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 servers are in fact known and trusted and not intruders intending accidentally or intentionally to masquerade as a legitimate server. The NTPv3 specification RFC-1305 defines a scheme using the Data Encryption Standard (DES) algorithm, commonly called DES-CBC. Subsequently, this scheme was replaced by the RSA Message Digest 5 (MD5) algorithm, commonly called keyed-MD5. Either algorithm computes a message digest or one-way hash which can be used to verify the client has the same key as the server.
NTPv4 includes the NTPv3 scheme, properly described as symmetric key cryptography and, in addition a new scheme based on public key cryptography and called Autokey. Public key cryptography is generally considered more secure than symmetric key cryptography, since the security is based on private and public values which are generated by each participant and where the private value is never revealed. Autokey uses X.509 public certificates, which can be produced by commercial services, utility programs in the OpenSSL software library or a utility program in the NTP software distribution.
While the algorithms for symmetric key cryptography are included in the NTPv4 software distribution, Autokey 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 separately using the key or autokey option on the peer, server, broadcast or manycastclient configuration commands, as described in the Server Options page, and the options described on this page. The ntp-keygen page describes the files required for the various authentication schemes. Further details are in the briefings, papers and reports at the NTP project page linked from www.ntp.org.
Keys and related information are specified in a keys file, usually called ntp.keys, which must be distributed and stored using secure means beyond the scope of the NTP protocol itself. Besides the keys used for ordinary NTP associations, additional keys can be used as passwords for the ntpq and ntpdc utility programs. Ordinarily, the ntp.keys file is generated by the ntp-keygen program, but it can be constructed using an ordinary text editor.
When ntpd is first started, it reads the key file specified by 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 a key remotely using ntpdc. The requestkey command selects the key ID used as the password for the ntpdc utility, while the controlkey command selects the key ID used as the password for the ntpq utility.
NTPv4 supports the Autokey security 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 using digital signatures and any of several digest/signature schemes. Optional identity schemes described on the Autokey Identity Schemes page are based on cryptographic challenge/response exchanges. Using these schemes provides strong security against replay with or without modification, spoofing, masquerade and most forms of clogging attacks. These schemes are described along with an executive summary, current status, briefing slides and reading list on the Autonomous Authentication page.
Autokey authenticates individual packets using cookies bound to the IP source and destination addresses. The cookies must have the same addresses 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.
NTP secure groups are used to define cryptographic compartments and security hierarchies. All hosts belonging to a secure group have the same group name but different host names. The string specified in the host option of the crypto command is the name of the host and in the name of the host key, sign key and certificate files. The string specified in the ident option of the crypto comand is the group name of all group hosts and in the name of the identity files. The file naming conventions are described on the ntp-keygen page.
Each group includes one or more trusted hosts (THs) operating at the root, or lowest stratum in the group. The group name is used in the subject and issuer fields of the TH trusted certificate. The host name is used in these fields for hosts other than THs.
All group hosts are configured to provide an unbroken path, called a certificate trail, from each host, possibly via intermediate hosts and ending at a TH. When a host starts up, it recursively retrieves the certificates along the trail in order to verify group membership and avoid masquerade and middleman attacks.
Secure groups can be configured as hierarchies where a TH of one group can be a client of one or more other groups operating at a lower stratum. In one scenario, groups RED and GREEN can be cryptographically distinct, but both be clients of group BLUE operating at a lower stratum. In another scenario, group CYAN can be a client of multiple groups YELLOW and MAGENTA, both operating at a lower stratum. There are many other scenarios, but all must be configured to include only acyclic certificate trails.
All configurations include a public/private host key pair and matching certificate. Absent an identity scheme, this is a Trusted Certificate (TC) scheme. There are three identity schemes, IFF, GQ and MV described on the Identity Schemes page. With these schemes all servers in the group have encrypted server identity keys, while clients have nonencrypted client identity parameters. The client parameters can be obtained from a trusted agent (TA), usually one of the THs of the lower stratum group. Further information on identity schemes is on the Autokey Identity Schemes page.
A specific combination of authentication and identity schemes is called a cryptotype, which applies to clients and servers separately. A group can be configured using more than one cryptotype combination, although not all combinations are interoperable. Note however that some cryptotype combinations may successfully interoperate with each other, but may not represent good security practice. The server and client cryptotypes are defined by the the following codes.
The compatible cryptotypes for clients and servers are listed in the following table.
| Client | Server | ||||
| NONE | AUTH | PC | TC | IDENT | |
| NONE | yes | yes* | yes* | yes* | yes* |
| AUTH | no | yes | no | no | no |
| PC | no | no | yes | no | no |
| TC | no | no | no | yes | yes |
| IDENT | no | no | no | no | yes |
* These combinations are not valid if the restriction list includes the notrust option.
Autokey has an intimidating number of configuration options, most of which are not necessary in typical scenarios. The simplest scenario consists of a secure group with one TH at the lowest stratum. For the simplest identity scheme TC, the TH generates host key and trusted certificate files using the ntp-keygen -T command, while the remaining group hosts use the same command with no options. All hosts use the crypto configuration command with no options. Configuration with passwords is described in the ntp-keygen page
When an identity scheme is included, for example IFF, the TH generates host key, trusted certificate and private identity keys files using the ntp-keygen -T -I -i group command, where group is the group name. The remaining group hosts use the same command with no options. All hosts use the crypto ident group configuration command.
Hosts with no dependent clients can retrieve public identity parameters from an archive or web page. The ntp-keygen can export these data using the -e option. Hosts with dependent clients other than the TH must retrieve copies of the TH private identity keys using secure means. The ntp-keygen can export these data using the -q option. In either case the data are installed as a file and then renamed using the name given as the first line in the file, but without the filestamp.
Consider a scenario involving three secure groups RED, GREEN and BLUE. RED and BLUE are typical of national laboratories providing certified time to the Internet at large. TH mort of RED and TH macabre of BLUE run NTP symmetric mode with each other for monitoring or backup. GREEN is typical of a large university providing certified time to the campus community. TH howland of GREEN is a client of both RED and BLUE. BLUE uses the IFF scheme, while both RED and GREEN use the GQ scheme, but with different keys.
BLUE TH macabre uses configuration commands
crypto pw qqsv ident blue
peer mort autokey
where qqsv is the password for macabre files. It generates BLUE files using the commands
ntp-keygen -p qqsv -T -G -i blue
ntp-keygen -p qqsv -e >ntpkey_gqpar_blue
The first line generates the host, trusted certificate and private GQ server files. The second generates the public GQ client file, which can have any nonconflicting mnemonic name.
RED TH mort uses configuration commands
crypto pw xxx ident red
peer macabre autokey
where xxx is the password for mort files. It generates RED files using the commands
ntp-keygen -p xxx -T -I -i red
ntp-keygen -p xxx -e >ntpkey_iffpar_red
GREEN TH howland uses configuration commands
crypto pw yyy ident green
server mort autokey
server macabre autokey
where yyy is the password for mort files. It generates GREEN files using the commands
ntp-keygen -p yyy -T -G -i green
ntp-keygen -p yyy -e >ntpkey_gqpar_green
ntp-keygen -p yyy -v zzz >zzz_ntpkey_gqkey_green
The first two lines serve the same purpose as the preceeding examples. The third line generats a copy of the private GREEN server file for use on another server in the same group, but encrypted with the zzz pasword.
Each TH in a group acting as a client of another group retrieves the public client file for that group from a public archive or web page using nonsecure means. In addition, each server in a group retrieves the private server file from the TH of that group, but it is encrypted and so can be sent using nonsecured means. The files are installed in the keys directory with name taken from the first line in the file, but without the filestamp
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 errors 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. Note that provisions to load leap second values from the NIST files have been removed. These provisions are now available whether or not the OpenSSL library is available. However, the functions that can download these values from servers remains available.