NTP-KEYGEN(8) BSD System Manager's Manual NTP-KEYGEN(8)NAMEntp-keygen — key generation program for ntpd
SYNOPSISntp-keygen [-deGgHIMnPT] [-c scheme] [-i name] [-p password]
[-S [RSA | DSA]] [-s name] [-v nkeys]
DESCRIPTION
This program generates cryptographic data files used by the NTPv4 authen‐
tication and identification schemes. It generates MD5 key files used in
symmetric key cryptography. In addition, if the OpenSSL software library
has been installed, it generates keys, certificate and identity files
used in public key cryptography. These files are used for cookie encryp‐
tion, digital signature and challenge/response identification algorithms
compatible with the Internet standard security infrastructure.
All files are in PEM-encoded printable ASCII format, so they can be
embedded as MIME attachments in mail to other sites and certificate
authorities. By default, files are not encrypted. The -p password
option specifies the write password and -q password option the read pass‐
word for previously encrypted files. The ntp-keygen program prompts for
the password if it reads an encrypted file and the password is missing or
incorrect. If an encrypted file is read successfully and no write pass‐
word is specified, the read password is used as the write password by
default.
The ntpd(8) configuration command crypto pw password specifies the read
password for previously encrypted files. The daemon expires on the spot
if the password is missing or incorrect. For convenience, if a file has
been previously encrypted, the default read password is the name of the
host running the program. If the previous write password is specified as
the host name, these files can be read by that host with no explicit
password.
File names begin with the prefix ntpkey_ and end with the postfix
_hostname.filestamp, where hostname is the owner name, usually the string
returned by the Unix gethostname() routine, and filestamp is the NTP sec‐
onds when the file was generated, in decimal digits. This both guaran‐
tees uniqueness and simplifies maintenance procedures, since all files
can be quickly removed by a rm ntpkey* command or all files generated at
a specific time can be removed by a rm *filestamp command. To further
reduce the risk of misconfiguration, the first two lines of a file con‐
tain the file name and generation date and time as comments.
All files are installed by default in the keys directory /usr/local/etc,
which is normally in a shared filesystem in NFS-mounted networks. The
actual location of the keys directory and each file can be overridden by
configuration commands, but this is not recommended. Normally, the files
for each host are generated by that host and used only by that host,
although exceptions exist as noted later on this page.
Normally, files containing private values, including the host key, sign
key and identification parameters, are permitted root read/write-only;
while others containing public values are permitted world readable.
Alternatively, files containing private values can be encrypted and these
files permitted world readable, which simplifies maintenance in shared
file systems. Since uniqueness is insured by the hostname and file name
extensions, the files for a NFS server and dependent clients can all be
installed in the same shared directory.
The recommended practice is to keep the file name extensions when
installing a file and to install a soft link from the generic names spec‐
ified elsewhere on this page to the generated files. This allows new
file generations to be activated simply by changing the link. If a link
is present, ntpd follows it to the file name to extract the filestamp.
If a link is not present, ntpd(8) extracts the filestamp from the file
itself. This allows clients to verify that the file and generation times
are always current. The ntp-keygen program uses the same timestamp
extension for all files generated at one time, so each generation is dis‐
tinct and can be readily recognized in monitoring data.
Running the program
The safest way to run the ntp-keygen program is logged in directly as
root. The recommended procedure is change to the keys directory, usually
/usr/local/etc, then run the program. When run for the first time, or if
all ntpkey files have been removed, the program generates a RSA host key
file and matching RSA-MD5 certificate file, which is all that is neces‐
sary in many cases. The program also generates soft links from the
generic names to the respective files. If run again, the program uses
the same host key file, but generates a new certificate file and link.
The host key is used to encrypt the cookie when required and so must be
RSA type. By default, the host key is also the sign key used to encrypt
signatures. When necessary, a different sign key can be specified and
this can be either RSA or DSA type. By default, the message digest type
is MD5, but any combination of sign key type and message digest type sup‐
ported by the OpenSSL library can be specified, including those using the
MD2, MD5, SHA, SHA1, MDC2 and RIPE160 message digest algorithms. How‐
ever, the scheme specified in the certificate must be compatible with the
sign key. Certificates using any digest algorithm are compatible with
RSA sign keys; however, only SHA and SHA1 certificates are compatible
with DSA sign keys.
Private/public key files and certificates are compatible with other
OpenSSL applications and very likely other libraries as well. Certifi‐
cates or certificate requests derived from them should be compatible with
extant industry practice, although some users might find the interpreta‐
tion of X509v3 extension fields somewhat liberal. However, the identifi‐
cation parameter files, although encoded as the other files, are probably
not compatible with anything other than Autokey.
Running the program as other than root and using the Unix su command to
assume root may not work properly, since by default the OpenSSL library
looks for the random seed file .rnd in the user home directory. However,
there should be only one .rnd, most conveniently in the root directory,
so it is convenient to define the $RANDFILE environment variable used by
the OpenSSL library as the path to /.rnd.
Installing the keys as root might not work in NFS-mounted shared file
systems, as NFS clients may not be able to write to the shared keys
directory, even as root. In this case, NFS clients can specify the files
in another directory such as /etc using the keysdir command. There is no
need for one client to read the keys and certificates of other clients or
servers, as these data are obtained automatically by the Autokey proto‐
col.
Ordinarily, cryptographic files are generated by the host that uses them,
but it is possible for a trusted agent (TA) to generate these files for
other hosts; however, in such cases files should always be encrypted.
The subject name and trusted name default to the hostname of the host
generating the files, but can be changed by command line options. It is
convenient to designate the owner name and trusted name as the subject
and issuer fields, respectively, of the certificate. The owner name is
also used for the host and sign key files, while the trusted name is used
for the identity files.
Trusted Hosts and Groups
Each cryptographic configuration involves selection of a signature scheme
and identification scheme, called a cryptotype, as explained in the
Authentication Options section of ntp.conf(5). The default cryptotype
uses RSA encryption, MD5 message digest and TC identification. First,
configure a NTP subnet including one or more low-stratum trusted hosts
from which all other hosts derive synchronization directly or indirectly.
Trusted hosts have trusted certificates; all other hosts have nontrusted
certificates. These hosts will automatically and dynamically build
authoritative certificate trails to one or more trusted hosts. A trusted
group is the set of all hosts that have, directly or indirectly, a cer‐
tificate trail ending at a trusted host. The trail is defined by static
configuration file entries or dynamic means described on the Automatic
NTP Configuration Options section of ntp.conf(5).
On each trusted host as root, change to the keys directory. To insure a
fresh fileset, remove all ntpkey files. Then run ntp-keygen-T to gener‐
ate keys and a trusted certificate. On all other hosts do the same, but
leave off the -T flag to generate keys and nontrusted certificates. When
complete, start the NTP daemons beginning at the lowest stratum and work‐
ing up the tree. It may take some time for Autokey to instantiate the
certificate trails throughout the subnet, but setting up the environment
is completely automatic.
If it is necessary to use a different sign key or different digest/signa‐
ture scheme than the default, run ntp-keygen with the -S type option,
where type is either RSA or DSA. The most often need to do this is when
a DSA-signed certificate is used. If it is necessary to use a different
certificate scheme than the default, run ntp-keygen with the -c scheme
option and selected scheme as needed. If ntp-keygen is run again without
these options, it generates a new certificate using the same scheme and
sign key.
After setting up the environment it is advisable to update certificates
from time to time, if only to extend the validity interval. Simply run
ntp-keygen with the same flags as before to generate new certificates
using existing keys. However, if the host or sign key is changed,
ntpd(8) should be restarted. When ntpd(8) is restarted, it loads any new
files and restarts the protocol. Other dependent hosts will continue as
usual until signatures are refreshed, at which time the protocol is
restarted.
Identity Schemes
As mentioned on the Autonomous Authentication page, the default TC iden‐
tity scheme is vulnerable to a middleman attack. However, there are more
secure identity schemes available, including PC, IFF, GQ and MV described
on the "Identification Schemes" page (maybe available at
http://www.eecis.udel.edu/%7emills/keygen.html). These schemes are based
on a TA, one or more trusted hosts and some number of nontrusted hosts.
Trusted hosts prove identity using values provided by the TA, while the
remaining hosts prove identity using values provided by a trusted host
and certificate trails that end on that host. The name of a trusted host
is also the name of its sugroup and also the subject and issuer name on
its trusted certificate. The TA is not necessarily a trusted host in
this sense, but often is.
In some schemes there are separate keys for servers and clients. A
server can also be a client of another server, but a client can never be
a server for another client. In general, trusted hosts and nontrusted
hosts that operate as both server and client have parameter files that
contain both server and client keys. Hosts that operate only as clients
have key files that contain only client keys.
The PC scheme supports only one trusted host in the group. On trusted
host alice run ntp-keygen-P -p password to generate the host key file
ntpkey_RSAkey_alice.filestamp and trusted private certificate file
ntpkey_RSA-MD5_cert_alice.filestamp. Copy both files to all group hosts;
they replace the files which would be generated in other schemes. On
each host bob install a soft link from the generic name ntpkey_host_bob
to the host key file and soft link ntpkey_cert_bob to the private cer‐
tificate file. Note the generic links are on bob, but point to files
generated by trusted host alice. In this scheme it is not possible to
refresh either the keys or certificates without copying them to all other
hosts in the group.
For the IFF scheme proceed as in the TC scheme to generate keys and cer‐
tificates for all group hosts, then for every trusted host in the group,
generate the IFF parameter file. On trusted host alice run ntp-keygen-T
-I -p password to produce her parameter file
ntpkey_IFFpar_alice.filestamp, which includes both server and client
keys. Copy this file to all group hosts that operate as both servers and
clients and install a soft link from the generic ntpkey_iff_alice to this
file. If there are no hosts restricted to operate only as clients, there
is nothing further to do. As the IFF scheme is independent of keys and
certificates, these files can be refreshed as needed.
If a rogue client has the parameter file, it could masquerade as a legit‐
imate server and present a middleman threat. To eliminate this threat,
the client keys can be extracted from the parameter file and distributed
to all restricted clients. After generating the parameter file, on alice
run ntp-keygen-e and pipe the output to a file or mail program. Copy or
mail this file to all restricted clients. On these clients install a
soft link from the generic ntpkey_iff_alice to this file. To further
protect the integrity of the keys, each file can be encrypted with a
secret password.
For the GQ scheme proceed as in the TC scheme to generate keys and cer‐
tificates for all group hosts, then for every trusted host in the group,
generate the IFF parameter file. On trusted host alice run ntp-keygen-T
-G -p password to produce her parameter file
ntpkey_GQpar_alice.filestamp, which includes both server and client keys.
Copy this file to all group hosts and install a soft link from the
generic ntpkey_gq_alice to this file. In addition, on each host bob
install a soft link from generic ntpkey_gq_bob to this file. As the GQ
scheme updates the GQ parameters file and certificate at the same time,
keys and certificates can be regenerated as needed.
For the MV scheme, proceed as in the TC scheme to generate keys and cer‐
tificates for all group hosts. For illustration assume trish is the TA,
alice one of several trusted hosts and bob one of her clients. On TA
trish run ntp-keygen-V n -p password, where n is the number of revokable
keys (typically 5) to produce the parameter file
ntpkeys_MVpar_trish.filestamp and client key files
ntpkeys_MVkeyd_trish.filestamp where d is the key number (0 < d < n).
Copy the parameter file to alice and install a soft link from the generic
ntpkey_mv_alice to this file. Copy one of the client key files to alice
for later distribution to her clients. It doesn't matter which client
key file goes to alice, since they all work the same way. Alice copies
the client key file to all of her cliens. On client bob install a soft
link from generic ntpkey_mvkey_bob to the client key file. As the MV
scheme is independent of keys and certificates, these files can be
refreshed as needed.
Command Line Options
-c scheme
Select certificate message digest/signature encryption scheme.
The scheme can be one of the following: RSA-MD2, RSA-MD5,
RSA-SHA, RSA-SHA1, RSA-MDC2, RSA-RIPEMD160, DSA-SHA, or DSA-SHA1.
Note that RSA schemes must be used with a RSA sign key and DSA
schemes must be used with a DSA sign key. The default without
this option is RSA-MD5.
-d Enable debugging. This option displays the cryptographic data
produced in eye-friendly billboards.
-e Write the IFF client keys to the standard output. This is
intended for automatic key distribution by mail.
-G Generate parameters and keys for the GQ identification scheme,
obsoleting any that may exist.
-g Generate keys for the GQ identification scheme using the existing
GQ parameters. If the GQ parameters do not yet exist, create
them first.
-H Generate new host keys, obsoleting any that may exist.
-I Generate parameters for the IFF identification scheme, obsoleting
any that may exist.
-i name
Set the suject name to name. This is used as the subject field
in certificates and in the file name for host and sign keys.
-M Generate MD5 keys, obsoleting any that may exist.
-P Generate a private certificate. By default, the program gener‐
ates public certificates.
-p password
Encrypt generated files containing private data with password and
the DES-CBC algorithm.
-q Set the password for reading files to password.
-S [RSA | DSA]
Generate a new sign key of the designated type, obsoleting any
that may exist. By default, the program uses the host key as the
sign key.
-s name
Set the issuer name to name. This is used for the issuer field
in certificates and in the file name for identity files.
-T Generate a trusted certificate. By default, the program gener‐
ates a non-trusted certificate.
-V nkeys
Generate parameters and keys for the Mu-Varadharajan (MV) identi‐
fication scheme.
Random Seed File
All cryptographically sound key generation schemes must have means to
randomize the entropy seed used to initialize the internal pseudo-random
number generator used by the library routines. The OpenSSL library uses
a designated random seed file for this purpose. The file must be avail‐
able when starting the NTP daemon and ntp-keygen program. If a site sup‐
ports OpenSSL or its companion OpenSSH, it is very likely that means to
do this are already available.
It is important to understand that entropy must be evolved for each gen‐
eration, for otherwise the random number sequence would be predictable.
Various means dependent on external events, such as keystroke intervals,
can be used to do this and some systems have built-in entropy sources.
Suitable means are described in the OpenSSL software documentation, but
are outside the scope of this page.
The entropy seed used by the OpenSSL library is contained in a file, usu‐
ally called .rnd, which must be available when starting the NTP daemon or
the ntp-keygen program. The NTP daemon will first look for the file
using the path specified by the randfile subcommand of the crypto config‐
uration command. If not specified in this way, or when starting the
ntp-keygen program, the OpenSSL library will look for the file using the
path specified by the RANDFILE environment variable in the user home
directory, whether root or some other user. If the RANDFILE environment
variable is not present, the library will look for the .rnd file in the
user home directory. If the file is not available or cannot be written,
the daemon exits with a message to the system log and the program exits
with a suitable error message.
Cryptographic Data Files
All other file formats begin with two lines. The first contains the file
name, including the generated host name and filestamp. The second con‐
tains the datestamp in conventional Unix date format. Lines beginning
with # are considered comments and ignored by the ntp-keygen program and
ntpd(8) daemon. Cryptographic values are encoded first using ASN.1
rules, then encrypted if necessary, and finally written PEM-encoded
printable ASCII format preceded and followed by MIME content identifier
lines.
The format of the symmetric keys file is somewhat different than the
other files in the interest of backward compatibility. Since DES-CBC is
deprecated in NTPv4, the only key format of interest is MD5 alphanumeric
strings. Following hte heard the keys are entered one per line in the
format
keyno type key
where keyno is a positive integer in the range 1-65,535, type is the
string MD5 defining the key format and key is the key itself, which is a
printable ASCII string 16 characters or less in length. Each character
is chosen from the 93 printable characters in the range 0x21 through 0x7f
excluding space and the ‘#’ character.
Note that the keys used by the ntpq(8) and ntpdc(8) programs are checked
against passwords requested by the programs and entered by hand, so it is
generally appropriate to specify these keys in human readable ASCII for‐
mat.
The ntp-keygen program generates a MD5 symmetric keys file
ntpkey_MD5key_hostname.filestamp. Since the file contains private shared
keys, it should be visible only to root and distributed by secure means
to other subnet hosts. The NTP daemon loads the file ntp.keys, so
ntp-keygen installs a soft link from this name to the generated file.
Subsequently, similar soft links must be installed by manual or automated
means on the other subnet hosts. While this file is not used with the
Autokey Version 2 protocol, it is needed to authenticate some remote con‐
figuration commands used by the ntpq(8) and ntpdc(8) utilities.
Bugs
It can take quite a while to generate some cryptographic values, from one
to several minutes with modern architectures such as UltraSPARC and up to
tens of minutes to an hour with older architectures such as SPARC IPC.
BSD May 17, 2006 BSD
