allow native and ssh-keygen public key check

This commit adds the possibibility to use either the native golang
libraries or ssh-keygen to check public keys. The check is adjusted
depending on the settings, so that only supported keys are let through.

This commit also brings back the blacklist feature, which was removed in
7ef9a05588. This allows to blacklist
algorythms or keys based on the key length. This works with the native
and the ssh-keygen way.

Because of #2179 it also includes a way to adjust the path to
ssh-keygen and the working directory for ssh-keygen. With this,
sysadmins should be able to adjust the settings in a way, that SELinux
is okay with it. In the worst case, they can switch to the native
implementation and only loose support for ed25519 keys at the moment.
There are some other places which need adjustment to utilize the
parameters and the native implementation, but this sets the ground work.

conf/app.ini

@@ -66,6 +66,13 @@ START_SSH_SERVER = false
 SSH_PORT = 22
 ; Root path of SSH directory
 SSH_ROOT_PATH = 
+; override engine choice to check public keys (default: 'ssh-keygen' when
+; DISABLE_SSH is set to false else 'native')
+SSH_PUBLICKEY_CHECK =
+; directory to create temporary files when using ssh-keygen (default: /tmp)
+SSH_WORK_PATH =
+; path to ssh-keygen (default: result of `which ssh-keygen`)
+SSH_KEYGEN_PATH =
 ; Disable CDN even in "prod" mode
 OFFLINE_MODE = false
 DISABLE_ROUTER_LOG = false
@@ -127,6 +134,15 @@ ENABLE_REVERSE_PROXY_AUTHENTICATION = false
 ENABLE_REVERSE_PROXY_AUTO_REGISTRATION = false
 ; Enable captcha validation for registration
 ENABLE_CAPTCHA = true
+; Do not check minimum key size with corresponding type
+ENABLE_MINIMUM_KEY_SIZE_CHECK = false
+
+; define allowed algorithms and their minimum key length (use -1 to disable a type)
+[service.minimum_key_sizes]
+ED25519 = 256
+ECDSA   = 256
+RSA     = 2048
+DSA     = 1024
 
 [webhook]
 ; Hook task queue length