rfc3711.txt

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      cryptographic context, according to Section 8.1.

   4. Determine the session keys and session salt (if they are used by
      the transform) as described in Section 4.3, using master key,
      master salt, key_derivation_rate, and session key-lengths in the
      cryptographic context with the index, determined in Steps 2 and 3.

   5. Encrypt the RTP payload to produce the Encrypted Portion of the
      packet (see Section 4.1, for the defined ciphers).  This step uses
      the encryption algorithm indicated in the cryptographic context,
      the session encryption key and the session salt (if used) found in
      Step 4 together with the index found in Step 2.

   6. If the MKI indicator is set to one, append the MKI to the packet.

   7. For message authentication, compute the authentication tag for the
      Authenticated Portion of the packet, as described in Section 4.2.
      This step uses the current rollover counter, the authentication





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RFC 3711                          SRTP                        March 2004


      algorithm indicated in the cryptographic context, and the session
      authentication key found in Step 4.  Append the authentication tag
      to the packet.

   8. If necessary, update the ROC as in Section 3.3.1, using the packet
      index determined in Step 2.

   To authenticate and decrypt an SRTP packet, the receiver SHALL do the
   following:

   1. Determine which cryptographic context to use as described in
      Section 3.2.3.

   2. Run the algorithm in Section 3.3.1 to get the index of the SRTP
      packet.  The algorithm uses the rollover counter and highest
      sequence number in the cryptographic context with the sequence
      number in the SRTP packet, as described in Section 3.3.1.

   3. Determine the master key and master salt.  If the MKI indicator in
      the context is set to one, use the MKI in the SRTP packet,
      otherwise use the index from the previous step, according to
      Section 8.1.

   4. Determine the session keys, and session salt (if used by the
      transform) as described in Section 4.3, using master key, master
      salt, key_derivation_rate and session key-lengths in the
      cryptographic context with the index, determined in Steps 2 and 3.

   5. For message authentication and replay protection, first check if
      the packet has been replayed (Section 3.3.2), using the Replay
      List and the index as determined in Step 2.  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 authentication tag, using the
      rollover counter from Step 2, the authentication algorithm
      indicated in the cryptographic context, and the session
      authentication key from Step 4.  If the result is "AUTHENTICATION
      FAILURE" (see Section 4.2), the packet MUST be discarded from
      further processing and the event SHOULD be logged.

   6. Decrypt the Encrypted Portion of the packet (see Section 4.1, for
      the defined ciphers), 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.






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RFC 3711                          SRTP                        March 2004


   7. Update the rollover counter and highest sequence number, s_l, in
      the cryptographic context as in Section 3.3.1, using the packet
      index estimated in Step 2.  If replay protection is provided, also
      update the Replay List as described in Section 3.3.2.

   8. When present, remove the MKI and authentication tag fields from
      the packet.

3.3.1.  Packet Index Determination, and ROC, s_l Update

   SRTP implementations use an "implicit" packet index for sequencing,
   i.e., not all of the index is explicitly carried in the SRTP packet.
   For the pre-defined transforms, the index i is used in replay
   protection (Section 3.3.2), encryption (Section 4.1), message
   authentication (Section 4.2), and for the key derivation (Section
   4.3).

   When the session starts, the sender side MUST set the rollover
   counter, ROC, to zero.  Each time the RTP sequence number, SEQ, wraps
   modulo 2^16, the sender side MUST increment ROC by one, modulo 2^32
   (see security aspects below).  The sender's packet index is then
   defined as

      i = 2^16 * ROC + SEQ.

   Receiver-side implementations use the RTP sequence number to
   determine the correct index of a packet, which is the location of the
   packet in the sequence of all SRTP packets.  A robust approach for
   the proper use of a rollover counter requires its handling and use to
   be well defined.  In particular, out-of-order RTP packets with
   sequence numbers close to 2^16 or zero must be properly handled.

   The index estimate is based on the receiver's locally maintained ROC
   and s_l values.  At the setup of the session, the ROC MUST be set to
   zero.  Receivers joining an on-going session MUST be given the
   current ROC value using out-of-band signaling such as key-management
   signaling.  Furthermore, the receiver SHALL initialize s_l to the RTP
   sequence number (SEQ) of the first observed SRTP packet (unless the
   initial value is provided by out of band signaling such as key
   management).

   On consecutive SRTP packets, the receiver SHOULD estimate the index
   as
         i = 2^16 * v + SEQ,

   where v is chosen from the set { ROC-1, ROC, ROC+1 } (modulo 2^32)
   such that i is closest (in modulo 2^48 sense) to the value 2^16 * ROC
   + s_l (see Appendix A for pseudocode).



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RFC 3711                          SRTP                        March 2004


   After the packet has been processed and authenticated (when enabled
   for SRTP packets for the session), the receiver MUST use v to
   conditionally update its s_l and ROC variables as follows.  If
   v=(ROC-1) mod 2^32, then there is no update to s_l or ROC.  If v=ROC,
   then s_l is set to SEQ if and only if SEQ is larger than the current
   s_l; there is no change to ROC.  If v=(ROC+1) mod 2^32, then s_l is
   set to SEQ and ROC is set to v.

   After a re-keying occurs (changing to a new master key), the rollover
   counter always maintains its sequence of values, i.e., it MUST NOT be
   reset to zero.

   As the rollover counter is 32 bits long and the sequence number is 16
   bits long, the maximum number of packets belonging to a given SRTP
   stream that can be secured with the same key is 2^48 using the pre-
   defined transforms.  After that number of SRTP packets have been sent
   with a given (master or session) key, the sender MUST NOT send any
   more packets with that key.  (There exists a similar limit for SRTCP,
   which in practice may be more restrictive, see Section 9.2.)  This
   limitation enforces a security benefit by providing an upper bound on
   the amount of traffic that can pass before cryptographic keys are
   changed.  Re-keying (see Section 8.1) MUST be triggered, before this
   amount of traffic, and MAY be triggered earlier, e.g., for increased
   security and access control to media.  Recurring key derivation by
   means of a non-zero key_derivation_rate (see Section 4.3), also gives
   stronger security but does not change the above absolute maximum
   value.

   On the receiver side, there is a caveat to updating s_l and ROC: if
   message authentication is not present, neither the initialization of
   s_l, nor the ROC update can be made completely robust.  The
   receiver's "implicit index" approach works for the pre-defined
   transforms as long as the reorder and loss of the packets are not too
   great and bit-errors do not occur in unfortunate ways.  In
   particular, 2^15 packets would need to be lost, or a packet would
   need to be 2^15 packets out of sequence before synchronization is
   lost.  Such drastic loss or reorder is likely to disrupt the RTP
   application itself.

   The algorithm for the index estimate and ROC update is a matter of
   implementation, and should take into consideration the environment
   (e.g., packet loss rate) and the cases when synchronization is likely
   to be lost, e.g., when the initial sequence number (randomly chosen
   by RTP) is not known in advance (not sent in the key management
   protocol) but may be near to wrap modulo 2^16.






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RFC 3711                          SRTP                        March 2004


   A more elaborate and more robust scheme than the one given above is
   the handling of RTP's own "rollover counter", see Appendix A.1 of
   [RFC3550].

3.3.2.  Replay Protection

   Secure replay protection is only possible when integrity protection
   is present.  It is RECOMMENDED to use replay protection, both for RTP
   and RTCP, as integrity protection alone cannot assure security
   against replay attacks.

   A packet is "replayed" when it is stored by an adversary, and then
   re-injected into the network.  When message authentication is
   provided, SRTP protects against such attacks through a Replay List.
   Each SRTP receiver maintains a Replay List, which conceptually
   contains the indices of all of the packets which have been received
   and authenticated.  In practice, the list can use a "sliding window"
   approach, so that a fixed amount of storage suffices for replay
   protection.  Packet indices which lag behind the packet index in the
   context by more than SRTP-WINDOW-SIZE can be assumed to have been
   received, where SRTP-WINDOW-SIZE is a receiver-side, implementation-
   dependent parameter and MUST be at least 64, but which MAY be set to
   a higher value.

   The receiver checks the index of an incoming packet against the
   replay list and the window.  Only packets with index ahead of the
   window, or, inside the window but not already received, SHALL be
   accepted.

   After the packet has been authenticated (if necessary the window is
   first moved ahead), the replay list SHALL be updated with the new
   index.

   The Replay List can be efficiently implemented by using a bitmap to
   represent which packets have been received, as described in the
   Security Architecture for IP [RFC2401].

3.4.  Secure RTCP

   Secure RTCP follows the definition of Secure RTP.  SRTCP adds three
   mandatory new fields (the SRTCP index, an "encrypt-flag", and the
   authentication tag) and one optional field (the MKI) to the RTCP
   packet definition.  The three mandatory fields MUST be appended to an
   RTCP packet in order to form an equivalent SRTCP packet.  The added
   fields follow any other profile-specific extensions.






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RFC 3711                          SRTP                        March 2004


   According to Section 6.1 of [RFC3550], there is a REQUIRED packet
   format for compound packets.  SRTCP MUST be given packets according
   to that requirement in the sense that the first part MUST be a sender
   report or a receiver report.  However, the RTCP encryption prefix (a
   random 32-bit quantity) specified in that Section MUST NOT be used
   since, as is stated there, it is only applicable to the encryption
   method specified in [RFC3550] and is not needed by the cryptographic
   mechanisms used in SRTP.

      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                         | |
   | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+
   | ~                     SRTCP MKI (OPTIONAL)                      ~ |
   | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
   | :                     authentication tag                        : |
   | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
   |                                                                   |
   +-- Encrypted Portion                    Authenticated Portion -----+


   Figure 2.  An example of the format of a Secure RTCP packet,
   consisting of an underlying RTCP compound packet with a Sender Report
   and SDES packet.