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RFC 4771

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Integrity Transform Carrying Roll-Over Counter for the Secure Real-time Transport Protocol (SRTP)


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Network Working Group                                      V. Lehtovirta
Request for Comments: 4771                                    M. Naslund
Category: Standards Track                                     K. Norrman
                                                            January 2007

             Integrity Transform Carrying Roll-Over Counter
           for 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 IETF Trust (2007).


   This document defines an integrity transform for Secure Real-time
   Transport Protocol (SRTP; see RFC 3711), 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 synchronize.  The mechanism also
   enhances SRTP operation in cases where there is a risk of losing
   sender-receiver synchronization.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
   2. The Transform ...................................................3
   3. Transform Modes .................................................5
   4. Parameter Negotiation ...........................................5
   5. Security Considerations .........................................7
   6. IANA Considerations ............................................10
   7. Acknowledgements ...............................................10
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10

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1.  Introduction

   When a receiver joins an ongoing SRTP [RFC3711] session, out-of-band
   signaling must provide the receiver with the value of the ROC the
   sender is currently using.  For instance, it can be transferred in
   the Common Header 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 not,
      however, join the service until after a few hours, at which point
      the sender's sequence number (SEQ) has wrapped around, and so the
      sender, meanwhile, has 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, as ROC is used
      as input in session key derivation.  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 signaling.

   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
      have 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 has recently wrapped
      around (say, SEQ = 0x0001).  The sender generates a MIKEY message
      and includes the current value of ROC (say, ROC = 1) in the MIKEY
      message.  The MIKEY message reaches the receiver, who reads the
      ROC value and initializes its local ROC to 1.  Now, if an 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 be out of synchronization.

   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 synchronization.

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   These problems have been recognized in, e.g., 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.

   One possible approach to address the issue could be to carry the ROC
   in the MKI (Master Key Identifier) 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; hence, care must be taken to avoid obvious
   attacks against the synchronization.

   In this document, a solution is presented where the ROC is carried in
   the authentication tag of a special integrity transform in selected
   SRTP packets.

   The benefit of this approach is that the functionality of fast and
   robust synchronization can be achieved as a separate integrity
   transform, using the hooks existing in SRTP.  Furthermore, when the
   ROC is transmitted to the receiver it needs to be integrity protected
   to avoid persistent denial-of-service (DoS) attacks or transmission
   errors that could bring the receiver out of synchronization.  (A DoS
   attack is regarded as persistent if it can last after the attacker
   has left the area; in this particular case, an attacker could modify
   the ROC in one packet and the victim would be out of synchronization
   until the next ROC is transmitted).  The above discussion leads to
   the conclusion that it makes sense to carry the ROC inside the
   authentication tag of an integrity transform.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The Transform

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

   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 MAC in the same way as is done
   when using the default integrity transform (i.e., HMAC-SHA1(auth_key,

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   Authenticated_portion || ROC)).  Next, the sender truncates the MAC
   by 32 bits to generate MAC_tr, i.e., MAC_tr is the tag_length - 32
   most significant bits of the MAC.  Next, the sender constructs the
   tag as TAG = ROC_sender || MAC_tr, where ROC_sender is the value of
   his local ROC, and appends the tag to the packet.  See the security
   considerations section for discussions on the effects of shortening
   the MAC.  In particular, note that a tag-length of 32 bits gives no
   security at all.

   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 except that during authentication processing
   ROC_local is replaced by ROC_sender (retrieved from the packet).
   This works as follows.  In the step where integrity protection is to
   be verified, if the SEQ is equal to 0 modulo R, the receiver extracts
   ROC_sender from the TAG and verifies the MAC computed (in the same
   way as if the default integrity transform was used) over the
   authenticated portion of the packet (as defined in [RFC3711]), but
   concatenated with ROC_sender instead of concatenated with the
   local_ROC.  The receiver generates MAC_tr for the MAC verification in
   the same way the sender did.  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 packet 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, so the receiver will not correct his ROC.

   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 Secure Real-time Transport Control Protocol (SRTCP) already
   carries the entire index in-band, 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.

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3.  Transform Modes

   The above transform only provides integrity protection for the
   packets that carry the ROC (this will be referred to as mode 1).  In
   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
   mode 2).

   Under some circumstances, it may be acceptable not to use integrity
   protection on any of the packets; this will be referred to as mode 3.
   Without integrity protection of the packets carrying the ROC, a DoS
   attack, which will prevail until the next correctly received ROC, is
   possible.  Make sure to carefully read the security considerations in
   Section 5 before using mode 3.

   In case no integrity protection is offered, i.e., mode 3, the
   following applies.  The receiver's SRTP layer SHOULD ignore the ROC
   value from the packet if the application layer can indicate to it
   that the local ROC is synchronized with the sender (hence, the packet
   would be processed using the local ROC).  Note that the received ROC
   still MUST be removed from the packet before continued processing.
   In this scenario, the application layer feedback to the SRTP layer
   need not be on a per-packet basis, and it can consist merely of a
   boolean value set by the application layer and read by the SRTP

   Thus, note the following difference.  Using mode 2 will integrity
   protect all RTP packets, but only add ROC to those having SEQ
   divisible by R.  Using mode 1 and setting R equal to one will also
   integrity protect all packets, but will in addition to that add ROC
   to each packet.  Modes 1 and 2 MUST compute the MAC in the same way
   as the pre-defined authentication transform for SRTP, i.e., HMAC-

   To comply with this specification, mode 1, mode 2, and mode 3 are
   MANDATORY to implement.  However, it is up to local policy to decide
   which mode(s) are allowed to be used.

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 mode and the rate, R, at which the ROC will be
   transmitted.  This can be done using, e.g., MIKEY [RFC3830].

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   To perform the parameter negotiation using MIKEY, three integrity
   transforms have been registered -- RCCm1, RCCm2, and RCCm3 in Table
   6.10.1.c of [RFC3830] -- for the three modes defined.

                  Table 1.  Integrity transforms

                      SRTP auth alg | Value
                      RCCm1         |     2
                      RCCm2         |     3
                      RCCm3         |     4

   Furthermore, the parameter R has been registered in Table 6.10.1.a of

              Table 2.  Integrity transform parameter

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

   The ROC transmission rate, R, is given in network byte order.  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.

   To have the ability to use different integrity transforms for SRTP
   and SRTCP, which is needed in connection to the use of RCC, the
   following additional parameters have been registered in Table
   6.10.1.a of [RFC3830]:

                    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 (types 14 and 15)
   are the same as for the "Authentication algorithm" parameter (type 2)
   in Table 6.10.1.a of RFC 3830 with the addition of the values found
   in Table 1 above.

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   The possible values for session authentication key lengths (types 16
   and 17) are the same as for the "Session Auth. key length" parameter
   (type 3) in Table 6.10.1.a of RFC 3830.

   The possible values for authentication tag lengths (types 18 and 19)
   are the same as for the "Authentication tag length" parameter (type
   11) in Table 6.10.1.a of RFC 3830 with the addition that the length
   of ROC MUST be included in the "Authentication tag length" parameter.
   This means that the minimum tag length when using RCC is 32 bits.

   To avoid ambiguities when introducing these new parameters that have
   overlapping functionality to existing parameters in Table 6.10.1.a of
   RFC 3830, 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 RCCm1, SRTP will use the RCCm1 algorithm, but since
   there is no specific algorithm chosen for SRTCP, the more generally
   specified one (HMAC-SHA-1) is used.

5.  Security Considerations

   An analogous method already exists in SRTCP (the SRTCP index is
   carried in each packet under integrity protection).  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
   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 Rth packet is integrity
   protected in mode 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.

   The use of mode 3 (NULL-MAC) introduces a vulnerability not present
   in RFC 3711; namely, if an attacker modifies the ROC, the
   modification will go undetected by the receiver, and the receiver
   will lose cryptographic synchronization until the next correct ROC is
   received.  This implies that an attacker can perform a DoS attack by
   only modifying every Rth packet.  Because of this, mode 3 MUST only
   be used after proper risk assessment of the underlying network.
   Besides the considerations in Section 9.5 and 9.5.1 of RFC 3711,
   additional requirements of the underlying transport network must be

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   o  The transport network must only consist of trusted domains.  That
      means that everyone on the path from the source to the destination
      is trusted not to modify or inject packets.

   o  The transport network must be protected from packet injection,
      i.e., it must be ensured that the only packets present on the path
      from the source to the destination(s) originate from trusted

   o  If the packets, on their way from the source to the
      destination(s), travel outside of a trusted domain, their
      integrity must be ensured (e.g., by using a Virtual Private
      Network (VPN) connection or a trusted leased line).

   In the (assumed common) case that the last link to the destination(s)
   is a wireless link, the possibility that an attacker injects forged
   packets here must be carefully considered before using mode 3.
   Especially, if used in a broadcast setting, many destinations would
   be affected by the attack.  However, unless R is big, this DoS attack
   would be similar in effect to radio jamming, which would be easier to

   It must also be noted that if the ROC is modified by an attacker and
   no integrity protection is used, the output of the decryption will
   not be useful to the upper layers, and these must be able to cope
   with data that appears random.  In the case integrity protection is
   used on the packets containing the ROC, and the ROC is modified by an
   attacker (and the receiver already has an approximation of the ROC,
   e.g., by getting it previously), the packet will be discarded and the
   receiver will not be able to decrypt correctly.  Note, however, that
   the situation is better in the latter case, since the receiver now
   can try different ROC values in a neighborhood around the approximate
   value he already has.

   As RCC is expected to be used in a broadcast setting where group
   membership will be based on access to a symmetric group key, it is
   important to point out the following.  With symmetric-key-based
   integrity protection, it may be as easy, if not easier, to get access
   to the integrity key (often a combination of a low-cost activity of
   purchasing a subscription and breaking the security of a terminal to
   extract the integrity key) as being able to transmit.

   A word of warning regarding the choice of length of the
   authentication tag:  Note that, in contrast to common MAC tags, there
   is a clear distinction made between the RCC authentication tag and
   the RCC MAC.  The tag is the container holding the MAC (and for some
   packets also the ROC), and the MAC is the output from the MAC-
   algorithm (i.e., HMAC-SHA1).  The length of the authentication tag

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   with the RCC transform includes the four-octet ROC in some packets.
   This means that for a tag-length of n octets, there is only room for
   a MAC of length n - 4, i.e., a tag-length of n octets does not
   provide a full n-octet integrity protection on all packets.  There
   are five cases:

      1. RCCm1 is used and tag-length is n.  For those packets that
         SEQ = 0 mod R, the ROC is carried in the tag and occupies four
         octets.  This leaves n - 4 octets for the MAC.

      2. RCCm1 is used and tag-length is n.  For those packets that
         SEQ != 0 mod R, there is no ROC carried in the tag.  For RCCm1
         there is no MAC on packets not carrying the ROC, so neither the
         length of the MAC nor the length of the tag has any relevance.

      3. RCCm2 is used and tag-length is n.  For those packets that
         SEQ = 0 mod R, the ROC is carried in the tag and occupies four
         octets.  This leaves n - 4 octets for the MAC (this is
         equivalent to case 1).

      4. RCCm2 is used and tag-length is n.  For those packets that
         SEQ != 0 mod R, there is no ROC carried in the tag.  This
         leaves n octets for the MAC.

      5. RCCm3 is used.  RCCm3 does not use any MAC, but the ROC still
         occupies four octets in the tag for packets with SEQ = 0 mod R,
         so the tag-length MUST be set to four.  For packets with
         SEQ != 0 mod R, neither the length of the MAC nor the length of
         the tag has any relevance.

   The conclusion is that in cases 1 and 3, the length of the MAC is
   shorter than the length of the authentication tag.  To achieve the
   same (or less) MAC forgery success probability on all packets when
   using RCCm1 or RCCm2, as with the default integrity transform in RFC
   3711, the tag-length must be set to 14 octets, which means that the
   length of MAC_tr is 10 octets.

   It is recommended to set the tag-length to 14 octets when RCCm1 or
   RCCm2 is used, and the tag-length MUST be set to four octets when
   RCCm3 is used.

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6.  IANA Considerations

   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.

   The value 2 for RCCm1, the value 3 for RCCm2, and the value 4 for
   RCCm3 have been registered in the SRTP auth alg namespace as
   specified in Table 1 in Section 4.

   The value 13 for ROC transmission rate has been registered in the
   SRTP Type namespace as specified in Table 2 in Section 4.

   The values 14 to 19 have been registered in the SRTP Type namespace
   according to Table 3 in Section 4.

7.  Acknowledgements

   We would like to thank Nigel Dallard, Lakshminath Dondeti, and David
   McGrew for fruitful comments and discussions.

8.  References

8.1.  Normative References

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

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

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

8.2.  Informative References

   [MBMS]     3GPP TS 33.246, "3G Security; Security of Multimedia
              Broadcast/ Multicast Service (MBMS)", October 2006.

   [BCMCS]    3GPP2 X.S0022-0, "Broadcast and Multicast Service in
              cdma2000 Wireless IP Network", February 2005.

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Authors' Addresses

   Vesa Lehtovirta
   Ericsson Research
   02420 Jorvas

   Phone:  +358 9 2993314

   Mats Naslund
   Ericsson Research
   SE-16480 Stockholm

   Phone:  +46 8 58533739

   Karl Norrman
   Ericsson Research
   SE-16480 Stockholm

   Phone:  +46 8 4044502

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