draft-ietf-ntp-network-time-security-04.txt   draft-ietf-ntp-network-time-security-05.txt 
NTP Working Group D. Sibold NTP Working Group D. Sibold
Internet-Draft PTB Internet-Draft PTB
Intended status: Standards Track S. Roettger Intended status: Standards Track S. Roettger
Expires: January 5, 2015 Expires: April 26, 2015 Google Inc
K. Teichel K. Teichel
PTB PTB
July 04, 2014 October 23, 2014
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-04.txt draft-ietf-ntp-network-time-security-05.txt
Abstract Abstract
This document describes the Network Time Security (NTS) protocol that This document describes the Network Time Security (NTS) protocol that
enables secure authentication of time servers using Network Time enables secure time synchronization with time servers using Network
Protocol (NTP) or Precision Time Protocol (PTP). Its design Time Protocol (NTP) or Precision Time Protocol (PTP). Its design
considers the special requirements of precise timekeeping, which are considers the special requirements of precise timekeeping, which are
described in Security Requirements of Time Protocols in Packet described in Security Requirements of Time Protocols in Packet
Switched Networks [I-D.ietf-tictoc-security-requirements]. Switched Networks [RFC7384].
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 5, 2015. This Internet-Draft will expire on April 26, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4 2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 5 4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 5
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 5 5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 5
5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 6 5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 5
6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6 6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Association Messages . . . . . . . . . . . . . . . . . . 6 6.1. Association Messages . . . . . . . . . . . . . . . . . . 6
6.1.1. Message Type: "client_assoc" . . . . . . . . . . . . 7 6.1.1. Message Type: "client_assoc" . . . . . . . . . . . . 7
6.1.2. Message Type: "server_assoc" . . . . . . . . . . . . 7 6.1.2. Message Type: "server_assoc" . . . . . . . . . . . . 7
6.2. Certificate Messages . . . . . . . . . . . . . . . . . . 7 6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 8
6.2.1. Message Type: "client_cert" . . . . . . . . . . . . . 8 6.2.1. Message Type: "client_cook" . . . . . . . . . . . . . 8
6.2.2. Message Type: "server_cert" . . . . . . . . . . . . . 8 6.2.2. Message Type: "server_cook" . . . . . . . . . . . . . 8
6.3. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 9 6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 9
6.3.1. Message Type: "client_cook" . . . . . . . . . . . . . 9 6.3.1. Message Type: "time_request" . . . . . . . . . . . . 9
6.3.2. Message Type: "server_cook" . . . . . . . . . . . . . 9 6.3.2. Message Type: "time_response" . . . . . . . . . . . . 9
6.4. Unicast Time Synchronisation Messages . . . . . . . . . . 10 6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 10
6.4.1. Message Type: "time_request" . . . . . . . . . . . . 10 6.4.1. Message Type: "client_bpar" . . . . . . . . . . . . . 10
6.4.2. Message Type: "time_response" . . . . . . . . . . . . 10 6.4.2. Message Type: "server_bpar" . . . . . . . . . . . . . 10
6.5. Broadcast Parameter Messages . . . . . . . . . . . . . . 11 6.5. Broadcast Messages . . . . . . . . . . . . . . . . . . . 11
6.5.1. Message Type: "client_bpar" . . . . . . . . . . . . . 11 6.5.1. Message Type: "server_broad" . . . . . . . . . . . . 11
6.5.2. Message Type: "server_bpar" . . . . . . . . . . . . . 11 6.6. Broadcast Key Check . . . . . . . . . . . . . . . . . . . 11
6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 12 6.6.1. Message Type: "client_keycheck" . . . . . . . . . . . 11
6.6.1. Message Type: "server_broad" . . . . . . . . . . . . 12 6.6.2. Message Type: "server_keycheck" . . . . . . . . . . . 12
7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 13 7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 12
7.1. The Client . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. The Client . . . . . . . . . . . . . . . . . . . . . . . 12
7.1.1. The Client in Unicast Mode . . . . . . . . . . . . . 13 7.1.1. The Client in Unicast Mode . . . . . . . . . . . . . 12
7.1.2. The Client in Broadcast Mode . . . . . . . . . . . . 15 7.1.2. The Client in Broadcast Mode . . . . . . . . . . . . 14
7.2. The Server . . . . . . . . . . . . . . . . . . . . . . . 16 7.2. The Server . . . . . . . . . . . . . . . . . . . . . . . 16
7.2.1. The Server in Unicast Mode . . . . . . . . . . . . . 16 7.2.1. The Server in Unicast Mode . . . . . . . . . . . . . 16
7.2.2. The Server in Broadcast Mode . . . . . . . . . . . . 17 7.2.2. The Server in Broadcast Mode . . . . . . . . . . . . 16
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 17 8. Server Seed Considerations . . . . . . . . . . . . . . . . . 17
8.1. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 17 8.1. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 17
8.2. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 17 8.2. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 17
8.3. Server Seed Lifetime . . . . . . . . . . . . . . . . . . 17 8.3. Server Seed Lifetime . . . . . . . . . . . . . . . . . . 17
9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 17 9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 17
9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 17 9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 17
9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 18 9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18 11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11.1. Initial Verification of the Server Certificates . . . . 18 11.1. Initial Verification of the Server Certificates . . . . 18
11.2. Revocation of Server Certificates . . . . . . . . . . . 19 11.2. Revocation of Server Certificates . . . . . . . . . . . 18
11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 19 11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 19
11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 19 11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 19
11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 19 11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 20 13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 21 13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 22 Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 22
Appendix B. Extension Fields . . . . . . . . . . . . . . . . . . 25 Appendix B. TICTOC Security Requirements . . . . . . . . . . . . 24
Appendix C. TICTOC Security Requirements . . . . . . . . . . . . 25 Appendix C. Broadcast Mode . . . . . . . . . . . . . . . . . . . 25
Appendix D. Broadcast Mode . . . . . . . . . . . . . . . . . . . 26 C.1. Server Preparations . . . . . . . . . . . . . . . . . . . 25
D.1. Server Preparations . . . . . . . . . . . . . . . . . . . 27 C.2. Client Preparation . . . . . . . . . . . . . . . . . . . 27
D.2. Client Preparation . . . . . . . . . . . . . . . . . . . 28 C.3. Sending Authenticated Broadcast Packets . . . . . . . . . 27
D.3. Sending Authenticated Broadcast Packets . . . . . . . . . 29 C.4. Authentication of Received Packets . . . . . . . . . . . 28
D.4. Authentication of Received Packets . . . . . . . . . . . 29 Appendix D. Random Number Generation . . . . . . . . . . . . . . 29
Appendix E. Random Number Generation . . . . . . . . . . . . . . 30 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
Time synchronization protocols are increasingly utilized to Time synchronization protocols are increasingly utilized to
synchronize clocks in networked infrastructures. The reliable synchronize clocks in networked infrastructures. The reliable
performance of such infrastructures can be degraded seriously by performance of such infrastructures can be degraded seriously by
successful attacks against the time synchronization protocol. successful attacks against the time synchronization protocol.
Therefore, time synchronization protocols applied in critical Therefore, time synchronization protocols have to be secured if they
infrastructures have to provide security measures to defeat possible are applied in environments that are prone to malicious attacks.
adversaries. Consequently, the widespread Network Time Protocol This can be accomplished by utilization of external security
(NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906] protocols like IPsec or by intrinsic security measures of the time
which shall ensure authenticity of the NTP server and integrity of synchronization protocol.
the protocol packets. Unfortunately, the autokey protocol exhibits
various severe security vulnerabilities as revealed in a thorough
analysis of the protocol [Roettger]. For the Precision Time Protocol
(PTP), Annex K of the standard document IEEE 1588 [IEEE1588] defines
an informative security protocol that is still in experimental state.
Because of autokey's security vulnerabilities and the absence of a The two most popular time synchronization protocols, the Network Time
standardized security protocol for PTP, these protocols cannot be Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP)
applied in environments in which compliance requirements demand [IEEE1588], currently do not provide adequate intrinsic security
authenticity and integrity protection. This document specifies a precautions. This document specifies security measures for NTP and
security protocol which ensures authenticity of the time server via a PTP which enable these protocols to verify authenticity of the time
Public Key Infrastructure (PKI) and integrity of the time server and integrity of the time synchronization protocol packets.
synchronization protocol packets and which therefore enables the
usage of NTP and PTP in such environments.
The protocol is specified with the prerequisite in mind that precise The protocol is specified with the prerequisite in mind that precise
timekeeping can only be accomplished with stateless time timekeeping can only be accomplished with stateless time
synchronization communication, which excludes the utilization of synchronization communication, which excludes the utilization of
standard security protocols like IPsec or TLS for time standard security protocols like IPsec or TLS for time
synchronization messages. This prerequisite corresponds with the synchronization messages. This prerequisite corresponds with the
requirement that a security mechanism for timekeeping must be requirement that a security mechanism for timekeeping must be
designed in such a way that it does not degrade the quality of the designed in such a way that it does not degrade the quality of the
time transfer [I-D.ietf-tictoc-security-requirements]. time transfer [RFC7384].
Note: Note:
The intent is to formulate the protocol to be applicable to NTP The intent is to formulate the protocol to be applicable to NTP
and also PTP. In the current state the draft focuses on the and also PTP. In the current state the specification focuses on
application to NTP. the application to NTP.
2. Security Threats 2. Security Threats
A profound analysis of security threats and requirements for NTP and A profound analysis of security threats and requirements for NTP and
PTP can be found in the "Security Requirements of Time Protocols in PTP can be found in the "Security Requirements of Time Protocols in
Packet Switched Networks" [I-D.ietf-tictoc-security-requirements]. Packet Switched Networks" [RFC7384].
3. Objectives 3. Objectives
The objectives of the NTS specification are as follows: The objectives of the NTS specification are as follows:
o Authenticity: NTS enables the client to authenticate its time o Authenticity: NTS enables the client to authenticate its time
server. servers.
o Integrity: NTS protects the integrity of time synchronization o Integrity: NTS protects the integrity of time synchronization
protocol packets via a message authentication code (MAC). protocol packets via a message authentication code (MAC).
o Confidentiality: NTS does not provide confidentiality protection o Confidentiality: NTS does not provide confidentiality protection
of the time synchronization packets. of the time synchronization packets.
o Modes of operation: All operational modes of NTP are supported. o Modes of operation: All operational modes of NTP are supported.
o Operational modes of PTP should be supported as far as possible. o Operational modes of PTP should be supported as far as possible.
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* Unsecured NTP associations shall not be affected. * Unsecured NTP associations shall not be affected.
* An NTP server that does not support NTS shall not be affected * An NTP server that does not support NTS shall not be affected
by NTS authentication requests. by NTS authentication requests.
4. Terms and Abbreviations 4. Terms and Abbreviations
MITM Man In The Middle MITM Man In The Middle
NTP Network Time Protocol NTP Network Time Protocol [RFC5905]
NTS Network Time Security NTS Network Time Security
PTP Precision Time Protocol PTP Precision Time Protocol [IEEE1588]
TESLA Timed Efficient Stream Loss-Tolerant Authentication TESLA Timed Efficient Stream Loss-Tolerant Authentication
5. NTS Overview 5. NTS Overview
5.1. Symmetric and Client/Server Mode 5.1. Symmetric and Client/Server Mode
NTS applies X.509 certificates to verify the authenticity of the time NTS applies X.509 certificates to verify the authenticity of the time
server and to exchange a symmetric key, the so-called cookie. This server and to exchange a symmetric key, the so-called cookie. This
cookie is then used to protect authenticity and integrity of the cookie is then used to protect authenticity and integrity of the
subsequent time synchronization packets by means of a Message subsequent time synchronization packets by means of a Message
Authentication Code (MAC), which is attached to each time Authentication Code (MAC), which is attached to each time
synchronization packet. The calculation of the MAC includes the synchronization packet. The calculation of the MAC includes the
whole time synchronization packet and the cookie which is shared whole time synchronization packet and the cookie which is shared
between client and server. It is calculated according to: between client and server. The cookie is calculated according to:
cookie = MSB_128 (HMAC(server seed, H(certificate of client))), cookie = MSB_128 (HMAC(server seed, H(certificate of client))),
with the server seed as key, where H is a hash function, and where 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 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 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 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 changes as long as the server seed stays the same, but the server
seed has to be refreshed periodically in order to provide key seed has to be refreshed periodically in order to provide key
freshness as required in [I-D.ietf-tictoc-security-requirements]. freshness as required in [RFC7384]. See Section 8 for details on the
See Section 8 for details on the seed refresh and Section 7.1.1 for seed refresh and Section 7.1.1 for the client's reaction to it.
the client's reaction to it.
The server does not keep a state of the client. Therefore it has to 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 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 client. To this end, the client has to attach the hash value of its
certificate to each request (see Section 6.4). certificate to each request (see Section 6.3).
5.2. Broadcast Mode 5.2. Broadcast Mode
Just as in the case of the client server mode and symmetric 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, authenticity and integrity of the NTP packets are ensured by a MAC,
which is attached to the NTP packet by the sender. The verification which is attached to the NTP packet by the sender. Verification of
of the authenticity is based on the TESLA protocol, in particular on the packets' authenticity is based on the TESLA protocol, in
its "Not Re-using Keys" scheme, see section 3.7.2 of [RFC4082]. particular on its "not re-using keys" scheme, see section 3.7.2 of
TESLA is based on a one-way chain of keys, where each key is the
output of a one-way function applied on the previous key in the [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 chain. The last element of the chain is shared securely with all
clients. The server splits time into intervals of uniform duration clients. The server splits time into intervals of uniform duration
and assigns each key to an interval in reverse order, starting with and assigns each key to an interval in reverse order, starting with
the penultimate. At each time interval, the server sends an NTP the penultimate. At each time interval, the server sends an NTP
broadcast packet appended by a MAC, calculated using the broadcast packet appended by a MAC, calculated using the
corresponding key, and the key of the previous disclosure interval. corresponding key, and the key of the previous disclosure interval.
The client verifies the MAC by buffering the packet until the The client verifies the MAC by buffering the packet until the
disclosure of the key in its associated disclosure interval. In 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 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 be loosely time synchronized to the server. This has to be
accomplished during the initial client server exchange between accomplished during the initial client server exchange between
broadcast client and server. For a more detailed description of the broadcast client and server. In addition, NTS uses another, more
TESLA protocol see Appendix D. 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 6. Protocol Messages
Note that this section currently describes the realization of the This section describes the types of messages needed for secure time
message format of NTS only for its utilization for NTP, in which the synchronization with NTS.
NTS specific data are enclosed in extension fields on top of NTP
packets. A specification of NTS messages for PTP would have to be
developed accordingly.
The steps described in Section 6.1 - Section 6.4 belong to the For some guidance on how these message types can be realized in
unicast mode, while Section 6.5 and Section 6.6 explain the steps 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. involved in the broadcast mode of NTS.
6.1. Association Messages 6.1. Association Messages
In this message exchange, the hash and signature algorithms that are In this message exchange, the hash and encryption algorithms that are
used throughout the protocol are negotiated. 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" 6.1.1. Message Type: "client_assoc"
The protocol sequence starts with the client sending an association The protocol sequence starts with the client sending an association
message, called client_assoc. This message contains message, called client_assoc. This message contains
o the NTS message ID "client_assoc", o the NTS message ID "client_assoc",
o the version number of NTS that the client wants to use (this o the version number of NTS that the client wants to use (this
SHOULD be the highest version number that it supports), SHOULD be the highest version number that it supports),
o the hostname of the client, o the hostname of the client,
o a selection of accepted hash algorithms, o a selection of accepted hash algorithms, and
o a selection of accepted encryption algorithms, and
o a selection of accepted algorithms for the signatures.
For NTP, this message is realized as a packet with an extension field o a selection of accepted encryption algorithms.
of type "association", which contains all this data.
6.1.2. Message Type: "server_assoc" 6.1.2. Message Type: "server_assoc"
This message is sent by the server upon receipt of client_assoc. It This message is sent by the server upon receipt of client_assoc. It
contains contains
o the NTS message ID "server_assoc", o the NTS message ID "server_assoc",
o the version number used for the rest of the protocol (which SHOULD o the version number used for the rest of the protocol (which SHOULD
be determined as the minimum over the client's suggestion in the be determined as the minimum over the client's suggestion in the
client_assoc message and the highest supported by the server), client_assoc message and the highest supported by the server),
o the hostname of the server, and o the hostname of the server, and
o the server's choice of algorithm for encryption, for the o the server's choice of algorithm for encryption and for
signatures and for cryptographic hashing , all of which MUST be cryptographic hashing, all of which MUST be chosen from the
chosen from the client's proposals. client's proposals.
In the case of NTP, the data is enclosed in a packet's extension
field, also of type "association".
6.2. Certificate Messages
In this message exchange, 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.
Discussion:
Note that in this step 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 [I-D.ietf-tictoc-security-requirements] depends on the chosen
trust anchor.
6.2.1. Message Type: "client_cert"
This message is sent by the client, after it successfully verified
the content of the received server_assoc message (see Section 7.1.1).
It contains
o the NTS message ID "client_cert",
o the negotiated version number,
o the client's hostname, and
o the signature algorithm negotiated during the association
messages.
In the case of NTP, the data is enclosed in a packet's extension
field of type "certificate request".
6.2.2. Message Type: "server_cert"
This message is sent by the server, upon receipt of a client_cert
message, if the version number and choice of methods communicated in
that message are actually supported by the server. It contains
o the NTS message ID "server_cert",
o the version number as transmitted in client_cert,
o a signature, calculated over the data listed above, with the o a signature, calculated over the data listed above, with the
server's private key and according to the signature algorithm server's private key and according to the signature algorithm
transmitted in server_cert, which is also used for the certificates which are included (see
below),
o all the information necessary to authenticate the server to the
client. This is a chain of certificates, which starts at the
server and goes up to a trusted authority, where each certificate
MUST be certified by the one directly following it.
This message is realized for NTP as a packet with extension field of o a chain of certificates, which starts at the server and goes up to
type "certificate" which holds all of the data listed above. a trusted authority, and each certificate MUST be certified by the
one directly following it.
6.3. Cookie Messages 6.2. Cookie Messages
During this message exchange, the server transmits a secret cookie to During this message exchange, the server transmits a secret cookie to
the client securely. The cookie will be used for integrity the client securely. The cookie will be used for integrity
protection during unicast time synchronization. protection during unicast time synchronization.
6.3.1. Message Type: "client_cook" 6.2.1. Message Type: "client_cook"
This message is sent by the client, upon successful authentication of This message is sent by the client, upon successful authentication of
the server. In this message, the client requests a cookie from the the server. In this message, the client requests a cookie from the
server. The message contains server. The message contains
o the NTS message ID "client_cook", o the NTS message ID "client_cook",
o the negotiated version number, o the negotiated version number,
o the negotiated signature algorithm, o the negotiated signature algorithm,
o the negotiated encryption algorithm, o the negotiated encryption algorithm,
o a 128-bit nonce, o a 128-bit nonce,
o the negotiated hash algorithm H, o the negotiated hash algorithm H,
o the client's certificate. o the client's certificate.
For NTP, an extension field of type "cookie request" holds the listed 6.2.2. Message Type: "server_cook"
data.
6.3.2. Message Type: "server_cook"
This message is sent by the server, upon receipt of a client_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, message. The server generates the hash of the client's certificate,
as conveyed during client_cook, in order to calculate the cookie as conveyed during client_cook, in order to calculate the cookie
according to Section 5.1. This message contains a concatenated according to Section 5.1. This message contains
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
o the NTS message ID "server_cook" o the NTS message ID "server_cook"
o the version number as transmitted in client_cook, o the version number as transmitted in client_cook,
o the nonce transmitted in client_cook, o a concatenated datum, which is encrypted with the client's public
key, according to the encryption algorithm transmitted in the
o the cookie, and client_cook message. The concatenated datum contains
o a signature, created with the server's private key, calculated
over
* all of the data listed above, and also
* the hash of the client's certificate. * the nonce transmitted in client_cook, and
This signature MUST be calculated according to the transmitted * the cookie.
signature algorithm from the client_cook message.
In the case of NTP, this is a packet with an extension field of type o a signature, created with the server's private key, calculated
"cookie transmit". over all of the data listed above. This signature MUST be
calculated according to the transmitted signature algorithm from
the client_cook message.
6.4. Unicast Time Synchronisation Messages 6.3. Unicast Time Synchronisation Messages
In this message exchange, the usual time synchronization process is In this message exchange, the usual time synchronization process is
executed, with the addition of integrity protection for all messages executed, with the addition of integrity protection for all messages
that the server sends. This message can be repeatedly exchanged as that the server sends. This message can be repeatedly exchanged as
often as the client desires and as long as the integrity of the often as the client desires and as long as the integrity of the
server's time responses is verified successfully. server's time responses is verified successfully.
6.4.1. Message Type: "time_request" 6.3.1. Message Type: "time_request"
This message is sent by the client when it requests time exchange. This message is sent by the client when it requests time exchange.
It contains It contains
o the NTS message ID "time_request", o the NTS message ID "time_request",
o the negotiated version number, o the negotiated version number,
o a 128-bit nonce, o a 128-bit nonce,
o the negotiated hash algorithm H, o the negotiated hash algorithm H,
o the hash of the client's certificate under H. o the hash of the client's certificate under H.
In the case of NTP the data is enclosed in the packet's extension 6.3.2. Message Type: "time_response"
field of type "time request".
6.4.2. Message Type: "time_response"
This message is sent by the server, after it received a time_request This message is sent by the server, after it received a time_request
message. Prior to this the server MUST recalculate the client's message. Prior to this the server MUST recalculate the client's
cookie by using the hash of the client's certificate and the cookie by using the hash of the client's certificate and the
transmitted hash algorithm. The message contains transmitted hash algorithm. The message contains
o the NTS message ID "time_response", o the NTS message ID "time_response",
o the version number as transmitted in time_request, o the version number as transmitted in time_request,
o the server's time synchronization response data, o the server's time synchronization response data,
o the nonce transmitted in time_request, o the 128-bit nonce transmitted in time_request,
o a MAC (generated with the cookie as key) for verification of all o a MAC (generated with the cookie as key) for verification of all
of the above data. of the above data.
In the case of NTP, this is a packet with the necessary time 6.4. Broadcast Parameter Messages
synchronization data and a new extension field of type "time
response". This packet has an appended MAC that is generated over
the time synchronization data and the extension field, with the
cookie as the key.
6.5. Broadcast Parameter Messages
In this message exchange, the client receives the necessary In this message exchange, the client receives the necessary
information to execute the TESLA protocol in a secured broadcast information to execute the TESLA protocol in a secured broadcast
association. The client can only initiate a secure broadcast association. The client can only initiate a secure broadcast
association after a successful unicast run, see Section 7.1.2. association after a successful unicast run, see Section 7.1.2.
See Appendix D for more details on TESLA. See Appendix C for more details on TESLA.
6.5.1. Message Type: "client_bpar" 6.4.1. Message Type: "client_bpar"
This message is sent by the client in order to establish a secured This message is sent by the client in order to establish a secured
time broadcast association with the server. It contains time broadcast association with the server. It contains
o the NTS message ID "client_bpar", o the NTS message ID "client_bpar",
o the version number negotiated during association in unicast mode, o the version number negotiated during association in unicast mode,
o the client's hostname, and o the client's hostname, and
o the signature algorithm negotiated during unicast. o the signature algorithm negotiated during unicast.
For NTP, this message is realized as a packet with an extension field 6.4.2. Message Type: "server_bpar"
of type "broadcast request".
6.5.2. Message Type: "server_bpar"
This message is sent by the server upon receipt of a client_bpar This message is sent by the server upon receipt of a client_bpar
message during the broadcast loop of the server. It contains message during the broadcast loop of the server. It contains
o the NTS message ID "server_bpar", o the NTS message ID "server_bpar",
o the version number as transmitted in the client_bpar message, o the version number as transmitted in the client_bpar message,
o the one-way function used for building the one-way key chain,
o the last key of the one-way key chain, and o the one-way functions used for building the key chain, and
o the disclosure schedule of the keys. This contains: o the disclosure schedule of the keys. This contains:
* the last key of the key chain,
* time interval duration, * time interval duration,
* the disclosure delay (number of intervals between use and * the disclosure delay (number of intervals between use and
disclosure of a key), disclosure of a key),
* the time at which the next time interval will start, and * the time at which the next time interval will start, and
* the next interval's associated index. * the next interval's associated index.
o The message also contains a signature signed by the server with o The message also contains a signature signed by the server with
its private key, verifying all the data listed above. its private key, verifying all the data listed above.
It is realized for NTP as a packet with an extension field of type 6.5. Broadcast Messages
"broadcast parameters", which contains all the given data.
6.6. Broadcast Message
Via this message, the server keeps sending broadcast time Via this message, the server keeps sending broadcast time
synchronization messages to all participating clients. synchronization messages to all participating clients.
6.6.1. Message Type: "server_broad" 6.5.1. Message Type: "server_broad"
This message is sent by the server over the course of its broadcast This message is sent by the server over the course of its broadcast
schedule. It is part of any broadcast association. It contains schedule. It is part of any broadcast association. It contains
o the NTS message ID "server_broad", o the NTS message ID "server_broad",
o the version number that the server's broadcast mode is working o the version number that the server's broadcast mode is working
under, under,
o time broadcast data, o time broadcast data,
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o the index that belongs to the current interval (and therefore o the index that belongs to the current interval (and therefore
identifies the current, yet undisclosed key), identifies the current, yet undisclosed key),
o the disclosed key of the previous disclosure interval (current o the disclosed key of the previous disclosure interval (current
time interval minus disclosure delay), time interval minus disclosure delay),
o a MAC, calculated with the key for the current time interval, o a MAC, calculated with the key for the current time interval,
verifying verifying
* the message ID, * the message ID,
* the version number, and * the version number, and
* the time data. * the time data.
For NTP, this message is realized as an NTP broadcast packet with the 6.6. Broadcast Key Check
time broadcast data and with an extension field of type "broadcast
message", which contains the rest of the listed data. The NTP packet This message exchange is performed for an additional check of packet
is then appended by a MAC verifying its contents. 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
o the NTS message ID "client_keycheck",
o the version number chosen for the broadcast,
o a 128-bit nonce,
o an interval number from the TESLA disclosure schedule,
o the hash algorithm H negotiated in unicast mode, and
o 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
o the NTS message ID "server_keycheck"
o the version number that the server's broadcast mode is working
under,
o the 128-bit nonce transmitted in the client_keycheck message,
o the interval number transmitted in the client_keycheck message,
and
o a MAC (generated with the cookie as key) for verification of all
of the above data.
7. Protocol Sequence 7. Protocol Sequence
7.1. The Client 7.1. The Client
7.1.1. The Client in Unicast Mode 7.1.1. The Client in Unicast Mode
For a unicast run, the client performs the following steps: For a unicast run, the client performs the following steps:
1. It sends a client_assoc message to the server. It MUST keep the 1. It sends a client_assoc message to the server. It MUST keep the
transmitted values for version number and algorithms available transmitted values for version number and algorithms available
for later checks. for later checks.
2. It waits for a reply in the form of a server_assoc message. 2. It waits for a reply in the form of a server_assoc message.
After receipt of the message it performs the following checks: After receipt of the message it performs the following checks:
* The client checks that the message contains a conform version * The client checks that the message contains a conform version
number. number.
* It also has to verify that the server has chosen the signature * It also verifies that the server has chosen the encryption and
and hash algorithms from its proposal sent in the client_assoc hash algorithms from its proposal sent in the client_assoc
message. message.
If one of the checks fails, the client MUST abort the run. * Furthermore, it performs authenticity checks on the
certificate chain and the signature for the version number.
3. The client then sends a client_cert message to the server. If one of the checks fails, the client MUST abort the run.
Again, it MUST remember the transmitted values for version number Discussion:
and algorithms for later checks.
4. It awaits a reply in the form of a server_cert message and Note that by performing the above message exchange and checks,
performs authenticity checks on the certificate chain and the the client validates the authenticity of its immediate NTP
signature for the version number. If one of these checks fails, server only. It does not recursively validate the
the client MUST abort the run. authenticity of each NTP server on the time synchronization
chain. Recursive authentication (and authorization) as
formulated in [RFC7384] depends on the chosen trust anchor.
5. Next, it sends a client_cook message to the server. The client 3. Next, it sends a client_cook message to the server. The client
MUST save the included nonce until the reply has been processed. MUST save the included nonce until the reply has been processed.
6. It awaits a reply in the form of a server_cook message; upon 4. It awaits a reply in the form of a server_cook message; upon
receipt it executes the following actions: receipt it executes the following actions:
* It decrypts the message with its own private key.
* It checks that the decrypted message is of the expected
format: the concatenation of version number, a 128 bit nonce,
a 128 bit cookie and a signature value.
* It verifies that the received version number matches the one * It verifies that the received version number matches the one
negotiated before. 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 * It verifies that the received nonce matches the nonce sent in
the client_cook message. the client_cook message.
* It verifies the signature using the server's public key. The
signature has to authenticate the version number, the nonce,
the cookie, and the hash of the client's certificate.
If one of those checks fails, the client MUST abort the run. If one of those checks fails, the client MUST abort the run.
7. The client sends a time_request message to the server. The 5. The client sends a time_request message to the server. The
client MUST save the included nonce and the transmit_timestamp client MUST save the included nonce and the transmit_timestamp
(from the time synchronization data) as a correlated pair for (from the time synchronization data) as a correlated pair for
later verification steps. later verification steps.
8. It awaits a reply in the form of a time_response message. Upon 6. It awaits a reply in the form of a time_response message. Upon
receipt, it checks: receipt, it checks:
* that the transmitted version number matches the one negotiated * that the transmitted version number matches the one negotiated
before, before,
* that the transmitted nonce belongs to a previous time_request * that the transmitted nonce belongs to a previous time_request
message, message,
* that the transmit_timestamp in that time_request message * that the transmit_timestamp in that time_request message
matches the corresponding time stamp from the synchronization matches the corresponding time stamp from the synchronization
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following steps: following steps:
1. It sends a client_bpar message to the server. It MUST remember 1. It sends a client_bpar message to the server. It MUST remember
the transmitted values for version number and signature the transmitted values for version number and signature
algorithm. algorithm.
2. It waits for a reply in the form of a server_bpar message after 2. It waits for a reply in the form of a server_bpar message after
which it performs the following checks: which it performs the following checks:
* The message must contain all the necessary information for the * The message must contain all the necessary information for the
TESLA protocol, as listed in Section 6.5.2. TESLA protocol, as listed in Section 6.4.2.
* Verification of the message's signature. * Verification of the message's signature.
If any information is missing or the server's signature cannot be If any information is missing or the server's signature cannot be
verified, the client MUST abort the broadcast run. If all checks verified, the client MUST abort the broadcast run. If all checks
are successful, the client MUST remember all the broadcast are successful, the client MUST remember all the broadcast
parameters received for later checks. parameters received for later checks.
3. The client awaits time synchronization data in the form of a 3. The client awaits time synchronization data in the form of a
server_broadcast message. Upon receipt, it performs the server_broadcast message. Upon receipt, it performs the
following checks: following checks:
1. Proof that the MAC is based on a key that is not yet 1. Proof that the MAC is based on a key that is not yet
disclosed. This is achieved via a disclosure schedule and disclosed (packet timeliness). This is achieved via a
requires the loose time synchronization. If verified, the combination of checks. First the disclosure schedule is
packet will be buffered for later authentication. Otherwise, used, which requires the loose time synchronization. If this
the client MUST discard it. Note that the time information is successful, the client gets a stronger guarantee via a key
included in the packet will not be used for synchronization check exchange: it sends a client_keycheck message and waits
until its authenticity could be verified. 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 whether it already knows the disclosed key. 2. The client checks that it does not already know the disclosed
If so, the client SHOULD discard the packet to avoid a buffer key. Otherwise, the client SHOULD discard the packet to
overrun. If not, the client verifies that the disclosed key avoid a buffer overrun. If verified, the client ensures that
belongs to the one-way key chain by applying the one-way the disclosed key belongs to the one-way key chain by
function until equality with a previous disclosed key is applying the one-way function until equality with a previous
verified. If falsified, the client MUST discard the packet. disclosed key is shown. If falsified, the client MUST
discard the packet.
3. If the disclosed key is legitimate the client verifies the 3. If the disclosed key is legitimate, then the client verifies
authenticity of any packet that it received during the the authenticity of any packet that it received during the
corresponding time interval. If authenticity of a packet is corresponding time interval. If authenticity of a packet is
verified it is released from the buffer and the packet's time verified it is released from the buffer and the packet's time
information can be utilized. If the verification fails information can be utilized. If the verification fails, then
authenticity is no longer given. In this case the client authenticity is no longer given. In this case the client
MUST request authentic time from the server by means of a MUST request authentic time from the server by means of a
unicast time request message. unicast time request message.
See RFC 4082[RFC4082] for a detailed description of the packet See RFC 4082[RFC4082] for a detailed description of the packet
verification process. verification process.
The client MUST restart the broadcast sequence with a client_bpar The client MUST restart the broadcast sequence with a client_bpar
message Section 6.5.1 if the one-way key chain expires. message Section 6.4.1 if the one-way key chain expires.
The client's behavior in broadcast mode can also be seen in Figure 2. The client's behavior in broadcast mode can also be seen in Figure 2.
7.2. The Server 7.2. The Server
7.2.1. The Server in Unicast Mode 7.2.1. The Server in Unicast Mode
To support unicast mode, the server MUST be ready to perform the To support unicast mode, the server MUST be ready to perform the
following actions: following actions:
o Upon receipt of a client_assoc message, the server constructs and o Upon receipt of a client_assoc message, the server constructs and
sends a reply in the form of a server_assoc message as described sends a reply in the form of a server_assoc message as described
in Section 6.1.2. in Section 6.1.2.
o Upon receipt of a client_cert message, the server checks whether
it supports the given signature algorithm. If so, it constructs
and sends a server_cert message as described in Section 6.2.2.
o Upon receipt of a client_cook message, the server checks whether o Upon receipt of a client_cook message, the server checks whether
it supports the given cryptographic algorithms. It then it supports the given cryptographic algorithms. It then
calculates the cookie according to the formula given in calculates the cookie according to the formula given in
Section 5.1. With this, it MUST construct a server_cook message Section 5.1. With this, it MUST construct a server_cook message
as described in Section 6.3.2. as described in Section 6.2.2.
o Upon receipt of a time_request message, the server re-calculates o Upon receipt of a time_request message, the server re-calculates
the cookie, then computes the necessary time synchronization data the cookie, then computes the necessary time synchronization data
and constructs a time_response message as given in Section 6.4.2. and constructs a time_response message as given in Section 6.3.2.
The server MUST refresh its server seed periodically (see The server MUST refresh its server seed periodically (see
Section 8.1). Section 8.1).
7.2.2. The Server in Broadcast Mode 7.2.2. The Server in Broadcast Mode
A broadcast server MUST also support unicast mode, in order to A broadcast server MUST also support unicast mode, in order to
provide the initial time synchronization which is a precondition for provide the initial time synchronization which is a precondition for
any broadcast association. To support NTS broadcast, the server MUST any broadcast association. To support NTS broadcast, the server MUST
additionally be ready to perform the following actions: additionally be ready to perform the following actions:
o Upon receipt of a client_bpar message, the server constructs and o Upon receipt of a client_bpar message, the server constructs and
sends a server_bpar message as described in Section 6.5.2. sends a server_bpar message as described in Section 6.4.2.
o 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.
o The server follows the TESLA protocol in all other aspects, by o The server follows the TESLA protocol in all other aspects, by
regularly sending server_broad messages as described in regularly sending server_broad messages as described in
Section 6.6.1, adhering to its own disclosure schedule. Section 6.5.1, adhering to its own disclosure schedule.
It is also the server's responsibility to watch for the expiration 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 date of the one-way key chain and generate a new key chain
accordingly. accordingly.
8. Server Seed Considerations 8. Server Seed Considerations
The server has to calculate a random seed which has to be kept The server has to calculate a random seed which has to be kept
secret. The server MUST generate a seed for each supported hash secret. The server MUST generate a seed for each supported hash
algorithm, see Section 9.1. algorithm, see Section 9.1.
8.1. Server Seed Refresh 8.1. Server Seed Refresh
According to the requirements in According to the requirements in [RFC7384] the server MUST refresh
[I-D.ietf-tictoc-security-requirements] the server MUST refresh each each server seed periodically. As a consequence, the cookie
server seed periodically. As a consequence, the cookie memorized by memorized by the client becomes obsolete. In this case the client
the client becomes obsolete. In this case the client cannot verify cannot verify the MAC attached to subsequent time response messages
the MAC attachted to subsequent time response messages and has to and has to respond accordingly by re-initiating the protocol with a
respond accordingly by re-initiating the protocol with a cookie cookie request (Section 6.2).
request (Section 6.3).
8.2. Server Seed Algorithm 8.2. Server Seed Algorithm
8.3. Server Seed Lifetime 8.3. Server Seed Lifetime
9. Hash Algorithms and MAC Generation 9. Hash Algorithms and MAC Generation
9.1. Hash Algorithms 9.1. Hash Algorithms
Hash algorithms are used at different points: calculation of the Hash algorithms are used at different points: calculation of the
skipping to change at page 18, line 40 skipping to change at page 18, line 21
is generated with the hash algorithm specified by the client (see is generated with the hash algorithm specified by the client (see
Section 9.1). Section 9.1).
10. IANA Considerations 10. IANA Considerations
11. Security Considerations 11. Security Considerations
11.1. Initial Verification of the Server Certificates 11.1. Initial Verification of the Server Certificates
The client has to verify the validity of the certificates during the The client has to verify the validity of the certificates during the
certification message exchange (Section 6.2). Since it generally has certification message exchange (Section 6.1.2). Since it generally
no reliable time during this initial communication phase, it is has no reliable time during this initial communication phase, it is
impossible to verify the period of validity of the certificates. impossible to verify the period of validity of the certificates.
Therefore, the client MUST use one of the following approaches: Therefore, the client MUST use one of the following approaches:
o The validity of the certificates is preconditioned. Usually this o The validity of the certificates is preconditioned. Usually this
will be the case in corporate networks. will be the case in corporate networks.
o The client ensures that the certificates are not revoked. To this o The client ensures that the certificates are not revoked. To this
end, the client uses the Online Certificate Status Protocol (OCSP) end, the client uses the Online Certificate Status Protocol (OCSP)
defined in [RFC6277]. defined in [RFC6277].
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connection to a reliable host. Another alternative is to request connection to a reliable host. Another alternative is to request
a time stamp from a Time Stamping Authority (TSA) by means of the a time stamp from a Time Stamping Authority (TSA) by means of the
Time-Stamp Protocol (TSP) defined in [RFC3161]. Time-Stamp Protocol (TSP) defined in [RFC3161].
11.2. Revocation of Server Certificates 11.2. Revocation of Server Certificates
According to Section 8.1, it is the client's responsibility to According to Section 8.1, it is the client's responsibility to
initiate a new association with the server after the server's initiate a new association with the server after the server's
certificate expires. To this end the client reads the expiration certificate expires. To this end the client reads the expiration
date of the certificate during the certificate message exchange date of the certificate during the certificate message exchange
(Section 6.2). Besides, certificates may also be revoked prior to (Section 6.1.2). Besides, certificates may also be revoked prior to
the normal expiration date. To increase security the client MAY the normal expiration date. To increase security the client MAY
verify the state of the server's certificate via OCSP periodically. verify the state of the server's certificate via OCSP periodically.
11.3. Usage of NTP Pools 11.3. Usage of NTP Pools
The certification based authentication scheme described in Section 6 The certification based authentication scheme described in Section 6
is not applicable to the concept of NTP pools. Therefore, NTS is not is not applicable to the concept of NTP pools. Therefore, NTS is not
able to provide secure usage of NTP pools. able to provide secure usage of NTP pools.
11.4. Denial-of-Service in Broadcast Mode 11.4. Denial-of-Service in Broadcast Mode
TESLA authentication buffers packets for delayed authentication. TESLA authentication buffers packets for delayed authentication.
This makes the protocol vulnerable to flooding attacks, causing the This makes the protocol vulnerable to flooding attacks, causing the
client to buffer excessive numbers of packets. To add stronger DoS client to buffer excessive numbers of packets. To add stronger DoS
protection to the protocol, client and server use the "Not Re-using 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 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 [RFC4082]. In this scheme the server never uses a key for the MAC
generation more than once. Therefore the client can discard any generation more than once. Therefore the client can discard any
packet that contains a disclosed key it knows already, thus packet that contains a disclosed key it knows already, thus
preventing memory flooding attacks. preventing memory flooding attacks.
Note, an alternative approach to enhance TESLA's resistance against Note that an alternative approach to enhance TESLA's resistance
DoS attacks involves the addition of a group MAC to each packet. against DoS attacks involves the addition of a group MAC to each
This requires the exchange of an additional shared key common to the packet. This requires the exchange of an additional shared key
whole group. This adds additional complexity to the protocol and common to the whole group. This adds additional complexity to the
hence is currently not considered in this document. protocol and hence is currently not considered in this document.
11.5. Delay Attack 11.5. Delay Attack
In a packet delay attack, an adversary with the ability to act as a In a packet delay attack, an adversary with the ability to act as a
MITM delays time synchronization packets between client and server MITM delays time synchronization packets between client and server
asymmetrically [I-D.ietf-tictoc-security-requirements]. This asymmetrically [RFC7384]. This prevents the client to measure the
prevents the client to measure the network delay, and hence its time network delay, and hence its time offset to the server, accurately
offset to the server, accurately [Mizrahi]. The delay attack does [Mizrahi]. The delay attack does not modify the content of the
not modifiy the content of the exchanged synchronization packets. exchanged synchronization packets. Therefore cryptographic means do
Therefore cryptographic means are not feasible to mitigate this not provide a feasible way to mitigate this attack. However, several
attack. However, serveral non-cryptographic precautions can be taken non-cryptographic precautions can be taken in order to detect this
in order to detect this attack. attack.
o Usage of multiple time servers: this enables the client to detect 1. Usage of multiple time servers: this enables the client to detect
the attack provided that the adversary is unable to delay the the attack provided that the adversary is unable to delay the
synchronizations packets between the majority of servers. This synchronizations packets between the majority of servers. This
approach is commonly used in NTP to exclude incorrect time servers approach is commonly used in NTP to exclude incorrect time
[RFC5905]. servers [RFC5905].
o Multiple communication paths: The client and server are utilizing 2. Multiple communication paths: The client and server are utilizing
different paths for packet exchange as described in the I-D different paths for packet exchange as described in the I-D
[I-D.shpiner-multi-path-synchronization]. The client can detect [I-D.shpiner-multi-path-synchronization]. The client can detect
the attack provided that the adversary is unable to manipulate the the attack provided that the adversary is unable to manipulate
majority of the available paths [Shpiner]. Note, that this the majority of the available paths [Shpiner]. Note that this
approach is not yet available, neither for NTP nor for PTP. approach is not yet available, neither for NTP nor for PTP.
o The introduction of a threshold value for the delay time of the 3. Usage of an encrypted connection: the client exchanges all
synchronization packets. The client can discard a time server if packets with the time server over an encrypted connection (e.g.
the packet delay time of this time server is larger than the IPsec). This measure does not mitigate the delay attack but it
threshold value. makes it more difficult for the adversary to identify the time
synchronization packets.
o Usage of an encrypted connection: the client exchanges all packets 4. For the unicast mode: Introduction of a threshold value for the
with the time server over an encrypted connection (e.g. IPsec). delay time of the synchronization packets. The client can
This measure does not mitigate the delay attack but it makes it discard a time server if the packet delay time of this time
more difficult for the adversary to identify the time server is larger than the threshold value.
synchronization packets.
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 12. Acknowledgements
The authors would like to thank Steven Bellovin, David Mills and Kurt The authors would like to thank Russ Housley, Steven Bellovin, David
Roeckx for discussions and comments on the design of NTS. Also, Mills and Kurt Roeckx for discussions and comments on the design of
thanks to Harlan Stenn for his technical review and specific text NTS. Also, thanks to Harlan Stenn for his technical review and
contributions to this document. specific text contributions to this document.
13. References 13. References
13.1. Normative References 13.1. Normative References
[IEEE1588] [IEEE1588]
IEEE Instrumentation and Measurement Society. TC-9 Sensor IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and synchronization protocol for networked measurement and
control systems", 2008. control systems", 2008.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February Hashing for Message Authentication", RFC 2104, February
skipping to change at page 21, line 21 skipping to change at page 21, line 28
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato, [RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp "Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001. Protocol (TSP)", RFC 3161, August 2001.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005. Transform Introduction", RFC 4082, June 2005.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. Specification", RFC 5905, June 2010.
[RFC5906] Haberman, B. and D. Mills, "Network Time Protocol Version
4: Autokey Specification", RFC 5906, June 2010.
[RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate [RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate
Status Protocol Algorithm Agility", RFC 6277, June 2011. Status Protocol Algorithm Agility", RFC 6277, June 2011.
13.2. Informative References 13.2. Informative References
[I-D.ietf-tictoc-security-requirements]
Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", draft-ietf-tictoc-security-
requirements-10 (work in progress), July 2014.
[I-D.shpiner-multi-path-synchronization] [I-D.shpiner-multi-path-synchronization]
Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi- Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
Path Time Synchronization", draft-shpiner-multi-path- Path Time Synchronization", draft-shpiner-multi-path-
synchronization-03 (work in progress), February 2014. synchronization-03 (work in progress), February 2014.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks [Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings of against time synchronization protocols", in Proceedings of
Precision Clock Synchronization for Measurement Control Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012. and Communication, ISPCS 2012, pp. 1-6, September 2012.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005. Requirements for Security", BCP 106, RFC 4086, June 2005.
[Roettger] [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Roettger, S., "Analysis of the NTP Autokey Procedures", Packet Switched Networks", RFC 7384, October 2014.
February 2012.
[Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", in Proceedings of Precision Clock Protocols", in Proceedings of Precision Clock
Synchronization for Measurement Control and Communication, Synchronization for Measurement Control and Communication,
ISPCS 2013, pp. 1-6, September 2013. ISPCS 2013, pp. 1-6, September 2013.
Appendix A. Flow Diagrams of Client Behaviour Appendix A. Flow Diagrams of Client Behaviour
+---------------------+ +---------------------+
|Association Messages | |Association Messages |
+----------+----------+ +----------+----------+
| |
v
+---------------------+
|Certificate Messages |
+----------+----------+
|
+------------------------------>o +------------------------------>o
| | | |
| v | v
| +---------------+ | +---------------+
| |Cookie Messages| | |Cookie Messages|
| +-------+-------+ | +-------+-------+
| | | |
| o<------------------------------+ | o<------------------------------+
| | | | | |
| v | | v |
skipping to change at page 25, line 5 skipping to change at page 24, line 5
| | | | | | | |
| v v | | v v |
| +-------------+ +-----------------+ | | +-------------+ +-----------------+ |
| |Sync. Process| |Discard Previous | | | |Sync. Process| |Discard Previous | |
| +------+------+ +--------+--------+ | | +------+------+ +--------+--------+ |
| | | | | | | |
+-----------+ +-----------------------------------+ +-----------+ +-----------------------------------+
Figure 2: The client's behaviour in NTS broadcast mode. Figure 2: The client's behaviour in NTS broadcast mode.
Appendix B. Extension Fields Appendix B. TICTOC Security Requirements
In Section 6, some new extension fields for NTP packets are
introduced. They are listed here again, for reference.
+------------------------+---------------+
| name | used in |
+------------------------+---------------+
| "association" | client_assoc |
| | server_assoc |
| | |
| "certificate request" | client_cert |
| | |
| "certificate" | server_cert |
| | |
| "cookie request" | client_cook |
| | |
| "cookie transmit" | server_cook |
| | |
| "time request" | time_request |
| | |
| "time response" | time_response |
| | |
| "broadcast request" | client_bpar |
| | |
| "broadcast parameters" | server_bpar |
| | |
| "broadcast message" | server_broad |
+------------------------+---------------+
Appendix C. TICTOC Security Requirements
The following table compares the NTS specifications against the The following table compares the NTS specifications against the
TICTOC security requirements [I-D.ietf-tictoc-security-requirements]. TICTOC security requirements [RFC7384].
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| Section | Requirement from I-D tictoc | Requirement | NTS | | Section | Requirement from I-D tictoc | Requirement | NTS |
| | security-requirements-05 | level | | | | security-requirements-05 | level | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.1 | Authentication of Servers | MUST | OK | | 5.1.1 | Authentication of Servers | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.1 | Authorization of Servers | MUST | OK | | 5.1.1 | Authorization of Servers | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.2 | Recursive Authentication of | MUST | OK | | 5.1.2 | Recursive Authentication of | MUST | OK |
skipping to change at page 26, line 44 skipping to change at page 25, line 14
| 5.10 | Secure mode | MUST | - | | 5.10 | Secure mode | MUST | - |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| | Hybrid mode | SHOULD | - | | | Hybrid mode | SHOULD | - |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
*) See discussion in section Section 11.5. *) See discussion in section Section 11.5.
Comparison of NTS sepecification against TICTOC security Comparison of NTS sepecification against TICTOC security
requirements. requirements.
Appendix D. Broadcast Mode Appendix C. Broadcast Mode
For the broadcast mode, NTS adopts the TESLA protocol, which is based For the broadcast mode, NTS adopts the TESLA protocol with some
on a one-way key chain. This appendix provides details on the customizations. This appendix provides details on the generation and
generation and usage of the one-way key chain collected and assembled usage of the one-way key chain collected and assembled from
from [RFC4082]. Note that NTS is using the "not re-using keys" [RFC4082]. Note that NTS is using the "not re-using keys" scheme of
scheme of TESLA as described in section 3.7.2. of [RFC4082]. TESLA as described in section 3.7.2. of [RFC4082].
D.1. Server Preparations C.1. Server Preparations
Server setup: Server setup:
1. The server determines a reasonable upper bound B on the network 1. The server determines a reasonable upper bound B on the network
delay between itself and an arbitrary client, measured in delay between itself and an arbitrary client, measured in
milliseconds. milliseconds.
2. It determines the number n+1 of keys in the one-way key chain. 2. It determines the number n+1 of keys in the one-way key chain.
This yields the number n of keys that are usable to authenticate This yields the number n of keys that are usable to authenticate
broadcast packets. This number n is therefore also the number of broadcast packets. This number n is therefore also the number of
skipping to change at page 27, line 34 skipping to change at page 26, line 4
4. The server generates a random key K_n. 4. The server generates a random key K_n.
5. Using a one-way function F, the server generates a one-way chain 5. Using a one-way function F, the server generates a one-way chain
of n+1 keys K_0, K_1, ..., K_{n} according to of n+1 keys K_0, K_1, ..., K_{n} according to
K_i = F(K_{i+1}). K_i = F(K_{i+1}).
6. Using another one-way function F', it generates a sequence of n+1 6. Using another one-way function F', it generates a sequence of n+1
MAC keys K'_0, K'_1, ..., K'_{n-1} according to MAC keys K'_0, K'_1, ..., K'_{n-1} according to
K'_i = F'(K_i). K'_i = F'(K_i).
7. Each MAC key K'_i is assigned to the time interval I_i. 7. Each MAC key K'_i is assigned to the time interval I_i.
8. The server determines the key disclosure delay d, which is the 8. The server determines the key disclosure delay d, which is the
number of intervals between using a key and disclosing it. Note number of intervals between using a key and disclosing it. Note
that although security is still provided for all choices d>0, the that although security is provided for all choices d>0, the
choice still makes a difference: choice still makes a difference:
* If d is chosen too short, the client might discards packets * If d is chosen too short, the client might discard packets
because it fails to verify that the key used for their MAC has because it fails to verify that the key used for their MAC has
not been yet disclosed. not been yet disclosed.
* If d is chosen too long, the received packets have to be * If d is chosen too long, the received packets have to be
buffered for a unnecessarily long time before they can be buffered for a unnecessarily long time before they can be
verified by the client and subsequently be utilized for time verified by the client and subsequently be utilized for time
synchronization. synchronization.
The server SHOULD calculate d according to The server SHOULD calculate d according to
d = ceil( 2*B / L) + 1, d = ceil( 2*B / L) + 1,
where ceil gives the smallest integer greater than or equal to where ceil gives the smallest integer greater than or equal to
its argument. its argument.
< - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - < - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Generation of Keys Generation of Keys
F F F F F F F F
K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <-------- K_n K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <------- K_n
| | | | | | | |
| | | | | | | |
| F' | F' | F' | F' | F' | F' | F' | F'
| | | | | | | |
v v v v v v v v
K'_0 K'_1 ... K'_{n-1} K'_n K'_0 K'_1 ... K'_{n-1} K'_n
[______________|____ ____|_________________|__________] [______________|____ ____|_________________|_______]
I_1 ... I_{n-1} I_n I_1 ... I_{n-1} I_n
Course of Time/Usage of Keys Course of Time/Usage of Keys
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ->
A Schematic explanation on the TESLA protocol's one-way key chain A Schematic explanation on the TESLA protocol's one-way key chain
D.2. Client Preparation C.2. Client Preparation
A client needs the following information in order to participate in a A client needs the following information in order to participate in a
TESLA broadcast. TESLA broadcast.
o One key K_i from the one-way key chain, which has to be o One key K_i from the one-way key chain, which has to be
authenticated as belonging to the server. Typically, this will be authenticated as belonging to the server. Typically, this will be
K_0. K_0.
o The disclosure schedule of the keys. This consists of: o The disclosure schedule of the keys. This consists of:
skipping to change at page 29, line 17 skipping to change at page 27, line 40
o An upper bound D_t on how far its own clock is "behind" that of o An upper bound D_t on how far its own clock is "behind" that of
the server. the server.
Note that if D_t is greater than (d - 1) * L, then some authentic Note that if D_t is greater than (d - 1) * L, then some authentic
packets might be discarded. If D_t is greater than d * L, then all packets might be discarded. If D_t is greater than d * L, then all
authentic packets will be discarded. In the latter case, the client authentic packets will be discarded. In the latter case, the client
should not participate in the broadcast, since there will be no should not participate in the broadcast, since there will be no
benefit in doing so. benefit in doing so.
D.3. Sending Authenticated Broadcast Packets C.3. Sending Authenticated Broadcast Packets
During each time interval I_i, the server sends one authenticated During each time interval I_i, the server sends one authenticated
broadcast packet P_i. This packet consists of: broadcast packet P_i. This packet consists of:
o a message M_i, o a message M_i,
o the index i (in case a packet arrives late), o the index i (in case a packet arrives late),
o a MAC authenticating the message M_i, with K'_i used as key, o a MAC authenticating the message M_i, with K'_i used as key,
o the key K_{i-d}, which is included for disclosure. o the key K_{i-d}, which is included for disclosure.
D.4. Authentication of Received Packets C.4. Authentication of Received Packets
When a client receives a packet P_i as described above, it first When a client receives a packet P_i as described above, it first
checks that it has not received a packet with associated index i checks that it has not received a packet with the same disclosed key
before. This is done to avoid replay/flooding attacks. A packet before. This is done to avoid replay/flooding attacks. A packet
that fails this test is discarded. that fails this test is discarded.
Next, the client checks that, according to the disclosure schedule Next, the client begins to check the packet's timeliness by ensuring
and with respect to the upper bound D_t determined above, the server that, according to the disclosure schedule and with respect to the
cannot have disclosed the key K_i yet. Specifically, it needs to upper bound D_t determined above, the server cannot have disclosed
check that the server's clock cannot read a time that is in time the key K_i yet. Specifically, it needs to check that the server's
interval I_{i+d} or later. Since it works under the assumption that clock cannot read a time that is in time interval I_{i+d} or later.
the server's clock is not more than D_t "ahead" of the client's Since it works under the assumption that the server's clock is not
clock, the client can calculate an upper bound t_i for the server's more than D_t "ahead" of the client's clock, the client can calculate
clock at the time when P_i arrived by an upper bound t_i for the server's clock at the time when P_i
arrived. This upper bound t_i is calculated according to
t_i = R + D_t, t_i = R + D_t,
where R is the client's clock at the arrival of P_i. This implies where R is the client's clock at the arrival of P_i. This implies
that at the time of arrival of P_i, the server could have been in that at the time of arrival of P_i, the server could have been in
interval I_x at most, with interval I_x at most, with
x = floor((t_i - T_1) / L), x = floor((t_i - T_1) / L) + 1,
where floor gives the greatest integer less than or equal to its where floor gives the greatest integer less than or equal to its
argument. The client now needs to verify that argument. The client now needs to verify that
x < i+d x < i+d
is valid (see also section 3.5 of [RFC4082]). If falsified, it is is valid (see also section 3.5 of [RFC4082]). If falsified, it is
discarded. discarded.
If the check above is successful, the client performs another more
rigorous check: it sends a key check request to the server (in the
form of a client_keycheck message), asking explicitly if K_i has
already been disclosed. It remembers the timestamp t_check of the
sending time of that request as well as the nonce it used correlated
with the interval number i. If it receives an answer from the server
stating that K_i has not yet been disclosed and it is able to verify
the HMAC on that response, then it deduces that K_i was undisclosed
at t_check and therefore also at R. In this case, the clients
accepts P_i as timely.
Next the client verifies that a newly disclosed key K_{i-d} belongs Next the client verifies that a newly disclosed key K_{i-d} belongs
to the one-way key chain. To this end it verifies identity with some to the one-way key chain. To this end it applies the one-way
earlier disclosed key by recursively applies the one-way function F function F to K_{i-d} until it can verify identity with an earlier
to K_{i-d} (see Clause 3.5 in RFC 4082, item 3). disclosed key (see Clause 3.5 in RFC 4082, item 3).
Next the client verifies that the transmitted time value s_i belongs
to the time interval I_i, by checking
T_i =< s_i, and
s_i < T_{i+1}.
If falsified, the packet MUST be discarded and the client MUST
reinitialize the broadcast mode with a unicast association (because a
falsification of this check yields that the packet was not generated
according to protocol, which suggests an attack).
If a packet P_i passes all tests listed above, it is stored for later If a packet P_i passes all tests listed above, it is stored for later
authentication. Also, if at this time there is a package with index authentication. Also, if at this time there is a package with index
i-d already buffered, then the client uses the disclosed key K_{i-d} i-d already buffered, then the client uses the disclosed key K_{i-d}
to derive K'_{i-d} and uses that to check the MAC included in package to derive K'_{i-d} and uses that to check the MAC included in package
P_{i-d}. On success, it regards M_{i-d} as authenticated. P_{i-d}. On success, it regards M_{i-d} as authenticated.
Appendix E. Random Number Generation Appendix D. Random Number Generation
At various points of the protocol, the generation of random numbers At various points of the protocol, the generation of random numbers
is required. The employed methods of generation need to be is required. The employed methods of generation need to be
cryptographically secure. See [RFC4086] for guidelines concerning cryptographically secure. See [RFC4086] for guidelines concerning
this topic. this topic.
Authors' Addresses Authors' Addresses
Dieter Sibold Dieter Sibold
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
Stephen Roettger Stephen Roettger
Google Inc
Email: stephen.roettger@googlemail.com Email: stephen.roettger@googlemail.com
Kristof Teichel Kristof Teichel
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8421 Phone: +49-(0)531-592-8421
Email: kristof.teichel@ptb.de Email: kristof.teichel@ptb.de
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