| NTP Working Group | D. Sibold |
| Internet-Draft | PTB |
| Intended status: Standards Track | S. Röttger |
| Expires: December 30, 2013 | TU-BS |
| June 28, 2013 |
Network Time Security
draft-ietf-ntp-network-time-security-00.txt
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].
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].
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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 [Röttger]. 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].
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].
The objectives of the autokey specifications are as follows:
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:Section 6.5).
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
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.
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.
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:
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.
In the broadcast mode the client requests the following information from the server:
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.
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.
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: [RFC4082] for a detailed description of the packet verification process.
See RFC 4082
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.
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 hash algorithms supported by the server has to fulfil the following requirements:
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).
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.
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an RFC.
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:
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.
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.
| [Röttger] | Röttger, S., "Analysis of the NTP Autokey Procedures", February 2012. |
| [I-D.ietf-tictoc-security-requirements] | Mizrahi, T., "Security Requirements of Time Protocols in Packet Switched Networks", Internet-Draft draft-ietf-tictoc-security-requirements-05, April 2013. |
The following table compares the NTS specifications against the TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
| Section | Requirement from I-D tictoc security-requirements-05 | Requirement level | NTS |
|---|---|---|---|
| 5.1 | Clock Identity Authentication and Authorization | MUST | OK |
| 5.1.1 | Authentication and Authorization of Masters | MUST | OK |
| 5.1.2 | Recursive Authentication and Authorization of Masters (Chain of Trust) | MUST | OK |
| 5.1.3 | Authentication and Authorization of Slaves | MAY | - |
| 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 associations. | SHOULD | OK |
| 5.6 | Performance: no degradation in quality of time transfer. | MUST | OK |
| Performance: lightweight computation | SHOULD | OK | |
| Performance: storage, bandwidth | SHOULD | OK | |
| 5.7 | Confidentiality protection | MAY | - |
| 5.8 | Protection against Packet Delay and Interception Attacks | SHOULD | - |
| 5.9.1 | Secure mode | MUST | OK (NTP) |
| 5.9.2 | Hybrid mode | MAY | OK (NTP) |