Movatterモバイル変換

Network Working Group                                      V. LehtovirtaRequest for Comments: 4771                                    M. NaslundCategory: Standards Track                                     K. Norrman                                                                Ericsson                                                            January 2007Integrity Transform Carrying Roll-Over Counterfor 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).Abstract   This document defines an integrity transform for Secure Real-time   Transport Protocol (SRTP; seeRFC 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 Contents1. Introduction ....................................................21.1. Terminology ................................................32. The Transform ...................................................33. Transform Modes .................................................54. Parameter Negotiation ...........................................55. Security Considerations .........................................76. IANA Considerations ............................................107. Acknowledgements ...............................................108. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................10Lehtovirta, et al.          Standards Track                     [Page 1]

RFC 4771          Roll-Over Counter Carrying Transform      January 20071.  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.Lehtovirta, et al.          Standards Track                     [Page 2]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 inRFC3711 (other than specifying master key), and a regularRFC 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 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 toRFC 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,Lehtovirta, et al.          Standards Track                     [Page 3]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 toRFC 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 toRFC 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 toRFC 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.Lehtovirta, et al.          Standards Track                     [Page 4]

RFC 4771          Roll-Over Counter Carrying Transform      January 20073.  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 inSection 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].Lehtovirta, et al.          Standards Track                     [Page 5]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 ofRFC 3830 with the addition of the values found   in Table 1 above.Lehtovirta, et al.          Standards Track                     [Page 6]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 ofRFC 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 ofRFC 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 ofRFC 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   inRFC 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 inSection 9.5 and 9.5.1 ofRFC 3711,   additional requirements of the underlying transport network must be   met.Lehtovirta, et al.          Standards Track                     [Page 7]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 tagLehtovirta, et al.          Standards Track                     [Page 8]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007   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 inRFC3711, 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.Lehtovirta, et al.          Standards Track                     [Page 9]

RFC 4771          Roll-Over Counter Carrying Transform      January 20076.  IANA Considerations   According toSection 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 inSection 4.   The value 13 for ROC transmission rate has been registered in the   SRTP Type namespace as specified in Table 2 inSection 4.   The values 14 to 19 have been registered in the SRTP Type namespace   according to Table 3 inSection 4.7.  Acknowledgements   We would like to thank Nigel Dallard, Lakshminath Dondeti, and David   McGrew for fruitful comments and discussions.8.  References8.1.  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.Lehtovirta, et al.          Standards Track                    [Page 10]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007Authors' Addresses   Vesa Lehtovirta   Ericsson Research   02420 Jorvas   Finland   Phone:  +358 9 2993314   EMail:  vesa.lehtovirta@ericsson.com   Mats Naslund   Ericsson Research   SE-16480 Stockholm   Sweden   Phone:  +46 8 58533739   EMail:  mats.naslund@ericsson.com   Karl Norrman   Ericsson Research   SE-16480 Stockholm   Sweden   Phone:  +46 8 4044502   EMail:  karl.norrman@ericsson.comLehtovirta, et al.          Standards Track                    [Page 11]

RFC 4771          Roll-Over Counter Carrying Transform      January 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Lehtovirta, et al.          Standards Track                    [Page 12]