Internet Engineering Task Force Lehtovirta, Naslund, Norrman (Ericsson) INTERNET-DRAFT EXPIRES: February 2007 September 2006 Integrity Transform Carrying Roll-over Counter Status of this memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire in February 2007. Abstract 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 synchronize. The mechanism also enhances SRTP operation in cases where there is a risk of loosing sender-receiver synchronization. INTERNET-DRAFT srtp-rcc September, 2006 TABLE OF CONTENTS 1. Introduction...................................................2 2. The transform..................................................4 3. Transform modes................................................5 4. Parameter negotiation..........................................6 5. Security Considerations........................................7 6. IANA Considerations...........................................10 7. Acknowledgements..............................................10 8. Author's Addresses............................................10 9. References....................................................11 1. Introduction When a receiver joins an ongoing SRTP session, out of band signalling must provide the receiver with the value of the ROC the sender is currently using. For instance, it can be transferred in the Security Policy 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 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 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 Lehtovirta et al. [Page 2] INTERNET-DRAFT srtp-rcc September, 2006 the security policy payload. The MIKEY message reaches the receiver, who reads the ROC value and initializes its local ROC 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 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 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. 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 DoS attacks or transmission errors bringing the receiver out of synchronization. (A DoS attack is regarded as persistent if it can last after the attacker has left the area, e.g., 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 that it makes sense to carry the ROC inside the authentication tag of an integrity transform. Lehtovirta et al. [Page 3] INTERNET-DRAFT srtp-rcc September, 2006 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 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 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 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. 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, and the receiver will not correct his ROC because of this. Lehtovirta et al. [Page 4] INTERNET-DRAFT srtp-rcc September, 2006 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 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 given transform only provides integrity protection for the packets that carry the ROC (this will be referred to 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 to not 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, that will prevail until the next correctly received ROC, is possible. It should be made 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 (the packet would hence 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 add ROC to each packet. 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. Lehtovirta et al. [Page 5] INTERNET-DRAFT srtp-rcc September, 2006 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, there is a need to register three integrity transforms, 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, 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. 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 [RFC3830]: Lehtovirta et al. [Page 6] INTERNET-DRAFT srtp-rcc September, 2006 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 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 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. 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 is transferred without encryption. (In normal SRTP operation, only Lehtovirta et al. [Page 7] INTERNET-DRAFT srtp-rcc September, 2006 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, NULL-MAC 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. . 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. . 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) originates from trusted sources. . If the packets, on their way from the source to the destination(s), travel outside of a trusted domain, their integrity must be assured (e.g., by using a 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 NULL-MAC. 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 the randomly looking data. 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 later case, since the receiver Lehtovirta et al. [Page 8] INTERNET-DRAFT srtp-rcc September, 2006 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 is in place when it comes to the choice of length of the authentication tag. It shall be noted 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 Lehtovirta et al. [Page 9] INTERNET-DRAFT srtp-rcc September, 2006 same (or less) MAC forgery success probability on all packets when using RCCm1 or RCCm2, as with the default integrity transform in RFC3711, the tag-length must be set to 14 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 Please add the following to the IANA registry at http://www.iana.org/assignments/mikey-payloads (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,the value 3 for RCCv2 and the value 4 for RCCv3 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 4. It is also requested to register the values 14 to 19 according to Table 3 in Section 4 to the SRTP Type namespace. 7. Acknowledgements We would like to thank Nigel Dallard, Lakshminath Dondeti and David McGrew for fruitful comments and discussions. 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: vesa.lehtovirta@ericsson.com Lehtovirta et al. [Page 10] INTERNET-DRAFT srtp-rcc September, 2006 Mats Naslund Ericsson Research SE-16480 Stockholm Phone: +46 8 58533739 Sweden EMail: mats.naslund@ericsson.com Karl Norrman Ericsson Research SE-16480 Stockholm Phone: +46 8 4044502 Sweden EMail: karl.norrman@ericsson.com 9. References Normative [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. [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", BCP 14, RFC2119, March 1997. Informative [MBMS] 3GPP TS 33.246, "Technical Specification 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Security; Security of Multimedia Broadcast/Multicast Service." [BCMCS] 3GPP2 X.S0022-0, "Broadcast and Multicast Service in cdma2000 Wireless IP network" 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 in BCP 78 and BCP 79. Lehtovirta et al. [Page 11] INTERNET-DRAFT srtp-rcc September, 2006 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 at http://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. Copyright Notice Copyright (C) The Internet Society (2006). 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. Disclaimer of Validity 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 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. Lehtovirta et al. [Page 12]