NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: September 4, 2015 Google Inc.
K. Teichel
PTB
March 3, 2015
Network Time Security
draft-ietf-ntp-network-time-security-07.txt
Abstract
This document describes Network Time Security (NTS), a collection of
measures that enable secure time synchronization with time servers
using protocols like the Network Time Protocol (NTP) or the Precision
Time Protocol (PTP). Its design considers the special requirements
of precise timekeeping which are described in Security Requirements
of Time Protocols in Packet Switched Networks [RFC7384].
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 4, 2015.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4
2.2. Common Terminology for PTP and NTP . . . . . . . . . . . 4
3. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4
4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Association Message Exchange . . . . . . . . . . . . . . 7
6.1.1. Goals of the Association Exchange . . . . . . . . . . 7
6.1.2. Message Type: "client_assoc" . . . . . . . . . . . . 7
6.1.3. Message Type: "server_assoc" . . . . . . . . . . . . 7
6.1.4. Procedure Overview of the Association Exchange . . . 8
6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 9
6.2.1. Goals of the Cookie Exchange . . . . . . . . . . . . 9
6.2.2. Message Type: "client_cook" . . . . . . . . . . . . . 10
6.2.3. Message Type: "server_cook" . . . . . . . . . . . . . 10
6.2.4. Procedure Overview of the Cookie Exchange . . . . . . 11
6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 12
6.3.1. Goals of the Unicast Time Synchronization Exchange . 12
6.3.2. Message Type: "time_request" . . . . . . . . . . . . 12
6.3.3. Message Type: "time_response" . . . . . . . . . . . . 13
6.3.4. Procedure Overview of the Unicast Time
Synchronization Exchange . . . . . . . . . . . . . . 13
6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 14
6.4.1. Goals of the Broadcast Parameter Exchange . . . . . . 15
6.4.2. Message Type: "client_bpar" . . . . . . . . . . . . . 15
6.4.3. Message Type: "server_bpar" . . . . . . . . . . . . . 15
6.4.4. Procedure Overview of the Broadcast Parameter
Exchange . . . . . . . . . . . . . . . . . . . . . . 16
6.5. Broadcast Time Synchronization Exchange . . . . . . . . . 17
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6.5.1. Goals of the Broadcast Time Synchronization Exchange 17
6.5.2. Message Type: "server_broad" . . . . . . . . . . . . 17
6.5.3. Procedure Overview of Broadcast Time Synchronization
Exchange . . . . . . . . . . . . . . . . . . . . . . 18
6.6. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 19
6.6.1. Goals of the Broadcast Keycheck Exchange . . . . . . 19
6.6.2. Message Type: "client_keycheck" . . . . . . . . . . . 20
6.6.3. Message Type: "server_keycheck" . . . . . . . . . . . 20
6.6.4. Procedure Overview of the Broadcast Keycheck Exchange 20
7. Server Seed Considerations . . . . . . . . . . . . . . . . . 21
8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 22
8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 22
8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
10.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 22
10.2. Initial Verification of the Server Certificates . . . . 23
10.3. Revocation of Server Certificates . . . . . . . . . . . 23
10.4. Mitigating Denial-of-Service for broadcast packets . . . 23
10.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 24
10.6. Random Number Generation . . . . . . . . . . . . . . . . 25
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. (informative) TICTOC Security Requirements . . . . . 27
Appendix B. (normative) Using TESLA for Broadcast-Type Messages 28
B.1. Server Preparation . . . . . . . . . . . . . . . . . . . 28
B.2. Client Preparation . . . . . . . . . . . . . . . . . . . 30
B.3. Sending Authenticated Broadcast Packets . . . . . . . . . 31
B.4. Authentication of Received Packets . . . . . . . . . . . 31
Appendix C. (informative) Dependencies . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
Time synchronization protocols are increasingly utilized to
synchronize clocks in networked infrastructures. Successful attacks
against the time synchronization protocol can seriously degrade the
reliable performance of such infrastructures. Therefore, time
synchronization protocols have to be secured if they are applied in
environments that are prone to malicious attacks. This can be
accomplished either by utilization of external security protocols,
like IPsec or TLS, or by intrinsic security measures of the time
synchronization protocol.
The two most popular time synchronization protocols, the Network Time
Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP)
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[IEEE1588], currently do not provide adequate intrinsic security
precautions. This document specifies security measures which enable
these and possibly other protocols to verify the authenticity of the
time server/master and the integrity of the time synchronization
protocol packets. The utilization of these measures for a given
specific time synchronisation protocol has to be described in a
separate document.
[RFC7384] specifies that a security mechanism for timekeeping must be
designed in such a way that it does not degrade the quality of the
time transfer. This implies that for time keeping the increase in
bandwidth and message latency caused by the security measures should
be small. Also, NTP as well as PTP work via UDP and connections are
stateless on the server/master side. Therefore, all security
measures in this document are designed in such a way that they add
little demand for bandwidth, that the necessary calculations can be
executed in a fast manner, and that the measures do not require a
server/master to keep state of a connection.
2. Terminology
2.1. Terms and Abbreviations
MITM Man In The Middle
NTS Network Time Security
TESLA Timed Efficient Stream Loss-tolerant Authentication
MAC Message Authentication Code
HMAC Keyed-Hash Message Authentication Code
2.2. Common Terminology for PTP and NTP
This document refers to different time synchronization protocols, in
particular to both the PTP and the NTP. Throughout the document the
term "server" applies to both a PTP master and an NTP server.
Accordingly, the term "client" applies to both a PTP slave and an NTP
client.
3. Security Threats
The document "Security Requirements of Time Protocols in Packet
Switched Networks" [RFC7384] contains a profound analysis of security
threats and requirements for time synchronization protocols.
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4. Objectives
The objectives of the NTS specification are as follows:
o Authenticity: NTS enables a client to authenticate its time
server(s).
o Integrity: NTS protects the integrity of time synchronization
protocol packets via a message authentication code (MAC).
o Confidentiality: NTS does not provide confidentiality protection
of the time synchronization packets.
o Authorization: NTS optionally enables the server to verify the
client's authorization.
o Request-Response-Consistency: NTS enables a client to match an
incoming response to a request it has sent. NTS also enables the
client to deduce from the response whether its request to the
server has arrived without alteration.
o Integration with protocols: NTS can be used to secure different
time synchronization protocols, specifically at least NTP and PTP.
A client or server running an NTS-secured version of a time
protocol does not negatively affect other participants who are
running unsecured versions of that protocol.
5. NTS Overview
NTS applies X.509 certificates to verify the authenticity of the time
server and to exchange a symmetric key, the so-called cookie. It
then uses the cookie to protect the 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))),
with the server seed as the key, where H is a hash function, and
where the function MSB_<b> cuts off the b most significant bits of
the result of the HMAC function. The client's certificate contains
the client's public key and enables the server to identify the
client, if client authorization is desired. The server seed is a
random value of bit length b that the server possesses, which has to
remain secret. The cookie never changes as long as the server seed
stays the same, but the server seed has to be refreshed periodically
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in order to provide key freshness as required in [RFC7384]. See
Section 7 for details on seed refreshing.
Since the server does not keep a state of the client, it has to
recalculate the cookie each time it receives a unicast time
synchronization request from the client. To this end, the client has
to attach the hash value of its certificate to each request (see
Section 6.3).
For broadcast-type messages, authenticity and integrity of the time
synchronization packets are also ensured by a MAC, which is attached
to the time synchronization packet by the sender. Verification of
the broadcast-type packets' authenticity is based on the TESLA
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,
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
element of the chain with all clients. The server splits time into
intervals of uniform duration and assigns each key to an interval in
reverse order, starting with the penultimate. At each time interval,
the server sends a broadcast packet appended by a MAC, calculated
using the corresponding key, and the key of the previous disclosure
interval. The client verifies the MAC by buffering the packet until
disclosure of the key in its associated disclosure interval occurs.
In order to be able to verify the timeliness of the packets, the
client has to be loosely time synchronized with the server. This has
to be accomplished before broadcast associations can be used. For
checking timeliness of packets, NTS uses another, more rigorous check
in addition to just the clock lookup used in the TESLA protocol. For
a more detailed description of how NTS employs and customizes TESLA,
see Appendix B.
6. Protocol Messages
This section describes the types of messages needed for secure time
synchronization with NTS.
For some guidance on how these message types can be realized in
practice, and integrated into the communication flow of existing time
synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a
companion document for NTS. Said document describes ASN.1 encodings
for those message parts that have to be added to a time
synchronization protocol for security reasons as well as CMS
(Cryptographic Message Syntax, see [RFC5652]) conventions that can be
used to get the cryptographic aspects right.
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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.
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,
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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 also verifies that the server has chosen the encryption and
hash algorithms from its proposal sent in the client_assoc
message.
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* Furthermore, it performs authenticity checks on the
certificate chain and the signature for the version number.
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.
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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.
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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
the client_cook message.
If one of those checks fails, the client MUST abort the run.
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+----------------------------+
| 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.
6.3. Unicast Time Synchronisation Messages
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 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
The unicast time synchronization exchange:
o exchanges (unicast) time synchronization data as specified by the
appropriate time synchronization protocol,
o guarantees to the client that the response originates from the
server and is based on the client's original, unaltered request.
6.3.2. Message Type: "time_request"
This message is sent by the client when it requests a time exchange.
It contains
o the NTS message ID "time_request",
o the negotiated version number,
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o a nonce,
o the negotiated hash algorithm H,
o the hash of the client's certificate under H.
6.3.3. Message Type: "time_response"
This message is sent by the server after it has received a
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
transmitted hash algorithm. The message contains
o the NTS message ID "time_response",
o the version number as transmitted in time_request,
o the server's time synchronization response data,
o the nonce transmitted in time_request,
o a MAC (generated with the cookie as key) for verification of all
of the above data.
6.3.4. Procedure Overview of the Unicast Time Synchronization Exchange
For a unicast time synchronization exchange, the following steps are
performed:
1. The client sends a time_request message to the server. The
client MUST save the included nonce and the transmit_timestamp
(from the time synchronization data) as a correlated pair for
later verification steps.
2. Upon receipt of a time_request message, the server re-calculates
the cookie, then computes the necessary time synchronization data
and constructs a time_response message as given in Section 6.3.3.
3. It awaits a reply in the form of a time_response message. Upon
receipt, it checks:
* that the transmitted version number matches the one negotiated
previously,
* that the transmitted nonce belongs to a previous time_request
message,
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* that the transmit_timestamp in that time_request message
matches the corresponding time stamp from the synchronization
data received in the time_response, and
* that the appended MAC verifies the received synchronization
data, version number and nonce.
If at least one of the first three checks fails (i.e. if the
version number does not match, if the client has never used the
nonce transmitted in the time_response message, or if it has used
the nonce with initial time synchronization data different from
that in the response), then the client MUST ignore this
time_response message. If the MAC is invalid, the client MUST do
one of the following: abort the run or go back to step 5 (because
the cookie might have changed due to a server seed refresh). If
both checks are successful, the client SHOULD continue time
synchronization by going back to step 7.
+-----------------------+
| o Re-generate cookie |
| o Assemble response |
| o Generate MAC |
+-----------+-----------+
|
<-+->
Server ----------------------------------------------->
/| \
time_ / \ time_
request / \ response
/ \|
Client ----------------------------------------------->
<------ Unicast time ------> <- Client-side ->
synchronization validity
exchange checks
Procedure for unicast time synchronization exchange.
6.4. 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.
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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,
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),
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* 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.
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.
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+---------------------+
| 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
The broadcast time synchronization exchange:
o transmits (broadcast) time synchronization data from the server to
the client as specified by the appropriate time synchronization
protocol,
o guarantees to the client that the received synchronization data
has arrived in a timely manner as required by the TESLA protocol
and is trustworthy enough to be stored for later checks,
o additionally guarantees authenticity of a certain broadcast
synchronization message in the client's storage.
6.5.2. Message Type: "server_broad"
This message is sent by the server over the course of its broadcast
schedule. It is part of any broadcast association. It contains
o the NTS message ID "server_broad",
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o the version number that the server is working under,
o time broadcast data,
o the index that belongs to the current interval (and therefore
identifies the current, yet undisclosed, key),
o the disclosed key of the previous disclosure interval (current
time interval minus disclosure delay),
o a MAC, calculated with the key for the current time interval,
verifying
* the message ID,
* the version number, and
* the time data.
6.5.3. Procedure Overview of Broadcast Time Synchronization Exchange
A broadcast time synchronization message exchange consists of the
following steps:
1. The server follows the TESLA protocol by regularly sending
server_broad messages as described in Section 6.5.2, adhering to
its own disclosure schedule.
2. The client awaits time synchronization data in the form of a
server_broadcast message. Upon receipt, it performs the
following checks:
* Proof that the MAC is based on a key that is not yet disclosed
(packet timeliness). This is achieved via a combination of
checks. First, the disclosure schedule is used, which
requires loose time synchronization. If this is successful,
the client obtains a stronger guarantee via a key check
exchange (see below). If its timeliness is verified, the
packet will be buffered for later authentication. Otherwise,
the client MUST discard it. Note that the time information
included in the packet will not be used for synchronization
until its authenticity could also be verified.
* The client checks that it does not already know the disclosed
key. Otherwise, the client SHOULD discard the packet to avoid
a buffer overrun. If this check is successful, the client
ensures that the disclosed key belongs to the one-way key
chain by applying the one-way function until equality with a
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previous disclosed key is shown. If it is falsified, the
client MUST discard the packet.
* If the disclosed key is legitimate, then the client verifies
the authenticity of any packet that it has received during the
corresponding time interval. If authenticity of a packet is
verified, then it is released from the buffer and its time
information can be utilized. If the verification fails, then
authenticity is not given. In this case, the client MUST
request authentic time from the server by means other than
broadcast messages. Also, the client MUST re-initialize the
broadcast sequence with a "client_bpar" message if the one-way
key chain expires, which it can check via the disclosure
schedule.
See RFC 4082[RFC4082] for a detailed description of the packet
verification process.
Server ---------------------------------->
\
\ server_
\ broad
\|
Client ---------------------------------->
< Broadcast > <- Client-side ->
time sync. validity and
exchange timeliness
checks
Procedure for broadcast time synchronization exchange.
6.6. Broadcast Keycheck
This message exchange is performed for an additional check of packet
timeliness in the course of the TESLA scheme, see Appendix B.
6.6.1. Goals of the Broadcast Keycheck Exchange
The keycheck exchange:
o guarantees to the client that the key belonging to the respective
TESLA interval communicated in the exchange had not been disclosed
before the client_keycheck message was sent.
o guarantees to the client the timeliness of any broadcast packet
secured with this key if it arrived before client_keycheck was
sent.
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6.6.2. Message Type: "client_keycheck"
A message of this type is sent by the client in order to initiate an
additional check of packet timeliness for the TESLA scheme. It
contains
o the NTS message ID "client_keycheck",
o the NTS version number negotiated during association,
o a nonce,
o an interval number from the TESLA disclosure schedule,
o the hash algorithm H negotiated during association, and
o the hash of the client's certificate under H.
6.6.3. Message Type: "server_keycheck"
A message of this type is sent by the server upon receipt of a
client_keycheck message during the broadcast loop of the server.
Prior to this, the server MUST recalculate the client's cookie by
using the hash of the client's certificate and the transmitted hash
algorithm. It contains
o the NTS message ID "server_keycheck"
o the version number as transmitted in "client_keycheck,
o the nonce transmitted in the client_keycheck message,
o the interval number transmitted in the client_keycheck message,
and
o a MAC (generated with the cookie as key) for verification of all
of the above data.
6.6.4. Procedure Overview of the Broadcast Keycheck Exchange
A broadcast keycheck message exchange consists of the following
steps:
1. The client sends a client_keycheck message. It MUST memorize the
nonce and the time interval number that it sends as a correlated
pair.
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2. Upon receipt of a client_keycheck message, the server looks up
whether it has already disclosed the key associated with the
interval number transmitted in that message. If it has not
disclosed it, it constructs and sends the appropriate
server_keycheck message as described in Section 6.6.3. For more
details, see also Appendix B.
3. The client awaits a reply in the form of a server_keycheck
message. On receipt, it performs the following checks:
* that the transmitted version number matches the one negotiated
previously,
* that the transmitted nonce belongs to a previous
client_keycheck message,
* that the TESLA interval number in that client_keycheck message
matches the corresponding interval number from the
server_keycheck, and
* that the appended MAC verifies the received data.
+----------------------+
| o Assemble response |
| o Re-generate cookie |
| o Generate MAC |
+-----------+----------+
|
<-+->
Server --------------------------------------------->
\ /| \
\ server_ client_ / \ server_
\ broad keycheck / \ keycheck
\| / \|
Client --------------------------------------------->
<-------- Extended broadcast time ------->
synchronization. exchange
<---- Keycheck exchange --->
Procedure for extended broadcast time synchronization exchange.
7. Server Seed Considerations
The server has to calculate a random seed which has to be kept
secret. The server MUST generate a seed for each supported hash
algorithm, see Section 8.1.
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According to the requirements in [RFC7384], the server MUST refresh
each server seed periodically. Consequently, the cookie memorized by
the client becomes obsolete. In this case, the client cannot verify
the MAC attached to subsequent time response messages and has to
respond accordingly by re-initiating the protocol with a cookie
request (Section 6.2).
8. Hash Algorithms and MAC Generation
8.1. Hash Algorithms
Hash algorithms are used at different points: calculation of the
cookie and the MAC, and hashing of the client's certificate. The
client and the server negotiate a hash algorithm H during the
association message exchange (Section 6.1) at the beginning. The
selected algorithm H is used for all hashing processes in that run.
In the TESLA scheme, hash algorithms are used as pseudo-random
functions to construct the one-way key chain. Here, the utilized
hash algorithm is communicated by the server and is non-negotiable.
Note:
Any hash algorithm is prone to be compromised in the future. A
successful attack on a hash algorithm would enable any NTS client
to derive the server seed from its own cookie. Therefore, the
server MUST have separate seed values for its different supported
hash algorithms. This way, knowledge gained from an attack on a
hash algorithm H can at least only be used to compromise such
clients who use hash algorithm H as well.
8.2. MAC Calculation
For the calculation of the MAC, client and server use a Keyed-Hash
Message Authentication Code (HMAC) approach [RFC2104]. The HMAC is
generated with the hash algorithm specified by the client (see
Section 8.1).
9. IANA Considerations
10. Security Considerations
10.1. Privacy
The payload of time synchronization protocol packets of two-way time
transfer approaches like NTP and PTP consists basically of time
stamps, which are not considered secret [RFC7384]. Therefore,
encryption of the time synchronization protocol packet's payload is
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not considered in this document. However, an attacker can exploit
the exchange of time synchronization protocol packets for topology
detection and inference attacks as described in
[I-D.iab-privsec-confidentiality-threat]. To make such attacks more
difficult, that draft recommends the encryption of the packet
payload. Yet, in the case of time synchronization protocols the
confidentiality protection of time synchronization packet's payload
is of secondary role since the packets meta data (IP addresses, port
numbers, possibly packet size and regular sending intervals) carry
more information than the payload. To enhance the privacy of the
time synchronization partners, the usage of tunnel protocols such as
IPsec and MACsec, where applicable, is therefore more suited than
confidentiality protection of the payload.
10.2. Initial Verification of the Server Certificates
The client has to verify the validity of the certificates during the
certification message exchange (Section 6.1.3). Since it generally
has no reliable time during this initial communication phase, it is
impossible to verify the period of validity of the certificates. To
solve this chicken-and-egg problem, the client as to rely on external
means.
10.3. Revocation of Server Certificates
According to Section 7, it is the client's responsibility to initiate
a new association with the server after the server's certificate
expires. To this end, the client reads the expiration date of the
certificate during the certificate message exchange (Section 6.1.3).
Furthermore, certificates may also be revoked prior to the normal
expiration date. To increase security the client MAY periodically
verify the state of the server's certificate via OCSP.
10.4. Mitigating Denial-of-Service for broadcast packets
TESLA authentication buffers packets for delayed authentication.
This makes the protocol vulnerable to flooding attacks, causing the
client to buffer excessive numbers of packets. To add stronger DoS
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
4082 [RFC4082]. In this scheme the server never uses a key for the
MAC generation more than once. Therefore, the client can discard any
packet that contains a disclosed key it already knows, thus
preventing memory flooding attacks.
Note that an alternative approach to enhance TESLA's resistance
against DoS attacks involves the addition of a group MAC to each
packet. This requires the exchange of an additional shared key
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common to the whole group. This adds additional complexity to the
protocol and hence is currently not considered in this document.
10.5. Delay Attack
In a packet delay attack, an adversary with the ability to act as a
MITM delays time synchronization packets between client and server
asymmetrically [RFC7384]. This prevents the client from accurately
measuring the network delay, and hence its time offset to the server
[Mizrahi]. The delay attack does not modify the content of the
exchanged synchronization packets. Therefore, cryptographic means do
not provide a feasible way to mitigate this attack. However, several
non-cryptographic precautions can be taken in order to detect this
attack.
1. Usage of multiple time servers: this enables the client to detect
the attack, provided that the adversary is unable to delay the
synchronization packets between the majority of servers. This
approach is commonly used in NTP to exclude incorrect time
servers [RFC5905].
2. Multiple communication paths: The client and server utilize
different paths for packet exchange as described in the I-D
[I-D.shpiner-multi-path-synchronization]. The client can detect
the attack, provided that the adversary is unable to manipulate
the majority of the available paths [Shpiner]. Note that this
approach is not yet available, neither for NTP nor for PTP.
3. Usage of an encrypted connection: the client exchanges all
packets with the time server over an encrypted connection (e.g.
IPsec). This measure does not mitigate the delay attack, but it
makes it more difficult for the adversary to identify the time
synchronization packets.
4. For unicast-type messages: Introduction of a threshold value for
the delay time of the synchronization packets. The client can
discard a time server if the packet delay time of this time
server is larger than the threshold value.
Additional provision against delay attacks has to be taken for
broadcast-type messages. This mode relies on the TESLA scheme which
is based on the requirement that a client and the broadcast server
are loosely time synchronized. Therefore, a broadcast client has to
establish time synchronization with its broadcast server before it
starts utilizing broadcast messages for time synchronization.
One possible way to achieve this initial synchronization is to
establish a unicast association with its broadcast server until time
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synchronization and calibration of the packet delay time is achieved.
After that, the client can establish a broadcast association with the
broadcast server and utilizes TESLA to verify integrity and
authenticity of any received broadcast packets.
An adversary who is able to delay broadcast packets can cause a time
adjustment at the receiving broadcast clients. If the adversary
delays broadcast packets continuously, then the time adjustment will
accumulate until the loose time synchronization requirement is
violated, which breaks the TESLA scheme. To mitigate this
vulnerability the security condition in TESLA has to be supplemented
by an additional check in which the client, upon receipt of a
broadcast message, verifies the status of the corresponding key via a
unicast message exchange with the broadcast server (see Appendix B.4
for a detailed description of this check). Note that a broadcast
client should also apply the above-mentioned precautions as far as
possible.
10.6. Random Number Generation
At various points of the protocol, the generation of random numbers
is required. The employed methods of generation need to be
cryptographically secure. See [RFC4086] for guidelines concerning
this topic.
11. Acknowledgements
The authors would like to thank Tal Mizrahi, Russ Housley, Steven
Bellovin, David Mills and Kurt Roeckx for discussions and comments on
the design of NTS. Also, thanks go to Harlan Stenn for his technical
review and specific text contributions to this document.
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
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[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, October 2014.
12.2. Informative References
[I-D.iab-privsec-confidentiality-threat]
Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", draft-iab-privsec-
confidentiality-threat-03 (work in progress), February
2015.
[I-D.ietf-ntp-cms-for-nts-message]
Sibold, D., Roettger, S., Teichel, K., and R. Housley,
"Protecting Network Time Security Messages with the
Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
for-nts-message-00 (work in progress), October 2014.
[I-D.shpiner-multi-path-synchronization]
Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
Path Time Synchronization", draft-shpiner-multi-path-
synchronization-03 (work in progress), February 2014.
[IEEE1588]
IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and
control systems", 2008.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings of
Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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[Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", in Proceedings of Precision Clock
Synchronization for Measurement Control and Communication,
ISPCS 2013, pp. 1-6, September 2013.
Appendix A. (informative) TICTOC Security Requirements
The following table compares the NTS specifications against the
TICTOC security requirements [RFC7384].
+---------+------------------------------+-------------+------------+
| Section | Requirement from RFC 7384 | Requirement | NTS |
| | | level | |
+---------+------------------------------+-------------+------------+
| 5.1.1 | Authentication of Servers | MUST | OK |
+---------+------------------------------+-------------+------------+
| 5.1.1 | Authorization of Servers | MUST | OK |
+---------+------------------------------+-------------+------------+
| 5.1.2 | Recursive Authentication of | MUST | OK |
| | Servers (Stratum 1) | | |
+---------+------------------------------+-------------+------------+
| 5.1.2 | Recursive Authorization of | MUST | OK |
| | Servers (Stratum 1) | | |
+---------+------------------------------+-------------+------------+
| 5.1.3 | Authentication and | MAY | Optional, |
| | Authorization of Clients | | Limited |
+---------+------------------------------+-------------+------------+
| 5.2 | Integrity protection | MUST | OK |
+---------+------------------------------+-------------+------------+
| 5.3 | Spoofing Prevention | MUST | OK |
+---------+------------------------------+-------------+------------+
| 5.4 | Protection from DoS attacks | SHOULD | OK |
| | against the time protocol | | |
+---------+------------------------------+-------------+------------+
| 5.5 | Replay protection | MUST | OK |
+---------+------------------------------+-------------+------------+
| 5.6 | Key freshness | MUST | OK |
+---------+------------------------------+-------------+------------+
| | Security association | SHOULD | OK |
+---------+------------------------------+-------------+------------+
| | Unicast and multicast | SHOULD | OK |
| | associations | | |
+---------+------------------------------+-------------+------------+
| 5.7 | Performance: no degradation | MUST | OK |
| | in quality of time transfer | | |
+---------+------------------------------+-------------+------------+
| | Performance: lightweight | SHOULD | OK |
| | computation | | |
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+---------+------------------------------+-------------+------------+
| | Performance: storage | SHOULD | OK |
+---------+------------------------------+-------------+------------+
| | Performance: bandwidth | SHOULD | OK |
+---------+------------------------------+-------------+------------+
| 5.8 | Confidentiality protection | MAY | NO |
+---------+------------------------------+-------------+------------+
| 5.9 | Protection against Packet | MUST | Limited*) |
| | Delay and Interception | | |
| | Attacks | | |
+---------+------------------------------+-------------+------------+
| 5.10 | Secure mode | MUST | OK |
+---------+------------------------------+-------------+------------+
| | Hybrid mode | SHOULD | - |
+---------+------------------------------+-------------+------------+
*) See discussion in Section 10.5.
Comparison of NTS specification against Security Requirements of Time
Protocols in Packet Switched Networks (RFC 7384)
Appendix B. (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
usage of the one-way key chain collected and assembled from
[RFC4082]. Note that NTS uses the "not re-using keys" scheme of
TESLA as described in Section 3.7.2. of [RFC4082].
B.1. Server Preparation
server setup:
1. The server determines a reasonable upper bound B on the network
delay between itself and an arbitrary client, measured in
milliseconds.
2. It determines the number n+1 of keys in the one-way key chain.
This yields the number n of keys that are usable to authenticate
broadcast packets. This number n is therefore also the number of
time intervals during which the server can send authenticated
broadcast messages before it has to calculate a new key chain.
3. It divides time into n uniform intervals I_1, I_2, ..., I_n.
Each of these time intervals has length L, measured in
milliseconds. In order to fulfill the requirement 3.7.2. of RFC
4082, the time interval L has to be shorter than the time
interval between the broadcast messages.
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4. The server generates a random key K_n.
5. Using a one-way function F, the server generates a one-way chain
of n+1 keys K_0, K_1, ..., K_{n} according to
K_i = F(K_{i+1}).
6. Using another one-way function F', it generates a sequence of n
MAC keys K'_0, K'_1, ..., K'_{n-1} according to
K'_i = F'(K_i).
7. Each MAC key K'_i is assigned to the time interval I_i.
8. The server determines the key disclosure delay d, which is the
number of intervals between using a key and disclosing it. Note
that although security is provided for all choices d>0, the
choice still makes a difference:
* If d is chosen too short, the client might discard packets
because it fails to verify that the key used for its MAC has
not yet been disclosed.
* If d is chosen too long, the received packets have to be
buffered for an unnecessarily long time before they can be
verified by the client and be subsequently utilized for time
synchronization.
The server SHOULD calculate d according to
d = ceil( 2*B / L) + 1,
where ceil yields the smallest integer greater than or equal to
its argument.
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< - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Generation of Keys
F F F F
K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <------- K_n
| | | |
| | | |
| F' | F' | F' | F'
| | | |
v v v v
K'_0 K'_1 ... K'_{n-1} K'_n
[______________|____ ____|_________________|_______]
I_1 ... I_{n-1} I_n
Course of Time/Usage of Keys
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ->
A schematic explanation of the TESLA protocol's one-way key chain
B.2. Client Preparation
A client needs the following information in order to participate in a
TESLA broadcast:
o One key K_i from the one-way key chain, which has to be
authenticated as belonging to the server. Typically, this will be
K_0.
o The disclosure schedule of the keys. This consists of:
* the length n of the one-way key chain,
* the length L of the time intervals I_1, I_2, ..., I_n,
* the starting time T_i of an interval I_i. Typically this is
the starting time T_1 of the first interval;
* the disclosure delay d.
o The one-way function F used to recursively derive the keys in the
one-way key chain,
o The second one-way function F' used to derive the MAC keys K'_0,
K'_1, ... , K'_n from the keys in the one-way chain.
o An upper bound D_t on how far its own clock is "behind" that of
the server.
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Note that if D_t is greater than (d - 1) * L, then some authentic
packets might be discarded. If D_t is greater than d * L, then all
authentic packets will be discarded. In the latter case, the client
should not participate in the broadcast, since there will be no
benefit in doing so.
B.3. Sending Authenticated Broadcast Packets
During each time interval I_i, the server sends at most one
authenticated broadcast packet P_i. Such a packet consists of:
o a message M_i,
o the index i (in case a packet arrives late),
o a MAC authenticating the message M_i, with K'_i used as key,
o the key K_{i-d}, which is included for disclosure.
B.4. Authentication of Received Packets
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
disclosed key. This is done to avoid replay/flooding attacks. A
packet that fails this test is discarded.
Next, the client begins to check the packet's timeliness by ensuring
that according to the disclosure schedule and with respect to the
upper bound D_t determined above, the server cannot have disclosed
the key K_i yet. Specifically, it needs to check that the server's
clock cannot read a time that is in time interval I_{i+d} or later.
Since it works under the assumption that the server's clock is not
more than D_t "ahead" of the client's clock, the client can calculate
an upper bound t_i for the server's clock at the time when P_i
arrived. This upper bound t_i is calculated according to
t_i = R + D_t,
where R is the client's clock at the arrival of P_i. This implies
that at the time of arrival of P_i, the server could have been in
interval I_x at most, with
x = floor((t_i - T_1) / L) + 1,
where floor gives the greatest integer less than or equal to its
argument. The client now needs to verify that
x < i+d
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is valid (see also Section 3.5 of [RFC4082]). If it is falsified, it
is discarded.
If the check above is successful, the client performs another more
rigorous check: it sends a key check request to the server (in the
form of a client_keycheck message), asking explicitly if K_i has
already been disclosed. It remembers the time stamp t_check of the
sending time of that request as well as the nonce it used correlated
with the interval number i. If it receives an answer from the server
stating that K_i has not yet been disclosed and it is able to verify
the HMAC on that response, then it deduces that K_i was undisclosed
at t_check and therefore also at R. In this case, the client accepts
P_i as timely.
Next the client verifies that a newly disclosed key K_{i-d} belongs
to the one-way key chain. To this end, it applies the one-way
function F to K_{i-d} until it can verify the identity with an
earlier disclosed key (see Clause 3.5 in RFC 4082, item 3).
Next the client verifies that the transmitted time value s_i belongs
to the time interval I_i, by checking
T_i =< s_i, and
s_i < T_{i+1}.
If it is falsified, the packet MUST be discarded and the client MUST
reinitialize its broadcast module by performing time synchronization
by other means than broadcast messages, and it MUST perform a new
broadcast parameter exchange (because a falsification of this check
yields that the packet was not generated according to protocol, which
suggests an attack).
If a packet P_i passes all the tests listed above, it is stored for
later authentication. Also, if at this time there is a package with
index i-d already buffered, then the client uses the disclosed key
K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in
package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.
Appendix C. (informative) Dependencies
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+---------+--------------+--------+-------------------------------+
| Issuer | Type | Owner | Description |
+---------+--------------+--------+-------------------------------+
| Server | private key | server | Used for server_assoc, |
| PKI | (signature) | | server_cook, server_bpar. |
| +--------------+--------+ The server uses the private |
| | public key | client | key to sign these messages. |
| | (signature) | | The client uses the public |
| +--------------+--------+ key to verify them. |
| | certificate | server | The certificate is used in |
| | | | server_assoc messages, for |
| | | | verifying authentication and |
| | | | (optionally) authorization. |
+---------+--------------+--------+-------------------------------+
| Client | private key | client | The server uses the client's |
| PKI | (encryption) | | public key to encrypt the |
| +--------------+--------+ content of server_cook |
| | public key | server | messages. The client uses |
| | (encryption) | | the private key to decrypt |
| +--------------+--------+ them. The certificate is |
| | certificate | client | sent in client_cook messages, |
| | | | where it is used for trans- |
| | | | portation of the public key |
| | | | as well as (optionally) for |
| | | | verification of client |
| | | | authorization. |
+---------+--------------+--------+-------------------------------+
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+------------<---------------+
| At least one |
V successful |
++====[ ]===++ ++=====^=====++
|| Cookie || ||Association||
|| Exchange || || Exchange ||
++====_ _===++ ++===========++
|
| At least one
| successful
V
++=======[ ]=======++
|| Unicast Time |>-----\ As long as further
|| Synchronization || | synchronization
|| Exchange(s) |<-----/ is desired
++=======_ _=======++
|
\ Other (unspecified)
Sufficient \ / methods which give
accuracy \ either or / sufficient accuracy
\----------\ /---------/
|
|
V
++========[ ]=========++
|| Broadcast ||
|| Parameter Exchange ||
++========_ _=========++
|
| One successful
| per client
V
++=======[ ]=======++
|| Broadcast Time |>--------\ As long as further
|| Synchronization || | synchronization
|| Reception |<--------/ is desired
++=======_ _=======++
|
/ \
either / \ or
/----------/ \-------------\
| |
V V
++========[ ]========++ ++========[ ]========++
|| Keycheck Exchange || || Keycheck Exchange ||
++===================++ || with TimeSync ||
++===================++
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Authors' Addresses
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de
Stephen Roettger
Google Inc.
Email: stephen.roettger@googlemail.com
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8421
Email: kristof.teichel@ptb.de
Sibold, et al. Expires September 4, 2015 [Page 35]