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keys: add support for CMAC keys

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Allow a cipher (AES128 or AES256) to be specified as the type of a key
in the key file to authenticate NTP packets with a CMAC instead of the
NTPv4 (RFC 5905) MAC using a hash function. This follows RFC 8573.
This commit is contained in:
Miroslav Lichvar
2019-09-17 16:59:55 +02:00
parent e8069a0179
commit 57957ab6cf
4 changed files with 127 additions and 56 deletions
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45
doc/chrony.conf.adoc
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@@ -96,7 +96,7 @@ interval in order to allow a burst with two requests.
*key* _ID_:::
The NTP protocol supports a message authentication code (MAC) to prevent
computers having their system time upset by rogue packets being sent to them.
The MAC is generated as a function of a password specified in the key file,
The MAC is generated as a function of a key specified in the key file,
which is specified by the <<keyfile,*keyfile*>> directive.
+
The *key* option specifies which key (with an ID in the range 1 through 2^32-1)
@@ -2017,8 +2017,10 @@ include @SYSCONFDIR@/chrony.d/*.conf
----
[[keyfile]]*keyfile* _file_::
This directive is used to specify the location of the file containing ID-key
pairs for authentication of NTP packets.
This directive is used to specify the location of the file containing symmetric
keys which are shared between NTP servers and clients, or peers, in order to
authenticate NTP packets with a message authentication code (MAC) using a
cryptographic hash function or cipher.
+
The format of the directive is shown in the example below:
+
@@ -2033,30 +2035,41 @@ format of the file is shown below:
10 tulip
11 hyacinth
20 MD5 ASCII:crocus
25 SHA1 HEX:1dc764e0791b11fa67efc7ecbc4b0d73f68a070c
25 SHA1 HEX:933F62BE1D604E68A81B557F18CFA200483F5B70
30 AES128 HEX:7EA62AE64D190114D46D5A082F948EC1
31 AES256 HEX:37DDCBC67BB902BCB8E995977FAB4D2B5642F5B32EBCEEE421921D97E5CBFE39
...
----
+
Each line consists of an ID, name of an authentication hash function (optional),
and a password. The ID can be any unsigned integer in the range 1 through
2^32-1. The default hash function is *MD5*, which is always supported.
Each line consists of an ID, optional type, and key.
+
The ID can be any positive integer in the range 1 through 2^32-1.
+
The type is a name of a cryptographic hash function or cipher which is used to
generate and verify the MAC. The default type is *MD5*, which is always
supported.
If *chronyd* was built with enabled support for hashing using a crypto library
(nettle, nss, or libtomcrypt), the following functions are available: *MD5*,
*SHA1*, *SHA256*, *SHA384*, *SHA512*. Depending on which library and version is
*chronyd* using, some or all of the following functions may also be available:
*SHA3-224*, *SHA3-256*, *SHA3-384*, *SHA3-512*, *TIGER*, *WHIRLPOOL*.
*chronyd* using, some of the following hash functions and ciphers may
also be available:
*SHA3-224*, *SHA3-256*, *SHA3-384*, *SHA3-512*, *TIGER*, *WHIRLPOOL*, *AES128*,
*AES256*.
+
The password can be specified as a string of characters not containing white
The key can be specified as a string of ASCII characters not containing white
space with an optional *ASCII:* prefix, or as a hexadecimal number with the
*HEX:* prefix. The maximum length of the line is 2047 characters.
If the type is a cipher, the length of the key must match the cipher (i.e. 128
bits for AES128 and 256 bits for AES256).
+
The password is used with the hash function to generate and verify a message
authentication code (MAC) in NTP packets. It is recommended to use SHA1, or
stronger, hash function with random passwords specified in the hexadecimal
format that have at least 128 bits. *chronyd* will log a warning to
syslog on start if a source is specified in the configuration file with a key
that has password shorter than 80 bits.
It is recommended to use randomly generated keys, specified in the hexadecimal
format, which are at least 128 bits long (i.e. they have at least 32 characters
after the *HEX:* prefix). *chronyd* will log a warning to syslog on start if a
source is specified in the configuration file with a key shorter than 80 bits.
+
The recommended key types are AES ciphers and SHA3 hash functions. MD5 should
be avoided unless no other type is supported on the server and client, or
peers.
+
The <<chronyc.adoc#keygen,*keygen*>> command of *chronyc* can be used to
generate random keys for the key file. By default, it generates 160-bit MD5 or