Network Time Protocol: autokey Version 2 Specification

In NTP the autokey protocol 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 . 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 .

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 . The major changes are: The bit length of server seed and cookie has been increased. The IP addresses of the synchronization partners in the calculation of the cookie have been replaced by the hash value of the client's public key. The identity schemes for the verification of the NTP server authenticity have been replaced by a hierarchical public key infrastructure (PKI) based on X.509 certificates.

A profound analysis of security threats and requirements for NTP and Precision Time Protocol (PTP) can be found in the I-D .

The objectives of the autokey specifications are as follows: Authenticity: Autokey enables the client to authenticate its NTP server or peer. Integrity: Autokey protects the integrity of time synchronization packets via a message authentication code (MAC). Confidentiality: Autokey does not provide confidentiality protection of the NTP packets. Modes of operation: All operational modes of NTP are supported (Client-Server, symmetric, broadcast). Hybrid mode: Both secure and insecure communication modes are possible for NTP servers and clients, respectively. 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.

Throughout this document the term "autokey" refers to autokey version 2.

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. To comply with 4.5.3 in the server seed has to be changed 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 ).

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: 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 requirement 4.1.2 of

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 ). The response of the server contains the cookie encrypted with the public key.

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.

Hash algorithms are used at three 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 support by the server. This list is notified during association message exchange (). The selected algorithm is used for all hashing processes in the protocol. The list of the server supported hash algorithms has to fulfill following requirements: it MUST NOT contain the MD5 or weaker algorithms, it MUST include SHA-256 or stronger algorithms.

For the calculation of the MAC client and server are using a Keyed-Hash Message Authentication Code (HMAC) approach . The HMAC is generated with the hash algorithm specified by the client (see ).

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 (). 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: The validity of the certificates is preconditioned. Usually this will be the case in corporation networks. The client ensures that the certificates are not revoked. To this end, the client uses the Online Certificate Status Protocol (OCSP) defined in . 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 .

Analysis of the NTP Autokey Procedures

The following table compares the autokey specifications against the tictoc security requirements . Section Requirement from I-D tictoc security-requirements-02 Type Autokey V2 4.1 Authentication of sender. MUST OK Authentication of master. MUST OK Recursive authentication MUST Open 1) Authentication of slaves. SHOULD OK PTP: Authentication of TCs. SHOULD N/A PTP: Authentication of Announce messages. SHOULD N/A 4.2 Integrity protection. MUST OK PTP: hop-by-hop integrity protection. MUST N/A PTP: end-to-end integrity protection. SHOULD N/A 4.3 Protection against DoS attacks. MUST NTP 2) 4.4 Replay protection. MUST NTP 2) 4.5 Security association. MUST OK Unicast and multicast associations. MUST OK Key freshness. MUST OK 4.6 Performance: no degradation in quality of time transfer. MUST OK Performance: lightweight. SHOULD YES Performance: storage, bandwidth. MUST OK 4.7 Confidentiality protection. MAY NO Protection against delay attacks. MAY NO 4.9 Secure mode. MUST NTP? 3) Hybrid mode. MAY YES 1) Refer to discussion in . 2) These requirements are fulfilled by the NTP on-wire protocol. 3) To be checked.