draft-ietf-ntp-network-time-security-05.txt   draft-ietf-ntp-network-time-security-06.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: April 26, 2015 Google Inc Expires: July 20, 2015 Google Inc.
K. Teichel K. Teichel
PTB PTB
October 23, 2014 January 16, 2015
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-05.txt draft-ietf-ntp-network-time-security-06.txt
Abstract Abstract
This document describes the Network Time Security (NTS) protocol that This document describes Network Time Security (NTS), a collection of
enables secure time synchronization with time servers using Network measures that enable secure time synchronization with time servers
Time Protocol (NTP) or Precision Time Protocol (PTP). Its design using protocols like the Network Time Protocol (NTP) or the Precision
considers the special requirements of precise timekeeping, which are Time Protocol (PTP). Its design considers the special requirements
described in Security Requirements of Time Protocols in Packet of precise timekeeping which are described in Security Requirements
Switched Networks [RFC7384]. of Time Protocols in Packet 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
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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 April 26, 2015. This Internet-Draft will expire on July 20, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . 4
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 5 6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 5
5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 5
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" . . . . . . . . . . . . 6
6.1.2. Message Type: "server_assoc" . . . . . . . . . . . . 7 6.1.2. Message Type: "server_assoc" . . . . . . . . . . . . 6
6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 8 6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 7
6.2.1. Message Type: "client_cook" . . . . . . . . . . . . . 8 6.2.1. Message Type: "client_cook" . . . . . . . . . . . . . 7
6.2.2. Message Type: "server_cook" . . . . . . . . . . . . . 8 6.2.2. Message Type: "server_cook" . . . . . . . . . . . . . 7
6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 9 6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 8
6.3.1. Message Type: "time_request" . . . . . . . . . . . . 9 6.3.1. Message Type: "time_request" . . . . . . . . . . . . 8
6.3.2. Message Type: "time_response" . . . . . . . . . . . . 9 6.3.2. Message Type: "time_response" . . . . . . . . . . . . 8
6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 10 6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 9
6.4.1. Message Type: "client_bpar" . . . . . . . . . . . . . 10 6.4.1. Message Type: "client_bpar" . . . . . . . . . . . . . 9
6.4.2. Message Type: "server_bpar" . . . . . . . . . . . . . 10 6.4.2. Message Type: "server_bpar" . . . . . . . . . . . . . 9
6.5. Broadcast Messages . . . . . . . . . . . . . . . . . . . 11 6.5. Broadcast Messages . . . . . . . . . . . . . . . . . . . 10
6.5.1. Message Type: "server_broad" . . . . . . . . . . . . 11 6.5.1. Message Type: "server_broad" . . . . . . . . . . . . 10
6.6. Broadcast Key Check . . . . . . . . . . . . . . . . . . . 11 6.6. Broadcast Key Check . . . . . . . . . . . . . . . . . . . 10
6.6.1. Message Type: "client_keycheck" . . . . . . . . . . . 11 6.6.1. Message Type: "client_keycheck" . . . . . . . . . . . 10
6.6.2. Message Type: "server_keycheck" . . . . . . . . . . . 12 6.6.2. Message Type: "server_keycheck" . . . . . . . . . . . 11
7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 12 7. Message Dependencies . . . . . . . . . . . . . . . . . . . . 11
7.1. The Client . . . . . . . . . . . . . . . . . . . . . . . 12 8. Server Seed Considerations . . . . . . . . . . . . . . . . . 12
7.1.1. The Client in Unicast Mode . . . . . . . . . . . . . 12 9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 13
7.1.2. The Client in Broadcast Mode . . . . . . . . . . . . 14 9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 13
7.2. The Server . . . . . . . . . . . . . . . . . . . . . . . 16 9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 13
7.2.1. The Server in Unicast Mode . . . . . . . . . . . . . 16 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7.2.2. The Server in Broadcast Mode . . . . . . . . . . . . 16 11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 17 11.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 17 11.2. Initial Verification of the Server Certificates . . . . 14
8.2. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 17 11.3. Revocation of Server Certificates . . . . . . . . . . . 14
8.3. Server Seed Lifetime . . . . . . . . . . . . . . . . . . 17 11.4. Mitigating Denial-of-Service for broadcast packets . . . 14
9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 17 11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 17 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 18 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 13.1. Normative References . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18 13.2. Informative References . . . . . . . . . . . . . . . . . 17
11.1. Initial Verification of the Server Certificates . . . . 18 Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 17
11.2. Revocation of Server Certificates . . . . . . . . . . . 18 Appendix B. Using TESLA for Broadcast-Type Messages . . . . . . 19
11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 19 B.1. Server Preparation . . . . . . . . . . . . . . . . . . . 19
11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 19 B.2. Client Preparation . . . . . . . . . . . . . . . . . . . 20
11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 19 B.3. Sending Authenticated Broadcast Packets . . . . . . . . . 21
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 B.4. Authentication of Received Packets . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 Appendix C. Random Number Generation . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 22
Appendix B. TICTOC Security Requirements . . . . . . . . . . . . 24
Appendix C. Broadcast Mode . . . . . . . . . . . . . . . . . . . 25
C.1. Server Preparations . . . . . . . . . . . . . . . . . . . 25
C.2. Client Preparation . . . . . . . . . . . . . . . . . . . 27
C.3. Sending Authenticated Broadcast Packets . . . . . . . . . 27
C.4. Authentication of Received Packets . . . . . . . . . . . 28
Appendix D. Random Number Generation . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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 have to be secured if they Therefore, time synchronization protocols have to be secured if they
are applied in environments that are prone to malicious attacks. are applied in environments that are prone to malicious attacks.
This can be accomplished by utilization of external security This can be accomplished either by utilization of external security
protocols like IPsec or by intrinsic security measures of the time protocols, like IPsec or TLS, or by intrinsic security measures of
synchronization protocol. the time synchronization protocol.
The two most popular time synchronization protocols, the Network Time The two most popular time synchronization protocols, the Network Time
Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP) Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP)
[IEEE1588], currently do not provide adequate intrinsic security [IEEE1588], currently do not provide adequate intrinsic security
precautions. This document specifies security measures for NTP and precautions. This document specifies security measures which enable
PTP which enable these protocols to verify authenticity of the time these protocols to verify the authenticity of the time server and the
server and integrity of the time synchronization protocol packets. integrity of the time synchronization protocol packets.
The protocol is specified with the prerequisite in mind that precise The given measures are specified with the prerequisite in mind that
timekeeping can only be accomplished with stateless time precise 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 [RFC7384]. time transfer [RFC7384].
Note: Note:
The intent is to formulate the protocol to be applicable to NTP It is recommended that details on how to apply NTS to specific
and also PTP. In the current state the specification focuses on time synchronization protocols be formulated in separate
the application to NTP. documents, with one separate document for each protocol.
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 time
PTP can be found in the "Security Requirements of Time Protocols in synchronization protocols can be found in the "Security Requirements
Packet Switched Networks" [RFC7384]. of Time Protocols in 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 a client/slave to authenticate its time
servers. server(s)/master(s).
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 Integration with protocols: NTS can be used to secure different
time synchronization protocols, specifically at least NTP and PTP.
o Operational modes of PTP should be supported as far as possible. An client or server running an NTS-secured version of a time
protocol does not negatively affect other participants who are
o Hybrid mode: Both secure and insecure communication modes are running unsecured versions of that protocol.
possible for NTP servers and clients, respectively.
o 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 4. Terms and Abbreviations
MITM Man In The Middle MITM Man In The Middle
NTP Network Time Protocol [RFC5905]
NTS Network Time Security NTS Network Time Security
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
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/master and to exchange a symmetric key, the so-called cookie.
cookie is then used to protect authenticity and integrity of the This cookie is then used to protect the authenticity and the
subsequent time synchronization packets by means of a Message integrity of subsequent unicast-type time synchronization packets.
Authentication Code (MAC), which is attached to each time This is done by means of a Message Authentication Code (MAC), which
synchronization packet. The calculation of the MAC includes the is attached to each time synchronization packet. The calculation of
whole time synchronization packet and the cookie which is shared the MAC includes the whole time synchronization packet and the cookie
between client and server. The cookie is calculated according to: which is shared between client and server. The cookie is calculated
according to:
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 [RFC7384]. See Section 8 for details on the
seed refresh and Section 7.1.1 for the client's reaction to it.
The server does not keep a state of the client. Therefore it has to cookie = MSB_<b> (HMAC(server seed, H(certificate of client))),
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 with the server seed as the key, where H is a hash function, and
where the function MSB_<b> cuts off the b most significant bits of
the result of the HMAC function. The server seed is a random value
of bit length b that the server possesses, 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 [RFC7384]. See
Section 8 for details on seed refreshing.
Just as in the case of the client server mode and symmetric mode, Since the server does not keep a state of the client, it has to
authenticity and integrity of the NTP packets are ensured by a MAC, recalculate the cookie each time it receives a unicast time
which is attached to the NTP packet by the sender. Verification of synchronization request from the client. To this end, the client has
the packets' authenticity is based on the TESLA protocol, in to attach the hash value of its certificate to each request (see
particular on its "not re-using keys" scheme, see section 3.7.2 of Section 6.3).
[RFC4082]. TESLA uses a one-way chain of keys, where each key is the For broadcast-type messages, authenticity and integrity of the time
output of a one-way function applied to the previous key in the synchronization packets are also ensured by a MAC, which is attached
chain. The last element of the chain is shared securely with all to the time synchronization packet by the sender. Verification of
clients. The server splits time into intervals of uniform duration the broadcast-type packets' authenticity is based on the TESLA
and assigns each key to an interval in reverse order, starting with protocol, in particular on its "not re-using keys" scheme, see
the penultimate. At each time interval, the server sends an NTP Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys,
broadcast packet appended by a MAC, calculated using the where each key is the output of a one-way function applied to the
corresponding key, and the key of the previous disclosure interval. previous key in the chain. The last element of the chain is shared
The client verifies the MAC by buffering the packet until the securely with all clients. The server splits time into intervals of
disclosure of the key in its associated disclosure interval. In uniform duration and assigns each key to an interval in reverse
order to be able to verify the validity of the key, the client has to order, starting with the penultimate. At each time interval, the
be loosely time synchronized to the server. This has to be server sends a broadcast packet appended by a MAC, calculated using
accomplished during the initial client server exchange between the corresponding key, and the key of the previous disclosure
broadcast client and server. In addition, NTS uses another, more interval. The client verifies the MAC by buffering the packet until
rigorous check to what is used in the TESLA protocol. For a more disclosure of the key in its associated disclosure interval occurs.
In order to be able to verify the validity of the key, the client has
to be loosely time synchronized with the server. This has to be
accomplished during the initial client server exchange between the
broadcast client and the server. In addition, NTS uses another, more
rigorous check than what is used in the TESLA protocol. For a more
detailed description of how NTS employs and customizes TESLA, see detailed description of how NTS employs and customizes TESLA, see
Appendix C. Appendix B.
6. Protocol Messages 6. Protocol Messages
This section describes the types of messages needed for secure time This section describes the types of messages needed for secure time
synchronization with NTS. synchronization with NTS.
For some guidance on how these message types can be realized in For some guidance on how these message types can be realized in
practice, for use with existing time synchronization protocols, see practice, and integrated into the communication flow of existing time
[I-D.ietf-ntp-cms-for-nts-messages], a companion document for NTS. synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a
Said document describes ASN.1 encodings for those message parts that companion document for NTS. Said document describes ASN.1 encodings
have to be added to a time synchronization protocol for security for those message parts that have to be added to a time
reasons as well as CMS (Cryptographic Message Syntax, see [RFC5652]) synchronization protocol for security reasons as well as CMS
conventions that can be used to get the cryptographic aspects right. (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 6.1. Association Messages
In this message exchange, the hash and encryption algorithms that are In this message exchange, the hash and encryption algorithms that are
used throughout the protocol are negotiated. Also, the client used throughout the protocol are negotiated. In addition , the
receives the certification chain up to a trusted anchor. With the client receives the certification chain up to a trusted anchor. With
established certification chain the client is able to verify the the established certification chain the client is able to verify the
server's signatures and, hence, authenticity of future NTS messages server's signatures and, hence, the authenticity of future NTS
from the server is ensured. 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),
skipping to change at page 7, line 34 skipping to change at page 6, line 46
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,
o the server's choice of algorithm for encryption and for o the server's choice of algorithm for encryption and for
cryptographic hashing, all of which MUST be chosen from the cryptographic hashing, all of which MUST be chosen from the
client's proposals. client's proposals,
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
which is also used for the certificates which are included (see which is also used for the certificates that are included (see
below), below), and
o a chain of certificates, which starts at the server and goes up to o a chain of certificates, which starts at the server and goes up to
a trusted authority, and each certificate MUST be certified by the a trusted authority; each certificate MUST be certified by the one
one directly following it. directly following it.
6.2. 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 later be used for integrity
protection during unicast time synchronization. protection during unicast time synchronization.
6.2.1. 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 nonce,
o the negotiated hash algorithm H, o the negotiated hash algorithm H,
o the client's certificate. o the client's certificate.
6.2.2. Message Type: "server_cook" 6.2.2. 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 according to Section 5. This message 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 a concatenated datum which is encrypted with the client's public
o a concatenated datum, which is encrypted with the client's public
key, according to the encryption algorithm transmitted in the key, according to the encryption algorithm transmitted in the
client_cook message. The concatenated datum contains client_cook message. The concatenated datum contains
* the nonce transmitted in client_cook, and * the nonce transmitted in client_cook, and
* the cookie. * the cookie.
o a signature, created with the server's private key, calculated o a signature, created with the server's private key, calculated
over all of the data listed above. This signature MUST be over all of the data listed above. This signature MUST be
calculated according to the transmitted signature algorithm from calculated according to the transmitted signature algorithm from
skipping to change at page 9, line 17 skipping to change at page 8, line 27
6.3. 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.
6.3.1. 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 a 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 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.
6.3.2. Message Type: "time_response" 6.3.2. 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 has received a
message. Prior to this the server MUST recalculate the client's time_request message. Prior to this the server MUST recalculate the
cookie by using the hash of the client's certificate and the client's 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.
6.4. Broadcast Parameter Messages 6.4. 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 Appendix C for more details on TESLA. See Appendix B for more details on TESLA.
6.4.1. 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 NTS 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.
6.4.2. Message Type: "server_bpar" 6.4.2. 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
skipping to change at page 11, line 17 skipping to change at page 10, line 24
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.
6.5.1. 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 is working under,
under,
o time broadcast data, o time broadcast data,
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.
6.6. Broadcast Key Check 6.6. Broadcast Key Check
This message exchange is performed for an additional check of packet This message exchange is performed for an additional check of packet
timeliness in the course of the TESLA scheme, see Appendix C. timeliness in the course of the TESLA scheme, see Appendix B.
6.6.1. Message Type: "client_keycheck" 6.6.1. Message Type: "client_keycheck"
A message of this type is sent by the client in order to initiate an 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 additional check of packet timeliness for the TESLA scheme. It
contains contains
o the NTS message ID "client_keycheck", o the NTS message ID "client_keycheck",
o the version number chosen for the broadcast, o the NTS version number negotiated during association in unicast
mode,
o a nonce,
o a 128-bit nonce,
o an interval number from the TESLA disclosure schedule, o an interval number from the TESLA disclosure schedule,
o the hash algorithm H negotiated in unicast mode, and o the hash algorithm H negotiated in unicast mode, and
o the hash of the client's certificate under H. o the hash of the client's certificate under H.
6.6.2. Message Type: "server_keycheck" 6.6.2. Message Type: "server_keycheck"
A message of this type is sent by the server upon receipt of a A message of this type is sent by the server upon receipt of a
client_keycheck message during the broadcast loop of the server. client_keycheck message during the broadcast loop of the server.
Prior to this the server MUST recalculate the client's cookie by Prior to this, the server MUST recalculate the client's cookie by
using the hash of the client's certificate and the transmitted hash using the hash of the client's certificate and the transmitted hash
algorithm. It contains algorithm. It contains
o the NTS message ID "server_keycheck" o the NTS message ID "server_keycheck"
o the version number that the server's broadcast mode is working o the version number as transmitted in "client_keycheck,
under,
o the 128-bit nonce transmitted in the client_keycheck message, o the nonce transmitted in the client_keycheck message,
o the interval number transmitted in the client_keycheck message, o the interval number transmitted in the client_keycheck message,
and and
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.
7. Protocol Sequence 7. Message Dependencies
+--------------------+
7.1. The Client |Association Exchange|
+--------------------+
7.1.1. The Client in Unicast Mode |
At least one successful
For a unicast run, the client performs the following steps: |
v
1. It sends a client_assoc message to the server. It MUST keep the +---------------+
transmitted values for version number and algorithms available |Cookie Exchange|
for later checks. +---------------+
|
2. It waits for a reply in the form of a server_assoc message. At least one successful
After receipt of the message it performs the following checks: |
v
* The client checks that the message contains a conform version +----------------------------------------+
number. |Unicast Time Synchronization Exchange(s)|
+----------------------------------------+
* It also verifies that the server has chosen the encryption and |
hash algorithms from its proposal sent in the client_assoc Until sufficient accuracy has been reached
message. |
v
* Furthermore, it performs authenticity checks on the +----------------------------+
certificate chain and the signature for the version number. |Broadcast Parameter Exchange|
+----------------------------+
If one of the checks fails, the client MUST abort the run. |
Discussion: One successful per client
|
Note that by performing the above message exchange and checks, v
the client validates the authenticity of its immediate NTP +----------------------------------------+
server only. It does not recursively validate the |Broadcast Time Synchronization Reception|
authenticity of each NTP server on the time synchronization +----------------------------------------+
chain. Recursive authentication (and authorization) as |
formulated in [RFC7384] depends on the chosen trust anchor. Whenever deemed necessary
|
3. Next, it sends a client_cook message to the server. The client v
MUST save the included nonce until the reply has been processed. +-----------------+
|Keycheck Exchange|
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 Figure 1.
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:
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 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.
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:
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
in Section 6.1.2.
o 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.
o 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
Section 8.1).
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:
o Upon receipt of a client_bpar message, the server constructs and
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
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 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 According to the requirements in [RFC7384], the server MUST refresh
each server seed periodically. Consequently, the cookie memorized by
According to the requirements in [RFC7384] the server MUST refresh the client becomes obsolete. In this case, the client cannot verify
each server seed periodically. As a consequence, the cookie the MAC attached to subsequent time response messages and has to
memorized by the client becomes obsolete. In this case the client respond accordingly by re-initiating the protocol with a cookie
cannot verify the MAC attached to subsequent time response messages request (Section 6.2).
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. 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
cookie and the MAC, and hashing of the client's certificate. Client cookie and the MAC, and hashing of the client's certificate. The
and server negotiate a hash algorithm H during the association client and the server negotiate a hash algorithm H during the
message exchange (Section 6.1) at the beginning of a unicast run. association message exchange (Section 6.1) at the beginning of a
The selected algorithm H is used for all hashing processes in that unicast run. The selected algorithm H is used for all hashing
run. processes in that run.
In broadcast mode, hash algorithms are used as pseudo random In the TESLA scheme, hash algorithms are used as pseudo-random
functions to construct the one-way key chain. Here, the utilized functions to construct the one-way key chain. Here, the utilized
hash algorithm is communicated by the server and non-negotiable. hash algorithm is communicated by the server and is non-negotiable.
The list of the hash algorithms supported by the server has to
fulfill the following requirements:
o it MUST NOT include SHA-1 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms.
Note Note:
Any hash algorithm is prone to be compromised in the future. A Any hash algorithm is prone to be compromised in the future. A
successful attack on a hash algorithm would enable any NTS client successful attack on a hash algorithm would enable any NTS client
to derive the server seed from their own cookie. Therefore, the to derive the server seed from its own cookie. Therefore, the
server MUST have separate seed values for its different supported server MUST have separate seed values for its different supported
hash algorithms. This way, knowledge gained from an attack on a hash algorithms. This way, knowledge gained from an attack on a
hash algorithm H can at least only be used to compromise such hash algorithm H can at least only be used to compromise such
clients who use hash algorithm H as well. clients who use hash algorithm H as well.
9.2. MAC Calculation 9.2. MAC Calculation
For the calculation of the MAC, client and server are using a Keyed- For the calculation of the MAC, client and server use a Keyed-Hash
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC Message Authentication Code (HMAC) approach [RFC2104]. The HMAC is
is generated with the hash algorithm specified by the client (see 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. Privacy
tbd
11.2. 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 6.1.2). Since it generally certification message exchange (Section 6.1.2). Since it generally
has 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.
skipping to change at page 18, line 40 skipping to change at page 14, line 27
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].
o The client requests a different service to get an initial time o The client requests a different service to get an initial time
stamp in order to be able to verify the certificates' periods of stamp in order to be able to verify the certificates' periods of
validity. To this end, it can, e.g., use a secure shell validity. To this end, it can, e.g., use a secure shell
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 11.3. 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 According to Section 8, it is the client's responsibility to initiate
is not applicable to the concept of NTP pools. Therefore, NTS is not a new association with the server after the server's certificate
able to provide secure usage of NTP pools. expires. To this end, the client reads the expiration date of the
certificate during the certificate message exchange (Section 6.1.2).
Furthermore, certificates may also be revoked prior to the normal
expiration date. To increase security the client MAY periodically
verify the state of the server's certificate via OCSP.
11.4. Denial-of-Service in Broadcast Mode 11.4. Mitigating Denial-of-Service for broadcast packets
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, the client and the server use the "not
keys" scheme of TESLA as pointed out in section 3.7.2 of RFC 4082 re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC
[RFC4082]. In this scheme the server never uses a key for the MAC 4082 [RFC4082]. In this scheme the server never uses a key for the
generation more than once. Therefore the client can discard any MAC 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 already knows, thus
preventing memory flooding attacks. preventing memory flooding attacks.
Note that an alternative approach to enhance TESLA's resistance Note that an alternative approach to enhance TESLA's resistance
against DoS attacks involves the addition of a group MAC to each against DoS attacks involves the addition of a group MAC to each
packet. This requires the exchange of an additional shared key packet. This requires the exchange of an additional shared key
common to the whole group. This adds additional complexity to the common to the whole group. This adds additional complexity to the
protocol and 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 [RFC7384]. This prevents the client to measure the asymmetrically [RFC7384]. This prevents the client from accurately
network delay, and hence its time offset to the server, accurately measuring the network delay, and hence its time offset to the server
[Mizrahi]. The delay attack does not modify the content of the [Mizrahi]. The delay attack does not modify the content of the
exchanged synchronization packets. Therefore cryptographic means do exchanged synchronization packets. Therefore, cryptographic means do
not provide a feasible way to mitigate this attack. However, several not provide a feasible way to mitigate this attack. However, several
non-cryptographic precautions can be taken in order to detect this non-cryptographic precautions can be taken in order to detect this
attack. attack.
1. 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 synchronization packets between the majority of servers. This
approach is commonly used in NTP to exclude incorrect time approach is commonly used in NTP to exclude incorrect time
servers [RFC5905]. servers [RFC5905].
2. Multiple communication paths: The client and server are utilizing 2. Multiple communication paths: The client and server utilize
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 attack, provided that the adversary is unable to manipulate
the 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.
3. Usage of an encrypted connection: the client exchanges all 3. Usage of an encrypted connection: the client exchanges all
packets with the time server over an encrypted connection (e.g. packets with the time server over an encrypted connection (e.g.
IPsec). This measure does not mitigate the delay attack but it IPsec). This measure does not mitigate the delay attack, but it
makes it more difficult for the adversary to identify the time makes it more difficult for the adversary to identify the time
synchronization packets. synchronization packets.
4. For the unicast mode: Introduction of a threshold value for the 4. For unicast-type messages: Introduction of a threshold value for
delay time of the synchronization packets. The client can the delay time of the synchronization packets. The client can
discard a time server if the packet delay time of this time discard a time server if the packet delay time of this time
server is larger than the threshold value. server is larger than the threshold value.
Additional provision against delay attacks has to be taken in the Additional provision against delay attacks has to be taken for
broadcast mode. This mode relies on the TESLA scheme which is based broadcast-type messages. This mode relies on the TESLA scheme which
on the requirement that a client and the broadcast server are loosely is based on the requirement that a client and the broadcast server
time synchronized. Therefore, a broadcast client has to establish are loosely time synchronized. Therefore, a broadcast client has to
time synchronization with its broadcast server before it maintains establish time synchronization with its broadcast server before it
time synchronization by utilization of the broadcast mode. To this starts utilizing broadcast messages for time synchronization. To
end it initially establishes a unicast association with its broadcast this end, it initially establishes a unicast association with its
server until time synchronization and calibration of the packet delay broadcast server until time synchronization and calibration of the
time is achieved. After that it establishes a broadcast association packet delay time is achieved. After that it establishes a broadcast
to the broadcast server and utilizes TESLA to verify integrity and association with the broadcast server and utilizes TESLA to verify
authenticity of any received broadcast packets. integrity and authenticity of any received broadcast packets.
An adversary who is able to delay broadcast packets can cause a time An adversary who is able to delay broadcast packets can cause a time
adjustment at the receiving broadcast clients. If the adversary adjustment at the receiving broadcast clients. If the adversary
delays broadcast packets continuously, then the time adjustment will delays broadcast packets continuously, then the time adjustment will
accumulate until the loose time synchronization requirement is accumulate until the loose time synchronization requirement is
violated, which breaks the TESLA scheme. To mitigate this violated, which breaks the TESLA scheme. To mitigate this
vulnerability the security condition in TESLA has to be supplemented vulnerability the security condition in TESLA has to be supplemented
by an additional check in which the client, upon receipt of a by an additional check in which the client, upon receipt of a
broadcast message, verifies the status of the corresponding key via a broadcast message, verifies the status of the corresponding key via a
unicast message exchange with the broadcast server (see section unicast message exchange with the broadcast server (see Appendix B.4
Appendix C.4 for a detailed description of this check). Note, that a for a detailed description of this check). Note that a broadcast
broadcast client should also apply the above mentioned precautions as client should also apply the above-mentioned precautions as far as
far as possible. possible.
12. Acknowledgements 12. Acknowledgements
The authors would like to thank Russ Housley, Steven Bellovin, David The authors would like to thank Russ Housley, Steven Bellovin, David
Mills and Kurt Roeckx for discussions and comments on the design of Mills and Kurt Roeckx for discussions and comments on the design of
NTS. Also, thanks to Harlan Stenn for his technical review and NTS. Also, thanks go to Harlan Stenn for his technical review and
specific text contributions to this document. specific text contributions to this document.
13. References 13. References
13.1. Normative References
[IEEE1588] 13.1. Normative References
IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and
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
1997. 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009. RFC 5652, September 2009.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, 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.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, October 2014.
13.2. Informative References 13.2. Informative References
[I-D.ietf-ntp-cms-for-nts-message]
Sibold, D., Roettger, S., Teichel, K., and R. Housley,
"Protecting Network Time Security Messages with the
Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
for-nts-message-00 (work in progress), October 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.
[IEEE1588]
IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and
control systems", 2008.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks [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.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Packet Switched Networks", RFC 7384, October 2014. Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[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. TICTOC Security Requirements
+---------------------+
|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-----------+
Figure 1: The client's behavior in NTS unicast mode.
+-----------------------------+
|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 | |
| +------+------+ +--------+--------+ |
| | | |
+-----------+ +-----------------------------------+
Figure 2: The client's behaviour in NTS broadcast mode.
Appendix B. TICTOC Security Requirements
The following table compares the NTS specifications against the The following table compares the NTS specifications against the
TICTOC security requirements [RFC7384]. 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 |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
skipping to change at page 24, line 27 skipping to change at page 18, line 17
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.2 | Recursive Authentication of | MUST | OK | | 5.1.2 | Recursive Authentication of | MUST | OK |
| | Servers (Stratum 1) | | | | | Servers (Stratum 1) | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.2 | Recursive Authorization of Servers | MUST | OK | | 5.1.2 | Recursive Authorization of Servers | MUST | OK |
| | (Stratum 1) | | | | | (Stratum 1) | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.1.3 | Authentication and Authorization | MAY | - | | 5.1.3 | Authentication and Authorization | MAY | - |
| | of Slaves | | | | | of Slaves | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.2 | Integrity protection. | MUST | OK | | 5.2 | Integrity protection | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.4 | Protection against DoS attacks | SHOULD | OK | | 5.4 | Protection against DoS attacks | SHOULD | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.5 | Replay protection | MUST | OK | | 5.5 | Replay protection | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.6 | Key freshness. | MUST | OK | | 5.6 | Key freshness | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| | Security association. | SHOULD | OK | | | Security association | SHOULD | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| | Unicast and multicast | SHOULD | OK | | | Unicast and multicast associations | SHOULD | OK |
| | associations. | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.7 | Performance: no degradation in | MUST | OK | | 5.7 | Performance: no degradation in | MUST | OK |
| | quality of time transfer. | | | | | quality of time transfer | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| | Performance: lightweight | SHOULD | OK | | | Performance: lightweight | SHOULD | OK |
| | computation | | | | | computation | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| | Performance: storage, bandwidth | SHOULD | OK | | | Performance: storage, bandwidth | SHOULD | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.7 | Confidentiality protection | MAY | NO | | 5.7 | Confidentiality protection | MAY | NO |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.9 | Protection against Packet Delay | SHOULD | NA*) | | 5.9 | Protection against Packet Delay | SHOULD | NA*) |
| | and Interception Attacks | | | | | and Interception Attacks | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 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 11.5.
Comparison of NTS sepecification against TICTOC security Comparison of NTS specification against TICTOC security requirements.
requirements.
Appendix C. Broadcast Mode Appendix B. Using TESLA for Broadcast-Type Messages
For the broadcast mode, NTS adopts the TESLA protocol with some For broadcast-type messages , NTS adopts the TESLA protocol with some
customizations. This appendix provides details on the generation and customizations. This appendix provides details on the generation and
usage of the one-way key chain collected and assembled from usage of the one-way key chain collected and assembled from
[RFC4082]. Note that NTS is using the "not re-using keys" scheme of [RFC4082]. Note that NTS uses the "not re-using keys" scheme of
TESLA as described in section 3.7.2. of [RFC4082]. TESLA as described in Section 3.7.2. of [RFC4082].
C.1. Server Preparations B.1. Server Preparation
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
time intervals during which the server can send authenticated time intervals during which the server can send authenticated
broadcast messages before it has to calculate a new key chain. broadcast messages before it has to calculate a new key chain.
3. It divides time into n uniform intervals I_1, I_2, ..., I_n. 3. It divides time into n uniform intervals I_1, I_2, ..., I_n.
Each of these time intervals has length L, measured in Each of these time intervals has length L, measured in
milliseconds. In order to fulfill the requirement 3.7.2. of RFC milliseconds. In order to fulfill the requirement 3.7.2. of RFC
4082 the time interval L has to be smaller than the time interval 4082, the time interval L has to be shorter than the time
between the broadcast messages. interval between the broadcast messages.
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
skipping to change at page 26, line 14 skipping to change at page 20, line 6
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 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 discard 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 its MAC has
not been yet disclosed. not yet been 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 an unnecessarily long time before they can be
verified by the client and subsequently be utilized for time verified by the client and be subsequently 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 yields 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 of the TESLA protocol's one-way key chain
C.2. Client Preparation B.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:
* the length n of the one-way key chain, * the length n of the one-way key chain,
* the length L of the time intervals I_1, I_2, ..., I_n, * the length L of the time intervals I_1, I_2, ..., I_n,
* the starting time T_i of an interval I_i. Typically this is * the starting time T_i of an interval I_i. Typically this is
the starting time T_1 of the first interval; the starting time T_1 of the first interval;
* the disclosure delay d. * the disclosure delay d.
o The one-way function F used to recursively derive the keys in the o The one-way function F used to recursively derive the keys in the
one-way key chain, one-way key chain,
skipping to change at page 27, line 40 skipping to change at page 21, line 26
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.
C.3. Sending Authenticated Broadcast Packets B.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.
C.4. Authentication of Received Packets B.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 the same disclosed key checks that it has not already received a packet with the same
before. This is done to avoid replay/flooding attacks. A packet disclosed key. This is done to avoid replay/flooding attacks. A
that fails this test is discarded. packet that fails this test is discarded.
Next, the client begins to check the packet's timeliness by ensuring Next, the client begins to check the packet's timeliness by ensuring
that, according to the disclosure schedule and with respect to the that according to the disclosure schedule and with respect to the
upper bound D_t determined above, the server cannot have disclosed upper bound D_t determined above, the server cannot have disclosed
the key K_i yet. Specifically, it needs to check that the server's the key K_i yet. Specifically, it needs to check that the server's
clock cannot read a time that is in time interval I_{i+d} or later. clock cannot read a time that is in time interval I_{i+d} or later.
Since it works under the assumption that the server's clock is not Since it works under the assumption that the server's clock is not
more than D_t "ahead" of the client's clock, the client can calculate more than D_t "ahead" of the client's clock, the client can calculate
an upper bound t_i for the server's clock at the time when P_i 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 arrived. This upper bound t_i is calculated according to
t_i = R + D_t, t_i = R + D_t,
skipping to change at page 28, line 35 skipping to change at page 22, line 21
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) + 1, 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 it is falsified, it
discarded. is discarded.
If the check above is successful, the client performs another more If the check above is successful, the client performs another more
rigorous check: it sends a key check request to the server (in the 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 form of a client_keycheck message), asking explicitly if K_i has
already been disclosed. It remembers the timestamp t_check of the already been disclosed. It remembers the time stamp t_check of the
sending time of that request as well as the nonce it used correlated 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 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 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 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 at t_check and therefore also at R. In this case, the client accepts
accepts P_i as timely. 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 applies the one-way to the one-way key chain. To this end, it applies the one-way
function F to K_{i-d} until it can verify identity with an earlier function F to K_{i-d} until it can verify the identity with an
disclosed key (see Clause 3.5 in RFC 4082, item 3). earlier disclosed key (see Clause 3.5 in RFC 4082, item 3).
Next the client verifies that the transmitted time value s_i belongs Next the client verifies that the transmitted time value s_i belongs
to the time interval I_i, by checking to the time interval I_i, by checking
T_i =< s_i, and T_i =< s_i, and
s_i < T_{i+1}. s_i < T_{i+1}.
If falsified, the packet MUST be discarded and the client MUST If it is falsified, the packet MUST be discarded and the client MUST
reinitialize the broadcast mode with a unicast association (because a reinitialize its broadcast module by performing a unicast time
falsification of this check yields that the packet was not generated synchronization as well as a new broadcast parameter exchange
according to protocol, which suggests an attack). (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 the tests listed above, it is stored for
authentication. Also, if at this time there is a package with index later authentication. Also, if at this time there is a package with
i-d already buffered, then the client uses the disclosed key K_{i-d} index i-d already buffered, then the client uses the disclosed key
to derive K'_{i-d} and uses that to check the MAC included in package K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in
P_{i-d}. On success, it regards M_{i-d} as authenticated. package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.
Appendix D. Random Number Generation Appendix C. 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 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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