TICTOC Working Group Tal Mizrahi Internet Draft Marvell Intended status: Informational Expires: August 2013 February 7, 2013 Security Requirements of Time Synchronization Protocols in Packet Switched Networks draft-ietf-tictoc-security-requirements-04.txt Abstract As time synchronization protocols are becoming increasingly common and widely deployed, concern about their exposure to various security threats is increasing. This document defines a set of security requirements for time synchronization protocols, focusing on the Precision Time Protocol (PTP) and the Network Time Protocol (NTP). This document also discusses the security impacts of time synchronization protocol practices, the time synchronization performance implications of external security practices, the dependencies between other security services and time synchronization. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 August 7, 2013. Tal Mizrahi Expires August 7, 2013 [Page 1] Internet-Draft Time Protocol Security Requirements February 2013 Copyright Notice Copyright (c) 2013 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. Conventions Used in this Document ............................ 5 2.1. Terminology ............................................. 5 2.2. Abbreviations ........................................... 5 2.3. Common Terminology for PTP and NTP ...................... 5 2.4. Terms used in this Document ............................. 5 3. Security Threats ............................................. 6 3.1. Threat Model ............................................ 7 3.1.1. Internal vs. External Attackers .................... 7 3.1.2. Man in the Middle (MITM) vs. Packet Injector ....... 7 3.2. Threat Analysis.......................................... 8 3.2.1. Packet Interception and Manipulation ............... 8 3.2.2. Spoofing ........................................... 8 3.2.3. Replay Attack ...................................... 8 3.2.4. Rogue Master Attack ................................ 8 3.2.5. Packet Interception and Removal .................... 9 3.2.6. Packet Delay Manipulation .......................... 9 3.2.7. Cryptographic Performance Attacks .................. 9 3.2.8. L2/L3 DoS Attacks .................................. 9 3.2.9. DoS Attacks against the Time Protocol .............. 9 3.2.10. Grandmaster Time Source Spoofing (e.g. GPS fraud) . 9 3.3. Threat Analysis Summary ................................ 10 4. Requirement Levels .......................................... 11 5. Security Requirements ....................................... 12 5.1. Clock Identity Authentication and Authorization ........ 12 5.1.1. Authentication and Authorization of Masters ....... 13 5.1.2. Recursive Authentication and Authorization of Masters (Chain of Trust) ......................................... 14 5.1.3. Authentication and Authorization of Slaves ........ 15 Tal Mizrahi Expires August 7, 2013 [Page 2] Internet-Draft Time Protocol Security Requirements February 2013 5.1.4. PTP: Authentication and Authorization of Transparent Clocks by Master ......................................... 15 5.1.5. PTP: Authentication and Authorization of Control Messages ................................................. 16 5.2. Data integrity ......................................... 17 5.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection 18 5.2.1.1. Hop-by-Hop Integrity Protection .............. 18 5.2.1.2. End-to-End Integrity Protection .............. 19 5.3. Availability ........................................... 19 5.4. Replay Protection ...................................... 20 5.5. Cryptographic Keys and Security Associations ........... 20 5.5.1. Key Freshness ..................................... 20 5.5.2. Security Association .............................. 21 5.5.3. Unicast and Multicast ............................. 21 5.6. Performance ............................................ 22 5.7. Confidentiality......................................... 22 5.8. Protection against Packet Delay and Interception Attacks 23 5.9. Combining Secured with Unsecured Nodes ................. 24 5.9.1. Secure Mode ....................................... 24 5.9.2. Hybrid Mode ....................................... 24 6. Summary of Requirements ..................................... 26 7. Additional security implications ............................ 27 7.1. Security and on-the-fly Timestamping ................... 27 7.2. PTP: Security and Two-Step Timestamping ................ 28 7.3. Intermediate Clocks .................................... 28 7.4. The Effect of External Security Protocols on Time Synchronization ............................................. 29 7.5. External Security Services Requiring Time Synchronization29 7.5.1. Timestamped Certificates .......................... 29 7.5.2. Time Synchronization as a Vulnerability ........... 30 8. Issues for Further Discussion ............................... 30 9. Security Considerations ..................................... 30 10. IANA Considerations......................................... 30 11. Acknowledgments ............................................ 30 12. References ................................................. 30 12.1. Normative References .................................. 30 12.2. Informative References ................................ 31 13. Contributing Authors ....................................... 32 1. Introduction As time synchronization protocols are becoming increasingly common and widely deployed, concern about the resulting exposure to various security threats is increasing. If a time synchronization protocol is compromised, the applications it serves are prone to a range of possible attacks including Denial-of-Service (DoS) or incorrect behavior. Tal Mizrahi Expires August 7, 2013 [Page 3] Internet-Draft Time Protocol Security Requirements February 2013 This document focuses on the security aspects of the Precision Time Protocol (PTP) [IEEE1588] and the Network Time Protocol [NTPv4]. The Network Time Protocol was defined with an inherent security protocol, defined in [NTPv4] and in [AutoKey]. [IEEE1588] includes an experimental security protocol, defined in Annex K of the standard, but this Annex was never formalized into a fully defined security protocol. While NTP includes an inherent security protocol, the absence of a standard security solution for PTP undoubtedly contributed to the wide deployment of unsecured time synchronization solutions. However, in some cases security mechanisms may not be strictly necessary, e.g., due to other security practices in place, or due to the architecture of the network. A time synchronization security solution, much like any security solution, is comprised of various building blocks, and must be carefully tailored for the specific system it is deployed in. Based on a system-specific threat assessment, the benefits of a security solution must be weighed against the potential risks, and based on this tradeoff an optimal security solution can be selected. This document attempts to add clarity to the time synchronization protocol security requirements discussion by addressing a series of questions: (1) What are the threats that need to be addressed for the time synchronization protocol, and thus what security services need to be provided? (e.g. a malicious NTP server or PTP master) (2) What external security practices impact the security and performance of time keeping, and what can be done to mitigate these impacts? (e.g. an IPsec tunnel in the synchronization traffic path) (3) What are the security impacts of time synchronization protocol practices? (e.g. on-the-fly modification of timestamps) (4) What are the dependencies between other security services and time synchronization? (e.g. which comes first - the certificate or the timestamp?) In light of the questions above, this document defines a set of requirements for security solutions for time synchronization protocols, focusing on PTP and NTP. Tal Mizrahi Expires August 7, 2013 [Page 4] Internet-Draft Time Protocol Security Requirements February 2013 2. Conventions Used in this Document 2.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. This document describes security requirements, and thus requirements are phrased in the document in the form "the security mechanism MUST/SHOULD/...". Note, that the phrasing does not imply that this document defines a specific security mechanism, but defines the requirements that every security mechanism should comply to. 2.2. Abbreviations BC Boundary Clock DoS Denial of Service MITM Man In The Middle NTP Network Time Protocol OC Ordinary Clock PTP Precision Time Protocol TC Transparent Clock 2.3. Common Terminology for PTP and NTP This document refers to both PTP and NTP. For the sake of consistency, throughout the document the term "master" applies to both a PTP master and an NTP server. Similarly, the term "slave" applies to both PTP slaves and NTP clients. The general term "clock" refers to masters, slaves and PTP Transparent Clocks (TC). The term "protocol packets" refers generically to PTP and NTP messages. 2.4. Terms used in this Document o Control packets - Packets used by the protocol to exchange information between clocks that is not strictly related to the time. NTP uses NTP Control Messages. PTP uses Announce, Signaling and Management messages. Tal Mizrahi Expires August 7, 2013 [Page 5] Internet-Draft Time Protocol Security Requirements February 2013 o End-to-end security - A security approach where secured packets sent from a source to a destination is not modified by intermediate nodes. o Grandmaster - A master that receives time information from a locally attached clock device, and not through the network. A grandmaster distributes its time to other clocks in the network. o Hop-by-hop security - A security approach where secured packets sent from a source to a destination may be modified by intermediate nodes. In this approach intermediate nodes share the encryption key with the source and destination, allowing them to re-encrypt or re-authenticate modified packets before relaying them to the destination. o Intermediate clock - A clock that receives timing information from a master, and sends timing information to other clocks. In NTP this term refers to an NTP server that is not a Stratum 1 server. In PTP this term refers to a BC or a TC. o Master - A clock that generates timing information to other clocks in the network. In NTP 'master' refers to an NTP server. In PTP 'master' refers to a master OC (aka grandmaster) or to a port of a BC that is in the master state. o Protocol packets - Packets used by the time protocol. The terminology used in this document distinguishes between time packets and control packets. o Secured clock - A clock that supports a security mechanism that complies to the requirements in this document. o Slave - A clock that receives timing information from a master. In NTP 'slave' refers to an NTP client. In PTP 'slave' refers to a slave OC, or to a port of a BC that is in the slave state. o Time packets - Protocol packets carrying time information. o Unsecured clock - A clock that does not support a security mechanism according to the requirements in this document. 3. Security Threats This section discusses the possible attacker types and analyzes various attacks against time synchronization protocols. Tal Mizrahi Expires August 7, 2013 [Page 6] Internet-Draft Time Protocol Security Requirements February 2013 The literature is rich with security threats of time synchronization protocols, e.g., [Traps], [AutoKey], [TM], [SecPTP], and [SecSen]. The threat analysis in this document is mostly based on [TM]. 3.1. Threat Model A time synchronization protocol can be attacked by various types of attackers. The analysis in this document classifies attackers according to 2 criteria, as described in 3.1.1. and 3 .1.2. 3.1.1. Internal vs. External Attackers In the context of internal and external attackers, the underlying assumption is that the time synchronization protocol is secured either by an encryption or an authentication mechanism, or both. Internal attackers either have access to a trusted segment of the network, or possess the encryption or authentication keys. An internal attack can also be performed by exploiting vulnerabilities in devices; for example, by installing malware, or obtaining credentials to reconfigure the device. Thus, an internal attacker can maliciously tamper with legitimate traffic in the network, as well as generate its own traffic and make it appear legitimate to its attacked nodes. External attackers, on the other hand, do not have the keys, and have access only to the encrypted or authenticated traffic. Obviously, in the absence of a security mechanism there is no distinction between internal and external attackers, since all attackers are internal in practice. 3.1.2. Man in the Middle (MITM) vs. Packet Injector MITM attackers are located in a position that allows interception and modification of in-flight protocol packets. It is assumed that an MITM attacker has physical access to a segment of the network, or has gained control of one of the nodes in the network. A traffic injector is not located in an MITM position, but can attack by generating protocol packets. An injector can reside either within the attacked network, or on an external network that is connected to the attacked network. An injector can also potentially eavesdrop on protocol packets sent as multicast, record them and replay them later. Tal Mizrahi Expires August 7, 2013 [Page 7] Internet-Draft Time Protocol Security Requirements February 2013 3.2. Threat Analysis 3.2.1. Packet Interception and Manipulation A packet interception and manipulation attack results when an MITM attacker intercepts timing protocol packets, alters them and relays them to their destination, allowing the attacker to maliciously tamper with the protocol. This can result in a situation where the time protocol is apparently operational but providing intentionally inaccurate information. 3.2.2. Spoofing In spoofing, an attacker masquerades as a legitimate node in the network by generating and transmitting protocol packets. For example, an attacker can impersonate the master, allowing malicious distribution of false timing information. As with packet interception and manipulation, this can result in a situation where the time protocol is apparently operational but providing intentionally inaccurate information. 3.2.3. Replay Attack In a replay attack, an attacker records protocol packets and replays them at a later time without any modification. This can also result in a situation where the time protocol is apparently operational but providing intentionally inaccurate information. 3.2.4. Rogue Master Attack In a rogue master attack, an attacker causes other nodes in the network to believe it is a legitimate master. As opposed to the spoofing attack, in the Rogue Master attack the attacker does not fake its identity, but rather manipulates the master election process using malicious control packets. For example, in PTP, an attacker can manipulate the Best Master Clock Algorithm (BMCA), and cause other nodes in the network to believe it is the most eligible candidate to be a grandmaster. In PTP, a possible variant of this attack is the rogue TC/BC attack. Similar to the rogue master attack, an attacker can cause victims to believe it is a legitimate TC or BC, allowing the attacker to manipulate the time information forwarded to the victims. Tal Mizrahi Expires August 7, 2013 [Page 8] Internet-Draft Time Protocol Security Requirements February 2013 3.2.5. Packet Interception and Removal A packet interception and removal attack results when an MITM attacker intercepts and drops protocol packets, preventing the destination node from receiving the some or all of the protocol packets. 3.2.6. Packet Delay Manipulation In a packet delay manipulation scenario, an MITM attacker intercepts protocol packets, and relays them to their destination after adding a maliciously computed delay. Note that the victim still receives one copy of each packet, contrary to the replay attack, where some or all of the packets may be received by the victim more than once. 3.2.7. Cryptographic Performance Attacks In cryptographic performance attacks, an attacker transmits fake protocol packet, causing high utilization of the cryptographic engine at the receiver, which attempts to verify the integrity of these fake packets. This DoS attack is applicable to all encryption and authentication protocols. However, when the time protocol uses a dedicated security mechanism implemented in a dedicated cryptographic engine, this attack can be applied to cause DoS specifically to the time protocol 3.2.8. L2/L3 DoS Attacks There are many possible Layer 2 and Layer 3 DoS attacks. As the target's availability is compromised, the timing protocol is affected accordingly. 3.2.9. DoS Attacks against the Time Protocol An attacker can attack a clock using an excessive number of time protocol packets, thus degrading the victim's performance. This attack can be implemented, for example, using the attacks described in 3.2.2. and 3 .2.4. 3.2.10. Grandmaster Time Source Spoofing (e.g. GPS fraud) In time source spoofing, an attacker spoofs the accurate time source of the grandmaster. For example, if the grandmaster uses a GPS based Tal Mizrahi Expires August 7, 2013 [Page 9] Internet-Draft Time Protocol Security Requirements February 2013 clock as its reference source, an attacker can spoof GPS satellite signals, causing the grandmaster to use a false reference time. Note that this attack is outside the scope of the time synchronization protocol. While various security measures can be taken to mitigate this attack, these measures are outside the scope of the security requirements defined in this document. 3.3. Threat Analysis Summary The two key factors to a threat analysis are the severity and the likelihood of each of the analyzed attacks. Table 1 summarizes the security attacks presented in 3.2. For each attack, the table specifies its impact, and its applicability to each of the attacker types presented in 3.1. Table 1 clearly shows the distinction between external and internal attackers, and motivates the usage of authentication and integrity protection, significantly reducing the impact of external attackers. The Impact column provides an intuition to the severity of each attack, and the relevant Attacker Type columns provide an intuition about the how difficult each attack is to implement, and hence about the likelihood of each attack. The impact column in Table 1 can have one of 3 values: o DoS - the attack causes denial of service to the attacked node, the impact of which is not restricted to the time synchronization protocol. o Accuracy degradation - the attack yields a degradation in the slave accuracy, but does not completely compromise the slaves' time and frequency. o False time - slaves align to a false time or frequency value due to the attack. Note that if the time synchronization service aligns to a false time, it may cause DoS to other applications that rely on accurate time. However, for the purpose of the analysis in this section we distinguish this implication from 'DoS', which refers to a DoS attack that is not necessarily aimed at the time synchronization protocol. All attacks that have a '+' for 'False Time' implicitly have a '+' for 'Accuracy Degradation'. Tal Mizrahi Expires August 7, 2013 [Page 10] Internet-Draft Time Protocol Security Requirements February 2013 The Attacker Type columns refer to the 4 possible combinations of the attacker types defined in 3.1. +-----------------------------+-------------------++-------------------+ | Attack | Impact || Attacker Type | | +-----+--------+----++---------+---------+ | |False|Accuracy| ||Internal |External | | |Time |Degrad. |DoS ||MITM|Inj.|MITM|Inj.| +-----------------------------+-----+--------+----++----+----+----+----+ |Interception and manipulation| + | | || + | | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Spoofing | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Replay attack | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Rogue master attack | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Interception and Removal | | + | || + | | + | | +-----------------------------+-----+--------+----++----+----+----+----+ |Packet delay manipulation | + | | || + | | + | | +-----------------------------+-----+--------+----++----+----+----+----+ |Crypt. performance attacks | | | + || + | + | + | + | +-----------------------------+-----+--------+----++----+----+----+----+ |L2/L3 DoS attacks | | | + || + | + | + | + | +-----------------------------+-----+--------+----++----+----+----+----+ |Time Protocol DoS attacks | | | + || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Master Time source spoofing | + | | || + | + | + | + | |(e.g. GPS spoofing) | | | || | | | | +-----------------------------+-----+--------+----++----+----+----+----+ Table 1 Threat Analysis - Summary The threats discussed in this section provide the background for the security requirements presented in Section 5 . 4. Requirement Levels The security requirements are presented in Section 5 . Each requirement is defined with a requirement level, in accordance with the requirement levels defined in [KEYWORDS]. Tal Mizrahi Expires August 7, 2013 [Page 11] Internet-Draft Time Protocol Security Requirements February 2013 The requirement levels in this document are affected by the following factors: o Impact: The possible impact of not implementing the requirement, as illustrated in the 'impact' column of Table 1. For example, a requirement that addresses a threat that can be implemented by an external injector is typically a 'MUST', since the threat can be implemented by all the attacker types analyzed in Section 3.1. o Difficulty of the corresponding attack: The level of difficulty of the possible attacks that become possible by not implementing the requirement. The level of difficulty is reflected in the 'Attacker Type' column of Table 1. For example, a requirement that addresses a threat that only compromises the availability of the protocol is typically no more than a 'SHOULD'. o Practical considerations: Various practical factors that may affect the requirement. For example, if a requirement is very difficult to implement, or is applicable to very specific scenarios, these factors may reduce the requirement level. Section 5. lists the requirements. For each requirement there is a short explanation about the reason for its requirement level. 5. Security Requirements This section defines the requirements of security mechanisms used for time synchronization protocols. These requirements are phrased in the form "the security mechanism MUST/SHOULD/MAY...". However, this document does not specify how these requirements can be met. While these requirements can be satisfied by defining explicit security mechanisms for time protocols, at least a subset of the requirements can be met by applying common security practices to the network or by using existing security protocols, such as [IPsec] or [MACsec]. Thus, security solutions that address these requirements are outside the scope of this document. 5.1. Clock Identity Authentication and Authorization Requirement Tal Mizrahi Expires August 7, 2013 [Page 12] Internet-Draft Time Protocol Security Requirements February 2013 The security mechanism MUST provide a means for each clock to authenticate the sender of a protocol packet. Requirement The security mechanism MUST provide a means for each clock to verify that the sender of a protocol packet is authorized to send a packet of this type. Requirement Level The requirements in this subsection address the spoofing attack (Section 3.2.2. ), and the rogue master attack (Section 3 .2.4. ). The requirement level of these requirements is 'MUST' since in the absence of these requirements the protocol is exposed to attacks that are easy to implement and have a high impact. Discussion Authentication refers to verifying the identity of the peer clock. Authorization, on the other hand, refers to verifying that the peer clock is permitted to play the role that it plays in the protocol. For example, some nodes may be permitted to be masters, while other nodes are only permitted to be slaves or TCs. It is noted that while the security mechanism is required to provide an authorization mechanism, the deployment of such a mechanism depends on the nature of the network. For example, a network that deploys PTP may consist of a set of identical OCs, where all clocks are equally permitted to be a master. In such a network an authorization mechanism may not be necessary. The following subsections describe 4 distinct cases of clock authentication. 5.1.1. Authentication and Authorization of Masters Requirement The security mechanism MUST support an authentication mechanism, allowing slaves to authenticate the identity of masters. Requirement The authentication mechanism MUST allow slaves to verify that the authenticated master is authorized to be a master. Tal Mizrahi Expires August 7, 2013 [Page 13] Internet-Draft Time Protocol Security Requirements February 2013 Requirement Level The requirements in this subsection address the spoofing attack (Section 3.2.2. ), and the rogue master attack (Section 3 .2.4. ). The requirement level of these requirements is 'MUST' since in the absence of these requirements the protocol is exposed to attacks that are easy to implement and have a high impact. Discussion Clocks authenticate masters in order to ensure the authenticity of the time source. It is important for a slave to verify the identity of the master, as well as to verify that the master is indeed authorized to be a master. 5.1.2. Recursive Authentication and Authorization of Masters (Chain of Trust) Requirement The security mechanism MUST support recursive authentication and authorization of the master, to be used in cases where time information is conveyed through intermediate clocks. Requirement Level The requirement in this subsection addresses the spoofing attack (Section 3.2.2. ), and the rogue master attack (Section 3 .2.4. ). The requirement level of this requirement is 'MUST' since in the absence of this requirement the protocol is exposed to attacks that are easy to implement and have a high impact. Discussion In some cases a slave is connected to an intermediate clock, that is not the primary time source. For example, in PTP a slave can be connected to a Boundary Clock (BC) or a Transparent Clock (TC), which in turn is connected to a grandmaster. A similar example in NTP is when a client is connected to a stratum 2 server, which is connected to a stratum 1 server. In both the PTP and the NTP cases, the slave authenticates the intermediate clock, and the intermediate clock authenticates the grandmaster. This inductive authentication process is referred to in [AutoKey] as proventication. Tal Mizrahi Expires August 7, 2013 [Page 14] Internet-Draft Time Protocol Security Requirements February 2013 Specifically in PTP, this requirement implies that if a slave is receives time information through a TC, it must authenticate the TC it is attached to, as well as authenticate the master it receives the time information from, as per Section 5.1.1. Similarly, if a TC receives time information through an attached TC, it must authenticate the attached TC. 5.1.3. Authentication and Authorization of Slaves Requirement The security mechanism MAY provide a means for a master to authenticate its slaves. Requirement Level The requirement in this subsection prevents DoS attacks against the master (Section 3.2.9. ). The requirement level of this requirement is 'MAY' since: o Its low impact, i.e., in the absence of this requirement the protocol is only exposed to DoS. o Practical considerations: requiring an NTP server to authenticate its clients may significantly impose on the server's performance. Discussion Slaves are authenticated by masters in order to verify that the slave is authorized to receive timing services from the master. Authentication of slaves prevents unauthorized clocks from receiving time services, and also reduces unnecessary load on the master, by preventing the master from serving unauthorized clocks. It could be argued that the authentication of slaves could put a higher load on the master then serving the unauthorized clock, and hence this requirement is a SHOULD. 5.1.4. PTP: Authentication and Authorization of Transparent Clocks by Master Requirement The security mechanism for PTP MAY provide a means for a master to authenticate the identity of the P2P TCs directly connected to it. Tal Mizrahi Expires August 7, 2013 [Page 15] Internet-Draft Time Protocol Security Requirements February 2013 Requirement Level The requirement in this subsection prevents DoS attacks against the master (Section 3.2.9. ). The requirement level of this requirement is 'MAY' for the same reasons specified in Section 5.1.3. Discussion P2P TCs that are one hop from the master use the PDelay_Req and PDelay_Resp handshake to compute the link delay between the master and TC. These TCs are authenticated by the master. Authentication of TCs, much like authentication of slaves, reduces unnecessary load on the master and peer TCs, by preventing the master from serving unauthorized clocks. 5.1.5. PTP: Authentication and Authorization of Control Messages Requirement The security mechanism for PTP MUST support authentication of Announce messages. The authentication mechanism MUST also verify that the sender is authorized to be a master. Requirement The security mechanism for PTP MUST support authentication and authorization of Management messages. Requirement The security mechanism MAY support authentication and authorization of Signaling messages. Requirement Level The requirements in this subsection address the spoofing attack (Section 3.2.2. ), and the rogue master attack (Section 3 .2.4. ). The requirement level of the first two requirements is 'MUST' since in the absence of these requirements the protocol is exposed to attacks that are easy to implement and have a high impact. Tal Mizrahi Expires August 7, 2013 [Page 16] Internet-Draft Time Protocol Security Requirements February 2013 The requirement level of the third requirement is 'MAY' since its impact greatly depends on the application for which the Signaling messages are used for. Discussion Master election is performed in PTP using the Best Master Clock Algorithm (BMCA). Each Ordinary Clock (OC) announces its clock attributes using Announce messages, and the best master is elected based on the information gathered from all the candidates. Announce messages must be authenticated in order to prevent rogue master attacks (Section 3.2.4. ). Note, that this subsection specifies a requirement that is not necessarily included in Section 5.1.1. or in Section 5.1.3. , since the BMCA is initiated before clocks have been defined as masters or slaves. Management messages are used to monitor or configure PTP clocks. Malicious usage of Management messages enables various attacks, such as the rogue master attack, or DoS attack. Signaling messages are used by PTP clocks to exchange information that is not strictly related to time information or to master selection, such as unicast negotiation. Authentication and authorization of Signaling message may be required in some systems, depending on the application these messages are used for. 5.2. Data integrity Requirement The security mechanism MUST protect the integrity of protocol packets. Requirement Level The requirement in this subsection addresses the packet interception and manipulation attack (Section 3.2.1. ). The requirement level of this requirement is 'MUST' since in the absence of this requirement the protocol is exposed to attacks that are easy to implement and have a high impact. Discussion While Section 5.1. refers to ensuring the identity an authorization of the source of a protocol packet, this subsection refers to ensuring that the packet arrived intact. The integrity protection Tal Mizrahi Expires August 7, 2013 [Page 17] Internet-Draft Time Protocol Security Requirements February 2013 mechanism ensures the authenticity and completeness of data from the data originator. 5.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection Requirement A security mechanism for PTP MUST support hop-by-hop integrity protection. Requirement A security mechanism for PTP SHOULD support end-to-end integrity protection. Requirement Level The requirement in this subsection addresses the packet interception and manipulation attack (Section 3.2.1. ). The requirement level of the first requirement is 'MUST' since in the absence of this requirement the protocol is exposed to attacks that are easy to implement and have a high impact. The requirement level of the first requirement is 'SHOULD' since in the presence of recursive authentication (Section 5.1.2. ) this requirement may be redundant. Discussion Specifically in PTP, when protocol packets are subject to modification by TCs, the integrity protection can be enforced in one of two approaches, end-to-end or hop-by-hop. 5.2.1.1. Hop-by-Hop Integrity Protection Each hop that needs to modify a protocol packet: o Verifies its integrity. o Modifies the packet, i.e., modifies the correctionField. o Re-generates the integrity protection, e.g., re-computes a Message Authentication Code. Tal Mizrahi Expires August 7, 2013 [Page 18] Internet-Draft Time Protocol Security Requirements February 2013 In the hop-by-hop approach, the integrity of protocol packets is protected by induction on the path from the originator to the receiver. This approach is simple, but allows rogue TCs to modify protocol packets. 5.2.1.2. End-to-End Integrity Protection In this approach, the integrity protection is maintained on the path from the originator of a protocol packet to the receiver. This allows the receiver to validate the protocol packet without the ability of intermediate TCs to manipulate the packet. Since TCs need to modify the correctionField, a separate integrity protection mechanism is used specifically for the correctionField. The end-to-end approach limits the TC's impact to the correctionField alone, while the rest of the protocol packet is protected on an end- to-end basis. It should be noted that this approach is more difficult to implement than the hop-by-hop approach, as it requires the correctionField to be protected separately from the other fields of the packet, possibly using different cryptographic mechanisms and keys. 5.3. Availability Requirement The security mechanism SHOULD include measures to mitigate DoS attacks against the time protocol. Requirement Level The requirement in this subsection prevents DoS attacks against the protocol (Section 3.2.9. ). The requirement level of this requirement is 'SHOULD' due to its low impact, i.e., in the absence of this requirement the protocol is only exposed to DoS. Discussion The protocol availability can be compromised by several different attacks. Tal Mizrahi Expires August 7, 2013 [Page 19] Internet-Draft Time Protocol Security Requirements February 2013 An attacker can inject protocol messages to implement the spoofing attack (Section 3.2.2. ) or the rogue master attack (Section 3.2.4. ), causing DoS to the victim (Section 3.2.9. ). An authentication mechanism (Section 5.1. ) limits these attacks strictly to internal attackers, and thus prevents external attackers from performing them. The DoS attacks described in Section 3.2.8. are performed at lower layers than the time synchronization protocol layer, and are thus outside the scope of the security requirements defined in this document. 5.4. Replay Protection Requirement The security mechanism MUST include a replay prevention mechanism. Requirement Level The requirement in this subsection prevents replay attacks (Section 3.2.3. ). The requirement level of this requirement is 'MUST' since in the absence of this requirement the protocol is exposed to attacks that are easy to implement and have a high impact. Discussion The replay attack (Section 3.2.3. ) can compromise both the integrity and availability of the protocol. Common encryption and authentication mechanisms include replay prevention mechanisms that typically use a monotonously increasing packet sequence number. 5.5. Cryptographic Keys and Security Associations 5.5.1. Key Freshness Requirement The cryptographic keys MUST be refreshed periodically. Requirement Level The requirement level of this requirement is 'MUST' since key freshness is an essential property for cryptographic algorithms, as discussed below. Tal Mizrahi Expires August 7, 2013 [Page 20] Internet-Draft Time Protocol Security Requirements February 2013 Discussion Key freshness guarantees that both sides share a common updated secret key. It also helps in preventing replay and playback attacks. Thus, it is important keys to be refreshed periodically. 5.5.2. Security Association Requirement The security protocol SHOULD support an association protocol where: o Two or more clocks authenticate each other. o The clocks generate and agree on a cryptographic session key. Requirement The association protocol SHOULD be periodically invoked. Each instance of the association protocol SHOULD produce a different session key. Requirement Level The requirement level of this requirement is 'SHOULD' since it may be expensive in terms of performance, especially in low-cost clocks. Discussion The security requirements in Section 5.1. and Section 5 .2. require usage of cryptographic mechanisms, deploying cryptographic keys. A security association is an essential building block in these mechanisms. 5.5.3. Unicast and Multicast Requirement The security mechanism SHOULD support security association protocols for unicast and for multicast associations. Requirement Level The requirement level of this requirement is 'SHOULD' since it may be expensive in terms of performance, especially in low-cost clocks. Discussion Tal Mizrahi Expires August 7, 2013 [Page 21] Internet-Draft Time Protocol Security Requirements February 2013 A unicast protocol requires an association protocol between two clocks, whereas a multicast protocol requires an association protocol among two or more clocks, where one of the clocks is a master. 5.6. Performance Requirement The security mechanism MUST be designed in such a way that it does not degrade the quality of the time transfer. Requirement The mechanism SHOULD minimize computational load. Requirement The mechanism also SHOULD minimize storage requirements of client state in the master, nor significantly increase bandwidth consumption. Discussion Performance efficiency is important since client restrictions often dictate a low processing and memory footprint, and because the server may have extensive fan-out. Note that the performance requirements refer to a time- synchronization-specific security mechanism. In systems where a security protocol is used for other types of traffic as well, this document does not place any performance requirements on the security protocol performance. For example, if IPsec encryption is used for securing all information between the master and slave node, including information that is not part of the time protocol, the requirements in this subsection are not necessarily applicable. 5.7. Confidentiality Requirement The security mechanism MAY provide confidentiality protection of the protocol packets. Requirement Level The requirement level of this requirement is 'MAY' since it does not prevent severe threats, as discussed below. Tal Mizrahi Expires August 7, 2013 [Page 22] Internet-Draft Time Protocol Security Requirements February 2013 Discussion In the context of time synchronization, confidentiality is typically of low importance, since timing information is typically not considered secret information. Confidentiality can play an important role when service providers charge their customers for time synchronization services, and thus an encryption mechanism can prevent eavesdroppers from obtaining the service without payment. Note that these cases are rather esoteric. Confidentiality can also prevent an MITM attacker from identifying protocol packets. Thus, confidentiality can assist in protecting the timing protocol against MITM attacks such as packet delay (Section 3.2.6. ), manipulation and interception and removal attacks. Note, that time protocols have predictable behavior even after encryption, such as packet transmission rates and packet lengths. Additional measure can be taken to mitigate encrypted traffic analysis by random padding of encrypted packets and by adding random dummy packets. Nevertheless, encryption does not prevent such MITM attacks, but rather makes these attacks more difficult to implement. 5.8. Protection against Packet Delay and Interception Attacks Requirement The security mechanism SHOULD include means to protect the protocol from MITM attacks that degrade the clock accuracy. Requirement Level The requirements in this subsection address MITM attacks such as the 3.2.1. ). The requirement level of this requirement is 'SHOULD'. In the absence of this requirement the protocol is exposed to attacks that are easy to implement and have a high impact. On the other hand, the implementation of this requirement depends on the topology and properties of the system, and is thus not necessarily applicable to all deployments. Discussion While this document does not define specific security solutions, we note that common practices for protection against MITM attacks use redundant masters (e.g. [NTPv4]), or redundant paths between the Tal Mizrahi Expires August 7, 2013 [Page 23] Internet-Draft Time Protocol Security Requirements February 2013 master and slave (e.g. [DelayAtt]). If one of the time sources indicates a time value that is significantly different than the other sources, it is assumed to be erroneous or under attack, and is therefore ignored. Thus, MITM attack prevention derives a requirement from the security mechanism, and a requirement from the network topology. While the security mechanism should support the ability to detect delay attacks, it is noted that in some networks it is not necessarily possible to provide the redundancy needed for such a detection mechanism. 5.9. Combining Secured with Unsecured Nodes Integrating a security mechanism into a time synchronized system is a complex process, and in some cases may require incremental deployment, where new equipment supports the security mechanism, and is required to interoperate with legacy equipment without the security features. 5.9.1. Secure Mode Requirement The security mechanism MUST support a secure mode, where only secured clocks are permitted to take part in the synchronization protocol. A protocol packet received from an unsecured clock MUST be discarded. Requirement Level The requirement level of this requirement is 'MUST' since the full capacity of the security requirements defined in this document can only be achieved in secure mode. Discussion While the requirement in this subsection is a bit similar to the one in 5.1. , it explicitly defines the secure mode, as opposed to the hybrid mode presented in the next subsection. 5.9.2. Hybrid Mode Requirement The security protocol MAY support a hybrid mode, where both secured and unsecured clocks are permitted to take part in the protocol. Tal Mizrahi Expires August 7, 2013 [Page 24] Internet-Draft Time Protocol Security Requirements February 2013 Requirement Level The requirement level of this requirement is a 'MAY', since it is not necessarily required in all systems. This document recommends to deploy the 'Secure Mode' described in Section 5.9.1. where possible. Discussion The hybrid mode allows both secured and unsecured clocks to take part in the synchronization protocol. NTP, for example, allows a mixture of secured and unsecured nodes. Requirement A master in the hybrid mode SHOULD be a secured clock. A secured slave in the hybrid mode SHOULD discard all protocol packets received from unsecured clocks. Requirement Level The requirement level of this requirement is a 'SHOULD', since it may not be applicable to all deployments. For example, a hybrid network may require the usage of unsecured masters or TCs. Discussion This requirement ensures that the existence of unsecured clocks does not compromise the security provided to secured clocks. Hence, secured slaves only "trust" protocol packets received from a secured clock. An unsecured slave can receive protocol packets either from unsecured clocks, or from secured clocks. Note that the latter does not apply when encryption is used. When integrity protection is used, the unsecured slave can receive secured packets ignoring the integrity protection. Note that the security scheme in [NTPv4] with [AutoKey] does not satisfy this requirement, since nodes prefer the server with the most accurate clock, and not necessarily the server that supports authentication. For example, a stratum 2 server is connected to two stratum 1 servers, Server A, supporting authentication, and server B, without authentication. If server B has a more accurate clock than A, the stratum 2 server chooses server B, in spite of the fact it does not support authentication. Tal Mizrahi Expires August 7, 2013 [Page 25] Internet-Draft Time Protocol Security Requirements February 2013 6. Summary of Requirements +-----------+---------------------------------------------+--------+ | Section | Requirement | Type | +-----------+---------------------------------------------+--------+ | 5.1. | Authentication & authorization of sender. | MUST | | +---------------------------------------------+--------+ | | Authentication & authorization of master. | MUST | | +---------------------------------------------+--------+ | | Recursive authentication & authorization. | MUST | | +---------------------------------------------+--------+ | | Authentication of slaves. | MAY | | +---------------------------------------------+--------+ | | PTP: Authentication of TCs by master. | MAY | | +---------------------------------------------+--------+ | | PTP: Authentication & authorization of | MUST | | | Announce messages. | | | +---------------------------------------------+--------+ | | PTP: Authentication & authorization of | MUST | | | Management messages. | | | +---------------------------------------------+--------+ | | PTP: Authentication & authorization of | MAY | | | Signaling messages. | | +-----------+---------------------------------------------+--------+ | 5.2. | Integrity protection. | MUST | | +---------------------------------------------+--------+ | | PTP: hop-by-hop integrity protection. | MUST | | +---------------------------------------------+--------+ | | PTP: end-to-end integrity protection. | SHOULD | +-----------+---------------------------------------------+--------+ | 5.3. | Protection against DoS attacks. | SHOULD | +-----------+---------------------------------------------+--------+ | 5.4. | Replay protection. | MUST | +-----------+---------------------------------------------+--------+ | 5.5. | Key freshness. | MUST | | +---------------------------------------------+--------+ | | Security association. | SHOULD | | +---------------------------------------------+--------+ | | Unicast and multicast associations. | SHOULD | +-----------+---------------------------------------------+--------+ | 5.6. | Performance: no degradation in quality of | MUST | Tal Mizrahi Expires August 7, 2013 [Page 26] Internet-Draft Time Protocol Security Requirements February 2013 | | time transfer. | | | +---------------------------------------------+--------+ | | Performance: computation load. | SHOULD | | +---------------------------------------------+--------+ | | Performance: storage, bandwidth. | SHOULD | +-----------+---------------------------------------------+--------+ | 5.7. | Confidentiality protection. | MAY | +-----------+---------------------------------------------+--------+ | 5.8. | Protection against delay and interception | SHOULD | | | attacks. | | +-----------+---------------------------------------------+--------+ | 5.9. | Secure mode. | MUST | | +---------------------------------------------+--------+ | | Hybrid mode. | MAY | +-----------+---------------------------------------------+--------+ Table 2 Summary of Security Requirements 7. Additional security implications This section discusses additional implications of the interaction between time synchronization protocols and security mechanisms. This section refers to time synchronization security mechanisms, as well as to "external" security mechanisms, i.e., security mechanisms that are not strictly related to the time synchronization protocol. 7.1. Security and on-the-fly Timestamping Time synchronization protocols often require protocol packets to be modified during transmission. Both NTP and PTP in one-step mode require clocks to modify protocol packets with the time of transmission. In the presence of a security mechanism, whether encryption or integrity protection: o During transmission the security protocol must be applied after integrating the timestamp into the packet. To allow high accuracy, timestamping is typically performed as close to the transmission or reception time as possible. However, since the security engine must be placed between the timestamping function and the physical interface, it may introduce non-deterministic latency that causes accuracy degradation. These performance aspects have been analyzed in the literature, e.g., in [1588IPsec] and [Tunnel]. Tal Mizrahi Expires August 7, 2013 [Page 27] Internet-Draft Time Protocol Security Requirements February 2013 7.2. PTP: Security and Two-Step Timestamping PTP supports a two-step mode of operation, where the time of transmission of protocol packets is communicated without modifying the packets. As opposed to one-step mode, two-step timestamping can be performed without the requirement to encrypt after timestamping. Note that if an encryption mechanism such as IPsec is used, it presents a challenge to the timestamping mechanism, since time protocol packets are encrypted when traversing the physical interface, and are thus impossible to identify. A possible solution to this problem [IPsecSync] is to include an indication in the encryption header that identifies time synchronization packets. 7.3. Intermediate Clocks A time synchronization protocol allows slaves to receive time information from an accurate time source. Time information is sent over a path that often traverses one or more intermediate clocks. o In NTP, time information originated from a stratum 1 server can be distributed to stratum 2 servers, and in turn distributed from the stratum 2 servers to NTP clients. In this case, the stratum 2 servers are a layer of intermediate clocks. o In PTP, BCs and TCs are intermediate nodes used to improve the accuracy of time information conveyed between the grandmaster and the slaves. A common rule of thumb in network security is that end-to-end security is the best policy, as it secures the entire path between the data originator and its receiver. The usage of intermediate nodes implies that if a security mechanism is deployed in the network, all intermediate nodes must possess the security key (hop-by-hop security) since they must be able to send time information to the slaves, or to modify time information sent through them. This inherent property of using intermediate clocks increases the system's exposure to internal threats, as there is a large number of nodes that possess the security keys. Tal Mizrahi Expires August 7, 2013 [Page 28] Internet-Draft Time Protocol Security Requirements February 2013 Thus, there is a tradeoff between the achievable clock accuracy of a system, and the robustness of its security solution. On one hand high clock accuracy calls for hop-by-hop involvement in the protocol, also known as on-path support. On the other hand, a robust security solution calls for end-to-end data protection. 7.4. The Effect of External Security Protocols on Time Synchronization Time synchronization protocols are often deployed in systems that use security mechanisms and protocols. A typical example is the 3GPP Femtocell network [3GPP], where IPsec is used for securing traffic between a Femtocell and the Femto Gateway. In some cases, all traffic between these two nodes may be secured by IPsec, including the time synchronization protocol traffic. This use-case is thoroughly discussed in [IPsecSync]. Another typical example is the usage of MACsec encryption ([MACsec]) in L2 networks that deploy time synchronization [AvbAssum]. The usage of external security mechanisms may affect time synchronization protocols as follows: o Timestamping accuracy can be affected, as described in 7.1. o If traffic is secured between two nodes in the network, no intermediate clocks can be used between these two nodes. In the [3GPP] example, if traffic between the Femtocell and the Femto Gateway is encrypted, then time protocol packets are sent over the underlying network without modification, and thus cannot enjoy the improved accuracy provided by intermediate clock nodes. 7.5. External Security Services Requiring Time Synchronization 7.5.1. Timestamped Certificates Certificate validation requires the sender and receiver to be roughly time synchronized. Thus, synchronization is required for establishing security protocols such as IKEv2 and TLS. An even stronger interdependence between a time synchronization protocol and a security mechanism is defined in [AutoKey], which defines mutual dependence between the acquired time information, and the authentication protocol that secures it. This bootstrapping behavior results from the fact that trusting the received time information requires a valid certificate, and validating a certificate requires knowledge of the time. Tal Mizrahi Expires August 7, 2013 [Page 29] Internet-Draft Time Protocol Security Requirements February 2013 7.5.2. Time Synchronization as a Vulnerability Cryptographic protocols often use time as an important factor in the cryptographic algorithm. If a time synchronization protocol is compromised, it may consequently cause expose the security protocols that rely on it to various attacks. For example, a successful attack on a time synchronization protocol may cause the attacked clocks to be synchronized to an early time. This erroneous time may expose cryptographic algorithms that rely on time to replay attacks. 8. Issues for Further Discussion The key distribution is outside the scope of this document. Although this is an essential element of any security system, it is outside the scope of this document. 9. Security Considerations The security considerations of network timing protocols are presented throughout this document. 10. IANA Considerations There are no new IANA considerations implied by this document. 11. Acknowledgments The authors gratefully acknowledge Stefano Ruffini, Doug Arnold, Kevin Gross, Dieter Sibold, Dan Grossman and Laurent Montini for their thorough review and helpful comments. The authors would also like to thank members of the TICTOC WG for providing feedback on the TICTOC mailing list. This document was prepared using 2-Word-v2.0.template.dot. 12. References 12.1. Normative References [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [NTPv4] Mills, D., Martin, J., Burbank, J., Kasch, W., "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. Tal Mizrahi Expires August 7, 2013 [Page 30] Internet-Draft Time Protocol Security Requirements February 2013 [AutoKey] Haberman, B., Mills, D., "Network Time Protocol Version 4: Autokey Specification", RFC 5906, June 2010. [IEEE1588] IEEE TC 9 Instrumentation and Measurement Society, "1588 IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems Version 2", IEEE Standard, 2008. 12.2. Informative References [Traps] Treytl, A., Gaderer, G., Hirschler, B., Cohen, R., "Traps and pitfalls in secure clock synchronization" in Proceedings of 2007 International Symposium for Precision Clock Synchronization for Measurement, Control and Communication, ISPCS 2007, pp. 18-24, 2007. [TM] T. Mizrahi, "Time synchronization security using IPsec and MACsec", ISPCS 2011, pp. 38-43, 2011. [SecPTP] J. Tsang, K. Beznosov, "A security analysis of the precise time protocol (short paper)," 8th International Conference on Information and Communication Security (ICICS 2006), pp. 50-59, 2006. [SecSen] S. Ganeriwal, C. Popper, S. Capkun, M. B. Srivastava, "Secure Time Synchronization in Sensor Networks", ACM Trans. Info. and Sys. Sec., Volume 11, Issue 4, July 2008. [AvbAssum] D. Pannell, "Audio Video Bridging Gen 2 Assumptions", IEEE 802.1 AVB Plenary, work in progress, May 2012. [IPsecSync] Y. Xu, "IPsec security for packet based synchronization", IETF, draft-xu-tictoc-ipsec- security-for-synchronization (work in progress), 2011. [3GPP] 3GPP, "Security of Home Node B (HNB) / Home evolved Node B (HeNB)", 3GPP TS 33.320 10.4.0 (work in progress), 2011. [1588IPsec] A. Treytl, B. Hirschler, "Securing IEEE 1588 by IPsec tunnels - An analysis", in Proceedings of 2010 International Symposium for Precision Clock Synchronization for Measurement, Control and Communication, ISPCS 2010, pp. 83-90, 2010. Tal Mizrahi Expires August 7, 2013 [Page 31] Internet-Draft Time Protocol Security Requirements February 2013 [Tunnel] A. Treytl, B. Hirschler, and T. Sauter, "Secure tunneling of high precision clock synchronisation protocols and other timestamped data", in Proceedings of the 8th IEEE International Workshop on Factory Communication Systems (WFCS), vol. ISBN 978-1-4244- 5461-7, pp. 303-313, 2010. [DelayAtt] T. Mizrahi, "A Game Theoretic Analysis of Delay Attacks against Time Synchronization Protocols", accepted, to appear in Proceedings of the International IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication, ISPCS, 2012. [MACsec] IEEE 802.1AE-2006, "IEEE Standard for Local and Metropolitan Area Networks - Media Access Control (MAC) Security", 2006. [IPsec] S. Kent, K. Seo, "Security Architecture for the Internet Protocol", IETF, RFC 4301, 2005. 13. Contributing Authors Karen O'Donoghue ISOC Email: odonoghue@isoc.org Authors' Addresses Tal Mizrahi Marvell 6 Hamada St. Yokneam, 20692 Israel Email: talmi@marvell.com Tal Mizrahi Expires August 7, 2013 [Page 32]