Network Time Security
draft-ietf-ntp-network-time-security-05.txt

Abstract

This document describes the Network Time Security (NTS) protocol that enables secure time synchronization with time servers using Network Time Protocol (NTP) or 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 April 26, 2015.

Copyright Notice

Copyright (c) 2014 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
• 2. Security Threats
• 3. Objectives
• 4. Terms and Abbreviations
• 5. NTS Overview
• 5.1. Symmetric and Client/Server Mode
• 5.2. Broadcast Mode
• 6. Protocol Messages
• 6.1. Association Messages
• 6.1.1. Message Type: "client_assoc"
• 6.1.2. Message Type: "server_assoc"
• 6.2. Cookie Messages
• 6.2.1. Message Type: "client_cook"
• 6.2.2. Message Type: "server_cook"
• 6.3. Unicast Time Synchronisation Messages
• 6.3.1. Message Type: "time_request"
• 6.3.2. Message Type: "time_response"
• 6.4. Broadcast Parameter Messages
• 6.4.1. Message Type: "client_bpar"
• 6.4.2. Message Type: "server_bpar"
• 6.5. Broadcast Messages
• 6.5.1. Message Type: "server_broad"
• 6.6. Broadcast Key Check
• 6.6.1. Message Type: "client_keycheck"
• 6.6.2. Message Type: "server_keycheck"
• 7. Protocol Sequence
• 7.1. The Client
• 7.1.1. The Client in Unicast Mode
• 7.1.2. The Client in Broadcast Mode
• 7.2. The Server
• 7.2.1. The Server in Unicast Mode
• 7.2.2. The Server in Broadcast Mode
• 8. Server Seed Considerations
• 8.1. Server Seed Refresh
• 8.2. Server Seed Algorithm
• 8.3. Server Seed Lifetime
• 9. Hash Algorithms and MAC Generation
• 9.1. Hash Algorithms
• 9.2. MAC Calculation
• 10. IANA Considerations
• 11. Security Considerations
• 11.1. Initial Verification of the Server Certificates
• 11.2. Revocation of Server Certificates
• 11.3. Usage of NTP Pools
• 11.4. Denial-of-Service in Broadcast Mode
• 11.5. Delay Attack
• 12. Acknowledgements
• 13. References
• 13.1. Normative References
• 13.2. Informative References
• Appendix A. Flow Diagrams of Client Behaviour
• Appendix B. TICTOC Security Requirements
• Appendix C. Broadcast Mode
• C.1. Server Preparations
• C.2. Client Preparation
• C.3. Sending Authenticated Broadcast Packets
• C.4. Authentication of Received Packets
• Appendix D. Random Number Generation
• Authors' Addresses

1. Introduction

Time synchronization protocols are increasingly utilized to synchronize clocks in networked infrastructures. The reliable performance of such infrastructures can be degraded seriously by successful attacks against the time synchronization protocol. Therefore, time synchronization protocols have to be secured if they are applied in environments that are prone to malicious attacks. This can be accomplished by utilization of external security protocols like IPsec 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 security measures for NTP and PTP which enable these protocols to verify authenticity of the time server and integrity of the time synchronization protocol packets.

The protocol is specified with the prerequisite in mind that precise timekeeping can only be accomplished with stateless time synchronization communication, which excludes the utilization of standard security protocols like IPsec or TLS for time synchronization messages. This prerequisite corresponds with the requirement that a security mechanism for timekeeping must be designed in such a way that it does not degrade the quality of the time transfer [RFC7384].

Note:

The intent is to formulate the protocol to be applicable to NTP and also PTP. In the current state the specification focuses on the application to NTP.

2. Security Threats

A profound analysis of security threats and requirements for NTP and PTP can be found in the "Security Requirements of Time Protocols in Packet Switched Networks" [RFC7384].

3. Objectives

The objectives of the NTS specification are as follows:

• Authenticity: NTS enables the client to authenticate its time servers.
• Integrity: NTS protects the integrity of time synchronization protocol packets via a message authentication code (MAC).
• Confidentiality: NTS does not provide confidentiality protection of the time synchronization packets.
• Modes of operation: All operational modes of NTP are supported.
• Operational modes of PTP should be supported as far as possible.
• Hybrid mode: Both secure and insecure communication modes are possible for NTP servers and clients, respectively.
• Compatibility:
  • Unsecured NTP associations shall not be affected.
  • An NTP server that does not support NTS shall not be affected by NTS authentication requests.

4. Terms and Abbreviations

MITM

Man In The Middle

NTP

Network Time Protocol [RFC5905]

NTS

Network Time Security

PTP

Precision Time Protocol [IEEE1588]

TESLA

Timed Efficient Stream Loss-Tolerant Authentication

5. NTS Overview

5.1. Symmetric and Client/Server Mode

NTS applies X.509 certificates to verify the authenticity of the time server and to exchange a symmetric key, the so-called cookie. This cookie is then used to protect authenticity and integrity of the subsequent time synchronization packets by means of a Message Authentication Code (MAC), which 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:[RFC7384]. See Section 8 for details on the seed refresh and Section 7.1.1 for the client's reaction to it.

cookie = MSB_128 (HMAC(server seed, H(certificate of client))),

with the server seed as key, where H is a hash function, and where the function MSB_128 cuts off the 128 most significant bits of the result of the HMAC function. The server seed is a 128 bit random value of the server, which has to be kept secret. The cookie never changes as long as the server seed stays the same, but the server seed has to be refreshed periodically in order to provide key freshness as required in

The server does not keep a state of the client. Therefore it has to recalculate the cookie each time it receives a request from the client. To this end, the client has to attach the hash value of its certificate to each request (see Section 6.3).

5.2. Broadcast Mode

Just as in the case of the client server mode and symmetric mode, authenticity and integrity of the NTP packets are ensured by a MAC, which is attached to the NTP packet by the sender. Verification of the 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 last element of the chain is shared securely with all clients. The server splits time into intervals of uniform duration and assigns each key to an interval in reverse order, starting with the penultimate. At each time interval, the server sends an NTP broadcast packet appended by a MAC, calculated using the corresponding key, and the key of the previous disclosure interval. The client verifies the MAC by buffering the packet until the disclosure of the key in its associated disclosure interval. In order to be able to verify the validity of the key, the client has to be loosely time synchronized to the server. This has to be accomplished during the initial client server exchange between broadcast client and server. In addition, NTS uses another, more rigorous check to what is 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, for use with existing time synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-messages], 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 as well as CMS (Cryptographic Message Syntax, see [RFC5652]) conventions that can be used to get the cryptographic aspects right.

Note that currently, the companion document describes realizations of NTS messages only for utilization with NTP, in which the NTS specific data are enclosed in extension fields on top of NTP packets. A specification of NTS messages for PTP will have to be developed accordingly.

The steps described in Section 6.1 - Section 6.3 belong to the unicast mode, while Section 6.4 and Section 6.5 explain the steps involved in the broadcast mode of NTS.

6.1. Association Messages

In this message exchange, the hash and encryption algorithms that are used throughout the protocol are negotiated. Also, 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, authenticity of future NTS messages from the server is ensured.

6.1.1. Message Type: "client_assoc"

The protocol sequence starts with the client sending an association message, called client_assoc. This message contains

• the NTS message ID "client_assoc",
• the version number of NTS that the client wants to use (this SHOULD be the highest version number that it supports),
• the hostname of the client,
• a selection of accepted hash algorithms, and
• a selection of accepted encryption algorithms.

6.1.2. Message Type: "server_assoc"

This message is sent by the server upon receipt of client_assoc. It contains

• the NTS message ID "server_assoc",
• 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),
• the hostname of the server, and
• the server's choice of algorithm for encryption and for cryptographic hashing, all of which MUST be chosen from the client's proposals.
• 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 which are included (see below),
• a chain of certificates, which starts at the server and goes up to a trusted authority, and each certificate MUST be certified by the one directly following it.

6.2. Cookie Messages

During this message exchange, the server transmits a secret cookie to the client securely. The cookie will be used for integrity protection during unicast time synchronization.

6.2.1. 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

• the NTS message ID "client_cook",
• the negotiated version number,
• the negotiated signature algorithm,
• the negotiated encryption algorithm,
• a 128-bit nonce,
• the negotiated hash algorithm H,
• the client's certificate.

6.2.2. 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 5.1. This message contains

• the NTS message ID "server_cook"
• the version number as transmitted in client_cook,
• 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.
• 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.

6.3. 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 can be repeatedly exchanged as often as the client desires and as long as the integrity of the server's time responses is verified successfully.

6.3.1. Message Type: "time_request"

This message is sent by the client when it requests time exchange. It contains

• the NTS message ID "time_request",
• the negotiated version number,
• a 128-bit nonce,
• the negotiated hash algorithm H,
• the hash of the client's certificate under H.

6.3.2. Message Type: "time_response"

This message is sent by the server, after it received a time_request message. Prior to this the server MUST recalculate the client's cookie by using the hash of the client's certificate and the transmitted hash algorithm. The message contains

• the NTS message ID "time_response",
• the version number as transmitted in time_request,
• the server's time synchronization response data,
• the 128-bit nonce transmitted in time_request,
• a MAC (generated with the cookie as key) for verification of all of the above data.

6.4. 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 a successful unicast run, see Section 7.1.2.

See Appendix C for more details on TESLA.

6.4.1. 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

• the NTS message ID "client_bpar",
• the version number negotiated during association in unicast mode,
• the client's hostname, and
• the signature algorithm negotiated during unicast.

6.4.2. 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

• the NTS message ID "server_bpar",
• the version number as transmitted in the client_bpar message,
• the one-way functions used for building the key chain, and
• 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.
• The message also contains a signature signed by the server with its private key, verifying all the data listed above.

6.5. Broadcast Messages

Via this message, the server keeps sending broadcast time synchronization messages to all participating clients.

6.5.1. 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

• the NTS message ID "server_broad",
• the version number that the server's broadcast mode is working under,
• time broadcast data,
• the index that belongs to the current interval (and therefore identifies the current, yet undisclosed key),
• the disclosed key of the previous disclosure interval (current time interval minus disclosure delay),
• a MAC, calculated with the key for the current time interval, verifying
  • the message ID,
  • the version number, and
  • the time data.

6.6. Broadcast Key Check

This message exchange is performed for an additional check of packet timeliness in the course of the TESLA scheme, see Appendix C.

6.6.1. 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

• the NTS message ID "client_keycheck",
• the version number chosen for the broadcast,
• a 128-bit nonce,
• an interval number from the TESLA disclosure schedule,
• the hash algorithm H negotiated in unicast mode, and
• the hash of the client's certificate under H.

6.6.2. 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 hash of the client's certificate and the transmitted hash algorithm. It contains

• the NTS message ID "server_keycheck"
• the version number that the server's broadcast mode is working under,
• the 128-bit nonce transmitted in the client_keycheck message,
• the interval number transmitted in the client_keycheck message, and
• a MAC (generated with the cookie as key) for verification of all of the above data.

7. Protocol Sequence

7.1. The Client

7.1.1. The Client in Unicast Mode

For a unicast run, the client performs the following steps: Figure 1.

1. It sends a client_assoc message to the server. It MUST keep the transmitted values for version number and algorithms available for later checks.
2. It 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 conform version number.
   • It also verifies that the server has chosen the encryption and hash algorithms from its proposal sent in the client_assoc message.
   • Furthermore, it performs authenticity checks on the certificate chain and the signature for the version number.

   If one of the checks fails, the client MUST abort the run. Discussion:

   Note that by performing the above message exchange and checks, the client validates the authenticity of its immediate NTP server only. It does not recursively validate the authenticity of each NTP server on the time synchronization chain. Recursive authentication (and authorization) as formulated in [RFC7384] depends on the chosen trust anchor.

3. Next, it sends a client_cook message to the server. The client MUST save the included nonce until the reply has been processed.
4. It 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 before.
   • 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 128 bit nonce and a 128 bit cookie.
   • 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.

5. 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.
6. It awaits a reply in the form of a time_response message. Upon receipt, it checks:
   • that the transmitted version number matches the one negotiated before,
   • 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
   • 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 go back to step 5 (because the cookie might have changed due to a server seed refresh). If both checks are successful, the client SHOULD continue time synchronization by going back to step 7.

The client's behavior in unicast mode is also expressed in

7.1.2. The Client in Broadcast Mode

To establish a secure broadcast association with a broadcast server, the client MUST initially authenticate the broadcast server and securely synchronize its time to it up to an upper bound for its time offset in unicast mode. After that, the client performs the following steps: Section 6.4.1 if the one-way key chain expires.

1. It sends a client_bpar message to the server. It MUST remember the transmitted values for version number and signature algorithm.
2. It waits for a reply in the form of a server_bpar message after which it performs the following checks:
   • The message must contain all the necessary information for the TESLA protocol, as listed in Section 6.4.2.
   • Verification of the message's signature.

   If any information is missing or 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.

3. The client awaits time synchronization data in the form of a server_broadcast message. Upon receipt, it performs the following checks:
   1. 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 the loose time synchronization. If this is successful, the client gets a stronger guarantee via a key check exchange: it sends a client_keycheck message and waits for the appropriate response. Note that it needs to memorize the nonce and the time interval number that it sends as a correlated pair. For more detail on both of the mentioned timeliness checks, see Appendix Appendix C.4. 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.
   2. 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 verified, 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 falsified, the client MUST discard the packet.
   3. If the disclosed key is legitimate, then the client verifies the authenticity of any packet that it received during the corresponding time interval. If authenticity of a packet is verified it is released from the buffer and the packet's time information can be utilized. If the verification fails, then authenticity is no longer given. In this case the client MUST request authentic time from the server by means of a unicast time request message.

   See RFC 4082

   [RFC4082] for a detailed description of the packet verification process.

The client MUST restart the broadcast sequence with a client_bpar message

The client's behavior in broadcast mode can also be seen in Figure 2.

7.2. The Server

7.2.1. The Server in Unicast Mode

To support unicast mode, the server MUST be ready to perform the following actions: Section 8.1).

• Upon receipt of a client_assoc message, the server constructs and sends a reply in the form of a server_assoc message as described in Section 6.1.2.
• 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 5.1. With this, it MUST construct a server_cook message as described in Section 6.2.2.
• Upon receipt of a time_request message, the server re-calculates the cookie, then computes the necessary time synchronization data and constructs a time_response message as given in Section 6.3.2.

The server MUST refresh its server seed periodically (see

7.2.2. The Server in Broadcast Mode

A broadcast server MUST also support unicast mode, in order to provide the initial time synchronization which is a precondition for any broadcast association. To support NTS broadcast, the server MUST additionally be ready to perform the following actions:

• Upon receipt of a client_bpar message, the server constructs and sends a server_bpar message as described in Section 6.4.2.
• Upon receipt of a client_keycheck message, the server looks up if 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 6.6.2. For more detail, see also Appendix C.
• The server follows the TESLA protocol in all other aspects, by regularly sending server_broad messages as described in Section 6.5.1, adhering to its own disclosure schedule.

It is also the server's responsibility to watch for the expiration date of the one-way key chain and generate a new key chain accordingly.

8. Server Seed Considerations

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 9.1.

8.1. Server Seed Refresh

According to the requirements in [RFC7384] the server MUST refresh each server seed periodically. As a consequence, 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 (Section 6.2).

8.2. Server Seed Algorithm

8.3. Server Seed Lifetime

9. Hash Algorithms and MAC Generation

9.1. Hash Algorithms

Hash algorithms are used at different points: calculation of the cookie and the MAC, and hashing of the client's certificate. Client and server negotiate a hash algorithm H during the association message exchange (Section 6.1) at the beginning of a unicast run. The selected algorithm H is used for all hashing processes in that run.

In broadcast mode, 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 non-negotiable.

The list of the hash algorithms supported by the server has to fulfill the following requirements:

• it MUST NOT include SHA-1 or weaker algorithms,
• it MUST include SHA-256 or stronger algorithms.

Note

Any hash algorithm is prone to be compromised in the future. A successful attack on a hash algorithm would enable any NTS client to derive the server seed from their 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.

9.2. MAC Calculation

For the calculation of the MAC, client and server are using a Keyed-Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC is generated with the hash algorithm specified by the client (see Section 9.1).

10. IANA Considerations

11. Security Considerations

11.1. Initial Verification of the Server Certificates

The client has to verify the validity of the certificates during the certification message exchange (Section 6.1.2). Since it generally has no reliable time during this initial communication phase, it is impossible to verify the period of validity of the certificates. Therefore, the client MUST use one of the following approaches:

• The validity of the certificates is preconditioned. Usually this will be the case in corporate networks.
• The client ensures that the certificates are not revoked. To this end, the client uses the Online Certificate Status Protocol (OCSP) defined in [RFC6277].
• The client requests a different service to get an initial time stamp in order to be able to verify the certificates' periods of validity. To this end, it can, e.g., use a secure shell connection to a reliable host. Another alternative is to request a time stamp from a Time Stamping Authority (TSA) by means of the Time-Stamp Protocol (TSP) defined in [RFC3161].

11.2. Revocation of Server Certificates

According to Section 8.1, 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 (Section 6.1.2). Besides, certificates may also be revoked prior to the normal expiration date. To increase security the client MAY verify the state of the server's certificate via OCSP periodically.

11.3. Usage of NTP Pools

The certification based authentication scheme described in Section 6 is not applicable to the concept of NTP pools. Therefore, NTS is not able to provide secure usage of NTP pools.

11.4. Denial-of-Service in Broadcast Mode

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, client and 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 knows already, thus preventing memory flooding attacks.

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.

11.5. 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 to measure the network delay, and hence its time offset to the server, accurately [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 synchronizations 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 are utilizing different paths for packet exchange as described in the I-D [I-D.shpiner-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 the unicast mode: 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 in the broadcast mode. 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 maintains time synchronization by utilization of the broadcast mode. To this end it initially establishes a unicast association with its broadcast server until time synchronization and calibration of the packet delay time is achieved. After that it establishes a broadcast association to the 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 section 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.

12. Acknowledgements

The authors would like to thank Russ Housley, Steven Bellovin, David Mills and Kurt Roeckx for discussions and comments on the design of NTS. Also, thanks to Harlan Stenn for his technical review and specific text contributions to this document.

13. References

13.1. Normative References

13.2. Informative References

Appendix A. Flow Diagrams of Client Behaviour

                     +---------------------+
                     |Association Messages |
                     +----------+----------+
                                |
+------------------------------>o
|                               |
|                               v
|                       +---------------+
|                       |Cookie Messages|
|                       +-------+-------+
|                               |
|                               o<------------------------------+
|                               |                               |
|                               v                               |
|                     +-------------------+                     |
|                     |Time Sync. Messages|                     |
|                     +---------+---------+                     |
|                               |                               |
|                               v                               |
|                            +-----+                            |
|                            |Check|                            |
|                            +--+--+                            |
|                               |                               |
|            /------------------+------------------\            |
|           v                   v                   v           |
|     .-----------.      .-------------.        .-------.       |
|    ( MAC Failure )    ( Nonce Failure )      ( Success )      |
|     '-----+-----'      '------+------'        '---+---'       |
|           |                   |                   |           |
|           v                   v                   v           |
|    +-------------+     +-------------+     +--------------+   |
|    |Discard Data |     |Discard Data |     |Sync. Process |   |
|    +-------------+     +------+------+     +------+-------+   |
|           |                   |                   |           |
|           |                   |                   v           |
+-----------+                   +------------------>o-----------+

                         +-----------------------------+
                         |Broadcast Parameter Messages |
                         +--------------+--------------+
                                        |
                                        o<--------------------------+
                                        |                           |
                                        v                           |
                         +-----------------------------+            |
                         |Broadcast Time Sync. Message |            |
                         +--------------+--------------+            |
                                        |                           |
+-------------------------------------->o                           |
|                                       |                           |
|                                       v                           |
|                             +-------------------+                 |
|                             |Key and Auth. Check|                 |
|                             +---------+---------+                 |
|                                       |                           |
|                      /----------------*----------------\          |
|                     v                                   v         |
|                .---------.                         .---------.    |
|               ( Verified  )                       ( Falsified )   |
|                '----+----'                         '----+----'    |
|                     |                                   |         |
|                     v                                   v         |
|              +-------------+                        +-------+     |
|              |Store Message|                        |Discard|     |
|              +------+------+                        +---+---+     |
|                     |                                   |         |
|                     v                                   +---------o
|             +---------------+                                     |
|             |Check Previous |                                     |
|             +-------+-------+                                     |
|                     |                                             |
|            /--------*--------\                                    |
|           v                   v                                   |
|      .---------.         .---------.                              |
|     ( Verified  )       ( Falsified )                             |
|      '----+----'         '----+----'                              |
|           |                   |                                   |
|           v                   v                                   |
|    +-------------+   +-----------------+                          |
|    |Sync. Process|   |Discard Previous |                          |
|    +------+------+   +--------+--------+                          |
|           |                   |                                   |
+-----------+                   +-----------------------------------+

< - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
                      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
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ->