kex: allow SNTRU-Prime (#8)

* Add SNTRU-Prime KEX (Post-quantum key exchange)

* Add the ChaCha20-Poly1305 AEAD

* Update the "Last Updated" date

* Update the Copyright year

* Update sshd_config

* feat(sshd): increased the priority of KexAlgorithm sntrup761x25519-sha512

---------

Co-authored-by: K4YT3X <github@k4yt3x.com>

Author: brxken128

sshd_config

@@ -1,11 +1,13 @@
 # Name: K4YT3X Hardened OpenSSH Configuration
 # Author: K4YT3X
+# Contributor: IceCodeNew
+# Contributor: brxken128
 # Date Created: October 5, 2020
-# Last Updated: December 21, 2023
+# Last Updated: February 8, 2024
 
 # Licensed under the GNU General Public License Version 3 (GNU GPL v3),
 #   available at: https://www.gnu.org/licenses/gpl-3.0.txt
-# (C) 2020-2023 K4YT3X
+# (C) 2020-2024 K4YT3X
 
 ########## Binding ##########
 
@@ -103,9 +105,10 @@ HostKeyAlgorithms rsa-sha2-512,rsa-sha2-256,ssh-ed25519
 MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com
 
 # short moduli should be deactivated before enabling the use of diffie-hellman-group-exchange-sha256
-# see this link for more details: https://github.com/k4yt3x/sshd_config#deactivating-short-diffie-hellman-moduli
-#KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256
-KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256
+#   see this link for more details: https://github.com/k4yt3x/sshd_config#deactivating-short-diffie-hellman-moduli
+# Chacha20-Poly1305 has been removed to mitigate the Terrapin attack: https://terrapin-attack.com/
+# ecdh-sha2-nistp* algorithms have been removed due to concerns around NIST P-curves' design: https://github.com/jtesta/ssh-audit/issues/213#issuecomment-1774204745
+KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512
 
 ########## Connection Preferences ##########