Internet DRAFT - draft-ietf-ntp-network-time-security
draft-ietf-ntp-network-time-security
NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: March 26, 2017 Google Inc.
K. Teichel
PTB
September 22, 2016
Network Time Security
draft-ietf-ntp-network-time-security-15
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 March 26, 2017.
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Copyright Notice
Copyright (c) 2016 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
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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 . . . . . . . . . . . . . . . . . . . . . . 5
4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Unicast Time Synchronisation Messages . . . . . . . . . . 7
6.1.1. Preconditions for the Unicast Time Synchronization
Exchange . . . . . . . . . . . . . . . . . . . . . . 7
6.1.2. Goals of the Unicast Time Synchronization Exchange . 8
6.1.3. Message Type: "time_request" . . . . . . . . . . . . 8
6.1.4. Message Type: "time_response" . . . . . . . . . . . . 8
6.1.5. Procedure Overview of the Unicast Time
Synchronization Exchange . . . . . . . . . . . . . . 9
6.2. Broadcast Time Synchronization Exchange . . . . . . . . . 10
6.2.1. Preconditions for the Broadcast Time Synchronization
Exchange . . . . . . . . . . . . . . . . . . . . . . 10
6.2.2. Goals of the Broadcast Time Synchronization Exchange 11
6.2.3. Message Type: "server_broad" . . . . . . . . . . . . 11
6.2.4. Procedure Overview of Broadcast Time Synchronization
Exchange . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 13
6.3.1. Preconditions for the Broadcast Keycheck Exchange . . 13
6.3.2. Goals of the Broadcast Keycheck Exchange . . . . . . 14
6.3.3. Message Type: "client_keycheck" . . . . . . . . . . . 14
6.3.4. Message Type: "server_keycheck" . . . . . . . . . . . 14
6.3.5. Procedure Overview of the Broadcast Keycheck Exchange 15
7. Server Seed, MAC Algorithms and Generating MACs . . . . . . . 16
7.1. Server Seed . . . . . . . . . . . . . . . . . . . . . . . 16
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7.2. MAC Algorithms . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.2. Initial Verification of the Server Certificates . . . . . 18
9.3. Revocation of Server Certificates . . . . . . . . . . . . 18
9.4. Mitigating Denial-of-Service for broadcast packets . . . 18
9.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 18
9.6. Random Number Generation . . . . . . . . . . . . . . . . 20
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. (informative) TICTOC Security Requirements . . . . . 22
Appendix B. (normative) Inherent Association Protocol Messages . 23
B.1. Overview of NTS with Inherent Association Protocol . . . 23
B.2. Access Message Exchange . . . . . . . . . . . . . . . . . 24
B.2.1. Goals of the Access Message Exchange . . . . . . . . 24
B.2.2. Message Type: "client_access" . . . . . . . . . . . . 24
B.2.3. Message Type: "server_access" . . . . . . . . . . . . 24
B.2.4. Procedure Overview of the Access Exchange . . . . . . 24
B.3. Association Message Exchange . . . . . . . . . . . . . . 25
B.3.1. Goals of the Association Exchange . . . . . . . . . . 25
B.3.2. Message Type: "client_assoc" . . . . . . . . . . . . 25
B.3.3. Message Type: "server_assoc" . . . . . . . . . . . . 26
B.3.4. Procedure Overview of the Association Exchange . . . 26
B.4. Cookie Message Exchange . . . . . . . . . . . . . . . . . 27
B.4.1. Goals of the Cookie Exchange . . . . . . . . . . . . 28
B.4.2. Message Type: "client_cook" . . . . . . . . . . . . . 28
B.4.3. Message Type: "server_cook" . . . . . . . . . . . . . 28
B.4.4. Procedure Overview of the Cookie Exchange . . . . . . 29
B.4.5. Broadcast Parameter Messages . . . . . . . . . . . . 30
Appendix C. (normative) Using TESLA for Broadcast-Type Messages 32
C.1. Server Preparation . . . . . . . . . . . . . . . . . . . 32
C.2. Client Preparation . . . . . . . . . . . . . . . . . . . 34
C.3. Sending Authenticated Broadcast Packets . . . . . . . . . 35
C.4. Authentication of Received Packets . . . . . . . . . . . 35
Appendix D. (informative) Dependencies . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
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
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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)
[IEEE1588], currently do not provide adequate intrinsic security
precautions. This document specifies generic 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 synchronization 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
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.
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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.
4. Objectives
The objectives of the NTS specification are as follows:
o Authenticity: NTS enables the 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 enables the client to verify its time server's
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
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 Applicability to Protocols: NTS can be used to secure different
time synchronization protocols, specifically at least NTP and PTP.
o Integration with Protocols: 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.
o Server-Side Statelessness: All security measures of NTS work
without creating the necessity for a server to keep state of a
connection.
o Prevention of Amplification Attacks: All communication introduced
by NTS offers protection against abuse for amplification denial-
of-service attacks.
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5. NTS Overview
NTS initially verifies the authenticity of the time server and
exchanges a symmetric key, the so-called cookie, as well as a key
input value (KIV). The KIV can be opaque for the client. After the
cookie and the KIV are exchanged, the client then uses them 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> (MAC(server seed, KIV)),
with the server seed as the key, where KIV is the client's key input
value, and where the application of the function MSB_<b> returns only
the b most significant bits. The server seed is a random value of
bit length b that the server possesses, which has to remain secret.
The cookie deterministically depends on KIV as long as the server
seed stays the same. The server seed has to be refreshed
periodically 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 its KIV to each request (see Section 6.1).
Note: The communication of the KIV and the cookie can be performed
between client and server directly, or via a third party key
distribution entity.
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. 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
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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 C.
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.
6.1. 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 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
Before this message exchange is available, there are some
requirements that the client and server need to meet:
o They MUST negotiate the 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.
One way of realizing these requirements is to use the Association and
Cookie Message Exchanges described in Appendix B.
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6.1.2. Goals of the Unicast Time Synchronization Exchange
The unicast time synchronization exchange:
o exchanges time synchronization data as specified by the
appropriate time synchronization protocol,
o guarantees authenticity and integrity of the request to the
server,
o guarantees authenticity and integrity of the response to the
client,
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.
It contains
o the NTS message ID "time_request",
o the negotiated version number,
o a nonce,
o the negotiated MAC algorithm,
o the client's key input value (for which the client knows the
associated cookie),
o optional: a MAC (generated with the cookie as key) for
verification of all of the above data.
6.1.4. 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 received key input value and the
transmitted MAC 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,
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o a MAC (generated with the cookie as key) for verification of all
of the above data.
6.1.5. 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. Optionally, the client protects the
request message with an appended MAC.
2. Upon receipt of a time_request message, the server performs the
following steps:
* It re-calculates the cookie.
* If the request message contains a MAC the server re-calculates
the MAC and compares this value with the MAC in the received
data.
+ If the re-calculated MAC does not match the MAC in the
received data the server MUST stop the processing of the
request.
+ If the re-calculated MAC matches the MAC in the received
data the server continues to process the request.
* The server computes the necessary time synchronization data
and constructs a time_response message as given in
Section 6.1.4.
3. The client 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,
* that the transmit_timestamp in that time_request message
matches the corresponding time stamp from the synchronization
data received in the time_response, and
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* 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 send another cookie
request (because the cookie might have changed due to a server
seed refresh). If both checks are successful, the client SHOULD
continue time synchronization.
+-----------------------+
| 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.
6.2. Broadcast Time Synchronization Exchange
6.2.1. Preconditions for the Broadcast Time Synchronization Exchange
Before this message exchange is available, there are some
requirements that the client and server need to meet:
o The client MUST receive all the information necessary to process
broadcast time synchronization messages from the server. This
includes
* the one-way functions used for building the key chain,
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* 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 communication of the data listed above MUST guarantee
authenticity of the server, as well as integrity and freshness of
the broadcast parameters to the client.
6.2.2. 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.
6.2.3. 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",
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),
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o a MAC, calculated with the key for the current time interval,
verifying
* the message ID,
* the version number, and
* the time data.
6.2.4. 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 6.2.3, 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
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
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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.
6.3. Broadcast Keycheck
This message exchange is performed for an additional check of packet
timeliness in the course of the TESLA scheme, see Appendix C.
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 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 realized via
the Association and Cookie Message Exchanges described in Appendix B
(Appendix B).
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6.3.2. 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.
6.3.3. 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 MAC algorithm negotiated during association,
o the client's key input value KIV, and
o optional: a MAC (generated with the cookie as key) for
verification of all of the above data.
6.3.4. 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 received key input value and the transmitted MAC 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,
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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.
6.3.5. 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.
2. Upon receipt of a client_keycheck message the server performs as
follows: If the client_keycheck message contains a MAC the server
re-calculates the MAC and compares this value with the MAC in the
received data.
* If the re-calculated MAC does not match the MAC in the
received data the server MUST stop the processing of the
request.
* If the re-calculated MAC matches the MAC in the received data
the server continues to process the request: It 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 6.3.4. For
more details, see also Appendix C.
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.
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+----------------------+
| 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, MAC Algorithms and Generating MACs
7.1. Server Seed
The server has to calculate a random seed which has to be kept
secret. The server MUST generate a seed for each supported MAC
algorithm, see Section 7.2.
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 (Appendix B.4).
7.2. MAC Algorithms
MAC algorithms are used for calculation of the cookie and the actual
MAC. The client and the server negotiate a MAC algorithm during the
association phase at the beginning. The selected algorithm MUST be
used for all cookie and MAC creation processes in that run.
Note: Any MAC algorithm is prone to be compromised in the future. A
successful attack on a MAC 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
MAC algorithms. This way, knowledge gained from an attack on a
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MAC algorithm can at least only be used to compromise such clients
who use this algorithm as well.
8. IANA Considerations
As mentioned, this document generically specifies security measures
whose utilization for any given specific time synchronization
protocol requires a separate document. Consequently, this document
itself does not have any IANA actions (TO BE REVIEWED).
9. Security Considerations
Aspects of security for time synchronization protocols are treated
throughout this document. For a comprehensive discussion of security
requirements in time synchronization contexts, refer to [RFC7384].
See Appendix A for a tabular overview of how NTS deals with those
requirements.
Additional NTS specific discussion of security issues can be found in
the following subsections.
Note: Any separate document describing the utilization of NTS to a
specific time synchronization protocol may additionally introduce
discussion of its own specific security considerations.
9.1. 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
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 [RFC7624]. 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 importance since the packet's 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.
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9.2. Initial Verification of the Server Certificates
The client may wish to verify the validity of certificates during the
initial association phase. 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 has to rely on external means.
9.3. 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 (Appendix B.3.3).
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 Online Certificate
Status Protocol (OCSP) Online Certificate Status Protocol (OCSP)
[RFC6960].
9.4. 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.
Discussion: 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 common to the whole group. This adds additional
complexity to the protocol and hence is currently not considered
in this document.
9.5. 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
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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.ietf-tictoc-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
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
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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 C.4
for a detailed description of this check). Note that a broadcast
client should also apply the above-mentioned precautions as far as
possible.
9.6. 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.
10. Acknowledgements
The authors would like to thank Tal Mizrahi, Russ Housley, Steven
Bellovin, David Mills, Kurt Roeckx, Rainer Bermbach, Martin Langer
and Florian Weimer for discussions and comments on the design of NTS.
Also, thanks go to Harlan Stenn and Richard Welty for their technical
review and specific text contributions to this document.
11. References
11.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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, DOI 10.17487/RFC4082,
June 2005, <http://www.rfc-editor.org/info/rfc4082>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>.
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11.2. Informative References
[I-D.ietf-ntp-cms-for-nts-message]
Sibold, D., Teichel, K., Roettger, S., and R. Housley,
"Protecting Network Time Security Messages with the
Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
for-nts-message-04 (work in progress), July 2015.
[I-D.ietf-tictoc-multi-path-synchronization]
Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
Path Time Synchronization", draft-ietf-tictoc-multi-path-
synchronization-02 (work in progress), April 2015.
[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 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[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, DOI 10.17487/RFC6960, June 2013,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC7624] 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", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<http://www.rfc-editor.org/info/rfc7624>.
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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 9.5.
Comparison of NTS specification against Security Requirements of Time
Protocols in Packet Switched Networks (RFC 7384)
Appendix B. (normative) Inherent Association Protocol Messages
This appendix presents a procedure that performs the association, the
cookie, and also the broadcast parameter message exchanges between a
client and a server. This procedure is one possible way to achieve
the preconditions listed in Sections Section 6.1.1, Section 6.2.1,
and Section 6.3.1 while taking into account the objectives given in
Section Section 4.
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. An additional required exchange in advance serves to limit
the amplification potential of the association message exchange.
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:
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cookie = MSB_<b> (MAC(server seed, H(certificate of client))),
Where the hash function H is the one used in the MAC algorithm. 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. Access Message Exchange
This message exchange serves only to prevent the next (association)
exchange from being abusable for amplification denial-of-service
attacks.
B.2.1. Goals of the Access Message Exchange
The access message exchange:
o transfers a secret value from the server to the client
(initiator),
o the secret value permits the client to initiate an association
message exchange.
B.2.2. Message Type: "client_access"
This message is sent by a client who intends to perform an
association exchange with the server in the future. It contains:
o the NTS message ID "client_access".
B.2.3. Message Type: "server_access"
This message is sent by the server on receipt of a client_access
message. It contains:
o the NTS message ID "server_access",
o an access key.
B.2.4. Procedure Overview of the Access Exchange
For an access exchange, the following steps are performed:
1. The client sends a client_access message to the server.
2. Upon receipt of a client_access, the server calculates the access
key. It then sends a reply in the form of a server_access
message. The server must either memorize the access key or
alternatively apply a means by which it can reconstruct the
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access key. Note that in both cases the access key must be
correlated with the address of the requester. Note also that if
the server memorizes the access key for a requester, it has to
keep state for a certain amount of time.
3. The client waits for a response in the form of a server_access
message. Upon receipt of one, it MUST memorize the included
access key.
B.3. Association Message Exchange
In this message exchange, the participants negotiate the MAC 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.3.1. 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.3.2. Message Type: "client_assoc"
This message is sent by the client if it wants to perform association
with a server. It contains
o the NTS message ID "client_assoc",
o a nonce,
o the access key obtained earlier via an access message exchange,
o the version number of NTS that the client wants to use (this
SHOULD be the highest version number that it supports),
o a selection of accepted MAC algorithms, and
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o a selection of accepted encryption algorithms.
B.3.3. 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 MAC 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 server's choice of algorithm for encryption and for MAC
creation, 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.3.4. 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 checks the
validity of the included access key. If it is not valid, the
server MUST abort communication. If it is valid, the server
constructs and sends a reply in the form of a server_assoc
message as described in Appendix B.3.3. Upon unsuccessful
negotiation for version number or algorithms the server_assoc
message MUST contain an error code.
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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
MAC 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 Check access key |
+------------------------+
| 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.4. Cookie Message Exchange
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.
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B.4.1. 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.4.2. 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 MAC algorithm,
o the client's certificate.
B.4.3. Message Type: "server_cook"
This message is sent by the server upon receipt of a client_cook
message. The server generates the hash (the used hash function is
the one used for the MAC algorithm) 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,
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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.4.4. 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.4.3.
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.
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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.
B.4.5. 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.4.5.1. 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.4.5.2. 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",
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o the NTS version number negotiated during association,
o a nonce, and
o the signature algorithm negotiated during association.
B.4.5.3. 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.4.5.4. 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.4.5.3.
3. The client waits for a reply in the form of a server_bpar
message, on which it performs the following checks:
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* The message must contain all the necessary information for the
TESLA protocol, as listed in Appendix B.4.5.3.
* 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
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].
C.1. Server Preparation
Server setup:
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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.
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.
It is RECOMMENDED that the server calculate d according to
d = ceil( 2*B / L) + 1,
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where ceil yields the smallest integer greater than or equal to
its argument.
< - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Generation of Keys
F F F F
K_0 <-------- K_1 <-------- ... <-------- K_{n-1} <------- K_n
| | | |
| | | |
| F' | F' | F' | F'
| | | |
v v v v
K'_0 K'_1 ... K'_{n-1} K'_n
[______________|____ ____|_________________|_______]
I_1 ... I_{n-1} I_n
Course of Time/Usage of Keys
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ->
A schematic explanation of the TESLA protocol's one-way key chain
C.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.
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o An upper bound D_t on how far its own clock is "behind" that of
the server.
Note that if D_t is greater than (d - 1) * L, then some authentic
packets might be discarded. If D_t is greater than d * L, then all
authentic packets will be discarded. In the latter case, the client
SHOULD NOT participate in the broadcast, since there will be no
benefit in doing so.
C.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.
C.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,
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where floor gives the greatest integer less than or equal to its
argument. The client now needs to verify that
x < i+d
is valid (see also Section 3.5 of [RFC4082]). If 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.
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Appendix D. (informative) Dependencies
+---------+--------------+--------+-------------------------------+
| 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. |
+---------+--------------+--------+-------------------------------+
This table shows the kind of cryptographic resources that NTS
participants of server and client role should have ready before NTS
communication starts.
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++===========================================++
|| ||
|| Secure Authentication and Cookie 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 ||
++===================++
This diagram shows the dependencies between the different message
exchanges and procedures which NTS offers.
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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
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