Keep a server SIV instance for each available algorithm.
Select AES-128-GCM-SIV if requested by NTS-KE client as the first
supported algorithm.
Instead of encoding the AEAD ID in the cookie, select the algorithm
according to the length of decrypted keys. (This can work as a long as
all supported algorithms use keys with different lengths.)
Since commit fdfcabd79b ("ntp: drop support for long NTPv4 MACs"), the
parser doesn't need to check validify of MACs in NTPv4 packets to
distinguish them from extension fields. Move the parser to ntp_core to
avoid having a separate iteration looking for non-authentication
extension fields.
Modify NNA_DecryptAuthEF() to not assume that the authenticator is the
last extension field in the packet as some extension fields specified in
future may need to be placed after the authenticator. The caller of the
function is supposed to verify the position.
When compiled with NTS support, don't require a SIV cipher to be always
supported (e.g. due to a different version of a library used for
building). Handle this case with a fatal message instead of crash.
Also, check the support early in the client unit test to prevent a hang.
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.
