Using the Network Time Security Specification to Secure the Network Time Protocol
draft-ietf-ntp-using-nts-for-ntp-00
NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: September 7, 2015 Google Inc
K. Teichel
PTB
March 06, 2015
Using the Network Time Security Specification to Secure the Network Time
Protocol
draft-ietf-ntp-using-nts-for-ntp-00.txt
Abstract
This document describes how to use the measures described in the
Network Time Security (NTS) specification to secure time
synchronization with servers using the Network Time Protocol (NTP).
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
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This Internet-Draft will expire on September 7, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4
4. Overview of NTS-Secured NTP . . . . . . . . . . . . . . . . . 4
4.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 4
4.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 4
5. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 4
5.1. The Client . . . . . . . . . . . . . . . . . . . . . . . 4
5.1.1. The Client in Unicast Mode . . . . . . . . . . . . . 4
5.1.2. The Client in Broadcast Mode . . . . . . . . . . . . 6
5.2. The Server . . . . . . . . . . . . . . . . . . . . . . . 8
5.2.1. The Server in Unicast Mode . . . . . . . . . . . . . 8
5.2.2. The Server in Broadcast Mode . . . . . . . . . . . . 9
6. Implementation Notes: ASN.1 Structures and Use of the CMS . . 9
6.1. Unicast Messages . . . . . . . . . . . . . . . . . . . . 9
6.1.1. Association Messages . . . . . . . . . . . . . . . . 9
6.1.2. Cookie Messages . . . . . . . . . . . . . . . . . . . 10
6.1.3. Time Synchronization Messages . . . . . . . . . . . . 10
6.2. Broadcast Messages . . . . . . . . . . . . . . . . . . . 11
6.2.1. Broadcast Parameter Messages . . . . . . . . . . . . 11
6.2.2. Broadcast Time Synchronization Message . . . . . . . 11
6.2.3. Broadcast Keycheck . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7.1. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 12
7.2. Server Seed Lifetime . . . . . . . . . . . . . . . . . . 12
7.3. Supported Hash Algorithms . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Flow Diagrams of Client Behaviour . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
One of the most popular time synchronization protocols, the Network
Time Protocol (NTP) [RFC5905], currently does not provide adequate
intrinsic security precautions. The Network Time Security draft
[I-D.ietf-ntp-network-time-security] specifies security measures
which can be used to enable time synchronization protocols to verify
authenticity of the time server and integrity of the time
synchronization protocol packets.
This document provides detail on how to specifically use those
measures to secure time synchronization between NTP clients and
servers.
2. Objectives
The objectives of the NTS specification are as follows:
o Authenticity: NTS enables an NTP client to authenticate its time
server(s).
o Integrity: NTS protects the integrity of NTP time synchronization
protocol packets via a message authentication code (MAC).
o Confidentiality: NTS does not provide confidentiality protection
of the time synchronization packets.
o Authorization: NTS optionally enables the server to verify the
client's authorization.
o Request-Response-Consistency: NTS enables a client to match an
incoming response to a request it has sent. NTS also enables the
client to deduce from the response whether its request to the
server has arrived without alteration.
o Modes of operation: Both the unicast and the broadcast mode of NTP
are supported.
o Hybrid mode: Both secure and insecure communication modes are
possible for both NTP servers and clients.
o Compatibility:
* Unsecured NTP associations are not be affected.
* An NTP server that does not support NTS are not affected by
NTS-secured authentication requests.
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3. Terms and Abbreviations
MITM Man In The Middle
NTP Network Time Protocol [RFC5905]
NTS Network Time Security
TESLA Timed Efficient Stream Loss-Tolerant Authentication
MAC Message Authentication Code
HMAC Keyed-Hash Message Authentication Code
4. Overview of NTS-Secured NTP
4.1. Symmetric and Client/Server Mode
The server does not keep a state of the client. NTS applies X.509
certificates to verify the authenticity of the time server and to
exchange a symmetric key, the so-called cookie. The "association"
and "cookie" message exchanges are utilized for this. In subsequent
"unicast time synchronization" message exchanges, the cookie is then
used to protect authenticity and integrity of NTP unicast time
synchronization packets. This is achieved by a MAC attached to each
time synchronization packet.
4.2. Broadcast Mode
After the client has accomplished the necessary initial time
synchronization via client-server mode, a "broadcast parameter"
message exchange is utilized to communicate the necessary broadcast
parameters to the client. Subsequently, "broadcast time
synchronization" message exchanges are utilized in combination with
optional "broadcast keycheck" exchanges to protect authenticity and
integrity of NTP broadcast time synchronization packets. This is
also achieved by MACs.
5. Protocol Sequence
5.1. The Client
5.1.1. The Client in Unicast Mode
For a unicast run, the client performs the following steps:
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1. It sends a client_assoc message to the server. It MUST keep the
transmitted nonce as well as the values for the version number
and algorithms available for later checks.
2. It waits for a reply in the form of a server_assoc message.
After receipt of the message it performs the following checks:
* The client checks that the message contains a conforming
version number.
* It checks that the nonce sent back by the server matches the
one transmitted in client_assoc,
* It also verifies that the server has chosen the encryption and
hash algorithms from its proposal sent in the client_assoc
message and that this proposal was not altered.
* Furthermore, it performs authenticity checks on the
certificate chain and the signature.
If one of the checks fails, the client MUST abort the run.
Discussion:
Note that by performing the above message exchange and checks,
the client validates the authenticity of its immediate NTP
server only. It does not recursively validate the
authenticity of each NTP server on the time synchronization
chain. Recursive authentication (and authorization) as
formulated in RFC 7384 [RFC7384] depends on the chosen trust
anchor.
3. Next it sends a client_cook message to the server. The client
MUST save the included nonce until the reply has been processed.
4. It 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.
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* 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.
5. 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.
6. 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 repeating the exchange of time_request and
time_response messages.
The client's behavior in unicast mode is also expressed in Figure 1.
5.1.2. The Client in Broadcast Mode
To establish a secure broadcast association with a broadcast server,
the client MUST initially authenticate the broadcast server and
securely synchronize its time with it up to an upper bound for its
time offset in unicast mode. After that, the client performs the
following steps:
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1. It 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. It waits for a reply in the form of a server_bpar message after
which it performs the following checks:
* The message must contain all the necessary information for the
TESLA protocol, as specified for a server_bpar message.
* 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.
3. The client awaits time synchronization data in the form of a
server_broadcast message. Upon receipt, it performs the
following checks:
1. 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: it sends a client_keycheck message and waits
for the appropriate response. Note that it needs to memorize
the nonce and the time interval number that it sends as a
correlated pair. For more detail on both of the mentioned
timeliness checks, see [I-D.ietf-ntp-network-time-security].
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.
2. 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 verified, 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.
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3. 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 it is released from the buffer and the packet's
time information can be utilized. If the verification fails,
then 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. 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.
The client MUST restart the broadcast sequence with a client_bpar
message ([I-D.ietf-ntp-network-time-security]) if the one-way key
chain expires.
The client's behavior in broadcast mode can also be seen in Figure 2.
5.2. The Server
5.2.1. The Server in Unicast Mode
To support unicast mode, the server MUST be ready to perform the
following actions:
o 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 [I-D.ietf-ntp-network-time-security].
o 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 4.1. With this, it MUST construct a server_cook message
as described in [I-D.ietf-ntp-network-time-security].
o 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
[I-D.ietf-ntp-network-time-security].
The server MUST refresh its server seed periodically (see
[I-D.ietf-ntp-network-time-security]).
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5.2.2. The Server in Broadcast Mode
A broadcast server MUST also support unicast mode in order to provide
the initial time synchronization, which is a precondition for any
broadcast association. To support NTS broadcast, the server MUST
additionally be ready to perform the following actions:
o Upon receipt of a client_bpar message, the server constructs and
sends a server_bpar message as described in
[I-D.ietf-ntp-network-time-security].
o 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
[I-D.ietf-ntp-network-time-security]. For more details, see also
[I-D.ietf-ntp-network-time-security].
o The server follows the TESLA protocol in all other aspects, by
regularly sending server_broad messages as described in
[I-D.ietf-ntp-network-time-security], adhering to its own
disclosure schedule.
It is also the server's responsibility to watch for the expiration
date of the one-way key chain and generate a new key chain
accordingly.
6. Implementation Notes: ASN.1 Structures and Use of the CMS
This section presents some hints about the structures of the
communication packets for the different message types when one wishes
to implement NTS for NTP. See document
[I-D.ietf-ntp-cms-for-nts-message] for descriptions of the archetypes
for CMS structures as well as for the ASN.1 structures that are
referenced here.
6.1. Unicast Messages
6.1.1. Association Messages
6.1.1.1. Message Type: "client_assoc"
This message is realized as an NTP packet with an extension field
which holds an "NTS-Plain" archetype CMS structure. This structure
contains in its core an NTS message object of the type
"ClientAssociationData", which holds all the data necessary for the
NTS security mechanisms.
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6.1.1.2. Message Type: "server_assoc"
Like "client_assoc", this message is realized as an NTP packet with
an extension field which holds an "NTS-Plain" archetype CMS
structure. This structure contains in its core an NTS message object
of the type "ServerAssociationData". The latter holds all the data
necessary for NTS.
6.1.2. Cookie Messages
6.1.2.1. Message Type: "client_cook"
This message type is realized as an NTP packet with an extension
field which holds a CMS structure of archetype "NTS-Certified",
containing in its core an NTS message object of the type
"ClientCookieData". The latter holds all the data necessary for the
NTS security mechanisms.
6.1.2.2. Message Type: "server_cook"
This message type is realized as an NTP packet with an extension
field containing a CMS structure of archetype "NTS-Signed-and-
Encrypted". The NTS message object in that structure is a
"ServerCookieData" object, which holds all data required by NTS for
this message type.
6.1.3. Time Synchronization Messages
6.1.3.1. Message Type: "time_request"
This message type is realized as an NTP packet which actually
contains regular NTP time synchronization data, as an unsecured NTP
packet from a client to a server would. Furthermore, the packet has
an extension field which contains an ASN.1 object of type
"TimeRequestSecurityData" (packed in a CMS structure of archetype
"NTS-Plain"), whose structure is as follows:
6.1.3.2. Message Type: "time_response"
This message is also realized as an NTP packet with regular NTP time
synchronization data. The packet also has an extension field which
contains an ASN.1 object of type "TimeResponseSecurityData".
Finally, this NTP packet has a MAC field which contains a Message
Authentication Code generated over the whole packet (including the
extension field).
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6.2. Broadcast Messages
6.2.1. Broadcast Parameter Messages
6.2.1.1. Message Type: "client_bpar"
This first broadcast message is realized as an NTP packet which is
empty except for an extension field which contains an ASN.1 object of
type "BroadcastParameterRequest" (packed in a CMS structure of
archetype "CMS-Plain"). This is sufficient to transport all data
specified by NTS.
6.2.1.2. Message Type: "server_bpar"
This message type is realized as an NTP packet whose extension field
carries the necessary CMS structure (archetype: "NTS-Signed"). The
NTS message object in this case is an ASN.1 object of type
"BroadcastParameterResponse".
6.2.2. Broadcast Time Synchronization Message
6.2.2.1. Message Type: "server_broad"
This message's realization works via an NTP packet which carries
regular NTP broadcast time data as well as an extension field, which
contains an ASN.1 object of type "BroadcastTime" (packed in a CMS
structure with archetype "NTS-Plain"). In addition to all this, this
packet has a MAC field which contains a Message Authentication Code
generated over the whole packet (including the extension field).
6.2.3. Broadcast Keycheck
6.2.3.1. Message Type: "client_keycheck"
This message is realized as an NTP packet with an extension field,
which transports a CMS structure of archetype "NTS-Plain", containing
an ASN.1 object of type "ClientKeyCheckSecurityData".
6.2.3.2. Message Type: "server_keycheck"
This message is also realized as an NTP packet with an extension
field, which contains an ASN.1 object of type
"ServerKeyCheckSecurityData" (packed in a CMS structure of archetype
"NTS-Plain"). Additionally, this NTP packet has a MAC field which
contains a Message Authentication Code generated over the whole
packet (including the extension field).
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7. Security Considerations
7.1. Usage of NTP Pools
The certification-based authentication scheme described in
[I-D.ietf-ntp-network-time-security] is not applicable to the concept
of NTP pools. Therefore, NTS is unable to provide secure usage of
NTP pools.
7.2. Server Seed Lifetime
tbd
7.3. Supported Hash Algorithms
The list of the hash algorithms supported by the server has to
fulfill the following requirements:
o it MUST NOT include SHA-1 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms.
8. Acknowledgements
The authors would like to thank Russ Housley, Steven Bellovin, David
Mills and Kurt Roeckx for discussions and comments on the design of
NTS. Also, thanks to Harlan Stenn for his technical review and
specific text contributions to this document.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, June 2005.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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9.2. Informative References
[I-D.ietf-ntp-cms-for-nts-message]
Sibold, D., Roettger, S., Teichel, K., and R. Housley,
"Protecting Network Time Security Messages with the
Cryptographic Message Syntax (CMS)", draft-ietf-ntp-cms-
for-nts-message-01 (work in progress), January 2015.
[I-D.ietf-ntp-network-time-security]
Sibold, D., Roettger, S., and K. Teichel, "Network Time
Security", draft-ietf-ntp-network-time-security-07 (work
in progress), March 2015.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, October 2014.
Appendix A. Flow Diagrams of Client Behaviour
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+---------------------+
|Association Messages |
+----------+----------+
|
+------------------------------>o
| |
| v
| +---------------+
| |Cookie Messages|
| +-------+-------+
| |
| o<------------------------------+
| | |
| v |
| +-------------------+ |
| |Time Sync. Messages| |
| +---------+---------+ |
| | |
| v |
| +-----+ |
| |Check| |
| +--+--+ |
| | |
| /------------------+------------------\ |
| v v v |
| .-----------. .-------------. .-------. |
| ( MAC Failure ) ( Nonce Failure ) ( Success ) |
| '-----+-----' '------+------' '---+---' |
| | | | |
| v v v |
| +-------------+ +-------------+ +--------------+ |
| |Discard Data | |Discard Data | |Sync. Process | |
| +-------------+ +------+------+ +------+-------+ |
| | | | |
| | | v |
+-----------+ +------------------>o-----------+
Figure 1: The client's behavior in NTS unicast mode.
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+-----------------------------+
|Broadcast Parameter Messages |
+--------------+--------------+
|
o<--------------------------+
| |
v |
+-----------------------------+ |
|Broadcast Time Sync. Message | |
+--------------+--------------+ |
| |
+-------------------------------------->o |
| | |
| v |
| +-------------------+ |
| |Key and Auth. Check| |
| +---------+---------+ |
| | |
| /----------------*----------------\ |
| v v |
| .---------. .---------. |
| ( Verified ) ( Falsified ) |
| '----+----' '----+----' |
| | | |
| v v |
| +-------------+ +-------+ |
| |Store Message| |Discard| |
| +------+------+ +---+---+ |
| | | |
| v +---------o
| +---------------+ |
| |Check Previous | |
| +-------+-------+ |
| | |
| /--------*--------\ |
| v v |
| .---------. .---------. |
| ( Verified ) ( Falsified ) |
| '----+----' '----+----' |
| | | |
| v v |
| +-------------+ +-----------------+ |
| |Sync. Process| |Discard Previous | |
| +------+------+ +--------+--------+ |
| | | |
+-----------+ +-----------------------------------+
Figure 2: The client's behaviour in NTS broadcast mode.
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Authors' Addresses
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de
Stephen Roettger
Google Inc
Email: stephen.roettger@googlemail.com
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8421
Email: kristof.teichel@ptb.de
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