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nts: make server and client AEAD algorithms configurable

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Add ntsaeads directive to specify a list of AEAD algorithms enabled for
NTS. The list is shared between the server and client. For the client it
also specifies the order of priority. The default is "30 15", matching
the previously hardcoded preference of AES-128-GCM-SIV (30) over
AES-SIV-CMAC-256 (15).
This commit is contained in:
Miroslav Lichvar
2024-10-03 14:25:53 +02:00
parent 79a790e6b5
commit 42fbf41686
6 changed files with 163 additions and 17 deletions
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65
doc/chrony.conf.adoc
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@@ -833,6 +833,34 @@ changes in the frequency and offset of the clock. The offsets in the
<<chronyc.adoc#sourcestats,*sourcestats*>> reports (and the _tracking.log_ and
_statistics.log_ files) may be smaller than the actual offsets.
[[ntsaeads1]]*ntsaeads* _ID_...::
This directive specifies a list of IDs of Authenticated Encryption with
Associated Data (AEAD) algorithms enabled for NTS authentication of NTP
messages. The algorithms are specified in decreasing order of priority.
Algorithms that are not supported by the installed version of the crypto
library (Nettle, GnuTLS) are ignored.
+
The following IDs are supported:
+
* 15: AES-SIV-CMAC-256
* 30: AES-128-GCM-SIV
{blank}::
+
The default list of IDs is _30 15_. AES-128-GCM-SIV is prefered over
AES-SIV-CMAC-256 for shorter keys, which makes NTS cookies shorter and improves
reliability of NTS in networks that block or limit rate of longer NTP messages.
+
The ID of the used algorithm is reported for each server by the
<<chronyc.adoc#authdata,*authdata*>> command.
+
An example of the directive is:
+
----
ntsaeads 15
----
+
This list is used also by the <<ntsaeads2,NTS server>>.
[[ntsdumpdir1]]*ntsdumpdir* _directory_::
This directive specifies a directory for the client to save NTS cookies it
received from the server in order to avoid making an NTS-KE request when
@@ -1779,6 +1807,43 @@ per process that the NTS server will accept. The default value is 100. The
maximum practical value is half of the system *FD_SETSIZE* constant (usually
1024).
[[ntsaeads2]]*ntsaeads* _ID_...::
This directive specifies a list of IDs of Authenticated Encryption with
Associated Data (AEAD) algorithms enabled for NTS authentication of NTP
messages. *chronyd* as a server uses the first enabled algorithm from the list
provided by the client. Algorithms that are not supported by the installed
version of the crypto library (Nettle, GnuTLS) are ignored.
+
The following IDs are supported:
+
* 15: AES-SIV-CMAC-256
* 30: AES-128-GCM-SIV
{blank}::
+
The default list of IDs is _30 15_. AES-128-GCM-SIV is prefered over
AES-SIV-CMAC-256 for shorter keys, which makes NTS cookies shorter and improves
reliability of NTS in networks that block or limit rate of longer NTP messages.
+
An example of the directive is:
+
----
ntsaeads 15
----
+
This list is used also by the <<ntsaeads1,NTS client>>.
+
Note the the NTS specification (RFC 8915) requires servers to support
AES-SIV-CMAC-256, i.e. 15 should be always included in the specified list.
+
The AES-128-GCM-SIV keys used by *chronyd* do not comply to RFC 8915 for
compatibility with older *chrony* clients unless the use of compliant keys is
negotiated with an
https://chrony-project.org/doc/spec/nts-compliant-128gcm.html[NTS-KE record].
Support for this record was added in version 4.6.1. As a client, *chronyd* can
interoperate with a server that uses compliant keys, but does not support the
negotiation, if it responds to incorrectly authenticated requests with an NTS
NAK.
[[ntsdumpdir2]]*ntsdumpdir* _directory_::
This directive specifies a directory where *chronyd* operating as an NTS server
can save the keys which encrypt NTS cookies provided to clients. The keys are