Rework the permissions model for SSH key restrictions

`allowed_key_types` is removed and the `minimum_<type>_bits` fields are
renamed to `<tech>_key_restriction`. A special sentinel value (`-1`) signifies
that the key type is disabled.

This also feeds through to the UI - checkboxes per key type are out, inline
selection of "forbidden" and "allowed" (i.e., no restrictions) are in.

As with the previous model, unknown key types are disallowed, even if the
underlying ssh daemon happens to support them. The defaults have also been
changed from the lowest known bit size to "no restriction". So if someone
does happen to have a 768-bit RSA key, it will continue to work on upgrade, at
least until the administrator restricts them.

3 files changed, 27 insertions, 37 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 8f766ba4f8d..803b48dd88a 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -744,7 +744,6 @@ module API
       expose(:default_snippet_visibility) { |setting, _options| Gitlab::VisibilityLevel.string_level(setting.default_snippet_visibility) }
       expose(:default_group_visibility) { |setting, _options| Gitlab::VisibilityLevel.string_level(setting.default_group_visibility) }
       expose :password_authentication_enabled, as: :signin_enabled
-      expose :allowed_key_types
     end
 
     class Release < Grape::Entity
diff --git a/lib/api/settings.rb b/lib/api/settings.rb
index 6ace0e1e390..01123e45ee0 100644
--- a/lib/api/settings.rb
+++ b/lib/api/settings.rb
@@ -122,11 +122,12 @@ module API
       optional :terminal_max_session_time, type: Integer, desc: 'Maximum time for web terminal websocket connection (in seconds). Set to 0 for unlimited time.'
       optional :polling_interval_multiplier, type: BigDecimal, desc: 'Interval multiplier used by endpoints that perform polling. Set to 0 to disable polling.'
 
-      optional :minimum_rsa_bits, type: Integer, values: Gitlab::SSHPublicKey.allowed_sizes('rsa'), desc: 'The minimum allowed bit length of an uploaded RSA key.'
-      optional :minimum_dsa_bits, type: Integer, values: Gitlab::SSHPublicKey.allowed_sizes('dsa'), desc: 'The minimum allowed bit length of an uploaded DSA key.'
-      optional :minimum_ecdsa_bits, type: Integer, values: Gitlab::SSHPublicKey.allowed_sizes('ecdsa'), desc: 'The minimum allowed curve size (in bits) of an uploaded ECDSA key.'
-      optional :minimum_ed25519_bits, type: Integer, values: Gitlab::SSHPublicKey.allowed_sizes('ed25519'), desc: 'The minimum allowed curve size (in bits) of an uploaded ED25519 key.'
-      optional :allowed_key_types, type: Array[String], values: Gitlab::SSHPublicKey.technology_names, desc: 'The SSH key types accepted by the application (`rsa`, `dsa`, `ecdsa` or `ed25519`).'
+      ApplicationSetting::SUPPORTED_KEY_TYPES.each do |type|
+        optional :"#{type}_key_restriction",
+                 type: Integer,
+                 values: ApplicationSetting.supported_key_restrictions(type),
+                 desc: "Restrictions on the complexity of uploaded #{type.upcase} keys. A value of #{ApplicationSetting::FORBIDDEN_KEY_VALUE} disables all #{type.upcase} keys."
+      end
 
       optional(*::ApplicationSettingsHelper.visible_attributes)
       at_least_one_of(*::ApplicationSettingsHelper.visible_attributes)
diff --git a/lib/gitlab/ssh_public_key.rb b/lib/gitlab/ssh_public_key.rb
index 2df31bcc246..a3f8730fb04 100644
--- a/lib/gitlab/ssh_public_key.rb
+++ b/lib/gitlab/ssh_public_key.rb
@@ -1,31 +1,20 @@
 module Gitlab
   class SSHPublicKey
-    TYPES = %w[rsa dsa ecdsa ed25519].freeze
-
-    Technology = Struct.new(:name, :allowed_sizes)
+    Technology = Struct.new(:name, :key_class, :supported_sizes)
 
     Technologies = [
-      Technology.new('rsa',     [1024, 2048, 3072, 4096]),
-      Technology.new('dsa',     [1024, 2048, 3072]),
-      Technology.new('ecdsa',   [256, 384, 521]),
-      Technology.new('ed25519', [256])
+      Technology.new(:rsa, OpenSSL::PKey::RSA, [1024, 2048, 3072, 4096]),
+      Technology.new(:dsa, OpenSSL::PKey::DSA, [1024, 2048, 3072]),
+      Technology.new(:ecdsa, OpenSSL::PKey::EC, [256, 384, 521]),
+      Technology.new(:ed25519, Net::SSH::Authentication::ED25519::PubKey, [256])
     ].freeze
 
-    def self.technology_names
-      Technologies.map(&:name)
-    end
-
     def self.technology(name)
-      Technologies.find { |ssh_key_technology| ssh_key_technology.name == name }
+      Technologies.find { |tech| tech.name.to_s == name.to_s }
     end
-    private_class_method :technology
 
-    def self.allowed_sizes(name)
-      technology(name).allowed_sizes
-    end
-
-    def self.allowed_type?(type)
-      technology_names.include?(type.to_s)
+    def self.supported_sizes(name)
+      technology(name)&.supported_sizes
     end
 
     attr_reader :key_text, :key
@@ -50,18 +39,7 @@ module Gitlab
     def type
       return unless valid?
 
-      case key
-      when OpenSSL::PKey::EC
-        :ecdsa
-      when OpenSSL::PKey::RSA
-        :rsa
-      when OpenSSL::PKey::DSA
-        :dsa
-      when Net::SSH::Authentication::ED25519::PubKey
-        :ed25519
-      else
-        raise "Unsupported key type: #{key.class}"
-      end
+      technology.name
     end
 
     def bits
@@ -80,5 +58,17 @@ module Gitlab
         raise "Unsupported key type: #{type}"
       end
     end
+
+    private
+
+    def technology
+      @technology ||=
+        begin
+          tech = Technologies.find { |tech| key.is_a?(tech.key_class) }
+          raise "Unsupported key type: #{key.class}" unless tech
+
+          tech
+        end
+    end
   end
 end