKEY] Arkko, Carrara, Lindholm, Naslund, Norrman, "MIKEY:
   Multimedia Internet KEYing", December 2003, <draft-ietf-msec-mikey-
   08.txt>


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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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   This draft expires in August 2004.










































Baugher, Carrara                                             [Page 17]


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