NTP Working Group D. Sibold Internet-Draft PTB Intended status: Standards Track S. Roettger Expires: August 28, 2016 Google Inc. K. Teichel PTB February 25, 2016 Network Time Security draft-ietf-ntp-network-time-security-13 Abstract This document describes Network Time Security (NTS), a collection of measures that enable secure time synchronization with time servers using protocols like the Network Time Protocol (NTP) or the Precision Time Protocol (PTP). Its design considers the special requirements of precise timekeeping which are described in Security Requirements of Time Protocols in Packet Switched Networks [RFC7384]. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on August 28, 2016. Sibold, et al. Expires August 28, 2016 [Page 1] Internet-Draft NTS February 2016 Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4 2.2. Common Terminology for PTP and NTP . . . . . . . . . . . 4 3. Security Threats . . . . . . . . . . . . . . . . . . . . . . 5 4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 6 6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Unicast Time Synchronisation Messages . . . . . . . . . . 7 6.1.1. Preconditions for the Unicast Time Synchronization Exchange . . . . . . . . . . . . . . . . . . . . . . 7 6.1.2. Goals of the Unicast Time Synchronization Exchange . 8 6.1.3. Message Type: "time_request" . . . . . . . . . . . . 8 6.1.4. Message Type: "time_response" . . . . . . . . . . . . 8 6.1.5. Procedure Overview of the Unicast Time Synchronization Exchange . . . . . . . . . . . . . . 9 6.2. Broadcast Time Synchronization Exchange . . . . . . . . . 10 6.2.1. Preconditions for the Broadcast Time Synchronization Exchange . . . . . . . . . . . . . . . . . . . . . . 10 6.2.2. Goals of the Broadcast Time Synchronization Exchange 11 6.2.3. Message Type: "server_broad" . . . . . . . . . . . . 11 6.2.4. Procedure Overview of Broadcast Time Synchronization Exchange . . . . . . . . . . . . . . . . . . . . . . 12 6.3. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 13 6.3.1. Preconditions for the Broadcast Keycheck Exchange . . 13 6.3.2. Goals of the Broadcast Keycheck Exchange . . . . . . 14 6.3.3. Message Type: "client_keycheck" . . . . . . . . . . . 14 6.3.4. Message Type: "server_keycheck" . . . . . . . . . . . 14 6.3.5. Procedure Overview of the Broadcast Keycheck Exchange 15 7. Server Seed, Hash Algorithms and Generating MACs . . . . . . 16 7.1. Server Seed . . . . . . . . . . . . . . . . . . . . . . . 16 Sibold, et al. Expires August 28, 2016 [Page 2] Internet-Draft NTS February 2016 7.2. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 16 7.3. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 17 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 9. Security Considerations . . . . . . . . . . . . . . . . . . . 17 9.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 17 9.2. Initial Verification of the Server Certificates . . . . . 18 9.3. Revocation of Server Certificates . . . . . . . . . . . . 18 9.4. Mitigating Denial-of-Service for broadcast packets . . . 18 9.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 19 9.6. Random Number Generation . . . . . . . . . . . . . . . . 20 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 11.1. Normative References . . . . . . . . . . . . . . . . . . 20 11.2. Informative References . . . . . . . . . . . . . . . . . 21 Appendix A. (informative) TICTOC Security Requirements . . . . . 22 Appendix B. (normative) Inherent Association Protocol Messages . 23 B.1. Overview of NTS with Inherent Association Protocol . . . 23 B.2. Access Message Exchange . . . . . . . . . . . . . . . . . 24 B.2.1. Goals of the Access Message Exchange . . . . . . . . 24 B.2.2. Message Type: "client_access" . . . . . . . . . . . . 24 B.2.3. Message Type: "server_access" . . . . . . . . . . . . 24 B.2.4. Procedure Overview of the Access Exchange . . . . . . 25 B.3. Association Message Exchange . . . . . . . . . . . . . . 25 B.3.1. Goals of the Association Exchange . . . . . . . . . . 25 B.3.2. Message Type: "client_assoc" . . . . . . . . . . . . 26 B.3.3. Message Type: "server_assoc" . . . . . . . . . . . . 26 B.3.4. Procedure Overview of the Association Exchange . . . 27 B.4. Cookie Message Exchange . . . . . . . . . . . . . . . . . 28 B.4.1. Goals of the Cookie Exchange . . . . . . . . . . . . 28 B.4.2. Message Type: "client_cook" . . . . . . . . . . . . . 29 B.4.3. Message Type: "server_cook" . . . . . . . . . . . . . 29 B.4.4. Procedure Overview of the Cookie Exchange . . . . . . 30 B.4.5. Broadcast Parameter Messages . . . . . . . . . . . . 31 Appendix C. (normative) Using TESLA for Broadcast-Type Messages 33 C.1. Server Preparation . . . . . . . . . . . . . . . . . . . 33 C.2. Client Preparation . . . . . . . . . . . . . . . . . . . 35 C.3. Sending Authenticated Broadcast Packets . . . . . . . . . 36 C.4. Authentication of Received Packets . . . . . . . . . . . 36 Appendix D. (informative) Dependencies . . . . . . . . . . . . . 38 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 1. Introduction Time synchronization protocols are increasingly utilized to synchronize clocks in networked infrastructures. Successful attacks against the time synchronization protocol can seriously degrade the reliable performance of such infrastructures. Therefore, time synchronization protocols have to be secured if they are applied in Sibold, et al. Expires August 28, 2016 [Page 3] Internet-Draft NTS February 2016 environments that are prone to malicious attacks. This can be accomplished either by utilization of external security protocols, like IPsec or TLS, or by intrinsic security measures of the time synchronization protocol. The two most popular time synchronization protocols, the Network Time Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP) [IEEE1588], currently do not provide adequate intrinsic security precautions. This document specifies generic security measures which enable these and possibly other protocols to verify the authenticity of the time server/master and the integrity of the time synchronization protocol packets. The utilization of these measures for a given specific time synchronization protocol has to be described in a separate document. [RFC7384] specifies that a security mechanism for timekeeping must be designed in such a way that it does not degrade the quality of the time transfer. This implies that for time keeping the increase in bandwidth and message latency caused by the security measures should be small. Also, NTP as well as PTP work via UDP and connections are stateless on the server/master side. Therefore, all security measures in this document are designed in such a way that they add little demand for bandwidth, that the necessary calculations can be executed in a fast manner, and that the measures do not require a server/master to keep state of a connection. 2. Terminology 2.1. Terms and Abbreviations MITM Man In The Middle NTS Network Time Security TESLA Timed Efficient Stream Loss-tolerant Authentication MAC Message Authentication Code HMAC Keyed-Hash Message Authentication Code 2.2. Common Terminology for PTP and NTP This document refers to different time synchronization protocols, in particular to both the PTP and the NTP. Throughout the document the term "server" applies to both a PTP master and an NTP server. Accordingly, the term "client" applies to both a PTP slave and an NTP client. Sibold, et al. Expires August 28, 2016 [Page 4] Internet-Draft NTS February 2016 3. Security Threats The document "Security Requirements of Time Protocols in Packet Switched Networks" [RFC7384] contains a profound analysis of security threats and requirements for time synchronization protocols. 4. Objectives The objectives of the NTS specification are as follows: o Authenticity: NTS enables the client to authenticate its time server(s). o Integrity: NTS protects the integrity of time synchronization protocol packets via a message authentication code (MAC). o Confidentiality: NTS does not provide confidentiality protection of the time synchronization packets. o Authorization: NTS enables the client to verify its time server's authorization. NTS optionally enables the server to verify the client's authorization as well. o Request-Response-Consistency: NTS enables a client to match an incoming response to a request it has sent. NTS also enables the client to deduce from the response whether its request to the server has arrived without alteration. o Applicability to Protocols: NTS can be used to secure different time synchronization protocols, specifically at least NTP and PTP. o Integration with Protocols: A client or server running an NTS- secured version of a time protocol does not negatively affect other participants who are running unsecured versions of that protocol. o Server-Side Statelessness: All security measures of NTS work without creating the necessity for a server to keep state of a connection. o Prevention of Amplification Attacks: All communication introduced by NTS offers protection against abuse for amplification denial- of-service attacks. Sibold, et al. Expires August 28, 2016 [Page 5] Internet-Draft NTS February 2016 5. NTS Overview NTS initially verifies the authenticity of the time server and exchanges a symmetric key, the so-called cookie, as well as a key input value (KIV). The KIV can be opaque for the client. After the cookie and the KIV are exchanged, the client then uses them to protect the authenticity and the integrity of subsequent unicast-type time synchronization packets. In order to do this, a Message Authentication Code (MAC) is attached to each time synchronization packet. The calculation of the MAC includes the whole time synchronization packet and the cookie which is shared between client and server. The cookie is calculated according to: cookie = MSB_ (HMAC(server seed, KIV)), with the server seed as the key, where KIV is the client's key input value, and where the application of the function MSB_ returns only the b most significant bits. The server seed is a random value of bit length b that the server possesses, which has to remain secret. The cookie deterministically depends on KIV as long as the server seed stays the same. The server seed has to be refreshed periodically in order to provide key freshness as required in [RFC7384]. See Section 7 for details on seed refreshing. Since the server does not keep a state of the client, it has to recalculate the cookie each time it receives a unicast time synchronization request from the client. To this end, the client has to attach its KIV to each request (see Section 6.1). Note: The communication of the KIV and the cookie can be performed between client and server directly, or via a third party key distribution entity. For broadcast-type messages, authenticity and integrity of the time synchronization packets are also ensured by a MAC, which is attached to the time synchronization packet by the sender. Verification of the broadcast-type packets' authenticity is based on the TESLA protocol, in particular on its "not re-using keys" scheme, see Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys, where each key is the output of a one-way function applied to the previous key in the chain. The server securely shares the last element of the chain with all clients. The server splits time into intervals of uniform duration and assigns each key to an interval in reverse order. At each time interval, the server sends a broadcast packet appended by a MAC, calculated using the corresponding key, and the key of the previous disclosure interval. The client verifies the Sibold, et al. Expires August 28, 2016 [Page 6] Internet-Draft NTS February 2016 MAC by buffering the packet until disclosure of the key in its associated disclosure interval occurs. In order to be able to verify the timeliness of the packets, the client has to be loosely time synchronized with the server. This has to be accomplished before broadcast associations can be used. For checking timeliness of packets, NTS uses another, more rigorous check in addition to just the clock lookup used in the TESLA protocol. For a more detailed description of how NTS employs and customizes TESLA, see Appendix C. 6. Protocol Messages This section describes the types of messages needed for secure time synchronization with NTS. For some guidance on how these message types can be realized in practice, and integrated into the communication flow of existing time synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a companion document for NTS. Said document describes ASN.1 encodings for those message parts that have to be added to a time synchronization protocol for security reasons. 6.1. Unicast Time Synchronisation Messages In this message exchange, the usual time synchronization process is executed, with the addition of integrity protection for all messages that the server sends. This message exchange can be repeatedly performed as often as the client desires and as long as the integrity of the server's time responses is verified successfully. 6.1.1. Preconditions for the Unicast Time Synchronization Exchange Before this message exchange is available, there are some requirements that the client and server need to meet: o They MUST negotiate the hash algorithm for the MAC used in the time synchronization messages. Authenticity and integrity of the communication MUST be ensured. o The client MUST know a key input value KIV. Authenticity and integrity of the communication MUST be ensured. o Client and server MUST exchange the cookie (which depends on the KIV as described in section Section 5). Authenticity, confidentiality and integrity of the communication MUST be ensured. One way of realizing these requirements is to use the Association and Cookie Message Exchanges described in Appendix B. Sibold, et al. Expires August 28, 2016 [Page 7] Internet-Draft NTS February 2016 6.1.2. Goals of the Unicast Time Synchronization Exchange The unicast time synchronization exchange: o exchanges time synchronization data as specified by the appropriate time synchronization protocol, o guarantees authenticity and integrity of the request to the server, o guarantees authenticity and integrity of the response to the client, o guarantees request-response-consistency to the client. 6.1.3. Message Type: "time_request" This message is sent by the client when it requests a time exchange. It contains o the NTS message ID "time_request", o the negotiated version number, o a nonce, o the negotiated hash algorithm H, o the client's key input value (for which the client knows the associated cookie), o optional: a MAC (generated with the cookie as key) for verification of all of the above data. 6.1.4. Message Type: "time_response" This message is sent by the server after it has received a time_request message. Prior to this the server MUST recalculate the client's cookie by using the received key input value and the transmitted hash algorithm. The message contains o the NTS message ID "time_response", o the version number as transmitted in time_request, o the server's time synchronization response data, o the nonce transmitted in time_request, Sibold, et al. Expires August 28, 2016 [Page 8] Internet-Draft NTS February 2016 o a MAC (generated with the cookie as key) for verification of all of the above data. 6.1.5. Procedure Overview of the Unicast Time Synchronization Exchange For a unicast time synchronization exchange, the following steps are performed: 1. The client sends a time_request message to the server. The client MUST save the included nonce and the transmit_timestamp (from the time synchronization data) as a correlated pair for later verification steps. Optionally, the client protects the request message with an appended MAC. 2. Upon receipt of a time_request message, the server performs the following steps: * It re-calculates the cookie. * If the request message contains a MAC the server re-calculates the MAC and compares this value with the MAC in the received data. + If the re-calculated MAC does not match the MAC in the received data the server MUST stop the processing of the request. + If the re-calculated MAC matches the MAC in the received data the server continues to process the request. * The server computes the necessary time synchronization data and constructs a time_response message as given in Section 6.1.4. 3. The client awaits a reply in the form of a time_response message. Upon receipt, it checks: * that the transmitted version number matches the one negotiated previously, * that the transmitted nonce belongs to a previous time_request message, * that the transmit_timestamp in that time_request message matches the corresponding time stamp from the synchronization data received in the time_response, and Sibold, et al. Expires August 28, 2016 [Page 9] Internet-Draft NTS February 2016 * that the appended MAC verifies the received synchronization data, version number and nonce. If at least one of the first three checks fails (i.e. if the version number does not match, if the client has never used the nonce transmitted in the time_response message, or if it has used the nonce with initial time synchronization data different from that in the response), then the client MUST ignore this time_response message. If the MAC is invalid, the client MUST do one of the following: abort the run or send another cookie request (because the cookie might have changed due to a server seed refresh). If both checks are successful, the client SHOULD continue time synchronization. +-----------------------+ | o Re-generate cookie | | o Assemble response | | o Generate MAC | +-----------+-----------+ | <-+-> Server -----------------------------------------------> /| \ time_ / \ time_ request / \ response / \| Client -----------------------------------------------> <------ Unicast time ------> <- Client-side -> synchronization validity exchange checks Procedure for unicast time synchronization exchange. 6.2. Broadcast Time Synchronization Exchange 6.2.1. Preconditions for the Broadcast Time Synchronization Exchange Before this message exchange is available, there are some requirements that the client and server need to meet: o The client MUST receive all the information necessary to process broadcast time synchronization messages from the server. This includes * the one-way functions used for building the key chain, Sibold, et al. Expires August 28, 2016 [Page 10] Internet-Draft NTS February 2016 * the last key of the key chain, * time interval duration, * the disclosure delay (number of intervals between use and disclosure of a key), * the time at which the next time interval will start, and * the next interval's associated index. o The communication of the data listed above MUST guarantee authenticity of the server, as well as integrity and freshness of the broadcast parameters to the client. 6.2.2. Goals of the Broadcast Time Synchronization Exchange The broadcast time synchronization exchange: o transmits (broadcast) time synchronization data from the server to the client as specified by the appropriate time synchronization protocol, o guarantees to the client that the received synchronization data has arrived in a timely manner as required by the TESLA protocol and is trustworthy enough to be stored for later checks, o additionally guarantees authenticity of a certain broadcast synchronization message in the client's storage. 6.2.3. Message Type: "server_broad" This message is sent by the server over the course of its broadcast schedule. It is part of any broadcast association. It contains o the NTS message ID "server_broad", o the version number that the server is working under, o time broadcast data, o the index that belongs to the current interval (and therefore identifies the current, yet undisclosed, key), o the disclosed key of the previous disclosure interval (current time interval minus disclosure delay), Sibold, et al. Expires August 28, 2016 [Page 11] Internet-Draft NTS February 2016 o a MAC, calculated with the key for the current time interval, verifying * the message ID, * the version number, and * the time data. 6.2.4. Procedure Overview of Broadcast Time Synchronization Exchange A broadcast time synchronization message exchange consists of the following steps: 1. The server follows the TESLA protocol by regularly sending server_broad messages as described in Section 6.2.3, adhering to its own disclosure schedule. 2. The client awaits time synchronization data in the form of a server_broadcast message. Upon receipt, it performs the following checks: * Proof that the MAC is based on a key that is not yet disclosed (packet timeliness). This is achieved via a combination of checks. First, the disclosure schedule is used, which requires loose time synchronization. If this is successful, the client obtains a stronger guarantee via a key check exchange (see below). If its timeliness is verified, the packet will be buffered for later authentication. Otherwise, the client MUST discard it. Note that the time information included in the packet will not be used for synchronization until its authenticity could also be verified. * The client checks that it does not already know the disclosed key. Otherwise, the client SHOULD discard the packet to avoid a buffer overrun. If this check is successful, the client ensures that the disclosed key belongs to the one-way key chain by applying the one-way function until equality with a previous disclosed key is shown. If it is falsified, the client MUST discard the packet. * If the disclosed key is legitimate, then the client verifies the authenticity of any packet that it has received during the corresponding time interval. If authenticity of a packet is verified, then it is released from the buffer and its time information can be utilized. If the verification fails, then authenticity is not given. In this case, the client MUST request authentic time from the server by means other than Sibold, et al. Expires August 28, 2016 [Page 12] Internet-Draft NTS February 2016 broadcast messages. Also, the client MUST re-initialize the broadcast sequence with a "client_bpar" message if the one-way key chain expires, which it can check via the disclosure schedule. See RFC 4082[RFC4082] for a detailed description of the packet verification process. Server ----------------------------------> \ \ server_ \ broad \| Client ----------------------------------> < Broadcast > <- Client-side -> time sync. validity and exchange timeliness checks Procedure for broadcast time synchronization exchange. 6.3. Broadcast Keycheck This message exchange is performed for an additional check of packet timeliness in the course of the TESLA scheme, see Appendix C. 6.3.1. Preconditions for the Broadcast Keycheck Exchange Before this message exchange is available, there are some requirements that the client and server need to meet: o They MUST negotiate the hash algorithm for the MAC used in the time synchronization messages. Authenticity and integrity of the communication MUST be ensured. o The client MUST know a key input value KIV. Authenticity and integrity of the communication MUST be ensured. o Client and server MUST exchange the cookie (which depends on the KIV as described in section Section 5). Authenticity, confidentiality and integrity of the communication MUST be ensured. These requirements conform to those for the unicast time synchronization exchange. Accordingly, they too can be realized via the Association and Cookie Message Exchanges described in Appendix B (Appendix B). Sibold, et al. Expires August 28, 2016 [Page 13] Internet-Draft NTS February 2016 6.3.2. Goals of the Broadcast Keycheck Exchange The keycheck exchange: o guarantees to the client that the key belonging to the respective TESLA interval communicated in the exchange had not been disclosed before the client_keycheck message was sent. o guarantees to the client the timeliness of any broadcast packet secured with this key if it arrived before client_keycheck was sent. 6.3.3. Message Type: "client_keycheck" A message of this type is sent by the client in order to initiate an additional check of packet timeliness for the TESLA scheme. It contains o the NTS message ID "client_keycheck", o the NTS version number negotiated during association, o a nonce, o an interval number from the TESLA disclosure schedule, o the hash algorithm H negotiated during association, o the client's key input value KIV, and o optional: a MAC (generated with the cookie as key) for verification of all of the above data. 6.3.4. Message Type: "server_keycheck" A message of this type is sent by the server upon receipt of a client_keycheck message during the broadcast loop of the server. Prior to this, the server MUST recalculate the client's cookie by using the received key input value and the transmitted hash algorithm. It contains o the NTS message ID "server_keycheck" o the version number as transmitted in "client_keycheck, o the nonce transmitted in the client_keycheck message, Sibold, et al. Expires August 28, 2016 [Page 14] Internet-Draft NTS February 2016 o the interval number transmitted in the client_keycheck message, and o a MAC (generated with the cookie as key) for verification of all of the above data. 6.3.5. Procedure Overview of the Broadcast Keycheck Exchange A broadcast keycheck message exchange consists of the following steps: 1. The client sends a client_keycheck message. It MUST memorize the nonce and the time interval number that it sends as a correlated pair. 2. Upon receipt of a client_keycheck message the server performs as follows: If the client_keycheck message contains a MAC the server re-calculates the MAC and compares this value with the MAC in the received data. * If the re-calculated MAC does not match the MAC in the received data the server MUST stop the processing of the request. * If the re-calculated MAC matches the MAC in the received data the server continues to process the request: It looks up whether it has already disclosed the key associated with the interval number transmitted in that message. If it has not disclosed it, it constructs and sends the appropriate server_keycheck message as described in Section 6.3.4. For more details, see also Appendix C. 3. The client awaits a reply in the form of a server_keycheck message. On receipt, it performs the following checks: * that the transmitted version number matches the one negotiated previously, * that the transmitted nonce belongs to a previous client_keycheck message, * that the TESLA interval number in that client_keycheck message matches the corresponding interval number from the server_keycheck, and * that the appended MAC verifies the received data. Sibold, et al. Expires August 28, 2016 [Page 15] Internet-Draft NTS February 2016 +----------------------+ | o Assemble response | | o Re-generate cookie | | o Generate MAC | +-----------+----------+ | <-+-> Server ---------------------------------------------> \ /| \ \ server_ client_ / \ server_ \ broad keycheck / \ keycheck \| / \| Client ---------------------------------------------> <-------- Extended broadcast time -------> synchronization exchange <---- Keycheck exchange ---> Procedure for extended broadcast time synchronization exchange. 7. Server Seed, Hash Algorithms and Generating MACs 7.1. Server Seed The server has to calculate a random seed which has to be kept secret. The server MUST generate a seed for each supported hash algorithm, see Section 7.2. According to the requirements in [RFC7384], the server MUST refresh each server seed periodically. Consequently, the cookie memorized by the client becomes obsolete. In this case, the client cannot verify the MAC attached to subsequent time response messages and has to respond accordingly by re-initiating the protocol with a cookie request (Appendix B.4). 7.2. Hash Algorithms Hash algorithms are used for calculation of the cookie and the MAC. The client and the server negotiate a hash algorithm H during the association phase at the beginning. The selected algorithm H MUST be used for all hashing processes in that run. In the TESLA scheme, hash algorithms are used as pseudo-random functions to construct the one-way key chain. Here, the utilized hash algorithm is communicated by the server and is non-negotiable. Note: Any hash algorithm is prone to be compromised in the future. A successful attack on a hash algorithm would enable any NTS Sibold, et al. Expires August 28, 2016 [Page 16] Internet-Draft NTS February 2016 client to derive the server seed from its own cookie. Therefore, the server MUST have separate seed values for its different supported hash algorithms. This way, knowledge gained from an attack on a hash algorithm H can at least only be used to compromise such clients who use hash algorithm H as well. 7.3. MAC Calculation For the calculation of the MAC, client and server MUST use a Keyed- Hash Message Authentication Code (HMAC) as described in [RFC2104]. The HMAC is generated with the hash algorithm specified by the client (see Section 7.2). The input values for the HMAC are the cookie and the content that has to be protected by NTS. 8. IANA Considerations As mentioned, this document generically specifies security measures whose utilization for any given specific time synchronization protocol requires a separate document. Consequently, this document itself does not have any IANA actions (TO BE REVIEWED). 9. Security Considerations Aspects of security for time synchronization protocols are treated throughout this document. For a comprehensive discussion of security requirements in time synchronization contexts, refer to [RFC7384]. See Appendix A for a tabular overview of how NTS deals with those requirements. Additional NTS specific discussion of security issues can be found in the following subsections. Note: Any separate document describing the utilization of NTS to a specific time synchronization protocol may additionally introduce discussion of its own specific security considerations. 9.1. Privacy The payload of time synchronization protocol packets of two-way time transfer approaches like NTP and PTP consists basically of time stamps, which are not considered secret [RFC7384]. Therefore, encryption of the time synchronization protocol packet's payload is not considered in this document. However, an attacker can exploit the exchange of time synchronization protocol packets for topology detection and inference attacks as described in [RFC7624]. To make such attacks more difficult, that draft recommends the encryption of the packet payload. Yet, in the case of time synchronization protocols the confidentiality protection of time synchronization Sibold, et al. Expires August 28, 2016 [Page 17] Internet-Draft NTS February 2016 packet's payload is of secondary importance since the packet's meta data (IP addresses, port numbers, possibly packet size and regular sending intervals) carry more information than the payload. To enhance the privacy of the time synchronization partners, the usage of tunnel protocols such as IPsec and MACsec, where applicable, is therefore more suited than confidentiality protection of the payload. 9.2. Initial Verification of the Server Certificates The client may wish to verify the validity of certificates during the initial association phase. Since it generally has no reliable time during this initial communication phase, it is impossible to verify the period of validity of the certificates. To solve this chicken- and-egg problem, the client has to rely on external means. 9.3. Revocation of Server Certificates According to Section 7, it is the client's responsibility to initiate a new association with the server after the server's certificate expires. To this end, the client reads the expiration date of the certificate during the certificate message exchange (Appendix B.3.3). Furthermore, certificates may also be revoked prior to the normal expiration date. To increase security the client MAY periodically verify the state of the server's certificate via Online Certificate Status Protocol (OCSP) Online Certificate Status Protocol (OCSP) [RFC6960]. 9.4. Mitigating Denial-of-Service for broadcast packets TESLA authentication buffers packets for delayed authentication. This makes the protocol vulnerable to flooding attacks, causing the client to buffer excessive numbers of packets. To add stronger DoS protection to the protocol, the client and the server use the "not re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC 4082 [RFC4082]. In this scheme the server never uses a key for the MAC generation more than once. Therefore, the client can discard any packet that contains a disclosed key it already knows, thus preventing memory flooding attacks. Discussion: Note that an alternative approach to enhance TESLA's resistance against DoS attacks involves the addition of a group MAC to each packet. This requires the exchange of an additional shared key common to the whole group. This adds additional complexity to the protocol and hence is currently not considered in this document. Sibold, et al. Expires August 28, 2016 [Page 18] Internet-Draft NTS February 2016 9.5. Delay Attack In a packet delay attack, an adversary with the ability to act as a MITM delays time synchronization packets between client and server asymmetrically [RFC7384]. This prevents the client from accurately measuring the network delay, and hence its time offset to the server [Mizrahi]. The delay attack does not modify the content of the exchanged synchronization packets. Therefore, cryptographic means do not provide a feasible way to mitigate this attack. However, several non-cryptographic precautions can be taken in order to detect this attack. 1. Usage of multiple time servers: this enables the client to detect the attack, provided that the adversary is unable to delay the synchronization packets between the majority of servers. This approach is commonly used in NTP to exclude incorrect time servers [RFC5905]. 2. Multiple communication paths: The client and server utilize different paths for packet exchange as described in the I-D [I-D.ietf-tictoc-multi-path-synchronization]. The client can detect the attack, provided that the adversary is unable to manipulate the majority of the available paths [Shpiner]. Note that this approach is not yet available, neither for NTP nor for PTP. 3. Usage of an encrypted connection: the client exchanges all packets with the time server over an encrypted connection (e.g. IPsec). This measure does not mitigate the delay attack, but it makes it more difficult for the adversary to identify the time synchronization packets. 4. For unicast-type messages: Introduction of a threshold value for the delay time of the synchronization packets. The client can discard a time server if the packet delay time of this time server is larger than the threshold value. Additional provision against delay attacks has to be taken for broadcast-type messages. This mode relies on the TESLA scheme which is based on the requirement that a client and the broadcast server are loosely time synchronized. Therefore, a broadcast client has to establish time synchronization with its broadcast server before it starts utilizing broadcast messages for time synchronization. One possible way to achieve this initial synchronization is to establish a unicast association with its broadcast server until time synchronization and calibration of the packet delay time is achieved. After that, the client can establish a broadcast association with the Sibold, et al. Expires August 28, 2016 [Page 19] Internet-Draft NTS February 2016 broadcast server and utilizes TESLA to verify integrity and authenticity of any received broadcast packets. An adversary who is able to delay broadcast packets can cause a time adjustment at the receiving broadcast clients. If the adversary delays broadcast packets continuously, then the time adjustment will accumulate until the loose time synchronization requirement is violated, which breaks the TESLA scheme. To mitigate this vulnerability the security condition in TESLA has to be supplemented by an additional check in which the client, upon receipt of a broadcast message, verifies the status of the corresponding key via a unicast message exchange with the broadcast server (see Appendix C.4 for a detailed description of this check). Note that a broadcast client should also apply the above-mentioned precautions as far as possible. 9.6. Random Number Generation At various points of the protocol, the generation of random numbers is required. The employed methods of generation need to be cryptographically secure. See [RFC4086] for guidelines concerning this topic. 10. Acknowledgements The authors would like to thank Tal Mizrahi, Russ Housley, Steven Bellovin, David Mills, Kurt Roeckx, Rainer Bermbach, Martin Langer and Florian Weimer for discussions and comments on the design of NTS. Also, thanks go to Harlan Stenn and Richard Welty for their technical review and specific text contributions to this document. 11. References 11.1. Normative References [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, . Sibold, et al. Expires August 28, 2016 [Page 20] Internet-Draft NTS February 2016 [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, DOI 10.17487/RFC4082, June 2005, . [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, October 2014, . 11.2. Informative References [I-D.ietf-ntp-cms-for-nts-message] Sibold, D., Teichel, K., Roettger, S., and R. Housley, "Protecting Network Time Security Messages with the Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms- for-nts-message-04 (work in progress), July 2015. [I-D.ietf-tictoc-multi-path-synchronization] Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi- Path Time Synchronization", draft-ietf-tictoc-multi-path- synchronization-02 (work in progress), April 2015. [IEEE1588] IEEE Instrumentation and Measurement Society. TC-9 Sensor Technology, "IEEE standard for a precision clock synchronization protocol for networked measurement and control systems", 2008. [Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks against time synchronization protocols", in Proceedings of Precision Clock Synchronization for Measurement Control and Communication, ISPCS 2012, pp. 1-6, September 2012. [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, . [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, . Sibold, et al. Expires August 28, 2016 [Page 21] Internet-Draft NTS February 2016 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013, . [RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T., Trammell, B., Huitema, C., and D. Borkmann, "Confidentiality in the Face of Pervasive Surveillance: A Threat Model and Problem Statement", RFC 7624, DOI 10.17487/RFC7624, August 2015, . [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time Protocols", in Proceedings of Precision Clock Synchronization for Measurement Control and Communication, ISPCS 2013, pp. 1-6, September 2013. Appendix A. (informative) TICTOC Security Requirements The following table compares the NTS specifications against the TICTOC security requirements [RFC7384]. +---------+------------------------------+-------------+------------+ | Section | Requirement from RFC 7384 | Requirement | NTS | | | | level | | +---------+------------------------------+-------------+------------+ | 5.1.1 | Authentication of Servers | MUST | OK | +---------+------------------------------+-------------+------------+ | 5.1.1 | Authorization of Servers | MUST | OK | +---------+------------------------------+-------------+------------+ | 5.1.2 | Recursive Authentication of | MUST | OK | | | Servers (Stratum 1) | | | +---------+------------------------------+-------------+------------+ | 5.1.2 | Recursive Authorization of | MUST | OK | | | Servers (Stratum 1) | | | +---------+------------------------------+-------------+------------+ | 5.1.3 | Authentication and | MAY | Optional, | | | Authorization of Clients | | Limited | +---------+------------------------------+-------------+------------+ | 5.2 | Integrity protection | MUST | OK | +---------+------------------------------+-------------+------------+ | 5.3 | Spoofing Prevention | MUST | OK | +---------+------------------------------+-------------+------------+ | 5.4 | Protection from DoS attacks | SHOULD | OK | | | against the time protocol | | | +---------+------------------------------+-------------+------------+ | 5.5 | Replay protection | MUST | OK | Sibold, et al. Expires August 28, 2016 [Page 22] Internet-Draft NTS February 2016 +---------+------------------------------+-------------+------------+ | 5.6 | Key freshness | MUST | OK | +---------+------------------------------+-------------+------------+ | | Security association | SHOULD | OK | +---------+------------------------------+-------------+------------+ | | Unicast and multicast | SHOULD | OK | | | associations | | | +---------+------------------------------+-------------+------------+ | 5.7 | Performance: no degradation | MUST | OK | | | in quality of time transfer | | | +---------+------------------------------+-------------+------------+ | | Performance: lightweight | SHOULD | OK | | | computation | | | +---------+------------------------------+-------------+------------+ | | Performance: storage | SHOULD | OK | +---------+------------------------------+-------------+------------+ | | Performance: bandwidth | SHOULD | OK | +---------+------------------------------+-------------+------------+ | 5.8 | Confidentiality protection | MAY | NO | +---------+------------------------------+-------------+------------+ | 5.9 | Protection against Packet | MUST | Limited*) | | | Delay and Interception | | | | | Attacks | | | +---------+------------------------------+-------------+------------+ | 5.10 | Secure mode | MUST | OK | +---------+------------------------------+-------------+------------+ | | Hybrid mode | SHOULD | - | +---------+------------------------------+-------------+------------+ *) See discussion in Section 9.5. Comparison of NTS specification against Security Requirements of Time Protocols in Packet Switched Networks (RFC 7384) Appendix B. (normative) Inherent Association Protocol Messages This appendix presents a procedure that performs the association, the cookie, and also the broadcast parameter message exchanges between a client and a server. This procedure is one possible way to achieve the preconditions listed in Sections Section 6.1.1, Section 6.2.1, and Section 6.3.1 while taking into account the objectives given in Section Section 4. B.1. Overview of NTS with Inherent Association Protocol This inherent association protocol applies X.509 certificates to verify the authenticity of the time server and to exchange the cookie. This is done in two separate message exchanges, described Sibold, et al. Expires August 28, 2016 [Page 23] Internet-Draft NTS February 2016 below. An additional required exchange in advance serves to limit the amplification potential of the association message exchange. A client needs a public/private key pair for encryption, with the public key enclosed in a certificate. A server needs a public/ private key pair for signing, with the public key enclosed in a certificate. If a participant intends to act as both a client and a server, it MUST have two different key pairs for these purposes. If this protocol is employed, the hash value of the client's certificate is used as the client's key input value, i.e. the cookie is calculated according to: cookie = MSB_ (HMAC(server seed, H(certificate of client))). The client's certificate contains the client's public key and enables the server to identify the client, if client authorization is desired. B.2. Access Message Exchange This message exchange serves only to prevent the next (association) exchange from being abusable for amplification denial-of-service attacks. B.2.1. Goals of the Access Message Exchange The access message exchange: o transfers a secret value from the server to the client (initiator), o the secret value permits the client to initiate an association message exchange. B.2.2. Message Type: "client_access" This message is sent by a client who intends to perform an association exchange with the server in the future. It contains: o the NTS message ID "client_access". B.2.3. Message Type: "server_access" This message is sent by the server on receipt of a client_access message. It contains: o the NTS message ID "server_access", Sibold, et al. Expires August 28, 2016 [Page 24] Internet-Draft NTS February 2016 o an access key. B.2.4. Procedure Overview of the Access Exchange For an access exchange, the following steps are performed: 1. The client sends a client_access message to the server. 2. Upon receipt of a client_access, the server calculates the access key. It then sends a reply in the form of a server_access message. The server must either memorize the access key or alternatively apply a means by which it can reconstruct the access key. Note that in both cases the access key must be correlated with the address of the requester. Note also that if the server memorizes the access key for a requester, it has to keep state for a certain amount of time. 3. The client waits for a response in the form of a server_access message. Upon receipt of one, it MUST memorize the included access key. B.3. Association Message Exchange In this message exchange, the participants negotiate the hash and encryption algorithms that are used throughout the protocol. In addition, the client receives the certification chain up to a trusted anchor. With the established certification chain the client is able to verify the server's signatures and, hence, the authenticity of future NTS messages from the server is ensured. B.3.1. Goals of the Association Exchange The association exchange: o enables the client to verify any communication with the server as authentic, o lets the participants negotiate NTS version and algorithms, o guarantees authenticity and integrity of the negotiation result to the client, o guarantees to the client that the negotiation result is based on the client's original, unaltered request. Sibold, et al. Expires August 28, 2016 [Page 25] Internet-Draft NTS February 2016 B.3.2. Message Type: "client_assoc" This message is sent by the client if it wants to perform association with a server. It contains o the NTS message ID "client_assoc", o a nonce, o the access key obtained earlier via an access message exchange, o the version number of NTS that the client wants to use (this SHOULD be the highest version number that it supports), o a selection of accepted hash algorithms, and o a selection of accepted encryption algorithms. B.3.3. Message Type: "server_assoc" This message is sent by the server upon receipt of client_assoc. It contains o the NTS message ID "server_assoc", o the nonce transmitted in client_assoc, o the client's proposal for the version number, selection of accepted hash algorithms and selection of accepted encryption algorithms, as transmitted in client_assoc, o the version number used for the rest of the protocol (which SHOULD be determined as the minimum over the client's suggestion in the client_assoc message and the highest supported by the server), o the server's choice of algorithm for encryption and for cryptographic hashing, all of which MUST be chosen from the client's proposals, o a signature, calculated over the data listed above, with the server's private key and according to the signature algorithm which is also used for the certificates that are included (see below), and o a chain of certificates, which starts at the server and goes up to a trusted authority; each certificate MUST be certified by the one directly following it. Sibold, et al. Expires August 28, 2016 [Page 26] Internet-Draft NTS February 2016 B.3.4. Procedure Overview of the Association Exchange For an association exchange, the following steps are performed: 1. The client sends a client_assoc message to the server. It MUST keep the transmitted values for the version number and algorithms available for later checks. 2. Upon receipt of a client_assoc message, the server checks the validity of the included access key. If it is not valid, the server MUST abort communication. If it is valid, the server constructs and sends a reply in the form of a server_assoc message as described in Appendix B.3.3. Upon unsuccessful negotiation for version number or algorithms the server_assoc message MUST contain an error code. 3. The client waits for a reply in the form of a server_assoc message. After receipt of the message it performs the following checks: * The client checks that the message contains a conforming version number. * It checks that the nonce sent back by the server matches the one transmitted in client_assoc, * It also verifies that the server has chosen the encryption and hash algorithms from its proposal sent in the client_assoc message and that this proposal was not altered. * Furthermore, it performs authenticity checks on the certificate chain and the signature. If one of the checks fails, the client MUST abort the run. Sibold, et al. Expires August 28, 2016 [Page 27] Internet-Draft NTS February 2016 +------------------------+ | o Check access key | +------------------------+ | o Choose version | | o Choose algorithms | | o Acquire certificates | | o Assemble response | | o Create signature | +-----------+------------+ | <-+-> Server ---------------------------> /| \ client_ / \ server_ assoc / \ assoc / \| Client ---------------------------> <------ Association -----> exchange Procedure for association and cookie exchange. B.4. Cookie Message Exchange During this message exchange, the server transmits a secret cookie to the client securely. The cookie will later be used for integrity protection during unicast time synchronization. B.4.1. Goals of the Cookie Exchange The cookie exchange: o enables the server to check the client's authorization via its certificate (optional), o supplies the client with the correct cookie and corresponding KIV for its association to the server, o guarantees to the client that the cookie originates from the server and that it is based on the client's original, unaltered request. o guarantees that the received cookie is unknown to anyone but the server and the client. Sibold, et al. Expires August 28, 2016 [Page 28] Internet-Draft NTS February 2016 B.4.2. Message Type: "client_cook" This message is sent by the client upon successful authentication of the server. In this message, the client requests a cookie from the server. The message contains o the NTS message ID "client_cook", o a nonce, o the negotiated version number, o the negotiated signature algorithm, o the negotiated encryption algorithm, o the negotiated hash algorithm H, o the client's certificate. B.4.3. Message Type: "server_cook" This message is sent by the server upon receipt of a client_cook message. The server generates the hash of the client's certificate, as conveyed during client_cook, in order to calculate the cookie according to Section 5. This message contains o the NTS message ID "server_cook" o the version number as transmitted in client_cook, o a concatenated datum which is encrypted with the client's public key, according to the encryption algorithm transmitted in the client_cook message. The concatenated datum contains * the nonce transmitted in client_cook, and * the cookie. o a signature, created with the server's private key, calculated over all of the data listed above. This signature MUST be calculated according to the transmitted signature algorithm from the client_cook message. Sibold, et al. Expires August 28, 2016 [Page 29] Internet-Draft NTS February 2016 B.4.4. Procedure Overview of the Cookie Exchange For a cookie exchange, the following steps are performed: 1. The client sends a client_cook message to the server. The client MUST save the included nonce until the reply has been processed. 2. Upon receipt of a client_cook message, the server checks whether it supports the given cryptographic algorithms. It then calculates the cookie according to the formula given in Section 5. The server MAY use the client's certificate to check that the client is authorized to use the secure time synchronization service. With this, it MUST construct a server_cook message as described in Appendix B.4.3. 3. The client awaits a reply in the form of a server_cook message; upon receipt it executes the following actions: * It verifies that the received version number matches the one negotiated beforehand. * It verifies the signature using the server's public key. The signature has to authenticate the encrypted data. * It decrypts the encrypted data with its own private key. * It checks that the decrypted message is of the expected format: the concatenation of a nonce and a cookie of the expected bit lengths. * It verifies that the received nonce matches the nonce sent in the client_cook message. If one of those checks fails, the client MUST abort the run. Sibold, et al. Expires August 28, 2016 [Page 30] Internet-Draft NTS February 2016 +----------------------------+ | o OPTIONAL: Check client's | | authorization | | o Generate cookie | | o Encrypt inner message | | o Generate signature | +-------------+--------------+ | <-+-> Server ---------------------------> /| \ client_ / \ server_ cook / \ cook / \| Client ---------------------------> <--- Cookie exchange --> Procedure for association and cookie exchange. B.4.5. Broadcast Parameter Messages In this message exchange, the client receives the necessary information to execute the TESLA protocol in a secured broadcast association. The client can only initiate a secure broadcast association after successful association and cookie exchanges and only if it has made sure that its clock is roughly synchronized to the server's. See Appendix C for more details on TESLA. B.4.5.1. Goals of the Broadcast Parameter Exchange The broadcast parameter exchange o provides the client with all the information necessary to process broadcast time synchronization messages from the server, and o guarantees authenticity, integrity and freshness of the broadcast parameters to the client. B.4.5.2. Message Type: "client_bpar" This message is sent by the client in order to establish a secured time broadcast association with the server. It contains o the NTS message ID "client_bpar", Sibold, et al. Expires August 28, 2016 [Page 31] Internet-Draft NTS February 2016 o the NTS version number negotiated during association, o a nonce, and o the signature algorithm negotiated during association. B.4.5.3. Message Type: "server_bpar" This message is sent by the server upon receipt of a client_bpar message during the broadcast loop of the server. It contains o the NTS message ID "server_bpar", o the version number as transmitted in the client_bpar message, o the nonce transmitted in client_bpar, o the one-way functions used for building the key chain, and o the disclosure schedule of the keys. This contains: * the last key of the key chain, * time interval duration, * the disclosure delay (number of intervals between use and disclosure of a key), * the time at which the next time interval will start, and * the next interval's associated index. o The message also contains a signature signed by the server with its private key, verifying all the data listed above. B.4.5.4. Procedure Overview of the Broadcast Parameter Exchange A broadcast parameter exchange consists of the following steps: 1. The client sends a client_bpar message to the server. It MUST remember the transmitted values for the nonce, the version number and the signature algorithm. 2. Upon receipt of a client_bpar message, the server constructs and sends a server_bpar message as described in Appendix B.4.5.3. 3. The client waits for a reply in the form of a server_bpar message, on which it performs the following checks: Sibold, et al. Expires August 28, 2016 [Page 32] Internet-Draft NTS February 2016 * The message must contain all the necessary information for the TESLA protocol, as listed in Appendix B.4.5.3. * The message must contain a nonce belonging to a client_bpar message that the client has previously sent. * Verification of the message's signature. If any information is missing or if the server's signature cannot be verified, the client MUST abort the broadcast run. If all checks are successful, the client MUST remember all the broadcast parameters received for later checks. +---------------------+ | o Assemble response | | o Create public-key | | signature | +----------+----------+ | <-+-> Server ---------------------------------------------> /| \ client_ / \ server_ bpar / \ bpar / \| Client ---------------------------------------------> <------- Broadcast ------> <- Client-side -> parameter validity exchange checks Procedure for unicast time synchronization exchange. Appendix C. (normative) Using TESLA for Broadcast-Type Messages For broadcast-type messages, NTS adopts the TESLA protocol with some customizations. This appendix provides details on the generation and usage of the one-way key chain collected and assembled from [RFC4082]. Note that NTS uses the "not re-using keys" scheme of TESLA as described in Section 3.7.2. of [RFC4082]. C.1. Server Preparation Server setup: Sibold, et al. Expires August 28, 2016 [Page 33] Internet-Draft NTS February 2016 1. The server determines a reasonable upper bound B on the network delay between itself and an arbitrary client, measured in milliseconds. 2. It determines the number n+1 of keys in the one-way key chain. This yields the number n of keys that are usable to authenticate broadcast packets. This number n is therefore also the number of time intervals during which the server can send authenticated broadcast messages before it has to calculate a new key chain. 3. It divides time into n uniform intervals I_1, I_2, ..., I_n. Each of these time intervals has length L, measured in milliseconds. In order to fulfill the requirement 3.7.2. of RFC 4082, the time interval L has to be shorter than the time interval between the broadcast messages. 4. The server generates a random key K_n. 5. Using a one-way function F, the server generates a one-way chain of n+1 keys K_0, K_1, ..., K_{n} according to K_i = F(K_{i+1}). 6. Using another one-way function F', it generates a sequence of n MAC keys K'_0, K'_1, ..., K'_{n-1} according to K'_i = F'(K_i). 7. Each MAC key K'_i is assigned to the time interval I_i. 8. The server determines the key disclosure delay d, which is the number of intervals between using a key and disclosing it. Note that although security is provided for all choices d>0, the choice still makes a difference: * If d is chosen too short, the client might discard packets because it fails to verify that the key used for its MAC has not yet been disclosed. * If d is chosen too long, the received packets have to be buffered for an unnecessarily long time before they can be verified by the client and be subsequently utilized for time synchronization. It is RECOMMENDED that the server calculate d according to d = ceil( 2*B / L) + 1, Sibold, et al. Expires August 28, 2016 [Page 34] Internet-Draft NTS February 2016 where ceil yields the smallest integer greater than or equal to its argument. < - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Generation of Keys F F F F K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <------- K_n | | | | | | | | | F' | F' | F' | F' | | | | v v v v K'_0 K'_1 ... K'_{n-1} K'_n [______________|____ ____|_________________|_______] I_1 ... I_{n-1} I_n Course of Time/Usage of Keys - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -> A schematic explanation of the TESLA protocol's one-way key chain C.2. Client Preparation A client needs the following information in order to participate in a TESLA broadcast: o One key K_i from the one-way key chain, which has to be authenticated as belonging to the server. Typically, this will be K_0. o The disclosure schedule of the keys. This consists of: * the length n of the one-way key chain, * the length L of the time intervals I_1, I_2, ..., I_n, * the starting time T_i of an interval I_i. Typically this is the starting time T_1 of the first interval; * the disclosure delay d. o The one-way function F used to recursively derive the keys in the one-way key chain, o The second one-way function F' used to derive the MAC keys K'_0, K'_1, ... , K'_n from the keys in the one-way chain. Sibold, et al. Expires August 28, 2016 [Page 35] Internet-Draft NTS February 2016 o An upper bound D_t on how far its own clock is "behind" that of the server. Note that if D_t is greater than (d - 1) * L, then some authentic packets might be discarded. If D_t is greater than d * L, then all authentic packets will be discarded. In the latter case, the client SHOULD NOT participate in the broadcast, since there will be no benefit in doing so. C.3. Sending Authenticated Broadcast Packets During each time interval I_i, the server sends at most one authenticated broadcast packet P_i. Such a packet consists of: o a message M_i, o the index i (in case a packet arrives late), o a MAC authenticating the message M_i, with K'_i used as key, o the key K_{i-d}, which is included for disclosure. C.4. Authentication of Received Packets When a client receives a packet P_i as described above, it first checks that it has not already received a packet with the same disclosed key. This is done to avoid replay/flooding attacks. A packet that fails this test is discarded. Next, the client begins to check the packet's timeliness by ensuring that according to the disclosure schedule and with respect to the upper bound D_t determined above, the server cannot have disclosed the key K_i yet. Specifically, it needs to check that the server's clock cannot read a time that is in time interval I_{i+d} or later. Since it works under the assumption that the server's clock is not more than D_t "ahead" of the client's clock, the client can calculate an upper bound t_i for the server's clock at the time when P_i arrived. This upper bound t_i is calculated according to t_i = R + D_t, where R is the client's clock at the arrival of P_i. This implies that at the time of arrival of P_i, the server could have been in interval I_x at most, with x = floor((t_i - T_1) / L) + 1, Sibold, et al. Expires August 28, 2016 [Page 36] Internet-Draft NTS February 2016 where floor gives the greatest integer less than or equal to its argument. The client now needs to verify that x < i+d is valid (see also Section 3.5 of [RFC4082]). If it is falsified, it is discarded. If the check above is successful, the client performs another more rigorous check: it sends a key check request to the server (in the form of a client_keycheck message), asking explicitly if K_i has already been disclosed. It remembers the time stamp t_check of the sending time of that request as well as the nonce it used correlated with the interval number i. If it receives an answer from the server stating that K_i has not yet been disclosed and it is able to verify the HMAC on that response, then it deduces that K_i was undisclosed at t_check and therefore also at R. In this case, the client accepts P_i as timely. Next the client verifies that a newly disclosed key K_{i-d} belongs to the one-way key chain. To this end, it applies the one-way function F to K_{i-d} until it can verify the identity with an earlier disclosed key (see Clause 3.5 in RFC 4082, item 3). Next the client verifies that the transmitted time value s_i belongs to the time interval I_i, by checking T_i =< s_i, and s_i < T_{i+1}. If it is falsified, the packet MUST be discarded and the client MUST reinitialize its broadcast module by performing time synchronization by other means than broadcast messages, and it MUST perform a new broadcast parameter exchange (because a falsification of this check yields that the packet was not generated according to protocol, which suggests an attack). If a packet P_i passes all the tests listed above, it is stored for later authentication. Also, if at this time there is a package with index i-d already buffered, then the client uses the disclosed key K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in package P_{i-d}. Upon success, it regards M_{i-d} as authenticated. Sibold, et al. Expires August 28, 2016 [Page 37] Internet-Draft NTS February 2016 Appendix D. (informative) Dependencies +---------+--------------+--------+-------------------------------+ | Issuer | Type | Owner | Description | +---------+--------------+--------+-------------------------------+ | Server | private key | server | Used for server_assoc, | | PKI | (signature) | | server_cook, server_bpar. | | +--------------+--------+ The server uses the private | | | public key | client | key to sign these messages. | | | (signature) | | The client uses the public | | +--------------+--------+ key to verify them. | | | certificate | server | The certificate is used in | | | | | server_assoc messages, for | | | | | verifying authentication and | | | | | (optionally) authorization. | +---------+--------------+--------+-------------------------------+ | Client | private key | client | The server uses the client's | | PKI | (encryption) | | public key to encrypt the | | +--------------+--------+ content of server_cook | | | public key | server | messages. The client uses | | | (encryption) | | the private key to decrypt | | +--------------+--------+ them. The certificate is | | | certificate | client | sent in client_cook messages, | | | | | where it is used for trans- | | | | | portation of the public key | | | | | as well as (optionally) for | | | | | verification of client | | | | | authorization. | +---------+--------------+--------+-------------------------------+ This table shows the kind of cryptographic resources that NTS participants of server and client role should have ready before NTS communication starts. Sibold, et al. Expires August 28, 2016 [Page 38] Internet-Draft NTS February 2016 ++===========================================++ || || || Secure Authentication and Cookie Exchange || || || ++=======_ _=================================++ | | At least one | successful V ++=======[ ]=======++ || Unicast Time |>-----\ As long as further || Synchronization || | synchronization || Exchange(s) |<-----/ is desired ++=======_ _=======++ | \ Other (unspecified) Sufficient \ / methods which give accuracy \ either or / sufficient accuracy \----------\ /---------/ | | V ++========[ ]=========++ || Broadcast || || Parameter Exchange || ++========_ _=========++ | | One successful | per client V ++=======[ ]=======++ || Broadcast Time |>--------\ As long as further || Synchronization || | synchronization || Reception |<--------/ is desired ++=======_ _=======++ | / \ either / \ or /----------/ \-------------\ | | V V ++========[ ]========++ ++========[ ]========++ || Keycheck Exchange || || Keycheck Exchange || ++===================++ || with TimeSync || ++===================++ This diagram shows the dependencies between the different message exchanges and procedures which NTS offers. Sibold, et al. Expires August 28, 2016 [Page 39] Internet-Draft NTS February 2016 Authors' Addresses Dieter Sibold Physikalisch-Technische Bundesanstalt Bundesallee 100 Braunschweig D-38116 Germany Phone: +49-(0)531-592-8420 Fax: +49-531-592-698420 Email: dieter.sibold@ptb.de Stephen Roettger Google Inc. Email: stephen.roettger@googlemail.com Kristof Teichel Physikalisch-Technische Bundesanstalt Bundesallee 100 Braunschweig D-38116 Germany Phone: +49-(0)531-592-8421 Email: kristof.teichel@ptb.de Sibold, et al. Expires August 28, 2016 [Page 40]