draft-ietf-ntp-network-time-security-00.txt   draft-ietf-ntp-network-time-security-01.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: December 29, 2013 TU-BS Expires: April 21, 2014 TU-BS
June 27, 2013 K. Teichel
PTB
October 18, 2013
Network Time Security Network Time Security
draft-ietf-ntp-network-time-security-00.txt draft-ietf-ntp-network-time-security-01.txt
Abstract Abstract
This document describes the Network Time Security (NTS) protocol that This document describes the Network Time Security (NTS) protocol that
enables secure authentication of time servers using Network Time enables secure authentication of time servers using Network Time
Protocol (NTP) or Precision Time Protocol (PTP). Its design Protocol (NTP) or Precision Time Protocol (PTP). Its design
considers the special requirements of precise timekeeping, which are considers the special requirements of precise timekeeping, which are
described in Security Requirements of Time Protocols in Packet described in Security Requirements of Time Protocols in Packet
Switched Network [I-D.ietf-tictoc-security-requirements]. Switched Network [I-D.ietf-tictoc-security-requirements].
skipping to change at page 1, line 42 skipping to change at page 1, line 44
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This Internet-Draft will expire on December 29, 2013. This Internet-Draft will expire on April 21, 2014.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 3 2. Security Threats . . . . . . . . . . . . . . . . . . . . . . 3
3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4 4. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4
5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 5. NTS Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 4 5.1. Symmetric and Client/Server Mode . . . . . . . . . . . . 4
5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 4 5.2. Broadcast Mode . . . . . . . . . . . . . . . . . . . . . 5
6. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 5 6. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Association Message . . . . . . . . . . . . . . . . . . . 5 6.1. Association Message . . . . . . . . . . . . . . . . . . . 5
6.2. Certificate Message . . . . . . . . . . . . . . . . . . . 5 6.2. Certificate Message . . . . . . . . . . . . . . . . . . . 5
6.3. Cookie Message . . . . . . . . . . . . . . . . . . . . . 6 6.3. Cookie Message . . . . . . . . . . . . . . . . . . . . . 6
6.4. Broadcast Parameter Message . . . . . . . . . . . . . . . 6 6.4. Broadcast Parameter Message . . . . . . . . . . . . . . . 6
6.5. Time Request Message . . . . . . . . . . . . . . . . . . 6 6.5. Time Request Message . . . . . . . . . . . . . . . . . . 7
6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 6 6.6. Broadcast Message . . . . . . . . . . . . . . . . . . . . 7
6.7. Restart of the Protocol Sequence . . . . . . . . . . . . 7 6.7. Server Seed Refresh . . . . . . . . . . . . . . . . . . . 7
7. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 7 7. Hash Algorithms and MAC Generation . . . . . . . . . . . . . 8
7.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 7 7.1. Hash Algorithms . . . . . . . . . . . . . . . . . . . . . 8
7.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 8 7.2. MAC Calculation . . . . . . . . . . . . . . . . . . . . . 8
8. Server Seed Considerations . . . . . . . . . . . . . . . . . 8 8. Server Seed Considerations . . . . . . . . . . . . . . . . . 8
8.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 8 8.1. Server Seed Algorithm . . . . . . . . . . . . . . . . . . 9
8.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 8 8.2. Server Seed Live Time . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8 10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Initial Verification of the Server Certificates . . . . 8 10.1. Initial Verification of the Server Certificates . . . . 9
10.2. Revocation of Server Certificates . . . . . . . . . . . 9 10.2. Revocation of Server Certificates . . . . . . . . . . . 9
10.3. Denial-of-Service in Broadcast Mode . . . . . . . . . . 9 10.3. Usage of NTP Pools . . . . . . . . . . . . . . . . . . . 10
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 10.4. Denial-of-Service in Broadcast Mode . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10 12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 10 12.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 10 Appendix A. TICTOC Security Requirements . . . . . . . . . . . . 11
Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . 11 Appendix B. Broadcast Mode . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Time synchronization protocols are more and more utilized to Time synchronization protocols are more and more utilized to
synchronize clocks in networked infrastructures. The reliable synchronize clocks in networked infrastructures. The reliable
performance of such infrastructures can be degraded seriously by performance of such infrastructures can be degraded seriously by
successful attacks against the time synchronization protocol. successful attacks against the time synchronization protocol.
Therefore, time synchronization protocols applied in critical Therefore, time synchronization protocols applied in critical
infrastructures have to provide security measures to defeat possible infrastructures have to provide security measures to defeat possible
adversaries. Consequently, the widespread Network Time Protocol adversaries. Consequently, the widespread Network Time Protocol
(NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906] (NTP) [RFC5905] was supplemented by the autokey protocol [RFC5906]
which shall ensure authenticity of the NTP server and integrity of which shall ensure authenticity of the NTP server and integrity of
the protocol packets. Unfortunately,the autokey protocol exhibits the protocol packets. Unfortunately,the autokey protocol exhibits
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the protocol packets. Unfortunately,the autokey protocol exhibits the protocol packets. Unfortunately,the autokey protocol exhibits
various severe security vulnerabilities as revealed in a thorough various severe security vulnerabilities as revealed in a thorough
analysis of the protocol [Roettger]. For the Precision Time Protocol analysis of the protocol [Roettger]. For the Precision Time Protocol
(PTP) Annex K of the standard document IEEE 1588 [IEEE1588] defines (PTP) Annex K of the standard document IEEE 1588 [IEEE1588] defines
an informative security protocol that is still in experimental state. an informative security protocol that is still in experimental state.
Because of autokey's security vulnerabilities and the absence of a Because of autokey's security vulnerabilities and the absence of a
standardized security protocol for PTP these protocols cannot be standardized security protocol for PTP these protocols cannot be
applied in environments in which compliance requirements demand applied in environments in which compliance requirements demand
authenticity and integrity protection. This document specifies a authenticity and integrity protection. This document specifies a
security protocol that ensures authenticity of the time server and security protocol which ensures authenticity of the time server via a
integrity of the time synchronisations protocol packets and hence Public Key Infrastructure and integrity of the time synchronization
enables the usage of NTP and PTP in such environments. protocol packets and which therefore enables the usage of NTP and PTP
in such environments.
The protocol is specified with the prerequisite in mind that precise The protocol is specified with the prerequisite in mind that precise
timekeeping can only be accomplished with stateless time timekeeping can only be accomplished with stateless time
synchronization communication, which excludes standard security synchronization communication, which excludes standard security
protocols like IPSec or TLS. This prerequisite corresponds with the protocols like IPsec or TLS. This prerequisite corresponds with the
requirement that a security mechanism for timekeeping must be 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 [I-D.ietf-tictoc-security-requirements]. time transfer [I-D.ietf-tictoc-security-requirements].
Note:
It is intended to formulate the protocol to be applicable to NTP
as well as PTP. In the current state the draft focuses on the
application to NTP.
2. Security Threats 2. Security Threats
A profound analysis of security threats and requirements for NTP and 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]. PTP can be found in the I-D [I-D.ietf-tictoc-security-requirements].
3. Objectives 3. Objectives
The objectives of the autokey specifications are as follows: The objectives of the NTS specifications are as follows:
o Authenticity: NTS enables the client to authenticate its time o Authenticity: NTS enables the client to authenticate its time
server server.
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 Modes of operation: All operational modes of NTP are supported o Modes of operation: All operational modes of NTP are supported.
o Operational modes of PTP should be supported as far as possible.
o Hybrid mode: Both secure and insecure communication modes are o Hybrid mode: Both secure and insecure communication modes are
possible for NTP servers and clients, respectively. possible for NTP servers and clients, respectively.
o Compatibility: o Compatibility:
* Unsecured NTP associations shall not be affected. * Unsecured NTP associations shall not be affected.
* An NTP server that does not support NTS shall not be affected * An NTP server that does not support NTS shall not be affected
by NTS authentication requests. by NTS authentication requests.
4. Terms and Abbreviations 4. Terms and Abbreviations
o TESLA: Time efficient stream loss-tolerant authentication o TESLA: Time efficient stream loss-tolerant authentication
5. NTS Overview 5. NTS Overview
5.1. Symmetric and Client/Server Mode 5.1. Symmetric and Client/Server Mode
Authenticity and integrity of the NTP packets are ensured by a Authenticity of the time server is verified once by a Public Key
Message Authentication Code (MAC), which is attached to the NTP Infrastructure. Authenticity and integrity of the NTP packets are
packet. The calculation of the MAC includes the whole NTP packet and then ensured by a Message Authentication Code (MAC), which is
the cookie which is shared between client and server. It is attached to the NTP packet. The calculation of the MAC includes the
calculated according to: 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))), cookie = MSB_128 (H(server seed || H(public key of client))),
where || indicates concatenation and in which H is a hash algorithm. where || indicates concatenation and in which H is a hash algorithm.
The function MSB_128 cuts off the 128 most significant bits of the 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 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 value of the server, which has to be kept secret. The cookie thus
never changes. The server seed has to be refreshed periodically. never changes as long as the server seed stays the same. The server
seed has to be refreshed periodically in order to provide key
freshness as required in [I-D.ietf-tictoc-security-requirements].
The server does not keep a state of the client. Therefore it has to 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 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 client. To this end, the client has to attach the hash value of its
public key to each request (see Section 6.5). public key to each request (see Section 6.5).
5.2. Broadcast Mode 5.2. Broadcast Mode
Just as in the case of the client server mode and symmetric 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, authenticity and integrity of the NTP packets are ensured by a MAC,
which is attached to the NTP packet by the sender. The verification which is attached to the NTP packet by the sender. The verification
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last element of the chain is shared securely with all clients. The last element of the chain is shared securely with all clients. The
server splits time into intervals of uniform duration and assigns server splits time into intervals of uniform duration and assigns
each key to an interval in reverse order, starting with the each key to an interval in reverse order, starting with the
penultimate. At each time interval, the server sends an NTP penultimate. At each time interval, the server sends an NTP
broadcast packet appended by a MAC, calculated using the broadcast packet appended by a MAC, calculated using the
corresponding key, and the key of the previous interval. The client corresponding key, and the key of the previous interval. The client
verifies the MAC by buffering the packet until the disclosure of the 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 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 of the key, the client has to be loosely time synchronized to the
server. This has to be accomplished during the initial client server server. This has to be accomplished during the initial client server
exchange between broadcast client and server. exchange between broadcast client and server. For a more detailed
description of the TESLA protocol see Appendix B.
6. Protocol Sequence 6. Protocol Sequence
6.1. Association Message 6.1. Association Message
The protocol sequence starts with the association message, in which The protocol sequence starts with the association message, in which
the client sends an NTP packet with an extension field of type the client sends an NTP packet with an extension field of type
association. It contains the hostname of the client and a status association. It contains the hostname of the client and a status
word which contains the algorithms used for the signatures and the word which contains the algorithms used for the signatures and the
status of the connection. The response contains the hostname of the status of the connection. The response contains the hostname of the
server and the algorithms for the signatures. The server notifies server and the algorithms for the signatures. The server notifies
the cryptographic hash algorithms which it supports. the cryptographic hash algorithms which it supports.
6.2. Certificate Message 6.2. Certificate Message
In this step, the client receives the certification chain up to the In this step, the client receives the certification chain up to a
trusted authority (TA). To this end, the client requests the trusted authority (TA). To this end, the client requests the
certificate for the subject name (hostname) of the NTP server. The certificate for the subject name (hostname) of the NTP server. The
response contains the certificate with the issuer name. If the response contains the certificate with the issuer name. If the
issuer name is different from the subject name, the client requests issuer name is different from the subject name, the client requests
the certificate for the issuer. This continues until it receives a the certificate for the issuer. This continues until it receives a
certificate which is issued by a TA. The client recognizes the TA certificate in which the subject name and the issuer name are
because it has a list of certificates which are accepted as TAs. The identical, which indicates that it is issued by a TA. The client
client has to check that each issuer is authorized to issue new then checks that the issuer is indeed on its list of issuers which
certificates. To this end, the certificates have to include the are accepted as TAs. The client has to check that each issuer in the
X.509v3 extension field "CA:TRUE". With the established certificate chain is authorized to issue new certificates. To this
certification chain the client is able to verify the server end, the certificates have to include the X.509v3 extension field
signatures and, hence, the authenticity of the server messages with "CA:TRUE". With the established certification chain the client is
extension fields is ensured. able to verify the server signatures and, hence, the authenticity of
the server messages with extension fields is ensured.
Discussion: Discussion:
Note that in this step the client validates the authenticity of Note that in this step the client validates the authenticity of
its NTP server only. It does not recursively validate the its NTP server only. It does not recursively validate the
authenticity of each NTP server on the time synchronization chain. authenticity of each NTP server on the time synchronization chain.
But each NTP server on the time synchronization chain validates But each NTP server on the time synchronization chain validates
the NTP server to which it is synchronized. This conforms to the the NTP server to which it is synchronized. This conforms to the
recursive authentication requirement in the TICTOC security recursive authentication requirement in the TICTOC security
requirements [I-D.ietf-tictoc-security-requirements]. requirements [I-D.ietf-tictoc-security-requirements].
6.3. Cookie Message 6.3. Cookie Message
The client requests a cookie from the server. It selects a hash The client requests a cookie from the server. It selects a hash
algorithm from the list of algorithms supported by the server. The algorithm from the list of algorithms supported by the server. The
request includes its public key and the selected hash algorithm. 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 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 (see Section 5.1). The response of the server contains the cookie
encrypted with the public key. and a signature of the cookie signed with the server's private key,
both encrypted with the client's public key.
6.4. Broadcast Parameter Message 6.4. Broadcast Parameter Message
In the broadcast mode the client requests the following information In the broadcast mode the client requests the following information
from the server: from the server:
o the last key of the one-way key chain, o the last key of the one-way key chain,
o the disclosure schedule of the following keys. This contains: o the disclosure schedule of the following keys. This contains:
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will start and its associated index, will start and its associated index,
* key disclosure delay (number of time intervals for which a key * key disclosure delay (number of time intervals for which a key
is valid). is valid).
The server will sign all transmitted properties so that the client is The server will sign all transmitted properties so that the client is
able to verify their authenticity. For this packet exchange a new able to verify their authenticity. For this packet exchange a new
extension field "broadcast parameters" is used. The client extension field "broadcast parameters" is used. The client
synchronizes its time with the server in the client server mode and 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 saves an upper bound of its time offset with respect to the time of
the server. See B for more details. the server. See Appendix B for more details.
6.5. Time Request Message 6.5. Time Request Message
The client request includes a new extension field "time request" The client request includes a new extension field "time request"
which contains the hash of its public key. The server needs the hash which contains the hash of its public key, a 128-bit nonce, and the
of the public key to recalculate the cookie for the client. The chosen hash algorithm . The server needs the hash of the public key
response is a normal NTP packet without extension field. It contains and the notified hash algorithm to recalculate the cookie for the
a MAC. client. The response is a NTP packet with a new extension field
"time response" which contains the nonce and a MAC generated over the
time synchronization data, the cookie and the nonce.
6.6. Broadcast Message 6.6. Broadcast Message
In broadcast mode the NTP packet includes a new extension field In broadcast mode the NTP packet includes a new extension field
"broadcast message" which contains the disclosed key of the previous "broadcast message" which contains the disclosed key of the previous
disclosure interval (current time interval minus disclosure delay). disclosure interval (current time interval minus disclosure delay).
The NTP packet is appended by a MAC, calculated with the key for the 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 current time interval. When a client receives a broadcast message it
has to perform the following tests: has to perform the following tests:
o Proof that the MAC is based on a key that is not yet disclosed. 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 This is achieved via a disclosure schedule, so this is where loose
otherwise it has to be discarded. time synchronization is required. If verified the packet will be
buffered for later authentication otherwise it has to be
discarded. Note that the time information included in the packet
will not be used for synchronization until their authenticity
could be verified.
o The client checks whether it already knows the disclosed key. If o The client checks whether it already knows the disclosed key. If
not, the client verifies its legitimacy. If falsified the packet so, the packet is discarded to avoid a buffer overrun. If not,
has to be discarded. the client verifies 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 verified. If falsified the packet has
to be discarded.
o If the disclosed key is legitimate the client verifies the o If the disclosed key is legitimate the client verifies the
authenticity of any packet that it received during the authenticity of any packet that it received during the
corresponding time interval. If authenticity of a packet is corresponding time interval. If authenticity of a packet is
verified it is released from the buffer. If the verification verified it is released from the buffer and the packet's time
fails authenticity is no longer given. In this case the client information can be utilized. If the verification fails
MUST request authentic time from the server by means of a unicast authenticity is no longer given. In this case the client MUST
time request message. 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 See RFC 4082[RFC4082] for a detailed description of the packet
verification process. verification process.
6.7. Restart of the Protocol Sequence 6.7. Server Seed Refresh
According to the requirements in According to the requirements in
[I-D.ietf-tictoc-security-requirements] the server has to refresh its [I-D.ietf-tictoc-security-requirements] the server has to refresh its
server seed periodically. As a consequence the cookie used in the server seed periodically. As a consequence the cookie used in the
time request messages becomes invalid. In this case the server has time request messages becomes invalid. In this case the client
to respond accordingly and the client has to restart the protocol cannot verify the attached MAC and has to respond accordingly by re-
with the association message. This is true for the unicast and initiating the protocol with a cookie request (Section 6.3). This is
broadcast mode, respectively. true for the unicast and broadcast mode, respectively.
Additionally, in broadcast mode the client has to restart the Additionally, in broadcast mode the client has to restart the
broadcast sequence with a time request message if the one-way key broadcast sequence with a time request message if the one-way key
chain expires. chain expires.
During certificate message exchange the client requests the During certificate message exchange the client reads the expiration
expiration date of the period of validity of the server certificate. date of the period of validity of the server certificate. The client
The client MAY restart the protocol sequence with the association MAY restart the protocol sequence with the association message before
message before the server certificate expires. the server certificate expires.
7. Hash Algorithms and MAC Generation 7. Hash Algorithms and MAC Generation
7.1. Hash Algorithms 7.1. 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 public key. The client cookie and the MAC, and hashing of the public key. The client
selects the hash algorithm from the list of hash algorithms which are selects the hash algorithm from the list of hash algorithms which are
supported by the server. This list is notified during the supported by the server. This list is notified during the
association message exchange (Section 6.1). The selected algorithm association message exchange (Section 6.1). The selected algorithm
is used for all hashing processes in the protocol. is used for all hashing processes in the protocol.
In the broadcast mode hash algorithm are used as pseudo random In the broadcast mode hash algorithm are used as pseudo random
function to construct the one-way key chain. functions to construct the one-way key chain.
The list of the hash algorithms supported by the server has to fulfil The list of the hash algorithms supported by the server has to
the following requirements: fulfill the following requirements:
o it MUST NOT contain the MD5 or weaker algorithms, o it MUST NOT include MD5 or weaker algorithms,
o it MUST include SHA-256 or stronger algorithms. o it MUST include SHA-256 or stronger algorithms.
7.2. MAC Calculation 7.2. MAC Calculation
For the calculation of the MAC client and server are using a Keyed- For the calculation of the MAC client and server are using a Keyed-
Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC Hash Message Authentication Code (HMAC) approach [RFC2104]. The HMAC
is generated with the hash algorithm specified by the client (see is generated with the hash algorithm specified by the client (see
Section 7.1). Section 7.1).
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10.1. Initial Verification of the Server Certificates 10.1. 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 6.2). Since it generally has certification message exchange (Section 6.2). Since it generally has
no reliable time during this initial communication phase, it is 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.
Therefore, the client MUST use one of the following approaches: Therefore, the client MUST use one of the following approaches:
o The validity of the certificates is preconditioned. Usually this o The validity of the certificates is preconditioned. Usually this
will be the case in corporation networks. will be the case in corporate networks.
o The client ensures that the certificates are not revoked. To this o The client ensures that the certificates are not revoked. To this
end, the client uses the Online Certificate Status Protocol (OCSP) end, the client uses the Online Certificate Status Protocol (OCSP)
defined in [RFC6277]. defined in [RFC6277].
o The client requests a different service to get an initial time o The client requests a different service to get an initial time
stamp in order to be able to verify the certificates' periods of stamp in order to be able to verify the certificates' periods of
validity. To this end, it can, e.g., use a secure shell validity. To this end, it can, e.g., use a secure shell
connection to a reliable host. Another alternative is to request connection to a reliable host. Another alternative is to request
a time stamp from a Time Stamping Authority (TSA) by means of the a time stamp from a Time Stamping Authority (TSA) by means of the
Time-Stamp Protocol (TSP) defined in [RFC3161]. Time-Stamp Protocol (TSP) defined in [RFC3161].
10.2. Revocation of Server Certificates 10.2. Revocation of Server Certificates
According to Section Section 6.7 it is the client's responsibility to According to Section Section 6.7 it is the client's responsibility to
initiate a new association with the server after the server's initiate a new association with the server after the server's
certificate expires. To this end the client reads the expiration certificate expires. To this end the client reads the expiration
date of the certificate during the certificate message exchange date of the certificate during the certificate message exchange
(Section 6.2). Besides, certificate may also be revoked prior to the (Section 6.2). Besides, certificates may also be revoked prior to
normal expiration date. To increase security the client MAY verify the normal expiration date. To increase security the client MAY
the state of the server's certificate via OCSP periodically. verify the state of the server's certificate via OCSP periodically.
10.3. Denial-of-Service in Broadcast Mode 10.3. Usage of NTP Pools
The certification based authentication scheme described in Section 6
is not applicable to the concept of NTP pools. Therefore, NTS is not
able to provide secure usage of NTP pools.
10.4. Denial-of-Service in Broadcast Mode
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 client and server SHALL use the "Not Re- 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 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 knows already, thus packet that contains a disclosed key it knows already, thus
preventing memory flooding attacks. preventing memory flooding attacks.
skipping to change at page 11, line 4 skipping to change at page 11, line 32
[I-D.ietf-tictoc-security-requirements] [I-D.ietf-tictoc-security-requirements]
Mizrahi, T., "Security Requirements of Time Protocols in Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", draft-ietf-tictoc-security- Packet Switched Networks", draft-ietf-tictoc-security-
requirements-05 (work in progress), April 2013. requirements-05 (work in progress), April 2013.
[Roettger] [Roettger]
Roettger, S., "Analysis of the NTP Autokey Procedures", Roettger, S., "Analysis of the NTP Autokey Procedures",
February 2012. February 2012.
Appendix A. TICTOC Security Requirements Appendix A. TICTOC Security Requirements
The following table compares the NTS specifications against the The following table compares the NTS specifications against the
TICTOC security requirements [I-D.ietf-tictoc-security-requirements]. TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
| Section | Requirement from I-D tictoc | Requirement | NTS | | Section | Requirement from I-D tictoc | Requirement | NTS |
| | security-requirements-05 | level | | | | security-requirements-05 | level | |
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
| 5.1 | Clock Identity Authentication | MUST | OK | | 5.1.1 | Authentication of Servers | MUST | OK |
| | and Authorization | | | | 5.1.1 | Authorization of Servers | MUST | - |
| 5.1.1 | Authentication and | MUST | OK | | 5.1.2 | Recursive Authentication of | MUST | NO |
| | Authorization of Masters | | | | | Servers (Stratum 1) | | |
| 5.1.2 | Recursive Authentication and | MUST | OK | | 5.1.2 | Recursive Authorization of | MUST | - |
| | Authorization of Masters | | | | | Servers (Stratum 1) | | |
| | (Chain of Trust) | | |
| 5.1.3 | Authentication and | MAY | - | | 5.1.3 | Authentication and | MAY | - |
| | Authorization of Slaves | | | | | Authorization of Slaves | | |
| 5.2 | Integrity protection. | MUST | OK | | 5.2 | Integrity protection. | MUST | OK |
| 5.3 | Protection against DoS attacks | SHOULD | - | | 5.3 | Protection against DoS attacks | SHOULD | OK |
| 5.4 | Replay protection | MUST | OK | | 5.4 | Replay protection | MUST | OK |
| | | | (NTP) |
| 5.5.1 | Key freshness. | MUST | OK | | 5.5.1 | Key freshness. | MUST | OK |
| 5.5.2 | Security association. | SHOULD | OK | | 5.5.2 | Security association. | SHOULD | OK |
| 5.5.3 | Unicast and multicast | SHOULD | OK | | 5.5.3 | Unicast and multicast | SHOULD | OK |
| | associations. | | | | | associations. | | |
| 5.6 | Performance: no degradation in | MUST | OK | | 5.6 | Performance: no degradation in | MUST | OK |
| | quality of time transfer. | | | | | quality of time transfer. | | |
| | Performance: lightweight | SHOULD | OK | | | Performance: lightweight | SHOULD | OK |
| | computation | | | | | computation | | |
| | Performance: storage, | SHOULD | OK | | | Performance: storage, | SHOULD | OK |
| | bandwidth | | | | | bandwidth | | |
| 5.7 | Confidentiality protection | MAY | - | | 5.7 | Confidentiality protection | MAY | NO |
| 5.8 | Protection against Packet | SHOULD | - | | 5.8 | Protection against Packet | SHOULD | NA*) |
| | Delay and Interception Attacks | | | | | Delay and Interception Attacks | | |
| 5.9.1 | Secure mode | MUST | OK | | 5.9.1 | Secure mode | MUST | - |
| | | | (NTP) | | 5.9.2 | Hybrid mode | MAY | - |
| 5.9.2 | Hybrid mode | MAY | OK |
| | | | (NTP) |
+---------+--------------------------------+---------------+--------+ +---------+--------------------------------+---------------+--------+
*) Ensured by NTP via multi-source configuration.
Comparsion of NTS sepecification against TICTOC security Comparsion of NTS sepecification against TICTOC security
requirements. requirements.
Appendix B. Broadcast Mode Appendix B. Broadcast Mode
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
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
skipping to change at line 532 skipping to change at page 12, line 42
Germany Germany
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
Stephen Roettger Stephen Roettger
Technische Universitaet Braunschweig Technische Universitaet Braunschweig
Email: stephen.roettger@googlemail.com Email: stephen.roettger@googlemail.com
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Email: kristof.teichel@ptb.de
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