< draft-ietf-ntp-network-time-security-08.txt		draft-ietf-ntp-network-time-security-09.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 6, 2015 Google Inc.		Expires: January 7, 2016 Google Inc.
	K. Teichel		K. Teichel
	PTB		PTB
	March 5, 2015		July 06, 2015
	Network Time Security		Network Time Security
	draft-ietf-ntp-network-time-security-08.txt		draft-ietf-ntp-network-time-security-09
	Abstract		Abstract
	This document describes Network Time Security (NTS), a collection of		This document describes Network Time Security (NTS), a collection of
	measures that enable secure time synchronization with time servers		measures that enable secure time synchronization with time servers
	using protocols like the Network Time Protocol (NTP) or the Precision		using protocols like the Network Time Protocol (NTP) or the Precision
	Time Protocol (PTP). Its design considers the special requirements		Time Protocol (PTP). Its design considers the special requirements
	of precise timekeeping which are described in Security Requirements		of precise timekeeping which are described in Security Requirements
	of Time Protocols in Packet Switched Networks [RFC7384].		of Time Protocols in Packet Switched Networks [RFC7384].
	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 September 6, 2015.		This Internet-Draft will expire on January 7, 2016.
	Copyright Notice		Copyright Notice
	Copyright (c) 2015 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.
	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
	skipping to change at page 2, line 30 ¶		skipping to change at page 2, line 30 ¶
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
	2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4		2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4
	2.2. Common Terminology for PTP and NTP . . . . . . . . . . . 4		2.2. Common Terminology for PTP and NTP . . . . . . . . . . . 4
	3. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4		3. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4
	4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5		4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5
	5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5		5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
	6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6		6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6
	6.1. Association Message Exchange . . . . . . . . . . . . . . 7		6.1. Unicast Time Synchronisation Messages . . . . . . . . . . 7
	6.1.1. Goals of the Association Exchange . . . . . . . . . . 7		6.1.1. Preconditions for the Unicast Time Synchronization
	6.1.2. Message Type: "client_assoc" . . . . . . . . . . . . 7		Exchange . . . . . . . . . . . . . . . . . . . . . . 7
	6.1.3. Message Type: "server_assoc" . . . . . . . . . . . . 8		6.1.2. Goals of the Unicast Time Synchronization Exchange . 7
	6.1.4. Procedure Overview of the Association Exchange . . . 8		6.1.3. Message Type: "time_request" . . . . . . . . . . . . 7
	6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 9		6.1.4. Message Type: "time_response" . . . . . . . . . . . . 8
	6.2.1. Goals of the Cookie Exchange . . . . . . . . . . . . 10		6.1.5. Procedure Overview of the Unicast Time
	6.2.2. Message Type: "client_cook" . . . . . . . . . . . . . 10		Synchronization Exchange . . . . . . . . . . . . . . 8
	6.2.3. Message Type: "server_cook" . . . . . . . . . . . . . 10		6.2. Broadcast Time Synchronization Exchange . . . . . . . . . 9
	6.2.4. Procedure Overview of the Cookie Exchange . . . . . . 11		6.2.1. Preconditions for the Broadcast Time Synchronization
	6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 12		Exchange . . . . . . . . . . . . . . . . . . . . . . 9
	6.3.1. Goals of the Unicast Time Synchronization Exchange . 12		6.2.2. Goals of the Broadcast Time Synchronization Exchange 10
	6.3.2. Message Type: "time_request" . . . . . . . . . . . . 12		6.2.3. Message Type: "server_broad" . . . . . . . . . . . . 10
	6.3.3. Message Type: "time_response" . . . . . . . . . . . . 13		6.2.4. Procedure Overview of Broadcast Time Synchronization
	6.3.4. Procedure Overview of the Unicast Time		Exchange . . . . . . . . . . . . . . . . . . . . . . 11
	Synchronization Exchange . . . . . . . . . . . . . . 13		6.3. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 12
	6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 14		6.3.1. Preconditions for the Broadcast Keycheck Exchange . . 12
	6.4.1. Goals of the Broadcast Parameter Exchange . . . . . . 15		6.3.2. Goals of the Broadcast Keycheck Exchange . . . . . . 13
	6.4.2. Message Type: "client_bpar" . . . . . . . . . . . . . 15		6.3.3. Message Type: "client_keycheck" . . . . . . . . . . . 13
	6.4.3. Message Type: "server_bpar" . . . . . . . . . . . . . 15		6.3.4. Message Type: "server_keycheck" . . . . . . . . . . . 13
	6.4.4. Procedure Overview of the Broadcast Parameter		6.3.5. Procedure Overview of the Broadcast Keycheck Exchange 14
	Exchange . . . . . . . . . . . . . . . . . . . . . . 16		7. Server Seed Considerations . . . . . . . . . . . . . . . . . 15
	6.5. Broadcast Time Synchronization Exchange . . . . . . . . . 17		8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 15
	6.5.1. Goals of the Broadcast Time Synchronization Exchange 17		8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 15
	6.5.2. Message Type: "server_broad" . . . . . . . . . . . . 17		8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 16
	6.5.3. Procedure Overview of Broadcast Time Synchronization		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
	Exchange . . . . . . . . . . . . . . . . . . . . . . 18		10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
	6.6. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 19		10.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 16
	6.6.1. Goals of the Broadcast Keycheck Exchange . . . . . . 19		10.2. Initial Verification of the Server Certificates . . . . 16
	6.6.2. Message Type: "client_keycheck" . . . . . . . . . . . 20		10.3. Revocation of Server Certificates . . . . . . . . . . . 17
	6.6.3. Message Type: "server_keycheck" . . . . . . . . . . . 20		10.4. Mitigating Denial-of-Service for broadcast packets . . . 17
	6.6.4. Procedure Overview of the Broadcast Keycheck Exchange 20		10.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 17
	7. Server Seed Considerations . . . . . . . . . . . . . . . . . 21		10.6. Random Number Generation . . . . . . . . . . . . . . . . 19
	8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 22		11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
	8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 22		12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
	8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 22		12.1. Normative References . . . . . . . . . . . . . . . . . . 19
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22		12.2. Informative References . . . . . . . . . . . . . . . . . 19
	10. Security Considerations . . . . . . . . . . . . . . . . . . . 22		Appendix A. (informative) TICTOC Security Requirements . . . . . 20
	10.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 22		Appendix B. (normative) Inherent Association Protocol Messages . 22
	10.2. Initial Verification of the Server Certificates . . . . 23		B.1. Overview of NTS with Inherent Association Protocol . . . 22
	10.3. Revocation of Server Certificates . . . . . . . . . . . 23		B.2. Association Message Exchange . . . . . . . . . . . . . . 22
	10.4. Mitigating Denial-of-Service for broadcast packets . . . 23		B.2.1. Goals of the Association Exchange . . . . . . . . . . 22
	10.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 24		B.2.2. Message Type: "client_assoc" . . . . . . . . . . . . 23
	10.6. Random Number Generation . . . . . . . . . . . . . . . . 25		B.2.3. Message Type: "server_assoc" . . . . . . . . . . . . 23
	11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25		B.2.4. Procedure Overview of the Association Exchange . . . 24
	12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25		B.3. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 25
	12.1. Normative References . . . . . . . . . . . . . . . . . . 25		B.3.1. Goals of the Cookie Exchange . . . . . . . . . . . . 25
	12.2. Informative References . . . . . . . . . . . . . . . . . 26		B.3.2. Message Type: "client_cook" . . . . . . . . . . . . . 26
	Appendix A. (informative) TICTOC Security Requirements . . . . . 27		B.3.3. Message Type: "server_cook" . . . . . . . . . . . . . 26
	Appendix B. (normative) Using TESLA for Broadcast-Type Messages 28		B.3.4. Procedure Overview of the Cookie Exchange . . . . . . 27
	B.1. Server Preparation . . . . . . . . . . . . . . . . . . . 28		B.3.5. Broadcast Parameter Messages . . . . . . . . . . . . 28
	B.2. Client Preparation . . . . . . . . . . . . . . . . . . . 30		Appendix C. (normative) Using TESLA for Broadcast-Type Messages 30
	B.3. Sending Authenticated Broadcast Packets . . . . . . . . . 31		C.1. Server Preparation . . . . . . . . . . . . . . . . . . . 31
	B.4. Authentication of Received Packets . . . . . . . . . . . 31		C.2. Client Preparation . . . . . . . . . . . . . . . . . . . 32
	Appendix C. (informative) Dependencies . . . . . . . . . . . . . 32		C.3. Sending Authenticated Broadcast Packets . . . . . . . . . 33
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35		C.4. Authentication of Received Packets . . . . . . . . . . . 33
			Appendix D. (informative) Dependencies . . . . . . . . . . . . . 35
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
	1. Introduction		1. Introduction
	Time synchronization protocols are increasingly utilized to		Time synchronization protocols are increasingly utilized to
	synchronize clocks in networked infrastructures. Successful attacks		synchronize clocks in networked infrastructures. Successful attacks
	against the time synchronization protocol can seriously degrade the		against the time synchronization protocol can seriously degrade the
	reliable performance of such infrastructures. Therefore, time		reliable performance of such infrastructures. Therefore, time
	synchronization protocols have to be secured if they are applied in		synchronization protocols have to be secured if they are applied in
	environments that are prone to malicious attacks. This can be		environments that are prone to malicious attacks. This can be
	accomplished either by utilization of external security protocols,		accomplished either by utilization of external security protocols,
	skipping to change at page 5, line 9 ¶		skipping to change at page 5, line 9 ¶
	3. Security Threats		3. Security Threats
	The document "Security Requirements of Time Protocols in Packet		The document "Security Requirements of Time Protocols in Packet
	Switched Networks" [RFC7384] contains a profound analysis of security		Switched Networks" [RFC7384] contains a profound analysis of security
	threats and requirements for time synchronization protocols.		threats and requirements for time synchronization protocols.
	4. Objectives		4. Objectives
	The objectives of the NTS specification are as follows:		The objectives of the NTS specification are as follows:
	o Authenticity: NTS enables a client to authenticate its time		o Authenticity: NTS enables the client to authenticate its time
	server(s).		server(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 Authorization: NTS optionally enables the server to verify the		o Authorization: NTS enables the client to verify its time server's
	client's authorization.		authorization. NTS optionally enables the server to verify the
			client's authorization as well.
	o Request-Response-Consistency: NTS enables a client to match an		o Request-Response-Consistency: NTS enables a client to match an
	incoming response to a request it has sent. NTS also enables the		incoming response to a request it has sent. NTS also enables the
	client to deduce from the response whether its request to the		client to deduce from the response whether its request to the
	server has arrived without alteration.		server has arrived without alteration.
	o Integration with protocols: NTS can be used to secure different		o Integration with protocols: NTS can be used to secure different
	time synchronization protocols, specifically at least NTP and PTP.		time synchronization protocols, specifically at least NTP and PTP.
	A client or server running an NTS-secured version of a time		A client or server running an NTS-secured version of a time
	protocol does not negatively affect other participants who are		protocol does not negatively affect other participants who are
	running unsecured versions of that protocol.		running unsecured versions of that protocol.
	5. NTS Overview		5. NTS Overview
	NTS applies X.509 certificates to verify the authenticity of the time		NTS initially verifies the authenticity of the time server and
	server and to exchange a symmetric key, the so-called cookie. A		exchanges a symmetric key, the so-called cookie, as well as a key
	client needs a public/private key pair for encryption, with the		input value (KIV). After the cookie and the KIV are exchanged, the
	public key enclosed in a certificate. A server needs a public/		client then uses them to protect the authenticity and the integrity
	private key pair for signing, with the public key enclosed in a		of subsequent unicast-type time synchronization packets. In order to
	certificate. If a participant intends to act as both a client and a		do this, a Message Authentication Code (MAC) is attached to each time
	server, it MUST have two different key pairs for these purposes.		synchronization packet. The calculation of the MAC includes the
			whole time synchronization packet and the cookie which is shared
			between client and server.
	After the cookie is exchanged, the client then uses it to protect the		The cookie is calculated according to:
	authenticity and the integrity of subsequent unicast-type time
	synchronization packets. In order to do this, a Message
	Authentication Code (MAC) is attached to each time synchronization
	packet. The calculation of the MAC includes the whole time
	synchronization packet and the cookie which is shared between client
	and server. The cookie is calculated according to:
	cookie = MSB_<b> (HMAC(server seed, H(certificate of client))),		cookie = MSB_<b> (HMAC(server seed, KIV)),
	with the server seed as the key, where H is a hash function, and		with the server seed as the key, where KIV is the client's key input
	where the function MSB_<b> cuts off the b most significant bits of		value, and where the application of the function MSB_<b> returns only
	the result of the HMAC function. The client's certificate contains		the b most significant bits. The server seed is a random value of
	the client's public key and enables the server to identify the		bit length b that the server possesses, which has to remain secret.
	client, if client authorization is desired. The server seed is a
	random value of bit length b that the server possesses, which has to		The cookie deterministically depends on KIV as long as the server
	remain secret. The cookie never changes as long as the server seed		seed stays the same. The server seed has to be refreshed
	stays the same, but the server seed has to be refreshed periodically		periodically in order to provide key freshness as required in
	in order to provide key freshness as required in [RFC7384]. See		[RFC7384]. See Section 7 for details on seed refreshing.
	Section 7 for details on seed refreshing.
	Since the server does not keep a state of the client, it has to		Since the server does not keep a state of the client, it has to
	recalculate the cookie each time it receives a unicast time		recalculate the cookie each time it receives a unicast time
	synchronization request from the client. To this end, the client has		synchronization request from the client. To this end, the client has
	to attach the hash value of its certificate to each request (see		to attach its KIV to each request (see Section 6.1).
	Section 6.3).
	For broadcast-type messages, authenticity and integrity of the time		For broadcast-type messages, authenticity and integrity of the time
	synchronization packets are also ensured by a MAC, which is attached		synchronization packets are also ensured by a MAC, which is attached
	to the time synchronization packet by the sender. Verification of		to the time synchronization packet by the sender. Verification of
	the broadcast-type packets' authenticity is based on the TESLA		the broadcast-type packets' authenticity is based on the TESLA
	protocol, in particular on its "not re-using keys" scheme, see		protocol, in particular on its "not re-using keys" scheme, see
	Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys,		Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys,
	where each key is the output of a one-way function applied to the		where each key is the output of a one-way function applied to the
	previous key in the chain. The server securely shares the last		previous key in the chain. The server securely shares the last
	element of the chain with all clients. The server splits time into		element of the chain with all clients. The server splits time into
	intervals of uniform duration and assigns each key to an interval in		intervals of uniform duration and assigns each key to an interval in
	reverse order, starting with the penultimate. At each time interval,		reverse order. At each time interval, the server sends a broadcast
	the server sends a broadcast packet appended by a MAC, calculated		packet appended by a MAC, calculated using the corresponding key, and
	using the corresponding key, and the key of the previous disclosure		the key of the previous disclosure interval. The client verifies the
	interval. The client verifies the MAC by buffering the packet until		MAC by buffering the packet until disclosure of the key in its
	disclosure of the key in its associated disclosure interval occurs.		associated disclosure interval occurs. In order to be able to verify
	In order to be able to verify the timeliness of the packets, the		the timeliness of the packets, the client has to be loosely time
	client has to be loosely time synchronized with the server. This has		synchronized with the server. This has to be accomplished before
	to be accomplished before broadcast associations can be used. For		broadcast associations can be used. For checking timeliness of
	checking timeliness of packets, NTS uses another, more rigorous check		packets, NTS uses another, more rigorous check in addition to just
	in addition to just the clock lookup used in the TESLA protocol. For		the clock lookup used in the TESLA protocol. For a more detailed
	a more detailed description of how NTS employs and customizes TESLA,		description of how NTS employs and customizes TESLA, see Appendix C.
	see 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, and integrated into the communication flow of existing time		practice, and integrated into the communication flow of existing time
	synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a		synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a
	companion document for NTS. Said document describes ASN.1 encodings		companion document for NTS. Said document describes ASN.1 encodings
	for those message parts that have to be added to a time		for those message parts that have to be added to a time
	synchronization protocol for security reasons as well as CMS		synchronization protocol for security reasons.
	(Cryptographic Message Syntax, see [RFC5652]) conventions that can be
	used to get the cryptographic aspects right.
	6.1. Association Message Exchange
	In this message exchange, the participants negotiate the hash and
	encryption algorithms that are used throughout the protocol. In
	addition, the client receives the certification chain up to a trusted
	anchor. With the established certification chain the client is able
	to verify the server's signatures and, hence, the authenticity of
	future NTS messages from the server is ensured.
	6.1.1. Goals of the Association Exchange
	The association exchange:
	o enables the client to verify any communication with the server as
	authentic,
	o lets the participants negotiate NTS version and algorithms,
	o guarantees authenticity of the negotiation result to the client,
	o guarantees to the client that the negotiation result is based on
	the client's original, unaltered request.
	6.1.2. Message Type: "client_assoc"
	The protocol sequence starts with the client sending an association
	message, called client_assoc. This message contains
	o the NTS message ID "client_assoc",
	o a nonce,
	o the version number of NTS that the client wants to use (this
	SHOULD be the highest version number that it supports),
	o the hostname of the client,
	o a selection of accepted hash algorithms, and
	o a selection of accepted encryption algorithms.
	6.1.3. Message Type: "server_assoc"
	This message is sent by the server upon receipt of client_assoc. It
	contains
	o the NTS message ID "server_assoc",
	o the nonce transmitted in client_assoc,
	o the client's proposal for the version number, selection of
	accepted hash algorithms and selection of accepted encryption
	algorithms, as transmitted in client_assoc,
	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
	client_assoc message and the highest supported by the server),
	o the hostname of the server,
	o the server's choice of algorithm for encryption and for
	cryptographic hashing, all of which MUST be chosen from the
	client's proposals,
	o a signature, calculated over the data listed above, with the
	server's private key and according to the signature algorithm
	which is also used for the certificates that are included (see
	below), and
	o a chain of certificates, which starts at the server and goes up to
	a trusted authority; each certificate MUST be certified by the one
	directly following it.
	6.1.4. Procedure Overview of the Association Exchange
	For an association exchange, the following steps are performed:
	1. The client sends a client_assoc message to the server. It MUST
	keep the transmitted values for the version number and algorithms
	available for later checks.
	2. 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 6.1.3. Upon unsuccessful negotiation for version
	number or algorithms the server_assoc message MUST contain an
	error code.
	3. The client waits for a reply in the form of a server_assoc
	message. After receipt of the message it performs the following
	checks:
	* The client checks that the message contains a conforming
	version number.
	* It checks that the nonce sent back by the server matches the
	one transmitted in client_assoc,
	* It also verifies that the server has chosen the encryption and
	hash algorithms from its proposal sent in the client_assoc
	message and that this proposal was not altered.
	* Furthermore, it performs authenticity checks on the
	certificate chain and the signature.
	If one of the checks fails, the client MUST abort the run.
	+------------------------+
	| o Choose version |
	| o Choose algorithms |
	| o Acquire certificates |
	| o Assemble response |
	| o Create signature |
	+-----------+------------+
	|
	<-+->
	Server --------------------------->
	/| \
	client_ / \ server_
	assoc / \ assoc
	/ \|
	Client --------------------------->
	<------ Association ----->
	exchange
	Procedure for association and cookie exchange.
	6.2. Cookie Messages
	During this message exchange, the server transmits a secret cookie to
	the client securely. The cookie will later be used for integrity
	protection during unicast time synchronization.
	6.2.1. Goals of the Cookie Exchange
	The cookie exchange:
	o enables the server to check the client's authorization via its
	certificate (optional),
	o supplies the client with the correct cookie for its association to
	the server,
	o guarantees to the client that the cookie originates from the
	server and that it is based on the client's original, unaltered
	request.
	o guarantees that the received cookie is unknown to anyone but the
	server and the client.
	6.2.2. Message Type: "client_cook"
	This message is sent by the client upon successful authentication of
	the server. In this message, the client requests a cookie from the
	server. The message contains
	o the NTS message ID "client_cook",
	o a nonce,
	o the negotiated version number,
	o the negotiated signature algorithm,
	o the negotiated encryption algorithm,
	o the negotiated hash algorithm H,
	o the client's certificate.
	6.2.3. Message Type: "server_cook"
	This message is sent by the server upon receipt of a client_cook
	message. The server generates the hash of the client's certificate,
	as conveyed during client_cook, in order to calculate the cookie
	according to Section 5. This message contains
	o the NTS message ID "server_cook"
	o the version number as transmitted in client_cook,
	o a concatenated datum which is encrypted with the client's public
	key, according to the encryption algorithm transmitted in the
	client_cook message. The concatenated datum contains
	* the nonce transmitted in client_cook, and
	* the cookie.
	o a signature, created with the server's private key, calculated
	over all of the data listed above. This signature MUST be
	calculated according to the transmitted signature algorithm from
	the client_cook message.
	6.2.4. Procedure Overview of the Cookie Exchange
	For a cookie exchange, the following steps are performed:
	1. The client sends a client_cook message to the server. The client
	MUST save the included nonce until the reply has been processed.
	2. Upon receipt of a client_cook message, the server checks whether
	it supports the given cryptographic algorithms. It then
	calculates the cookie according to the formula given in
	Section 5. The server MAY use the client's certificate to check
	that the client is authorized to use the secure time
	synchronization service. With this, it MUST construct a
	server_cook message as described in Section 6.2.3.
	3. The client 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 beforehand.
	* 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		6.1. Unicast Time Synchronisation Messages
	the client_cook message.
	If one of those checks fails, the client MUST abort the run.		In this message exchange, the usual time synchronization process is
			executed, with the addition of integrity protection for all messages
			that the server sends. This message exchange can be repeatedly
			performed as often as the client desires and as long as the integrity
			of the server's time responses is verified successfully.
	+----------------------------+		6.1.1. Preconditions for the Unicast Time Synchronization Exchange
	| o OPTIONAL: Check client's |
	| authorization |
	| o Generate cookie |
	| o Encrypt inner message |
	| o Generate signature |
	+-------------+--------------+
	|
	<-+->
	Server --------------------------->		Before this message exchange is available, there are some
	/| \		requirements that the client and server need to meet:
	client_ / \ server_
	cook / \ cook
	/ \|
	Client --------------------------->
	<--- Cookie exchange -->		o They MUST negotiate the hash algorithm for the MAC used in the
			time synchronization messages. Authenticity and integrity of the
			communication MUST be ensured.
	Procedure for association and cookie exchange.		o The client MUST know a key input value KIV. Authenticity and
			integrity of the communication MUST be ensured.
	6.3. Unicast Time Synchronisation Messages		o Client and server MUST exchange the cookie (which depends on the
			KIV as described in section Section 5). Authenticity,
			confidentiality and integrity of the communication MUST be
			ensured.
	In this message exchange, the usual time synchronization process is		One way of realising these requirements is to use the Association and
	executed, with the addition of integrity protection for all messages		Cookie Message Exchanges described in Appendix B.
	that the server sends. This message can be repeatedly exchanged as
	often as the client desires and as long as the integrity of the
	server's time responses is verified successfully.
	6.3.1. Goals of the Unicast Time Synchronization Exchange		6.1.2. Goals of the Unicast Time Synchronization Exchange
	The unicast time synchronization exchange:		The unicast time synchronization exchange:
	o exchanges (unicast) time synchronization data as specified by the		o exchanges (unicast) time synchronization data as specified by the
	appropriate time synchronization protocol,		appropriate time synchronization protocol,
	o guarantees to the client that the response originates from the		o guarantees authenticity and integrity of the response to the
	server and is based on the client's original, unaltered request.		client,
	6.3.2. Message Type: "time_request"		o guarantees request-response-consistency to the client.
			6.1.3. Message Type: "time_request"
	This message is sent by the client when it requests a 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 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 client's key input value (for which the client knows the
			associated cookie).
	6.3.3. Message Type: "time_response"		6.1.4. Message Type: "time_response"
	This message is sent by the server after it has received a		This message is sent by the server after it has received a
	time_request message. Prior to this the server MUST recalculate the		time_request message. Prior to this the server MUST recalculate the
	client's cookie by using the hash of the client's certificate and the		client's cookie by using the received key input value 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 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.
	6.3.4. Procedure Overview of the Unicast Time Synchronization Exchange		6.1.5. Procedure Overview of the Unicast Time Synchronization Exchange
	For a unicast time synchronization exchange, the following steps are		For a unicast time synchronization exchange, the following steps are
	performed:		performed:
	1. The client sends a time_request message to the server. The		1. The client sends a time_request message to the server. The
	client MUST save the included nonce and the transmit_timestamp		client MUST save the included nonce and the transmit_timestamp
	(from the time synchronization data) as a correlated pair for		(from the time synchronization data) as a correlated pair for
	later verification steps.		later verification steps.
	2. Upon receipt of a time_request message, the server re-calculates		2. 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 6.3.3.		and constructs a time_response message as given in Section 6.1.4.
	3. It awaits a reply in the form of a time_response message. Upon		3. The client awaits a reply in the form of a time_response message.
	receipt, it checks:		Upon receipt, it checks:
	* that the transmitted version number matches the one negotiated		* that the transmitted version number matches the one negotiated
	previously,		previously,
	* that the transmitted nonce belongs to a previous time_request		* that the transmitted nonce belongs to a previous time_request
	message,		message,
	* that the transmit_timestamp in that time_request message		* that the transmit_timestamp in that time_request message
	matches the corresponding time stamp from the synchronization		matches the corresponding time stamp from the synchronization
	data received in the time_response, and		data received in the time_response, and
	* that the appended MAC verifies the received synchronization		* that the appended MAC verifies the received synchronization
	data, version number and nonce.		data, version number and nonce.
	If at least one of the first three checks fails (i.e. if the		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		version number does not match, if the client has never used the
	nonce transmitted in the time_response message, or if it has used		nonce transmitted in the time_response message, or if it has used
	the nonce with initial time synchronization data different from		the nonce with initial time synchronization data different from
	that in the response), then the client MUST ignore this		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 send another cookie
	the cookie might have changed due to a server seed refresh). If		request (because the cookie might have changed due to a server
	both checks are successful, the client SHOULD continue time		seed refresh). If both checks are successful, the client SHOULD
	synchronization by going back to step 7.		continue time synchronization.
	+-----------------------+		+-----------------------+
	| o Re-generate cookie |		| o Re-generate cookie |
	| o Assemble response |		| o Assemble response |
	| o Generate MAC |		| o Generate MAC |
	+-----------+-----------+		+-----------+-----------+
	|		|
	<-+->		<-+->
	Server ----------------------------------------------->		Server ----------------------------------------------->
	skipping to change at page 14, line 44 ¶		skipping to change at page 9, line 44 ¶
	request / \ response		request / \ response
	/ \|		/ \|
	Client ----------------------------------------------->		Client ----------------------------------------------->
	<------ Unicast time ------> <- Client-side ->		<------ Unicast time ------> <- Client-side ->
	synchronization validity		synchronization validity
	exchange checks		exchange checks
	Procedure for unicast time synchronization exchange.		Procedure for unicast time synchronization exchange.
	6.4. Broadcast Parameter Messages		6.2. Broadcast Time Synchronization Exchange
	In this message exchange, the client receives the necessary
	information to execute the TESLA protocol in a secured broadcast
	association. The client can only initiate a secure broadcast
	association after successful association and cookie exchanges and
	only if it has made sure that its clock is roughly synchronized to
	the server's.
	See Appendix B for more details on TESLA.
	6.4.1. Goals of the Broadcast Parameter Exchange
	The broadcast parameter exchange
	o provides the client with all the information necessary to process
	broadcast time synchronization messages from the server, and
	o guarantees authenticity, integrity and freshness of the broadcast
	parameters to the client.
	6.4.2. Message Type: "client_bpar"
	This message is sent by the client in order to establish a secured
	time broadcast association with the server. It contains
	o the NTS message ID "client_bpar",
	o the NTS version number negotiated during association,
	o a nonce,
	o the client's hostname, and
	o the signature algorithm negotiated during association.
	6.4.3. Message Type: "server_bpar"
	This message is sent by the server upon receipt of a client_bpar
	message during the broadcast loop of the server. It contains
	o the NTS message ID "server_bpar",
	o the version number as transmitted in the client_bpar message,		6.2.1. Preconditions for the Broadcast Time Synchronization Exchange
	o the nonce transmitted in client_bpar,		Before this message exchange is available, there are some
			requirements that the client and server need to meet:
	o the one-way functions used for building the key chain, and		o The client MUST receive all the information necessary to process
			broadcast time synchronization messages from the server. This
			includes
	o the disclosure schedule of the keys. This contains:		* the one-way functions used for building the key chain,
	* the last key of the key chain,		* the last key of the key chain,
	* time interval duration,		* time interval duration,
	* the disclosure delay (number of intervals between use and		* the disclosure delay (number of intervals between use and
	disclosure of a key),		disclosure of a key),
	* the time at which the next time interval will start, and		* the time at which the next time interval will start, and
	* the next interval's associated index.		* the next interval's associated index.
	o The message also contains a signature signed by the server with		o The communication of the data listed above MUST guarantee
	its private key, verifying all the data listed above.		authenticity of the server, as well as integrity and freshness of
			the broadcast parameters to the client.
	6.4.4. Procedure Overview of the Broadcast Parameter Exchange
	A broadcast parameter exchange consists of the following steps:
	1. The client sends a client_bpar message to the server. It MUST
	remember the transmitted values for the nonce, the version number
	and the signature algorithm.
	2. Upon receipt of a client_bpar message, the server constructs and
	sends a server_bpar message as described in Section 6.4.3.
	3. The client waits for a reply in the form of a server_bpar
	message, on which it performs the following checks:
	* The message must contain all the necessary information for the
	TESLA protocol, as listed in Section 6.4.3.
	* The message must contain a nonce belonging to a client_bpar
	message that the client has previously sent.
	* Verification of the message's signature.
	If any information is missing or if 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.
	+---------------------+
	| o Assemble response |
	| o Create public-key |
	| signature |
	+----------+----------+
	|
	<-+->
	Server --------------------------------------------->
	/| \
	client_ / \ server_
	bpar / \ bpar
	/ \|
	Client --------------------------------------------->
	<------- Broadcast ------> <- Client-side ->
	parameter validity
	exchange checks
	Procedure for unicast time synchronization exchange.
	6.5. Broadcast Time Synchronization Exchange
	Via a stream of messages of the following message type, the server
	keeps sending broadcast time synchronization messages to all
	participating clients.
	6.5.1. Goals of the Broadcast Time Synchronization Exchange		6.2.2. Goals of the Broadcast Time Synchronization Exchange
	The broadcast time synchronization exchange:		The broadcast time synchronization exchange:
	o transmits (broadcast) time synchronization data from the server to		o transmits (broadcast) time synchronization data from the server to
	the client as specified by the appropriate time synchronization		the client as specified by the appropriate time synchronization
	protocol,		protocol,
	o guarantees to the client that the received synchronization data		o guarantees to the client that the received synchronization data
	has arrived in a timely manner as required by the TESLA protocol		has arrived in a timely manner as required by the TESLA protocol
	and is trustworthy enough to be stored for later checks,		and is trustworthy enough to be stored for later checks,
	o additionally guarantees authenticity of a certain broadcast		o additionally guarantees authenticity of a certain broadcast
	synchronization message in the client's storage.		synchronization message in the client's storage.
	6.5.2. Message Type: "server_broad"		6.2.3. 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 is working under,		o the version number that the server 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		verifying
	* the message ID,		* the message ID,
	* the version number, and		* the version number, and
	* the time data.		* the time data.
	6.5.3. Procedure Overview of Broadcast Time Synchronization Exchange		6.2.4. Procedure Overview of Broadcast Time Synchronization Exchange
	A broadcast time synchronization message exchange consists of the		A broadcast time synchronization message exchange consists of the
	following steps:		following steps:
	1. The server follows the TESLA protocol by regularly sending		1. The server follows the TESLA protocol by regularly sending
	server_broad messages as described in Section 6.5.2, adhering to		server_broad messages as described in Section 6.2.3, adhering to
	its own disclosure schedule.		its own disclosure schedule.
	2. The client awaits time synchronization data in the form of a		2. 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:
	* Proof that the MAC is based on a key that is not yet disclosed		* Proof that the MAC is based on a key that is not yet disclosed
	(packet timeliness). This is achieved via a combination of		(packet timeliness). This is achieved via a combination of
	checks. First, the disclosure schedule is used, which		checks. First, the disclosure schedule is used, which
	requires loose time synchronization. If this is successful,		requires loose time synchronization. If this is successful,
	skipping to change at page 19, line 36 ¶		skipping to change at page 12, line 34 ¶
	\|		\|
	Client ---------------------------------->		Client ---------------------------------->
	< Broadcast > <- Client-side ->		< Broadcast > <- Client-side ->
	time sync. validity and		time sync. validity and
	exchange timeliness		exchange timeliness
	checks		checks
	Procedure for broadcast time synchronization exchange.		Procedure for broadcast time synchronization exchange.
	6.6. Broadcast Keycheck		6.3. Broadcast Keycheck
	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 B.		timeliness in the course of the TESLA scheme, see Appendix C.
	6.6.1. Goals of the Broadcast Keycheck Exchange		6.3.1. Preconditions for the Broadcast Keycheck Exchange
			Before this message exchange is available, there are some
			requirements that the client and server need to meet:
			o They MUST negotiate the hash algorithm for the MAC used in the
			time synchronization messages. Authenticity and integrity of the
			communication MUST be ensured.
			o The client MUST know a key input value KIV. Authenticity and
			integrity of the communication MUST be ensured.
			o Client and server MUST exchange the cookie (which depends on the
			KIV as described in section Section 5). Authenticity,
			confidentiality and integrity of the communication MUST be
			ensured.
			These requirements conform to those for the unicast time
			synchronization exchange. Accordingly, they too can be realised via
			the Association and Cookie Message Exchanges described in Appendix B
			(Appendix B).
			6.3.2. Goals of the Broadcast Keycheck Exchange
	The keycheck exchange:		The keycheck exchange:
	o guarantees to the client that the key belonging to the respective		o guarantees to the client that the key belonging to the respective
	TESLA interval communicated in the exchange had not been disclosed		TESLA interval communicated in the exchange had not been disclosed
	before the client_keycheck message was sent.		before the client_keycheck message was sent.
	o guarantees to the client the timeliness of any broadcast packet		o guarantees to the client the timeliness of any broadcast packet
	secured with this key if it arrived before client_keycheck was		secured with this key if it arrived before client_keycheck was
	sent.		sent.
	6.6.2. Message Type: "client_keycheck"		6.3.3. 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 NTS version number negotiated during association,		o the NTS version number negotiated during association,
	o a nonce,		o a 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 during association, and		o the hash algorithm H negotiated during association, and
	o the hash of the client's certificate under H.		o the client's key input value KIV.
	6.6.3. Message Type: "server_keycheck"		6.3.4. 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 received key input value 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 as transmitted in "client_keycheck,		o the version number as transmitted in "client_keycheck,
	o the 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.
	6.6.4. Procedure Overview of the Broadcast Keycheck Exchange		6.3.5. Procedure Overview of the Broadcast Keycheck Exchange
	A broadcast keycheck message exchange consists of the following		A broadcast keycheck message exchange consists of the following
	steps:		steps:
	1. The client sends a client_keycheck message. It MUST memorize the		1. The client sends a client_keycheck message. It MUST memorize the
	nonce and the time interval number that it sends as a correlated		nonce and the time interval number that it sends as a correlated
	pair.		pair.
	2. Upon receipt of a client_keycheck message, the server looks up		2. Upon receipt of a client_keycheck message, the server looks up
	whether it has already disclosed the key associated with the		whether it has already disclosed the key associated with the
	interval number transmitted in that message. If it has not		interval number transmitted in that message. If it has not
	disclosed it, it constructs and sends the appropriate		disclosed it, it constructs and sends the appropriate
	server_keycheck message as described in Section 6.6.3. For more		server_keycheck message as described in Section 6.3.4. For more
	details, see also Appendix B.		details, see also Appendix C.
	3. The client awaits a reply in the form of a server_keycheck		3. The client awaits a reply in the form of a server_keycheck
	message. On receipt, it performs the following checks:		message. On receipt, it performs the following checks:
	* that the transmitted version number matches the one negotiated		* that the transmitted version number matches the one negotiated
	previously,		previously,
	* that the transmitted nonce belongs to a previous		* that the transmitted nonce belongs to a previous
	client_keycheck message,		client_keycheck message,
	skipping to change at page 22, line 10 ¶		skipping to change at page 15, line 36 ¶
	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 8.1.		algorithm, see Section 8.1.
	According to the requirements in [RFC7384], the server MUST refresh		According to the requirements in [RFC7384], the server MUST refresh
	each server seed periodically. Consequently, the cookie memorized by		each server seed periodically. Consequently, the cookie memorized by
	the client becomes obsolete. In this case, the client cannot verify		the client becomes obsolete. In this case, the client cannot verify
	the MAC attached to subsequent time response messages and has to		the MAC attached to subsequent time response messages and has to
	respond accordingly by re-initiating the protocol with a cookie		respond accordingly by re-initiating the protocol with a cookie
	request (Section 6.2).		request (Appendix B.3).
	8. Hash Algorithms and MAC Generation		8. Hash Algorithms and MAC Generation
	8.1. Hash Algorithms		8.1. Hash Algorithms
	Hash algorithms are used at different points: calculation of the		Hash algorithms are used for calculation of the cookie and the MAC.
	cookie and the MAC, and hashing of the client's certificate. The		The client and the server negotiate a hash algorithm H during the
	client and the server negotiate a hash algorithm H during the		association phase at the beginning. The selected algorithm H is used
	association message exchange (Section 6.1) at the beginning. The		for all hashing processes in that run.
	selected algorithm H is used for all hashing processes in that run.
	In the TESLA scheme, 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 is non-negotiable.		hash algorithm is communicated by the server and is non-negotiable.
	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 its own cookie. Therefore, the		to derive the server seed from its own cookie. Therefore, the
	skipping to change at page 23, line 11 ¶		skipping to change at page 16, line 37 ¶
	transfer approaches like NTP and PTP consists basically of time		transfer approaches like NTP and PTP consists basically of time
	stamps, which are not considered secret [RFC7384]. Therefore,		stamps, which are not considered secret [RFC7384]. Therefore,
	encryption of the time synchronization protocol packet's payload is		encryption of the time synchronization protocol packet's payload is
	not considered in this document. However, an attacker can exploit		not considered in this document. However, an attacker can exploit
	the exchange of time synchronization protocol packets for topology		the exchange of time synchronization protocol packets for topology
	detection and inference attacks as described in		detection and inference attacks as described in
	[I-D.iab-privsec-confidentiality-threat]. To make such attacks more		[I-D.iab-privsec-confidentiality-threat]. To make such attacks more
	difficult, that draft recommends the encryption of the packet		difficult, that draft recommends the encryption of the packet
	payload. Yet, in the case of time synchronization protocols the		payload. Yet, in the case of time synchronization protocols the
	confidentiality protection of time synchronization packet's payload		confidentiality protection of time synchronization packet's payload
	is of secondary role since the packets meta data (IP addresses, port		is of secondary importance since the packet's meta data (IP
	numbers, possibly packet size and regular sending intervals) carry		addresses, port numbers, possibly packet size and regular sending
	more information than the payload. To enhance the privacy of the		intervals) carry more information than the payload. To enhance the
	time synchronization partners, the usage of tunnel protocols such as		privacy of the time synchronization partners, the usage of tunnel
	IPsec and MACsec, where applicable, is therefore more suited than		protocols such as IPsec and MACsec, where applicable, is therefore
	confidentiality protection of the payload.		more suited than confidentiality protection of the payload.
	10.2. Initial Verification of the Server Certificates		10.2. Initial Verification of the Server Certificates
	The client has to verify the validity of the certificates during the		The client may wish to verify the validity of certificates during the
	certification message exchange (Section 6.1.3). Since it generally		initial association phase. Since it generally has no reliable time
	has no reliable time during this initial communication phase, it is		during this initial communication phase, it is impossible to verify
	impossible to verify the period of validity of the certificates. To		the period of validity of the certificates. To solve this chicken-
	solve this chicken-and-egg problem, the client as to rely on external		and-egg problem, the client has to rely on external means.
	means.
	10.3. Revocation of Server Certificates		10.3. Revocation of Server Certificates
	According to Section 7, it is the client's responsibility to initiate		According to Section 7, it is the client's responsibility to initiate
	a new association with the server after the server's certificate		a new association with the server after the server's certificate
	expires. To this end, the client reads the expiration date of the		expires. To this end, the client reads the expiration date of the
	certificate during the certificate message exchange (Section 6.1.3).		certificate during the certificate message exchange (Appendix B.2.3).
	Furthermore, certificates may also be revoked prior to the normal		Furthermore, certificates may also be revoked prior to the normal
	expiration date. To increase security the client MAY periodically		expiration date. To increase security the client MAY periodically
	verify the state of the server's certificate via OCSP.		verify the state of the server's certificate via Online Certificate
			Status Protocol (OCSP) Online Certificate Status Protocol (OCSP)
			[RFC6960].
	10.4. Mitigating Denial-of-Service for broadcast packets		10.4. 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, the client and the server use the "not		protection to the protocol, the client and the server use the "not
	re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC		re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC
	4082 [RFC4082]. In this scheme the server never uses a key for the		4082 [RFC4082]. In this scheme the server never uses a key for the
	MAC 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 already knows, 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		Discussion: Note that an alternative approach to enhance TESLA's
	against DoS attacks involves the addition of a group MAC to each		resistance against DoS attacks involves the addition of a group
	packet. This requires the exchange of an additional shared key		MAC to each packet. This requires the exchange of an additional
	common to the whole group. This adds additional complexity to the		shared key common to the whole group. This adds additional
	protocol and hence is currently not considered in this document.		complexity to the protocol and hence is currently not considered
			in this document.
	10.5. Delay Attack		10.5. 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 from accurately		asymmetrically [RFC7384]. This prevents the client from accurately
	measuring the network delay, and hence its time offset to the server		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
	skipping to change at page 24, line 27 ¶		skipping to change at page 18, line 7 ¶
	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
	synchronization 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 utilize		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.ietf-tictoc-multi-path-synchronization]. The client can
	the attack, provided that the adversary is unable to manipulate		detect the attack, provided that the adversary is unable to
	the majority of the available paths [Shpiner]. Note that this		manipulate the majority of the available paths [Shpiner]. Note
	approach is not yet available, neither for NTP nor for PTP.		that this 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 unicast-type messages: Introduction of a threshold value for		4. For unicast-type messages: Introduction of a threshold value for
	the 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
	skipping to change at page 25, line 17 ¶		skipping to change at page 18, line 46 ¶
	authenticity of any received broadcast packets.		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 Appendix B.4		unicast message exchange with the broadcast server (see Appendix C.4
	for a detailed description of this check). Note that a broadcast		for a detailed description of this check). Note that a broadcast
	client should also apply the above-mentioned precautions as far as		client should also apply the above-mentioned precautions as far as
	possible.		possible.
	10.6. Random Number Generation		10.6. 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.
	11. Acknowledgements		11. Acknowledgements
	The authors would like to thank Tal Mizrahi, Russ Housley, Steven		The authors would like to thank Tal Mizrahi, Russ Housley, Steven
	Bellovin, David Mills and Kurt Roeckx for discussions and comments on		Bellovin, David Mills, Kurt Roeckx, Rainer Bermbach, Martin Langer
	the design of NTS. Also, thanks go to Harlan Stenn for his technical		and Florian Weimer for discussions and comments on the design of NTS.
	review and specific text contributions to this document.		Also, thanks go to Harlan Stenn for his technical review and specific
			text contributions to this document.
	12. References		12. References
	12.1. Normative References		12.1. Normative References
	[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.
	[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.		[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
	Briscoe, "Timed Efficient Stream Loss-Tolerant		Briscoe, "Timed Efficient Stream Loss-Tolerant
	Authentication (TESLA): Multicast Source Authentication		Authentication (TESLA): Multicast Source Authentication
	Transform Introduction", RFC 4082, June 2005.		Transform Introduction", RFC 4082, June 2005.
	[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
	RFC 5652, September 2009.
	[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in		[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
	Packet Switched Networks", RFC 7384, October 2014.		Packet Switched Networks", RFC 7384, October 2014.
	12.2. Informative References		12.2. Informative References
	[I-D.iab-privsec-confidentiality-threat]		[I-D.iab-privsec-confidentiality-threat]
	Barnes, R., Schneier, B., Jennings, C., Hardie, T.,		Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
	Trammell, B., Huitema, C., and D. Borkmann,		Trammell, B., Huitema, C., and D. Borkmann,
	"Confidentiality in the Face of Pervasive Surveillance: A		"Confidentiality in the Face of Pervasive Surveillance: A
	Threat Model and Problem Statement", draft-iab-privsec-		Threat Model and Problem Statement", draft-iab-privsec-
	confidentiality-threat-03 (work in progress), February		confidentiality-threat-07 (work in progress), May 2015.
	2015.
	[I-D.ietf-ntp-cms-for-nts-message]		[I-D.ietf-ntp-cms-for-nts-message]
	Sibold, D., Roettger, S., Teichel, K., and R. Housley,		Sibold, D., Teichel, K., Roettger, S., and R. Housley,
	"Protecting Network Time Security Messages with the		"Protecting Network Time Security Messages with the
	Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-		Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
	for-nts-message-00 (work in progress), October 2014.		for-nts-message-03 (work in progress), April 2015.
	[I-D.shpiner-multi-path-synchronization]		[I-D.ietf-tictoc-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-ietf-tictoc-multi-path-
	synchronization-03 (work in progress), February 2014.		synchronization-02 (work in progress), April 2015.
	[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.
	[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.
	[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network		[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
	Time Protocol Version 4: Protocol and Algorithms		Time Protocol Version 4: Protocol and Algorithms
	Specification", RFC 5905, June 2010.		Specification", RFC 5905, June 2010.
			[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
			Galperin, S., and C. Adams, "X.509 Internet Public Key
			Infrastructure Online Certificate Status Protocol - OCSP",
			RFC 6960, June 2013.
	[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. (informative) TICTOC Security Requirements		Appendix A. (informative) 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].
	skipping to change at page 28, line 25 ¶		skipping to change at page 22, line 5 ¶
	| 5.10 | Secure mode | MUST | OK |		| 5.10 | Secure mode | MUST | OK |
	+---------+------------------------------+-------------+------------+		+---------+------------------------------+-------------+------------+
	| | Hybrid mode | SHOULD | - |		| | Hybrid mode | SHOULD | - |
	+---------+------------------------------+-------------+------------+		+---------+------------------------------+-------------+------------+
	*) See discussion in Section 10.5.		*) See discussion in Section 10.5.
	Comparison of NTS specification against Security Requirements of Time		Comparison of NTS specification against Security Requirements of Time
	Protocols in Packet Switched Networks (RFC 7384)		Protocols in Packet Switched Networks (RFC 7384)
	Appendix B. (normative) Using TESLA for Broadcast-Type Messages		Appendix B. (normative) Inherent Association Protocol Messages
	For broadcast-type messages , NTS adopts the TESLA protocol with some		One option for completing association, cookie exchange, and also
			broadcast parameter exchange between a client and server is to use
			the message exchanges listed below.
			B.1. Overview of NTS with Inherent Association Protocol
			This inherent association protocol applies X.509 certificates to
			verify the authenticity of the time server and to exchange the
			cookie. This is done in two separate message exchanges, described
			below. A client needs a public/private key pair for encryption, with
			the public key enclosed in a certificate. A server needs a public/
			private key pair for signing, with the public key enclosed in a
			certificate. If a participant intends to act as both a client and a
			server, it MUST have two different key pairs for these purposes.
			If this protocol is employed, the hash value of the client's
			certificate is used as the client's key input value, i.e. the cookie
			is calculated according to:
			cookie = MSB_<b> (HMAC(server seed, H(certificate of client))).
			The client's certificate contains the client's public key and enables
			the server to identify the client, if client authorization is
			desired.
			B.2. Association Message Exchange
			In this message exchange, the participants negotiate the hash and
			encryption algorithms that are used throughout the protocol. In
			addition, the client receives the certification chain up to a trusted
			anchor. With the established certification chain the client is able
			to verify the server's signatures and, hence, the authenticity of
			future NTS messages from the server is ensured.
			B.2.1. Goals of the Association Exchange
			The association exchange:
			o enables the client to verify any communication with the server as
			authentic,
			o lets the participants negotiate NTS version and algorithms,
			o guarantees authenticity and integrity of the negotiation result to
			the client,
			o guarantees to the client that the negotiation result is based on
			the client's original, unaltered request.
			B.2.2. Message Type: "client_assoc"
			The protocol sequence starts with the client sending an association
			message, called client_assoc. This message contains
			o the NTS message ID "client_assoc",
			o a nonce,
			o the version number of NTS that the client wants to use (this
			SHOULD be the highest version number that it supports),
			o the hostname of the client,
			o a selection of accepted hash algorithms, and
			o a selection of accepted encryption algorithms.
			B.2.3. Message Type: "server_assoc"
			This message is sent by the server upon receipt of client_assoc. It
			contains
			o the NTS message ID "server_assoc",
			o the nonce transmitted in client_assoc,
			o the client's proposal for the version number, selection of
			accepted hash algorithms and selection of accepted encryption
			algorithms, as transmitted in client_assoc,
			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
			client_assoc message and the highest supported by the server),
			o the hostname of the server,
			o the server's choice of algorithm for encryption and for
			cryptographic hashing, all of which MUST be chosen from the
			client's proposals,
			o a signature, calculated over the data listed above, with the
			server's private key and according to the signature algorithm
			which is also used for the certificates that are included (see
			below), and
			o a chain of certificates, which starts at the server and goes up to
			a trusted authority; each certificate MUST be certified by the one
			directly following it.
			B.2.4. Procedure Overview of the Association Exchange
			For an association exchange, the following steps are performed:
			1. The client sends a client_assoc message to the server. It MUST
			keep the transmitted values for the version number and algorithms
			available for later checks.
			2. 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 Appendix B.2.3. Upon unsuccessful negotiation for version
			number or algorithms the server_assoc message MUST contain an
			error code.
			3. The client waits for a reply in the form of a server_assoc
			message. After receipt of the message it performs the following
			checks:
			* The client checks that the message contains a conforming
			version number.
			* It checks that the nonce sent back by the server matches the
			one transmitted in client_assoc,
			* It also verifies that the server has chosen the encryption and
			hash algorithms from its proposal sent in the client_assoc
			message and that this proposal was not altered.
			* Furthermore, it performs authenticity checks on the
			certificate chain and the signature.
			If one of the checks fails, the client MUST abort the run.
			+------------------------+
			| o Choose version |
			| o Choose algorithms |
			| o Acquire certificates |
			| o Assemble response |
			| o Create signature |
			+-----------+------------+
			|
			<-+->
			Server --------------------------->
			/| \
			client_ / \ server_
			assoc / \ assoc
			/ \|
			Client --------------------------->
			<------ Association ----->
			exchange
			Procedure for association and cookie exchange.
			B.3. Cookie Messages
			During this message exchange, the server transmits a secret cookie to
			the client securely. The cookie will later be used for integrity
			protection during unicast time synchronization.
			B.3.1. Goals of the Cookie Exchange
			The cookie exchange:
			o enables the server to check the client's authorization via its
			certificate (optional),
			o supplies the client with the correct cookie and corresponding KIV
			for its association to the server,
			o guarantees to the client that the cookie originates from the
			server and that it is based on the client's original, unaltered
			request.
			o guarantees that the received cookie is unknown to anyone but the
			server and the client.
			B.3.2. Message Type: "client_cook"
			This message is sent by the client upon successful authentication of
			the server. In this message, the client requests a cookie from the
			server. The message contains
			o the NTS message ID "client_cook",
			o a nonce,
			o the negotiated version number,
			o the negotiated signature algorithm,
			o the negotiated encryption algorithm,
			o the negotiated hash algorithm H,
			o the client's certificate.
			B.3.3. Message Type: "server_cook"
			This message is sent by the server upon receipt of a client_cook
			message. The server generates the hash of the client's certificate,
			as conveyed during client_cook, in order to calculate the cookie
			according to Section 5. This message contains
			o the NTS message ID "server_cook"
			o the version number as transmitted in client_cook,
			o a concatenated datum which is encrypted with the client's public
			key, according to the encryption algorithm transmitted in the
			client_cook message. The concatenated datum contains
			* the nonce transmitted in client_cook, and
			* the cookie.
			o a signature, created with the server's private key, calculated
			over all of the data listed above. This signature MUST be
			calculated according to the transmitted signature algorithm from
			the client_cook message.
			B.3.4. Procedure Overview of the Cookie Exchange
			For a cookie exchange, the following steps are performed:
			1. The client sends a client_cook message to the server. The client
			MUST save the included nonce until the reply has been processed.
			2. Upon receipt of a client_cook message, the server checks whether
			it supports the given cryptographic algorithms. It then
			calculates the cookie according to the formula given in
			Section 5. The server MAY use the client's certificate to check
			that the client is authorized to use the secure time
			synchronization service. With this, it MUST construct a
			server_cook message as described in Appendix B.3.3.
			3. The client 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 beforehand.
			* 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 nonce and a cookie of the
			expected bit lengths.
			* 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.
			+----------------------------+
			| o OPTIONAL: Check client's |
			| authorization |
			| o Generate cookie |
			| o Encrypt inner message |
			| o Generate signature |
			+-------------+--------------+
			|
			<-+->
			Server --------------------------->
			/| \
			client_ / \ server_
			cook / \ cook
			/ \|
			Client --------------------------->
			<--- Cookie exchange -->
			Procedure for association and cookie exchange.
			B.3.5. Broadcast Parameter Messages
			In this message exchange, the client receives the necessary
			information to execute the TESLA protocol in a secured broadcast
			association. The client can only initiate a secure broadcast
			association after successful association and cookie exchanges and
			only if it has made sure that its clock is roughly synchronized to
			the server's.
			See Appendix C for more details on TESLA.
			B.3.5.1. Goals of the Broadcast Parameter Exchange
			The broadcast parameter exchange
			o provides the client with all the information necessary to process
			broadcast time synchronization messages from the server, and
			o guarantees authenticity, integrity and freshness of the broadcast
			parameters to the client.
			B.3.5.2. Message Type: "client_bpar"
			This message is sent by the client in order to establish a secured
			time broadcast association with the server. It contains
			o the NTS message ID "client_bpar",
			o the NTS version number negotiated during association,
			o a nonce,
			o the client's hostname, and
			o the signature algorithm negotiated during association.
			B.3.5.3. Message Type: "server_bpar"
			This message is sent by the server upon receipt of a client_bpar
			message during the broadcast loop of the server. It contains
			o the NTS message ID "server_bpar",
			o the version number as transmitted in the client_bpar message,
			o the nonce transmitted in client_bpar,
			o the one-way functions used for building the key chain, and
			o the disclosure schedule of the keys. This contains:
			* the last key of the key chain,
			* time interval duration,
			* the disclosure delay (number of intervals between use and
			disclosure of a key),
			* the time at which the next time interval will start, and
			* the next interval's associated index.
			o The message also contains a signature signed by the server with
			its private key, verifying all the data listed above.
			B.3.5.4. Procedure Overview of the Broadcast Parameter Exchange
			A broadcast parameter exchange consists of the following steps:
			1. The client sends a client_bpar message to the server. It MUST
			remember the transmitted values for the nonce, the version number
			and the signature algorithm.
			2. Upon receipt of a client_bpar message, the server constructs and
			sends a server_bpar message as described in Appendix B.3.5.3.
			3. The client waits for a reply in the form of a server_bpar
			message, on which it performs the following checks:
			* The message must contain all the necessary information for the
			TESLA protocol, as listed in Appendix B.3.5.3.
			* The message must contain a nonce belonging to a client_bpar
			message that the client has previously sent.
			* Verification of the message's signature.
			If any information is missing or if 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.
			+---------------------+
			| o Assemble response |
			| o Create public-key |
			| signature |
			+----------+----------+
			|
			<-+->
			Server --------------------------------------------->
			/| \
			client_ / \ server_
			bpar / \ bpar
			/ \|
			Client --------------------------------------------->
			<------- Broadcast ------> <- Client-side ->
			parameter validity
			exchange checks
			Procedure for unicast time synchronization exchange.
			Appendix C. (normative) Using TESLA for Broadcast-Type Messages
			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 uses 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].
	B.1. Server Preparation		C.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.
	skipping to change at page 29, line 33 ¶		skipping to change at page 32, line 5 ¶
	* 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 its MAC has		because it fails to verify that the key used for its MAC has
	not yet been 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 an unnecessarily long time before they can be		buffered for an unnecessarily long time before they can be
	verified by the client and be subsequently utilized for time		verified by the client and be subsequently utilized for time
	synchronization.		synchronization.
	The server SHOULD calculate d according to		It is RECOMMENDED that the server calculate d according to
	d = ceil( 2*B / L) + 1,		d = ceil( 2*B / L) + 1,
	where ceil yields 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
	skipping to change at page 30, line 24 ¶		skipping to change at page 32, line 31 ¶
	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 of the TESLA protocol's one-way key chain		A schematic explanation of the TESLA protocol's one-way key chain
	B.2. Client Preparation		C.2. Client Preparation
	A client needs the following information in order to participate in a		A client needs the following information in order to participate in a
	TESLA broadcast:		TESLA broadcast:
	o One key K_i from the one-way key chain, which has to be		o One key K_i from the one-way key chain, which has to be
	authenticated as belonging to the server. Typically, this will be		authenticated as belonging to the server. Typically, this will be
	K_0.		K_0.
	o The disclosure schedule of the keys. This consists of:		o The disclosure schedule of the keys. This consists of:
	skipping to change at page 31, line 8 ¶		skipping to change at page 33, line 14 ¶
	o The second one-way function F' used to derive the MAC keys K'_0,		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.		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		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.
	B.3. Sending Authenticated Broadcast Packets		C.3. Sending Authenticated Broadcast Packets
	During each time interval I_i, the server sends at most one		During each time interval I_i, the server sends at most one
	authenticated broadcast packet P_i. Such a packet consists of:		authenticated broadcast packet P_i. Such a 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.
	B.4. Authentication of Received Packets		C.4. Authentication of Received Packets
	When a client receives a packet P_i as described above, it first		When a client receives a packet P_i as described above, it first
	checks that it has not already received a packet with the same		checks that it has not already received a packet with the same
	disclosed key. This is done to avoid replay/flooding attacks. A		disclosed key. This is done to avoid replay/flooding attacks. A
	packet 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
	skipping to change at page 32, line 44 ¶		skipping to change at page 35, line 5 ¶
	broadcast parameter exchange (because a falsification of this check		broadcast parameter exchange (because a falsification of this check
	yields that the packet was not generated according to protocol, which		yields that the packet was not generated according to protocol, which
	suggests an attack).		suggests an attack).
	If a packet P_i passes all the tests listed above, it is stored for		If a packet P_i passes all the tests listed above, it is stored for
	later authentication. Also, if at this time there is a package with		later authentication. Also, if at this time there is a package with
	index i-d already buffered, then the client uses the disclosed key		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		K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in
	package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.		package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.
	Appendix C. (informative) Dependencies		Appendix D. (informative) Dependencies
	+---------+--------------+--------+-------------------------------+		+---------+--------------+--------+-------------------------------+
	| Issuer | Type | Owner | Description |		| Issuer | Type | Owner | Description |
	+---------+--------------+--------+-------------------------------+		+---------+--------------+--------+-------------------------------+
	| Server | private key | server | Used for server_assoc, |		| Server | private key | server | Used for server_assoc, |
	| PKI | (signature) | | server_cook, server_bpar. |		| PKI | (signature) | | server_cook, server_bpar. |
	| +--------------+--------+ The server uses the private |		| +--------------+--------+ The server uses the private |
	| | public key | client | key to sign these messages. |		| | public key | client | key to sign these messages. |
	| | (signature) | | The client uses the public |		| | (signature) | | The client uses the public |
	| +--------------+--------+ key to verify them. |		| +--------------+--------+ key to verify them. |
	| | certificate | server | The certificate is used in |		| | certificate | server | The certificate is used in |
End of changes. 81 change blocks.
	541 lines changed or deleted		634 lines changed or added
This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/