





Network Working Group                                         M. Baugher
Request for Comments: 4383                                         Cisco
Category: Standards Track                                     E. Carrara
                                           Royal Institute of Technology
                                                           February 2006


 The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA)
           in the Secure Real-time Transport Protocol (SRTP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo describes the use of the Timed Efficient Stream Loss-
   tolerant Authentication (RFC 4082) 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 ......................................5
      4.1. The TESLA Extension ........................................5
      4.2. SRTP Packet Format .........................................6
      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. Acknowledgements ...............................................16
   9. References .....................................................17
      9.1. Normative References ......................................17
      9.2. Informative 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 group
   key), and that the packet was sent by a source within the group.





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   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 that 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 that
   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.

   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 its processing rules.  TESLA is




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   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.




























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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].

   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,
      which 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)), which 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.









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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).

     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.






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   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, the SRTP MAC covers up to the MKI field but does not
   include the MKI.  It is necessary for packet integrity that the
   SRTP-TESLA MAC 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
   MAC and SRTP-MAC) contiguous, we would need to redefine the SRTP
   specification to include the MKI in SRTP-MAC coverage.  This change
   is impossible, 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 TESLA 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 (TESLA PRF), implementing the one-way
         function F(x) in TESLA (to derive the keys in the chain), and
         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.

     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.  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.

     4.  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.



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

     6.  the beginning of the session T_0.

     7.  the interval duration T_int (in msec).

     8.  the key disclosure delay d (in number of intervals).

     9.  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).

     10. a 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.

     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-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 Sections 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 per Section 3.3
      of [RFC3711]).  Note, however, that 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
      (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 as follows:

      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 that the TESLA verification be performed
   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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RFC 4383                       TESLA-SRTP                  February 2006


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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RFC 4383                       TESLA-SRTP                  February 2006


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].  That is:

   - 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 predefined authentication transform in SRTP, HMAC-SHA1 [RFC2104],
   is also used to generate the TESLA MAC.  For SRTP (and respectively
   for SRTCP), the HMAC SHALL be applied to the key in the TESLA chain
   corresponding to a particular time interval, and to 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 predefined HMAC-SHA1 authentication algorithm MAY
   be replaced with an alternative algorithm that is specified in a
   future Internet RFC.

4.7.  PRFs

   TESLA requires a pseudo-random function (PRF) 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 PRF 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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RFC 4383                       TESLA-SRTP                  February 2006


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 setup (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).  That is, 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, note that the
   settings defined in this memo can have a large impact on bandwidth,
   as they add 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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   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 table.

   Parameter                        Mandatory-to-support     Default
   ---------                        --------------------     -------
   TESLA PRF                              HMAC-SHA1         HMAC-SHA1
   BIT-OUTPUT LENGTH n_p                     160               160
   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 and the TESLA PRF.  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
   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 because 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



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   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.  Acknowledgements

   The authors would like to thank Ran Canetti, Karl Norrman, Mats
   Naslund, Fredrik Lindholm, David McGrew, and Bob Briscoe for their
   valuable help.



















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9.  References

9.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

9.2.  Informative References

   [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.

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, April 2005.














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Authors' Addresses

   Questions and comments should be directed to the authors and
   msec@ietf.org.

   Mark Baugher
   Cisco Systems, Inc.
   5510 SW Orchid Street
   Portland, OR 97219 USA

   Phone:  +1 408-853-4418
   EMail:  mbaugher@cisco.com


   Elisabetta Carrara
   Royal Institute of Technology
   Stockholm
   Sweden

   EMail:  carrara@kth.se































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Full Copyright Statement

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