| NTP Working Group | D. Sibold |
| Internet-Draft | PTB |
| Intended status: Standards Track | S. Röttger |
| Expires: August 27, 2013 | TU-BS |
| February 23, 2013 |
Network Time Protocol: autokey Version 2 Specification
draft-sibold-autokey-02
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.
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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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.
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 [Röttger]. 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].
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 Röttger [Röttger]. The major changes are:
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].
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.
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: B or [RFC4082] for a detailed description of the packet verification process.
See
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:
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:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC6277] | Santesson, S. and P. Hallam-Baker, "Online Certificate Status Protocol Algorithm Agility", RFC 6277, June 2011. |
| [RFC3161] | Adams, C., Cain, P., Pinkas, D. and R. Zuccherato, "Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)", RFC 3161, August 2001. |
| [Röttger] | Röttger, S., "Analysis of the NTP Autokey Procedures", February 2012. |
| [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. |
| [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. |
| [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. |
The following table compares the autokey specifications against the TICTOC security requirements [I-D.ietf-tictoc-security-requirements].
| Section | Requirement from I-D tictoc security-requirements-04 | Requirement level | Autokey V2 |
|---|---|---|---|
| 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) |
Comparison between TICTOC security requirements and autokey.