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].
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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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
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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,
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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,
skipping to change at page 18, line 23 skipping to change at page 11, line 19
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.
skipping to change at page 20, line 43 skipping to change at page 14, line 14
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. 
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