MSEC V. Roca Internet-Draft A. Francillon Intended status: Standards Track S. Faurite Expires: September 3, 2007 INRIA March 2, 2007 Use of TESLA in the ALC and NORM Protocols draft-ietf-msec-tesla-for-alc-norm-01.txt 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 on September 3, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Roca, et al. Expires September 3, 2007 [Page 1] Internet-Draft TESLA in ALC and NORM March 2007 Abstract This document explains how to integrate the TESLA source authentication and packet integrity protocol to the ALC and NORM content delivery protocols. This document only considers the authentication/integrity of the packets generated by the session's sender. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Conventions Used in this Document . . . . . . . . . . . . 5 1.2. Terminology and Notations . . . . . . . . . . . . . . . . 5 2. Using TESLA with ALC and NORM . . . . . . . . . . . . . . . . 7 2.1. ALC and NORM Specificities that Impact TESLA . . . . . . . 7 2.2. The Need for Secure Time Synchronization . . . . . . . . . 8 2.2.1. Direct Time Synchronization . . . . . . . . . . . . . 8 2.2.2. Indirect Time Synchronization . . . . . . . . . . . . 8 3. Time Synchronization and Delay Bound Calculations . . . . . . 10 3.1. Delay Bound Calculation in Direct Time Synchronization Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Delay Bound Calculation in Indirect time Synchronization Mode . . . . . . . . . . . . . . . . . . . 10 3.2.1. Single time reference . . . . . . . . . . . . . . . . 10 3.2.2. Multiple time references . . . . . . . . . . . . . . . 11 4. Sender Operations . . . . . . . . . . . . . . . . . . . . . . 12 4.1. TESLA Parameters . . . . . . . . . . . . . . . . . . . . . 12 4.1.1. Key Chains . . . . . . . . . . . . . . . . . . . . . . 12 4.1.2. Time Interval Schedule . . . . . . . . . . . . . . . . 13 4.2. TESLA Messages and Authentication Tags . . . . . . . . . . 13 4.2.1. Bootstrap Information . . . . . . . . . . . . . . . . 14 4.2.2. Direct Time Synchronization Response . . . . . . . . . 15 4.2.3. Authentication Tag . . . . . . . . . . . . . . . . . . 15 4.3. TESLA Messages and Authentication Tag Format . . . . . . . 15 4.3.1. Bootstrap Information Format . . . . . . . . . . . . . 16 4.3.2. Format of a Direct Time Synchronization Response . . . 23 4.3.3. Format of a Standard Authentication Tag . . . . . . . 25 4.3.4. Format of an Authentication Tag with a New Key Chain Commitment . . . . . . . . . . . . . . . . . . . 26 4.3.5. Format of an Authentication Tag with an Old Chain Last Key Disclosure . . . . . . . . . . . . . . . . . 26 5. Receiver Operations . . . . . . . . . . . . . . . . . . . . . 28 5.1. Initialization of a Receiver . . . . . . . . . . . . . . . 28 5.1.1. Processing the Bootstrap Information Message . . . . . 28 5.1.2. Time Synchronization . . . . . . . . . . . . . . . . . 28 5.1.3. Format of a Direct Time Synchronization Request . . . 29 5.2. Authentication of Received Packets . . . . . . . . . . . . 30 Roca, et al. Expires September 3, 2007 [Page 2] Internet-Draft TESLA in ALC and NORM March 2007 6. Integration in the ALC and NORM Protocols . . . . . . . . . . 31 6.1. Authentication Header Extension Format . . . . . . . . . . 31 6.2. Use of Authentication Header Extensions . . . . . . . . . 33 6.2.1. EXT_AUTH Header Extension of Type Bootstrap Information . . . . . . . . . . . . . . . . . . . . . 33 6.2.2. EXT_AUTH Header Extension of Type Bootstrap Information . . . . . . . . . . . . . . . . . . . . . 33 6.2.3. EXT_AUTH Header Extension of Type Direct Time Synchronization Request . . . . . . . . . . . . . . . 34 6.2.4. EXT_AUTH Header Extension of Type Direct Time Synchronization Response . . . . . . . . . . . . . . . 35 6.3. Managing Silent Periods . . . . . . . . . . . . . . . . . 35 7. Security Considerations . . . . . . . . . . . . . . . . . . . 36 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9.1. Normative References . . . . . . . . . . . . . . . . . . . 38 9.2. Informative References . . . . . . . . . . . . . . . . . . 38 Appendix A. IANA Considerations . . . . . . . . . . . . . . . . . 40 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41 Intellectual Property and Copyright Statements . . . . . . . . . . 42 Roca, et al. Expires September 3, 2007 [Page 3] Internet-Draft TESLA in ALC and NORM March 2007 1. Introduction Many applications using multicast and broadcast communications require that each receiver be able to authenticate the source of any packet it receives as well as its integrity. For instance, ALC [draft-ietf-rmt-pi-alc-revised] and NORM [draft-ietf-rmt-pi-norm-revised] are two Content Delivery Protocols (CDP) designed to transfer reliably objects (e.g. files) between a session's sender and several receivers. The NORM protocol is based on bidirectional transmissions. Each receiver acknowledges data received or, in case of packet erasures, asks for retransmissions. The ALC protocol defines unidirectional transmissions. Reliability can be achieved by means of cyclic transmissions of the content within a carousel, or by the use of proactive Forward Error Correction codes (FEC), or by the joint use of these mechanisms. Being purely unidirectional, ALC is massively scalable, while NORM is intrinsically limited in terms of the number of receivers that can be handled in a session. Both protocols have in common the fact that they operate at application level, on top of an erasure channel (e.g. the Internet) where packets can be lost (erased) during the transmission. With some use case, an attacker might impersonate the the ALC or NORM session's sender and inject forged packets to the receivers, thereby corrupting the objects reconstructed by the receivers. The situation is much more complex in case of group communications than it is with unicast communications. Indeed, in the latter case a simple solution exist: the sender and receiver can share a secret key to compute a Message Authentication Code (MAC) of all messages exchanged. This is no longer feasible in case of a multicast and broadcast communications since sharing a group key between the sender and all receivers and using the same MAC scheme means that any group member can impersonate the sender and send forged messages to other receivers. The usual solution to provide the source authentication and message integrity services in case of multicast and broadcast communications consists in relying on asymmetric cryptography and using digital signatures. Yet this solution is limited by high computational costs and high transmission overheads. The Timed Efficient Stream Loss- tolerant Authentication protocol (TESLA) is an alternative solution that provides the two required services, while being compatible with high rate transmissions over lossy channels. This document explains how to integrate the TESLA source authentication and packet integrity protocol to the ALC and NORM content delivery protocols. Since the FLUTE application [RFC3926] is Roca, et al. Expires September 3, 2007 [Page 4] Internet-Draft TESLA in ALC and NORM March 2007 built on top of ALC, it will directly benefit from the services offered by TESLA at the transport layer. This specification only considers the authentication/integrity of the packets generated by the session's sender. This specification does not consider the packets that will be generated by receivers, for instance the feedback packets of NORM. Adding authentication/ integrity to the packets sent by receivers is outside the scope of this document. Of course, this remark does not apply to ALC where transmissions are purely unidirectional. For more information on the TESLA protocol and its principles, please refer to [RFC4082][Perrig04]. For more information on ALC, NORM and FLUTE, please refer to [draft-ietf-rmt-pi-alc-revised], [draft-ietf-rmt-bb-lct-revised], [draft-ietf-rmt-pi-norm-revised] and [RFC3926]. 1.1. 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 [RFC2119]. 1.2. Terminology and Notations The following notations and definitions are used throughout this document: o PRF is the Pseudo Random Function; o MAC is the Message Authentication Code; o HMAC is the Keyed-Hash Message Authentication Code; o t_s is the sender local time value at some absolute time; o t_r is the receiver local time value at the same absolute time; o T_0, the start time corresponding to the beginning of the session (NTP timestamp); o T_int, the interval duration (in milliseconds); o d, the key disclosure delay (in number of intervals); o D_t, the upper bound of the lag of the receiver's clock with respect to the clock of the sender; Roca, et al. Expires September 3, 2007 [Page 5] Internet-Draft TESLA in ALC and NORM March 2007 o S_sr, an estimated bound of the clock drift between the sender and a receiver throughout the duration of the session; o D^0_t, the upper bound of the lag of the sender's clock with respect to the time reference in indirect time synchronization mode; o D^R_t, the upper bound of the lag of the receiver s's clock with respect to the time reference in indirect time synchronization mode; o D_err, an upper bound of the time error between all the time references, in indirect time synchronization mode; o N is the number of keys in a key chain. When several chains are used, all the chains MUST have the same length, N; o N_tx_old_kck is the number of intervals during which the last key of the old key chain SHOULD be sent, after switching to a new key chain and waiting for the disclosure delay d; o N_tx_new_kcc is the number of intervals during which the commitment to a new key chain SHOULD be sent, before switching to the new key chain; Roca, et al. Expires September 3, 2007 [Page 6] Internet-Draft TESLA in ALC and NORM March 2007 2. Using TESLA with ALC and NORM 2.1. ALC and NORM Specificities that Impact TESLA The ALC and NORM protocols have features and requirements that largely impact the way TESLA can be used. In case of ALC: o ALC is massively scalable: nothing in the protocol specification limits the number of receivers that join a session. Therefore an ALC session potentially includes a huge number (e.g. millions or more) of receivers; o ALC can work on top of purely unidirectional transport channels: this is one of the assets of ALC, and examples of unidirectional channels include satellite (even if a back channel might exist in some use cases) and DVB-H systems; o ALC defines an on-demand content delivery model [draft-ietf-rmt-pi-alc-revised] where receivers can arrive at any time, at their own discretion, download the content and leave the session. Other models (e.g. push or streaming) are also defined; o ALC sessions are potentially very long: with some use cases a session can last several months during which the content is continuously transmitted within a carousel. The content can be either static (e.g. a software update) or dynamic (e.g. a web site). Depending on the use case, some of the above features may not apply. For instance ALC can also be used over a bidirectional channel, or ALC can be used for small groups. In case of NORM: o NORM has been designed for limited or medium size sessions: Indeed, NORM relies on feedback messages and the sender may collapse if the feedback message rate is too high; o NORM requires a bidirectional transport channel: the back channel is not necessarily a high rate channel since only low to medium rate control traffic will flow on it. Networks with an asymmetric connectivity (e.g. a high rate satellite downlink and a low-rate RTC based return channel) are appropriate; Roca, et al. Expires September 3, 2007 [Page 7] Internet-Draft TESLA in ALC and NORM March 2007 2.2. The Need for Secure Time Synchronization The security offered by TESLA relies heavily on time. Therefore the session's sender and each receiver need to be time synchronized in a secure way. To that purpose, two general methods exist: o direct time synchronization, and o indirect time synchronization. 2.2.1. Direct Time Synchronization When direct time synchronization is used, each receiver asks the sender for a time synchronization. To that purpose, a receiver sends a "Direct Time Synchronization Request" (Section 5.1.3). The sender then directly answers to each request with a "Direct Time Synchronization Response" (Section 4.3.2), signing this reply. Upon receiving this response, a receiver first verifies the signature, and then calculates an upper bound of the lag of his clock with respect to the clock of the sender, D_t. The details on how to calculate D_t are given in Section 3.1. This synchronization method is both simple and secure. Yet there are two potential issues: o a bidirectional channel MUST exist between the sender and each receiver, o the sender may collapse if the incoming request rate is too high. Direct time synchronization is not be an issue with NORM since (1) bidirectional communications already take place, and (2) NORM scalability is anyway limited. But direct time synchronization is potentially incompatible with ALC since (1) there might not be a back channel to the session's sender, and (2) there are potentially a huge number of receivers and therefore a risk that the sender collapses. 2.2.2. Indirect Time Synchronization When indirect time synchronization is used, the sender and each receiver must synchronize securely via an external time reference. Several possibilities exist: o sender and receivers can synchronize through a NTPv3 (Network Time Protocol version 3) [RFC1305] hierarchy of servers. The authentication mechanism of NTPv3 MUST be used in order to Roca, et al. Expires September 3, 2007 [Page 8] Internet-Draft TESLA in ALC and NORM March 2007 authenticate each NTP message individually. It prevents for instance an attacker to impersonate a NTP server; o similarly sender and receivers can synchronize through a NTPv4 (Network Time Protocol version 4) [draft-ietf-ntp-ntpv4-proto] hierarchy of servers. The Autokey security protocol of NTPv4 MUST be used in order to authenticate each NTP message individually; o similarly, they can synchronize through a SNTPv4 (Simple Network Time Protocol version 4) [RFC4330] hierarchy of servers. The authentication features of SNTPv4 must then be used. Note that TESLA only needs a loose (but secure) time synchronization, which is in line with the time synchronization service offered by SNTP; o they can synchronize through a GPS or Galileo (or similar) device that also provide a high precision time reference. This time reference is in general trusted, yet depending on the use case, this trust will or not be acceptable; o they can synchronize thanks to a dedicated hardware, embedded on each sender and receiver, that offers a clock with a time-drift that is negligible in front of the TESLA time accuracy requirements. This feature enables a device to synchronize its embedded clock with the official time reference from time to time, when feasible (in an extreme case once, at manufacturing time), and then to remain autonomous for some time, depending on the known maximum clock drift. A bidirectional channel is required by the NTP/SNTP schemes. On the opposite, with the GPS/Galileo and high precision clock schemes, no such assumption is made. In situations where ALC is used on purely unidirectional transport channels (Section 2.1), using the NTP/SNTP schemes is not possible. Another aspect is the scalability requirement of ALC, and to a lesser extent of NORM. From this point of view, the above mechanisms usually do not raise any problem, unlike the direct time synchronization schemes. Therefore, using indirect time synchronization is a good candidate, in particular with ALC. The details on how to calculate an upper bound of the lag of a receiver's clock with respect to the clock of the sender, D_t, are given in Section 3.2. Roca, et al. Expires September 3, 2007 [Page 9] Internet-Draft TESLA in ALC and NORM March 2007 3. Time Synchronization and Delay Bound Calculations 3.1. Delay Bound Calculation in Direct Time Synchronization Mode In direct time synchronization mode, synchronization between a receiver and the sender follows the following protocol [RFC4082]: o The receiver sends a "Direct Time Synchronization Request" message to the sender, that includes t_r, the receiver local time at the moment of sending (Section 5.1.3). o Upon receipt of this message, the sender records its local time, t_s, and sends to the receiver a "Direct Time Synchronization Response" that includes t_r (taken from the request) and t_s (Section 4.3.2), signing this reply. o Upon receiving this response, the receiver first verifies that he actually sent a request with t_r and then checks the signature. Then he calculates D_t = t_s - t_r + S_sr, where S_sr is an estimated bound of the clock drift between the sender and the receiver throughout the duration of the session. After this initial synchronization, at any point throughout the session, the receiver knows that: T_s < T_r + D_t, where T_s is the current time at the sender and T_r is the current time at the receiver. 3.2. Delay Bound Calculation in Indirect time Synchronization Mode In indirect time synchronization, the sender and the receivers must synchronize indirectly with one or several time references. 3.2.1. Single time reference Let's assume that there is a single time reference. o The sender uses a direct time synchronization scheme (but not necessarily the one specified in Section 3.1, see below) to calculate D^0_t, the upper bound of the lag of the sender's clock with respect to the time reference. This D^0_t value is communicated to receivers within the Bootstrap Information (Section 4.3.1). o Similarly, a receiver R uses a direct time synchronization scheme (same remark) to calculate D^R_t, the upper bound of the lag of the receiver's clock with respect to the time reference. Roca, et al. Expires September 3, 2007 [Page 10] Internet-Draft TESLA in ALC and NORM March 2007 o Then, for receiver R, the overall upper bound of the lag of the receiver's clock with respect to the clock of the sender, D_t, is the sum: D_t = D^0_t + D^R_t. The way to calculate D^0_t and D^R_t depends on the time synchronization mechanism used (Section 2.2.2). In some cases, it is part of the time synchronization scheme specifications. In other cases, it can be achieved by using the direct time synchronization scheme of Section 3.1, for instance in case synchronization is achieved via a group controller [RFC4082]. 3.2.2. Multiple time references Let's now assume that there are several time references (e.g. several (S)NTP servers). The sender and receivers use the direct time synchronization scheme to synchronize with the various time references. It results in D^0_t and D^R_t. Let D_err be an upper bound of the time error between all the time references. Then, the overall value of D_t within receiver R is set to the sum: D_t = D^0_t + D^R_t + D_err. In some cases, the D_t value is is part of the time synchronization scheme specifications. For instance NTPv3 [RFC1305] defines algorithms that are "capable of accuracies in the order of a millisecond, even after extended periods when synchronization to primary reference sources has been lost". In practice, depending on the NTP server stratum, the accuracy might be a little bit worse. In that case, D_t = security_factor * (1ms + 1ms), where the security_factor is meant to compensate several sources of inaccuracy in NTP. ----- Editor's note: is this D_t = security_factor * (1ms + 1ms) rule of thumb acceptable? ----- Roca, et al. Expires September 3, 2007 [Page 11] Internet-Draft TESLA in ALC and NORM March 2007 4. Sender Operations 4.1. TESLA Parameters 4.1.1. Key Chains The sender divides the time into uniform intervals of duration T_int. The sender then computes a one-way key chain of N keys, and assigns one key from the chain to each interval in sequence. In order to compute this chain, he must first select a Primary Key, K_N, and a PRF function, f. The functions F and F' are then defined as: F(k)=f_k(0) and F'(k)=f_k(1). The sender computes all the keys of key chain, starting with K_N, using: K_{i-1} = F(K_i). The key for MAC calculation can then be derived from the corresponding K_i key by K'_i=F'(K_i). The randomness of the Primary Key, K_N, is vital to the security since no one should be able to guess it. The key chain has a finite length, N, so the TESLA session must finish before the end of the key chain. But the longer the key chain, the higher the memory and computation required to cope with it. Another solution consists in switching to a new key chain when necessary [Perrig04]. To do so, the sender commits the new key chain with the old key chain. Let's say that the old key chain stops at K_N and the new key chain starts at K_{N+1}. Then the sender includes the commitment F(K_{N+1}) to the new key chain to packets authenticated with the old key chain. This commitment SHOULD be sent during N_tx_new_kcc intervals before the end of the old key chain. Since several packets are usually sent during a interval, the sender should alternate between sending a disclosed key of the old key chain, and the commitment to the new key chain. The receiver will keep the commitment, until the key K_{N+1} is disclosed, at interval N+1+d. Then the receiver will be able to test the validity of that key by computing F(K_{N+1}) and comparing it to the commitment. When the key chain is changed, it becomes impossible to recover a previous key from the old key chain. This is a problem if the receiver lost the packets disclosing the last key of the old key chain. A solution consists in re-sending the last key, K_N, of the old key chain. This SHOULD be done during N_tx_old_kck intervals at the beginning of the new key chain, after the disclosure delay d. Since several packets are usually sent during a interval, the sender should alternate between sending a disclosed key of the new key chain, and the last key of the old key chain. Roca, et al. Expires September 3, 2007 [Page 12] Internet-Draft TESLA in ALC and NORM March 2007 In some cases a receiver having experienced a very long disconnection might have lost the commitment of the new chain. Therefore this receiver will be unable to authenticate any packet related to the new chain and all the following ones. The solution for this receiver to catch up consists in receiving the bootstrap information. This can happen by waiting for the next periodic transmission (indirect time synchronization), by requesting it (direct time synchronization), or through an external mechanism (Section 4.2.1). 4.1.2. Time Interval Schedule The sender must determine the following parameters: o T_0, the start time corresponding to the beginning of the session; o T_int, the interval duration, usually ranging from 100 milliseconds to 1 second; o d, the key disclosure delay (in number of intervals). It is the time to wait before disclosing a key; o N, the number of keys in a key chain; The correct choice of T_int, d, and N is crucial for the usability of the scheme. For instance, a T_int * d product that is too long will cause excessive delay in the authentication process. A T_int * d product that is is too short will cause too many packets to be unverifiable by some receivers. A N * T_int product that is too small will cause the sender to switch too often to new key chains. A N that is too long with respect to the expected session duration, if this latter is known, will require the sender to compute too many keys without using them all. [RFC4082] (sections 3.2 and 3.6) gives general guidelines for initializing these parameters. 4.2. TESLA Messages and Authentication Tags At a sender, TESLA produces two types of signaling information: o The bootstrap information, which is a digitally signed packet containing all the information required to bootstrap TESLA at a receiver. o The authentication tag, which is sent in all packets (see Section 6 for exceptions) and contains the MAC of the packet. Roca, et al. Expires September 3, 2007 [Page 13] Internet-Draft TESLA in ALC and NORM March 2007 4.2.1. Bootstrap Information In order to initialize the TESLA component at a receiver, the sender must communicate some key information. This TESLA bootstrap information MUST be securely transmitted. In particular a receiver must be able to check the packet source and the packet integrity using standard protocols. Any digital signature will do. The bootstrap information can be: o unicast to a receiver during a direct time synchronization request/response exchange; o broadcast to all receivers. This is typically the case in indirect time synchronization mode. It can also be used in direct time synchronization mode, for instance when a large number of clients arrive at the same time, in which case it is more efficient to answer globally. Let's consider situations where the bootstrap information is broadcast. This message should be broadcast at the beginning of the session, before data packets are actually sent. This is particularly important with ALC or NORM sessions in ``push'' mode, when all clients join the session in advance. For improved reliability, bootstrap information might be sent a certain number of times. Afterwards, a periodic broadcast of the bootstrap information message could be useful when: o the ALC session uses an ``on-demand'' mode, clients arriving at their own discretion; o some clients experience an intermittent connectivity. This is particularly true when several key chains are used in an ALC or NORM session, since there is a risk that some receivers lose all the commitments to the new key chain. A balance must be found between the signaling overhead and the maximum initial waiting time at the receiver before starting the delayed authentication process. A frequency of a few seconds for the transmission of this bootstrap information is often a reasonable value. In some use cases, the bootstrapping information MAY be sent to receivers through an external mechanism, for instance in a way similar to [RFC4442]. How to do that is outside the scope of this document. Roca, et al. Expires September 3, 2007 [Page 14] Internet-Draft TESLA in ALC and NORM March 2007 4.2.2. Direct Time Synchronization Response In Direct Time Synchronization, upon receipt of a synchronization request, the sender records its local time, t_s, and sends a response message that contains both t_r and t_s (Section 3.1). This message is unicast to the receiver. This Direct Time Synchronization Response message MUST be securely transmitted, in particular the receiver must be able to check the packet source and the packet integrity using standard protocols. Any digital signature will do. The receiver MUST also be able the associate this response and his request, which is the reason why t_r is included in the message. The Direct Time Synchronization Response messages are distinct from the Bootstrap Information message. Therefore, if a large number of receivers try to initialize their TESLA component at the same time (a reasonable assumption in "push" mode), a single Bootstrap Information message can be broadcast to all of them. In some situations, when there is a limited number of receivers, a sender can also choose to unicast a Bootstrap Information message to each client individually after the direct time synchronization response message. The choice is outside the scope of this document. Note that a single session might include receivers that use the direct time synchronization mode while others use indirect mode. 4.2.3. Authentication Tag Every authenticated packet must have an authentication tag containing: o the index of the interval (which is also the index of the key used for computing the MAC of this packet: i; o the MAC of the message: MAC(K'_i, M), where K'_i=F'(K_i); o and either a disclosed key or a commitment to a new key chain. The computation of MAC(K'_i, M), includes the ALC or NORM header (with the various header extensions) and the payload when applicable. The UDP/IP/MAC headers are not included. During this computation, the MAC(K'_i, M) field of the authentication tag (see Section 4.3.3 Section 4.3.4 Section 4.3.5) MUST be set to 0. 4.3. TESLA Messages and Authentication Tag Format This section specifies the format of the various kinds of TESLA messages and authentication tags sent by the session's sender. Roca, et al. Expires September 3, 2007 [Page 15] Internet-Draft TESLA in ALC and NORM March 2007 4.3.1. Bootstrap Information Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved |A|B| # Cert| T_int | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | d | PRF Type | HMAC Func Type| Signature Type| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key length | Signature Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + T_0 + | (NTP timestamp) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | N (Number of Keys in chain) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K_i) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ K_i +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Signature Extension ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |P| | +-+ D^0_t Extension (optional, present if A==1) + | (NTP timestamp diff, positive if P==1, negative if p==0) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ NTP Extension (optional, present if B==1) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Bootstrap information format. The format of the bootstrap information is depicted in Figure 1. The fields are: "Reserved" field (10 bits): This is a reserved field that MUST be set to zero in this specification. "A" flag (1 bit): Roca, et al. Expires September 3, 2007 [Page 16] Internet-Draft TESLA in ALC and NORM March 2007 A==0 indicates that the P flag and D^0_t field are not present. A==1 indicates that the P flag and D^0_t field are present (which is required in. indirect time synchronization mode). "B" flag (1 bit): B==0 indicates that the NTP extension is not present. B==1 indicates that the NTP extension is present. "# Cert." (Number of Certificates) field (8 bits): The "Number of Certificates" indicates the number of certificates present in the signature extension. "T_int" field (8 bits): T_int is an unsigned integer that defines the interval duration (in milliseconds). "d" field (8 bits): d is an unsigned integer that defines the key disclosure delay (in number of intervals). d MUST be greater or equal to 2. "PRF Type" field (8 bits): "PRF Type" is the reference number of the f function used to derive the F (for key chain) and F' (for MAC keys) functions (Appendix A). "HMAC Function Type" field (8 bits): The "HMAC Function Type" is the reference number of the function used to compute the HMAC of the packets (Appendix A). "Signature Type" field (8 bits): The "Signature Type" is the reference number (Appendix A) of the digital signature used to authenticate this bootstrap information and included in the "Signature Extension" field. "Key Length" field (16 bits): The "Key length" is an unsigned integer that indicates the length in bits of key K_i. "Signature Length" field (16 bits): Roca, et al. Expires September 3, 2007 [Page 17] Internet-Draft TESLA in ALC and NORM March 2007 The "Signature Length" is an unsigned integer that indicates the signature field size in bytes in the "Signature Extension" field. This is NOT the total length of the "Signature Extension" field. "T_0" field (8 bits): "T_0" is an NTP timestamp that indicates the time when this session begun. "N" field (8 bits): "N" is an unsigned integer that indicates the number of keys in the current key chain. "i" (Interval Index of K_i) field (8 bits): "i" is an unsigned integer that indicates the interval index associated to the key disclosed in this bootstrap information, K_i. For performance reasons, the sender SHOULD always send a bootstrap information with the highest possible index i since this will reduce the required computation needed to validate key K_j with j > i. But using interval O and K_0 remains valid. In any case, if j is the current interval index, then it is REQUIRED that i < j - d. "K_i" field (variable size): "K_i" is the key corresponding to interval i. If j is the current interval index, then it is REQUIRED that i < j - d. "Signature Extension" (variable size): The "Signature Extension" is mandatory. It is described in Section 4.3.1.1. It's format depends on the "# of certificates" field, and the total length of this extension is given by "Signature length". "P" flag (optional, 1 bit if present): The "P" flag is optional. It is only used in indirect time synchronization mode when the A flag is 1. This flag indicates whether the D^0_t NTP timestamp difference is positive (P==1) or negative (P==0). "D^0_t" field (optional, 63 bits if present): Roca, et al. Expires September 3, 2007 [Page 18] Internet-Draft TESLA in ALC and NORM March 2007 The "D^0_t" field is optional (controlled by the A flag). It is only used in indirect time synchronization mode. It is the upper bound of the lag of the sender's clock with respect to the time reference. When several time references are specified (e.g. several NTP servers), then D^0_t is the maximum upper bound of the lag with each time reference. D^0_t is composed of two unsigned integers, as with NTP timestamps: the first 31 bits give the time difference in seconds (the MSB is used by the P flag), and the remaining 32 bits give the sub-second time difference. ----- Editor's note: a first alternative would be to use floating point arithmetic, IEEE754 for carrying D^0_t. NTP timestamp difference is usually performed with double floating point arithmetic internally (at least in TESLA and NTPv4), so it makes sense. A second alternative would be to use a signed integer representing the difference in sub-second units (e.g. in milliseconds). This is simple but it requires NTP timestamp/ms conversions on both sides. The use of the "P" flag seems simpler... ----- "NTP Extension" (optional, variable size): The "NTP extension" is optional (controlled by the B flag). It is only used in indirect time synchronization mode with (S)NTP synchronization. This extension is described in Section 4.3.1.2. Note that the first seven 32-bit words are mandatory fixed length fields. The K_i and Signature Extension fields are mandatory but variable length fields. The remaining D^0_t and NTP Extension fields are only present in indirect synchronization mode. 4.3.1.1. Signature Extension Format The Signature Extension format can be deduced from the "# Cert" and "Signature Length" field of the Bootstrapping Information generic part. Note that "Signature Length" only refers to the "Signature" field and DOES NOT include the various certificates. Roca, et al. Expires September 3, 2007 [Page 19] Internet-Draft TESLA in ALC and NORM March 2007 The signature extension format when the "# of certificates" field is strictly greater than 0 (2 in this example) is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cert 1 Type | Cert 1 Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Certificate 1 +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cert 2 Type | Cert 2 Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Certificate 2 +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Signature +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Signature extension format when #cert==2. In Figure 2: "Cert Type" (8 bits): The type of certificate identifies the algorithm used for the certificate (see Appendix A). "Cert Length" (8 bits): The certificate length is the length in bytes of the certificate. "Certificate" field (variable size): The certificate field contains a certificate signed by an external authority and that certifies the sender's public key. This field is padded (with 0) up to a multiple of 32 bits. "Signature" field (variable size): The signature field contains a digital signature using the type and length specified in the main part of the bootstrap information message. This field is padded (with 0) up to a multiple of 32 bits. Roca, et al. Expires September 3, 2007 [Page 20] Internet-Draft TESLA in ALC and NORM March 2007 The signature extension format when the "# of certificates" field is zero (i.e. when no certificate is provided) is: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Signature +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Signature extension format when #cert==0. 4.3.1.2. NTP Extension Format In some use cases, when NTP or SNTP is used in indirect time synchronization mode, the session's sender must have a way to indicate to receivers one or more NTP server that will act as time reference (Section 5.1.2). Roca, et al. Expires September 3, 2007 [Page 21] Internet-Draft TESLA in ALC and NORM March 2007 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Total Length | # of Entries | Reserved (0) | Cert Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FQDN 1 Length | Cert 1 Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ NTP Server 1 FQDN +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ NTP Certificate 1 (optional) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FQDN 2 Length | Cert 2 Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ NTP Server 2 FQDN +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ NTP Certificate 2 (optional) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Optional NTP information format, when two NTP servers are specified Figure 4 shows the optional NTP information format, when two NTP servers are specified: "Total Length" field (8 bits): The total length is the total length in units of 32 bit words of this NTP information extension. "# of Entries" field (8 bits): The number of entries is the number of NTP entries. "Cert Type" field (8 bits): The type of certificates identifies the algorithm used for all the certificates that are provided in this NTP Extension (see Appendix A). "FQDN Length" field (8 bits): Roca, et al. Expires September 3, 2007 [Page 22] Internet-Draft TESLA in ALC and NORM March 2007 The FQDN length is the number of bytes of the NTP server fully qualified domain name. "Cert Length" field (8 bits): The certificate length is the length in bytes of the (optional) NTP certificate field. "NTP Server FQDN" field (variable length): The NTP server FQDN is a string containing the NTP server Fully Qualified Domain Name (e.g. "ntp.foo.bar."). This field is padded (with 0) up to a multiple of 32 bits. "NTP Certificate" field (optional, variable size): The NTP Certificate is optional. The content delivery sender can use it to self-certify the NTP public key. The "Certificate Length" field indicates whether this field is present or not. This field is padded (with 0) up to a multiple of 32 bits. ----- Editor's note: Providing only NTP Server FQDN is perhaps too limitative. It should be possible to use either a FQDN or an IPv4/IPv6 address. ----- 4.3.2. Format of a Direct Time Synchronization Response Roca, et al. Expires September 3, 2007 [Page 23] Internet-Draft TESLA in ALC and NORM March 2007 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + t_s (NTP timestamp) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + t_r (NTP timestamp) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Rsvd | # Cert| Signature Type| Signature Length | +-------+-------------------------------------------------------+ | | ~ Signature extension ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Format of a Direct Time Synchronization Response The response to a direct time synchronization request contains the following information: "Rsvd" (Reserved) field (4 bits): This is a reserved field that MUST be set to zero in this specification. "t_s" (NTP timestamp, 64 bits): t_s is an NTP timestamp that corresponds to the sender local time value when receiving the direct time synchronization request message; "t_r" (NTP timestamp, 64 bits): t_r is an NTP timestamp that contains the receiver local time value received in the direct time synchronization request message; "# Cert." (Number of Certificates) field (8 bits): The Number of Certificates indicates the number of certificates present in the signature extension. "Signature Type" field (8 bits): Roca, et al. Expires September 3, 2007 [Page 24] Internet-Draft TESLA in ALC and NORM March 2007 The Signature Type is the reference number (Appendix A) of the digital signature used to authenticate this message and included in the "Signature Extension" field. "Signature Length" field (16 bits): Signature length is an unsigned integer that indicates the signature field size in bytes in the "Signature Extension" field. This is NOT the total length of the "Signature Extension" field. "Signature Extension" (variable size): The signature extension is described in Section 4.3.1.1. It's format depends on the "# of certificates" field, and the total length of this extension is given by "Signature length". 4.3.3. Format of a Standard Authentication Tag 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Disclosed Key K_{i-d} +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Format of the authentication tag Figure 6 shows the format of the authentication tag: "i" optional field (32 bits): i is the interval index associated to the key (K'_i) used to compute the MAC of this packet. This field is optional. It is included if the "I" field within the TESLA EXT_AUTH header extension is 1, and not included if the "I" field is 0. "Disclosed Key" (variable size): Roca, et al. Expires September 3, 2007 [Page 25] Internet-Draft TESLA in ALC and NORM March 2007 The "Disclosed Key" is the key used for interval i-d: K_{i-d}; MAC(K'_i, M) (variable size): MAC(K'_i, M) is the message authentication code of the current packet, including the ALC or NORM header (with the header extensions if any) and the payload (when applicable). 4.3.4. Format of an Authentication Tag with a New Key Chain Commitment During the last N_tx_new_kcc intervals of the current key chain, the sender MUST send a commitment to the next key chain. This is done by replacing the disclosed key of the authentication tag with the new key chain commitment, F(K_{N+1}). Figure 7 shows the corresponding format. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ New Key Commitment F(K_{N+1}) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Format of the authentication tag with a new key chain commitment 4.3.5. Format of an Authentication Tag with an Old Chain Last Key Disclosure During the first N_tx_old_kck intervals of the new key chain after the disclosing interval, d, the sender MUST send a commitment to the old key chain. This is done by replacing the disclosed key of the authentication tag with the last key of the old chain. Figure 8 shows the corresponding format. Roca, et al. Expires September 3, 2007 [Page 26] Internet-Draft TESLA in ALC and NORM March 2007 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Last Key of Old Chain, K_N +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Format of the authentication tag with an old chain last key disclosure Roca, et al. Expires September 3, 2007 [Page 27] Internet-Draft TESLA in ALC and NORM March 2007 5. Receiver Operations 5.1. Initialization of a Receiver A receiver must be initialized before being able to authenticate the source of incoming packets. Two actions must be performed: o receive and process a bootstrap information message, and o calculate an upper bound of the sender's local time, and to that purpose, he must perform time synchronization. 5.1.1. Processing the Bootstrap Information Message A receiver must first receive a packet containing the bootstrap information, digitally signed by the sender, and verify its signature. Because the packet is signed, the receiver also needs to know the public key of the sender. The present document does not specify how the public key of the sender is communicated reliably and in a secure way to all possible receivers. Once the bootstrap information has been verified, the receiver can initialize its TESLA component. The receiver SHOULD then ignore the following bootstrap information messages, if any. There is an exception though: when a new key chain is used and a receiver missed all the commitments for this new key chain, then this latter SHOULD process any new Bootstrap information message. Before TESLA has been initialized, a receiver MUST ignore all packets other than the bootstrap information message. Yet, a receiver MAY chose to buffer incoming packets, recording the reception time of each packet, and proceed with delayed authentication later, once the receiver will be fully initialized. In that case, the buffer must be carefully sized in order to prevent memory starvation (e.g. an attacker who sends faked packets before the session actually starts can exhaust the memory of receivers who do not limit the maximum incoming buffer size). 5.1.2. Time Synchronization First of all, the receiver must know whether the ALC or NORM session relies on direct or indirect time synchronization. This information is communicated by an out-of-band mechanism (for instance when describing the various parameters of a FLUTE session in case of ALC). In some cases, both mechanisms might be acceptable in the same session. Roca, et al. Expires September 3, 2007 [Page 28] Internet-Draft TESLA in ALC and NORM March 2007 5.1.2.1. Direct Time Synchronization In case of a direct time synchronization, a receiver MUST first synchronize with the sender. To that purpose, the receiver sends a direct time synchronization request message. This message includes the local time (NTP timestamp) at the receiver when sending the message. This timestamp will be integrated in the sender's response. Figure 9 details the direct time synchronization message format. 5.1.2.2. Indirect Time Synchronization With the indirect time synchronization method, the sender MAY provide in its bootstrap information, the URL or IP address of the NTP server(s) he trusts along with an OPTIONAL certificate for each NTP server. When several NTP servers are specified, a receiver MUST choose one of them. This document does not specify how the choice is made, but for the sake of scalability, the clients SHOULD NOT use the same server if several possibilities are offered. The NTP synchronization between the NTP server and the receiver MUST be authenticated, either using the certificate provided by the content delivery server, or another certificate the client may obtain for this NTP server. Then the receiver computes the time offset between itself and the NTP server chosen. Note that the receiver does not need to update the local time, since this operation would often require some privileges, computing the time offset is sufficient. Since the offset between the server and the time reference is indicated in the bootstrap information message, the receiver can now calculate an upper bound of the sender's local time. 5.1.3. Format of a Direct Time Synchronization Request 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + t_r (NTP timestamp) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Format of a Direct Time Synchronization Request The direct time synchronization request (Figure 9) contains the following information: Roca, et al. Expires September 3, 2007 [Page 29] Internet-Draft TESLA in ALC and NORM March 2007 "t_r" (NTP timestamp, 64 bits): t_r is an NTP timestamp that contains the receiver local time value when sending this direct time synchronization request message; 5.2. Authentication of Received Packets The receiver can now authenticate incoming packets. To that purpose, he MUST follow different steps ([RFC4082] section 3.5): 1. The receiver parses the different packet headers. If the TESLA authentication tag is not present, the receiver MUST reject the packet. 2. Then proceed with the TESLA safe test: (1) check that the key used to compute the MAC of this packet has not already been disclosed, and (2) check the disclosed key by computing the necessary number of PRF functions to obtain a previously safe disclosed key. If any of these two tests fail, the receiver MUST reject the packet. 3. Then, according to the [draft-ietf-rmt-bb-lct-revised], when applicable, perform congestion control even if the packet has not yet been authenticated. If this feature leads to a potential DoS attack, it does not compromise the security features offered by TESLA and enables a rapid reaction in front of congestion problems. 4. Then buffer the packet for a later authentication, once the corresponding key will be received or deduced from another key. 5. If the disclosed key is a new one, then the receiver can authenticate previously stored packets using this key or any key derived from this one. 6. If a packet fails to be authenticated, then this packet MUST be rejected. 7. If a packet is successfully authenticated, then the receiver continues processing it. ----- Editor's note: [RFC4082] explains that unauthenticated packets SHOULD be destroyed, and if not this is at the own risk of the receiver. We choose the other strategy, requiring that unsafe packets be destroyed when the client decides to use TESLA. But the client can at any time choose to continue an ALC or NORM session in unsafe mode, ignoring TESLA extensions. ----- Roca, et al. Expires September 3, 2007 [Page 30] Internet-Draft TESLA in ALC and NORM March 2007 6. Integration in the ALC and NORM Protocols 6.1. Authentication Header Extension Format The integration of TESLA in ALC or NORM is similar and relies on the header extension mechanism defined in both protocols. More precisely we further specify the EXT_AUTH=1 header extension defined in [draft-ietf-rmt-bb-lct-revised]. ----- Editor's note: All authentication schemes using the EXT_AUTH header extension MUST reserve the same 4 bit "Scheme" field after the HET/HEL fields. This way, several authentication schemes can be used in the same ALC or NORM session. For instance, in case of NORM, TESLA can be used for the downstream traffic while another scheme is used for the upstream traffic. ----- Several fields are added in addition to the HET (Header Extension Type) and HEL (Header Extension Length) fields (Figure 10). 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL |Scheme | V |I| Resvd |Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ ~ | Content | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Format of the TESLA EXT_AUTH header extension. The fields of the TESLA EXT_AUTH header extension are: "Scheme ID" (Authentication Scheme Identifier) field (4 bits): The "Scheme ID" identifies the source authentication scheme or protocol in use. The value 0 is reserved for TESLA. "V" (Version) field (4 bits): "Version" identifies the version number of the TESLA authentication scheme. The value 0 is reserved for the current specification of TESLA. "Resvd" (Reserved) field (5 bits): Roca, et al. Expires September 3, 2007 [Page 31] Internet-Draft TESLA in ALC and NORM March 2007 The "Resvd" field is not used in the current specification and MUST be set to zero by the sender. "I" (Interval Index Present) field (1 bit): The "Interval Index Present" field indicates whether the "Interval Index" field is present (I=1) or not (I=0) in the three authentication information messages (see Section 4.3.3 for Type=1, see Section 4.3.4 for Type=2, see Section 4.3.5> for Type=3). This "I" field MUST be set to zero in all other types. "Type" field (3 bits): The "Type" field identifies the type of TESLA information carried in this header extension. This specification defines the following types: * 0: bootstrap information, sent by the sender periodically or after a direct time synchronization request; * 1: authentication tag for the on-going key chain, sent by the sender along with each packet; * 2: authentication tag with a new key chain commitment, sent by the sender when approaching the end of a key chain; * 3: authentication tag with an old chain last key disclosure, sent by the sender some time after moving to a new key chain; * 4: direct time synchronization request, sent by a NORM receiver. This type of message is invalid in case of an ALC session since ALC is restricted to unidirectional transmissions. Yet an external mechanism may provide the direct time synchronization functionality. How this is done is out of the scope of this document; * 5: direct time synchronization response, sent by a NORM sender. This type of message is invalid in case of an ALC session since ALC is restricted to unidirectional transmissions. Yet an external mechanism may provide the direct time synchronization functionality. How this is done is out of the scope of this document; "Content" field (variable length, multiple of 32 bits): This is the TESLA information carried in the header extension, whose type is given by the "Type" field. Roca, et al. Expires September 3, 2007 [Page 32] Internet-Draft TESLA in ALC and NORM March 2007 All receivers MUST recognize EXT_AUTH but MAY NOT be able to parse its content, for instance because they do not include the TESLA building block. In that case these receivers MUST ignore the EXT_AUTH extensions. In case of NORM, the packets sent by receivers MAY contain a direct synchronization request but MUST NOT contain any of the other four TESLA EXT_AUTH header extensions. 6.2. Use of Authentication Header Extensions Each packet sent by the session's sender MUST contain exactly one TESLA EXT_AUTH header extension. 6.2.1. EXT_AUTH Header Extension of Type Bootstrap Information The "bootstrap information" TESLA EXT_AUTH (Type=0) MUST be sent in a stand-alone control packet, rather than in a packet containing application data. The reason for that is the large size of this bootstrap information. By using stand-alone packets, the maximum payload size of data packets is only affected by the (mandatory) authentication information header extension. With NORM, the "bootstrap information" TESLA EXT_AUTH MUST be sent in a NORM_INFO message. With ALC, the "bootstrap information" TESLA EXT_AUTH MUST be sent in a control packet, i.e. containing no encoding symbol. 6.2.2. EXT_AUTH Header Extension of Type Bootstrap Information The three "authentication information" TESLA EXT_AUTH (Type=1, 2, or 3) MUST be attached to the ALC or NORM packet (data or control packet) that they protect. Depending on the "I" field, two possibilities exist for each of these three messages. Roca, et al. Expires September 3, 2007 [Page 33] Internet-Draft TESLA in ALC and NORM March 2007 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL |Scheme | V |1| 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Disclosed Key K_{i-d} +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Format of the authentication information with the Interval Index field (I=1). 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL |Scheme | V |0| 0 | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Disclosed Key K_{i-d} +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ MAC(K'_i, M) +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: Format of the authentication information without the Interval Index field (I=0). Figure 11 and Figure 12 show the format of the authentication tag respectively with(without) the Interval Index field. 6.2.3. EXT_AUTH Header Extension of Type Direct Time Synchronization Request With NORM, the direct time synchronization request TESLA EXT_AUTH (Type=4) is sent by a receiver in a (TBD) NORM packet (see editor's note below). There is no authentication information header extension in this case since this draft only considers the authentication/ integrity of the packets generated by the session's sender. Roca, et al. Expires September 3, 2007 [Page 34] Internet-Draft TESLA in ALC and NORM March 2007 ----- Editor's note: what type of NORM packet should be used to that purpose? NORM_REPORT is one possibility. ----- With ALC, the direct time synchronization request information cannot be included in an ALC packet, since ALC is restricted to unidirectional transmissions, from the session's sender to the receivers. An external mechanism, out of the scope of this document, must be used with ALC for carrying direct time synchronization requests to the session's sender. 6.2.4. EXT_AUTH Header Extension of Type Direct Time Synchronization Response With NORM, the "direct time synchronization response" TESLA EXT_AUTH MUST be sent in a NORM_INFO message. With ALC, the "direct time synchronization response" TESLA EXT_AUTH MUST be sent in a control packet, i.e. containing no encoding symbol. 6.3. Managing Silent Periods An ALC or NORM sender may stop transmitting packet for some time, for various reasons. It can be the end of the session and all packets have already been sent, or the use-case may consist in a succession of busy periods (when fresh objects are available) followed by silent periods. In both cases, this is an issue since the authentication of the packets sent during the last d intervals requires that the associated keys be revealed, which can only take place after d additional intervals. To resolve this boundary problem, the session's sender MUST sent null packets containing the TESLA EXT_AUTH header extension along with the authentication information (Type=1) for at least d intervals after the end of the regular ALC or NORM packet transmissions. The transmission frequency of these null packets must be sufficient to guaranty that each receiver receives at least that containing the last key with a sufficiently high probability. Roca, et al. Expires September 3, 2007 [Page 35] Internet-Draft TESLA in ALC and NORM March 2007 7. Security Considerations The security of the TESLA protocol is discussed in [RFC4082]. Security considerations specific to its use in ALC and NORM remain TBD... Roca, et al. Expires September 3, 2007 [Page 36] Internet-Draft TESLA in ALC and NORM March 2007 8. Acknowledgments The authors are grateful to Ran Canetti and David L. Mills for their valuable comments when preparing this document. Roca, et al. Expires September 3, 2007 [Page 37] Internet-Draft TESLA in ALC and NORM March 2007 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [RFC3926] Paila, T., Luby, M., Lehtonen, R., Roca, V., and R. Walsh, "FLUTE - File Delivery over Unidirectional Transport", RFC 3926, October 2004. [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005. [draft-ietf-rmt-bb-lct-revised] Luby, M., Watson, M., and L. Vicisano, "Layered Coding Transport (LCT) Building Block", draft-ietf-rmt-bb-lct-revised-05.txt (work in progress), February 2007. [draft-ietf-rmt-pi-alc-revised] Luby, M., Watson, M., and L. Vicisano, "Asynchronous Layered Coding (ALC) Protocol Instantiation", draft-ietf-rmt-pi-alc-revised-04.txt (work in progress), February 2007. [draft-ietf-rmt-pi-norm-revised] Adamson, B., Bormann, C., Handley, M., and J. Macker, "Negative-acknowledgment (NACK)-Oriented Reliable Multicast (NORM) Protocol", draft-ietf-rmt-pi-norm-revised-03.txt (work in progress), September 2006. 9.2. Informative References [Perrig04] Perrig, A. and J. Tygar, "Secure Broadcast Communication in Wired and Wireless Networks", Kluwer Academic Publishers ISBN 0-7923-7650-1, 2004. [RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992. [RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI", RFC 4330, January 2006. Roca, et al. Expires September 3, 2007 [Page 38] Internet-Draft TESLA in ALC and NORM March 2007 [RFC4442] Fries, S. and H. Tschofenig, "Bootstrapping Timed Efficient Stream Loss-Tolerant Authentication (TESLA)", RFC 4442, March 2006. [draft-ietf-ntp-ntpv4-proto] Burbank, J., "The Network Time Protocol Version 4 Protocol Specification", draft-ietf-ntp-ntpv4-proto-01.txt (work in progress), October 2005. Roca, et al. Expires September 3, 2007 [Page 39] Internet-Draft TESLA in ALC and NORM March 2007 Appendix A. IANA Considerations This document requires an IANA registration for the following attributes: Cryptographic pseudo-random function, TESLA-PRF: The currently defined values are: +--------------+-------+ | PRF function | Value | +--------------+-------+ | HMAC-SHA1 | 0 | | | | | HMAC-SHA256 | 1 | +--------------+-------+ Cryptographic message authentication code (MAC): The currently defined values are: +--------------+-------+ | MAC function | Value | +--------------+-------+ | HMAC-SHA1 | 0 | | | | | HMAC-SHA256 | 1 | +--------------+-------+ Signature type: The currently defined values are: +------------------------------------+-------+ | Signature type | Value | +------------------------------------+-------+ | PKCS #1: RSA Cryptography Standard | 0 | +------------------------------------+-------+ Certificate type: The currently defined values are: +------------------------------------+-------+ | Certificate type | Value | +------------------------------------+-------+ | PKCS #1: RSA Cryptography Standard | 0 | +------------------------------------+-------+ Roca, et al. Expires September 3, 2007 [Page 40] Internet-Draft TESLA in ALC and NORM March 2007 Authors' Addresses Vincent Roca INRIA 655, av. de l'Europe Zirst; Montbonnot ST ISMIER cedex 38334 France Email: vincent.roca@inrialpes.fr URI: http://planete.inrialpes.fr/~roca/ Aurelien Francillon INRIA 655, av. de l'Europe Zirst; Montbonnot ST ISMIER cedex 38334 France Email: aurelien.francillon@inrialpes.fr URI: http://planete.inrialpes.fr/~francill/ Sebastien Faurite INRIA 655, av. de l'Europe Zirst; Montbonnot ST ISMIER cedex 38334 France Email: faurite@lcpc.fr Roca, et al. Expires September 3, 2007 [Page 41] Internet-Draft TESLA in ALC and NORM March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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 in BCP 78 and BCP 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 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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Roca, et al. Expires September 3, 2007 [Page 42]