draft-ietf-ntp-network-time-security-06.txt   draft-ietf-ntp-network-time-security-07.txt 
NTP Working Group D. Sibold NTP Working Group D. Sibold
Internet-Draft PTB Internet-Draft PTB
Intended status: Standards Track S. Roettger Intended status: Standards Track S. Roettger
Expires: July 20, 2015 Google Inc. Expires: September 4, 2015 Google Inc.
K. Teichel K. Teichel
PTB PTB
January 16, 2015 March 3, 2015
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-06.txt draft-ietf-ntp-network-time-security-07.txt
Abstract Abstract
This document describes Network Time Security (NTS), a collection of This document describes Network Time Security (NTS), a collection of
measures that enable secure time synchronization with time servers measures that enable secure time synchronization with time servers
using protocols like the Network Time Protocol (NTP) or the Precision using protocols like the Network Time Protocol (NTP) or the Precision
Time Protocol (PTP). Its design considers the special requirements Time Protocol (PTP). Its design considers the special requirements
of precise timekeeping which are described in Security Requirements of precise timekeeping which are described in Security Requirements
of Time Protocols in Packet Switched Networks [RFC7384]. of Time Protocols in Packet Switched Networks [RFC7384].
skipping to change at page 1, line 44 skipping to change at page 1, line 44
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4
4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4 2.2. Common Terminology for PTP and NTP . . . . . . . . . . . 4
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Security Threats . . . . . . . . . . . . . . . . . . . . . . 4
6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 5 4. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Association Messages . . . . . . . . . . . . . . . . . . 6 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1.1. Message Type: "client_assoc" . . . . . . . . . . . . 6 6. Protocol Messages . . . . . . . . . . . . . . . . . . . . . . 6
6.1.2. Message Type: "server_assoc" . . . . . . . . . . . . 6 6.1. Association Message Exchange . . . . . . . . . . . . . . 7
6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 7 6.1.1. Goals of the Association Exchange . . . . . . . . . . 7
6.2.1. Message Type: "client_cook" . . . . . . . . . . . . . 7 6.1.2. Message Type: "client_assoc" . . . . . . . . . . . . 7
6.2.2. Message Type: "server_cook" . . . . . . . . . . . . . 7 6.1.3. Message Type: "server_assoc" . . . . . . . . . . . . 7
6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 8 6.1.4. Procedure Overview of the Association Exchange . . . 8
6.3.1. Message Type: "time_request" . . . . . . . . . . . . 8 6.2. Cookie Messages . . . . . . . . . . . . . . . . . . . . . 9
6.3.2. Message Type: "time_response" . . . . . . . . . . . . 8 6.2.1. Goals of the Cookie Exchange . . . . . . . . . . . . 9
6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 9 6.2.2. Message Type: "client_cook" . . . . . . . . . . . . . 10
6.4.1. Message Type: "client_bpar" . . . . . . . . . . . . . 9 6.2.3. Message Type: "server_cook" . . . . . . . . . . . . . 10
6.4.2. Message Type: "server_bpar" . . . . . . . . . . . . . 9 6.2.4. Procedure Overview of the Cookie Exchange . . . . . . 11
6.5. Broadcast Messages . . . . . . . . . . . . . . . . . . . 10 6.3. Unicast Time Synchronisation Messages . . . . . . . . . . 12
6.5.1. Message Type: "server_broad" . . . . . . . . . . . . 10 6.3.1. Goals of the Unicast Time Synchronization Exchange . 12
6.6. Broadcast Key Check . . . . . . . . . . . . . . . . . . . 10 6.3.2. Message Type: "time_request" . . . . . . . . . . . . 12
6.6.1. Message Type: "client_keycheck" . . . . . . . . . . . 10 6.3.3. Message Type: "time_response" . . . . . . . . . . . . 13
6.6.2. Message Type: "server_keycheck" . . . . . . . . . . . 11 6.3.4. Procedure Overview of the Unicast Time
7. Message Dependencies . . . . . . . . . . . . . . . . . . . . 11 Synchronization Exchange . . . . . . . . . . . . . . 13
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 12 6.4. Broadcast Parameter Messages . . . . . . . . . . . . . . 14
9. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 13 6.4.1. Goals of the Broadcast Parameter Exchange . . . . . . 15
9.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 13 6.4.2. Message Type: "client_bpar" . . . . . . . . . . . . . 15
9.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 13 6.4.3. Message Type: "server_bpar" . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 6.4.4. Procedure Overview of the Broadcast Parameter
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13 Exchange . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 13 6.5. Broadcast Time Synchronization Exchange . . . . . . . . . 17
11.2. Initial Verification of the Server Certificates . . . . 14 6.5.1. Goals of the Broadcast Time Synchronization Exchange 17
11.3. Revocation of Server Certificates . . . . . . . . . . . 14 6.5.2. Message Type: "server_broad" . . . . . . . . . . . . 17
11.4. Mitigating Denial-of-Service for broadcast packets . . . 14 6.5.3. Procedure Overview of Broadcast Time Synchronization
11.5. Delay Attack . . . . . . . . . . . . . . . . . . . . . . 15 Exchange . . . . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 6.6. Broadcast Keycheck . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.6.1. Goals of the Broadcast Keycheck Exchange . . . . . . 19
13.1. Normative References . . . . . . . . . . . . . . . . . . 16 6.6.2. Message Type: "client_keycheck" . . . . . . . . . . . 20
13.2. Informative References . . . . . . . . . . . . . . . . . 17 6.6.3. Message Type: "server_keycheck" . . . . . . . . . . . 20
Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 17 6.6.4. Procedure Overview of the Broadcast Keycheck Exchange 20
Appendix B. Using TESLA for Broadcast-Type Messages . . . . . . 19 7. Server Seed Considerations . . . . . . . . . . . . . . . . . 21
B.1. Server Preparation . . . . . . . . . . . . . . . . . . . 19 8. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 22
B.2. Client Preparation . . . . . . . . . . . . . . . . . . . 20 8.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 22
B.3. Sending Authenticated Broadcast Packets . . . . . . . . . 21 8.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 22
B.4. Authentication of Received Packets . . . . . . . . . . . 21 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
Appendix C. Random Number Generation . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 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 1. Introduction
Time synchronization protocols are increasingly utilized to Time synchronization protocols are increasingly utilized to
synchronize clocks in networked infrastructures. The reliable synchronize clocks in networked infrastructures. Successful attacks
performance of such infrastructures can be degraded seriously by against the time synchronization protocol can seriously degrade the
successful attacks against the time synchronization protocol. reliable performance of such infrastructures. Therefore, time
Therefore, time synchronization protocols have to be secured if they synchronization protocols have to be secured if they are applied in
are applied in environments that are prone to malicious attacks. environments that are prone to malicious attacks. This can be
This can be accomplished either by utilization of external security accomplished either by utilization of external security protocols,
protocols, like IPsec or TLS, or by intrinsic security measures of like IPsec or TLS, or by intrinsic security measures of the time
the time synchronization protocol. synchronization protocol.
The two most popular time synchronization protocols, the Network Time The two most popular time synchronization protocols, the Network Time
Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP) Protocol (NTP) [RFC5905] and the Precision Time Protocol (PTP)
[IEEE1588], currently do not provide adequate intrinsic security [IEEE1588], currently do not provide adequate intrinsic security
precautions. This document specifies security measures which enable precautions. This document specifies security measures which enable
these protocols to verify the authenticity of the time server and the these and possibly other protocols to verify the authenticity of the
integrity of the time synchronization protocol packets. 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.
The given measures are specified with the prerequisite in mind that [RFC7384] specifies that a security mechanism for timekeeping must be
precise timekeeping can only be accomplished with stateless time
synchronization communication, which excludes the utilization of
standard security protocols, like IPsec or TLS, for time
synchronization messages. This prerequisite corresponds with the
requirement that a security mechanism for timekeeping must be
designed in such a way that it does not degrade the quality of the designed in such a way that it does not degrade the quality of the
time transfer [RFC7384]. 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.
Note: 2. Terminology
It is recommended that details on how to apply NTS to specific 2.1. Terms and Abbreviations
time synchronization protocols be formulated in separate
documents, with one separate document for each protocol.
2. Security Threats MITM Man In The Middle
A profound analysis of security threats and requirements for time NTS Network Time Security
synchronization protocols can be found in the "Security Requirements
of Time Protocols in Packet Switched Networks" [RFC7384].
3. Objectives 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.
4. Objectives
The objectives of the NTS specification are as follows: The objectives of the NTS specification are as follows:
o Authenticity: NTS enables a client/slave to authenticate its time o Authenticity: NTS enables a client to authenticate its time
server(s)/master(s). server(s).
o Integrity: NTS protects the integrity of time synchronization o Integrity: NTS protects the integrity of time synchronization
protocol packets via a message authentication code (MAC). protocol packets via a message authentication code (MAC).
o Confidentiality: NTS does not provide confidentiality protection o Confidentiality: NTS does not provide confidentiality protection
of the time synchronization packets. of the time synchronization packets.
o Authorization: NTS optionally enables the server to verify the
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 o Integration with protocols: NTS can be used to secure different
time synchronization protocols, specifically at least NTP and PTP. time synchronization protocols, specifically at least NTP and PTP.
An client or server running an NTS-secured version of a time A client or server running an NTS-secured version of a time
protocol does not negatively affect other participants who are protocol does not negatively affect other participants who are
running unsecured versions of that protocol. running unsecured versions of that protocol.
4. Terms and Abbreviations
MITM Man In The Middle
NTS Network Time Security
TESLA Timed Efficient Stream Loss-tolerant Authentication
5. NTS Overview 5. NTS Overview
NTS applies X.509 certificates to verify the authenticity of the time NTS applies X.509 certificates to verify the authenticity of the time
server/master and to exchange a symmetric key, the so-called cookie. server and to exchange a symmetric key, the so-called cookie. It
This cookie is then used to protect the authenticity and the then uses the cookie to protect the authenticity and the integrity of
integrity of subsequent unicast-type time synchronization packets. subsequent unicast-type time synchronization packets. In order to do
This is done by means of a Message Authentication Code (MAC), which this, a Message Authentication Code (MAC) is attached to each time
is attached to each time synchronization packet. The calculation of synchronization packet. The calculation of the MAC includes the
the MAC includes the whole time synchronization packet and the cookie whole time synchronization packet and the cookie which is shared
which is shared between client and server. The cookie is calculated between client and server. The cookie is calculated according to:
according to:
cookie = MSB_<b> (HMAC(server seed, H(certificate of client))), cookie = MSB_<b> (HMAC(server seed, H(certificate of client))),
with the server seed as the key, where H is a hash function, and 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 where the function MSB_<b> cuts off the b most significant bits of
the result of the HMAC function. The server seed is a random value the result of the HMAC function. The client's certificate contains
of bit length b that the server possesses, which has to be kept the client's public key and enables the server to identify the
secret. The cookie never changes as long as the server seed stays client, if client authorization is desired. The server seed is a
the same, but the server seed has to be refreshed periodically in random value of bit length b that the server possesses, which has to
order to provide key freshness as required in [RFC7384]. See remain secret. The cookie never changes as long as the server seed
Section 8 for details on seed refreshing. stays the same, but 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 Since the server does not keep a state of the client, it has to
recalculate the cookie each time it receives a unicast time recalculate the cookie each time it receives a unicast time
synchronization request from the client. To this end, the client has synchronization request from the client. To this end, the client has
to attach the hash value of its certificate to each request (see to attach the hash value of its certificate to each request (see
Section 6.3). Section 6.3).
For broadcast-type messages, authenticity and integrity of the time For broadcast-type messages, authenticity and integrity of the time
synchronization packets are also ensured by a MAC, which is attached synchronization packets are also ensured by a MAC, which is attached
to the time synchronization packet by the sender. Verification of to the time synchronization packet by the sender. Verification of
the broadcast-type packets' authenticity is based on the TESLA the broadcast-type packets' authenticity is based on the TESLA
protocol, in particular on its "not re-using keys" scheme, see protocol, in particular on its "not re-using keys" scheme, see
Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys, Section 3.7.2 of [RFC4082]. TESLA uses a one-way chain of keys,
where each key is the output of a one-way function applied to the where each key is the output of a one-way function applied to the
previous key in the chain. The last element of the chain is shared previous key in the chain. The server securely shares the last
securely with all clients. The server splits time into intervals of element of the chain with all clients. The server splits time into
uniform duration and assigns each key to an interval in reverse intervals of uniform duration and assigns each key to an interval in
order, starting with the penultimate. At each time interval, the reverse order, starting with the penultimate. At each time interval,
server sends a broadcast packet appended by a MAC, calculated using the server sends a broadcast packet appended by a MAC, calculated
the corresponding key, and the key of the previous disclosure using the corresponding key, and the key of the previous disclosure
interval. The client verifies the MAC by buffering the packet until interval. The client verifies the MAC by buffering the packet until
disclosure of the key in its associated disclosure interval occurs. disclosure of the key in its associated disclosure interval occurs.
In order to be able to verify the validity of the key, the client has In order to be able to verify the timeliness of the packets, the
to be loosely time synchronized with the server. This has to be client has to be loosely time synchronized with the server. This has
accomplished during the initial client server exchange between the to be accomplished before broadcast associations can be used. For
broadcast client and the server. In addition, NTS uses another, more checking timeliness of packets, NTS uses another, more rigorous check
rigorous check than what is used in the TESLA protocol. For a more in addition to just the clock lookup used in the TESLA protocol. For
detailed description of how NTS employs and customizes TESLA, see a more detailed description of how NTS employs and customizes TESLA,
Appendix B. see Appendix B.
6. Protocol Messages 6. Protocol Messages
This section describes the types of messages needed for secure time This section describes the types of messages needed for secure time
synchronization with NTS. synchronization with NTS.
For some guidance on how these message types can be realized in For some guidance on how these message types can be realized in
practice, and integrated into the communication flow of existing time practice, and integrated into the communication flow of existing time
synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a synchronization protocols, see [I-D.ietf-ntp-cms-for-nts-message], a
companion document for NTS. Said document describes ASN.1 encodings companion document for NTS. Said document describes ASN.1 encodings
for those message parts that have to be added to a time for those message parts that have to be added to a time
synchronization protocol for security reasons as well as CMS synchronization protocol for security reasons as well as CMS
(Cryptographic Message Syntax, see [RFC5652]) conventions that can be (Cryptographic Message Syntax, see [RFC5652]) conventions that can be
used to get the cryptographic aspects right. used to get the cryptographic aspects right.
6.1. Association Messages 6.1. Association Message Exchange
In this message exchange, the hash and encryption algorithms that are In this message exchange, the participants negotiate the hash and
used throughout the protocol are negotiated. In addition , the encryption algorithms that are used throughout the protocol. In
client receives the certification chain up to a trusted anchor. With addition, the client receives the certification chain up to a trusted
the established certification chain the client is able to verify the anchor. With the established certification chain the client is able
server's signatures and, hence, the authenticity of future NTS to verify the server's signatures and, hence, the authenticity of
messages from the server is ensured. future NTS messages from the server is ensured.
6.1.1. Message Type: "client_assoc" 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 The protocol sequence starts with the client sending an association
message, called client_assoc. This message contains message, called client_assoc. This message contains
o the NTS message ID "client_assoc", o the NTS message ID "client_assoc",
o a nonce,
o the version number of NTS that the client wants to use (this o the version number of NTS that the client wants to use (this
SHOULD be the highest version number that it supports), SHOULD be the highest version number that it supports),
o the hostname of the client, o the hostname of the client,
o a selection of accepted hash algorithms, and o a selection of accepted hash algorithms, and
o a selection of accepted encryption algorithms. o a selection of accepted encryption algorithms.
6.1.2. Message Type: "server_assoc" 6.1.3. Message Type: "server_assoc"
This message is sent by the server upon receipt of client_assoc. It This message is sent by the server upon receipt of client_assoc. It
contains contains
o the NTS message ID "server_assoc", o the NTS message ID "server_assoc",
o the nonce transmitted in client_assoc,
o the client's proposal for the version number, selection of
accepted hash algorithms and selection of accepted encryption
algorithms, as transmitted in client_assoc,
o the version number used for the rest of the protocol (which SHOULD o the version number used for the rest of the protocol (which SHOULD
be determined as the minimum over the client's suggestion in the be determined as the minimum over the client's suggestion in the
client_assoc message and the highest supported by the server), client_assoc message and the highest supported by the server),
o the hostname of the server, o the hostname of the server,
o the server's choice of algorithm for encryption and for o the server's choice of algorithm for encryption and for
cryptographic hashing, all of which MUST be chosen from the cryptographic hashing, all of which MUST be chosen from the
client's proposals, client's proposals,
o a signature, calculated over the data listed above, with the o a signature, calculated over the data listed above, with the
server's private key and according to the signature algorithm server's private key and according to the signature algorithm
which is also used for the certificates that are included (see which is also used for the certificates that are included (see
below), and below), and
o a chain of certificates, which starts at the server and goes up to 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 a trusted authority; each certificate MUST be certified by the one
directly following it. 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.
* 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 6.2. Cookie Messages
During this message exchange, the server transmits a secret cookie to During this message exchange, the server transmits a secret cookie to
the client securely. The cookie will later be used for integrity the client securely. The cookie will later be used for integrity
protection during unicast time synchronization. protection during unicast time synchronization.
6.2.1. Message Type: "client_cook" 6.2.1. Goals of the Cookie Exchange
The cookie exchange:
o enables the server to check the client's authorization via its
certificate (optional),
o supplies the client with the correct cookie for its association to
the server,
o guarantees to the client that the cookie originates from the
server and that it is based on the client's original, unaltered
request.
o guarantees that the received cookie is unknown to anyone but the
server and the client.
6.2.2. Message Type: "client_cook"
This message is sent by the client upon successful authentication of This message is sent by the client upon successful authentication of
the server. In this message, the client requests a cookie from the the server. In this message, the client requests a cookie from the
server. The message contains server. The message contains
o the NTS message ID "client_cook", o the NTS message ID "client_cook",
o a nonce,
o the negotiated version number, o the negotiated version number,
o the negotiated signature algorithm, o the negotiated signature algorithm,
o the negotiated encryption algorithm, o the negotiated encryption algorithm,
o a nonce,
o the negotiated hash algorithm H, o the negotiated hash algorithm H,
o the client's certificate. o the client's certificate.
6.2.2. Message Type: "server_cook" 6.2.3. Message Type: "server_cook"
This message is sent by the server upon receipt of a client_cook This message is sent by the server upon receipt of a client_cook
message. The server generates the hash of the client's certificate, message. The server generates the hash of the client's certificate,
as conveyed during client_cook, in order to calculate the cookie as conveyed during client_cook, in order to calculate the cookie
according to Section 5. This message contains according to Section 5. This message contains
o the NTS message ID "server_cook" o the NTS message ID "server_cook"
o the version number as transmitted in client_cook, o the version number as transmitted in client_cook,
o a concatenated datum which is encrypted with the client's public o a concatenated datum which is encrypted with the client's public
key, according to the encryption algorithm transmitted in the key, according to the encryption algorithm transmitted in the
client_cook message. The concatenated datum contains client_cook message. The concatenated datum contains
* the nonce transmitted in client_cook, and * the nonce transmitted in client_cook, and
* the cookie. * the cookie.
o a signature, created with the server's private key, calculated o a signature, created with the server's private key, calculated
over all of the data listed above. This signature MUST be over all of the data listed above. This signature MUST be
skipping to change at page 8, line 17 skipping to change at page 11, line 5
* the nonce transmitted in client_cook, and * the nonce transmitted in client_cook, and
* the cookie. * the cookie.
o a signature, created with the server's private key, calculated o a signature, created with the server's private key, calculated
over all of the data listed above. This signature MUST be over all of the data listed above. This signature MUST be
calculated according to the transmitted signature algorithm from calculated according to the transmitted signature algorithm from
the client_cook message. the client_cook message.
6.2.4. Procedure Overview of the Cookie Exchange
For a cookie exchange, the following steps are performed:
1. The client sends a client_cook message to the server. The client
MUST save the included nonce until the reply has been processed.
2. Upon receipt of a client_cook message, the server checks whether
it supports the given cryptographic algorithms. It then
calculates the cookie according to the formula given in
Section 5. The server MAY use the client's certificate to check
that the client is authorized to use the secure time
synchronization service. With this, it MUST construct a
server_cook message as described in Section 6.2.3.
3. The client awaits a reply in the form of a server_cook message;
upon receipt it executes the following actions:
* It verifies that the received version number matches the one
negotiated beforehand.
* It verifies the signature using the server's public key. The
signature has to authenticate the encrypted data.
* It decrypts the encrypted data with its own private key.
* It checks that the decrypted message is of the expected
format: the concatenation of a 128 bit nonce and a 128 bit
cookie.
* It verifies that the received nonce matches the nonce sent in
the client_cook message.
If one of those checks fails, the client MUST abort the run.
+----------------------------+
| o OPTIONAL: Check client's |
| authorization |
| o Generate cookie |
| o Encrypt inner message |
| o Generate signature |
+-------------+--------------+
|
<-+->
Server --------------------------->
/| \
client_ / \ server_
cook / \ cook
/ \|
Client --------------------------->
<--- Cookie exchange -->
Procedure for association and cookie exchange.
6.3. Unicast Time Synchronisation Messages 6.3. Unicast Time Synchronisation Messages
In this message exchange, the usual time synchronization process is In this message exchange, the usual time synchronization process is
executed, with the addition of integrity protection for all messages executed, with the addition of integrity protection for all messages
that the server sends. This message can be repeatedly exchanged as that the server sends. This message can be repeatedly exchanged as
often as the client desires and as long as the integrity of the often as the client desires and as long as the integrity of the
server's time responses is verified successfully. server's time responses is verified successfully.
6.3.1. Message Type: "time_request" 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. This message is sent by the client when it requests a time exchange.
It contains It contains
o the NTS message ID "time_request", o the NTS message ID "time_request",
o the negotiated version number, o the negotiated version number,
o a nonce, o a nonce,
o the negotiated hash algorithm H, o the negotiated hash algorithm H,
o the hash of the client's certificate under H. o the hash of the client's certificate under H.
6.3.2. Message Type: "time_response" 6.3.3. Message Type: "time_response"
This message is sent by the server after it has received a This message is sent by the server after it has received a
time_request message. Prior to this the server MUST recalculate the time_request message. Prior to this the server MUST recalculate the
client's cookie by using the hash of the client's certificate and the client's cookie by using the hash of the client's certificate and the
transmitted hash algorithm. The message contains transmitted hash algorithm. The message contains
o the NTS message ID "time_response", o the NTS message ID "time_response",
o the version number as transmitted in time_request, o the version number as transmitted in time_request,
skipping to change at page 9, line 4 skipping to change at page 13, line 22
This message is sent by the server after it has received a This message is sent by the server after it has received a
time_request message. Prior to this the server MUST recalculate the time_request message. Prior to this the server MUST recalculate the
client's cookie by using the hash of the client's certificate and the client's cookie by using the hash of the client's certificate and the
transmitted hash algorithm. The message contains transmitted hash algorithm. The message contains
o the NTS message ID "time_response", o the NTS message ID "time_response",
o the version number as transmitted in time_request, o the version number as transmitted in time_request,
o the server's time synchronization response data, o the server's time synchronization response data,
o the nonce transmitted in time_request, o the nonce transmitted in time_request,
o a MAC (generated with the cookie as key) for verification of all o a MAC (generated with the cookie as key) for verification of all
of the above data. of the above data.
6.3.4. Procedure Overview of the Unicast Time Synchronization Exchange
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,
* 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 6.4. Broadcast Parameter Messages
In this message exchange, the client receives the necessary In this message exchange, the client receives the necessary
information to execute the TESLA protocol in a secured broadcast information to execute the TESLA protocol in a secured broadcast
association. The client can only initiate a secure broadcast association. The client can only initiate a secure broadcast
association after a successful unicast run. association after successful association and cookie exchanges and
only if it has made sure that its clock is roughly synchronized to
the server's.
See Appendix B for more details on TESLA. See Appendix B for more details on TESLA.
6.4.1. Message Type: "client_bpar" 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 This message is sent by the client in order to establish a secured
time broadcast association with the server. It contains time broadcast association with the server. It contains
o the NTS message ID "client_bpar", o the NTS message ID "client_bpar",
o the NTS version number negotiated during association in unicast o the NTS version number negotiated during association,
mode,
o a nonce,
o the client's hostname, and o the client's hostname, and
o the signature algorithm negotiated during unicast. o the signature algorithm negotiated during association.
6.4.2. Message Type: "server_bpar" 6.4.3. Message Type: "server_bpar"
This message is sent by the server upon receipt of a client_bpar This message is sent by the server upon receipt of a client_bpar
message during the broadcast loop of the server. It contains message during the broadcast loop of the server. It contains
o the NTS message ID "server_bpar", o the NTS message ID "server_bpar",
o the version number as transmitted in the client_bpar message, 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 one-way functions used for building the key chain, and
o the disclosure schedule of the keys. This contains: o the disclosure schedule of the keys. This contains:
* the last key of the key chain, * the last key of the key chain,
* time interval duration, * time interval duration,
* the disclosure delay (number of intervals between use and * the disclosure delay (number of intervals between use and
disclosure of a key), disclosure of a key),
* the time at which the next time interval will start, and * the time at which the next time interval will start, and
* the next interval's associated index. * the next interval's associated index.
o The message also contains a signature signed by the server with o The message also contains a signature signed by the server with
its private key, verifying all the data listed above. its private key, verifying all the data listed above.
6.5. Broadcast Messages 6.4.4. Procedure Overview of the Broadcast Parameter Exchange
Via this message, the server keeps sending broadcast time A broadcast parameter exchange consists of the following steps:
synchronization messages to all participating clients.
6.5.1. Message Type: "server_broad" 1. The client sends a client_bpar message to the server. It MUST
remember the transmitted values for the nonce, the version number
and the signature algorithm.
2. Upon receipt of a client_bpar message, the server constructs and
sends a server_bpar message as described in Section 6.4.3.
3. The client waits for a reply in the form of a server_bpar
message, on which it performs the following checks:
* The message must contain all the necessary information for the
TESLA protocol, as listed in Section 6.4.3.
* The message must contain a nonce belonging to a client_bpar
message that the client has previously sent.
* Verification of the message's signature.
If any information is missing or if the server's signature cannot
be verified, the client MUST abort the broadcast run. If all
checks are successful, the client MUST remember all the broadcast
parameters received for later checks.
+---------------------+
| o Assemble response |
| o Create public-key |
| signature |
+----------+----------+
|
<-+->
Server --------------------------------------------->
/| \
client_ / \ server_
bpar / \ bpar
/ \|
Client --------------------------------------------->
<------- Broadcast ------> <- Client-side ->
parameter validity
exchange checks
Procedure for unicast time synchronization exchange.
6.5. Broadcast Time Synchronization Exchange
Via a stream of messages of the following message type, the server
keeps sending broadcast time synchronization messages to all
participating clients.
6.5.1. Goals of the Broadcast Time Synchronization Exchange
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 This message is sent by the server over the course of its broadcast
schedule. It is part of any broadcast association. It contains schedule. It is part of any broadcast association. It contains
o the NTS message ID "server_broad", o the NTS message ID "server_broad",
o the version number that the server is working under, o the version number that the server is working under,
o time broadcast data, o time broadcast data,
o the index that belongs to the current interval (and therefore o the index that belongs to the current interval (and therefore
identifies the current, yet undisclosed, key), identifies the current, yet undisclosed, key),
o the disclosed key of the previous disclosure interval (current o the disclosed key of the previous disclosure interval (current
time interval minus disclosure delay), time interval minus disclosure delay),
skipping to change at page 10, line 43 skipping to change at page 18, line 23
o a MAC, calculated with the key for the current time interval, o a MAC, calculated with the key for the current time interval,
verifying verifying
* the message ID, * the message ID,
* the version number, and * the version number, and
* the time data. * the time data.
6.6. Broadcast Key Check 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
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 This message exchange is performed for an additional check of packet
timeliness in the course of the TESLA scheme, see Appendix B. timeliness in the course of the TESLA scheme, see Appendix B.
6.6.1. Message Type: "client_keycheck" 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.
6.6.2. Message Type: "client_keycheck"
A message of this type is sent by the client in order to initiate an A message of this type is sent by the client in order to initiate an
additional check of packet timeliness for the TESLA scheme. It additional check of packet timeliness for the TESLA scheme. It
contains contains
o the NTS message ID "client_keycheck", o the NTS message ID "client_keycheck",
o the NTS version number negotiated during association in unicast o the NTS version number negotiated during association,
mode,
o a nonce, o a nonce,
o an interval number from the TESLA disclosure schedule, o an interval number from the TESLA disclosure schedule,
o the hash algorithm H negotiated in unicast mode, and o the hash algorithm H negotiated during association, and
o the hash of the client's certificate under H. o the hash of the client's certificate under H.
6.6.2. Message Type: "server_keycheck" 6.6.3. Message Type: "server_keycheck"
A message of this type is sent by the server upon receipt of a A message of this type is sent by the server upon receipt of a
client_keycheck message during the broadcast loop of the server. client_keycheck message during the broadcast loop of the server.
Prior to this, the server MUST recalculate the client's cookie by Prior to this, the server MUST recalculate the client's cookie by
using the hash of the client's certificate and the transmitted hash using the hash of the client's certificate and the transmitted hash
algorithm. It contains algorithm. It contains
o the NTS message ID "server_keycheck" o the NTS message ID "server_keycheck"
o the version number as transmitted in "client_keycheck, o the version number as transmitted in "client_keycheck,
o the nonce transmitted in the client_keycheck message, o the nonce transmitted in the client_keycheck message,
o the interval number transmitted in the client_keycheck message, o the interval number transmitted in the client_keycheck message,
and and
o a MAC (generated with the cookie as key) for verification of all o a MAC (generated with the cookie as key) for verification of all
of the above data. of the above data.
7. Message Dependencies 6.6.4. Procedure Overview of the Broadcast Keycheck Exchange
+--------------------+
|Association Exchange|
+--------------------+
|
At least one successful
|
v
+---------------+
|Cookie Exchange|
+---------------+
|
At least one successful
|
v
+----------------------------------------+
|Unicast Time Synchronization Exchange(s)|
+----------------------------------------+
|
Until sufficient accuracy has been reached
|
v
+----------------------------+
|Broadcast Parameter Exchange|
+----------------------------+
|
One successful per client
|
v
+----------------------------------------+
|Broadcast Time Synchronization Reception|
+----------------------------------------+
|
Whenever deemed necessary
|
v
+-----------------+
|Keycheck Exchange|
+-----------------+
8. Server Seed Considerations 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 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 The server has to calculate a random seed which has to be kept
secret. The server MUST generate a seed for each supported hash secret. The server MUST generate a seed for each supported hash
algorithm, see Section 9.1. algorithm, see Section 8.1.
According to the requirements in [RFC7384], the server MUST refresh According to the requirements in [RFC7384], the server MUST refresh
each server seed periodically. Consequently, the cookie memorized by each server seed periodically. Consequently, the cookie memorized by
the client becomes obsolete. In this case, the client cannot verify the client becomes obsolete. In this case, the client cannot verify
the MAC attached to subsequent time response messages and has to the MAC attached to subsequent time response messages and has to
respond accordingly by re-initiating the protocol with a cookie respond accordingly by re-initiating the protocol with a cookie
request (Section 6.2). request (Section 6.2).
9. Hash Algorithms and MAC Generation 8. Hash Algorithms and MAC Generation
9.1. Hash Algorithms 8.1. Hash Algorithms
Hash algorithms are used at different points: calculation of the Hash algorithms are used at different points: calculation of the
cookie and the MAC, and hashing of the client's certificate. The cookie and the MAC, and hashing of the client's certificate. The
client and the server negotiate a hash algorithm H during the client and the server negotiate a hash algorithm H during the
association message exchange (Section 6.1) at the beginning of a association message exchange (Section 6.1) at the beginning. The
unicast run. The selected algorithm H is used for all hashing selected algorithm H is used for all hashing processes in that run.
processes in that run.
In the TESLA scheme, hash algorithms are used as pseudo-random In the TESLA scheme, hash algorithms are used as pseudo-random
functions to construct the one-way key chain. Here, the utilized functions to construct the one-way key chain. Here, the utilized
hash algorithm is communicated by the server and is non-negotiable. hash algorithm is communicated by the server and is non-negotiable.
Note: Note:
Any hash algorithm is prone to be compromised in the future. A Any hash algorithm is prone to be compromised in the future. A
successful attack on a hash algorithm would enable any NTS client successful attack on a hash algorithm would enable any NTS client
to derive the server seed from its own cookie. Therefore, the to derive the server seed from its own cookie. Therefore, the
server MUST have separate seed values for its different supported server MUST have separate seed values for its different supported
hash algorithms. This way, knowledge gained from an attack on a hash algorithms. This way, knowledge gained from an attack on a
hash algorithm H can at least only be used to compromise such hash algorithm H can at least only be used to compromise such
clients who use hash algorithm H as well. clients who use hash algorithm H as well.
9.2. MAC Calculation 8.2. MAC Calculation
For the calculation of the MAC, client and server use a Keyed-Hash For the calculation of the MAC, client and server use a Keyed-Hash
Message Authentication Code (HMAC) approach [RFC2104]. The HMAC is Message Authentication Code (HMAC) approach [RFC2104]. The HMAC is
generated with the hash algorithm specified by the client (see generated with the hash algorithm specified by the client (see
Section 9.1). Section 8.1).
10. IANA Considerations 9. IANA Considerations
11. Security Considerations 10. Security Considerations
11.1. Privacy 10.1. Privacy
tbd 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
[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.
11.2. Initial Verification of the Server Certificates 10.2. Initial Verification of the Server Certificates
The client has to verify the validity of the certificates during the The client has to verify the validity of the certificates during the
certification message exchange (Section 6.1.2). Since it generally certification message exchange (Section 6.1.3). Since it generally
has no reliable time during this initial communication phase, it is has no reliable time during this initial communication phase, it is
impossible to verify the period of validity of the certificates. impossible to verify the period of validity of the certificates. To
Therefore, the client MUST use one of the following approaches: solve this chicken-and-egg problem, the client as to rely on external
means.
o The validity of the certificates is preconditioned. Usually this
will be the case in corporate networks.
o The client ensures that the certificates are not revoked. To this
end, the client uses the Online Certificate Status Protocol (OCSP)
defined in [RFC6277].
o The client requests a different service to get an initial time
stamp in order to be able to verify the certificates' periods of
validity. To this end, it can, e.g., use a secure shell
connection to a reliable host. Another alternative is to request
a time stamp from a Time Stamping Authority (TSA) by means of the
Time-Stamp Protocol (TSP) defined in [RFC3161].
11.3. Revocation of Server Certificates 10.3. Revocation of Server Certificates
According to Section 8, it is the client's responsibility to initiate According to Section 7, it is the client's responsibility to initiate
a new association with the server after the server's certificate a new association with the server after the server's certificate
expires. To this end, the client reads the expiration date of the expires. To this end, the client reads the expiration date of the
certificate during the certificate message exchange (Section 6.1.2). certificate during the certificate message exchange (Section 6.1.3).
Furthermore, certificates may also be revoked prior to the normal Furthermore, certificates may also be revoked prior to the normal
expiration date. To increase security the client MAY periodically expiration date. To increase security the client MAY periodically
verify the state of the server's certificate via OCSP. verify the state of the server's certificate via OCSP.
11.4. Mitigating Denial-of-Service for broadcast packets 10.4. Mitigating Denial-of-Service for broadcast packets
TESLA authentication buffers packets for delayed authentication. TESLA authentication buffers packets for delayed authentication.
This makes the protocol vulnerable to flooding attacks, causing the This makes the protocol vulnerable to flooding attacks, causing the
client to buffer excessive numbers of packets. To add stronger DoS client to buffer excessive numbers of packets. To add stronger DoS
protection to the protocol, the client and the server use the "not protection to the protocol, the client and the server use the "not
re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC re-using keys" scheme of TESLA as pointed out in Section 3.7.2 of RFC
4082 [RFC4082]. In this scheme the server never uses a key for the 4082 [RFC4082]. In this scheme the server never uses a key for the
MAC generation more than once. Therefore, the client can discard any MAC generation more than once. Therefore, the client can discard any
packet that contains a disclosed key it already knows, thus packet that contains a disclosed key it already knows, thus
preventing memory flooding attacks. preventing memory flooding attacks.
Note that an alternative approach to enhance TESLA's resistance Note that an alternative approach to enhance TESLA's resistance
against DoS attacks involves the addition of a group MAC to each against DoS attacks involves the addition of a group MAC to each
packet. This requires the exchange of an additional shared key packet. This requires the exchange of an additional shared key
common to the whole group. This adds additional complexity to the common to the whole group. This adds additional complexity to the
protocol and hence is currently not considered in this document. protocol and hence is currently not considered in this document.
11.5. Delay Attack 10.5. Delay Attack
In a packet delay attack, an adversary with the ability to act as a In a packet delay attack, an adversary with the ability to act as a
MITM delays time synchronization packets between client and server MITM delays time synchronization packets between client and server
asymmetrically [RFC7384]. This prevents the client from accurately asymmetrically [RFC7384]. This prevents the client from accurately
measuring the network delay, and hence its time offset to the server measuring the network delay, and hence its time offset to the server
[Mizrahi]. The delay attack does not modify the content of the [Mizrahi]. The delay attack does not modify the content of the
exchanged synchronization packets. Therefore, cryptographic means do exchanged synchronization packets. Therefore, cryptographic means do
not provide a feasible way to mitigate this attack. However, several not provide a feasible way to mitigate this attack. However, several
non-cryptographic precautions can be taken in order to detect this non-cryptographic precautions can be taken in order to detect this
attack. attack.
skipping to change at page 15, line 48 skipping to change at page 24, line 48
4. For unicast-type messages: Introduction of a threshold value for 4. For unicast-type messages: Introduction of a threshold value for
the delay time of the synchronization packets. The client can the delay time of the synchronization packets. The client can
discard a time server if the packet delay time of this time discard a time server if the packet delay time of this time
server is larger than the threshold value. server is larger than the threshold value.
Additional provision against delay attacks has to be taken for Additional provision against delay attacks has to be taken for
broadcast-type messages. This mode relies on the TESLA scheme which broadcast-type messages. This mode relies on the TESLA scheme which
is based on the requirement that a client and the broadcast server is based on the requirement that a client and the broadcast server
are loosely time synchronized. Therefore, a broadcast client has to are loosely time synchronized. Therefore, a broadcast client has to
establish time synchronization with its broadcast server before it establish time synchronization with its broadcast server before it
starts utilizing broadcast messages for time synchronization. To starts utilizing broadcast messages for time synchronization.
this end, it initially establishes a unicast association with its
broadcast server until time synchronization and calibration of the One possible way to achieve this initial synchronization is to
packet delay time is achieved. After that it establishes a broadcast establish a unicast association with its broadcast server until time
association with the broadcast server and utilizes TESLA to verify synchronization and calibration of the packet delay time is achieved.
integrity and authenticity of any received broadcast packets. 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 An adversary who is able to delay broadcast packets can cause a time
adjustment at the receiving broadcast clients. If the adversary adjustment at the receiving broadcast clients. If the adversary
delays broadcast packets continuously, then the time adjustment will delays broadcast packets continuously, then the time adjustment will
accumulate until the loose time synchronization requirement is accumulate until the loose time synchronization requirement is
violated, which breaks the TESLA scheme. To mitigate this violated, which breaks the TESLA scheme. To mitigate this
vulnerability the security condition in TESLA has to be supplemented vulnerability the security condition in TESLA has to be supplemented
by an additional check in which the client, upon receipt of a by an additional check in which the client, upon receipt of a
broadcast message, verifies the status of the corresponding key via a broadcast message, verifies the status of the corresponding key via a
unicast message exchange with the broadcast server (see Appendix B.4 unicast message exchange with the broadcast server (see Appendix B.4
for a detailed description of this check). Note that a broadcast for a detailed description of this check). Note that a broadcast
client should also apply the above-mentioned precautions as far as client should also apply the above-mentioned precautions as far as
possible. possible.
12. Acknowledgements 10.6. Random Number Generation
The authors would like to thank Russ Housley, Steven Bellovin, David At various points of the protocol, the generation of random numbers
Mills and Kurt Roeckx for discussions and comments on the design of is required. The employed methods of generation need to be
NTS. Also, thanks go to Harlan Stenn for his technical review and cryptographically secure. See [RFC4086] for guidelines concerning
specific text contributions to this document. this topic.
13. References 11. Acknowledgements
13.1. Normative References 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- [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February Hashing for Message Authentication", RFC 2104, February
1997. 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. [RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005. Transform Introduction", RFC 4082, June 2005.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009. RFC 5652, September 2009.
[RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate
Status Protocol Algorithm Agility", RFC 6277, June 2011.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, October 2014. Packet Switched Networks", RFC 7384, October 2014.
13.2. Informative References 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] [I-D.ietf-ntp-cms-for-nts-message]
Sibold, D., Roettger, S., Teichel, K., and R. Housley, Sibold, D., Roettger, S., Teichel, K., and R. Housley,
"Protecting Network Time Security Messages with the "Protecting Network Time Security Messages with the
Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms- Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
for-nts-message-00 (work in progress), October 2014. for-nts-message-00 (work in progress), October 2014.
[I-D.shpiner-multi-path-synchronization] [I-D.shpiner-multi-path-synchronization]
Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi- Shpiner, A., Tse, R., Schelp, C., and T. Mizrahi, "Multi-
Path Time Synchronization", draft-shpiner-multi-path- Path Time Synchronization", draft-shpiner-multi-path-
skipping to change at page 17, line 44 skipping to change at page 27, line 10
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. Specification", RFC 5905, June 2010.
[Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", in Proceedings of Precision Clock Protocols", in Proceedings of Precision Clock
Synchronization for Measurement Control and Communication, Synchronization for Measurement Control and Communication,
ISPCS 2013, pp. 1-6, September 2013. ISPCS 2013, pp. 1-6, September 2013.
Appendix A. TICTOC Security Requirements Appendix A. (informative) TICTOC Security Requirements
The following table compares the NTS specifications against the The following table compares the NTS specifications against the
TICTOC security requirements [RFC7384]. TICTOC security requirements [RFC7384].
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| Section | Requirement from I-D tictoc | Requirement | NTS | | Section | Requirement from RFC 7384 | Requirement | NTS |
| | security-requirements-05 | level | | | | | level | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.1.1 | Authentication of Servers | MUST | OK | | 5.1.1 | Authentication of Servers | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.1.1 | Authorization of Servers | MUST | OK | | 5.1.1 | Authorization of Servers | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.1.2 | Recursive Authentication of | MUST | OK | | 5.1.2 | Recursive Authentication of | MUST | OK |
| | Servers (Stratum 1) | | | | | Servers (Stratum 1) | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.1.2 | Recursive Authorization of Servers | MUST | OK | | 5.1.2 | Recursive Authorization of | MUST | OK |
| | (Stratum 1) | | | | | Servers (Stratum 1) | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.1.3 | Authentication and Authorization | MAY | - | | 5.1.3 | Authentication and | MAY | Optional, |
| | of Slaves | | | | | Authorization of Clients | | Limited |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.2 | Integrity protection | MUST | OK | | 5.2 | Integrity protection | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.4 | Protection against DoS attacks | SHOULD | OK | | 5.3 | Spoofing Prevention | MUST | OK |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.5 | Replay protection | MUST | OK | | 5.4 | Protection from DoS attacks | SHOULD | OK |
+---------+------------------------------------+-------------+------+ | | against the time protocol | | |
| 5.6 | Key freshness | MUST | OK | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | 5.5 | Replay protection | MUST | OK |
| | Security association | SHOULD | OK | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | 5.6 | Key freshness | MUST | OK |
| | Unicast and multicast associations | SHOULD | OK | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | | Security association | SHOULD | OK |
| 5.7 | Performance: no degradation in | MUST | OK | +---------+------------------------------+-------------+------------+
| | quality of time transfer | | | | | Unicast and multicast | SHOULD | OK |
+---------+------------------------------------+-------------+------+ | | associations | | |
| | Performance: lightweight | SHOULD | OK | +---------+------------------------------+-------------+------------+
| | computation | | | | 5.7 | Performance: no degradation | MUST | OK |
+---------+------------------------------------+-------------+------+ | | in quality of time transfer | | |
| | Performance: storage, bandwidth | SHOULD | OK | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | | Performance: lightweight | SHOULD | OK |
| 5.7 | Confidentiality protection | MAY | NO | | | computation | | |
+---------+------------------------------------+-------------+------+ +---------+------------------------------+-------------+------------+
| 5.9 | Protection against Packet Delay | SHOULD | NA*) | | | Performance: storage | SHOULD | OK |
| | and Interception Attacks | | | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | | Performance: bandwidth | SHOULD | OK |
| 5.10 | Secure mode | MUST | - | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | 5.8 | Confidentiality protection | MAY | NO |
| | Hybrid mode | SHOULD | - | +---------+------------------------------+-------------+------------+
+---------+------------------------------------+-------------+------+ | 5.9 | Protection against Packet | MUST | Limited*) |
| | Delay and Interception | | |
| | Attacks | | |
+---------+------------------------------+-------------+------------+
| 5.10 | Secure mode | MUST | OK |
+---------+------------------------------+-------------+------------+
| | Hybrid mode | SHOULD | - |
+---------+------------------------------+-------------+------------+
*) See discussion in Section 11.5. *) See discussion in Section 10.5.
Comparison of NTS specification against TICTOC security requirements. Comparison of NTS specification against Security Requirements of Time
Protocols in Packet Switched Networks (RFC 7384)
Appendix B. Using TESLA for Broadcast-Type Messages Appendix B. (normative) Using TESLA for Broadcast-Type Messages
For broadcast-type messages , NTS adopts the TESLA protocol with some For broadcast-type messages , NTS adopts the TESLA protocol with some
customizations. This appendix provides details on the generation and customizations. This appendix provides details on the generation and
usage of the one-way key chain collected and assembled from usage of the one-way key chain collected and assembled from
[RFC4082]. Note that NTS uses the "not re-using keys" scheme of [RFC4082]. Note that NTS uses the "not re-using keys" scheme of
TESLA as described in Section 3.7.2. of [RFC4082]. TESLA as described in Section 3.7.2. of [RFC4082].
B.1. Server Preparation B.1. Server Preparation
Server setup: server setup:
1. The server determines a reasonable upper bound B on the network 1. The server determines a reasonable upper bound B on the network
delay between itself and an arbitrary client, measured in delay between itself and an arbitrary client, measured in
milliseconds. milliseconds.
2. It determines the number n+1 of keys in the one-way key chain. 2. It determines the number n+1 of keys in the one-way key chain.
This yields the number n of keys that are usable to authenticate This yields the number n of keys that are usable to authenticate
broadcast packets. This number n is therefore also the number of broadcast packets. This number n is therefore also the number of
time intervals during which the server can send authenticated time intervals during which the server can send authenticated
broadcast messages before it has to calculate a new key chain. broadcast messages before it has to calculate a new key chain.
skipping to change at page 19, line 40 skipping to change at page 29, line 12
4082, the time interval L has to be shorter than the time 4082, the time interval L has to be shorter than the time
interval between the broadcast messages. interval between the broadcast messages.
4. The server generates a random key K_n. 4. The server generates a random key K_n.
5. Using a one-way function F, the server generates a one-way chain 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 of n+1 keys K_0, K_1, ..., K_{n} according to
K_i = F(K_{i+1}). K_i = F(K_{i+1}).
6. Using another one-way function F', it generates a sequence of n+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 MAC keys K'_0, K'_1, ..., K'_{n-1} according to
K'_i = F'(K_i). K'_i = F'(K_i).
7. Each MAC key K'_i is assigned to the time interval I_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 8. The server determines the key disclosure delay d, which is the
number of intervals between using a key and disclosing it. Note number of intervals between using a key and disclosing it. Note
that although security is provided for all choices d>0, the that although security is provided for all choices d>0, the
choice still makes a difference: choice still makes a difference:
skipping to change at page 21, line 28 skipping to change at page 31, line 13
the server. the server.
Note that if D_t is greater than (d - 1) * L, then some authentic Note that if D_t is greater than (d - 1) * L, then some authentic
packets might be discarded. If D_t is greater than d * L, then all packets might be discarded. If D_t is greater than d * L, then all
authentic packets will be discarded. In the latter case, the client authentic packets will be discarded. In the latter case, the client
should not participate in the broadcast, since there will be no should not participate in the broadcast, since there will be no
benefit in doing so. benefit in doing so.
B.3. Sending Authenticated Broadcast Packets B.3. Sending Authenticated Broadcast Packets
During each time interval I_i, the server sends one authenticated During each time interval I_i, the server sends at most one
broadcast packet P_i. This packet consists of: authenticated broadcast packet P_i. Such a packet consists of:
o a message M_i, o a message M_i,
o the index i (in case a packet arrives late), o the index i (in case a packet arrives late),
o a MAC authenticating the message M_i, with K'_i used as key, o a MAC authenticating the message M_i, with K'_i used as key,
o the key K_{i-d}, which is included for disclosure. o the key K_{i-d}, which is included for disclosure.
B.4. Authentication of Received Packets B.4. Authentication of Received Packets
skipping to change at page 22, line 48 skipping to change at page 32, line 32
earlier disclosed key (see Clause 3.5 in RFC 4082, item 3). earlier disclosed key (see Clause 3.5 in RFC 4082, item 3).
Next the client verifies that the transmitted time value s_i belongs Next the client verifies that the transmitted time value s_i belongs
to the time interval I_i, by checking to the time interval I_i, by checking
T_i =< s_i, and T_i =< s_i, and
s_i < T_{i+1}. s_i < T_{i+1}.
If it is falsified, the packet MUST be discarded and the client MUST If it is falsified, the packet MUST be discarded and the client MUST
reinitialize its broadcast module by performing a unicast time reinitialize its broadcast module by performing time synchronization
synchronization as well as a new broadcast parameter exchange by other means than broadcast messages, and it MUST perform a new
(because a falsification of this check yields that the packet was not broadcast parameter exchange (because a falsification of this check
generated according to protocol, which suggests an attack). 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 If a packet P_i passes all the tests listed above, it is stored for
later authentication. Also, if at this time there is a package with later authentication. Also, if at this time there is a package with
index i-d already buffered, then the client uses the disclosed key index i-d already buffered, then the client uses the disclosed key
K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in K_{i-d} to derive K'_{i-d} and uses that to check the MAC included in
package P_{i-d}. Upon success, it regards M_{i-d} as authenticated. package P_{i-d}. Upon success, it regards M_{i-d} as authenticated.
Appendix C. Random Number Generation Appendix C. (informative) Dependencies
+---------+--------------+--------+-------------------------------+
At various points of the protocol, the generation of random numbers | Issuer | Type | Owner | Description |
is required. The employed methods of generation need to be +---------+--------------+--------+-------------------------------+
cryptographically secure. See [RFC4086] for guidelines concerning | Server | private key | server | Used for server_assoc, |
this topic. | 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. |
+---------+--------------+--------+-------------------------------+
+------------<---------------+
| 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 ||
++===================++
Authors' Addresses Authors' Addresses
Dieter Sibold Dieter Sibold
Physikalisch-Technische Bundesanstalt Physikalisch-Technische Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
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