Initialize the unused part of shorter server NTS keys (AES-128-GCM-SIV)
loaded from ntsdumpdir to avoid sending uninitialized data in requests
to the NTS-KE helper process.
Do that also for newly generated keys in case the memory will be
allocated dynamically.
Fixes: b1230efac3 ("nts: add support for encrypting cookies with AES-128-GCM-SIV")
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.)
When the request has an unrecognized critical record before the
NEXT_PROTOCOL and AEAD_ALGORITHM records, respond with error 0
(unrecognized critical record) instead of 1 (bad request).
When the request has multiple NEXT_PROTOCOL or AEAD_ALGORITHM records,
respond with error 1 (bad request).
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.
The server session instances are reused for different clients. Separate
the server name from the label used in log messages and set it on each
start of the session.
