NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: August 25, 2013 TU-BS
February 23, 2013
Network Time Protocol: autokey Version 2 Specification
draft-sibold-autokey-02
Abstract
This document describes a security protocol that enables
authenticated time synchronization using Network Time Protocol (NTP).
Autokey Version 2 obsoletes NTP autokey protocol RFC 5906 [RFC5906]
which suffers from various security vulnerabilities. Its design
considers the special requirements that are related to the task of
precise timekeeping.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 25, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
Sibold & Roettger Expires August 25, 2013 [Page 1]
Internet-Draft NTP autokey V2 February 2013
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Differences from the original autokey . . . . . . . . . . 3
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . . 3
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terms and abbreviations . . . . . . . . . . . . . . . . . . . 4
5. Autokey 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
7. Hash algorithms and MAC generation . . . . . . . . . . . . . . 7
7.1. Hash algorithms . . . . . . . . . . . . . . . . . . . . . 7
7.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 7
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
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. TICTOC Security Requirements . . . . . . . . . . . . . 9
Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
In NTP [RFC5905] the autokey protocol [RFC5906] was introduced to
provide authenticity to NTP servers and to ensure integrity of time
synchronization. It is designed to meet the specific communication
requirements of precise timekeeping and therefore does not compromise
timekeeping precision.
Sibold & Roettger Expires August 25, 2013 [Page 2]
Internet-Draft NTP autokey V2 February 2013
This document focuses on a new definition of the autokey protocol for
NTP, autokey version 2. The necessity to renew the autokey
specification arises from various severe security vulnerabilities
that have been found in a thorough analysis of the protocol
[Roettger]. The new specification is based on the same assumptions
as the original autokey specification. In particular, the
prerequisite is 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].
1.1. Differences from the original autokey
Autokey version 2 is a major redraft of the original autokey
specification. It is intended to mitigate security vulnerabilities
of the original specification and it is based on the suggestions in
the analysis of Roettger [Roettger]. The major changes are:
o The bit length of server seed and cookie has been increased.
o The IP addresses of the synchronization partners in the
calculation of the cookie have been replaced by the hash value of
the client's public key.
o The identity schemes for the verification of the NTP server
authenticity have been replaced by a hierarchical public key
infrastructure (PKI) based on X.509 certificates.
2. Security Threats
A profound analysis of security threats and requirements for NTP and
Precision Time Protocol (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: Autokey enables the client to authenticate its NTP
server or peer.
o Integrity: Autokey protects the integrity of time synchronization
packets via a message authentication code (MAC).
o Confidentiality: Autokey does not provide confidentiality
Sibold & Roettger Expires August 25, 2013 [Page 3]
Internet-Draft NTP autokey V2 February 2013
protection of the NTP packets.
o Modes of operation: All operational modes of NTP are supported
(client server, symmetric, broadcast).
o Hybrid mode: Both secure and insecure communication modes are
possible for NTP servers and clients, respectively.
o Compatibility:
* Interoperation with autokey version 1 is not given.
* NTP associations without authentication shall not be affected.
* An NTP server that does not support autokey version 2 shall not
be affected by autokey version 2 authentication requests.
4. Terms and abbreviations
o Throughout this document the term "autokey" refers to autokey
version 2.
o TESLA: Time efficient stream loss-tolerant authentication
5. Autokey 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
Sibold & Roettger Expires August 25, 2013 [Page 4]
Internet-Draft NTP autokey V2 February 2013
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
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:
Sibold & Roettger Expires August 25, 2013 [Page 5]
Internet-Draft NTP autokey V2 February 2013
Note that in this step the client validate 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].
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 Appendix 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
Sibold & Roettger Expires August 25, 2013 [Page 6]
Internet-Draft NTP autokey V2 February 2013
The NTP broadcast 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:
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 Appendix B or [RFC4082] for a detailed description of the packet
verification process.
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.
In the broadcast mode hash algorithm are used as pseudo random
function to construct the one-way key chain.
The list of the server supported hash algorithms has to fulfill
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).
Sibold & Roettger Expires August 25, 2013 [Page 7]
Internet-Draft NTP autokey V2 February 2013
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
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.
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. Acknowledgements
12. References
12.1. Normative References
[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.
Sibold & Roettger Expires August 25, 2013 [Page 8]
Internet-Draft NTP autokey V2 February 2013
[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
Synchronization Protocols in Packet Switched Networks",
Internet-Draft draft-ietf-tictoc-security-requirements-04,
February 2013.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[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.
[Roettger]
Roettger, S., "Analysis of the NTP Autokey Procedures",
February 2012.
Appendix A. TICTOC Security Requirements
The following table compares the autokey specifications against the
TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
Sibold & Roettger Expires August 25, 2013 [Page 9]
Internet-Draft NTP autokey V2 February 2013
+----------+------------------------------+---------------+---------+
| Section | Requirement from I-D tictoc | Requirement | Autokey |
| | security-requirements-04 | level | V2 |
+----------+------------------------------+---------------+---------+
| 5.1 | Clock Identity | MUST | OK |
| | Authentication 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 | SHOULD | - |
| | attacks | | |
| 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 | MUST | OK |
| | in 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) |
+----------+------------------------------+---------------+---------+
Comparison between TICTOC security requirements and autokey.
Appendix B. Broadcast Mode
Authors' Addresses
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig, D-38116
Germany
Phone: +49-(0)531-592-8420
Email: dieter.sibold@ptb.de
Sibold & Roettger Expires August 25, 2013 [Page 10]
Internet-Draft NTP autokey V2 February 2013
Stephen Roettger
Technische Universitaet Braunschweig
Email: stephen.roettger@googlemail.com
Sibold & Roettger Expires August 25, 2013 [Page 11]