Implement KexAlgorithms settings, based on Key exchange section of

https://stribika.github.io/2015/01/04/secure-secure-shell.html

Note, that on some systems it is uncertain if they will have a new
enough version of openssh installed, so on those a version test is done
to see before setting them.

templates/sshd_config/CentOS_6.erb

@@ -151,6 +151,9 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/CentOS_7.erb

@@ -165,6 +165,9 @@ AllowGroups <%= s %>
 #  CheckHostIP no
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Debian_jessie.erb

@@ -112,6 +112,7 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+KexAlgorithms curve25519-sha256@libssh.org
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Debian_sid.erb

@@ -112,6 +112,7 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+KexAlgorithms curve25519-sha256@libssh.org
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Debian_wheezy.erb

@@ -115,6 +115,9 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/FreeBSD.erb

@@ -153,6 +153,9 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Gentoo.erb

@@ -148,6 +148,9 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/OpenBSD.erb

@@ -129,6 +129,9 @@ AllowGroups <%= s %>
 #	ForceCommand cvs server
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Ubuntu.erb

@@ -116,6 +116,9 @@ AllowGroups <%= s %>
 <%- end -%>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>

templates/sshd_config/Ubuntu_lucid.erb

@@ -119,6 +119,9 @@ AllowGroups <%= s %>
 PrintMotd <%= scope.lookupvar('sshd::print_motd') %>
 
 <% if scope.lookupvar('sshd::hardened') == 'yes' -%>
+<% if (scope.function_versioncmp([scope.lookupvar('::ssh_version'),'6.5'])) >= 0 -%>
+KexAlgorithms curve25519-sha256@libssh.org
+<% end -%>
 Ciphers aes256-ctr
 MACs hmac-sha1
 <% end -%>