Internet Engineering Task Force        Lehtovirta, Naslund, Norrman
   EXPIRES: August 2006                                  February 2006

             Integrity Transform Carrying Roll-over Counter

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   This document defines an integrity transform for SRTP [RFC3711],
   which allows the roll-over counter (ROC) to be transmitted in SRTP
   packets as part of the authentication tag.  The need for sending the
   ROC in SRTP packets arises in situations where the receiver joins an
   ongoing SRTP session, and needs to quickly and robustly get into
   synchronization. The mechanism also enhances SRTP operation in cases
   where there is a risk of loosing sender-receiver synchronization.

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    1. Introduction...................................................2
    2. The transform..................................................3
    3. Transform versions.............................................4
    4. Parameter negotiation..........................................5
    5. Security Considerations........................................6
    6. IANA Considerations............................................7
    8. Author's Addresses.............................................7
    9. References.....................................................8

1. Introduction

   When a user joins an ongoing SRTP session, he must be given, using
   out of band signalling, the value of the ROC the sender is currently
   using.  For instance, it can be transferred in the Security Payload
   of a MIKEY [RFC3830] message.  In some cases the receiver will not
   be able to synchronize his ROC with the one used by the sender even
   if it is signaled to him out of band.  Examples of where
   synchronization failure will appear are:

   1. The receiver receives the ROC in a MIKEY message together with
      a key required for a particular continuous service.  He does,
      however, not join the service until after a few hours, at which
      point the sender's sequence number (SEQ) has wrapped around, and
      the sender hence has meanwhile increased the value of ROC.  When
      the user joins the service he grabs the SEQ from the first seen
      SRTP packet and prepends the ROC to build the index.  If
      integrity protection is used, the packet will be discarded.  If
      there is no integrity protection, the packet may (if key
      derivation rate is non-zero) be decrypted using the wrong session
      key.  In either case, the receiver will not have its ROC
      synchronized with the sender, and it is not possible to recover
      without out-of-band signalling.

   2. If the receiver leaves the session (due to being out of radio
      coverage or because of a user action), and does not start
      receiving traffic from the service again until after 2^{15}
      packets has been sent, the receiver will be out of
      synchronization (for the same reasons as in example 1).

   3. The receiver joins a service when the SEQ is close after
      wraparound, say SEQ = 0x0001.  The sender generates a MIKEY
      message, and includes the current value of ROC, say ROC = 1, in
      the security policy payload. The MIKEY message reaches the
      receiver, who reads the ROC value and initializes its local ROC

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      to 1.  Now, if a SRTP packet prior to wraparound, i.e., with a
      SEQ lower than 0, say SEQ = 0xffff, was delayed and reaches the
      receiver as the first SRTP packet he sees, the receiver will
      initialize its highest received sequence number, s_l, to 0xffff.
      Next the receiver will receive SRTP packets with sequence numbers
      larger than zero, and will deduce that the SEQ has wrapped.
      Hence, the receiver will incorrectly update the ROC and will be
      out of synch.

   4. Similarly to (3), since the initial SEQ is selected at random by
      the sender, it may happen to be selected as a value very close to
      0xffff.  In this case, should the first few packets be lost, the
      receiver may similarly end up out of synch.

   These problems have been recognized in 3GPP2 and 3GPP, where SRTP is
   used for streaming media protection in their respective
   multicast/broadcast solutions [BCMCS][MBMS].  Problem 4 actually
   exists inherently due to the way SEQ initialization is done in RTP.

   To avoid problems, 3GPP2 have chosen to carry the ROC in the MKI
   field of each SRTP packet.  This has the advantage that the receiver
   immediately knows the entire index for a packet.  Unfortunately, the
   MKI has no semantics in RFC 3711 (other than specifying master key),
   and a regular RFC 3711 compliant implementation would not be able to
   make use of the information carried in the MKI.  Furthermore, the
   MKI field is not integrity protected, and hence care must be taken
   to avoid obvious attacks against the synchronization.

   3GPP has agreed on a solution carrying the ROC in selected SRTP
   packets. In this solution the ROC is carried in the authentication
   tag of a special integrity transform.

   The benefit of this approach is that the functionality of fast and
   robust synchronization can be standardized as a separate integrity
   transform in IETF, using the hooks existing in SRTP.  This way,
   there will not be interoperability problems, and other solutions
   than the 3GPP multicast/broadcast multimedia service can make use of
   the functionality as desired.  Furthermore, when the ROC is
   transmitted to the receiver it needs to be integrity protected, to
   avoid DoS attacks or transmission errors bringing the receiver out
   of synch.  Hence, it makes sense to carry the ROC inside the
   authentication tag of an integrity transform.

2. The transform

   The transform, hereafter called Roll-over Counter Carrying Transform
   (or RCC for short), works as follows.

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   The sender processes the RTP packet according to RFC 3711.  When
   applying the message integrity transform, the sender checks if the
   SEQ is equal to 0 modulo some non-zero integer constant R.  If that
   is the case, the sender computes the default integrity transform
   over the authenticated portion of the packet(i.e. packet || ROC
   sender), obtaining the value MAC.  Next the sender constructs the
   tag as TAG = ROC_sender || MAC, where ROC_sender is the value of his
   local ROC, and appends the tag to the packet.

   If the SEQ is not equal to 0 mod R, the sender just proceeds to
   process the packet according to RFC 3711 without performing the
   actions in the previous paragraph.

   The value R is the rate at which the ROC is included in the SRTP
   packets.  Since the ROC consumes four octets, this gives the
   possibility to use it sparsely.

   When the receiver receives an SRTP packet, it processes the packet
   according to RFC 3711. In the step where integrity protection is to
   be verified, if the SEQ is equal to 0 modulo R, the receiver
   verifies the MAC using the default integrity transform, but does not
   include the four octets at the end of the packet containing the
   sender's ROC value.  According to RFC 3711, the receiver shall
   include his own ROC value in the MAC calculation.  In RCC, however,
   the receiver replaces his local ROC value by the value found in the
   packet in the MAC calculation.  Note that the session key used in
   the MAC calculation is dependent on the ROC, and during the
   derivation of the session integrity key, the ROC found in the packet
   under consideration MUST be used.  If the verification is
   successful, the receiver sets his local ROC equal to the ROC carried
   in the packet.  If the MAC does not verify, the packets MUST be
   dropped.  The rationale for using the ROC from the packet in the MAC
   calculation is that if the receiver has an incorrect ROC value, MAC
   verification will fail, and the receiver will not correct his ROC
   because of this.

   If the SEQ is not equal to 0 mod R, the receiver just proceeds to
   process the packet according to RFC 3711 without performing the
   actions in the previous paragraph.

   Since SRTCP already carries the entire index inband, there is no
   reason to apply this transform to SRTCP.  Hence, the transform SHALL
   only be applied to SRTP, and SHALL NOT be used with SRTCP.

3. Transform versions

   The above given transform only provides integrity protection for the
   packets that carry the ROC (this will be referred to version 1).  In

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   the cases where there is a need to integrity protect all the
   packets, the packets that do not have SEQ equal to 0 mod R, MUST be
   protected using the default integrity transform (this will be
   referred to as version 2).

   Thus, note the following difference. Using version 2 will integrity
   protect all RTP packets, but only add ROC to those having SEQ
   divisible by R.  Using version 1 and setting R equal to one, will
   also integrity protect all packets, but will in addition add ROC to
   each packet.

4. Parameter negotiation

   RCC requires that a few parameters are signaled out of band.  The
   parameters that must be in place before the transform can be used
   are integrity transform version and the rate, R, at which the ROC
   will be transmitted.  This can be done using, e.g., MIKEY [RFC3830].

   To perform the parameter negotiation using MIKEY, there is a need to
   register two integrity transforms, RCCv1 and RCCv2 in Table 6.10.1.c
   of [RFC3830].

                       Table 1. Integrity transforms

                           SRTP auth alg | Value
                           RCCv1         |     2
                           RCCv2         |     3

   Furthermore, the parameter R, must be registered in Table 6.10.1.a
   of [RFC3830].

                   Table 2. Integrity transform parameter

             Type | Meaning                     | Possible values
              13  | ROC transmission rate       |  16-bit integer

   The ROC transmission rate, R, is given with the leftmost bit being
   the most significant.  R MUST be a non-zero unsigned integer.  If
   the ROC transmission rate is not included in the negotiation, the
   default value of 1 SHALL be used.

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   To be able to use different integrity transforms for SRTP and SRTCP,
   which is needed in connection to the use of RCC, the following
   additional parameters must be registered in Table 6.10.1.a of

                       Table 3. Integrity parameters

           Type | Meaning                     | Possible values
            14  | SRTP Auth. algorithm        | see below
            15  | SRTCP Auth. algorithm       | see below
            16  | SRTP Session Auth. key len  | see below
            17  | SRTCP Session Auth. key len | see below
            18  | SRTP Authentication tag len | see below
            19  | SRTCP Authentication tag len| see below

   The possible values for authentication algorithms (type 14 and 15)
   are the same as for the "Authentication algorithm" parameter (type
   2) in Table 6.10.1.a of RFC3830 with the addition of the values
   found in Table 1 above.

   The possible values for session authentication key lengths (type 16
   and 17) are the same as for the "Session Auth. key length" parameter
   (type 3) in Table 6.10.1.a of RFC3830.

   The possible values for authentication tag lengths (type 18 and 19)
   are the same as for the "Authentication tag length" parameter (type
   11) in Table 6.10.1.a of RFC3830.

   To avoid ambiguities when introducing these new parameters that have
   overlapping functionality to existing parameters in Table 6.10.1.a
   of RFC3830, the following approach MUST be taken: If any of the
   parameter types 14-19 (specifying behavior specific to SRTP or
   SRTCP) and a corresponding general parameter (type 2, 3, or 11) are
   both present in the policy, the more specific parameter SHALL have
   precedence. For example, if the "Authentication algorithm" parameter
   (type 2) is set to HMAC-SHA-1 and the "SRTP Auth. Algorithm" (type
   14) is set to RCCV1, SRTP will use the RCCV1 algorithm, but since
   there is no specific algorithm chosen for SRTCP, the more generally
   specified one (HMAC-SHA-1) is used for SRTCP.

5. Security Considerations

   An analogous method already exists in SRTCP (the SRTCP index is
   carried in each packet under integrity protection) and to the best
   of our knowledge, the only security consideration introduced here is
   that the entire SRTP index (ROC || SEQ) will become public since it

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   is transferred without encryption. (In normal SRTP operation, only
   the SEQ-part of the index is disclosed). However, RFC 3711 does not
   identify a need for encrypting the SRTP index.

   It is important to realize that only every R:th packet is integrity
   protected in version 1, so unless R = 1, the mechanism should be
   seen for what it is: a way to improve sender-receiver
   synchronization, and not a replacement for integrity protection.

6. IANA Considerations

   Please add the following to the IANA registry at (This paragraph to be
   removed after IANA processing).

   According to Section 10 of RFC 3830, IETF consensus is required to
   register values in the range 0-240 in the SRTP auth alg namespace
   and the SRTP Type namespace.

   It is requested to register the value 2 for RCCv1 and the value 3
   for RCCv2 in the SRTP auth alg namespace as specified in Table 1 in
   Section 4.

   It is also requested to register the value 13 for ROC transmission
   rate in the SRTP Type namespace as specified in Table 2 in Section

   It is also requested to register the values 14 to 19 according to
   Table 3 in Section 4 to the SRTP Type namespace.

8. Author's Addresses

   Questions and comments should be directed to the authors:

      Vesa Lehtovirta
      Ericsson Research
      02420 Jorvas           Phone:  +358 9 2993314
      Finland                EMail:

      Mats Naslund
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 58533739
      Sweden                 EMail:

      Karl Norrman
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 4044502
      Sweden                 EMail:

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9. References


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

   [RFC3711] Baugher et al., "The Secure Real-time Transport Protocol
   (SRTP)", RFC3711, March 2004.


   [MBMS] 3GPP TS 33.246, "Technical Specification 3rd Generation
   Partnership Project; Technical Specification Group Services and
   System Aspects; Security; Security of Multimedia Broadcast/Multicast

   [BCMCS] 3GPP2 X.S0022-0, "Broadcast and Multicast Service in
   cdma2000 Wireless IP network"

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