This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 3233
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

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.

   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,
   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 replay check and authentication processing
ROC_local is replaced by ROC_sender (retrieved from the packet).

EID 3233 (Verified) is as follows:

Section: 2

Original Text:

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).

Corrected Text:

When the receiver receives an SRTP packet, it processes the packet
according to RFC 3711 except that during replay check and authentication processing
ROC_local is replaced by ROC_sender (retrieved from the packet).
While this is typo, it has the unfortunate side effect of creating a possibility for a replay attack where the attacker injects a previous message, possibly causing the receiver to loose synch on the ROC value. This is prevented if the receiver uses ROC_sender in place of ROC_local during both authentication _and_ replay check.

We thank David McGrew for spotting this error.
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. 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 layer. 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- SHA1. 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]. 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 [RFC3830]. 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. 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 met. 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 sources. 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 perform. 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 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. 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. Authors' Addresses Vesa Lehtovirta Ericsson Research 02420 Jorvas Finland Phone: +358 9 2993314 EMail: Mats Naslund Ericsson Research SE-16480 Stockholm Sweden Phone: +46 8 58533739 EMail: Karl Norrman Ericsson Research SE-16480 Stockholm Sweden Phone: +46 8 4044502 EMail: Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. 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