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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: April 2006                                    October 2005





                        The Use of TESLA in SRTP
                  <draft-ietf-msec-srtp-tesla-05.txt>



Status of this memo


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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

   This memo describes the use of the Timed Efficient Stream Loss-
   tolerant Authentication (RFC4082) 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..................7
   4.4. SRTP Processing..............................................8
   4.4.1 Sender Processing...........................................9
   4.4.2 Receiver Processing.........................................9
   4.5. SRTCP Packet Format.........................................11
   4.6. TESLA MAC...................................................13
   4.7. PRFs........................................................13
   5. TESLA Bootstrapping and Cleanup...............................14
   6. SRTP TESLA Default parameters.................................14
   7. Security Considerations.......................................15
   8. IANA Considerations...........................................16
   9. Acknowledgements..............................................16
   10. Author's Addresses...........................................17
   11. References...................................................17


1. Introduction

   Multicast and broadcast communications 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
   [RFC4046] 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



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   group key), and that the packet was sent by a source within the
   group.

   Some applications cannot tolerate source ambiguity and need to
   identify 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 [RFC4082].

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 is 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]
   and [RFC4082].  This specification uses the same definitions as
   TESLA for common terms and assumes that the reader is familiar with
   the TESLA algorithms and protocols [RFC4082].


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 be 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.



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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
   [RFC3711], its packet structure, and processing rules.  TESLA is an
   alternative message-authentication algorithm that authenticates
   messages from the source when a key is shared among two or more
   receivers.


3. TESLA

   TESLA provides delayed per-packet data authentication and is
   specified in [RFC4082].

   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
   [RFC4082].  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. [RFC3547,
   RFC3830].  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 delayed-authentication [RFC4082].

4.1. The TESLA extension

   TESLA is an OPTIONAL authentication transform for SRTP.  When used,
   TESLA adds the fields shown in Figure 1 per-packet.  The fields
   added by TESLA are called "TESLA authentication extensions," 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].







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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              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 of the current packet
      (and other 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 [RFC4082].  A
       Section 4.3 parameter establishes the size of this field.

   TESLA MAC (Message Authentication Code): variable length, MANDATORY
       The MAC computed using the key K'_i (derived from K_i)
       [RFC4082], which is disclosed in a subsequent packet (in the
       Disclosed Key field).  The MAC coverage is defined in Section
       4.6.  A Section 4.3 parameter establishes the size of this
       field.

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 (SRTP MAC).














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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 | | |
+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| |                            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.

   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.

   The "TESLA Authenticated Portion" of an SRTP packet consists of the
   RTP header and the Encrypted Portion of the SRTP packet. As shown in
   Figure 2, SRTP HMAC-SHA1 covers up to the MKI field but does not



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   include the MKI.  It is necessary for packet integrity that the
   SRTP-TESLA tag be covered by the SRTP integrity check.  SRTP does
   not cover the MKI field (because it does not need to be covered for
   SRTP packet integrity).  In order to make the two tags (SRTP-TESLA
   and SRTP-HMAC_SHA1) contiguous, we would need to redefine the SRTP
   specification to include the MKI in HMAC-SHA1 coverage.  This change
   is impossible and so the MKI field separates the TESLA MAC from the
   SRTP MAC in the packet layout of Figure 2.  This change to the
   packet format presents no problem to an implementation that supports
   the new SRTP-TESLA authentication transform.

   The lengths of the Disclosed Key and TELSA MAC fields are Section
   4.3 parameters.  As in SRTP, fields that follow the packet payload
   are not necessarily aligned on 32-bit boundaries.


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-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 for the default value.

     2. a non-negative integer n_p, 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 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 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
        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 for
        the default value.

     6. a non-negative integer n_m, determining the length of the
        output of the TESLA MAC, see Section 6 for the default value.



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     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)

     10. the upper bound D_t (in sec) on the lag of the receiver clock
        relative to the sender clock (this quantity has to be
        calculated by the peers out-of-band)

     11. non-negative integer n_c, determining the length of the key
        chain, K_0...K_n-1 of [RFC4082] (see also Section 6 of this
        document), which is determined based upon the expected duration
        of the stream.

     12. 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-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 [RFC4082]. 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
   [RFC4082]).

   As pointed out in [RFC4082], if the packet is deemed "unsafe", then
   the receiver considers the packet unauthenticated. It should discard
   unsafe packets but, at its own risk, it may choose to use them
   unverified. Hence, if the safe condition does not hold, it is
   RECOMMENDED to discard the packet and log the event.





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4.4.1 Sender Processing

   The sender processing is as described in Section 3.3 of [RFC3711, up
   to step 5 inclusive.  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 identifier (as i field), the disclosed key of the
   chain for the previous disclosure interval (i.e. the key for
   interval i is disclosed in interval i+d), 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.

   8. When TESLA is applied, and if the SRTP authentication (external
   tag) is required (for DoS), 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 rollover counter (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 inclusive.

   To authenticate and replay-protect the current packet, the
   processing is as follows:

     First check if the packet has been replayed (as for Section 3.3 of
     [RFC3711]). Note however, 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.




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     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. Recall that a key for interval i
     is disclosed during interval i+d, i.e. the same key is disclosed
     in packets sent over d intervals of length t_int.  If the interval
     identifier i from the packet (Section 4.1) has advanced more than
     d intervals from the highest value of i that has been received,
     then packets have been lost and one or more keys MUST be computed
     as described in Section 3.2, second paragraph, of the TESLA
     specification [RFC4082]. The computation is performed recursively
     for all disclosed keys that have been lost, from the newly-
     received interval to the last-received interval.

     When a newly-disclosed key is received or computed, 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.

   (Note that the order of decryption and TESLA verification is not
   mandated. It is RECOMMENDED to perform the  TESLA verification
   before decryption.  TESLA application designers might choose to
   implement optimistic processing techniques such as notification of
   TESLA verification results after decryption or even after plaintext
   processing.  Optimistic verification is beyond the scope of this
   document.)

   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).





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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.  This means that the SRTP
   (external) MAC is MANDATORY also when TESLA is used.

   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) byte 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) and the Encrypted
   Portion of the SRTCP packet.

   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.























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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                         | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ |
| |                              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 when additional fields are added to a packet, it will
   increase the packet size and thus the RTCP average packet size.







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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 and SRTP Encrypted Portion, see Figure 2) of the packet
   concatenated with the rollover counter (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 and SRTCP Encrypted
   Portion).

   The normal authentication tag (OPTIONAL for SRTP, MANDATORY for
   SRTCP) 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 in Section 4.2)

   - for SRTCP: Authenticated Portion (with the extended definition of
   SRTCP Authentication Portion as in 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.

   As with SRTP, the pre-defined HMAC-SHA1 authentication algorithm MAY
   be replaced with an alternative algorithm that is specified in a
   future Internet RFC.

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.

   Other PRFs can be chosen, and their use SHALL follow the common
   guidelines in [RFC3711] when adding new security parameters.



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5. TESLA Bootstrapping and Cleanup

   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 (it could for example be part of the key
   management protocol).

   A critical factor for the security of TESLA is that the sender and
   receiver need to be loosely synchronized. TESLA requires a bound on
   clock drift to be known (D_t).  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.

   It also should be noted that TESLA has some reliability requirements
   in that a key is disclosed for a packet in a subsequent packet,
   which can get lost. Since a key in a lost packet can be derived from
   a future packet, TESLA is robust to packet loss.  This key stream
   stops, however, when the key-bearing data stream packets stop at the
   conclusion of the RTP session.  To avoid this nasty boundary
   condition, send null packets with TESLA keys for one entire key-
   disclosure period following the interval in which the stream ceases:
   Null packets SHOULD be sent for d intervals of duration t_int (items
   8 and 9 of Section 4.3).  The rate of null packets SHOULD be the
   average rate of the session media stream.


6. SRTP TESLA Default parameters

   Key management procedures establish SRTP TESLA operating parameters,
   which are listed in section 4.3 of this document.  The operating
   parameters appear in the SRTP cryptographic context and have the
   default values that are described in this section.  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




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   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.

   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
      BIT-OUTPUT LENGTH n_p             160                  160

   TESLA MAC KEY OWF (F'(F(x)))      HMAC-SHA1             HMAC-SHA1
      BIT-OUTPUT LENGTH n_f             160                  160

   TESLA MAC                         HMAC-SHA1             HMAC-SHA1
      (TRUNCATED) BIT-OUTPUT LENGTH n_m  80                   80


   As shown above, TESLA implementations MUST support HMAC-SHA1
   [RFC2104] 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 [RFC4082].

                    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 [RFC4082].

                           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 on delayed authentication are
   discussed in [PCST].  TESLA requires receiver buffering before



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   authentication, therefore the receiver can suffer a denial of
   service attack due to a flood of bogus packets.  To address this
   problem, the external SRTP MAC, based on the group key, MAY be used
   in addition to the TESLA MAC.  The short size of the SRTP MAC
   (default 32 bits) is motivated by the fact that that MAC is purely
   for DoS prevention from attackers external to the group. The shorter
   output tag means that an attacker has a better chance of getting a
   forged packet accepted, which is about 2^31 attempts on average.  As
   a first line of defense against a denial of service attack, a short
   tag is probably adequate; a victim will likely have ample evidence
   that it is under attack before accepting a forged packet, which will
   subsequently fail the TESLA check.  [RFC4082] describes other
   mechanisms that can be used to prevent DoS, in place of the external
   group-key MAC. If used, they need to be added as processing steps
   (following the guidelines of [RFC4082]).

   The use of TESLA in SRTP defined in this specification is subject to
   the security considerations discussed in the SRTP specification
   [RFC3711] and in the TESLA specification [RFC4082]. In particular,
   the TESLA security is dependent on the computation of the "safety
   condition" as defined in Section 3.5 of [RFC4082].

   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.

   The length of the TESLA MAC is by default 80 bits. RFC 2104 requires
   the MAC length to be at least 80 bits and at least half the output
   size of the underlying hash function.  The SHA-1 output size is 160
   bits, so both of these requirements are met with the 80 bit MAC
   specified in this document.  Note that IPsec implementations tend to
   use 96 bits for their MAC values to align the header with a 64 bit
   boundary.  Both MAC sizes are well beyond the reach of current
   cryptanalytic techniques.


8. IANA Considerations

   No IANA registration is required.


9. Acknowledgements

   The authors would like to thanks Ran Canetti, Karl Norrman, Mats
   Naslund, Fredrik Lindholm, David McGrew, and Bob Briscoe 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
      SE-16480 Stockholm     Phone:  +46 8 50877040
      Sweden                 EMail:  elisabetta.carrara@ericsson.com


11. References

   Normative

   [RFC1305] Mills D., Network Time Protocol (Version 3) Specification,
   Implementation and Analysis, Internet Engineering Task Force, RFC
   1305, March, 1992.

   [RFC2104] Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
   Message Authentication," Internet Engineering Task Force, RFC 2104,
   February, 1997.

   [RFC2119] Bradner, Keywords to Use in RFCs to Indicate Requirement
   Levels, Internet Engineering Task Force,  RFC 2119, March 1997.

   [RFC3711] Baugher, McGrew, Naslund, Carrara, Norrman, "The Secure
   Real-time Transport Protocol", Internet Engineering Task Force, RFC
   3711, March 2004.

   [RFC4082] Perrig, Song, Canetti, Tygar, Briscoe, "TESLA: Multicast
   Source Authentication Transform Introduction", Internet Engineering
   Task Force, RFC 4082, June 2005.

   Informative

   [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.






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   [RFC3547] Baugher, Weis, Hardjono, Harney, "The Group Domain of
   Interpretation", Internet Engineering Task Force, RFC 3547, July
   2003.

   [RFC3830] Arkko et al., "MIKEY: Multimedia Internet KEYing", RFC
   3830, Internet Engineering Task Force, August 2004.

   [RFC4046] Baugher, Canetti, Dondeti, Lindholm, "MSEC Group Key
   Management Architecture", Internet Engineering Task Force, April
   2005.


Copyright Notice

   Copyright (C) The Internet Society (2005).  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 April 2006.





















Baugher, Carrara                                             [Page 18]


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