NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: December 29, 2013 TU-BS
June 27, 2013
Network Time Security
draft-ietf-ntp-network-time-security-00.txt
Abstract
This document describes the Network Time Security (NTS) protocol that
enables secure authentication of time servers using Network Time
Protocol (NTP) or 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 Network [I-D.ietf-tictoc-security-requirements].
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.
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This Internet-Draft will expire on December 29, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 3
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 4
5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 4
6. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Association Message . . . . . . . . . . . . . . . . . . . 5
6.2. Certificate Message . . . . . . . . . . . . . . . . . . . 5
6.3. Cookie Message . . . . . . . . . . . . . . . . . . . . . 6
6.4. Broadcast Parameter Message . . . . . . . . . . . . . . . 6
6.5. Time Request Message . . . . . . . . . . . . . . . . . . 6
6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 6
6.7. Restart of the Protocol Sequence . . . . . . . . . . . . 7
7. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 7
7.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 7
7.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 8
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 8
8.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 8
8.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10.1. Initial Verification of the Server Certificates . . . . 8
10.2. Revocation of Server Certificates . . . . . . . . . . . 9
10.3. Denial-of-Service in Broadcast Mode . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 10
Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
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Time synchronization protocols are more and more utilized to
synchronize clocks in networked infrastructures. The reliable
performance of such infrastructures can be degraded seriously by
successful attacks against the time synchronization protocol.
Therefore, time synchronization protocols applied in critical
infrastructures have to provide security measures to defeat possible
adversaries. Consequently, the widespread Network Time Protocol
(NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
which shall ensure authenticity of the NTP server and integrity of
the protocol packets. Unfortunately,the autokey protocol exhibits
various severe security vulnerabilities as revealed in a thorough
analysis of the protocol [Roettger]. For the Precision Time Protocol
(PTP) Annex K of the standard document IEEE 1588 [IEEE1588] defines
an informative security protocol that is still in experimental state.
Because of autokey's security vulnerabilities and the absence of a
standardized security protocol for PTP these protocols cannot be
applied in environments in which compliance requirements demand
authenticity and integrity protection. This document specifies a
security protocol that ensures authenticity of the time server and
integrity of the time synchronisations protocol packets and hence
enables the usage of NTP and PTP in such environments.
The protocol is specified with the prerequisite in mind that precise
timekeeping can only be accomplished with stateless time
synchronization communication, which excludes standard security
protocols like IPSec or TLS. 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
time transfer [I-D.ietf-tictoc-security-requirements].
2. Security Threats
A profound analysis of security threats and requirements for NTP and
PTP can be found in the I-D [I-D.ietf-tictoc-security-requirements].
3. Objectives
The objectives of the autokey specifications are as follows:
o Authenticity: NTS enables the client to authenticate its time
server
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.
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o Modes of operation: All operational modes of NTP are supported
o Hybrid mode: Both secure and insecure communication modes are
possible for NTP servers and clients, respectively.
o Compatibility:
* Unsecured NTP associations shall not be affected.
* An NTP server that does not support NTS shall not be affected
by NTS authentication requests.
4. Terms and Abbreviations
o TESLA: Time efficient stream loss-tolerant authentication
5. NTS Overview
5.1. Symmetric and Client/Server Mode
Authenticity and integrity of the NTP packets are ensured by a
Message Authentication Code (MAC), which is attached to the NTP
packet. The calculation of the MAC includes the whole NTP packet and
the cookie which is shared between client and server. It is
calculated according to:
cookie = MSB_128 (H(server seed || H(public key of client))),
where || indicates concatenation and in which H is a hash algorithm.
The function MSB_128 cuts off the 128 most significant bits of the
result of the hash function. The server seed is a 128 bit random
value of the server, which has to be kept secret. The cookie thus
never changes. The server seed has to be refreshed periodically.
The server does not keep a state of the client. Therefore it has to
recalculate the cookie each time it receives a request from the
client. To this end, the client has to attach the hash value of its
public key to each request (see Section 6.5).
5.2. Broadcast Mode
Just as in the case of the client server mode and symmetric mode,
authenticity and integrity of the NTP packets are ensured by a MAC,
which is attached to the NTP packet by the sender. The verification
of the authenticity is based on the TESLA protocol [RFC4082]. TESLA
is based on a one-way chain of keys, where each key is the output of
a one-way function applied on the previous key in the chain. The
last element of the chain is shared securely with all clients. The
server splits time into intervals of uniform duration and assigns
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each key to an interval in reverse order, starting with the
penultimate. At each time interval, the server sends an NTP
broadcast packet appended by a MAC, calculated using the
corresponding key, and the key of the previous interval. The client
verifies the MAC by buffering the packet until the disclosure of the
key in the next interval. In order to be able to verify the validity
of the key, the client has to be loosely time synchronized to the
server. This has to be accomplished during the initial client server
exchange between broadcast client and server.
6. Protocol Sequence
6.1. Association Message
The protocol sequence starts with the association message, in which
the client sends an NTP packet with an extension field of type
association. It contains the hostname of the client and a status
word which contains the algorithms used for the signatures and the
status of the connection. The response contains the hostname of the
server and the algorithms for the signatures. The server notifies
the cryptographic hash algorithms which it supports.
6.2. Certificate Message
In this step, the client receives the certification chain up to the
trusted authority (TA). To this end, the client requests the
certificate for the subject name (hostname) of the NTP server. The
response contains the certificate with the issuer name. If the
issuer name is different from the subject name, the client requests
the certificate for the issuer. This continues until it receives a
certificate which is issued by a TA. The client recognizes the TA
because it has a list of certificates which are accepted as TAs. The
client has to check that each issuer is authorized to issue new
certificates. To this end, the certificates have to include the
X.509v3 extension field "CA:TRUE". With the established
certification chain the client is able to verify the server
signatures and, hence, the authenticity of the server messages with
extension fields is ensured.
Discussion:
Note that in this step the client validates the authenticity of
its NTP server only. It does not recursively validate the
authenticity of each NTP server on the time synchronization chain.
But each NTP server on the time synchronization chain validates
the NTP server to which it is synchronized. This conforms to the
recursive authentication requirement in the TICTOC security
requirements [I-D.ietf-tictoc-security-requirements].
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6.3. Cookie Message
The client requests a cookie from the server. It selects a hash
algorithm from the list of algorithms supported by the server. The
request includes its public key and the selected hash algorithm. The
hash of the public key is used by the server to calculate the cookie
(see Section 5.1). The response of the server contains the cookie
encrypted with the public key.
6.4. Broadcast Parameter Message
In the broadcast mode the client requests the following information
from the server:
o the last key of the one-way key chain,
o the disclosure schedule of the following keys. This contains:
* time interval duration, time at which the next time interval
will start and its associated index,
* key disclosure delay (number of time intervals for which a key
is valid).
The server will sign all transmitted properties so that the client is
able to verify their authenticity. For this packet exchange a new
extension field "broadcast parameters" is used. The client
synchronizes its time with the server in the client server mode and
saves an upper bound of its time offset with respect to the time of
the server. See B for more details.
6.5. Time Request Message
The client request includes a new extension field "time request"
which contains the hash of its public key. The server needs the hash
of the public key to recalculate the cookie for the client. The
response is a normal NTP packet without extension field. It contains
a MAC.
6.6. Broadcast Message
In broadcast mode the NTP packet includes a new extension field
"broadcast message" which contains the disclosed key of the previous
disclosure interval (current time interval minus disclosure delay).
The NTP packet is appended by a MAC, calculated with the key for the
current time interval. When a client receives a broadcast message it
has to perform the following tests:
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o Proof that the MAC is based on a key that is not yet disclosed.
If verified the packet will be buffered for later authentication
otherwise it has to be discarded.
o The client checks whether it already knows the disclosed key. If
not, the client verifies its legitimacy. If falsified the packet
has to be discarded.
o If the disclosed key is legitimate the client verifies the
authenticity of any packet that it received during the
corresponding time interval. If authenticity of a packet is
verified it is released from the buffer. If the verification
fails authenticity is no longer given. In this case the client
MUST request authentic time from the server by means of a unicast
time request message.
See RFC 4082[RFC4082] for a detailed description of the packet
verification process.
6.7. Restart of the Protocol Sequence
According to the requirements in
[I-D.ietf-tictoc-security-requirements] the server has to refresh its
server seed periodically. As a consequence the cookie used in the
time request messages becomes invalid. In this case the server has
to respond accordingly and the client has to restart the protocol
with the association message. This is true for the unicast and
broadcast mode, respectively.
Additionally, in broadcast mode the client has to restart the
broadcast sequence with a time request message if the one-way key
chain expires.
During certificate message exchange the client requests the
expiration date of the period of validity of the server certificate.
The client MAY restart the protocol sequence with the association
message before the server certificate expires.
7. Hash Algorithms and MAC Generation
7.1. Hash Algorithms
Hash algorithms are used at different points: calculation of the
cookie and the MAC, and hashing of the public key. The client
selects the hash algorithm from the list of hash algorithms which are
supported by the server. This list is notified during the
association message exchange (Section 6.1). The selected algorithm
is used for all hashing processes in the protocol.
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In the broadcast mode hash algorithm are used as pseudo random
function to construct the one-way key chain.
The list of the hash algorithms supported by the server has to fulfil
the following requirements:
o it MUST NOT contain the MD5 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms.
7.2. MAC Calculation
For the calculation of the MAC client and server are using a Keyed-
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC
is generated with the hash algorithm specified by the client (see
Section 7.1).
8. Server Seed Considerations
The server has to calculate a random seed which has to be kept secret
and which has to be changed periodically. The server has to generate
a seed for each supported hash algorithm.
8.1. Server Seed Algorithm
8.2. Server Seed Live Time
9. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
10. Security Considerations
10.1. Initial Verification of the Server Certificates
The client has to verify the validity of the certificates during the
certification message exchange (Section 6.2). Since it generally has
no reliable time during this initial communication phase, it is
impossible to verify the period of validity of the certificates.
Therefore, the client MUST use one of the following approaches:
o The validity of the certificates is preconditioned. Usually this
will be the case in corporation networks.
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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].
10.2. Revocation of Server Certificates
According to Section Section 6.7 it is the client's responsibility to
initiate a new association with the server after the server's
certificate expires. To this end the client reads the expiration
date of the certificate during the certificate message exchange
(Section 6.2). Besides, certificate may also be revoked prior to the
normal expiration date. To increase security the client MAY verify
the state of the server's certificate via OCSP periodically.
10.3. Denial-of-Service in Broadcast Mode
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 client and server SHALL 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 knows already, thus
preventing memory flooding attacks.
Note, 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.
11. Acknowledgements
12. References
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12.1. Normative References
[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.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.D., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC5906] Haberman, B. and D. Mills, "Network Time Protocol Version
4: Autokey Specification", RFC 5906, June 2010.
[RFC6277] Santesson, S. and P. Hallam-Baker, "Online Certificate
Status Protocol Algorithm Agility", RFC 6277, June 2011.
12.2. Informative References
[I-D.ietf-tictoc-security-requirements]
Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", draft-ietf-tictoc-security-
requirements-05 (work in progress), April 2013.
[Roettger]
Roettger, S., "Analysis of the NTP Autokey Procedures",
February 2012.
Appendix A. TICTOC Security Requirements
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The following table compares the NTS specifications against the
TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
+---------+--------------------------------+---------------+--------+
| Section | Requirement from I-D tictoc | Requirement | NTS |
| | security-requirements-05 | level | |
+---------+--------------------------------+---------------+--------+
| 5.1 | Clock Identity Authentication | MUST | OK |
| | and Authorization | | |
| 5.1.1 | Authentication and | MUST | OK |
| | Authorization of Masters | | |
| 5.1.2 | Recursive Authentication and | MUST | OK |
| | Authorization of Masters | | |
| | (Chain of Trust) | | |
| 5.1.3 | Authentication and | MAY | - |
| | Authorization of Slaves | | |
| 5.2 | Integrity protection. | MUST | OK |
| 5.3 | Protection against DoS attacks | SHOULD | - |
| 5.4 | Replay protection | MUST | OK |
| | | | (NTP) |
| 5.5.1 | Key freshness. | MUST | OK |
| 5.5.2 | Security association. | SHOULD | OK |
| 5.5.3 | Unicast and multicast | SHOULD | OK |
| | associations. | | |
| 5.6 | Performance: no degradation in | MUST | OK |
| | quality of time transfer. | | |
| | Performance: lightweight | SHOULD | OK |
| | computation | | |
| | Performance: storage, | SHOULD | OK |
| | bandwidth | | |
| 5.7 | Confidentiality protection | MAY | - |
| 5.8 | Protection against Packet | SHOULD | - |
| | Delay and Interception Attacks | | |
| 5.9.1 | Secure mode | MUST | OK |
| | | | (NTP) |
| 5.9.2 | Hybrid mode | MAY | OK |
| | | | (NTP) |
+---------+--------------------------------+---------------+--------+
Comparsion of NTS sepecification against TICTOC security
requirements.
Appendix B. Broadcast Mode
Authors' Addresses
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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
Technische Universitaet Braunschweig
Email: stephen.roettger@googlemail.com
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