HMAC-authenticated Diffie-Hellman for MIKEY May 2004 Internet Engineering Task Force - MSEC WG Internet Draft M. Euchner Intended Category: Proposed Standard Expires: December 2004 May 2004 HMAC-authenticated Diffie-Hellman for MIKEY Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. The distribution of this memo is unlimited. Comments should be sent to the MSEC WG mailing list at msec@securemulticast.org and to the author. Abstract This document describes a light-weight point-to-point key management protocol variant for the multimedia Internet keying (MIKEY). In particular, the classic Diffie-Hellman key agreement protocol is used for key establishment in conjunction with a keyed hash (HMAC-SHA1) Euchner Expires =96 December 2004 [Page 1] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 for achieving mutual authentication and message integrity of the key management messages exchanged. This MIKEY variant is called the HMAC-authenticated Diffie-Hellmann (DHHMAC). It addresses the security and performance constraints of multimedia key management in MIKEY. Conventions used in this document 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 [2]. Table of Contents 1. Introduction................................................3 1.1. Definitions.................................................5 1.2. Abbreviations...............................................6 2. Scenario....................................................7 2.1. Applicability...............................................8 3. DHHMAC Security Protocol...................................10 3.1. TGK re-keying..............................................12 4. DHHMAC payload formats.....................................12 4.1. Common header payload (HDR)................................13 4.2. Key data transport payload (KEMAC).........................14 4.3. ID payload (ID)............................................15 5. Security Considerations....................................15 5.1. Security environment.......................................15 5.2. Threat model...............................................15 5.3. Security features and properties...........................18 5.4. Assumptions................................................22 5.5. Residual risk..............................................23 IANA considerations.............................................25 Intellectual Property Rights....................................25 References......................................................26 Normative References............................................26 Informative References..........................................26 Acknowledgments.................................................28 Conclusions.....................................................28 Full Copyright Statement........................................29 Expiration Date.................................................29 Revision History................................................30 Euchner Expires =96 December 2004 [Page 2] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 Author's Addresses..............................................31 1. Introduction There is work done in IETF to develop key management schemes. For example, IKE [14] is a widely accepted unicast scheme for IPsec, and the MSEC WG is developing other schemes, addressed to group communication [24], [25]. For reasons discussed below, there is however a need for a scheme with low latency, suitable for demanding cases such as real-time data over heterogeneous networks, and small interactive groups. As pointed out in MIKEY (see [3]), secure real-time multimedia applications demand a particular adequate light-weight key management scheme that cares for how to securely and efficiently establish dynamic session keys in a conversational multimedia scenario. In general, MIKEY scenarios cover peer-to-peer, simple-one-to-many and small-sized groups. MIKEY in particular, describes three key management schemes for the peer-to-peer case that all finish their task within one round trip: - a symmetric key distribution protocol (MIKEY-PS) based upon pre-shared master keys; - a public-key encryption-based key distribution protocol (MIKEY-PK) assuming a public-key infrastructure with RSA-based (Rivest, Shamir and Adleman) private/public keys and digital certificates; - and a Diffie-Hellman key agreement protocol (MIKEY-DHSIGN) deploying digital signatures and certificates. All these three key management protocols are designed such that they complete their work within just one round trip. This requires depending on loosely synchronized clocks and deploying timestamps within the key management protocols. However, it is known [7] that each of the three key management schemes has its subtle constraints and limitations: - The symmetric key distribution protocol (MIKEY-PS) is simple to Euchner Expires =96 December 2004 [Page 3] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 implement but does not nicely scale in any larger configuration of potential peer entities due to the need of mutually pre- assigned shared master secrets. Moreover, the security provided does not achieve the property of perfect forward secrecy; i.e. compromise of the shared master secret would render past and even future session keys susceptible to compromise. Further, the generation of the session key happens just at the initiator. Thus, the responder has to fully trust the initiator on choosing a good and secure session secret; the responder neither is able to participate in the key generation nor to influence that process. This is considered as a specific limitation in less trusted environments. - The public-key encryption scheme (MIKEY-PK) depends upon a public-key infrastructure that certifies the private-public keys by issuing and maintaining digital certificates. While such a key management scheme provides full scalability in large networked configurations, public-key infrastructures are still not widely available and in general, implementations are significantly more complex. Further, additional round trips might be necessary for each side in order to ascertain verification of the digital certificates. Finally, as in the symmetric case, the responder depends completely upon the initiator choosing good and secure session keys. - The third MIKEY-DHSIGN key management protocol deploys the Diffie-Hellman key agreement scheme and authenticates the exchange of the Diffie-Hellman half-keys in each direction by using a digital signature upon. As in the previous method, this introduces the dependency upon a public-key infrastructure with its strength on scalability but also the limitations on computational costs in performing the asymmetric long-integer operations and the potential need for additional communication for verification of the digital certificates. Euchner Expires =96 December 2004 [Page 4] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 However, the Diffie-Hellman key agreement protocol is known for its subtle security strengths in that it is able to provide full perfect forward secrecy (PFS) and further have both parties actively involved in session key generation. This special security property - despite the somewhat higher computational costs - makes Diffie-Hellman techniques attractive in practice. In order to overcome some of the limitations as outlined above, a special need has been recognized for another efficient key agreement protocol variant in MIKEY. This protocol variant aims to provide the capability of perfect forward secrecy as part of a key agreement with low latency without dependency on a public-key infrastructure. This document describes such a fourth light-weight key management scheme for MIKEY that could somehow be seen as a synergetic optimization between the pre-shared key distribution scheme and the Diffie-Hellman key agreement. The idea of that protocol is to apply the Diffie-Hellman key agreement but instead of deploying a digital signature for authenticity of the exchanged keying material rather uses a keyed- hash upon using symmetrically pre-assigned shared secrets. This combination of security mechanisms is called the HMAC-authenticated Diffie-Hellman (DH) key agreement for MIKEY (DHHMAC). The DHHMAC variant closely follows the design and philosophy of MIKEY and reuses MIKEY protocol payload components and MIKEY mechanisms to its maximum benefit and for best compatibility. Like the MIKEY Diffie-Hellman protocol, DHHMAC does not scale beyond a point-to-point constellation; thus, both MIKEY Diffie-Hellman protocols do not support group-based keying for any group size larger than two entities. 1.1. Definitions The definitions and notations in this document are aligned with MIKEY, see [3] and [3] sections 1.3 - 1.4. Euchner Expires =96 December 2004 [Page 5] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 All large integer computations in this document should be understood as being mod p within some fixed group G for some large prime p; see [3] section 3.3; however, the DHHMAC protocol is applicable in general to other appropriate finite, cyclical groups as well. It is assumed that a pre-shared key s is known by both entities (initiator and responder). The authentication key auth_key is derived from the pre-shared secret s using the pseudo-random function PRF; see [3] sections 4.1.3 and 4.1.5. In this text, [X] represents an optional piece of information. Generally throughout the text, X SHOULD be present unless certain circumstance MAY allow X being optional and not be present thereby resulting in weaker security potentially. Likewise [X, Y] represents an optional compound piece of information where the pieces X and Y SHOULD be either both present or MAY optionally be both absent. 1.2. Abbreviations auth_key pre-shared authentication key, PRF-derived from pre-shared key s. DH Diffie-Hellman DHi public Diffie-Hellman half key g^(xi) of Initiatior DHr public Diffie-Hellman half key g^(xr) of Responder DHHMAC HMAC-authenticated Diffie-Hellman DoS Denial-of-service G Diffie-Hellman group HDR MIKEY common header payload HMAC keyed Hash Message Authentication Code HMAC-SHA1 HMAC using SHA1 as hash function (160-bit result) IDi Identity of initiator IDr Identity of receiver IKE Internet Key Exchange IPSec Internet Protocol Security MIKEY Multimedia Internet KEYing MIKEY-DHHMAC MIKEY Diffie-Hellman key management protocol using HMAC MIKEY-DHSIGN MIKEY Diffie-Hellman key agreement protocol MIKEY-PK MIKEY public-key encryption-based key distribution protocol MIKEY-PS MIKEY pre-shared key distribution protocol p Diffie-Hellman prime modulus Euchner Expires =96 December 2004 [Page 6] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 PRF MIKEY pseudo-random function (see [3] section 4.1.3.) RSA Rivest, Shamir and Adleman s pre-shared key SDP Session Description Protocol SOI Son-of-IKE, IKEv2 SP MIKEY Security Policy (Parameter) Payload T timestamp TEK Traffic Encryption Key TGK MIKEY TEK Generation Key as the common Diffie- Hellman shared secret TLS Transport Layer Security xi secret, (pseudo) random Diffie-Hellman key of the Initiator xr secret, (pseudo) random Diffie-Hellman key of the Responder 2. Scenario The HMAC-authenticated Diffie-Hellman key agreement protocol (DHHMAC) for MIKEY addresses the same scenarios and scope as the other three key management schemes in MIKEY address. DHHMAC is applicable in a peer-to-peer group where no access to a public-key infrastructure can be assumed available. Rather, pre- shared master secrets are assumed available among the entities in such an environment. In a pair-wise group, it is assumed that each client will be setting up a session key for its outgoing links with it's peer using the DH- MAC key agreement protocol. As is the case for the other three MIKEY key management protocol, DHHMAC assumes loosely synchronized clocks among the entities in the small group. Note: To synchronize the clocks in a secure manner, some operational or procedural means are recommended. However, MIKEY-DHHMAC does not describe any secure time synchronization measures and leaves such tasks to the discretion of the implementation. Euchner Expires =96 December 2004 [Page 7] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 2.1. Applicability MIKEY-DHHMAC as well as the other MIKEY key management protocols are optimized and targeted for the purpose of multimedia applications with application-level key management needs under real-time session setup and session management constraints. As the MIKEY-DHHMAC key management protocol terminates in one roundtrip, DHHMAC is applicable for integration into two-way handshake session- or call signaling protocols such as a) SIP/SDP (see [5]) where the encoded MIKEY messages are encapsulated and transported in SDP containers of the SDP offer/answer handshake, b) H.323 (see [22]) where the encoded MIKEY messages are transported in the H.225.0 fast start call signaling handshake. MIKEY-DHHMAC is offered as option to the other MIKEY key management variants (MIKEY-pre-shared, MIKEY-public-key and MIKEY-DH-SIGN) for all those cases where DHHMAC has its peculiar strengths (see section 5). 2.1.1. Usage in H.235 This section provides informative overview how MIKEY-DHHMAC can be applied in some H.323-based multimedia environments. Generally, MIKEY is applicable for multimedia applications including IP telephony. [22] describes various use cases of the MIKEY key management protocols (MIKEY-PS, MIKEY-PK, MIKEY-DHSIGN and MIKEY- DHHMAC) with the purpose to establish TGK keying material among H.323 endpoints. The TGKs are then used for media encryption by applying SRTP [27]. Addressed scenarios include point-to-point with one or more intermediate gatekeepers (trusted or partially trusted) in- between. One particular use case addresses MIKEY-DHHMAC to establish a media connection from an endpoint B calling (through a gatekeeper) to another endpoint A that is located within that same gatekeeper zone. While EP-A and EP-B typically do not share any auth_key a priori, some separate protocol exchange means are achieved outside the actual call setup procedure to establish an auth_key for the time while Euchner Expires =96 December 2004 [Page 8] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 endpoints are being registered with the gatekeeper; such protocols exist [22] but are not shown in this document. The auth_key between the endpoints is being used to authenticate and integrity protect the MIKEY-DHHMAC messages. To establish a call, it is assumed that endpoint B has obtained permission from the gatekeeper (not shown). Endpoint B as the caller builds the MIKEY-DHHMAC I_message(see section 3) and sends the I_message encapsulated within the H.323-SETUP to endpoint A. A routing gatekeeper (GK) would forward this message to endpoint B; in case of a non-routing gatekeeper, endpoint B sends the SETUP directly to endpoint A. In either case, H.323 inherent security mechanisms [28] are applied to protect the (encapsulation) message during transfer. This is not depicted here. The receiving endpoint A is able to verify the conveyed I_message and can compute a TGK. Assuming that endpoint A would accept the call, EP-A then builds the MIKEY-DHHMAC R_message and sends the response as part of the CallProceeding-to-Connect message back to the calling endpoint B (possibly through a routing gatekeeper). Endpoint B processes the conveyed R_message to compute the same TGK as the called endpoint A. 1.) EP-B -> (GK) -> EP-A: SETUP(I_fwd_message [, I_rev_message]) 2.) EP-A -> (GK) -> EP-B: CallProceeding-to-CONNECT(R_fwd_message [, R_rev_message]) Notes: If it is necessary to establish directional TGKs for full- duplex links in both directions B->A and A->B, then the calling endpoint B instantiates the DHHMAC protocol twice: once in the direction B->A using I_fwd_message and another run in parallel in the direction A->B using I_rev_message. In that case, two MIKEY-DHHMAC I_messages are encapsulated within SETUP (I_fwd_message and I_rev_message) and two MIKEY-DHHMAC R_messages (R_fwd_message and R_rev_message) are encapsulted within CallProceeding-to-CONNECT. The I_rev_message corresponds with the I_fwd_message. Alternatively, the called endpoint A may instantiate the DHHMAC protocol in a separate run with endpoint B (not shown); however, this requires a third handshake to complete. For more details on how the MIKEY protocols may be deployed with H.235, please refer to [22]. Euchner Expires =96 December 2004 [Page 9] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 2.2. Relation to GKMARCH The Group key management architecture (GKMARCH) [26] describes a generic architecture for multicast security group key management protocols. In the context of this architecture, MIKEY-DHHMAC may operate as a registration protocol. The main entities involved in the architecture are a group controller/key server (GCKS), the receiver(s), and the sender(s). Due to the pair-wise nature of the Diffie-Hellman operation and the 1-roundtrip constraint, usage of MIKEY-DHHMAC rules out any deployment as a group key management protocol with more than two group entities. Only the degenerate case with two peers is possible where for example the responder acts as the group controller. Note that MIKEY does not provide re-keying in the GKMARCH sense, only updating of the keys by normal unicast messages. 3. DHHMAC Security Protocol The following figure defines the security protocol for DHHMAC: Initiator Responder I_message =3D HDR, T, RAND, [IDi], IDr, {SP}, DHi, KEMAC I_message -----------------------> R_message =3D HDR, T, [IDr], IDi, DHr, DHi, KEMAC R_message <---------------------- TGK =3D g^(xi * yi) TGK =3D g^(xi * yi) Figure 1: HMAC-authenticated Diffie-Hellman key based exchange, where xi and xr are (pseudo) randomly chosen respectively by the initiator and the responder. Euchner Expires =96 December 2004 [Page 10] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 The DHHMAC key exchange SHALL be done according to Figure 1. The initiator chooses a (pseudo) random value xi, and sends an HMACed message including g^(xi) and a timestamp to the responder. It is recommended that the initiator SHOULD always include the identity payloads IDi and IDr within the I_message; unless the receiver can defer the initiator's identity by some other means, then IDi MAY optionally be left out. The initiator SHALL always include the recipient=92s identity. The group parameters (e.g., the group G) are a set of parameters chosen by the initiator. The responder chooses a (pseudo) random positive integer xr, and sends an HMACed message including g^(xr) and the timestamp to the initiator. The responder SHALL always include the initiator's identity IDi regardless of whether the I_message conveyed any IDi. It is recommended that the responder SHOULD always include the identity payload IDr within the R_message; unless the initiator can defer the reponder's identity by some other means, then IDr MAY optionally be left out. Both parties then calculate the TGK, g^(xi * xr). The HMAC authentication is due to provide authentication of the DH half-keys, and is necessary to avoid man-in-the-middle attacks. This approach is less expensive than digitally signed Diffie- Hellman. It requires first of all, that both sides compute one exponentiation and one HMAC, then one HMAC verification and finally another Diffie-Hellman exponentiation. With off-line pre-computation, the initial Diffie-Hellman half-key MAY be computed before the key management transaction and thereby MAY further reduce the overall round trip delay as well as reduce the risk of denial-of-service attacks. Processing of the TGK SHALL be accomplished as described in MIKEY [3] chapter 4. The computed HMAC result SHALL be conveyed in the KEMAC payload field where the MAC fields holds the HMAC result. The HMAC shall be computed over the entire message excluding the MAC field using auth_key, see also section 4.2. Euchner Expires =96 December 2004 [Page 11] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 3.1. TGK re-keying TGK re-keying for DHHMAC generally proceeds as described in [3] section 4.5. Specifically, figure 2 provides the message fields for DHHMAC update message. Initiator Responder I_message =3D HDR, T, [IDi], IDr, {SP}, [DHi], KEMAC I_message -----------------------> R_message =3D HDR, T, [IDr], IDi, [DHr, DHi], KEMAC R_message <---------------------- [TGK =3D g^(xi * yi)] [TGK =3D g^(xi * yi)] Figure 2: DHHMAC update message TGK re-keying supports two procedures: a) True re-keying by exchanging new and fresh Diffie-Hellman half- keys. For this, the initiator SHALL provide a new, fresh DHi and the responder SHALL respond with a new, fresh DHr and the received DHi. b) Non-key related information update without any Diffie-Hellman half-keys included in the exchange. Such transaction does not change the actual TGK but updates other information like security policy parameters for example. To only update the non-key related information, [DHi] and [DHr, DHi] SHALL be left out. 4. DHHMAC payload formats Euchner Expires =96 December 2004 [Page 12] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 This section specifies the payload formats and data type values for DHHMAC, see also [3] chapter 6 for a definition of the MIKEY payloads. The following referenced MIKEY payloads are used for DH-MAC: * Common header payload (HDR), see section 4.1 and [3] section 6.1 * SRTP ID sub-payload, see [3] section 6.1.1, * Key data transport payload (KEMAC), see section 4.2 and [3] section 6.2 * DH data payload, see [3] section 6.4 * Timestamp payload, [3] section 6.6 * ID payload, [3] section 6.7 * Security Policy payload (SP), [3] section 6.10 * RAND payload (RAND), [3] section 6.11 * Error payload (ERR), [3] section 6.12 * General Extension Payload, [3] section 6.15 4.1. Common header payload (HDR) Referring to [3] section 6.1, for DHHMAC the following data types SHALL be used: Data type | Value | Comment ------------------------------------------------------------- DHHMAC init | 7 | Initiator's DHHMAC exchange message DHHMAC resp | 8 | Responder's DHHMAC exchange message Error | 6 | Error message, see [3] section 6.12 Euchner Expires =96 December 2004 [Page 13] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 The next payload field shall be one of the following values: Next payload| Value | Section ---------------------------------------------------------------- Last payload| 0 | - KEMAC | 1 | section 4.2 and [3] section 6.2 DH | 3 | [3] section 6.4 T | 5 | [3] section 6.6 ID | 6 | [3] section 6.7 SP | 10 | [3] section 6.10 RAND | 11 | [3] section 6.11 ERR | 12 | [3] section 6.12 General Ext.| 21 | [3] section 6.15 Other defined next payload values defined in [3] SHALL not be applied to DHHMAC. The responder in case of a decoding error or of a failed HMAC authentication verification SHALL apply the Error payload data type. 4.2. Key data transport payload (KEMAC) DHHMAC SHALL apply this payload for conveying the HMAC result along with the indicated authentication algorithm. KEMAC when used in conjunction with DHHMAC SHALL not convey any encrypted data; thus Encr alg SHALL be set to 2 (NULL), Encr data len shall be set to 0 and Encr data SHALL be left empty. The AES key wrap method (see [23]) SHALL not be applied for DHHMAC. For DHHMAC, this key data transport payload SHALL be the last payload in the message. Note that the Next payload field SHALL be set to Last payload. The HMAC is then calculated over the entire MIKEY message excluding the MAC field using auth_key as described in [3] section 5.2 and then stored within MAC field. MAC alg | Value | Comments ------------------------------------------------------------------ HMAC-SHA-1 | 0 | Mandatory, Default (see [4]) NULL | 1 | Very restricted use, see | [3] section 4.2.4 Euchner Expires =96 December 2004 [Page 14] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 HMAC-SHA-1 is the default hash function that MUST be implemented as part of the DHHMAC. The length of the HMAC-SHA-1 result is 160 bits. 4.3. ID payload (ID) For DHHMAC, this payload SHALL only hold a non-certificate based identity. 5. Security Considerations This document addresses key management security issues throughout. For a comprehensive explanation of MIKEY security considerations, please refer to MIKEY [3] section 9. In addition to that, this document addresses security issues according to [8] where the following security considerations apply in particular to this document: 5.1. Security environment Generally, the DHHMAC security protocol described in this document focuses primarily on communication security; i.e. the security issues concerned with the MIKEY DHHMAC protocol. Nevertheless, some system security issues are of interest as well that are not explicitly defined by the DHHMAC protocol, but should be provided locally in practice. The system where the DHHMAC protocol entity runs upon shall provide the capability to generate (pseudo) random numbers as input to the Diffie-Hellman operation (see [9], [15]). Further, the system shall be capable of storing the generated (pseudo) random data, secret data, keys and other secret security parameters securely (i.e. confidential and safe from unauthorized tampering). 5.2. Threat model The threat model that this document adheres to cover the issues of end-to-end security in the Internet generally; without ruling out the Euchner Expires =96 December 2004 [Page 15] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 possibility that MIKEY DHHMAC be deployed in a corporate, closed IP environment. This also includes the possibility that MIKEY DHHMAC be deployed on a hop-by-hop basis with some intermediate trusted "MIKEY DHHMAC proxies" involved. Since DHHMAC is a key management protocol, the following security threats are of concern: * Unauthorized interception of plain TGKs. This threat shall not occur. Nevertheless, for DHHMAC this threat does not occur since the TGK is not actually transmitted on the wire (not even in encrypted fashion). * Eavesdropping of other, transmitted keying information: DHHMAC protocol does not explicitly transmit the TGK at all. Rather, by the Diffie-Hellman "encryption" operation, that conceals the secret (pseudo) random values, only partial information (i.e. the DH- half key) for construction of the TGK is transmitted. It is fundamentally assumed that availability of such Diffie-Hellman half-keys to an eavesdropper does not result in any substantial security risk; see 5.4. Further, the DHHMAC carries other data such as timestamps, (pseudo) random values, identification information or security policy parameters; eavesdropping of any such data is considered not to yield any significant security risk. * Masquerade of either entity: This security threat must be avoided and if a masquerade attack would be attempted, appropriate detection means must be in place. DHHMAC addresses this threat by providing mutual peer entity authentication. * Man-in-the-middle attacks: Such attacks threaten the security of exchanged, non-authenticated messages. Man-in-the-middle attacks usually come with masquerade and or loss of message integrity (see below). Man-in-the-middle attacks must be avoided, and if present or attempted must be detected appropriately. DHHMAC addresses this threat by providing mutual peer entity authentication and message integrity. * Loss of integrity: This security threat relates to unauthorized replay, deletion, insertion and manipulation of messages. While any such attacks Euchner Expires =96 December 2004 [Page 16] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 cannot be avoided they must be detected at least. DHHMAC addresses this threat by providing message integrity. * Bidding-down attacks: When multiple key management protocols each of a distinct security level are offered (e.g., such as is possible by SDP [5]), avoiding bidding-down attacks is of concern. DHHMAC addresses this threat by reusing the MIKEY mechanism as described in [3] section 7.1, where all key management protocol identifiers must be listed within the MIKEY General Extension Payload. The protocol identifier for DHHMAC shall be "mikeydhhmac". The General Extension Payload must be integrity-protected with the HMAC using the shared secret. Some potential threats are not within the scope of this threat model: * Passive and off-line cryptanalysis of the Diffie-Hellman algorithm: Under certain reasonable assumptions (see 5.4 below) it is widely believed that DHHMAC is sufficiently secure and that such attacks be infeasible although the possibility of a successful attack cannot be ruled out completely. * Non-repudiation of the receipt or of the origin of the message: These are not requirements of this environment and thus related countermeasures are not provided at all. * Denial-of-service or distributed denial-of-service attacks: Some considerations are given on some of those attacks, but DHHMAC does not claim to provide full countermeasure against any of those attacks. For example, stressing the availability of the entities are not thwarted by means of the key management protocol; some other local countermeasures should be applied. Further, some DoS attacks are not countered such as interception of a valid DH- request and its massive instant duplication. Such attacks might at least be countered partially by some local means that are outside the scope of this document. * Identity protection: Like MIKEY, identity protection is not a major design requirement for MIKEY-DHHMAC either, see [3]. No security protocol is known so far, that is able to provide the objectives of DHHMAC as stated in section 5.3 including identity protection within just a single roundtrip. MIKEY-DHHMAC trades identity protection for better Euchner Expires =96 December 2004 [Page 17] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 security for the keying material and shorter roundtrip time. Thus, MIKEY-DHHMAC does not provide identity protection on its own but may inherit such property from a security protocol underneath that actually features identity protection. On the other hand, it is expected that MIKEY-DHHMAC is typically being deployed within SDP/SIP ([20], [5]); both those protocols do not provide end-to-end identity protection either. The DHHMAC security protocol (see section 3) and the TGK re-keying security protocol (see section 3.1) provide the option not to supply identity information. This option is only applicable if some other means are available of supplying trustworthy identity information; e.g., by relying on secured links underneath of MIKEY that supply trustworthy identity information otherwise. However, it is understood that without identity information present, the MIKEY key management security protocols might be subject to security weaknesses such as masquerade, impersonation and reflection attacks particularly in end-to-end scenarios where no other secure means of assured identity information is provided. Leaving identity fields optional if possible thus should not be seen as a privacy method either, but rather as a protocol optimization feature. 5.3. Security features and properties With the security threats in mind, this draft provides the following security features and yields the following properties: * Secure key agreement with the establishment of a TGK at both peers: This is achieved using an authenticated Diffie-Hellman key management protocol. * Peer-entity authentication (mutual): This authentication corroborates that the host/user is authentic in that possession of a pre-assigned secret key is proven using keyed HMAC. The authentication occurs on the request and on the response message, thus authentication is mutual. The HMAC computation corroborates for authentication and message integrity of the exchanged Diffie-Hellman half-keys and associated Euchner Expires =96 December 2004 [Page 18] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 messages. The authentication is absolutely necessary in order to avoid man-in-the-middle attacks on the exchanged messages in transit and in particular, on the otherwise non-authenticated exchanged Diffie-Hellman half keys. Note: This document does not address issues regarding authorization; this feature is not provided explicitly. However, DHHMAC authentication means support and facilitate realization of authorization means (local issue). * Cryptographic integrity check: The cryptographic integrity check is achieved using a message digest (keyed HMAC). It includes the exchanged Diffie-Hellman half-keys but covers the other parts of the exchanged message as well. Both mutual peer entity authentication and message integrity provide effective countermeasure against man-in-the-middle attacks. The initiator may deploy a local timer that fires when the awaited response message did not arrive timely. This is to detect deletion of entire messages. * Replay protection of the messages is achieved using embedded timestamps. * Limited DoS protection: Rapid checking of the message digest allows verifying the authenticity and integrity of a message before launching CPU intensive Diffie-Hellman operations or starting other resource consuming tasks. This protects against some denial-of-service attacks: malicious modification of messages and spam attacks with (replayed or masqueraded) messages. DHHMAC probably does not explicitly counter sophisticated distributed, large-scale denial- of-service attacks that compromise system availability for example. Some DoS protection is provided by inclusion of the initiator=92s identity payload in the I_message. This allows the recipient to filter out those (replayed) I_messages that are not targeted for him and avoids the recipient from creating unnecessary MIKEY sessions. * Perfect-forward secrecy (PFS): Other than the MIKEY pre-shared and public-key based key distribution protocols, the Diffie-Hellman key agreement protocol features a security property called perfect forward secrecy. That Euchner Expires =96 December 2004 [Page 19] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 is, that even if the long-term pre-shared key would be compromised at some point in time, this would not render past or future session keys compromised. Neither the MIKEY pre-shared nor the MIKEY public-key protocol variants are able to provide the security property of perfect- forward secrecy. Thus, none of the other MIKEY protocols is able to substitute the Diffie-Hellman PFS property. As such, DHHMAC but also digitally signed DH provides a far superior security level over the pre-shared or public-key based key distribution protocol in that respect. * Fair, mutual key contribution: The Diffie-Hellman key management protocol is not a strict key distribution protocol per se with the initiator distributing a key to its peers. Actually, both parties involved in the protocol exchange are able to equally contribute to the common Diffie- Hellman TEK traffic generating key. This reduces the risk of either party cheating or unintentionally generating a weak session key. This makes the DHHMAC a fair key agreement protocol. One may view this property as an additional distributed security measure that is increasing security robustness over the case where all the security depends just on the proper implementation of a single entity. In order for Diffie-Hellman key agreement to be secure, each party shall generate its xi or xr values using a strong, unpredictable pseudo-random generator if a source of true randomness is not available. Further, these values xi or xr shall be kept private. It is recommended that these secret values be destroyed once the common Diffie-Hellman shared secret key has been established. * Efficiency and performance: Like the MIKEY-public key protocol, the MIKEY DHHMAC key agreement protocol securely establishes a TGK within just one roundtrip. Other existing key management techniques like IPSEC-IKE [14], IPSEC-IKEv2 [21] and TLS [13] and other schemes are not deemed adequate in addressing sufficiently those real-time and security requirements; they all use more than a single roundtrip. All the MIKEY key management protocols are able to complete their task of security policy parameter negotiation including key-agreement or key distribution in one roundtrip. However, the MIKEY pre-shared Euchner Expires =96 December 2004 [Page 20] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 and the MIKEY public-key protocol both are able to complete their task even in a half-round trip when the confirmation messages are omitted. Using HMAC in conjunction with a strong one-way hash function such as SHA1 may be achieved more efficiently in software than expensive public-key operations. This yields a particular performance benefit of DHHMAC over signed DH or the public-key encryption protocol. If a very high security level is desired for long-term secrecy of the negotiated Diffie-Hellman shared secret, longer hash values may be deployed such as SHA256, SHA384 or SHA512 provide, possibly in conjunction with stronger Diffie-Hellman groups. This is left as for further study. For the sake of improved performance and reduced round trip delay either party may off-line pre-compute its public Diffie-Hellman half-key. On the other side and under reasonable conditions, DHHMAC consumes more CPU cycles than the MIKEY pre-shared key distribution protocol. The same might hold true quite likely for the MIKEY public-key distribution protocol (depending on choice of the private and public key lengths). As such, it can be said that DHHMAC provides sound performance when compared with the other MIKEY protocol variants. The use of optional identity information (with the constraints stated in section 5.2) and optional Diffie-Hellman half-key fields provides a means to increase performance and shorten the consumed network bandwidth. * Security infrastructure: This document describes the HMAC-authenticated Diffie-Hellman key agreement protocol that completely avoids digital signatures and the associated public-key infrastructure as would be necessary for the X.509 RSA public-key based key distribution protocol or the digitally signed Diffie-Hellman key agreement protocol as described in MIKEY. Public-key infrastructures may not always be available in certain environments nor may they be deemed adequate for real- time multimedia applications when taking additional steps for Euchner Expires =96 December 2004 [Page 21] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 certificate validation and certificate revocation methods with additional round-trips into account. DHHMAC does not depend on PKI nor do implementations require PKI standards and thus is believed to be much simpler than the more complex PKI facilities. DHHMAC is particularly attractive in those environments where provisioning of a pre-shared key has already been accomplished. * NAT/Firewall-friendliness: DHHMAC is able to operate smoothly through firewall/NAT devices as long as the protected identity information of the end entity is not an IP /transport address. Of course, DHHMAC does not necessarily require a firewall/NAT to operate. * Scalability: Like the MIKEY signed Diffie-Hellman protocol, DHHMAC does not scale to any larger configurations beyond peer-to-peer groups. 5.4. Assumptions This document states a couple of assumptions upon which the security of DHHMAC significantly depends. It is assumed, that * the parameters xi, xr, s and auth_key are to be kept secret. * the pre-shared key s has sufficient entropy and cannot be effectively guessed. * the pseudo-random function (PRF) is secure, yields indeed the pseudo-random property and maintains the entropy. * a sufficiently large and secure Diffie-Hellman group is applied. * the Diffie-Hellman assumption holds saying basically that even with knowledge of the exchanged Diffie-Hellman half-keys and knowledge of the Diffie-Hellman group, it is infeasible to compute the TGK or to derive the secret parameters xi or xr. The latter is also called the discrete logarithm assumption. Please see [7], [11] or [12] for more background information regarding the Diffie-Hellman problem and its computational complexity assumptions. Euchner Expires =96 December 2004 [Page 22] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 * the hash function (SHA1) is secure; i.e. that it is computationally infeasible to find a message which corresponds to a given message digest, or to find two different messages that produce the same message digest. * the HMAC algorithm is secure and does not leak the auth_key. In particular, the security depends on the message authentication property of the compression function of the hash function H when applied to single blocks (see [6]). * A source capable of producing sufficiently many bits of (pseudo) randomness is available. * The systems upon which DHHMAC runs are sufficiently secure. The assumptions MUST be met as far as they can be enforced. 5.5. Residual risk Although these detailed assumptions are non-negligible, security experts generally believe that all these assumptions are reasonable and that the assumptions made can be fulfilled in practice with little or no expenses. The mathematical and cryptographic assumptions upon the properties of the PRF, the Diffie-Hellman algorithm (discrete log-assumption), the HMAC and SHA1 algorithms have not been proved yet nor have they been disproved by the time of this writing. Thus, a certain residual risk remains, which might threaten the overall security at some unforeseeable time in the future. The DHHMAC would be compromised as soon as * the discrete logarithm problem could be solved efficiently, * the hash function could be subverted (efficient collisions become feasible), * the HMAC method be broken (leaking the auth_key), Euchner Expires =96 December 2004 [Page 23] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 * systematic brute force attacks are effective by which an attacker attempts to discover the shared secret. It is assumed that the shared secret yields sufficient entropy to make such attacks infeasible, * or some other yet unknown attacking technique will be discovered. The Diffie-Hellman mechanism is a generic security technique that is not only applicable to groups of prime order or of characteristic two. This is because of the fundamental mathematical assumption that the discrete logarithm problem is also a very hard one in general groups. This enables Diffie-Hellman to be deployed also for GF(p)*, for sub-groups of sufficient size and for groups upon elliptic curves. RSA does not allow such generalization, as the core mathematical problem is a different one (large integer factorization). RSA asymmetric keys tend to become increasingly lengthy (1536 bits and more) and thus very computational intensive. Neverthess, elliptic curve Diffie-Hellman (ECDH) allows to cut-down key lengths substantially (say 170 bits or more) while maintaining at least the security level and providing even significant performance benefits in practice. Moreover, it is believed that elliptic curve techniques provide much better protection against side channel attacks due to the inherent redundancy in the projective coordinates. For all these reasons, one may view elliptic-curve-based Diffie-Hellman as being more "future-proof" and robust against potential threats than RSA. Note, that an elliptic-curve Diffie-Hellman variant of MIKEY remains for further study. It is not recommended to deploy DHHMAC for any other usage than depicted in section 2. Otherwise any such misapplication might lead to unknown, undefined properties. 5.6. Authorization and Trust Model Basically, similar remarks on authorization as stated in [3] section 4.3.2. hold also for DHHMAC. However, as noted before, this key management protocol does not serve full groups. One may view the pre-established shared secret to yield some pre- established trust relationship between the initiator and the Euchner Expires =96 December 2004 [Page 24] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 responder. This results in a much simpler trust model for DHHMAC than would be the case for some generic group key management protocol and potential group entities without any pre-defined trust relationship. The common group controller in conjunction with the assumption of a shared key simplifies the communication setup of the entities. One may view the pre-established trust relationship through the pre- shared secret as some means for pre-granted, implied authorization. This document does not define any particular authorization means but leaves this subject to the application. IANA considerations This document does not define its own new name spaces for DHHMAC, rather additional values for DHHMAC are defined as part of the MIKEY fields. Thus, close alignment between DHHMAC values and MIKEY values shall be maintained; see also [3] section 10. Intellectual Property Rights This proposal is in full conformity with [RFC-2026]. The author is aware of related intellectual property rights currently being held by Infineon. Pursuant to the provisions of [RFC-2026], the author represents that he has disclosed the existence of any proprietary or intellectual property rights in the contribution that are reasonably and personally known to the author. The author does not represent that he personally knows of all potentially pertinent proprietary and intellectual property rights owned or claimed by the organizations he represents or third parties. The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made Euchner Expires =96 December 2004 [Page 25] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. References Normative References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, K. Norrman; "MIKEY: Multimedia Internet KEYing", Internet Draft , Work in Progress (MSEC WG), IETF, December 2003. [4] NIST, FIBS-PUB 180-1, "Secure Hash Standard", April 1995, http://csrc.nist.gov/fips/fip180-1.ps. [5] J. Arkko, E. Carrara et al: "Key Management Extensions for SDP and RTSP", Internet Draft , Work in Progress (MMUSIC WG), IETF, April 2004. [6] H. Krawczyk, M. Bellare, R. Canetti: "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. Informative References [7] A.J. Menezes, P. van Oorschot, S. A. Vanstone: "Handbook of Applied Cryptography", CRC Press 1996. [8] E. Rescorla, B. Korver: " Guidelines for Writing RFC Text on Security Considerations", RFC 3552, IETF, July 2003. [9] D. Eastlake, S. Crocker: "Randomness Recommendations for Security", RFC 1750, IETF, December 1994. [10] S.M. Bellovin, C. Kaufman, J. I. Schiller: "Security Mechanisms for the Internet", RFC 3631, IETF, December 2003. Euchner Expires =96 December 2004 [Page 26] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 [11] Ueli M. Maurer, S. Wolf: "The Diffie-Hellman Protocol", Designs, Codes, and Cryptography, Special Issue Public Key Cryptography, Kluwer Academic Publishers, vol. 19, pp. 147-171, 2000. ftp://ftp.inf.ethz.ch/pub/crypto/publications/MauWol00c.ps [12] Discrete Logarithms and the Diffie-Hellman Protocol; http://www.crypto.ethz.ch/research/ntc/dldh/ [13] T. Dierks, C. Allen: "The TLS Protocol Version 1.0.", RFC 2246, IETF, January 1999. [14] D. Harkins, D. Carrel: "The Internet Key Exchange (IKE).", RFC 2409, IETF, November 1998. [15] Donald E. Eastlake, Jeffrey I. Schiller, Steve Crocker: "Randomness Requirements for Security"; ; Work in Progress, IETF, April 2004. [16] J. Schiller: "Strong Security Requirements for Internet Engineering Task Force Standard Protocols", RFC 3365, IETF, 2002. [17] C. Meadows: "Advice on Writing an Internet Draft Amenable to Security Analysis", Work in Progress , IRTF, October 2002. [18] T. Narten: "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, IETF, October 1998. [19] J. Reynolds: "Instructions to Request for Comments (RFC) Authors", Work in Progress, , IETF, August 2003. [20] J. Rosenberg et all: "SIP: Session Initiation Protocol", RFC 3261, IETF, June 2002. [21] Ch. Kaufman: "Internet Key Exchange (IKEv2) Protocol", Work in Progress (IPSEC WG), , IETF, March 2004. [22] Draft ITU-T Recommendation H.235 Annex G: "Usage of the MIKEY Key Management Protocol for the Secure Real Time Transport Protocol (SRTP) within H.235"; 5/2004. Euchner Expires =96 December 2004 [Page 27] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 [23] Schaad, J., Housley R.: "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, IETF, September 2002. [24] Baugher, M., Weis, B., Hardjono, T., Harney, H.: "The Group Domain of Interpretation", RFC 3547, IETF, July 2003. [25] Harney, H., Colegrove, A., Harder, E., Meth, U., Fleischer, R.: "Group Secure Association Key Management Protocol", , Internet Draft, Work in Progress (MSEC WG). [26] Baugher, M., Canetti, R., Dondeti, L., and Lindholm, F.: "Group Key Management Architecture", < draft-ietf-msec-gkmarch-07.txt>, Internet Draft, Work in Progress (MSEC WG). [27] Baugher, McGrew, Oran, Blom, Carrara, Naslund: "The Secure Real- time Transport Protocol", RFC 3711, IETF, March 2004. [28] ITU-T Recommendation H.235V3Amd1, Security and encryption for H- series (H.323 and other H.245-based) multimedia terminals, (04/2004) Acknowledgments This document incorporates kindly review feedback by Steffen Fries and Fredrick Lindholm and general feedback by the MSEC WG. Conclusions Key management for environments and applications with real-time and performance constraints are becoming of interest. Existing key management techniques like IPSEC-IKE [14] and IPSEC-IKEv2 [22], TLS [13] and other schemes are not deemed adequate in addressing sufficiently those real-time and security requirements. MIKEY defines three key management security protocols addressing real-time constraints. DHHMAC as described in this document defines a fourth MIKEY variant aiming at the same target. Euchner Expires =96 December 2004 [Page 28] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 While each of the four key management protocols has its own merits there are also certain limitations of each approach. As such there is no single ideal solution and none of the variants is able to subsume the other remaining variants. It is concluded that DHHMAC features useful security and performance properties that none of the other three MIKEY variants is able to provide. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. Expiration Date This Internet Draft expires on 30 December 2004. Euchner Expires =96 December 2004 [Page 29] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 Revision History Changes against draft-ietf-msec-mikey-dhhmac-05.txt: * HMAC-SHA1-96 option removed (see section 1.2, 4.2, 5.3,). This option does not really provide much gain; removal reduces number of options. * IDr added to I_message for DoS protection of the recipient; see section 3, 3.1, 5.3. * References updated. Changes against draft-ietf-msec-mikey-dhhmac-04.txt: * Introduction section modified: PFS property of DH, requirement for 4th MIKEY key management variant motivated. * MIKEY-DHSIGN, MIKEY-PK and MIKEY-PS added to section 1.2 Abbreviations. * Note on secure time synchronization added to section 2.0. * New section 2.2 "Relation to GMKARCH" added. * New section 2.1.1 "Usage in H.235" added: this section outlines a use case of DHHMAC in the context of H.235. * Trade-off between identity-protection and security & performance added to section 5.1. * New section 5.6 "Authorization and Trust Model" added. * Some further informative references added. Changes against draft-ietf-msec-mikey-dhhmac-03.txt: * RFC 3552 available; some references updated. Changes against draft-ietf-msec-mikey-dhhmac-02.txt: * text allows both random and pseudo-random values. * exponentiation ** changed to ^. * Notation aligned with MIKEY-07. * Clarified that the HMAC is calculated over the entire MIKEY message excluding the MAC field. * Section 4.2: The AES key wrap method SHALL not be applied. * Section 1: Relationship with other, existing work mentioned. * Euchner Expires =96 December 2004 [Page 30] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 Changes against draft-ietf-msec-mikey-dhhmac-01.txt: * bidding-down attacks addressed (see section 5.2). * optional [X], [X, Y] defined and clarified (see section 1.1, 5.3). * combination of options defined in key update procedure (see section 3.1). * ID payloads clarified (see section 3 and 5.2). * relationship with MIKEY explained (roundtrip, performance). * new section 2.1 on applicability of DHHMAC for SIP/SDP and H.323 added. * more text due to DH resolution incorporated in section 5.3 regarding PFS, security robustness of DH, generalization capability of DH to general groups in particular EC and "future-proofness". * a few editorials and nits. * references adjusted and cleaned-up. Changes against draft-ietf-msec-mikey-dhhmac-00.txt: * category set to proposed standard. * identity protection clarified. * aligned with MIKEY-05 DH protocol, notation and with payload * some editorials and nits. Changes against draft-euchner-mikey-dhhmac-00.txt: * made a MSEC WG draft * aligned with MIKEY-03 DH protocol, notation and with payload formats * clarified that truncated HMAC actually truncates the HMAC result rather than the SHA1 intermediate value. * improved security considerations section completely rewritten in the spirit of [8]. * IANA consideration section added * a few editorial improvements and corrections * IPR clarified and IPR section changed. Author's Addresses Euchner Expires =96 December 2004 [Page 31] HMAC-authenticated Diffie-Hellman for MIKEY May 2004 Martin Euchner Email: martin_euchner@hotmail.com Phone: +49 89 722 55790 Hofmannstr. 51 Fax: +49 89 722 62366 81359 Munich, Germany Euchner Expires =96 December 2004 [Page 32]