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doc: warn about MD5 keys not protecting extension fields

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Add a warning to the chrony.conf man page that MD5 keys cannot protect
NTP extension fields due to the length extension attack.
This commit is contained in:
Miroslav Lichvar 2024-10-08 14:13:13 +02:00
parent b9f5278846
commit cd65e32cf0
1 changed files with 5 additions and 1 deletions
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6
doc/chrony.conf.adoc
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@ -2821,7 +2821,11 @@ 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.
peers. A major weakness of MD5 for the NTP MAC is a length extension attack,
where a man-in-the-middle attacker can add arbitrary extension fields to the
NTP message and update the MAC to pass the verification of the extended
message. The *extfield* option (enabling processing of the specified extension
field) should not be used for NTP sources authenticated with an MD5 key.
+
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