from Alice's Adventures in Wonderland, Lewis Carroll
Our resident cryptographer; now you see him, now you don't.
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Authentication support allows the NTP client to verify that the server is in fact known and trusted and not an intruder intending accidentally or on purpose to masquerade as that server. The NTPv3 specification RFC-1305 defines a scheme which provides cryptographic authentication of received NTP packets. Originally, this was done using the Data Encryption Standard (DES) algorithm operating in Cipher Block Chaining (CBC) mode, commonly called DES-CBC. Subsequently, this was replaced by the RSA Message Digest 5 (MD5) algorithm using a private key, commonly called keyed-MD5. Either algorithm computes a message digest, or one-way hash, which can be used to verify the server has the correct private key and key identifier.
NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryptography, and, in addition, provides a new Autokey scheme based on public key cryptography. Public key cryptography is generally considered more secure than symmetric key cryptography, since the security is based on a private value which is generated by each host and never revealed. With the exception of the group key described later, all key distribution and management functions involve only public values, which considerably simplifies key distribution and storage. Public key management is based on X.509 certificates, which can be provided by commercial services or produced by utility programs in the OpenSSL software library or the NTPv4 distribution.
While the algorithms for symmetric key cryptography are included in the NTPv4 distribution, public key cryptography requires the OpenSSL software library to be installed before building the NTP distribution. This library is available from http://www.openssl.org and can be installed using the procedures outlined in the Building and Installing the Distribution page. Once installed, the configure and build process automatically detects the library and links the library routines required.
Authentication is configured separately for each association using the key or autokey subcommand on the peer, server, broadcast and manycastclient configuration commands as described in the Configuration Options page. The authentication options described below specify the locations of the key files, if other than default, which symmetric keys are trusted and the interval between various operations, if other than default.
Authentication is always enabled, although ineffective if not configured as described below. If a NTP packet arrives including a message authentication code (MAC), it is accepted only if it passes all cryptographic checks. The checks require correct key ID, key value and message digest. If the packet has been modified in any way or replayed by an intruder, it will fail one or more of these checks and be discarded. Furthermore, the Autokey scheme requires a preliminary protocol exchange to obtain the server certificate, verify its credentials and initialize the protocol
The auth flag controls whether new associations or remote configuration commands require cryptographic authentication. This flag can be set or reset by the enable and disable commands and also by remote configuration commands sent by a ntpdc program running on another machine. If this flag is enabled, which is the default case, new broadcast/manycast client and symmetric passive associations and remote configuration commands must be cryptographically authenticated using either symmetric key or public key cryptography. If this flag is disabled, these operations are effective even if not cryptographic authenticated. It should be understood that operating with the auth flag disabled invites a significant vulnerability where a rogue hacker can masquerade as a truechimer and seriously disrupt system timekeeping. It is important to note that this flag has no purpose other than to allow or disallow a new association in response to new broadcast and symmetric active messages and remote configuration commands and, in particular, the flag has no effect on the authentication process itself.
The security model and protocol schemes for both symmetric key and public key cryptography are summarized below; further details are in the briefings, papers and reports at the NTP project page linked from www.ntp.org.
When ntpd is first started, it reads the key file specified in the keys configuration command and installs the keys in the key cache. However, individual keys must be activated with the trustedkey command before use. This allows, for instance, the installation of possibly several batches of keys and then activating or deactivating each batch remotely using ntpdc. This also provides a revocation capability that can be used if a key becomes compromised. The requestkey command selects the key used as the password for the ntpdc utility, while the controlkey command selects the key used as the password for the ntpq utility.
NTPv4 supports the original NTPv3 symmetric key scheme described in RFC-1305 and in addition the Autokey protocol, which is based on public key cryptography. The Autokey Version 2 protocol described on the Autokey Protocol page verifies packet integrity using MD5 message digests and verifies the source with digital signatures and any of several digest/signature schemes. Optional identity schemes described on the Identity Schemes page and based on cryptographic challenge/response algorithms are also available. Using these schemes provides strong security against replay with or without modification, spoofing, masquerade and most forms of clogging attacks.
The Autokey protocol has several modes of operation corresponding to the various NTP modes supported. Most modes use a special cookie which can be computed independently by the client and server, but encrypted in transmission. All modes use in addition a variant of the S-KEY scheme, in which a pseudo-random key list is generated and used in reverse order. These schemes are described along with an executive summary, current status, briefing slides and reading list on the Autonomous Authentication page.
The specific cryptographic environment used by Autokey servers and clients is determined by a set of files and soft links generated by the ntp-keygen program. This includes a required host key file, required host certificate file and optional sign key file, leapsecond file and identity scheme files. The digest/signature scheme is specified in the X.509 certificate along with the matching sign key. There are several schemes available in the OpenSSL software library, each identified by a specific string such as md5WithRSAEncryption, which stands for the MD5 message digest with RSA encryption scheme. The current NTP distribution supports all the schemes in the OpenSSL library, including those based on RSA and DSA digital signatures.
NTP secure groups can be used to define cryptographic compartments and security hierarchies. It is important that every host in the group be able to construct a certificate trail to one or more trusted hosts in the same group. Each group host runs the Autokey protocol to obtain the certificates for all hosts along the trail to one or more trusted hosts. This requires the configuration file in all hosts to be engineered so that, even under anticipated failure conditions, the NTP subnet will form such that every group host can find a trail to at least one trusted host.
It is important to note that Autokey does not use DNS to resolve addresses, since DNS can't be completely trusted until the name servers have synchronized clocks. The cryptographic name used by Autokey to bind the host identity credentials and cryptographic values must be independent of interface, network and any other naming convention. The name appears in the host certificate in either or both the subject and issuer fields, so protection against DNS compromise is essential.
By convention, the name of an Autokey host is the name returned by the Unix gethostname() system call or equivalent in other systems. By the system design model, there are no provisions to allow alternate names or aliases. However, this is not to say that DNS aliases, different names for each interface, etc., are constrained in any way.
It is also important to note that Autokey verifies authenticity using the host name, network address and public keys, all of which are bound together by the protocol specifically to deflect masquerade attacks. For this reason Autokey includes the source and destinatino IP addresses in message digest computations and so the same addresses must be available at both the server and client. For this reason operation with network address translation schemes is not possible. This reflects the intended robust security model where government and corporate NTP servers are operated outside firewall perimeters.
Autokey has an intimidating number of options, most of which are not necessary in typical scenarios. The simplest configuration consists of a subnet with one or more servers at the same low stratum acting as trusted hosts and with dependent clients at higher strata and sharing a single secure group and identity scheme. Each trusted host generates a host key, trusted certificate and group key. Each client generates a host key, normal certificate and installs the group key of each trusted host using secure means and renames it as the name of the trusted host.
For example, trusted host Alice generates keys using
ntp-keygen -H -T -I -p xyz
where H specifies a new host key, T the trusted certificate, I the IFF identity scheme and p the password used to encrypt the private key files. The group key file is ntpkey_IFFpar_alice.filestamp, where filestamp represents the NTP time in seconds when the file was generated.
Host Bob generate keys using
ntp-keygen -H -p abc
where abc is different for each group host. The trusted host generates a password-protected group key using
ntp-keygen -q xyz -p abc -e >temp
where xyz is the trusted host password, abc is the password supplied by the client and temp is a temporary file. This file is transmitted to Bob using secure means and renamed to the fully qualified host name for Alice preceded by the string ntpkey_iff_.
A specific combination of authentication scheme (none, symmetric key, public key) and identity scheme is called a cryptotype, although not all combinations are compatible. There may be management configurations where the clients, servers and peers may not all support the same cryptotypes. A secure NTPv4 subnet can be configured in many ways while keeping in mind the principles explained above and in this section. Note however that some cryptotype combinations may successfully interoperate with each other, but may not represent good security practice.
The cryptotype of an association is determined at the time of mobilization, either at configuration time or some time later when a message of appropriate cryptotype arrives. When mobilized by a server or peer configuration command and no key or autokey subcommands are present, the association is not authenticated; if the key subcommand is present, the association is authenticated using the symmetric key ID specified; if the autokey subcommand is present, the association is authenticated using Autokey.
The cryptographic values used by the Autokey protocol are incorporated as a set of files generated by the ntp-keygen utility program, including symmetric key, host key and public certificate files, as well as sign key, identity parameters and leapseconds files. Alternatively, host and sign keys and certificate files can be generated by the OpenSSL utilities and certificates can be imported from public certificate authorities. Note that symmetric keys are necessary for the ntpq and ntpdc utility programs. The remaining files are necessary only for the Autokey protocol.
Certificates imported from OpenSSL or public certificate authorities have certian limitations. The certificate should be in ASN.1 syntax, X.509 Version 3 format and encoded in PEM, which is the same format used by OpenSSL. The overall length of the certificate encoded in ASN.1 must not exceed 1024 bytes. The subject distinguished name field (CN) is the fully qualified name of the host on which it is used; the remaining subject fields are ignored. The certificate extension fields must not contain either a subject key identifier or a issuer key identifier field; however, an extended key usage field for a trusted host must contain the value trustRoot;. Other extension fields are ignored.
Errors can occur due to mismatched configurations, unexpected restarts, expired certificates and unfriendly people. In most cases the protocol state machine recovers automatically by retransmission, timeout and restart, where necessary. Some errors are due to mismatched keys, digest schemes or identity schemes and must be corrected by installing the correct media and/or correcting the configuration file. One of the most common errrors is expired certificates, which must be regenerated and signed at least once per year using the ntp-keygen program.
The following error codes are reported via the NTP control and monitoring protocol trap mechanism.
See the ntp-keygen page.
The NIST provides a file documenting the epoch for all historic occasions of leap second insertion since 1972. The leapsecond table shows each epoch of insertion along with the offset of International Atomic Time (TAI) with respect to Coordinated Universal Time (UTC), as disseminated by NTP. The table can be obtained directly from NIST national time servers using ftp as the ASCII file pub/leap-seconds.
While not strictly a security function, the Autokey protocol provides means to securely retrieve the leapsecond table from a server or peer. Servers load the leapsecond table directly from the file specified in the crypto command, with default ntpkey_leap, while clients can obtain the table indirectly from the servers using the Autokey protocol. Once loaded, the table can be provided on request to other clients and servers.