When the NTS client and server negotiated use of AES-128-GCM-SIV keys,
the keys exported from the TLS session and used for authentication and
encryption of NTP messages do not comply to RFC8915. The exporter
context value specified in the section 5.1 of RFC8915 function is
incorrect. It is a hardcoded string which contains 15 (AES-SIV-CMAC-256)
instead of 30 (AES-128-GCM-SIV). This causes chrony to not interoperate
with NTS implementations that follow RFC8915 correctly. (At this time,
there doesn't seem to be another implementation with AES-128-GCM-SIV
support yet.)
Replace the string with a proper construction of the exporter context
from a specified AEAD ID and next protocol.
Keep using the incorrect AEAD ID for AES-128-GCM-SIV to not break
compatibility with existing chrony servers and clients. A new NTS-KE
record will be added to negotiate the compliant exporter context.
Reported-by: Martin Mayer <martin.mayer@m2-it-solutions.de>
Add a context structure for the algorithm and keys established by
NTS-KE. Modify the client to save the context and reset the SIV key to
the C2S/S2C key before each request/response instead of keeping two SIV
instances.
This will make it easier for the server to support different algorithms
and allow the client to save the context with cookies to disk.
Make the NTS-KE retry interval exponentially increasing, using a factor
provided by the NKE session. Use shorter intervals when the server is
refusing TCP connections or the connection is closed or timing out
before the TLS handshake.
