draft-ietf-ntp-network-time-security-03.txt   draft-ietf-ntp-network-time-security-04.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: September 20, 2014 Expires: January 5, 2015
K. Teichel K. Teichel
PTB PTB
March 19, 2014 July 04, 2014
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-03.txt draft-ietf-ntp-network-time-security-04.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 authentication of time servers using Network Time
Protocol (NTP) or Precision Time Protocol (PTP). Its design 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 [I-D.ietf-tictoc-security-requirements].
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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-
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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 September 20, 2014. This Internet-Draft will expire on January 5, 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 . . . . . . . . . . . . . . . . . . . . . 5 5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 6
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" . . . . . . . . . . . . 6 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. Certificate Messages . . . . . . . . . . . . . . . . . . 7
6.2.1. Message type: "client_cert" . . . . . . . . . . . . . 7 6.2.1. Message Type: "client_cert" . . . . . . . . . . . . . 8
6.2.2. Message type: "server_cert" . . . . . . . . . . . . . 8 6.2.2. Message Type: "server_cert" . . . . . . . . . . . . . 8
6.3. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 8 6.3. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 9
6.3.1. Message type: "client_cook" . . . . . . . . . . . . . 8 6.3.1. Message Type: "client_cook" . . . . . . . . . . . . . 9
6.3.2. Message type: "server_cook" . . . . . . . . . . . . . 8 6.3.2. Message Type: "server_cook" . . . . . . . . . . . . . 9
6.4. Unicast Time Synchronisation Messages . . . . . . . . . . 9 6.4. Unicast Time Synchronisation Messages . . . . . . . . . . 10
6.4.1. Message type: "time_request" . . . . . . . . . . . . 9 6.4.1. Message Type: "time_request" . . . . . . . . . . . . 10
6.4.2. Message type: "time_response" . . . . . . . . . . . . 9 6.4.2. Message Type: "time_response" . . . . . . . . . . . . 10
6.5. Broadcast Parameter Messages . . . . . . . . . . . . . . 10 6.5. Broadcast Parameter Messages . . . . . . . . . . . . . . 11
6.5.1. Message type: "client_bpar" . . . . . . . . . . . . . 10 6.5.1. Message Type: "client_bpar" . . . . . . . . . . . . . 11
6.5.2. Message type: "server_bpar" . . . . . . . . . . . . . 10 6.5.2. Message Type: "server_bpar" . . . . . . . . . . . . . 11
6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 11 6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 12
6.6.1. Message type: "server_broad" . . . . . . . . . . . . 11 6.6.1. Message Type: "server_broad" . . . . . . . . . . . . 12
7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 11 7. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 13
7.1. The client . . . . . . . . . . . . . . . . . . . . . . . 11 7.1. The Client . . . . . . . . . . . . . . . . . . . . . . . 13
7.1.1. The client in unicast mode . . . . . . . . . . . . . 11 7.1.1. The Client in Unicast Mode . . . . . . . . . . . . . 13
7.1.2. The client in broadcast mode . . . . . . . . . . . . 13 7.1.2. The Client in Broadcast Mode . . . . . . . . . . . . 15
7.2. The server . . . . . . . . . . . . . . . . . . . . . . . 14 7.2. The Server . . . . . . . . . . . . . . . . . . . . . . . 16
7.2.1. The server in unicast mode . . . . . . . . . . . . . 14 7.2.1. The Server in Unicast Mode . . . . . . . . . . . . . 16
7.2.2. The server in broadcast mode . . . . . . . . . . . . 14 7.2.2. The Server in Broadcast Mode . . . . . . . . . . . . 17
7.3. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 15 8. Server Seed Considerations . . . . . . . . . . . . . . . . . 17
8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 15 8.1. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 17
8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 15 8.2. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 17
8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 16 8.3. Server Seed Lifetime . . . . . . . . . . . . . . . . . . 17
9. Server Seed Considerations . . . . . . . . . . . . . . . . . 16 9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 17
9.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 16 9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 17
9.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 16 9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16 11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11.1. Initial Verification of the Server Certificates . . . . 16 11.1. Initial Verification of the Server Certificates . . . . 18
11.2. Revocation of Server Certificates . . . . . . . . . . . 17 11.2. Revocation of Server Certificates . . . . . . . . . . . 19
11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 17 11.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 19
11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 17 11.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 18 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 18 13.1. Normative References . . . . . . . . . . . . . . . . . . 20
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19 13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 19 Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 22
Appendix B. Extension fields . . . . . . . . . . . . . . . . . . 22 Appendix B. Extension Fields . . . . . . . . . . . . . . . . . . 25
Appendix C. TICTOC Security Requirements . . . . . . . . . . . . 22 Appendix C. TICTOC Security Requirements . . . . . . . . . . . . 25
Appendix D. Broadcast Mode . . . . . . . . . . . . . . . . . . . 23 Appendix D. Broadcast Mode . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 D.1. Server Preparations . . . . . . . . . . . . . . . . . . . 27
D.2. Client Preparation . . . . . . . . . . . . . . . . . . . 28
D.3. Sending Authenticated Broadcast Packets . . . . . . . . . 29
D.4. Authentication of Received Packets . . . . . . . . . . . 29
Appendix E. Random Number Generation . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
Time synchronization protocols are utilized more and more 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 applied in critical
infrastructures have to provide security measures to defeat possible infrastructures have to provide security measures to defeat possible
adversaries. Consequently, the widespread Network Time Protocol adversaries. Consequently, the widespread Network Time Protocol
(NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906] (NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
which shall ensure authenticity of the NTP server and integrity of which shall ensure authenticity of the NTP server and integrity of
the protocol packets. Unfortunately, the autokey protocol exhibits the protocol packets. Unfortunately, the autokey protocol exhibits
various severe security vulnerabilities as revealed in a thorough various severe security vulnerabilities as revealed in a thorough
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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 [I-D.ietf-tictoc-security-requirements].
Note: Note:
The intent is to formulate the protocol to be applicable to NTP as The intent is to formulate the protocol to be applicable to NTP
well as PTP. In the current state the draft focuses on the and also PTP. In the current state the draft focuses on the
application to NTP. 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 I-D [I-D.ietf-tictoc-security-requirements]. PTP can be found in the "Security Requirements of Time Protocols in
Packet Switched Networks" [I-D.ietf-tictoc-security-requirements].
3. Objectives 3. Objectives
The objectives of the NTS specifications 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. server.
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.
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o Compatibility: o Compatibility:
* 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
o TESLA: Timed Efficient Stream Loss-Tolerant Authentication MITM Man In The Middle
NTP Network Time Protocol
NTS Network Time Security
PTP Precision Time Protocol
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
Authenticity of the time server is verified once by utilization of NTS applies X.509 certificates to verify the authenticity of the time
X.509 certificates. Authenticity and integrity of the NTP packets server and to exchange a symmetric key, the so-called cookie. This
are then ensured by a Message Authentication Code (MAC), which is cookie is then used to protect authenticity and integrity of the
attached to the NTP packet. The calculation of the MAC includes the subsequent time synchronization packets by means of a Message
whole NTP packet and the cookie which is shared between client and Authentication Code (MAC), which is attached to each time
server. It is calculated according to: synchronization packet. The calculation of the MAC includes the
whole time synchronization packet and the cookie which is shared
between client and server. It is calculated according to:
cookie = MSB_128 (HMAC(server seed, H(public key 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 [I-D.ietf-tictoc-security-requirements].
See Section 9 for details on the seed refresh and Section 7.1.1 for See Section 8 for details on the 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
public key to each request (see Section 6.4). certificate to each request (see Section 6.4).
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. The verification
of the authenticity is based on the TESLA protocol, in particular on of the authenticity is based on the TESLA protocol, in particular on
its "Not Re-using Keys" scheme, see section 3.7.2 of [RFC4082]. its "Not Re-using Keys" scheme, see section 3.7.2 of [RFC4082].
TESLA is based on a one-way chain of keys, where each key is the 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 output of a one-way function applied on the previous key in the
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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. For a more detailed description of the
TESLA protocol see Appendix D. TESLA protocol see Appendix D.
6. Protocol Messages 6. Protocol Messages
Note that this section currently describes realization of the message Note that this section currently describes the realization of the
format of NTS only for its utilization for NTP, in which the NTS message format of NTS only for its utilization for NTP, in which the
specific data are enclosed in extension fields on top of NTP packets. NTS specific data are enclosed in extension fields on top of NTP
A specification of NTS messages for PTP would have to be developed packets. A specification of NTS messages for PTP would have to be
accordingly. developed accordingly.
The steps described in Section 6.1 - Section 6.4 belong to the The steps described in Section 6.1 - Section 6.4 belong to the
unicast mode, while Section 6.5 and Section 6.6 explain the steps unicast mode, while Section 6.5 and Section 6.6 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 step, the hash and signature algorithms that are used for the In this message exchange, the hash and signature algorithms that are
rest of the protocol are negotiated. used throughout the protocol are negotiated.
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 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 hash algorithms, and o a selection of accepted hash algorithms,
o a selection of accepted encryption algorithms, and
o a selection of accepted algorithms for the signatures. o a selection of accepted algorithms for the signatures.
For NTP, this message is realized as a packet with an extension field For NTP, this message is realized as a packet with an extension field
of type "association", which contains all this data. 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 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 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 the signatures and o the server's choice of algorithm for encryption, for the
cryptographic hash algorithm, both of which MUST be chosen from signatures and for cryptographic hashing , all of which MUST be
the client's proposals. chosen from the client's proposals.
In the case of NTP, the data is enclosed in a packet's extension In the case of NTP, the data is enclosed in a packet's extension
field, also of type "association". field, also of type "association".
6.2. Certificate Messages 6.2. Certificate Messages
In this step, the client receives the certification chain up to a In this message exchange, the client receives the certification chain
trusted anchor. With the established certification chain the client up to a trusted anchor. With the established certification chain the
is able to verify the server signatures and, hence, the authenticity client is able to verify the server's signatures and, hence,
of the server messages with extension fields is ensured. authenticity of future NTS messages from the server is ensured.
Discussion: Discussion:
Note that in this step the client validates the authenticity of Note that in this step the client validates the authenticity of
its immediate NTP server only. It does not recursively validate its immediate NTP server only. It does not recursively validate
the authenticity of each NTP server on the time synchronization the authenticity of each NTP server on the time synchronization
chain. Recursive authentication (and authorization) as formulated chain. Recursive authentication (and authorization) as formulated
in [I-D.ietf-tictoc-security-requirements] depends on the chosen in [I-D.ietf-tictoc-security-requirements] depends on the chosen
trust anchor. trust anchor.
6.2.1. Message type: "client_cert" 6.2.1. Message Type: "client_cert"
This message is sent by the client, after it successfully verified This message is sent by the client, after it successfully verified
the content of the received server_assoc message (see Section 7.1.1). the content of the received server_assoc message (see Section 7.1.1).
It contains It contains
o the NTS message ID "client_cert",
o the negotiated version number, o the negotiated version number,
o the client's hostname, and o the client's hostname, and
o the signature algorithm negotiated during the association o the signature algorithm negotiated during the association
messages. messages.
It is realized as an NTP packet with extension field of type In the case of NTP, the data is enclosed in a packet's extension
"certificate request" for the necessary data. field of type "certificate request".
6.2.2. Message type: "server_cert" 6.2.2. Message Type: "server_cert"
This message is sent by the server, upon receipt of a client_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 message, if the version number and choice of methods communicated in
that message are actually supported by the server. It contains 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
server's private key and according to the signature algorithm
transmitted in server_cert,
o all the information necessary to authenticate the server to the o all the information necessary to authenticate the server to the
client. This is a chain of certificates, which starts at the client. This is a chain of certificates, which starts at the
server and goes up to a trusted authority, where each certificate server and goes up to a trusted authority, where each certificate
MUST be certified by the one directly following it. MUST be certified by the one directly following it.
This message is realized for NTP as a packet with extension field of This message is realized for NTP as a packet with extension field of
type "certificate" which holds the certification data. type "certificate" which holds all of the data listed above.
6.3. Cookie Messages 6.3. Cookie Messages
During this step, the server transmits a secret cookie to the client During this message exchange, the server transmits a secret cookie to
securely. The cookie will be used for integrity protection during the client securely. The cookie will be used for integrity
unicast time synchronization. protection during unicast time synchronization.
6.3.1. Message type: "client_cook" 6.3.1. 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. It contains server. The message contains
o the NTS message ID "client_cook",
o the negotiated version number, o the negotiated version number,
o the hash algorithm H negotiated between client and server during o the negotiated signature algorithm,
the association messages,
o the client's public key. o the negotiated encryption algorithm,
o a 128-bit nonce,
o the negotiated hash algorithm H,
o the client's certificate.
For NTP, an extension field of type "cookie request" holds the listed For NTP, an extension field of type "cookie request" holds the listed
data. data.
6.3.2. Message type: "server_cook" 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 hash of the client's public key, as included in message. The server generates the hash of the client's certificate,
client_cook, is used by the server to calculate the cookie (see as conveyed during client_cook, in order to calculate the cookie
Section 5.1). This message contains according to Section 5.1. This message contains a concatenated
datum, which is encrypted with the client's public key, according to
the encryption algorithm transmitted in the client_cook message. The
concatenated datum contains
o a concatenated pair, encrypted with the client's public key, where o the NTS message ID "server_cook"
the pair consists of
* the cookie, and
* a signature of the cookie signed with the server's private key. o the version number as transmitted in client_cook,
o the nonce transmitted in client_cook,
o the cookie, and
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.
This signature MUST be calculated according to the transmitted
signature algorithm from the client_cook message.
In the case of NTP, this is a packet with an extension field of type In the case of NTP, this is a packet with an extension field of type
"cookie transmit". "cookie transmit".
6.4. Unicast Time Synchronisation Messages 6.4. Unicast Time Synchronisation Messages
In this step, the usual time synchronization process is executed, In this message exchange, the usual time synchronization process is
with the addition of integrity protection for all messages that the executed, with the addition of integrity protection for all messages
server sends. This step can be repeated as often as the client that the server sends. This message can be repeatedly exchanged as
desires and as long as the integrity of the server's time responses often as the client desires and as long as the integrity of the
is verified successfully. Secure time synchronization by repetition server's time responses is verified successfully.
of this step is the goal of a unicast run.
6.4.1. Message type: "time_request" 6.4.1. 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.
To send this message, the client MUST have received server_cook and It contains
successfully verified the cookie via the server's signature. It
contains
o the negotiated version number, o the NTS message ID "time_request",
o the time synchronization data that the client wants to transmit, 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 public key under H. o the hash of the client's certificate under H.
It is realized as an NTP packet with the time synchronization data In the case of NTP the data is enclosed in the packet's extension
and an additional extension field of type "time request" for the rest field of type "time request".
of the information.
6.4.2. Message type: "time_response" 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. The server uses the hash of the client's public key and the message. Prior to this the server MUST recalculate the client's
transmitted hash algorithm to recalculate the cookie for the client. cookie by using the hash of the client's certificate and the
This message contains transmitted hash algorithm. The message contains
o the NTS message ID "time_response",
o the version number as transmitted in time_request,
o the server's time synchronization response data, o the server's time synchronization response data,
o the nonce transmitted in time_request, o the nonce transmitted in time_request,
o a MAC (generated with the cookie as key) for verification of the
above.
It is realized as an NTP packet with the necessary time o a MAC (generated with the cookie as key) for verification of all
synchronization data and with a new extension field of type "time of the above data.
In the case of NTP, this is a packet with the necessary time
synchronization data and a new extension field of type "time
response". This packet has an appended MAC that is generated over response". This packet has an appended MAC that is generated over
the time synchronization data and the extension field, with the the time synchronization data and the extension field, with the
cookie as the key. cookie as the key.
6.5. Broadcast Parameter Messages 6.5. Broadcast Parameter Messages
In this step, the client receives the necessary information to In this message exchange, the client receives the necessary
execute the TESLA protocol in a secured broadcast association. The information to execute the TESLA protocol in a secured broadcast
client can only initiate a secure broadcast association after a association. The client can only initiate a secure broadcast
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 D for more details on TESLA.
6.5.1. Message type: "client_bpar" 6.5.1. 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 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 For NTP, this message is realized as a packet with an extension field
of type "broadcast request". of type "broadcast request".
6.5.2. Message type: "server_bpar" 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 version number as transmitted in the client_bpar message,
o the one-way function used for building the one-way key chain, 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 last key of the one-way key chain, and
o the disclosure schedule of the keys. This contains: o the disclosure schedule of the keys. This contains:
* 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),
skipping to change at page 11, line 17 skipping to change at page 12, line 27
* 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 It is realized for NTP as a packet with an extension field of type
"broadcast parameters", which contains all the given data. "broadcast parameters", which contains all the given data.
6.6. Broadcast Message 6.6. Broadcast Message
In this step, the server keeps sending broadcast time synchronization Via this message, the server keeps sending broadcast time
messages to all participating clients. synchronization messages to all participating clients.
6.6.1. Message type: "server_broad" 6.6.1. 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 version number that the server's broadcast mode is working
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 the time data verifying
The message is realized as an NTP broadcast packet with the time * the message ID,
broadcast data and with an extension field of type "broadcast * the version number, and
* the time data.
For NTP, this message is realized as an NTP broadcast packet with the
time broadcast data and with an extension field of type "broadcast
message", which contains the rest of the listed data. The NTP packet message", which contains the rest of the listed data. The NTP packet
is then appended by a MAC verifying the time data, but not the is then appended by a MAC verifying its contents.
extension field.
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. 1. It sends a client_assoc message to the server. It MUST keep the
transmitted values for version number and algorithms available
for later checks.
2. It waits for a reply in the form of a server_assoc message. 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 message MUST contain a conform version number. * The client checks that the message contains a conform version
number.
* The client has to verify that the server has chosen the * It also has to verify that the server has chosen the signature
signature and hash algorithms from its proposal sent in the and hash algorithms from its proposal sent in the client_assoc
client_assoc message. message.
If one of the checks fails, the client MUST abort the run. If one of the checks fails, the client MUST abort the run.
3. The client then sends a client_cert message to the server. 3. The client then sends a client_cert message to the server.
Again, it MUST remember the transmitted values for version number
and algorithms for later checks.
4. It awaits a reply in the form of a server_cert message and 4. It awaits a reply in the form of a server_cert message and
performs an authenticity check. If this check fails, the client performs authenticity checks on the certificate chain and the
MUST abort the run. signature for the version number. If one of these checks fails,
the client MUST abort the run.
5. Next, it sends a client_cook message to the server. 5. Next, it sends a client_cook message to the server. The client
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 6. 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 decrypts the message with its own private key.
* It checks that the decrypted message has the format of a 128 * It checks that the decrypted message is of the expected
bit Cookie concatenated with its own signature value, format: the concatenation of version number, a 128 bit nonce,
verifiable with the server's public key. a 128 bit cookie and a signature value.
If the check fails, the client MAY abort the run. * It verifies that the received version number matches the one
negotiated before.
7. The client sends a time_request message to the server. * It verifies that the received nonce matches the nonce sent in
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.
7. 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.
8. It awaits a reply in the form of a time_response message. Upon 8. It awaits a reply in the form of a time_response message. Upon
receipt, it checks: receipt, it checks:
* that the transmitted nonce belongs to the previous * that the transmitted version number matches the one negotiated
time_request message and . before,
* that the appended MAC verifies the time data and the * that the transmitted nonce belongs to a previous time_request
transmitted nonce. message,
If the nonce is invalid, the client MUST ignore this * 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 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 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 the cookie might have changed due to a server seed refresh). If
both checks are successful, the client SHOULD continue time both checks are successful, the client SHOULD continue time
synchronization by going back to step 7. synchronization by going back to step 7.
The client's behaviour in unicast mode is also expressed in Figure 1. The client's behavior in unicast mode is also expressed in Figure 1.
7.1.2. The client in broadcast mode 7.1.2. The Client in Broadcast Mode
To establish a secure broadcast association with a broadcast server, To establish a secure broadcast association with a broadcast server,
the client MUST initially authenticate the broadcast server and the client MUST initially authenticate the broadcast server and
securely synchronize its time to it up to an upper bound for its time securely synchronize its time to it up to an upper bound for its time
offset in unicast mode. After that, the client performs the offset in unicast mode. After that, the client performs the
following steps: following steps:
1. It sends a client_bpar message to the server. 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 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 6.5.2.
* Verification of the message's signature. * Verification of the message's signature.
If any information is missing or cannot be verified as signed by If any information is missing or the server's signature cannot be
the server, the client MUST abort the broadcast run. 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 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, so disclosed. This is achieved via a disclosure schedule and
this is where loose time synchronization is required. If requires the loose time synchronization. If verified, the
verified the packet will be buffered for later packet will be buffered for later authentication. Otherwise,
authentication. Otherwise, the client MUST discard it. Note the client MUST discard it. Note that the time information
that the time information included in the packet will not be included in the packet will not be used for synchronization
used for synchronization until its authenticity could be until its authenticity could be verified.
verified.
2. The client checks whether it already knows the disclosed key. 2. The client checks whether it already knows the disclosed key.
If so, the client SHOULD discard the packet to avoid a buffer If so, the client SHOULD discard the packet to avoid a buffer
overrun. If not, the client verifies that the disclosed key overrun. If not, the client verifies that the disclosed key
belongs to the one-way key chain by applying the one-way belongs to the one-way key chain by applying the one-way
function until equality with a previous disclosed key is function until equality with a previous disclosed key is
verified. If falsified, the client MUST discard the packet. verified. 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 the client verifies the
authenticity of any packet that it received during 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
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's behaviour in broadcast mode can also be seen in The client MUST restart the broadcast sequence with a client_bpar
Figure 2. message Section 6.5.1 if the one-way key chain expires.
7.2. The server The client's behavior in broadcast mode can also be seen in Figure 2.
The server's behaviour is not as easy to express in sequential terms 7.2. The Server
as the client's, not even for a single association with one client.
This is because the server does not keep state of any connection.
7.2.1. The server in unicast mode 7.2.1. The Server in Unicast Mode
A broadcast server MUST also support unicast mode, in order to To support unicast mode, the server MUST be ready to perform the
provide the initial time synchronization is a precondition for any following actions:
broadcast association. 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 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 o Upon receipt of a client_cert message, the server checks whether
it supports the given signature algorithm. If so, it constructs it supports the given signature algorithm. If so, it constructs
and sends a server_cert message as described in Section 6.2.2. and sends a server_cert message as described in Section 6.2.2.
o Upon receipt of a client_cook message, the server calculates the o Upon receipt of a client_cook message, the server checks whether
cookie according to the formula given in Section 5.1. With this, it supports the given cryptographic algorithms. It then
it constructs a server_cook message as described in Section 6.3.2. 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 6.3.2.
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 6.4.2.
Also, it must adhere to the rule of server seed refreshing, as given The server MUST refresh its server seed periodically (see
in [1]. More information on that can be found in Section 7.3. Section 8.1).
7.2.2. The server in broadcast mode 7.2.2. The Server in Broadcast Mode
To support NTS broadcast, the server MUST be ready to perform the A broadcast server MUST also support unicast mode, in order to
following actions: 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 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 6.5.2.
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 6.6.1, 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.
7.3. Server Seed Refresh 8. Server Seed Considerations
The server has to calculate a random seed which has to be kept
secret. The server MUST generate a seed for each supported hash
algorithm, see Section 9.1.
8.1. Server Seed Refresh
According to the requirements in According to the requirements in
[I-D.ietf-tictoc-security-requirements] the server has to refresh its [I-D.ietf-tictoc-security-requirements] the server MUST refresh each
server seed periodically. As a consequence the cookie used in the server seed periodically. As a consequence, the cookie memorized by
time request messages becomes invalid. In this case the client the client becomes obsolete. In this case the client cannot verify
cannot verify the attached MAC and has to respond accordingly by re- the MAC attachted to subsequent time response messages and has to
initiating the protocol with a cookie request (Section 6.3). This is respond accordingly by re-initiating the protocol with a cookie
true for the unicast and broadcast mode, respectively. request (Section 6.3).
Additionally, in broadcast mode the client has to restart the 8.2. Server Seed Algorithm
broadcast sequence with a time request message if the one-way key
chain expires.
During certificate message exchange the client reads the expiration 8.3. Server Seed Lifetime
date of the period of validity of the server certificate. The client
MAY restart the protocol sequence with the association message before
the server certificate expires.
8. Hash Algorithms and MAC Generation 9. Hash Algorithms and MAC Generation
8.1. 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 public key. Client and server cookie and the MAC, and hashing of the client's certificate. Client
negotiate a hash algorithm H during the association message exchange and server negotiate a hash algorithm H during the association
(Section 6.1) at the beginning of a unicast run. The selected message exchange (Section 6.1) at the beginning of a unicast run.
algorithm H is used for all hashing processes in that run. The selected algorithm H is used for all hashing processes in that
run.
In broadcast mode, hash algorithms are used as pseudo random In broadcast mode, 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 non-negotiable.
The list of the hash algorithms supported by the server has to The list of the hash algorithms supported by the server has to
fulfill the following requirements: fulfill the following requirements:
o it MUST NOT include MD5 or weaker algorithms, o it MUST NOT include SHA-1 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms.
8.2. MAC Calculation
For the calculation of the MAC client and server are using a Keyed- o it MUST include SHA-256 or stronger algorithms.
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC
is generated with the hash algorithm specified by the client (see
Section 8.1).
9. Server Seed Considerations Note
The server has to calculate a random seed which has to be kept secret Any hash algorithm is prone to be compromised in the future. A
and which MUST be changed periodically. The server MUST generate a successful attack on a hash algorithm would enable any NTS client
seed for each supported hash algorithm. to derive the server seed from their own cookie. Therefore, the
server MUST have separate seed values for its different supported
hash algorithms. This way, knowledge gained from an attack on a
hash algorithm H can at least only be used to compromise such
clients who use hash algorithm H as well.
9.1. Server Seed Algorithm 9.2. MAC Calculation
9.2. Server Seed Live Time For the calculation of the MAC, client and server are using a Keyed-
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC
is generated with the hash algorithm specified by the client (see
Section 9.1).
10. IANA Considerations 10. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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 6.2). Since it generally has
no reliable time during this initial communication phase, it is 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:
skipping to change at page 17, line 9 skipping to change at page 19, line 14
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 11.2. Revocation of Server Certificates
According to Section 7.3, 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 6.2). 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 SHALL use the "Not Re- protection to the protocol, client and server use the "Not Re-using
using Keys" scheme of TESLA as pointed out in section 3.7.2 of RFC Keys" scheme of TESLA as pointed out in section 3.7.2 of RFC 4082
4082 [RFC4082]. In this scheme the server never uses a key for the [RFC4082]. In this scheme the server never uses a key for the MAC
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, an alternative approach to enhance TESLA's resistance against
DoS attacks involves the addition of a group MAC to each packet. DoS attacks involves the addition of a group MAC to each packet.
This requires the exchange of an additional shared key common to the This requires the exchange of an additional shared key common to the
whole group. This adds additional complexity to the protocol and whole group. This adds additional complexity to the protocol and
hence is currently not considered in this document. hence is currently not considered in this document.
11.5. Delay Attack
In a packet delay attack, an adversary with the ability to act as a
MITM delays time synchronization packets between client and server
asymmetrically [I-D.ietf-tictoc-security-requirements]. This
prevents the client to measure the network delay, and hence its time
offset to the server, accurately [Mizrahi]. The delay attack does
not modifiy the content of the exchanged synchronization packets.
Therefore cryptographic means are not feasible to mitigate this
attack. However, serveral non-cryptographic precautions can be taken
in order to detect this attack.
o Usage of multiple time servers: this enables the client to detect
the attack provided that the adversary is unable to delay the
synchronizations packets between the majority of servers. This
approach is commonly used in NTP to exclude incorrect time servers
[RFC5905].
o Multiple communication paths: The client and server are utilizing
different paths for packet exchange as described in the I-D
[I-D.shpiner-multi-path-synchronization]. The client can detect
the attack provided that the adversary is unable to manipulate the
majority of the available paths [Shpiner]. Note, that this
approach is not yet available, neither for NTP nor for PTP.
o The introduction of a threshold value for the delay time of the
synchronization packets. The client can discard a time server if
the packet delay time of this time server is larger than the
threshold value.
o Usage of an encrypted connection: the client exchanges all packets
with the time server over an encrypted connection (e.g. IPsec).
This measure does not mitigate the delay attack but it makes it
more difficult for the adversary to identify the time
synchronization packets.
12. Acknowledgements 12. Acknowledgements
The authors would like to thank David Mills and Kurt Roeckx for The authors would like to thank Steven Bellovin, David Mills and Kurt
discussions and comments on the design of NTS. Also, thanks to Roeckx for discussions and comments on the design of NTS. Also,
Harlan Stenn for his technical review and specific text contributions thanks to Harlan Stenn for his technical review and specific text
to this document. 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.
skipping to change at page 18, line 46 skipping to change at page 21, line 36
4: Autokey Specification", RFC 5906, June 2010. 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] [I-D.ietf-tictoc-security-requirements]
Mizrahi, T., "Security Requirements of Time Protocols in Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", draft-ietf-tictoc-security- Packet Switched Networks", draft-ietf-tictoc-security-
requirements-05 (work in progress), April 2013. requirements-10 (work in progress), July 2014.
[I-D.shpiner-multi-path-synchronization]
Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
Path Time Synchronization", draft-shpiner-multi-path-
synchronization-03 (work in progress), February 2014.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings of
Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[Roettger] [Roettger]
Roettger, S., "Analysis of the NTP Autokey Procedures", Roettger, S., "Analysis of the NTP Autokey Procedures",
February 2012. February 2012.
13.3. URIs [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", in Proceedings of Precision Clock
[1] I-D.ietf-tictoc-security-requirements Synchronization for Measurement Control and Communication,
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 v
+---------------------+ +---------------------+
|Certificate Messages | |Certificate Messages |
+----------+----------+ +----------+----------+
skipping to change at page 20, line 46 skipping to change at page 23, line 46
| '-----+-----' '------+------' '---+---' | | '-----+-----' '------+------' '---+---' |
| | | | | | | | | |
| v v v | | v v v |
| +-------------+ +-------------+ +--------------+ | | +-------------+ +-------------+ +--------------+ |
| |Discard Data | |Discard Data | |Sync. Process | | | |Discard Data | |Discard Data | |Sync. Process | |
| +-------------+ +------+------+ +------+-------+ | | +-------------+ +------+------+ +------+-------+ |
| | | | | | | | | |
| | | v | | | | v |
+-----------+ +------------------>o-----------+ +-----------+ +------------------>o-----------+
Figure 1: The client's behaviour in NTS unicast mode. Figure 1: The client's behavior in NTS unicast mode.
+-----------------------------+ +-----------------------------+
|Broadcast Parameter Messages | |Broadcast Parameter Messages |
+--------------+--------------+ +--------------+--------------+
| |
o<--------------------------+ o<--------------------------+
| | | |
v | v |
+-----------------------------+ | +-----------------------------+ |
|Broadcast Time Sync. Message | | |Broadcast Time Sync. Message | |
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| | | | | | | |
| 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. Extension Fields
In Section 6, some new extension fields for NTP packets are In Section 6, some new extension fields for NTP packets are
introduced. They are listed here again, for reference. introduced. They are listed here again, for reference.
+------------------------+---------------+ +------------------------+---------------+
| name | used in | | name | used in |
+------------------------+---------------+ +------------------------+---------------+
| "association" | client_assoc | | "association" | client_assoc |
| | server_assoc | | | server_assoc |
| | | | | |
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| | 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 |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.3 | Protection against DoS attacks | SHOULD | OK | | 5.4 | Protection against DoS attacks | SHOULD | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.4 | Replay protection | MUST | OK | | 5.5 | Replay protection | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.5.1 | Key freshness. | MUST | OK | | 5.6 | Key freshness. | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.5.2 | Security association. | SHOULD | OK | | | Security association. | SHOULD | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.5.3 | Unicast and multicast | SHOULD | OK | | | Unicast and multicast | SHOULD | OK |
| | associations. | | | | | associations. | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.6 | 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 |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.8 | Protection against Packet Delay | SHOULD | NA*) | | 5.9 | Protection against Packet Delay | SHOULD | NA*) |
| | and Interception Attacks | | | | | and Interception Attacks | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.9.1 | Secure mode | MUST | - | | 5.10 | Secure mode | MUST | - |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
| 5.9.2 | Hybrid mode | MAY | - | | | Hybrid mode | SHOULD | - |
+---------+------------------------------------+-------------+------+ +---------+------------------------------------+-------------+------+
*) Ensured by NTP via multi-source configuration. *) See discussion in section Section 11.5.
Comparsion of NTS sepecification against TICTOC security Comparison of NTS sepecification against TICTOC security
requirements. requirements.
Appendix D. Broadcast Mode Appendix D. Broadcast Mode
For the broadcast mode, NTS adopts the TESLA protocol, which is based
on a one-way key chain. This appendix provides details on the
generation and usage of the one-way key chain collected and assembled
from [RFC4082]. Note that NTS is using the "not re-using keys"
scheme of TESLA as described in section 3.7.2. of [RFC4082].
D.1. Server Preparations
Server setup:
1. The server determines a reasonable upper bound B on the network
delay between itself and an arbitrary client, measured in
milliseconds.
2. It determines the number n+1 of keys in the one-way key chain.
This yields the number n of keys that are usable to authenticate
broadcast packets. This number n is therefore also the number of
time intervals during which the server can send authenticated
broadcast messages before it has to calculate a new key chain.
3. It divides time into n uniform intervals I_1, I_2, ..., I_n.
Each of these time intervals has length L, measured in
milliseconds. In order to fulfill the requirement 3.7.2. of RFC
4082 the time interval L has to be smaller than the time interval
between the broadcast messages.
4. The server generates a random key K_n.
5. Using a one-way function F, the server generates a one-way chain
of n+1 keys K_0, K_1, ..., K_{n} according to
K_i = F(K_{i+1}).
6. Using another one-way function F', it generates a sequence of n+1
MAC keys K'_0, K'_1, ..., K'_{n-1} according to
K'_i = F'(K_i).
7. Each MAC key K'_i is assigned to the time interval I_i.
8. The server determines the key disclosure delay d, which is the
number of intervals between using a key and disclosing it. Note
that although security is still provided for all choices d>0, the
choice still makes a difference:
* If d is chosen too short, the client might discards packets
because it fails to verify that the key used for their MAC has
not been yet disclosed.
* If d is chosen too long, the received packets have to be
buffered for a unnecessarily long time before they can be
verified by the client and subsequently be utilized for time
synchronization.
The server SHOULD calculate d according to
d = ceil( 2*B / L) + 1,
where ceil gives the smallest integer greater than or equal to
its argument.
< - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Generation of Keys
F F F F
K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <-------- K_n
| | | |
| | | |
| F' | F' | F' | F'
| | | |
v v v v
K'_0 K'_1 ... K'_{n-1} K'_n
[______________|____ ____|_________________|__________]
I_1 ... I_{n-1} I_n
Course of Time/Usage of Keys
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - >
A Schematic explanation on the TESLA protocol's one-way key chain
D.2. Client Preparation
A client needs the following information in order to participate in a
TESLA broadcast.
o One key K_i from the one-way key chain, which has to be
authenticated as belonging to the server. Typically, this will be
K_0.
o The disclosure schedule of the keys. This consists of:
* the length n of the one-way key chain,
* the length L of the time intervals I_1, I_2, ..., I_n,
* the starting time T_i of an interval I_i. Typically this is
the starting time T_1 of the first interval;
* the disclosure delay d.
o The one-way function F used to recursively derive the keys in the
one-way key chain,
o The second one-way function F' used to derive the MAC keys K'_0,
K'_1, ... , K'_n from the keys in the one-way chain.
o An upper bound D_t on how far its own clock is "behind" that of
the server.
Note that if D_t is greater than (d - 1) * L, then some authentic
packets might be discarded. If D_t is greater than d * L, then all
authentic packets will be discarded. In the latter case, the client
should not participate in the broadcast, since there will be no
benefit in doing so.
D.3. Sending Authenticated Broadcast Packets
During each time interval I_i, the server sends one authenticated
broadcast packet P_i. This packet consists of:
o a message M_i,
o the index i (in case a packet arrives late),
o a MAC authenticating the message M_i, with K'_i used as key,
o the key K_{i-d}, which is included for disclosure.
D.4. Authentication of Received Packets
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
before. This is done to avoid replay/flooding attacks. A packet
that fails this test is discarded.
Next, the client checks that, according to the disclosure schedule
and with respect to the upper bound D_t determined above, the server
cannot have disclosed the key K_i yet. Specifically, it needs to
check that the server's clock cannot read a time that is in time
interval I_{i+d} or later. Since it works under the assumption that
the server's clock is not more than D_t "ahead" of the client's
clock, the client can calculate an upper bound t_i for the server's
clock at the time when P_i arrived by
t_i = R + D_t,
where R is the client's clock at the arrival of P_i. This implies
that at the time of arrival of P_i, the server could have been in
interval I_x at most, with
x = floor((t_i - T_1) / L),
where floor gives the greatest integer less than or equal to its
argument. The client now needs to verify that
x < i+d
is valid (see also section 3.5 of [RFC4082]). If falsified, it is
discarded.
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
earlier disclosed key by recursively applies the one-way function F
to K_{i-d} (see Clause 3.5 in RFC 4082, item 3).
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
i-d already buffered, then the client uses the disclosed key K_{i-d}
to derive K'_{i-d} and uses that to check the MAC included in package
P_{i-d}. On success, it regards M_{i-d} as authenticated.
Appendix E. Random Number Generation
At various points of the protocol, the generation of random numbers
is required. The employed methods of generation need to be
cryptographically secure. See [RFC4086] for guidelines concerning
this topic.
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
skipping to change at page 24, line 17 skipping to change at page 31, line 4
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
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
Email: kristof.teichel@ptb.de Email: kristof.teichel@ptb.de
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